Study on a Thermally Crosslinking Clay-Free Weak Gel Water-Based Drilling Fluids

In this study, a thermally crosslinking clay-free weak gel water-based drilling fluid based on salt-responsive polymers and crosslinking agents was investigated, a promising and feasible strategy. Firstly, a salt-tolerant polymer was synthesized using N, N-dimethylacrylamide (DMAA), [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfonopropyl) ammonium hydroxide (DMAPS), and acrylamide (AM). BPEI10000 was selected as the thermal crosslinking agent. The optimal crosslinking could be achieved at 180 °C and 36% NaCl when the RMFL concentration was 2.0%, and the BPEI10000 concentration was 0.1%. Performance evaluation demonstrated that the crosslinking between RMFL and BPEI10000 could enhance the AV, PV, and YP of the RMFL(BPEI10000)/CF-WBDFs after aging at 180 °C for 16 h, and reduce FLAPI. The RMFL(BPEI10000)/CF-WBDFs exhibited appropriate shear-thinning behavior, viscoelasticity, thixotropy, and recoverable viscosity under high-temperature, high-salinity, and high-pressure conditions. Mechanism analysis revealed that RMFL and BPEI10000 could form a predominantly negatively charged, three-dimensional crosslinking weak gel at high temperatures. The crosslinking weak gel could form dense filter cakes, improving rheological properties and reducing filtration loss of CFWBDFs in high-temperature, high-salinity environments. This paper proposed a novel method to address the technical challenge of rheological performance failure of CFWBDFs, offering valuable insights for subsequent investigations.