Impact of thermodynamic hypotheses on the calculation of the quantity of CO₂ stored in a saline aquifer

Impact des hypothèses thermodynamiques sur le calcul de la quantité de CO₂ stockée dans un aquifère salin

IPRA (FR 2952) LaTEP EA 1932, Université de Pau et des Pays de l’Adour/E2S UPPA, 64000 Pau, France.

Abstract

The aim of this paper is to study the impact of thermodynamics models on the CO₂ storage capacity of an aquifer, by using modelling and simulation of transport phenomena in porous media. The aquifer is represented here by a simplified three phase isothermal non-reacting medium composed by an inert solid phase and two binary (CO₂ and H2O) fluid phases. The mathematical description is classically developed by the volume averaging method. For each phase, the conservation equations of mass, momentum and energy alongside thermodynamic laws and boundary conditions are first written. They are then integrated over a representative elementary volume in order to establish the description at the local scale. In a thermodynamic point of view, two kinds of models have been implemented and compared. The first one assumes that the gas and liquid phases are ideal. On the contrary, in the second approach, non-ideal thermodynamics is taken into account by calculating an activity coefficient (γ-φ approach) for the liquid phase and a fugacity coefficient (PengRobinson approach) for the gas phase. Results show, among other things, that the amount of CO₂ dissolved in the liquid phase is significantly reduced with the non-ideal approach compared to the ideal case. These results highlight the interest of considering non-ideal phases in more complex models.