Abstract: To address the challenge of wellbore instability in lacustrineshale gas horizontal wells, a low free-water activity water-based drilling fluid system PSW-2 for shale gas horizontal wells was developed based on the principles of reducing liquid activity, inhibiting hydration, and enhancing shale sealing. Rheological, sealing, and inhibition tests were conducted, along with evaluations of practical application effectiveness. The study shows that the API filtration loss of PSW-2 drilling fluids with different densities is below 3.2 mL, with a sealing efficiency greater than 86%, a rolling recovery rate higher than 95%, and a linear expansion rate below 1.38%. The fluid exhibits good rheological properties, sealing capability, and inhibition performance. Field practice indicates that the PSW-2 drilling fluid maintains stable performance during application, with the API filtration loss kept below 3.0 mL. It provides effective wellbore stability for long sections of mudstone, carbonaceous mudstone and shale, which has achieved an 87.5% success rate in 8 shale gas horizontal wells. Future development trends include enhancing the sealing capability for nano-scale and micro-scale fractures and improving the sealing and collapse prevention ability of drilling fluids through composite weighting methods, while further reducing drilling fluid costs. This technological achievement provides technical support for low-cost and efficient drilling in shale gas horizontal wells.

Key words: water-based drilling fluid, lacustrine shale, shale gas, weakly cemented formations, wellbore instability, Yan′an Block