Guangxu Zhou, Chuanliang Yan, Yuanfang Cheng
Shale exhibits a semipermeable membrane effect, but it does not behave as an ideal semipermeable membrane. Membrane efficiency is a key parameter for characterizing the semipermeable membrane properties of shale and provides an important basis for wellbore stability analysis, showing significant engineering value. In this study, commonly used water-based and oil-based drilling fluids were selected. Shale samples wrapped with an artificial semipermeable membrane were immersed in these drilling fluids, and their mechanical properties were subsequently measured. Meanwhile, variations in water content were jointly measured using gravimetric measurements and nuclear magnetic resonance (NMR), and the relationships among drilling fluid type, rock strength, water content, and membrane efficiency were investigated. The results show that rock strength was highest after immersion in the oil-based drilling fluid, followed by the polyamine drilling fluid and the polyol drilling fluid. The attenuation law of rock strength induced by shale hydration damage was determined, and the relationship between rock strength and water content was established. A method for calculating the rock hydration damage coefficient based on the NMR T 2 signal was proposed, and its accuracy was experimentally verified. Furthermore, a rock strength-based method for characterizing shale membrane efficiency was established. This method enables membrane efficiency to be calculated using rock strength, drilling fluid activity, and formation water activity and can characterize the variation in membrane efficiency during wellbore stability analysis. Among the tested drilling fluids, the oil-based drilling fluid exhibited the highest membrane efficiency, approximately 0.71, followed by the polyamine drilling fluid, with a membrane efficiency of approximately 0.60. As the soaking time increased, the membrane efficiency continuously decreased.