Multifractal Analysis for the Acoustic Emission Energy Dissipation of Shale after Long-Term Immersion in Fracturing Fluids with Various pH

Abstract

The interaction between shale and fracturing fluids will change the internal pore structure and adsorption capacity of shale, which have an important impact on shale gas productivity. This study aims to investigate the variations in acoustic emission (AE) energy and multifractal characteristics of shale under different pH values of fracturing fluids with implications for shale gas productivity. A series of immersion experiments were carried out on shale with fracturing fluids, with pH values of 6, 7, and 8, respectively. The uniaxial compression tests and AE tests were conducted on shale under fracturing fluid immersion with pH values of 6, 7, and 8. The multifractal theory is applied to analyze the evolution of fracture closure and propagation during uniaxial compression based on AE signals. The study also examines the deformation and crack evolution of shale after long-term fracturing fluid immersion, with particular emphasis on fluid−rock reactions. The results show that the maximum cumulative AE energy of shale after immersion is higher than that of intact shale. The focus is on understanding the fracture evolution process of shale during uniaxial compression at different stages. The fluid−rock reactions determine the deformation and crack evolution of shale after longterm fracturing fluid immersion. The increase of shale spectrum width Δα after immersion indicates that the distribution of AE signals is more uneven after immersion. The Δf value of shale after acid fracturing fluid soaking changes more significantly than those of neutral or alkaline fracturing fluids. All the fractal spectra of shale samples generally show a left-skewed distribution. The values of spectrum parameters are affected by soaking fluid, soaking time, and compression stages, resulting in the variation of multifractal characteristics. Hydration expansion of clay minerals is dominant in the middle and late period of immersion. The research results have a certain guidance for understanding the deformation and crack evolution and the influence of rock−fluid reactions on the failure process of shale samples.