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

Poor sweep efficiency seriously limits field applications of CO2-EOR, especially in fractured reservoirs. The investigation of CO2 production performance and the method to control CO2 production becomes a key to successfully run a CO2-EOR project. In this study, artificial sandstone cores were used to perform a series of CO2 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 CO2 production performance. Two-stage gas channeling control with three different scenarios, including PLS gel and ethylenediamine, starch gel and ethylenediamine, and starch gel and CO2 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 CO2 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 CO2 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 CO2 within high permeability layer could successfully tune injected CO2 to flood low permeability layer. This study could help field engineers determine the optimal timing of CO2 production control, and also offered an optimal method to improve the conformance for CO2-EOR.