Experimental Investigation on the Behavior of Supercritical CO_2 during Reservoir Depressurization

摘要

CO_2 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 CO_2 exsolution and expansion in the reservoir. In this study, we conducted a core-scale experimental investigation on the depressurization of CO_2-containing sandstone using NMR equipment. Three different series of experiments were designed to investigate the influence of the depressurization rate and the initial CO_2 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 CO_2 (named as CO_2(sc) and CO_2(g)> respectively). It was found that when the aqueous phase saturated initially, the exsolution behavior strongly depended on the depressurization rate. When the CO_2 and aqueous phase coexisting initially, the expansion of the CO_2(sc/g) contributed to the incremental CO_2 saturation in the core only when the CO_2 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 CO_2.