Investigations into the effect of hydraulic fracturing fluids viscosity on the fracture's propagations in unconventional gas reservoir

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The viscous oil, water, and liquid-CO2 fracturing fluids were evaluated under both ambient and uniaxial stress conditions. Experiments involving water and oil show that cracks propagate within the load orientation, perpendicular to the sedimentary plane, and along the sediment plane under the uniaxial load conditions. Results imply that in-situ stress conditions greatly affect the development of cracks. Fractures induced by water showed several branches starting from the loading axis. On fluorescent microscopy, however, hydraulic fractures using dense oil produced straight fractures with minimal branching. Viscous oil caused the Mode I fracture according to a statistical analysis of the P wave direction within sound emission frequencies. By contrast, liquid CO2 and water produced the Mode II fracture. Unlike the other two fluids, under loading conditions liquid-CO2 injection had no effect on the propagation of cracks. The low viscosity indicates that low-viscosity fluids, such CO2, cause extensive, branched cracks mostly Mode II fractures. For these reasons, shale gas generation beats slick water injections. Hydraulic fracturing Fluids density Unconventional reservoir Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1. Introduction Shale gas is a naturally occurring gas found in shale formations, which are fine-grained sedimentary rocks that serve as significant reservoirs for natural gas and petroleum. Shallowly permeable shale significantly impedes gas migration through rocks of the reservoir to the wells of the production. Economic feasibility of gas produced from shale extraction relies about the efficiency of the stimulation within the unconventional reservoirs. Recent studies indicate that horizontal drilling employing a multi-stage hydraulic fracturing approach demonstrates significant efficiency in unconventional gas reservoirs [ 1 ]. Hydraulic fracturing generates and maintains fractures by injecting fluid at pressures exceeding the rock's failure stress. In 2023, Liu et al., established the methodology for quantifying hydraulic fracturing stress. Wei et la., (2023) and Jiang et. al., (2024), advanced discipline by presenting an initial variation of the hydraulic fracture concept and examining fracture management. A multitude of experimental and theoretical research have been conducted and examined [ 2 , 3 , 4 ]. Diverse geological processes sculpt strata and reveal complex stress fields [ 5 ]. The magnitude and orientation of the principal stresses, along with the shape and direction of the fracture, determine the pressure range required to create and maintain a fracture in hydraulically fractured rocks. The anisotropy of the rock's strength is a significant factor influencing the trajectory of hydraulic fracturing in rock. The 2021 study by Liu et al. examined the propagation of fractures during the hydraulic fracturing of oil shale. Their findings indicate that fractures influenced by anisotropic characteristics and existing in-situ stress conditions exhibit elliptical shapes and extend in several directions [ 6 ]. Li et al. [ 7 ] examined hydraulic fracturing utilizing three distinct fracturing fluids: water, carbon dioxide, and viscous oil. Low-viscosity fluids, such as carbon dioxide, were anticipated to induce substantial cracks with numerous branches. The fissures in the shale, generated by carbon dioxide due to its greater surface area, which is more effective than water, enhance shale gas production. The aim of this study is to investigate acoustic emissions and the effect of fluid viscosity on the unconventional shale gas reservoir through an analysis of shale core fracture propagation under the influence of sedimentary layer stress and anisotropies. This research examines the microscopic properties and modes of fractures in relation to the viscosities of water, viscous oil, and CO 2 . The findings could provide valuable insights for projects in the energy industry, including greenhouse gas storage and shale gas production. 2. Experimental Section 2.1 Cores and Experimental Setup The experiments made use of six shale cores with dimensions of 80 millimeters in diameter and roughly 165-millimeter height. During the excavation of a drift at a coal mine in Iraq, samples were gathered 310 meters below sea level. Each block comprised sandy shale taken from a coal-bearing formation. The block surfaces' drilling orientation was perpendicular to the sedimentary planes. For hydraulic fracturing, a 12 mm diameter hole was vertically drilled midway on the side of the core. Figure 1 shows the cores shown using a Cartesian coordinate system; the X-axis runs across the hole that was drilled; the Y-axis runs opposite the other two axes. A Z-axis is orthogonal with the sediment line and coincides with the cylindrical axis. Every sample's P wave velocity was measured along all three designated axes. Apart from cores A-1 and Am-12, P wave velocity readings show perpendicular anisotropy connected with sedimentary bedding (Table 1 ). Using uniaxial strains of 1.25 MPa as well as 3.25 MPa across a z-axis (σz), hydraulic fracturing tests were carried out under ambient conditions with stress applied perpendicular to the sedimentary planes. A total of four 10-millimeter gauges for strain had been attached onto the core prior to load constancy testing. Table 1 Anisotropic velocity of P-waves across shale samples across different axes. No. of Cores Speed of the P-wave (Km/s) x-axis y-axis z-axis A-1 3.21 3.14 3.58 A-3 3.14 3.15 2.78 A-4 3.25 3.26 2.99 A-5 3.67 3.38 2.21 A-7 2.90 3.14 2.88 A-9 2.85 3.60 2.84 Am-1 3.30 3.40 2.88 Am-6 3.38 3.25 3.14 Am-7 3.29 3.27 3.08 Am-12 3.57 3.78 3.99 Table 1 lists the anisotropic velocity of P-waves in shale samples along several axes. Monitoring and describing fractures depend on exact location of source events [ 8 ]. Event matching and event localization both depend much on velocity anisotropy [ 8 ]. Ignoring velocity anisotropy could cause major error in the event of localization. The p-wave velocity and the site of acoustic emission in the shale sample were analyzed in three dimensions employing the three distinct categories of fracturing fluids: water, viscous oil, and CO2. Each experiment was thoroughly explained utilizing numerous samples. Calculating a percentage of the compressional beginning movement compared to total count of initial motions improves fracture mode diagnosis considering the polarity of the early P-wave movements. While Mode II for a plane shear fracture predicts 50%, the expected percentage for a tensile crack is 100%. Experimental results show that whilst Mode II fractures are produced by injecting water and L-CO2, Mode I fractures result from viscous oil injection. 2.2 Methods of Fluid Injection Hydraulic fracturing used three fluid categories: water, liquid carbon dioxide (L-CO2) and viscous oil. Lubricant used was a particular kind of vehicle transmission oil more especially, Super Transmission Oil. Whereas the Liquid CO2 showed around one-tenth that of water, the viscous oil showed a viscosity 250 times more in the temperature test. The geomechanically properties of the shale sample and fluids properties explain in Table 2 . The packer, with a 35 mm pressurising section, was precisely situated at the centre of the injection hole. Fluids were administered at constant the rate of flow of one mL/min, with injection pressure documented at 0.11-second periods. An adjustable discharge rate linked to the oracle by connector tubes using a fluid injection syringe pump. A pressure transducer near the packer tracked fluid pressure during injection. 2.3 Methods for Detecting Acoustic Emissions, Pressure, and Temperature Eighteen cylindrical sensors was used to identify events of sonic emission. With a 4-millimeter diameter and a 5-mm length, each sensor displayed a resonance frequency of about 500 kh. 39 dB from the preamplifier and 50 dB from the signal conditioner were combined to amplify the found acoustic emission signals to 88 dB, due to very high noise. The conditioner of the signal was tuned to about 28 dB to lower high noise levels in testing using liquid CO2 as a fracturing fluid. An Analogue/ digital (A/D) converter wrote the amplified signals onto a hard disk. With an 18 bit resolution and a 0.14 µs sampling time, the event record length was 1850 words. Post-event recording with a dead time of one ms was implemented to reduce too high hard disk noise generated by vibrations after a significant acoustic emissions event. Recording acoustic emission events started when any of the eighteen sensors sensed a signal above 4 V. Usually, these triggered occurrences combine several signals. Throughout the injecting process, adverse effects were seen and recorded. Using P-wave arrival timings at sensors, the hypocenters of acoustic emissions were calculated and matched with visual detection data to describe the orientation and form of fractures on the core surfaces. We achieved this by means of a least squares approach. A statistical examination of the various polarities for the P-wave initiation and their associated acoustic emission waveform, utilising Kakadjian et al. (2021) and Iferobia et al. (2022), enhances the understanding of the fractured mode [ 8 , 9 ]. Their polarity facilitated being categorised as either compression or dilatation. Each acoustic emission event documented the fundamental counts of compressional and dilatational movements. The initial compressional motion ratio relative to the total of initial compressional as well as dilatational motions can be determined through computations. 