Transport of CO₂ in heterogeneous porous media: Spatio-temporal variation of trapping mechanisms

We present the results of core-flooding experiments by assessing the impact of capillary heterogeneity on residual and dissolution CO₂ trapping. Two types of injection tests with different rates are conducted: single-phase (supercritical CO₂) injection and co-injection (equilibrated CO₂ and brine) cases are considered for drainage tests. In case of the single-phase injection, CO₂ builds up at a low capillary pressure (P_c) zone located upstream of a high P_c zone as CO₂ was hindered to transport across the barriers. The experiments were conducted under capillary-dominated condition, and as capillary number (N_c) was increased with greater injection rate, the increased viscous force led higher CO₂ saturation in fine matrix implying that high P_c zone was more sensitive to the injection rate than the low P_c zone. The co-injection, however, leads to greater CO₂ saturation at the high P_c zone. The observed disparity of CO₂ saturation patterns in these two experiments resulted from the induced pressure gradient across the core as well as the greater capillary forces in high P_c zone. The following imbibition test, which was designed to reproduce conditions for the post-injection period, characterized three regimes of spatio-temporal variation of trapping mechanisms as follows: (1) displacement of mobile CO₂ by the injected brine and concurrent CO₂ dissolution into the fresh brine; (2) preservation of immobile CO₂ as a residual trapping; and (3) gradual CO₂ dissolution into the fresh brine.