The Effect of Shale Swelling on In Situ Stress during Hydraulic Fracturing

The interaction between shale and working fluids during hydraulic fracturing has drawn significant attention, as it can induce shale swelling. However, the impact of this swelling on the evolution of in situ stresses remains poorly understood, posing a challenge for accurate stress evaluation. This study combines experimental and numerical simulation methods to investigate this effect. Swelling experiments confirm that the penetration of fluid into fractures causes shale hydration and swelling. A postfracturing shale gas well model was established numerically. The results demonstrate that shale swelling significantly alters the in situ stress field within the stimulated reservoir volume (SRV), affecting both the magnitude and orientation of the stresses. With increasing swelling, the in situ stress tends to shift from compressive to tensile, creating a stress deficit and triggering stress redistribution. This effect intensifies with the degree of swelling but is negligible outside the SRV area. Furthermore, enhanced swelling leads to more extensive stress alterations after multistage fracturing, characterized by more pronounced tensile stress attributes. As fracturing progresses, the swelling effect accumulates, resulting in progressively greater cumulative changes in the in situ stress distribution.