2.4 Analysis of fractures utilizing fluorescent resin In fluorescence microscopy (Xu, 2024), the application of a fluorescent agent and resin enhances the viewing of fractures and pores in rocks. This section elucidates the microscopic examination of shale specimens subjected to fracturing via viscous oil and water injection [ 11 ]. To enhance the resin's strength, cores have been immersed into resin under vacuum about one week and subsequently subjected to temperatures exceeding 90°C. The cores were subsequently sectioned perpendicular to the fracture plane formed by hydraulic fracturing and readied for ultraviolet light microscopic examination. 3. Results and Discussion Shale cores were subjected to three hydraulic fracturing fluids under ambient circumstances and between 1.25 and 3.25 MPa uniaxial pressures. Examining the results produced twelve tests that validated their dependability. Table (2) displays the analysis findings; the last columns indicate the induced fracture orientation depending on stress and fracturing fluid. Table 2 Summarizes shale hydraulic fracturing experiments. Number of Core Applied stress σz (MPa) Fracturing fluid (measured in millipascals) Injections per minute (ml/min) Failure Pressure (MPa) Identified acoustic emission sources Fracture orientation A-1 0 Water (1) 1 5.26 144 Aligned with bedding A-3 0 Water (1) 1 8.68 82 Aligned with bedding A-2 1.25 Water (1) 1 13.14 170 Aligned with bedding A-4 3.25 Water (1) 1 16.88 207 Aligned with bedding A-5 0 Oil (270) 1 9.98 93 Aligned with bedding A-7 1.25 Oil (270) 1 8.89 268 Direction of loading A-9 3.25 Oil (270) 1 8.87 322 Direction of loading Am-1 3.25 L-CO2 (0.1) 1 6.98 339 Inclined orientation Am-6 3.25 L-CO2 (0.1) 1 6.88 48 Inclined orientation Am-7 3.25 L-CO2 (0.1) 1 6.09 170 Inclined orientation Am-12 0 L-CO2 (0.1) 1 8.17 250 Inclined orientation The results of a typical case are described in this part together with the designated loading circumstances and fracturing fluid. The quantity and quality of the found acoustic emission sources was the sole factor applied to choose the representative example for investigation of the recorded surface fractures. From 1 second before the failure to 15 seconds following, the found sources of sonic emissions span. Under constant stress circumstances, the development of cracks in shale during hydraulic fracturing with L-CO2 displayed anomalies. The change within the fracturing mode depends on the viscosity of a breaking fluid. Refer to Table 2 ; L-CO2 has a viscosity of two hundred and seventy times that of viscous oil and one- tenth that of water. L-CO2's fluid character allows it to pass via small cracks, so fracture propagation is very sensitive to slight core defects. As pressure drops during fracture development, CO2's low viscosity helps it to phase change from liquid to gas. The increased compressibility of CO2 gas relative to its liquid form may encourage the interconnection of flaws inside a core, therefore producing fractures that propagate at oblique angles and emit sounds much louder than those produced during water or oil injection in fracturing. Studies showed that low-viscosity fluids, such CO2, usually show Mode II fracture characteristics and are prone to cause large-scale fractures with several branching. particularly with relation to the sensitivity of fracture propagation. Core flaws including microscopic voids that influence this sensitivity define Mode II dominant fractures following CO2 injection. By producing more efficient fractures for the extraction of shale gas using CO2 fracturing than with conventional slick water fracturing, surface area is projected to be much increased. Moreover, the addition of CO2 could help shale gas to be produced and recovered (Lei et al., 2022). This phenomenon results from shale's inclination for CO2 that is approximately five times more important than methane (Lei et al. 2022), therefore allowing CO2 to replace adsorbed methane (shale gas) to be quite effective. The results imply that CO2 fracturing offers a novel method of shale gas extraction [ 12 ]. 3.1 Assessment of Observations in Ambient Conditions In the tests involving cores A-1 and A-3, water was the fluid causing the fracture; no load was used and 0 MPa tension was imparted along the Z axis. The pressure profile A-3 shown in Fig. 2 shows that pressure building and failure occurred at 8.97 MPa during the hydraulic fracturing test. Figures 3 a show that the two core cracks ran horizontally across a sedimentary plane and were evident on the surface of the cores. The sensors are located and spatially mapped acoustic emission events in three dimensions during core splitting. Figure 3 b shows core A-3's acoustic emission source projected onto the horizontal plane (X-Y plane), therefore highlighting both XZ and YZ as two vertical planes. Vertical plane acoustic emission sources show horizontal dispersion. The recorded fracture patterns on the core surfaces coincide with the acoustic emission data. Injecting the viscous oil (A-5), having a higher viscosity than water, causes the crack to progress horizontally along the sedimentary bedding. This activity was like using water as the fracture fluid. Unlike water, the fracture aperture was much more obvious and bigger when oil was used as the fracturing fluid (Fig. 4 a). The expected orientations of acoustic emission sources in the Y-Z plane (Fig. 4 b) confirm the hypothesis that the fracture route deviates from its surface projection. Recognized as a low viscosity fracturing fluid, L-CO2 was injected into core Am-12 across the Z axis under 0 MPa pressure. Because the propagation of the fracture was not horizontal aligned to the sedimentary strata, at the site of failure a notable audible acoustic emission occurred. This stands quite apart from cores A-3 and A-5. Figure 5 a depicts a complicated inclined fracture surface with a big opening bisecting the core. Figure 5 b shows that the fracture spread from vertical at 15° to 30° according to locations of acoustic emission sources. This surface crack was noted. 3.2 Assessment of Observations Under Uniaxial Loading Under uniaxial load between the 1.25 to 3.25 MPa range along the Z axis, surface fractures developed aligned with the applied load, perpendicular to the sedimentary plane. Water was the fracturing fluid in main A-2 and A-4 hydraulic fracturing. Figure 6 presents the basic A-4. Both locations illustrate that the applied stress directed the development of fractures. Using oil as the fluid for fracturing at stress values such as (1.25, 3.25 MPa) across z-axis, evaluation of supplemental samples A-7 and A-9 revealed that the fracture propagated vertically in a direction perpendicular to the sedimentary plane. This is compatible with the results on water, the fluid used in hydraulic fracturing. Figure 7 shows case of core A-9. The pattern of acoustic emission sources and the fracture trace on the core surfaces imply that the site of the fracture spans vertically in the Two planes: X-Z as well Y-Z. L-CO2 as fracturing fluid across cores Am-1, Am-6, and Am-7 was used in final tests under 3.25 MPa stress along the z-axis. Under loading, core Am-1 showed a surface fracture that intersected the two horizontal bedding planes connected through an inclined fracture. Unlike those buried in oil and water, Fig. 8 shows each of the last two cores extended practically vertically with a small slope. This relates to central Am-7. The Y-Z plane distribution of acoustic emission source points shows that the fracture spread at an angle other than the vertical loading direction (Fig. 8 b). Unlike research using water and oil, loading conditions and the existence of bedding planes have no effect on fracture propagation in trials using L-CO2 as a fracturing fluid. Results show that among the investigated cores containing L-CO2, fracture propagation differs even under homogeneous stress circumstances. The unique characteristics of L-CO2 explain its erratic behavior independent of the loading conditions. 