Multi-scale simulation of gas transport in organic shale with the lattice Boltzmann method

摘要

Successful hydrocarbon production from shale gas reservoir in the United States has initiated interest in its exploration and development around the world. However, a lack of scientific knowledge hinders the proposal of this energy alternative. The aim of this study is to develop an innovative multi-scale flow simulation tool to characterize the hydrocarbon transport in organic shale and to provide a comprehensive work-flow in estimating its permeability. Flow simulations based on lattice Boltzmann (LB) method are carried out at pore-scale and core-scale to investigate the gas transport behaviour, and the problems of gas slippage, gas desorption/adsorption, surface diffusion and pore morphology at both scales are addressed accordingly. Specifically, a pore-scale multiple-relaxation-time (MRT) LB model is adopted to study the gas flow in micro-pores and a new boundary condition, which combines Langmuir adsorption theory with kinetic boundary condition, is proposed to capture the gas slippage and the surface diffusion of adsorbed gas. Later, this pore-scale MRT LB model is applied to study the gas transport in reconstructed shale samples and to identify the effect of pore morphology on gas transport. At last, considering the huge computational cost involved in the application of the pore-scale LB model to a larger computation domain, an effective core-scale generalized LB model is employed for gas flow in the shale matrix consisting of the organic matter (OM) and inorganic matter (IOM). The dusty gas model (DGM) combined with the generalized Maxwell-Stefan model (GMS) are adopted to account for the multiple transport mechanisms in each component. The simulation results show that the gas transport in micro-pores of shale matrix is influenced by multiple effects. The adsorbed gas and its surface diffusion have a profound influence on gas transport in shale matrix, and the apparent permeability can be either enhanced or reduced depending on variations in surface diffusion. In addition, gas exhibits different flow behaviours in organic matter (OM) and inorganic matter (IOM). The pore structure of the shale matrix, the total organic content (TOC), and the pore size distribution control the gas transport and the apparent permeability. The current study is expected to lead to new developments in studying fluid dynamics in micro-porous media and the developed simulation technique of multi-scale flow processes offers both theoretical and practical significance.