How pore structure evolves in shale gas extraction: A new fractal model

Abstract

The reservoir can be divided into four components with different properties: inorganic matrix, hydraulic fracture, natural fracture and kerogen according to the internal structure characteristics and the natural gas migration mode after hydraulic fracturing. Previous studies usually focus on the relationship between reservoir porosity and permeability, but ignore the influence of reservoir microstructure on gas production. The microstructure of different reservoir components can be described by corresponding fractal dimensions. The structure of fractures can be characterized by fractal dimension of fractures and the fractal dimension of pores can characterize the pore structure of inorganic matrix. Considering the complexity of mass transfer between components and structural deformation, in order to describe the evolution of reservoir microstructure and the influence of various factors on gas production, a fully coupled multi-scale fractal model is established. To verify that the model is reliable, the simulation results contrast with the conclusions of field tests. The effects of adsorption parameters, initial diffusion coefficient and compressibility coefficient on gas production and pore structure evolution were studied. The results show that:(1) when the adsorption constant decreases, the variation of the fractal dimension of pore structure becomes more obvious, and the shale reservoir's gas production and pore structure are most sensitive to the volume constant; (2) the deformation of pore structure increases with the increase of effective diffusion coefficient; (3) the change of compressibility of a single medium only has a major impact on the evolution of its own structure.