Quantitative investigation of phase characteristic effects on CO2 breakthrough pressures in unsaturated Low-Permeability sandstone

Abstract

Study on CO2 breakthrough pressures in caprocks under formation temperatures (T) and pressures (P), which can make CO2 in gaseous, liquid and supercritical states, are vital to the seal ability of caprocks for CO2 sequestration. Laboratory testing is necessary to obtain breakthrough pressures as there is no in situ method to accurately estimate the factors that can affect its variability. In this study, we use the step-by-step method to study CO2 breakthrough pressures in unsaturated low-permeability sandstone, and carried out 9 cases of CO2 breakthrough pressure experiments under multiple P-T conditions that can make CO2 in gaseous, liquid and supercritical states, respectively. The variation of CO2 breakthrough pressures (Pbt) were analyzed based on 4 properties of multi-phase flow: CO2-H2O interfacial tension (γ), CO2-H2O-rock wettability, CO2-H2O viscosity ratio (M), and CO2-H2O density difference. The results show that CO2 breakthrough pressure and breakthrough time are positively correlated with experimental temperatures and pressures. The main property affecting CO2 breakthrough pressures is the CO2-H2O viscosity ratio, which is more affected by the P-T conditions (or CO2 phase). Moreover, the breakthrough pressures of CO2 under 9 groups of multiple P-T conditions (which can make CO2 in various phases) is Supercritical CO2 > Liquid CO2 > Gaseous CO2. This study has important guiding significance for the selection of target caprock for CO2 geo-sequestration.