Abstract
Multi-gas transport mechanisms coexist in nanopores of shale gas reservoirs with complex pore size distribution and different gas-storage processes, including continuous flow, slip flow and transition flow of bulk gas and surface diffusion for adsorbed gas. The force between gas molecules and the volume of the gas molecules themselves cannot be negligible in shale gas reservoirs with high pressure and nanoscale pores, which influences gas transfer and must be taken into account as a real gas effect. During depressurization development of shale gas reservoirs, the adsorbed gas desorption and the decrease of an adsorption layer influence gas transport. Meanwhile, due to the stress dependence, the decreases of intrinsic permeability, porosity and a pore diameter also influence gas transport. In this work, a unified model for describing gas transport in organic nanopores of shale gas reservoirs is presented, accounting for the effects of coupling the real gas effect, stress dependence and adsorption layer on gas transport. The unified model is developed by coupling a bulk gas transport model and an adsorbed gas surface diffusion model. The bulk gas transport model is validated with published molecular simulation data, and the adsorbed gas surface diffusion model is validated with published experimental data. The results show that (1) in comparison with the previous models, the bulk gas transport model presented on the basis of the weighted superposition of slip flow and Knudsen diffusion in this paper can more reasonably describe known bulk gas transport processes, (2) surface diffusion is an important transfer mechanism, and its contribution of gas transport cannot be negligible and even dominates under the condition of smaller nanopores, and (3) the effect of stress dependence on fluid flow of shale gas reservoirs is significantly different from that of conventional reservoirs, which is related to not only the shale matrix mechanical properties and the effective stress, but also the gas transport mechanisms. The model can capture the physical mechanism of the interaction between bulk gas transport and adsorbed gas surface diffusion, and provide insights on the effect of coupling the real gas effect, stress dependence and adsorption layer on gas transport. The model allows for extrapolation from laboratory tests with the conditions of low pressure to field conditions of high pressure. The model is highly recommended to describe accurately the reservoir behavior during gas production in shale gas reservoirs and to design production scenarios smartly. The work is important and timely for development of new generation shale-gas reservoir-flow simulators.
