Abstract
Enhanced oil recovery schemes that control gas mobility (including WAG and SWAG) are often considered among viable options for oil recovery under challenging flow conditions of carbonate reservoirs. Miscible gas injection would be beneficial to oil recovery, however, in some cases, the reservoir pressure and availability of injectants are among the limiting factors. In this paper, we present the results of a series of immiscible and miscible gas injection coreflood experiments performed using a limestone reservoir core under different injection strategies. The experiments were performed under reservoir conditions using live crude oil. The core wettability was restored by ageing the core in crude oil for several weeks under reservoir temperature. Hydrocarbon gas was used as the immiscible injectant, and CO2 was used in the miscible experiments. To study the impact of the order of fluid injection, multiple WAG injection cycles were performed in separate experiments after water or gas injection. To investigate the impact of gravity on gas injection and WAG after gas injection on core scale, we have performed these experiment in both vertical and horizontal orientation. SWAG injection experiment with the same ratio of gas and water was also investigated. While saturation history affected the amount of trapped gas in the immiscible (high IFT) WAG injection experiments, only small amounts of gas were trapped regardless of injection direction. Regardless of the amount of trapped gas, no additional oil was recovered during water injection periods. Therefore, no meaningful linear relationship was observed between three-phase residual oil saturation and trapped gas saturation. Under immiscible conditions, gravity-stable gas injection recovered more than 45% of IOIP. In comparison, waterflooding produced 61% of IOIP. WAG injection after waterflooding only managed to produce 6% additional oil. Subsequent WAG cycles after first gas injection could only reduce the residual oil saturation to that observed for waterflooding (Sorw). SWAG injection improved the gas sweep efficiency considerably, however, the recovery factor at the end of the experiment was lower than waterflooding. Miscibility significantly improved the performance of WAG injection. The first miscible gas slug injected after waterflood, led to the recovery of about 70% of Sorw. Similar to WAG under immiscible conditions, trapped gas did not contribute to additional oil recovery in water injection periods thereafter. The observed difference in residual oil saturation versus trapped gas saturation relationship in carbonate rocks with those of sandstone reservoirs regardless of miscibility emphasises the importance of developing relevant three phase models specific to oil-wet carbonate reservoirs. The results provide rarely available experimental data that can help improve the available models for simulating and predicting the performance of WAG injection in oil-wet carbonates.