3.3 Assessment of Fracturing Mode Uniaxial loading of 3.25 MPa was applied to cores A-4 (water), A-9 (oil), and Am-7 (L-CO2) to investigate the effect of differing fracturing fluid viscosities on the fracturing mode. This method was utilized because, unlike cores that remained unloaded due to the reduced impact of sedimentary layers, fracture propagation under load demonstrated increased stability and was markedly influenced by fluid viscosity. Drop a ball of steel onto one side of a steel plate to apply polarity calibration evaluation, therefore enabling the classification of the first arrivals for the P-wave into two categories: compressional or dilatational. But sensors were installed on the other side. The impact on the opposite side of the plate created a compressional wave the sensors sensed. Every first P-wave arrival showed compressional motion on the upward track. Thirty noteworthy acoustic events including the injection of viscous oil, water, and L-CO2 were investigated to ascertain the percentages for compressional to total starting motion. These occurrences were chosen because they would activate at least twelve of the sixteen motion sensors. The study found that, in contrast to the lower ratios of 35–55% observed for less viscous water and L-CO2 injections, the compressional motion ratios for viscous oil injection were much higher, at 70–80%. Figure 9 . With in-plane shear, Mode II calls for a minimum ratio of 50%; Mode I, marked by tensile failure, calls for 100%. The results show that whilst viscous oil will generate Mode I fractures, water and L-CO2 will cause Mode II fractures. Underwater, the samples split mostly showing variation from a main fracture; the fractures ran in the loading direction. Moreover, the development of shear stress on an inclined plane in respect to the loading direction shows that the findings of acoustic emission monitoring coincide with the variations in fracture properties. Figure 9 shows that the frequency of Mode II acoustic emission events increases with decreasing viscosity of the fluid. 3.4 Microscopic Analysis of Fractures Figures 3 a and 4 a show that the A-5 specimen's (oil) fracture aperture is rather larger than that of the A-3 specimen (water) due to its higher density. Similar fractures found in the two samples under 0 MPa force most likely result from fracture propagation along the sedimentary planes. When a load of 3.25 MPa is applied, core A-4, split by water, shows steeper angles in respect to the loading direction than core A-9, split by oil (Fig. 10 ). Fractures travel through sedimentary layers from the origin at the bottom of photo. Figure 11 shows scanning lines at 0.5 mm intervals to count fracture branches. Variations in viscosity cause variances in a certain number branches produced via oil and water, which are quantified statistically. The fractured samples were sectioned; an 8 mm segment through injection's aperture along the fissure trajectory was investigated using 22 scan lines at 0.5 mm intervals (Fig. 10 ). Fractures travel through sedimentary layers from the origin at the bottom of photo. Figure 11 shows scanning lines at 0.5 mm intervals to count fracture branches. The direction of fracture propagation is strongly influenced by the anisotropy of rock strength and the general in situ stress conditions. The results show that significant horizontal stress and horizontal alignment of shale strata suggest that hydraulic fracture will spread horizontally. On the other hand, fractures usually run vertically, perpendicular to sedimentary layers. When vertical tension much exceeds horizontal stress, deep shale formations can pierce adjacent layers. The statistical analysis defines the variations in the general branch count between water and oil about viscosity changes. Following sectioning the broken samples, an 8 mm segment across the injection hole's position was examined under 22 checking lines at 0.5-millimeter intervals (Fig. 10 , 11 ). Study of fractures across the scanning lines (Fig. 12 ) shows that oil injection generates branches at certain points along the fracture course. Branching brought about by water injection crosses the fracture path at several spots. Water injections have produced 55 fracture branches; over 38 branches have come out from oil injections. This remark has only relevance to the injection site. As the angle increases, the shear stress on inclined fracture surfaces reaches its maximum value at a 45-degree angle relative to the loading axis. Consequently, as Fig. 12 shows, shear fracture is more readily facilitated by fracture features with trajectories more than by oil. Examining the fractures across the scanning lines (Fig. 12 ) shows that water injections generate a variable quantity of branches across any place along the crack path, whereas oil injection generates branches at limited positions along the crack path. Though this observation is limited to the vicinity of the injection hole, the overall number of fracture branches (55) arising from water injection exceeds that (38) from oil injection. The distribution of acoustic emission sources and the propagation of cracks from surface fissures revealed a correlation in the experimental data. Under uniaxial strains orienting perpendicular to the sedimentary plane, cracks formed in tests using oil or water as fracturing fluid. Conversely, it were discharged and relocated over the sedimentary layers. Elasticity is introduced as a fracture moves toward peak compressive stress; in the setting of uniaxial loading, the path of fracture propagation found in this study corresponds with the ideas of elastic theory. Unlike elastic theory, which holds that fractures could spread in any direction, the fracture happened independent of external load over the weakest plane defined by the sediment bedding. 4. CONCLUSIONS In this study, we analyzed the P-wave first motion polarity ratios of acoustic emission events, fracture properties identified through fluorescence techniques, and the interplay between the distribution of acoustic emissions and the viscosity of fracturing fluids. Our findings yield several key conclusions regarding fracture behavior under varying conditions, which is as follows: The distribution of surface fractures and acoustic emission sources indicates that, during uniaxial loading experiments using oil and water as fracturing fluids, fractures predominantly developed in alignment with the load direction and orthogonal to the sedimentary plane. In unloaded tests, fractures traversed the sedimentary plane, underscoring the significant influence of in situ stress conditions on fracture orientation. This behavior was not observed in experiments utilizing liquid CO 2 as a fracturing medium, where the low viscosity likely contributed to oblique fracture development, aligning with the orientation of minimal principal stress and the sedimentary plane. Analysis of P-wave initial motions revealed a polarity ratio indicative of Mode I fractures, which are typically associated with viscous oil injection. In contrast, the introduction of water and liquid CO 2 predominantly resulted in Mode II fractures. Microscopic examinations demonstrated that high-frequency viscous oil injection generates straight fractures with minimal branching, whereas hydraulic fractures induced by water injection exhibit multiple branching pathways emanating from the load axis. This observation is consistent with previous findings that highlight variations in crack modes as a function of fluid viscosity. The low viscosity of liquid CO 2 likely accounts for the observed differences in fracture propagation across various fluid fracturing techniques. Low-viscosity fluids such as CO 2 tend to produce significant fractures in granite specimens, characterized by extensive branching, with Mode II fractures being the most prevalent type. These results align with our established trends. The use of liquid CO 2 for shale gas extraction offers a competitive advantage over conventional slick water fracturing methods, aligning with established trends in fracture behavior. Declarations Author Contribution Ali Falah and Arezou Jafari, completed the experimental work. And Reza Gharibshahi complete the analysis work. References Li S, Zhou Z, Nie H, Zhang L, Song T, Liu W, Li H, Xu Q, Wei S, Tao S (2022) Distribution characteristics, exploration and development, geological theories research progress and exploration directions of shale gas in China. China Geol 5:110–135 Liu Q, Sun M, Sun X, Liu B, Ostadhassan M, Huang W, Chen X, Pan Z (2023) Pore network characterization of shale reservoirs through state-of-the-art X-ray computed tomography: A review. Gas Sci Eng 113:204967 Wei J, Zhang A, Li J, Shang D, Zhou X (2023) Study on microscale pore structure and bedding fracture characteristics of shale oil reservoir. