Experimental Investigation on the Behavior of Supercritical CO₂ during Reservoir Depressurization

CO₂ sequestration in saline aquifers is a promising way to address climate change. However, the pressure of the sequestration reservoir may decrease in practice, which induces CO₂ exsolution and expansion in the reservoir. In this study, we conducted a core-scale experimental investigation on the depressurization of CO₂-containing sandstone using NMR equipment. Three different series of experiments were designed to investigate the influence of the depressurization rate and the initial CO2 states on the dynamics of different trapping mechanisms. The pressure range of the depressurization was from 10.5 to 4.0 MPa, which covered the supercritical and gaseous states of the CO₂ (named as CO₂(sc) and CO₂(g), respectively). It was found that when the aqueous phase saturated initially, the exsolution behavior strongly depended on the depressurization rate. When the CO₂ and aqueous phase coexisting initially, the expansion of the CO₂(sc/g) contributed to the incremental CO₂ saturation in the core only when the CO₂ occurred as residually trapped. It indicates that the reservoir depressurization has the possibility to convert the solubility trapping to the residual trapping phase, and/or convert the residual trapping to mobile CO₂.