On gas transport modes in matrix-fracture systems with arbitrary multiscale configurations

Abstract

Tight shale reservoirs exhibit high heterogeneity and strong anisotropy in multiscale pore/fracture networks, with highly variable properties. The local equilibrium or non-equilibrium states vary spatially and are strongly controlled by the gas transport modes at each scale. A fundamental understanding of the coupling effects of gas flow in heterogeneous porous media with arbitrary scale ratios is crucial but not yet available. Here, we systematically and theoretically study the gas transport modes and gas flow velocity in multiscale matrix-fracture systems using the asymptotic homogenization method. A series of exact scaling laws for the gas velocity in heterogeneous porous media with arbitrary multiscale configurations are established, and the local equilibrium/non-equilibrium effects at each scale are analyzed in detail. It is shown that the gas transport modes between two adjacent porous media can be classified into four distinct types governed by two characteristic time scales (rather than two types as commonly reported). We demonstrate an ultrahigh pressure gradient in a thin depressurized zone in the matrix that can reach ${10}^3\sim {10}^5$ times the macroscopic pressure gradient, greatly increasing gas flow rates by three to five orders of magnitude. The hydraulically-created fractures not only provide preferential flow pathways, but more importantly, they increase the gas velocity in the matrix (which does not contain any fractures) by several orders of magnitude. The work also sheds light on the discrepancy between the observed high gas production and the experimentally measured low permeability in drilled cores.