Low salinity water alternate gas injection process for Alaskan viscous oil EOR

Abstract

Carbon dioxide has excellent oil swelling and viscosity reducing characteristics. CO₂ injection alternated with water has shown substantial incremental recovery over waterflood for the Alaska North Slope (ANS) viscous oil reservoirs. However, for any project, the ultimate CO₂ slug size is finite and once the apportioned solvent volume is used up, the reservoir oil rates gradually revert to the low waterflood rates during the later life of a field. Low salinity waterflooding (LSWF) has also shown some promise based on corefloods and single well tracer tests in North Slope light oil reservoirs. However, two challenges impede its implementation as a standalone enhanced oil recovery (EOR) option on the North Slope: 1) slow response; the delay prolonged with increasing oil viscosity and 2) large upfront investments for the processing and transport of source water. This study proposes a hybrid EOR scheme, the low salinity water alternate gas (LSWAG) process, for the viscous fields of the ANS. The process was modeled by coupling geochemical and ion exchange reactions to a CO₂-WAG type pattern model of the Schrader Bluff O sand. The Schrader Bluff reservoir has been classified suitable for low salinity EOR based on its permeability, temperature, clay content, and oil and formation water properties. Oil recovery through wettability alteration was modeled through ion exchange at the clay sites. Multiphase compositional flow simulation was run using numerical dispersion control. LSWAG forecast for 50 years following 36 years of high salinity waterflood recovered 15% OOIP more oil over high salinity waterflood and 4% incremental over high salinity WAG. This translates to an improvement of 58% and 11% over waterflood and conventional WAG respectively. Higher oil rates were observed during later life due to increased oil relative permeability caused by the low salinity mechanism. Furthermore, very low solvent utilization values were seen for LSWAG which can be tied to the higher ultimate oil recovery potential of using low salinity water over conventional waterflood. In summary, LSWAG outperformed LSWF and conventional WAG by synthesizing the oil swelling and viscosity reduction advantages of CO₂ with lower residual oil benefits of LSWF, while overcoming the challenges of the late response of LSWF and low waterflood oil rates during later life in a conventional WAG flood.