Rock-core scale modeling of initial water saturation effects on CO2 breakthrough pressure in CO2 geo-sequestration

Abstract

CO2 breakthrough pressure plays an important role in the capacity, safety and caprock evaluation of CO2 geo-sequestration. Many laboratory experiments have been put forward to investigate the breakthrough pressure and some functional relationships are proposed. Meanwhile, some numerical simulation theories on two-phase flow have also been presented by scholars. However, up to now, no numerical simulation method for CO2 breakthrough pressure has been put forward. In this study, a numerical method based on step-by-step method that can be used to predict CO2 breakthrough pressure in rock core is proposed. We use COMSOL Multiphysics, a numerical simulation software for multiple physical fields, to investigate the effects of initial water saturations in rock core on CO2 breakthrough pressure. The simulation results show that the breakthrough time of CO2 are 17, 21, 37.5, 65 and 70 h, and the breakthrough pressures of CO2 are 1.9, 2.2, 3.3, 5.1 and 5.4 MPa for initial water saturations of 0.2, 0.4, 0.6, 0.8 and 1, respectively. Moreover, both breakthrough pressure and breakthrough time are exponentially related to the initial water saturation of rock core. Based on the comparison with the published experimental data, it is proved that the breakthrough pressure data of CO2 in this numerical simulation is reasonable. Meanwhile, sensitivity studies are conducted on interfacial tensions (20, 25 and 30 mN/m) in CO2-H2O system and m (0.3, 0.4 and 0.5)/l (1.4, 1.7 and 2) values in van Genuchten (VG) model. The results show that, (1) under the condition of high saturation, the effect of interfacial tension on breakthrough will be more obvious; low interfacial tension in CO2-H2O system has an adverse impact on CO2 breakthrough, the higher the interfacial tension, the lower the breakthrough pressure and breakthrough time of CO2. (2) Under the condition of high saturations, the influence of parameters m and l in VG model on CO2 breakthrough will be more obvious; at the same saturation, the greater the m/l parameters, the greater the breakthrough pressure and breakthrough time of CO2. This study provides numerical information for further studies of comprehensive and more complex numerical simulations on CO2 breakthrough pressure in rock core, CO2 geo-sequestration as well as the caprock evaluation.