A Pore-Scale Study on the Shale-Gas Transport with CO2 Injection Applyingthe Lattice Boltzmann Method

CO2 injection into the shale formation has the potential of enhanced shale-gas recovery and CO2sequestration. The gas transport exhibits slippage and adsorption phenomena because the shale formation contains numerous nano pores and organic matters. And the transport mechanism could become more complex when considers the interactions between different gas components. To provide microscopic investigations on the shale-gas transport with CO2 injection, a LB model is developed to simulate the transport process of multi-component shale-gas. To characterize the multi-component shale-gas transport, a multi-relaxation-time LB model is developed. The interactions between CO2 and CH4 is described by introducing a diffusion force into the evolution equation. The relaxation times are determined by considering the Knudsen effect. The gas slippage is described by the bounce-back combined with the full diffusive boundary condition. The gas adsorption effect near the organic matters is captured by introducing an adsorption force between gas and organic matter nodes. In this work, we first validated the numerical model with several benchmark problems. Then the CH4-CO2 mixture transport during CO2 injection in a micro-tube is simulated. The effects of several influential factors, including the Knudsen number (Kn), adsorption effects, on the CO2 molar fraction distribution (CMFD) along the domain were analyzed. The independent impact of KN was first analyzed. Simulation results showed that when it is less than 0.05, the slippage effect is weak, resulting in piston-like CMFD. As it increases, the slippage effect is significantly increased, and the Knudsen layer becomes a flowing channel for CO2 to flow bypass the front. Thus, the trend of CMFD function changes from the piston-like form to the linear form. Because the adsorption strengths of organic matters on CO2 is stronger than CH4, we also considered the adsorption effects on the gas transport in organic tube. The results showed that when the KN is larger than 0.1, the slippage effects could become stronger due to adsorption, especially for CO2. The transport of the adsorbed CO2 could contribute a large portion of the gas transport in the small pores. The adsorbed CO2 failed to displace the CH4 at the center of pores, weakening the CO2-EGR performance. The LB model proposed in this study is capable to simulate the multi-component shale-gas transport, including the components interactions, slippage, and adsorption phenomena. The effects of the KN and adsorption phenomenon were quantitively analyzed by simulating the CH4-CO2 mixture transport during CO2 injection in a micro-tube.