Effects of Normal Stress and Clay Content on the Frictional Properties of Reservoir Rocks Under Fully Saturated Conditions

The large magnitude micro-seismicity observed during the process of hydraulic fracturing is likely to be associated with the activation of pre-existing faults. To better understand the mechanism of fault stability, shear tests were performed on powdered gouge of shale and sandstone reservoir rocks from four different oilfields in China using the double direct shearing geometry, at three successive normal stresses, 10, 20 and 40 MPa, and under fully saturated conditions, aiming to investigate the effects of normal stress and clay contents on frictional strength and stability of faults. The velocity step tests were performed from 0.1 to 100 μm/s to assess the frictional stability by calculating the value of a − b. It is observed that the frictional strength decreases with higher clay content and a transition from velocity weakening behavior (a – b < 0) to velocity strengthening behavior (a – b > 0) at a clay content of 15–20%. The frictional strength generally decreases slightly with higher normal stress. The critical slip displacement Dc shows an increasing trend with the increase of clay content at both low and high normal stress, while an opposite trend appears under medium normal stress.