Study of Injection Rate on Fracture Network Complexity During Hydraulic Fracturing: Experimental Insights from Concrete, Sandstone and Bituminous Coal

This study examines how the injection rate affects fracture complexity during hydraulic fracturing using water as the fracturing fluid. Experiments were conducted on concrete, impermeable sandstone, and brittle bituminous coal across a wide range of injection rates. The results showed that both low (< 1.0mL/min) and high (≥5.0mL/min) injection rates produced simple, planar fractures with limited branching. In contrast, an intermediate rate of 2.2mL/min consistently generated the most complex fracture networks. Fracture complexity was evaluated using 3D-scanned surface roughness quantified by the Joint Roughness Coefficient (JRC), which reached its highest values at the optimum injection rate. Although water is sometimes considered less effective than unconventional fluids, the findings demonstrate that it performs well in moderately permeable rocks with sufficient tensile strength. However, water was less effective in brittle coal and highly impermeable sandstone, where more compressible, low-viscosity fluids such as supercritical CO2 or nitrogen may be advantageous. Overall, three fracture behavior regimes were identified: a low-rate regime producing simple fractures, a mid-rate optimal regime producing complex networks, and a high-rate regime where fracture complexity decreased. Thin fractures dominated because controlled injection conditions focused on observing fracture behavior rather than merely inducing fracture, as reflected by the relatively high post-fracture injection pressure.