Hybrid CO2 EOR using polymer-assisted carbonated low salinity waterflood to improve CO2 deliverability and mobility

Abstract

A hybrid polymer-assisted carbonated low-salinity waterflood (PCLSWF) is a low-salinity waterflood-based enhanced oil recovery (EOR) co-injecting CO2 and polymer. In the hybrid process, the low salinity of make-up brine enhances CO2 deliverability into a reservoir due to the salting-out phenomenon. This PCLSWF introduces the following synergetic effects: 1) wettability modification; 2) decreasing oil viscosity; 3) increasing brine viscosity. These effects are evaluated in core- and pilot-scaled systems. First, the PCLSWF induces ion-exchange of Ca2+ and improves wettability following MIE (multi-components ionic exchange) theory. The wettability modification effect is enhanced in the pilot-scaled system because the low pH of the PLCSWF dissolves more cemented calcite mineral producing Ca2+ and resulting in more ion-exchange of Ca2+. Secondly, a lot of dissolved CO2 in brine is transferred into oil, consequently decreasing the viscosity and density of oil. Finally, polymer injection increases the viscosity of the displacing fluid. Although it undergoes mechanical and chemical degradation, it still improves mobility ratio and sweep efficiency. In the pilot-scaled system, the PCLSWF enhances oil recovery up to 12%, 6%, and 2% over the low salinity waterflood (LSWF), low salinity polymer flood (LSPF), and carbonated low salinity waterflood (CLSWF) respectively. The optimization process maximizes the efficiency of the PCLSWF, and it is recommended that tertiary PCLSWF is deployed following the secondary CLSWF. The optimized injection design of the PCLSWF increases the net present value up to 18% more than the LSWF.