Implications of the Phase-Solubility Behaviour on the Performance Predictions of the CO₂ Trapping in Depleted Gas Reservoirs and Aquifers

The phase behaviour modelling of gaseous/super critical CO₂ mixtures with reservoir brine exhibits constraints regarding thermodynamical complexity of the system as well as insufficiencies in computing approaches. The modelling of physical properties of mixtures of CO₂ with CH₄ in a depleted gas reservoir is not problematic. In this study a generic reservoir model was created with the aid of a commercial reservoir simulator ECLIPSE, which uses different approaches on modelling the phase behaviour of CO₂ containing geological systems. The model reservoir, adapted to storage properties of real cases, was assumed to be infinite acting, homogenous and isothermal. A constant rate of CO₂ injection was considered. The critical saturation for the flow of gas and water, salinity and brine composition as well as the permeability anisotropy were defined as main parameters influencing the solution trapping of CO₂. The chemical interactions between the system components as well as the hysteresis effects of mechanical trapping were neglected.

The phase/solubility calculations are most critical for the prediction of solution trapping capacity of the aquifers. The migration capacity of the higher density plumes is dependent on the solubility modelling of CO₂ in brine. The results indicate an obvious sensitivity to the varying solubility of CO₂ in various types of brines (NaCl, CaCl₂) with changing salinities. The ratio of vertical to horizontal permeability (k_v/k_h) has a significant effect; at low k_v/k_h CO₂ tends to migrate laterally, whereas an increase in this ratio enhances the vertical gas migration leading to more CO₂ dissolution trapping.