Abstract
The competitive adsorption of CH(4)/CO(2) in shale has consistently garnered significant attention as a means to enhance the recovery efficiency of shale gas reservoirs. A theoretical formula for a binary gas competitive adsorption rate model was formulated to investigate the adsorption and desorption characteristics and the evolution patterns of carbon dioxide (CO(2)) displacing methane (CH(4)) in shale. This formula was integrated into the lattice Boltzmann method (LBM) to simulate the displacement process of CH(4) by CO(2), addressing competitive adsorption challenges associated with binary gases exhibiting adsorption/desorption behaviors. The research findings reveal that the proposed theoretical model accurately captures the competitive adsorption dynamics between CO(2) and CH(4), elucidating the patterns of adsorption and desorption characteristics during the displacement of CH(4) by CO(2). This insight is pivotal for understanding the microscopic mechanisms underlying CO(2)-induced CH(4) displacement. The displacement process is dynamic, marked by concurrent adsorption and desorption of CO(2) and CH(4), ultimately converging to an equilibrium state where CO(2) adsorption and CH(4) desorption coexist. Notably, the time taken for CO(2) to attain equilibrium is marginally delayed compared to CH(4). Moreover, the concentration of injected CO(2) substantially influences the dynamics of CO(2) replacing CH(4), as the CO(2) injection concentration increases, both the adsorption rate of CO(2) and the desorption rate of CH(4) augment, while the time required to reach equilibrium in the adsorption/desorption process diminishes. This implies that CO(2) injection effectively facilitates CH(4) desorption. Additionally, models with higher porosity exhibit enhanced permeability, resulting in accelerated adsorbate diffusion rates and improved displacement efficiency. The heterogeneity of the pore structure exerts a pronounced impact on the velocity distribution within the flow field, which in turn significantly influences the concentration field distribution of CO(2) and CH(4). It is worth noting that the distribution characteristics of the two gases within the flow field and their concentrations within the particles are complementary. The results underscore that carbon dioxide injection can enhance methane desorption, thereby improving shale gas recovery.