Damage Mechanisms of Oil Shale Induced by Drilling Fluid Invasion under Coupled Stress-Seepage Conditions

Shale oil exploration and development is important to meet our global energy demand. To improve wellbore stability in oil shale formations, this study investigates the damage mechanisms associated with drilling fluid invasion into oil shale under coupled stress-seepage conditions. A rock pretreatment procedure was established and a coupled stress-seepage experimental system was constructed. Oil shale damage experiments were conducted considering rock stress state, drilling fluid seepage conditions, drilling fluid type, and bedding angle. The results show that a quantitative correspondence can be established between the saturation of Chang 7 oil shale cores and ambient relative humidity and water potential. The oil shale exhibits pronounced anisotropy, and the coupled stress-seepage response is strongly influenced by this anisotropy. The strength of samples using oil-based drilling fluids is significantly higher than using water-based drilling fluids, with increases reaching up to 71%, confirming the superior performance of oil-based drilling fluids. During the coupled stress-seepage process, an increase in axial stress promotes crack propagation, facilitates drilling fluid invasion, and weakens the rock strength by as much as 88%, whereas an increase in confining pressure compacts the bedding and fractures, suppresses drilling fluid invasion, and consequently enhances rock strength, with gains of about 16%. A critical stress point exists at which the effects of axial stress and confining pressure on rock strength offset each other.