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

Abstract

Abstract 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 multiphase multicomponent transport model for primary production and gas injection in shale reservoirs considering dual scale porous medium and fluid-solid interactions in nanopores. 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. The calculated fluid thermodynamic properties and transmissibility values are then integrated into the multiphase multicomponent transport model grounded in the Maxwell-Stefan theory to simulate primary production and gas injection processes. Our research underscores the precision of density functional theory in capturing intricate fluid inhomogeneities within nanopores, which is overlooked by the cubic equation of state. The fluid system within varying pores can be classified into confined fluid and bulk fluid, separated 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 are difficult to extract 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 results in efficient adsorption on pore walls, displacing propane from nanopores.