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6496204","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":452655577,"identity":"7871afc0-f2ca-4c2a-9b18-49df82141671","order_by":0,"name":"Ali Falah","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+UlEQVRIiWNgGAWjYDACdsaGDwwFDAn8zMwHHwD5PHwEtTAzNs5gMGBIkGxnSzYAaWEjrIWBEazF4DyPmgRIgKAW/mbmxoYfBnZ5Bod52Cq/5tjJsDEwP3x0A48WicOMjY09BsnFkod5j92W3ZYMdBibsXEOPmsOM7Y/4DFgTuw7zJd2W3IbM1ALD5s0Pi3yIFv+GNQnNhzmMSuW3FZPWIsBUEszj8HhxAlALYwftx0mrMUQpEXG4HjizGa2ZGnGbcd52JgJ+EXuePvDxjcV1Yn9/IcPfvy5rdqen7354WO83kcGzDxgkljlIMD4gxTVo2AUjIJRMGIAAA+sR5CVtHSKAAAAAElFTkSuQmCC","orcid":"","institution":"Tarbiat Modares University","correspondingAuthor":true,"prefix":"","firstName":"Ali","middleName":"","lastName":"Falah","suffix":""},{"id":452655578,"identity":"b6281256-7908-4de7-b621-b42ef3727c0d","order_by":1,"name":"Arezou Jafari","email":"","orcid":"","institution":"Tarbiat Modares University","correspondingAuthor":false,"prefix":"","firstName":"Arezou","middleName":"","lastName":"Jafari","suffix":""},{"id":452655579,"identity":"6c868e22-8f4e-4b96-8b92-080e2cede89d","order_by":2,"name":"Reza Gharibshahi","email":"","orcid":"","institution":"Tarbiat Modares University","correspondingAuthor":false,"prefix":"","firstName":"Reza","middleName":"","lastName":"Gharibshahi","suffix":""}],"badges":[],"createdAt":"2025-04-21 12:53:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6496204/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6496204/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":82253250,"identity":"2490e903-ea99-4ea0-8686-10f61aba89dc","added_by":"auto","created_at":"2025-05-08 10:33:19","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":18659,"visible":true,"origin":"","legend":"\u003cp\u003eGraph as well picture illustrating the basics of coordinates and dimensions\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/aaf33d0048fda14ac7f42d28.jpg"},{"id":82254141,"identity":"f612871d-28ff-4a15-b719-09029a91cac8","added_by":"auto","created_at":"2025-05-08 10:41:19","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":34310,"visible":true,"origin":"","legend":"\u003cp\u003eShale core A-3 (water) test profile with a loading of 0 MPa.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/dd47185b136c96202db76178.jpg"},{"id":82253253,"identity":"781ac57a-264a-432e-95a9-22650a457db7","added_by":"auto","created_at":"2025-05-08 10:33:19","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":43457,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Image illustrating an enlarged perspective of the identified fracture (red box) on the surface and (b) positions of acoustic emission sources in core A-3 under 0 MPa loading and water infusion.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/dde2d89e36398f62eb75cc03.jpg"},{"id":82253255,"identity":"c209d44b-0496-4f8d-b42d-96f3c0098c0b","added_by":"auto","created_at":"2025-05-08 10:33:19","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":41856,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) \u003c/strong\u003eLocations of Core A-5 acoustic emission sources at Zero MPa loading and oil injection, and (b) an enlarged image of the detected surface fracture (red box).\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/8f361ad773e46ae1e1aac59a.jpg"},{"id":82253256,"identity":"5a0849f6-2621-405b-9732-c980aea2a325","added_by":"auto","created_at":"2025-05-08 10:33:20","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":33939,"visible":true,"origin":"","legend":"\u003cp\u003eshows (a) the surface fracture (red box) and (b) core Am-12 acoustic emission source positions under 0 MPa loading and L-CO2 injection.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/0090160876a88139a2792385.jpg"},{"id":82254143,"identity":"fdfaacad-7738-4449-8e18-6c784b907b0e","added_by":"auto","created_at":"2025-05-08 10:41:20","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":33489,"visible":true,"origin":"","legend":"\u003cp\u003eillustrates (a) the surface fracture (red box) and (b) the locations of acoustic emission sources for core A-4 under loading of 3.25 MPa and injection of water.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/69ad127104fe27e012d99601.jpg"},{"id":82253258,"identity":"ba834808-d520-4be8-a613-7a9deb8ea8e8","added_by":"auto","created_at":"2025-05-08 10:33:20","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":29385,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Locations of surface fractures (indicated by the red box) and (b) the acoustic emission source points for core A-9, observed under a loading of 1.25 MPa and during oil injection.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/0a05f2df34432d6244aeeca8.jpg"},{"id":82254526,"identity":"bfa770c7-3b33-427e-85a9-049af6cf5f08","added_by":"auto","created_at":"2025-05-08 10:49:20","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":31484,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Surface fracture (indicated by the red box) and (b) core Am-7 acoustic emission source locations observed under loading of 1.25 MPa and the injection of L-CO2.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/a89743daf726825523312931.jpg"},{"id":82253267,"identity":"758bd1e2-59da-40b6-85c4-6d5488a85743","added_by":"auto","created_at":"2025-05-08 10:33:20","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":40364,"visible":true,"origin":"","legend":"\u003cp\u003eillustrates the ratios of compressional initial motion polarity across 30 acoustic emission events for every hydraulic fracture experiment conducted with liquid carbon dioxide, viscous oil, and water.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/40409a3e436dba8347978c85.jpg"},{"id":82253261,"identity":"ec594da5-ead0-4c10-bcda-1f8fa42591b1","added_by":"auto","created_at":"2025-05-08 10:33:20","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":52567,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence microscopy showing fracture morphology after water injection into cores A-4.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/ac6141098967cb34fd549695.jpg"},{"id":82254150,"identity":"66817adc-bb6d-4cf8-82e2-c5a478a58ac2","added_by":"auto","created_at":"2025-05-08 10:41:20","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":60810,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence microscopy showing fracture morphology after oil injection into cores A-9.\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/cffcf2f96b0bb2b4dc8a19bf.jpg"},{"id":82254145,"identity":"ab2f1438-5c8f-4af4-8bc4-a9edc37c717c","added_by":"auto","created_at":"2025-05-08 10:41:20","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":47657,"visible":true,"origin":"","legend":"\u003cp\u003eWater and viscous oil fracture branch count at an equivalent distance.\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/88bd8fe928819681a40dc828.jpg"},{"id":84097815,"identity":"e6ddaf48-d8d8-44bf-b965-444a20414734","added_by":"auto","created_at":"2025-06-06 18:01:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1461235,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6496204/v1/d630c261-f225-4380-ad03-6b9c3064e9c4.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigations into the effect of hydraulic fracturing fluids viscosity on the fracture's propagations in unconventional gas reservoir","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eShale gas is a naturally occurring gas found in shale formations, which are fine-grained sedimentary rocks that serve as significant reservoirs for natural gas and petroleum. Shallowly permeable shale significantly impedes gas migration through rocks of the reservoir to the wells of the production. Economic feasibility of gas produced from shale extraction relies about the efficiency of the stimulation within the unconventional reservoirs. Recent studies indicate that horizontal drilling employing a multi-stage hydraulic fracturing approach demonstrates significant efficiency in unconventional gas reservoirs [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHydraulic fracturing generates and maintains fractures by injecting fluid at pressures exceeding the rock's failure stress. In 2023, Liu et al., established the methodology for quantifying hydraulic fracturing stress. Wei et la., (2023) and Jiang et. al., (2024), advanced discipline by presenting an initial variation of the hydraulic fracture concept and examining fracture management. A multitude of experimental and theoretical research have been conducted and examined [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDiverse geological processes sculpt strata and reveal complex stress fields [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The magnitude and orientation of the principal stresses, along with the shape and direction of the fracture, determine the pressure range required to create and maintain a fracture in hydraulically fractured rocks. The anisotropy of the rock's strength is a significant factor influencing the trajectory of hydraulic fracturing in rock. The 2021 study by Liu et al. examined the propagation of fractures during the hydraulic fracturing of oil shale. Their findings indicate that fractures influenced by anisotropic characteristics and existing in-situ stress conditions exhibit elliptical shapes and extend in several directions [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eLi et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] examined hydraulic fracturing utilizing three distinct fracturing fluids: water, carbon dioxide, and viscous oil. Low-viscosity fluids, such as carbon dioxide, were anticipated to induce substantial cracks with numerous branches. The fissures in the shale, generated by carbon dioxide due to its greater surface area, which is more effective than water, enhance shale gas production.