Primary recovery remains as low as 5-10 % of original oil in place (OOIP) in tight oil reservoirs, even with horizontal wells and massively hydraulical fracturing applied. Water flood helps to maintain the pressure and CO2 contributes to oil swelling, viscosity reduction and wettability alteration; in addition, CO2 and water have a chance to improve oil recovery. Furthermore, a water alternating gas (WAG) process gives a higher oil recovery compared to continuous water or gas injection.
The WAG performance can be improved by mobility control, wettability alteration and interfaical tension management. Chemical additives like polymer or foam can help to improve mobility, but they are limited to large porous media. The common pore diameter is approximately 30 nm to 2,000 nm in tight sandstone reservoirs and 2nm to 50nm in shale reservoirs. Alkaline can cause a reduction in interfacial tension. However, a candidate for alkaline flood should have an acid number above 0.5 mg OH- /g oil, corresponding to oil with API below 30. The surfactant particles with a diameter of around 10nm to 30nm can reduce interfacial tension while nanoparticles with a diameter of 1nm to 7 nm can affect disjoining pressure at interface and alter wettability; both of them can be candidate additives in improving WAG performance. Moreover, low salinity water exchanges ions in a reservoir, resulting in water film instability and wettability alteration. It can be an alternative solution in improving WAG performance.
In this paper, an analytical model of the WAG process is studied. Afterwards, numerical reservoir simulations are made for surfactant, low salinity water and nanofluid additives in improving WAG performance.