Mechanistic Study of Enhanced Oil Recovery by Gas, WAG and SWAG Injections in Mixed-Wet Rocks: Effect of Gas/Oil IFT

We report the results of a comprehensive series of coreflood experiments carried out at three different levels of gas/oil IFT namely, ultra-low, intermediate, and high gas/oil IFT values of 0.04, 0.15, and 2.70 mN m⁻¹ in mixed-wet rocks. Coreflood experiments included waterflooding (WF), gas injection (GI) and two WAG injection scenarios at each IFT value in the first series of WAG experiments, fluid injection started with water injection (I) followed by gas injection (D), and this cyclic injection of water and gas was repeated in four cycles (WAG-IDIDIDID). In the second series of WAG experiments, the test started with gas injection (D) followed by water injection (I), and this cyclic injection of water and gas was repeated four times (WAG-DIDIDIDI). In addition to these experiments, for the high and ultra-low gas/oil IFT systems, SWAG injection experiments have also been performed with SWAG ratio of unity (Q_g/Q_w = 1).

The results showed that the performance of GI was higher in the case of lower IFT condition compared to high-IFT system. The beneficial effect of gas/oil IFT was more pronounced in high permeable mixed-wet rock than it was in low permeable mixed-wet system. For all IFT values tested, WF performance was better than GI under mixed-wet condition. The results also showed that under mixed-wet conditions, for the three gas/oil IFT levels tested, WAG injections outperformed WF and GI. For the ultra-low IFT condition, oil recovery by the WAG-IDIDIDID experiment was higher than that of the WAG-DIDIDIDI experiment. However, at the other two IFT values, WAG-DIDIDIDI outperformed WAG-IDIDIDID injection scenario. For WAG-IDIDIDID, the lower the gas/oil IFT the higher the ultimate oil recovery; conversely, for the WAG-DIDIDIDI injection scenario, oil recovery performance was better for the high IFT condition rather than the ultra-low IFT case. Our results show considerably higher injectivities during WF periods of the ultra-low IFT WAG injections compared to high-IF1T WAG injections. In general, injectivity was lower for the WAG-DI injection scenarios compared to the WAG-ID. The effect of gas/oil IFT on oil recovery was more significant under three-phase flow (WAG injections) compared to the two-phase flow (GI). Trapped gas saturations S_gt (for the same S_gi) were found to be higher under higher IFT conditions, and the trend of S_gt vs. S_gi curve was significantly affected by the sequence of fluid injection during WAG injection (DIDIDIDI or IDIDIDID). This is especially true for intermediate and high IFT conditions.

The results show that trapping models such as Land, Carlson, and Jerauld models cannot capture the observed trend of trapped gas saturations accurately, under the conditions of our experiments. This is especially true for the WAG-DIDIDIDI injection scenarios in which, contrary to the WAG-IDIDIDID injections, S_gtw values are not necessarily higher for the case with higher initial gas saturation. This shows the importance of developing new trapping models for non-water-wet systems. In addition, the results show that the reduction coefficient in S_or adjustment formula of the WAG-Hysteresis model (proposed by Larsen and Skauge) is a function of both gas/oil IFT and fluid injection sequence and it also depends on the rock permeability. These further highlight the importance of performing laboratory experiments under representative reservoir and operational conditions.