\u003c/p\u003e \u003cp\u003eThe aim of this study is to investigate acoustic emissions and the effect of fluid viscosity on the unconventional shale gas reservoir through an analysis of shale core fracture propagation under the influence of sedimentary layer stress and anisotropies. This research examines the microscopic properties and modes of fractures in relation to the viscosities of water, viscous oil, and CO\u003csub\u003e2\u003c/sub\u003e. The findings could provide valuable insights for projects in the energy industry, including greenhouse gas storage and shale gas production.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Cores and Experimental Setup\u003c/h2\u003e \u003cp\u003eThe experiments made use of six shale cores with dimensions of 80 millimeters in diameter and roughly 165-millimeter height. During the excavation of a drift at a coal mine in Iraq, samples were gathered 310 meters below sea level. Each block comprised sandy shale taken from a coal-bearing formation. The block surfaces' drilling orientation was perpendicular to the sedimentary planes.\u003c/p\u003e \u003cp\u003eFor hydraulic fracturing, a 12 mm diameter hole was vertically drilled midway on the side of the core. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the cores shown using a Cartesian coordinate system; the X-axis runs across the hole that was drilled; the Y-axis runs opposite the other two axes. A Z-axis is orthogonal with the sediment line and coincides with the cylindrical axis.\u003c/p\u003e \u003cp\u003eEvery sample's P wave velocity was measured along all three designated axes. Apart from cores A-1 and Am-12, P wave velocity readings show perpendicular anisotropy connected with sedimentary bedding (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Using uniaxial strains of 1.25 MPa as well as 3.25 MPa across a z-axis (σz), hydraulic fracturing tests were carried out under ambient conditions with stress applied perpendicular to the sedimentary planes. A total of four 10-millimeter gauges for strain had been attached onto the core prior to load constancy testing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAnisotropic velocity of P-waves across shale samples across different axes.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNo. of Cores\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eSpeed of the P-wave (Km/s)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ex-axis\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ey-axis\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ez-axis\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eA-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.21\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.14\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.58\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-3\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.14\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.15\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.78\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-4\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.26\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.99\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-5\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.67\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.38\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.21\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-7\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e2.90\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.14\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.88\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e2.85\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.60\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.84\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.30\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.40\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e2.88\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-6\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.38\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e3.14\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-7\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.29\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.27\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e3.08\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-12\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.57\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e3.78\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e3.99\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e lists the anisotropic velocity of P-waves in shale samples along several axes. Monitoring and describing fractures depend on exact location of source events [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Event matching and event localization both depend much on velocity anisotropy [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Ignoring velocity anisotropy could cause major error in the event of localization. The p-wave velocity and the site of acoustic emission in the shale sample were analyzed in three dimensions employing the three distinct categories of fracturing fluids: water, viscous oil, and CO2. Each experiment was thoroughly explained utilizing numerous samples.\u003c/p\u003e \u003cp\u003eCalculating a percentage of the compressional beginning movement compared to total count of initial motions improves fracture mode diagnosis considering the polarity of the early P-wave movements. While Mode II for a plane shear fracture predicts 50%, the expected percentage for a tensile crack is 100%. Experimental results show that whilst Mode II fractures are produced by injecting water and L-CO2, Mode I fractures result from viscous oil injection.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Methods of Fluid Injection\u003c/h2\u003e \u003cp\u003eHydraulic fracturing used three fluid categories: water, liquid carbon dioxide (L-CO2) and viscous oil. Lubricant used was a particular kind of vehicle transmission oil more especially, Super Transmission Oil. Whereas the Liquid CO2 showed around one-tenth that of water, the viscous oil showed a viscosity 250 times more in the temperature test. The geomechanically properties of the shale sample and fluids properties explain in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe packer, with a 35 mm pressurising section, was precisely situated at the centre of the injection hole. Fluids were administered at constant the rate of flow of one mL/min, with injection pressure documented at 0.11-second periods. An adjustable discharge rate linked to the oracle by connector tubes using a fluid injection syringe pump. A pressure transducer near the packer tracked fluid pressure during injection.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Methods for Detecting Acoustic Emissions, Pressure, and Temperature\u003c/h2\u003e \u003cp\u003eEighteen cylindrical sensors was used to identify events of sonic emission. With a 4-millimeter diameter and a 5-mm length, each sensor displayed a resonance frequency of about 500 kh.\u003c/p\u003e \u003cp\u003e39 dB from the preamplifier and 50 dB from the signal conditioner were combined to amplify the found acoustic emission signals to 88 dB, due to very high noise. The conditioner of the signal was tuned to about 28 dB to lower high noise levels in testing using liquid CO2 as a fracturing fluid. An Analogue/ digital (A/D) converter wrote the amplified signals onto a hard disk. With an 18 bit resolution and a 0.14 \u0026micro;s sampling time, the event record length was 1850 words. Post-event recording with a dead time of one ms was implemented to reduce too high hard disk noise generated by vibrations after a significant acoustic emissions event. Recording acoustic emission events started when any of the eighteen sensors sensed a signal above 4 V. Usually, these triggered occurrences combine several signals.