FRACTURING INDUCED SHALE SOFTENING MECHANISM AND PREVENTION TECHNOLOGY

Hydraulic fracturing is a critical technology for the development of shale oil and gas reservoirs. However, shale tends to soften after water-rock interactions, which not only reduces the effectiveness of reservoir stimulation but also leads to proppant settlement within hydraulic fractures. As a result, fractures lose support and reclose, causing a significant decline in shale productivity due to loss of fracture conductivity. This study utilizes core samples from the Longmaxi Formation shale reservoir in the Sichuan Basin. The shale softening process under actual geological conditions was simulated, and variations in mechanical properties under different soaking durations were compared through micro-indentation experiments. The mechanisms underlying water-rock interaction-induced softening were investigated by analyzing shale composition, fracturing fluid properties, and surface characteristics. A method for mitigating shale softening was proposed and experimentally validated. The results show that during the hydraulic fracturing of Longmaxi shale, the content of carbonate minerals decreases while clay mineral content increases. The shale surface is predominantly composed of oxygen-containing functional groups that readily form hydrogen bonds with water molecules, promoting water adsorption onto the shale surface and penetration into clay mineral interlayers, resulting in surface hydration. The negatively charged shale surface attracts cation adsorption, while free anions accumulate in the interlayer aqueous solution of clay minerals, generating osmotic pressure that draws more fracturing fluid into the interlayers and induces osmotic hydration. By incorporating 3% by volume of a shale reinforcement agent into the fracturing fluid prior to fracturing, water molecule adsorption on shale mineral surfaces is effectively inhibited. This treatment increases the average wetting angle of the fracturing fluid by 33.04%, reduces the average cation exchange capacity of shale by 28.63%, decreases the average Zeta potential by 31.21%, and reduces the shale erosion rate by 96.37%, 89.80%, and 53.05%, respectively. This study demonstrates that the proposed shale reinforcement agent effectively inhibits shale softening and helps maintain fracture conductivity.