Experimental Investigation on the Shear Behavior of Different Faults in Depleted Gas Storage Reservoirs

Gas storage in depleted gas reservoirs has become a core facility for ensuring energy security and a key means of guaranteeing a safe and stable supply of energy. Steep pressure rise and fall cyclic fluctuations caused by strong injection and production are likely to lead to the destabilization of the geological structure of the gas storage reservoir. Among the geological formations, fault activation is a serious threat to the safety of gas storage reservoirs. In this study, faults with different filling types were depicted by real downhole cores. Through a series of shear tests, the effects of normal stress, filling thickness and fault angles on the lithology of rocks on both sides were investigated. (1) A novel testing method was developed for finely engraving faults on downhole cores, allowing for the simulation of real reservoir conditions. (2) An increase in normal stress results in enhanced shear strength, which in turn elevates the critical initiation stress of the fault. (3) Shear strength decreases with an increasing amount of fault mud, indicating that the critical initiation stress in faults filled with minor amounts of fault mud is higher than that in faults filled with significant amounts of fault mud. (4) For equal amounts of fault mud, the shear strength of fault specimens at a 40° angle exceeds that of specimens at a 10° angle. This implies that a greater degree of fault undulation corresponds to a higher critical slip initiation stress, reducing the likelihood of fault slip and enhancing stability. (5) The shear strength of fault specimens composed of sandstone-mudstone combinations is lower than that of specimens containing sandstone-sandstone combinations, suggesting that the critical slip initiation stress for sandstone-mudstone combination faults is comparatively lower.