Experimental and theoretical investigation of oil/gas relative permeabilities hysteresis in the presence of immobile water: under low oil/gas IFT and mixed-wet conditions

Accurate determination of relative permeability values and their hysteresis is crucial for obtaining a reliable prediction of the performance of water-alternating-gas (WAG) injection in oil reservoirs. In this paper we report two series of gas/oil relative permeability curves obtained from coreflood experiments carried out in a mixed-wet core under a very low oil/gas interfacial tension (IFT) of 0.04mN.m-1. The first set of the corefloods began by oil injection (imbibition) in the core saturated with gas and immobile water (Swi). This was followed by a period of gas injection (drainage) and this sequential injection of oil and gas continued and in total, three imbibition and two drainage periods were carried out. In the second series of experiments, the core was initially saturated with oil and immobile water and the experiment started with a gas injection followed by cycles of drainage and imbibitions.

The measured pressure drop and production data were history matched through simulation analysis to obtain krg and kro values for each of the imbibition and drainage cycles. The results show that both the oil and the gas relative permeability curves show cycle-dependent hysteresis despite the very low gas/oil IFT. Therefore, the current assumption in existing models (such as Land, Carlson and Killough) that the drainage scanning kr curves follow the preceding imbibition curve is not supported by our coreflood experiments.

When compared to our measured data, Carlson model predictions for krg in imbibition direction are poor. Killough model predictions underestimate krg and overestimate kro especially near trapped gas saturation regions. Beattie et al. hysteresis model is able to capture the krg and kro behavior that we observed in our experiments qualitatively, but it is still unable to predict the value of the observed hysteresis. The results suggest that for mixed-wet systems, it is necessary to consider irreversible hysteresis loops for both the wetting and non-wetting phases. Such capability currently does not exist in reservoir simulators due to lack of appropriate predictive tools.