\u003c/p\u003e \u003cp\u003eThroughout the injecting process, adverse effects were seen and recorded. Using P-wave arrival timings at sensors, the hypocenters of acoustic emissions were calculated and matched with visual detection data to describe the orientation and form of fractures on the core surfaces. We achieved this by means of a least squares approach.\u003c/p\u003e \u003cp\u003eA statistical examination of the various polarities for the P-wave initiation and their associated acoustic emission waveform, utilising Kakadjian et al. (2021) and Iferobia et al. (2022), enhances the understanding of the fractured mode [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Their polarity facilitated being categorised as either compression or dilatation. Each acoustic emission event documented the fundamental counts of compressional and dilatational movements. The initial compressional motion ratio relative to the total of initial compressional as well as dilatational motions can be determined through computations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Analysis of fractures utilizing fluorescent resin\u003c/h2\u003e \u003cp\u003eIn fluorescence microscopy (Xu, 2024), the application of a fluorescent agent and resin enhances the viewing of fractures and pores in rocks. This section elucidates the microscopic examination of shale specimens subjected to fracturing via viscous oil and water injection [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo enhance the resin's strength, cores have been immersed into resin under vacuum about one week and subsequently subjected to temperatures exceeding 90\u0026deg;C. The cores were subsequently sectioned perpendicular to the fracture plane formed by hydraulic fracturing and readied for ultraviolet light microscopic examination.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eShale cores were subjected to three hydraulic fracturing fluids under ambient circumstances and between 1.25 and 3.25 MPa uniaxial pressures. Examining the results produced twelve tests that validated their dependability. Table\u0026nbsp;(2) displays the analysis findings; the last columns indicate the induced fracture orientation depending on stress and fracturing fluid.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummarizes shale hydraulic fracturing experiments.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of Core\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eApplied stress σz (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFracturing fluid (measured in millipascals)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInjections per minute (ml/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFailure Pressure (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eIdentified acoustic emission sources\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eFracture orientation\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eA-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eWater (1)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.26\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e144\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAligned with bedding\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eA-3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eWater (1)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.68\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e82\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAligned with bedding\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e1.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eWater (1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e13.14\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e170\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eAligned with bedding\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-4\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eWater (1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e16.88\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e207\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eAligned with bedding\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-5\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e0\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eOil (270)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e9.98\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e93\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eAligned with bedding\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-7\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e1.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eOil (270)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e8.89\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e268\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eDirection of loading\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eA-9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eOil (270)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e8.87\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e322\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eDirection of loading\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eL-CO2 (0.1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e6.98\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e339\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eInclined orientation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-6\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eL-CO2 (0.1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e6.88\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e48\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eInclined orientation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-7\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e3.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eL-CO2 (0.1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e6.09\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e170\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eInclined orientation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAm-12\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e0\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eL-CO2 (0.1)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e8.17\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e250\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eInclined orientation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe results of a typical case are described in this part together with the designated loading circumstances and fracturing fluid. The quantity and quality of the found acoustic emission sources was the sole factor applied to choose the representative example for investigation of the recorded surface fractures. From 1 second before the failure to 15 seconds following, the found sources of sonic emissions span.\u003c/p\u003e \u003cp\u003eUnder constant stress circumstances, the development of cracks in shale during hydraulic fracturing with L-CO2 displayed anomalies. The change within the fracturing mode depends on the viscosity of a breaking fluid. Refer to Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; L-CO2 has a viscosity of two hundred and seventy times that of viscous oil and one- tenth that of water. L-CO2's fluid character allows it to pass via small cracks, so fracture propagation is very sensitive to slight core defects. As pressure drops during fracture development, CO2's low viscosity helps it to phase change from liquid to gas. The increased compressibility of CO2 gas relative to its liquid form may encourage the interconnection of flaws inside a core, therefore producing fractures that propagate at oblique angles and emit sounds much louder than those produced during water or oil injection in fracturing.\u003c/p\u003e \u003cp\u003eStudies showed that low-viscosity fluids, such CO2, usually show Mode II fracture characteristics and are prone to cause large-scale fractures with several branching. particularly with relation to the sensitivity of fracture propagation. Core flaws including microscopic voids that influence this sensitivity define Mode II dominant fractures following CO2 injection.