A pore-network-based upscaling framework for the nanoconfined phase behavior in shale rocks

The presence of extensive nanopores in organic-rich shale introduces unique thermodynamic fluid phase behaviors owing to large pressure differentials across fluid-fluid interfaces and strong fluid-wall interactions. While the nanoconfined phase behavior has been extensively studied in a single nanopore, its manifestation in complex nanopore networks remains poorly understood and rigorously derived macroscopic phase behavior formulations are not yet available. We develop a novel upscaling framework for deriving macroscopic phase behavior formulations in realistic nanopore networks (e.g., obtained from high-resolution digital images of shale samples). The framework employs a generalized phase equilibrium model that explicitly accounts for the impact of capillary pressure and multicomponent adsorption in each pore. Assuming thermodynamic equilibrium across the pore network, macroscopic phase behavior variables for the entire pore network are then derived by integrating the variables from the individual pores. This leads to a macroscopic network-scale phase equilibrium model that naturally accounts for the size- and geometry-dependent nanoconfinement effects of a complex pore structure. Simulated phase behaviors using three multiscale pore networks demonstrate that (1) the phase behavior in a pore network—controlled by the multiscale pore structure—significantly deviates from that in a single nanopore and (2) due to capillary trapping of the liquid phase and competitive adsorption on the pore wall, heavier components tend to reside in smaller pores and suppress the bubble point pressure therein. The upscaled phase behavior model shares the same mathematical structure as that of a standard phase behavior model and can thus be readily incorporated in commercial reservoir simulators.