Inhomogeneous Fluid Transport Modeling of Gas Injection in Shale Reservoirs Considering Fluid-Solid Interaction and Pore Size Distribution

Gas injection presents unique enhanced oil recovery (EOR) mechanisms in shale reservoirs compared to conventional reservoirs due to the complex nature of fluid transport and fluid-solid interaction in nanopores. We propose a multiscale continuum-scale multiphase multicomponent transport model for primary production and gas injection in shale reservoirs, leveraging pore-scale information from density functional theory considering fluid-solid interactions and pore size distribution (PSD). The shale matrix is separated into macropore and nanopore based on pore size distribution. The density functional theory is employed, accounting for fluid-solid interactions, to compute the inhomogeneous fluid density distribution and phase behavior within multiscale matrix. These fluid thermodynamic properties and transmissibility values are then integrated into the multiphase multicomponent transport model grounded in the Maxwell-Stefan theory, facilitating the simulation of primary production and gas injection processes. Our research underscores the precision of density functional theory in capturing intricate fluid inhomogeneities within nanopores, a capability overlooked by the cubic equation of state. The fluid system within varying pores can be classified into confined fluid and bulk fluid, delineated by a pore width threshold of 30 nm. Distinct fluid compositions are observed in macropores and nanopores, with heavy components exhibiting a preference for distribution in nanopores due to stronger fluid-solid interactions compared to light components. During primary production period, the robust fluid-solid interactions in nanopores impede the mobility of heavy components, leading to their confinement. Consequently, heavy components within nanopores pose challenges for extraction during primary production processes. During the CO₂ injection period, the injected CO₂ induces a significant alteration in fluid composition within both macropores and nanopores, promoting fluid redistribution. The competitive fluid-solid interaction of CO₂, particularly with propane, results in efficient adsorption on pore walls, displacing propane from nanopores.