Relative permeability characterization for water-alternating-gas (WAG) injection in oil reservoirs

Large quantities of oil usually remain in oil reservoirs after conventional water floods. A significant part of this remaining oil can still be economically recovered by Water- Alternating-Gas (WAG) injection. WAG injection involves drainage and imbibition processes taking place sequentially, hence the numerical simulation of the WAG process requires reliable knowledge of three-phase relative permeability (kr) accounting for cyclic hysteresis effects. In this study, the results of a series of unsteady-state two-phase displacements and WAG coreflood experiments were employed to investigate the behaviour of three-phase kr and hysteresis effects in the WAG process. The experiments were carried out on two different cores with different characteristics and wettability conditions. An in-house coreflood simulator was developed in order to obtain three-phase relative permeability values directly from unsteady-state WAG experiments by history matching the measured recovery and differential pressure profiles. The results show that three-phase gas relative permeability is reduced in consecutive gas injection cycles and consequently the gas mobility and injectivity drops significantly with successive gas injections during the WAG process, under different rock conditions. The trend of hysteresis in krg partly contradicts the existing hysteresis models available in the literature. The three-phase water relative permeability (krw) of the water-wet core does not exhibit considerable hysteresis effect during different water injections, whereas the mixed-wet core shows slight cyclic hysteresis. This may indicate a slight increase of the water injectivity in the subsequent water injections in the WAG process under mixed-wet conditions. Insignificant hysteresis is observed in the oil relative permeability (kro) during different gas injection cycles, for both water-wet and mixed-wet rocks. However, a considerable cyclic hysteresis effect in kro is observed during water injection cycles of WAG, which is attributed to the reduction of the residual oil saturation during successive water injections. kro of the water-wet core exhibits much more cyclic hysteresis effect than that of the mixed wet core. No models currently exist in reservoir simulators that can capture the observed cyclic hysteresis effect in oil relative permeability for the WAG process. Investigation of relative permeability data obtained from these displacement tests at different rock conditions revealed that there is a significant discrepancy between two-phase and three-phase relative permeability of all fluids. This highlights that not only the three-phase relative permeability of the intermiate phase (oil), the three-phase kr of the wetting phase (water) and non-wetting phase (gas) are also functions of two independent saturations.