\u003c/p\u003e \u003cp\u003eBy producing more efficient fractures for the extraction of shale gas using CO2 fracturing than with conventional slick water fracturing, surface area is projected to be much increased. Moreover, the addition of CO2 could help shale gas to be produced and recovered (Lei et al., 2022). This phenomenon results from shale's inclination for CO2 that is approximately five times more important than methane (Lei et al. 2022), therefore allowing CO2 to replace adsorbed methane (shale gas) to be quite effective. The results imply that CO2 fracturing offers a novel method of shale gas extraction [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Assessment of Observations in Ambient Conditions\u003c/h2\u003e \u003cp\u003eIn the tests involving cores A-1 and A-3, water was the fluid causing the fracture; no load was used and 0 MPa tension was imparted along the Z axis. The pressure profile A-3 shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows that pressure building and failure occurred at 8.97 MPa during the hydraulic fracturing test. Figures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea show that the two core cracks ran horizontally across a sedimentary plane and were evident on the surface of the cores.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe sensors are located and spatially mapped acoustic emission events in three dimensions during core splitting. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb shows core A-3's acoustic emission source projected onto the horizontal plane (X-Y plane), therefore highlighting both XZ and YZ as two vertical planes. Vertical plane acoustic emission sources show horizontal dispersion. The recorded fracture patterns on the core surfaces coincide with the acoustic emission data.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eInjecting the viscous oil (A-5), having a higher viscosity than water, causes the crack to progress horizontally along the sedimentary bedding. This activity was like using water as the fracture fluid. Unlike water, the fracture aperture was much more obvious and bigger when oil was used as the fracturing fluid (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The expected orientations of acoustic emission sources in the Y-Z plane (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) confirm the hypothesis that the fracture route deviates from its surface projection.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRecognized as a low viscosity fracturing fluid, L-CO2 was injected into core Am-12 across the Z axis under 0 MPa pressure. Because the propagation of the fracture was not horizontal aligned to the sedimentary strata, at the site of failure a notable audible acoustic emission occurred. This stands quite apart from cores A-3 and A-5. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea depicts a complicated inclined fracture surface with a big opening bisecting the core. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb shows that the fracture spread from vertical at 15\u0026deg; to 30\u0026deg; according to locations of acoustic emission sources. This surface crack was noted.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Assessment of Observations Under Uniaxial Loading\u003c/h2\u003e \u003cp\u003eUnder uniaxial load between the 1.25 to 3.25 MPa range along the Z axis, surface fractures developed aligned with the applied load, perpendicular to the sedimentary plane. Water was the fracturing fluid in main A-2 and A-4 hydraulic fracturing. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents the basic A-4. Both locations illustrate that the applied stress directed the development of fractures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUsing oil as the fluid for fracturing at stress values such as (1.25, 3.25 MPa) across z-axis, evaluation of supplemental samples A-7 and A-9 revealed that the fracture propagated vertically in a direction perpendicular to the sedimentary plane. This is compatible with the results on water, the fluid used in hydraulic fracturing. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows case of core A-9. The pattern of acoustic emission sources and the fracture trace on the core surfaces imply that the site of the fracture spans vertically in the Two planes: X-Z as well Y-Z.\u003c/p\u003e \u003cp\u003eL-CO2 as fracturing fluid across cores Am-1, Am-6, and Am-7 was used in final tests under 3.25 MPa stress along the z-axis. Under loading, core Am-1 showed a surface fracture that intersected the two horizontal bedding planes connected through an inclined fracture. Unlike those buried in oil and water, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows each of the last two cores extended practically vertically with a small slope. This relates to central Am-7. The Y-Z plane distribution of acoustic emission source points shows that the fracture spread at an angle other than the vertical loading direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUnlike research using water and oil, loading conditions and the existence of bedding planes have no effect on fracture propagation in trials using L-CO2 as a fracturing fluid. Results show that among the investigated cores containing L-CO2, fracture propagation differs even under homogeneous stress circumstances. The unique characteristics of L-CO2 explain its erratic behavior independent of the loading conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Assessment of Fracturing Mode\u003c/h2\u003e \u003cp\u003eUniaxial loading of 3.25 MPa was applied to cores A-4 (water), A-9 (oil), and Am-7 (L-CO2) to investigate the effect of differing fracturing fluid viscosities on the fracturing mode. This method was utilized because, unlike cores that remained unloaded due to the reduced impact of sedimentary layers, fracture propagation under load demonstrated increased stability and was markedly influenced by fluid viscosity.\u003c/p\u003e \u003cp\u003eDrop a ball of steel onto one side of a steel plate to apply polarity calibration evaluation, therefore enabling the classification of the first arrivals for the P-wave into two categories: compressional or dilatational. But sensors were installed on the other side. The impact on the opposite side of the plate created a compressional wave the sensors sensed. Every first P-wave arrival showed compressional motion on the upward track.\u003c/p\u003e \u003cp\u003eThirty noteworthy acoustic events including the injection of viscous oil, water, and L-CO2 were investigated to ascertain the percentages for compressional to total starting motion. These occurrences were chosen because they would activate at least twelve of the sixteen motion sensors. The study found that, in contrast to the lower ratios of 35\u0026ndash;55% observed for less viscous water and L-CO2 injections, the compressional motion ratios for viscous oil injection were much higher, at 70\u0026ndash;80%. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWith in-plane shear, Mode II calls for a minimum ratio of 50%; Mode I, marked by tensile failure, calls for 100%. The results show that whilst viscous oil will generate Mode I fractures, water and L-CO2 will cause Mode II fractures.\u003c/p\u003e \u003cp\u003eUnderwater, the samples split mostly showing variation from a main fracture; the fractures ran in the loading direction. Moreover, the development of shear stress on an inclined plane in respect to the loading direction shows that the findings of acoustic emission monitoring coincide with the variations in fracture properties. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows that the frequency of Mode II acoustic emission events increases with decreasing viscosity of the fluid.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Microscopic Analysis of Fractures\u003c/h2\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea show that the A-5 specimen's (oil) fracture aperture is rather larger than that of the A-3 specimen (water) due to its higher density. Similar fractures found in the two samples under 0 MPa force most likely result from fracture propagation along the sedimentary planes. When a load of 3.25 MPa is applied, core A-4, split by water, shows steeper angles in respect to the loading direction than core A-9, split by oil (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFractures travel through sedimentary layers from the origin at the bottom of photo. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows scanning lines at 0.5 mm intervals to count fracture branches.\u003c/p\u003e \u003cp\u003eVariations in viscosity cause variances in a certain number branches produced via oil and water, which are quantified statistically. The fractured samples were sectioned; an 8 mm segment through injection's aperture along the fissure trajectory was investigated using 22 scan lines at 0.5 mm intervals (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFractures travel through sedimentary layers from the origin at the bottom of photo. