Effects of Fracturing Fluids on Properties of Shale Reservoir: A Case Study of the Longmaxi Formation in the Sichuan Basin

Hydraulic fracturing is widely used for developing shale reservoirs with low porosity and permeability. Large volumes of fracturing fluids are injected into reservoirs, yet the impact of these fluids on shale is not entirely understood. This study investigates the effects of commonly used fracturing fluids on the fundamental properties of shale during the shut-in period using experimental methods. Shale samples are collected from the Longmaxi Formation in the Sichuan Basin. Two types of fracturing fluids (guar gel and slickwater) are prepared for tests. The effects of these fluids on shale’s mineral composition, pore distribution, and fracture structure are analyzed using a range of techniques, including X-ray diffraction, nuclear magnetic resonance, nitrogen adsorption-desorption, and X-ray computed tomography scanning. The results show that the shale is composed of quartz, siderite, and clay minerals. The reservoir’s pore structure is relatively uniform, with a higher proportion of small pores and a predominance of wedge-shaped pore types. The porosity ranges from 1.8% to 4.33%, with an average pore diameter varying between 10.8 nm and 24.8 nm. More fracturing fluid enters the reservoir as shut-in time increases. Initially, fluid invasion occurs rapidly, but the volume of infiltrated fluid stabilizes after 15 days. The fracturing fluids cause chemical reactions and hydration of clay minerals. Both fracturing fluids lead to a decrease in the proportion of clay minerals and an increase in the proportion of quartz. After soaking in guar gel, the shale’s surface area and pore volume decrease while the average pore diameter increases. The breakdown of guar gel leads to a residue that blocks pore spaces, resulting in lower surface porosity. In contrast, slickwater increases surface area and pore volume while reducing the average pore diameter. Slickwater also promotes the development of fractures, with larger pores forming around them. The results suggest that slickwater is more effective than guar gel in improving shale’s pore structure.