Hydrofracturing fluid-weakened shear behavior of sandstone joints

The weakened shear behavior of sandstones induced by hydrofracturing fluids significantly affects the activation of natural fractures during hydraulic stimulation in hydrocarbon and geothermal exploitations. This paper presents an experimental study of characterizing the sandstone joints treated by water-based fracturing fluid (WFF) and acid mixture solution (AMS) treatment that simulate rock joint-hydrofracturing fluid interactions in deep reservoirs during short shut-off times on wells after hydraulic fracturing. Three-dimensional (3D) optical scanning method coupled with 3D rigid engraving was utilized to create replicas of natural rock joint surfaces, and direct shear tests were conducted to probe the shearing behavior of untreated and WFF/AMS-treated replicas with three different roughness levels and under two high normal stresses of 15 and 30 MPa. Microindentation and X-ray diffraction were then performed to investigate the degradation of mechanical properties and potential alterations in composition and microstructure of the hydrofracturing fluid-treated joint surfaces. The results indicate that short-term interactions between the hydrofracturing fluids and sandstone can lead to a reduction in mechanical properties and frictional resistance of the joint surface, ultimately weakening its shearing properties. The decline in mechanical properties observed in the AMS-treated sample is attributed predominantly to the dissolution of QFC (quartz, feldspar, and calcite) and swelling of clay minerals. For the WFF-treated sample, clay mineral swelling and the residual guanidine gum on the joint surfaces remain the primary factors contributing to the reduction in shearing resistance. At high normal stresses of 15 and 30 MPa, joints with different roughnesses exhibit asperity shearing off as the dominant mechanism. Furthermore, an increase in joint roughness leads to a higher friction angle on the joint surface. However, higher normal loads could result in a reduction in apparent cohesion. The results of this study provide valuable insights into the interactions between hydrofracturing fluids and sandstone, as well as the behavior of natural fracture activation during hydraulic fracturing.