Study on anisotropy of Longmaxi shale using hydraulic fracturing experiment

Abstract

Hydraulic fracturing reservoir reconstruction technology is crucial in the development of shale gas exploitation techniques. Large quantities of high-pressure fluids injected into shale reservoirs significantly alter compressional (P) and shear (S) wave velocities, rock mechanical parameters, and anisotropic characteristics. In this study, differentiated hydraulic fracturing petrophysical experiments were carried out on Longmaxi Formation shale under pseudo-triaxial stress loading conditions. The effects of stress loading methods, and water-rock physical and chemical reactions on P- and S-wave velocities and rock mechanical parameters were compared. The experimental results showed that isotropic stress loading may increase the P- and S-wave velocities and Young’s modulus of dry shale kldnsample. Furthermore, it may lead to a weakening of the corresponding anisotropy. In contrast, differential stress loading was able to improve the anisotropy of Young’s modulus and accelerate the decrease in the compressive strength of shale in the vertical bedding direction. The water-rock physical and chemical reactions prompted by hydraulic fracturing was found to “soften” shale samples and reduce Young’s modulus. The influence of this “soften” effect on the compressional and shear wave velocities of shale was negligible, whilst there was a significant decrease in the anisotropy characteristics of Thomsen parameters, Young’s modulus, and Poisson’s ratio. The negative linear relationship between the Poisson’s ratios of the shale samples was also observed to lose sensitivity to stress loading, as a result of the “soften” effect of fracturing fluid on shale. The results of this study provide a reliable reference point and data support for future research on the mechanical properties of Longmaxi shale rocks.