English abstract
Coinciding with the increase in world demand for fossil fuels, hydrocarbon liquid production from the existing oil fields should be increased in the coming years. Although water flooding (WF) is an efficient secondary oil recovery method for most oil reservoirs in the world, it results in low oil recovery due to the low microscopic sweep efficiency. Hence, Enhanced Oil Recovery (EOR) methods are required to be applied after the primary and secondary flooding phases as considerable amount of oil are still unrecovered. Continuous Gas Injection (CGI) is one of the most effective EOR methods in improvement of oil recovery because of the ability of gases such as CO2 to be dissolved in the oil and increase oil swelling and decrease oil viscosity, which makes the oil lighter and easier to be displaced to producers. However, CO2 flooding often results in low macroscopic sweep efficiency due to its unfavorable mobility ratio. In order to address the drawbacks of WF and CO2 flooding and maximize oil recovery, several CO2-brine combinations have been developed. Simultaneous Water and Gas (SWAG) injection and Carbonáted Water Injection (CWI) are proved by different lab and field experiments to be attractive EOR options in fields under WF process. In this study, the performance of SWAG and CWI scenarios was investigated with different Water-Gas Ratios (WGR) at the same experimental temperature of 140°F and pressure of 500 psi. A total of four coreflood tests with three different immiscible CO2-EOR scenarios were conducted in Berea sandstone cores. The core flooding results are combined with other characteristics tests such as: CO2 solubility in oil and brine, oil swelling factor, oil viscosity reduction, CO2 diffusion coefficient, and gas utilization factor in order to provide comprehensive analysis and to investigate different oil recovery mechanisms behind EOR scenarios. Furthermore, the results of oil recovery factor together with the gas utilization factor were compared to choose the best scenario. Our results indicated that SWAG with WGR of 2.4:1 has the highest oil recovery factor of 36%, whereas the SWAG with WGR of 1.2:1 has the lowest oil recovery factor of 28%. Therefore, it's proven that WGR strongly affects the amount of oil recovered by SWAG process. However, a water bank might be formed ahead of the CO2-water mixture in SWAG process with the higher WGR of 2.4:1. Additionally, CWI scenario results in high oil recovery factor of 34% after WF. It is also proven that oil swelling plays a major role during CO2-based EOR scenarios. Although, CO2 flooding has a high oil recovery of 35%, it consumes high amount of gas. In comparison with CO2 flooding alone, the CO2-brine mixtures have lower gas utilization factor. Therefore, the immiscible SWAG with WGR between 2.4:1 and 3.6:1 is the best scenario during a CO2 based EOR.
