Study on the nonlinear coupling seepage model of shale reservoir with discrete networks

Abstract

Gas shale is rich in natural fractures, in which the percolation mechanisms are complicated. Consequently, it’s the key scientific problem of accurately evaluating shale gas production to figure out the law of flow in fractured shale reservoirs. In this paper, a multi-continuum/discrete fracture model was built to describe the shale reservoir space after hydraulic fracturing, and a nonlinear coupled seepage flow mathematical model is established based on the adsorbed gas and free gas coupling, including the nonlinear non-equilibrium desorption and surface diffusion of adsorbed gas, viscous flow and Knudsen diffusion mechanism of free gas. The model believes that the surface diffusion of adsorbed gas occurs on the surface of the organic matter, and non-equilibrium desorption occurs between the adsorbed gas and the pores of the inorganic matrix. There are viscous flow and Knudsen diffusion in the pores of the inorganic matrix. But there is only viscous flow in natural and hydraulic fractures. Through the numerical simulation results of a fractured horizontal well, the time-space evolution process of the net desorption rate of the adsorbed gas in the hydraulic fractured shale reservoir was revealed. It was found that the region of the maximum net desorption rate gradually spread from the artificial network to the matrix. Through the analysis of multiple pressure systems, it was found that the diffusion of free gas in natural fractures, free gas in inorganic matrix pores and adsorbed gas in organic matter lagged successively. Through the flow analysis of natural fracture field, it is found that the flow patterns, outside the envelope of discrete fracture network, appear in succession with time: elliptical radial flow, linear flow and non-linear flow caused by primary fracture interference. This study reveals the percolation mechanism of fractured shale reservoirs and provides a scientific basis for the exploitation of the shale gas reservoir.