Evaluation of CO2 Low Salinity Water-Alternating-Gas for enhanced oil recovery

Low Salinity Waterflooding (LSW) is an emerging attractive enhanced oil recovery (EOR) method because of its oil recovery performance and relatively simple, environmentally friendly implementation, when compared with conventional high salinity waterflooding and EOR approaches. More importantly, another advantage of LSW is that it can be integrated with other EOR methods (in hybrid LSW processes), i.e. chemical or miscible gas flooding. The merits of combining LSW with CO2 injection is investigated in this paper, and a novel EOR method, Low Salinity Water Alternating CO2 (CO2 LSWAG), is proposed. CO2 LSWAG injection promotes the synergy of the mechanisms underlying these methods which further enhances oil recovery and overcomes the late production problems frequently encountered in conventional WAG. CO2 LSWAG has been evaluated in both one-dimensional and full-field scale with positive results compared with conventional high salinity WAG. To investigate the advantages of CO2 LSWAG, a comprehensive ion exchange model associated with geochemical processes has been implemented and coupled to the multi-phase multi-component flow equations in an equation-of-state compositional simulator. 1D simulation of different CO2 LSWAG schemes are first conducted to understand the combined effects of solubility of CO2 in brine, dissolution of carbonate minerals, ion exchange, and wettability alteration. CO2 LSWAG performance is then evaluated on a field scale through an integrated workflow that includes geological modeling, multi-phase multi component reservoir flow modeling and process optimization. The simulation results indicate that CO2 LSWAG has the highest oil recovery compared to conventional high salinity waterflood, high salinity WAG, continuous CO2 flooding, and low salinity waterflood. A number of geological realizations are generated to assess the geological uncertainty effect, in particular clay distribution uncertainties, on CO2 LSWAG efficiency. CO2 LSWAG injection strategies are optimized by identifying key WAG parameters. Finally, CO2 LSWAG is evaluated in the full field scale for a North Sea reservoir and the simulation results shows that CO2 LSWAG yields about 4.5% incremental OOIP compared to the conventional high salinity WAG. The proposed workflow demonstrates the synergy between CO2 WAG and LSW. Built in a robust reservoir simulator, it serves as a powerful tool for screening, design, optimization, and uncertainty assessment of the process. CO2 LSWAG is a promising EOR technique as it not only combines the benefits of CO2 injection and low salinity water floods but also promotes the synergy between these processes through the interactions between geochemical reactions associated with CO2 injection, ion exchange process, and wettability alteration. This paper demonstrates the merits of this process through modeling, optimization and uncertainty assessment. (C) 2016 Elsevier B.V. All rights reserved.