Analytical Model for Multicomponent Gas Transport Behaviors in Shale Nanopores

Abstract

Despite shale gas being a mixture of multicomponent gas species, studies address gas transport in shale based on a single gas component. This study investigates the behavior of multicomponent gas transport during shale gas production through a developed analytical model for multiple flow mechanisms considering CO2-CH4. The model is validated using experimental data and CMG simulation results, considering the effects of pressure, pore size distribution, competitive adsorption, stress dependence permeability, and mole concentration on multicomponent gas transport. Results show that at a high-pressure gradient, the flow rate for bulk flow (slip flow, transition flow, and Knudsen diffusion) is high in pores >10 nm, reaching 0.18 kg/m2.s, whereas CO2 displays higher mobility than CH4. Under the same conditions, surface diffusion reveals a significant flow in pores <10 nm dominated by CH4 than CO2. As pressure decreases, CO2 and CH4 fractions in the bulk flow and surface diffusion interchangeably increase and decrease in all pores affected by various factors, including the CO2-CH4 competitive adsorption. It could therefore be inferred that the flow characteristics of multicomponent gas in a shale formation dynamically change depending on the production conditions and reservoir parameters, which, on the other hand, are found to affect one another in the cause of the production process. The model in this manuscript shows how to use theory to predict shale gas production in a way that is both accurate and efficient.