Miscibility Management for CO2 EOR and Storage at Depleted Oil Reservoirs

Most reservoirs have experienced pressure depletion below original bubble point pressure during primary production, which are then repressurized with waterflood. To maximize the utilization of anthropogenic CO₂ for miscible EOR in these depleted oil reservoirs, we investigated the effect of impurity of anthropogenic CO₂ stream and production history of depleted reservoir on the reservoir fluids’ characteristic, especially with regard to miscibility with the injectants.

The reservoir fluid phase behavior studies were carried out to evaluate the potential of applying miscible CO2 flooding in depleted reservoirs, taking NHK reservoir (in Assam of India) as an example. The rising bubble apparatus (RBA) and slim tube test were employed to determine CO2 MMP. The CO2 MMP with the reservoir fluid were monitored as it produced gas through a differential liberation procedure. The CO2 stream with impurities of methane and ethane was investigated to determine their sensitivity on the CO2 miscibility. PVT simulation study were then carried out to validate the laboratory tests.

In NHK reservoir, the MMP of CO2 with live oil before pressure depletion was determined to be above 4000 psi. After loss of dissolved gas through multiple-step differential liberation procedure, the MMP was dramatically reduced. The MMP measurements on the deferentially liberated oils demonstrate that this process actually lowers the MMP and hence is beneficial to the miscible gas flood. Moreover, CO2 with 5% lean gas contaminants (4.5% C1+0.5% C2) only slightly altered the MMP. When C1+C2 content increased to 10%, the CO2 MMP increased significantly. This indicates a negative effect exists when either CO2 is mixed with trapped lean gas or the anthropogenic gas purity is low. In all, this study provides a strategy for optimizing the MMP of a CO2-crude oil mixture in a depleted reservoir so that CO2 can be utilized more effectively in a miscible displacement process.