Conformance control for CO2-EOR in naturally fractured low permeability oil reservoirs.

Poor sweep efficiency seriously limits field applications of CO 2 -EOR, especially in fractured reservoirs. The investigation of CO 2 production performance and the method to control CO 2 production becomes a key to successfully run a CO 2 -EOR project. In this study, artificial sandstone cores were used to perform a series of CO 2 flooding experiments at reservoir conditions of pressure, temperature and formation water salinity. Injection pressure and rock heterogeneity were taken into account to study their effect on CO 2 production performance. Two-stage gas channeling control with three different scenarios, including PLS gel and ethylenediamine, starch gel and ethylenediamine, and starch gel and CO 2 foam, was presented to improve the conformance in 3-D fractured-core models. Based on production performance and experimental observation, three production stages were clearly stated, including gas-free production stage, oil/gas co-production stage, and gas channeling stage. Oil/gas co-production stage contributed the most to oil recovery. This finding was very consistent with the practical production performance of CO 2 flooding in oil fields. The conformance improvement and the increase in injection pressure (within permissible range bearing in mind of fracturing pressure) could remarkably enhance the oil recovery at oil/gas co-production stage. In 3-D radial flow experiments, the two-stage gas channeling control presented great potential in delaying gas production and improving CO 2 flooding efficiency. The combination of starch gel and ethylenediamine achieved the best result. The starch gel could effectively block the fracture, while the viscous reactant of ethylenediamine and in-situ CO 2 within high permeability layer could successfully tune injected CO 2 to flood low permeability layer. This study could help field engineers determine the optimal timing of CO 2 production control, and also offered an optimal method to improve the conformance for CO 2 -EOR. [ABSTRACT FROM AUTHOR]