Compositional Modeling of CO2 Injection in a Nanoporous Unconventional Reservoir Core

Abstract

This paper presents a one-dimensional radial compositional numerical model that simulates CO2 injection into a decane-saturated nanoporous medium. The theoretical framework couples advective mass transport and diffusive molar fluxes as the primary mechanisms and incorporates molecular adsorption and sieving as secondary nanoscale phenomena. The Peng–Robinson Equation of State (EOS) is used to determine phase equilibrium, compressibility, and related properties. Emphasis is placed on the "soaking period," during which molecular diffusion dominates mass transport in a static system. Numerical results quantify the interaction between CO2 and decane in nanoporous media and characterize the resulting pressure behavior over time. Simulations show that CO2 dissolution in decane significantly alters liquid-phase properties, resulting in substantial pressure decline during the soaking period. During early times, pressure depletion follows a linear relationship with the square root of time, indicating diffusion-controlled transport. Model validation reproduces the pressure-decay trends observed in laboratory experiments on Eagle Ford rock samples, as reported by Fu et al. (2021). The results are consistent with the experimental observations. The numerical results are also compared with our adaptation of the analytical method by Sheikha et al. (2005) for gas diffusion into bitumen—not a porous medium—which is a linearized version of the nonlinear compositional model. Our analytical method is shown to provide reasonable estimates of the diffusion coefficient and oil recovery when gas diffusion into the matrix is the dominant transport mechanism. Overall, this paper highlights the importance of fully compositional numerical modeling for accurate evaluation of diffusion-dominated transport, particularly in high-pressure, high-temperature (HPHT) environments.