Impact of Slickwater Fracturing Fluid on Pore Structure and Micromechanical Properties of Clay-Rich Shale Using Fluid Intrusion and AFM Experiments

Multistage hydraulic fracturing leads to prolonged interactions between shale reservoirs and slickwater fracturing fluids, resulting in changes to the pore structure and micromechanical properties of the shale. However, systematic studies of the impact of shale mineral composition on hydraulic fracturing remain limited. This research investigates the effects of fracturing fluids on reservoirs with different mineral compositions, focusing on the Wufeng-Longmaxi shale in northern Guizhou, China. Comprehensive analyses were conducted on core samples before and after immersion in fracturing fluid, utilizing various testing methods, including field emission scanning electron microscopy, adsorption experiments, mercury intrusion porosimetry, and atomic force microscopy to characterize the pore structure, surface morphology, and modulus values of the shale. Additionally, sulfur-carbon analysis, vitrinite reflectance testing, and X-ray diffraction were employed to assess the mineral composition and geochemical parameters of the shale. The results indicate that after reacting with the fracturing fluid, clay minerals exhibit swelling and dispersion, and carbonate minerals undergo dissolution, while quartz remains largely unchanged. Furthermore, the better the retention of the pore volume and specific surface area in the shale, the more rapidly the modulus values decrease. Clay-rich shales retain more pores compared with quartz-rich and organic-rich shales, facilitating shale gas migration. However, the modulus values of clay-rich shales significantly decrease compared to those of quartz-rich shales, which may undermine the effectiveness of proppants, resulting in fracture closure and reduced permeability. Therefore, maintaining the modulus values of clay-rich shales is crucial for sustainable extraction of shale gas. The addition of clay stabilizers to the fracturing fluid may help preserve the modulus values and porosity of the shale reservoir postfracturing.