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows scanning lines at 0.5 mm intervals to count fracture branches.\u003c/p\u003e \u003cp\u003eThe direction of fracture propagation is strongly influenced by the anisotropy of rock strength and the general in situ stress conditions. The results show that significant horizontal stress and horizontal alignment of shale strata suggest that hydraulic fracture will spread horizontally. On the other hand, fractures usually run vertically, perpendicular to sedimentary layers. When vertical tension much exceeds horizontal stress, deep shale formations can pierce adjacent layers.\u003c/p\u003e \u003cp\u003eThe statistical analysis defines the variations in the general branch count between water and oil about viscosity changes. Following sectioning the broken samples, an 8 mm segment across the injection hole's position was examined under 22 checking lines at 0.5-millimeter intervals (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e,\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). Study of fractures across the scanning lines (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e) shows that oil injection generates branches at certain points along the fracture course. Branching brought about by water injection crosses the fracture path at several spots. Water injections have produced 55 fracture branches; over 38 branches have come out from oil injections. This remark has only relevance to the injection site.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs the angle increases, the shear stress on inclined fracture surfaces reaches its maximum value at a 45-degree angle relative to the loading axis. Consequently, as Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e shows, shear fracture is more readily facilitated by fracture features with trajectories more than by oil.\u003c/p\u003e \u003cp\u003eExamining the fractures across the scanning lines (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e) shows that water injections generate a variable quantity of branches across any place along the crack path, whereas oil injection generates branches at limited positions along the crack path. Though this observation is limited to the vicinity of the injection hole, the overall number of fracture branches (55) arising from water injection exceeds that (38) from oil injection.\u003c/p\u003e \u003cp\u003eThe distribution of acoustic emission sources and the propagation of cracks from surface fissures revealed a correlation in the experimental data. Under uniaxial strains orienting perpendicular to the sedimentary plane, cracks formed in tests using oil or water as fracturing fluid. Conversely, it were discharged and relocated over the sedimentary layers. Elasticity is introduced as a fracture moves toward peak compressive stress; in the setting of uniaxial loading, the path of fracture propagation found in this study corresponds with the ideas of elastic theory. Unlike elastic theory, which holds that fractures could spread in any direction, the fracture happened independent of external load over the weakest plane defined by the sediment bedding.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. CONCLUSIONS","content":"\u003cp\u003eIn this study, we analyzed the P-wave first motion polarity ratios of acoustic emission events, fracture properties identified through fluorescence techniques, and the interplay between the distribution of acoustic emissions and the viscosity of fracturing fluids. Our findings yield several key conclusions regarding fracture behavior under varying conditions, which is as follows:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eThe distribution of surface fractures and acoustic emission sources indicates that, during uniaxial loading experiments using oil and water as fracturing fluids, fractures predominantly developed in alignment with the load direction and orthogonal to the sedimentary plane.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn unloaded tests, fractures traversed the sedimentary plane, underscoring the significant influence of in situ stress conditions on fracture orientation. This behavior was not observed in experiments utilizing liquid CO\u003csub\u003e2\u003c/sub\u003e as a fracturing medium, where the low viscosity likely contributed to oblique fracture development, aligning with the orientation of minimal principal stress and the sedimentary plane.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAnalysis of P-wave initial motions revealed a polarity ratio indicative of Mode I fractures, which are typically associated with viscous oil injection. In contrast, the introduction of water and liquid CO\u003csub\u003e2\u003c/sub\u003e predominantly resulted in Mode II fractures.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eMicroscopic examinations demonstrated that high-frequency viscous oil injection generates straight fractures with minimal branching, whereas hydraulic fractures induced by water injection exhibit multiple branching pathways emanating from the load axis. This observation is consistent with previous findings that highlight variations in crack modes as a function of fluid viscosity.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe low viscosity of liquid CO\u003csub\u003e2\u003c/sub\u003e likely accounts for the observed differences in fracture propagation across various fluid fracturing techniques.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eLow-viscosity fluids such as CO\u003csub\u003e2\u003c/sub\u003e tend to produce significant fractures in granite specimens, characterized by extensive branching, with Mode II fractures being the most prevalent type. These results align with our established trends.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe use of liquid CO\u003csub\u003e2\u003c/sub\u003e for shale gas extraction offers a competitive advantage over conventional slick water fracturing methods, aligning with established trends in fracture behavior.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAli Falah and Arezou Jafari, completed the experimental work. And Reza Gharibshahi complete the analysis work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLi S, Zhou Z, Nie H, Zhang L, Song T, Liu W, Li H, Xu Q, Wei S, Tao S (2022) Distribution characteristics, exploration and development, geological theories research progress and exploration directions of shale gas in China. 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ACS Omega 7(16):13644\u0026ndash;13653\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Hydraulic fracturing, Fluids density, Unconventional reservoir","lastPublishedDoi":"10.21203/rs.3.rs-6496204/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6496204/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHydraulic fracturing was carried out using cylindrical anisotropic shale cores aligned perpendicular to the sediment plane. The viscous oil, water, and liquid-CO2 fracturing fluids were evaluated under both ambient and uniaxial stress conditions. Experiments involving water and oil show that cracks propagate within the load orientation, perpendicular to the sedimentary plane, and along the sediment plane under the uniaxial load conditions. Results imply that in-situ stress conditions greatly affect the development of cracks. Fractures induced by water showed several branches starting from the loading axis. On fluorescent microscopy, however, hydraulic fractures using dense oil produced straight fractures with minimal branching. Viscous oil caused the Mode I fracture according to a statistical analysis of the P wave direction within sound emission frequencies. By contrast, liquid CO2 and water produced the Mode II fracture. Unlike the other two fluids, under loading conditions liquid-CO2 injection had no effect on the propagation of cracks. The low viscosity indicates that low-viscosity fluids, such CO2, cause extensive, branched cracks mostly Mode II fractures. For these reasons, shale gas generation beats slick water injections.\u003c/p\u003e","manuscriptTitle":"Investigations into the effect of hydraulic fracturing fluids viscosity on the fracture's propagations in unconventional gas reservoir","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-08 10:33:15","doi":"10.21203/rs.3.rs-6496204/v1","editorialEvents":[{"type":"communityComments","content":1}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9a60268b-344f-4305-a7f3-68e70bb48a24","owner":[],"postedDate":"May 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-06-06T17:53:25+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-08 10:33:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6496204","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6496204","identity":"rs-6496204","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}