Modelling CO2 injection into a saline aquifer taking into account non-equilibrium phase transitions

Abstract

<p>Carbon dioxide storage in saline aquifers becomes an increasingly important scenario of subsurface utilization aimed at mitigating the effects of climate change. The storage of CO<sub>2</sub> can be organized by means of its injection through a few wellbores. A rather complicated reservoirs simulations are needed to estimate the potential well injectivity and reservoir capacity. Usually, the modelling is performed by means of conventional reservoir simulators, e.g. MUFITS [1]. The mathematical models employed in the simulators can account for many physical phenomena including phase exchanges between the brine and gas phases. The standard modelling approach assumes that the phase transitions occur under local thermodynamic equilibrium. This implies that if injected CO<sub>2</sub> flows into a grid block saturated with brine, then the gas instantly mixes with all brine in that grid block. Thus, all gas appearing in the grid blocks is instantly distributed over the entire block. This is a rather rough assumption given that the typical size of the grid blocks is a few tens of meters in the lateral directions. Thus, the phase equilibrium between gas and brine requires a considerable time to establish over such a large grid block. Furthermore, on the pore scale the establishment of the phase equilibrium is controlled by a rather slow process of molecular diffusion in tiny pores, which also delay the establishment of the equilibrium. The standard modelling approaches can overestimate the solubility trapping because they do not account for such phenomena.</p><p>In the present study, we aim at extending our MUFITS software [1] to the non-equilibrium phase transitions by implementing the model proposed in [2]. The extension is based on the modified compositional modelling approach that assumes using the cubic equation of state for predicting the fluid phase equilibria. Instead of equilibrating the chemical potentials in the phases, we assume that the difference between the potentials is not zero and it relaxes with time. If gas appears in a grid block, then the thermodynamic system is disturbed and the equilibrium breaks down, i.e. the chemical potentials are not equal. Then, if no more gas flows into the block, then the difference of the chemical potentials relaxes over time and the thermodynamic equilibrium establishes with some delay. The grid block is thus characterized with an extra relaxation parameter showing the rate of the thermodynamic system equilibration. For example, it can be a function of the pore sizes and diffusivities. We provide several examples of CO<sub>2</sub> injection comparing the modelling results under the assumption of local equilibrium with those basing on the proposed non-equilibrium approach.</p><p>The authors acknowledge funding from the Russian Science Foundation under grant # 19-71-10051.</p><p>[1] Afanasyev A., Vedeneeva E., 2021. Compositional modeling of multicomponent gas injection into saline aquifers with the MUFITS simulator. J. Nat. Gas Sci. Eng. 94. 103988.</p><p>[2] Indrupskiy I., Lobanova O., Zubov V., 2017. Non-equilibrium phase behavior of hydrocarbons in compositional simulations and upscaling. <em>Comput. Geosci.</em>, 21, 5–6, 1173–1188.</p>