Compositional Numerical Simulation of CO2-enhanced Gas Recovery in very Low-pressure Partially-depleted Reservoirs

Injection of CO2 into depleted gas reservoirs offers the potential to securely store carbon dioxide while enhancing hydrocarbon recovery. Mathias et al. (2014) developed a two-layer vertical-equilibrium model for the injection of carbon dioxide into a low-pressure (<1 MPa) reservoir. In contrast to previous vertical-equilibrium models, the compressibility of all components was fully accounted for and non-Darcy effects were also considered. However, they ignored the effects of compositional changes and mixing between phases. This study seeks to extend the study of Mathias et al. (2014) by incorporating compositional effects, using a Method of Lines (MOL) three-component two-phase numerical simulator. MOL requires formulation of derivatives of the Primary Variables (PV) with respect to time. This often gives rise to the need for evaluating partial derivatives of some of the flow variables (in particular the bulk fluid density per unit volume of rock) with respect to the PVs. In this work, it will be shown how it is possible to evaluate these partial derivatives analytically (as opposed to e.g. conventional finite differencing). Moreover, following an assumption of constant equilibrium ratios for CO2 and CH4, a ternary flash calculator is developed providing closed-form relationships for exact interpolation between equations-of-state for CO2+H2O and CH4+H2O binary mixtures. Overall, the numerical results confirm the finding of Mathias et al. (2014), that pressure build-up, during CO2 injection into low pressure depleted gas reservoirs, can be expected to be largely unaffected by heat transport and other associated thermodynamic effects. Reference: Mathias, S. A., McElwaine, J. N., & Gluyas, J. G. (2014). Heat transport and pressure buildup during carbon dioxide injection into depleted gas reservoirs. Journal of Fluid Mechanics, 756, 89-109.