Oil recovery by water-alternating-gas (WAG) injection under low gas-oil IFT in water-wet and mixed-wet rocks

We report the results of a comprehensive series of coreflood experiments carried out to investigate the physical mechanisms involved in WAG injection and to quantify the amount of additional oil recovery by WAG injection. A series of experiments were first carried out in a sandstone core under its natural water-wet conditions. These included water injection, gas injection and also WAG injection. To investigate the impact of wettability on the performance of the above injection strategies, the wettability of the same core was changed to mixed-wet and a similar set of experiments were carried out and the results were compared with the water-wet results. The same fluid system which represented a very low gas-oil IFT (interfacial tension) system was used in both the water-wet and the mixed-wet experiments. The results show that in both the water-wet and mixed-wet cores, the performance of WAG injection is better than water injection and gas injection alone. It was found that changing the rock wettability from water-wet to mixed-wet significantly improved the performance of water injection. In both wettability systems, the breakthrough (BT) of the gas during gas injection happened sooner than the BT of water in water injection experiments, but the ultimate oil recovery by gas injection was considerably higher than that obtained by water injection in the water-wet system. In the mixed-wet system, gas injection recovered considerably less oil compared to gas injection in the water-wet rock. In the case of mixed-wet system gas injection performance was less than water injection. The results provide new insight and novel experimental data for demonstration of the performance of WAG injection in a system that closely resembles a real reservoirs scenario. The results particularly highlight the impact of wettability as this important parameter is usually not correctly represented in laboratory experiments.