One major problem in the numerical simulation of the water-alternating-gas (WAG) process is the uncertainty associated with the changes in three-phase relative permeability (kr) values in the sequence of drainage and imbibition cycles known as cyclic hysteresis. In this work we have investigated the effect of cyclic injection of water and gas on three-phase kr of water, gas and oil under both water-wet and mixed-wet conditions. Two different sandstone cores with one order of magnitude difference in absolute permeability (65 mD and 1000 mD) have been used in the coreflood experiments.
In the base study, WAG injection started with water flooding (I) followed by gas injection (D) and this cyclic injection was repeated three times (IDIDID). To investigate the effect of immobile water saturation, the IDIDID injection scenario was repeated with lower immobile water saturation. To study the effect of saturation history, another WAG experiment was performed which started with the primary gas flooding (D) followed by water injection (I) and this cyclic injection were repeated as DIDIDIDI. For all of the experiments, three-phase relative permeabilities were obtained analytically from the coreflood data using an extension of Buckley-Leveret formula to three-phase flow.
The results show irreversible kr hysteresis loops for water and gas and saturation history dependency for oil in processes involving cyclic injection under three-phase flow conditions. Generally, krg reduced during successive cycles for all WAG experiments. For the IDIDID injection sequence and in water-wet system, it was found that krg shows larger hysteresis effect for the case with higher immobile water saturation. krw hysteresis was larger in the DIDIDIDI injection scenario compared to the IDIDID case. krg and krw hysteresis effects were larger for the 1000 mD core compared to the low permeability (65 mD) sample. The most important hysteresis effect for both krg and krw was observed for the transition from two-phase to three-phase. As WAG cycles were repeated, krg and krw hysteresis reduced became minimal after the 2nd water injection in IDIDID and 2nd gas injection in DIDIDIDI injection sequences.
Comparison of our experimental observations with predication of existing models highlights some serious shortcomings of reservoir simulators in simulation of oil recovery processes involving three-phase flow and flow reversal. The paper also offers some novel insights into cyclic hysteresis behaviour and offers explanations based of our understanding of the pore-scale and core-scale displacement mechanisms involved in three-phase flow and WAG injection.