A laboratory investigation of CO2 influence on solvent-assisted polymer flooding for improving viscous oil recovery on Alaska North Slope

Abstract

In this study, the CO2 influence on enhancing Alaska North Slope (ANS) viscous oil recovery for polymer flooding with assistance of solvents is investigated. Seven coreflooding tests were conducted at reservoir conditions to evaluate ANS viscous oil recovery performance of five solvent-assisted EOR methods including LHS (light hydrocarbon solvent)-alternating-LSW (low salinity water) flooding, CO2-enriched LHS-alternating-LSW flooding, LHS-alternating-LSP (low salinity polymer) flooding, CO2-enriched LHS-alternating-LSP flooding, and CO2-enriched LHS-LSW-LSP flooding. Besides, the CO2 influence on the performance of solvent-assisted EOR methods is investigated. In addition, Constant Composition Expansion (CCE) tests were performed at reservoir temperatue of 85 °F to explore mechanisms of oil recovery improvement with solvent injection in EOR methods of this study. Moreover, water ion analyses of DI-water/natural-sand and DI-water/natural-sand/CO2 systems were carried out to discover the complex reaction between CO2, sand, and LSP solution for better understanding underlying mechanisms of solvent-assisted EOR methods. Coreflooding experiment results demonstrate that five proposed EOR methods all present remarkable production performance with recovery factor ranging from 79.73% to 92.23%. The main EOR mechanisms can be attributed to oil swelling and viscosity reduction effects resulting in the enhancement of microscopic displacement efficiency during solvent flooding and the improvement of macroscopic sweep efficiency of subsequent LSW/LSP flooding resulting from obtaining better mobility control. Considering the recovery factor from the 1st solvent flooding, no obvious improvement was observed with solvent containing CO2. However, higher oil recovery improvement was obtained from the subsequent LSW/LSP flooding of tests using solvent with CO2 than that without CO2, mainly resulting from better mobility control due to the greater viscosity reduction effect by CO2-enriched LHS. Nevertheless, a severe blockage occurred at the production end in the CO2-enriched LHS-alternating-LSP flooding test, which was likely due to the polymer precipitation. CCE test results present that more solvent could be dissolved into the ANS viscous oil by increasing the pressure of the system, which causes greater oil swelling and viscosity reduction effects. The saturation pressure was affected little by adding CO2 in LHS, while the presence of CO2 had a negative impact on oil swelling effect but a positive influence on oil viscosity reduction effect. In addition, water ion analyses results show that, after introducing CO2 to the DI-water/natural-sand system, a great amount of multivalent cations were generated probably due to the dissolution of minerals in the natural sand, which may explain the polymer precipitation. Therefore, the blockage issue at the production end could be solved by injecting LSW as a spacer between CO2-enriched LHS flooding and LSP flooding to reduce the concentration of generated cations.