Multi-field coupling deformation of rock and multi-scale flow of gas in shale gas extraction

Abstract Shale is a kind of multi-scale medium to store shale gas. The analysis of its structural changes during gas extraction is helpful to increase shale gas production. In this study, the shale reservoir is divided into three systems according to the scale: kerogen system, pore system and fracture system. Based on the fractal geometry theory, the fractal permeability models of the pore system and fracture system are established according to the fracture length and pore diameter geometric characteristics. The diffusion model of kerogen system was established according to the nano-pore structure characteristic of kerogen with adsorption and desorption mechanism. A multi-scale fluid-solid coupling model was established by combining the gas mass transfer equation with the matrix deformation equation. The accuracy of the model is verified by the horizontal well gas production data. The results show that:(1) the effective stress causes the evolution of reservoir internal structure and permeability; (2) local mechanical property changes of the reservoir will spread to the entire reservoir; (3) adsorption effect dominates the storage capacity of shale structure. This study provides a new method for revealing the mechanism of gas flow in rock and improving gas extraction rate.