LAUSR

Abstract The physical and chemical properties of hard brittle shale and its mechanical properties under different drilling fluid systems are of great practical significance to the in-depth study of shale borehole stability. Microscopic geological characteristics of hard brittle shale and the changes of physical and chemical properties after soaking different drilling fluids are revealed. After soaking in different drilling fluid systems, the mechanical properties have changed. After soaking in distilled water, water expands along micro-cracks due to the structural composition of rocks, eventually lead to cracking along the structural plane. After soaking in polysulfonate drilling fluid, the sample is destroyed and the strength of rock sample is weakened. After soaking in organic salt drilling fluid, the strength of rock sample weakens, drilling fluid fills the fracture surface to a certain extent. After soaking in oil-based drilling fluid, the strength of rock sample weakens, and oil-based drilling fluid fills and closes the fracture surface. The research results are of great significance to the analysis of borehole instability mechanism of hard brittle shale and drilling fluid optimization of shale gas drilling.