Multiscale simulations of shale gas transport in micro/nano-porous shale matrix considering pore structure influence

Unravelling the transport characteristic of shale gas in actual extraction process is significantly important to improve the gas recovery efficiency and production. But the geological complexity of shale formations in particular makes it challenging to comprehensively cognize the transport mechanisms at multiscale. Traditional shale gas transport model based on circle nanopores system would be failed to capture the realistic transport process of shale gas in the matrix consisting of abundant micro/nano-pores with complex pore structures. Towards this end, herein multiscale simulations were performed to investigate the transport characteristic and mechanism of shale gas in micro/nano-porous shale matrix combining the molecular dynamics (MD) simulations, analytical model and pore network model. MD simulations of shale gas (methane) transport in nanopores demonstrated that the transport behavior is determined by the competition between gas-wall interaction (gas diffusion) and gas-gas intermolecular interaction (viscous flow). Considering different pore structures (e.g., circle, square, triangular, and slit), we proposed a multiscale analytical model with the coupling of continuum flow theory and diffusion effect (Knudsen diffusion and surface diffusion), which is well verified by results from MD simulations and exhibits the practicability of predicting shale gas transport from nanoscale to macroscale. Furthermore, with the aid of pore network model, micro/nano-porous structure was constructed to simulate gas transport in shale matrix. It was found that traditional simulations of shale gas transport in matrix based on circular nanopores would underestimate the transport capacity, for example, that of slit nanopore system is enhanced by as much as 126%. In particular, shale gas transport in shale matrix can be remarkably influenced by the reservoir pressure. At early stage of exploitation, the shale gas transport in matrix is mainly contributed by macropores (H > 100 nm) with the dominant transport characteristic of viscous flow. With the decrease of pressure in continuous exploitation, nanopores (H < 10 nm) will become the primary flow paths due to the enormously enhanced diffusion effect. The intrinsic tendency of our simulated multiscale transport may be useful for understanding the transport behavior and extraction of shale gas in porous shale formations.