Experimental and Theoretical Investigation of Water/Gas Relative Permeability Hysteresis: Applicable to Water Alternating Gas (WAG) Injection and Gas Storage Processes

Abstract

AbstractAccurate determination of relative permeability hysteresis is needed for reliable prediction of WAG injection. We report two series of gas/water kr hysteresis curves obtained from corefloods under mixed-wet conditions. The first set began by water injection (imbibition: I) in the core saturated with hydrocarbon gas and immobile water. Then, the injection of gas (Drainage: D) and water continued sequentially and in total, three imbibitions and two drainages were carried out (IDIDI). In the second series, the core was initially 100% saturated with water and the experiment started with drainage (gas injection) followed by successive imbibitions (water) and drainages (DIDIDI) periods. The measured pressure drop and production data were history matched to obtain krg and krw values for each imbibition and drainage. The results show cycle-dependent hysteresis for both krg and krw curves. Therefore, the current assumption in existing hysteresis models that the drainage scanning curves follow the preceding imbibition curve is not supported by our experiments. Historic behaviour of both krg and krw is qualitatively different for these two series of experiments. This shows that unlike water-wet systems, relative permeability historic behaviour in mixed-wet system can be a function of injection scenario (saturation history). In the IDIDI series, both krg and krw decreased as the alternation between imbibition and drainage injection continued. In the DIDID series, no significant hysteresis was observed for krw, but krg in drainage cycles were higher than the corresponding values in preceding imbibition cycles. The results reveal that, none of the widely used hysteresis models (e.g., Carlson, Killough) is able to predict the observed cyclic kr hysteresis for alternating injection of gas and water. The results suggest that for mixed-wet systems it is necessary to consider irreversible hysteresis loops for both the wetting and nonwetting phases. In addition to WAG injection, the results presented in this paper and the conclusions drawn also have applications in underground hydrocarbon gas storage which usually involves cyclic pressurization (drainage) and depressurization (imbibition) on annual basis.