Water- Alternating- Gas (WAG) injection in waterflooded reservoirs can increase oil recovery and extend the life of these reservoirs. Reliable reservoir simulations are needed to predict the performance of WAG injection before field implementation. This requires accurate sets of relative permeability (kr) and capillary pressure (Pc) functions for each fluid phase, in a three-phase flow regime. The WAG process also involves another major complication, hysteresis, which is caused by flow reversal happening during WAG injection. Hysteresis is one of the most phenomena manipulating the performance of WAG injection and hence, it has to be carefully accounted for.

In this study we have benefited from the results of a series of coreflood experiments that we have been running since 1997 as a part of the Characterization of Three-Phase Flow and WAG Injection JIP (joint industry project) at Heriot-Watt University. In particular we focus on a WAG experiment carried out on a water wet core to obtain three-phase relative permeability values for oil, water and gas. The relative permeabilities exhibit significant and irreversible hysteresis for, oil, water and gas. The observed hysteresis, which is due to the cyclic injection of water and gas during the experiment, is not predicted by the existing hysteresis models.

We present a new three-phase relative permeability model coupled with hysteresis effect for modelling of the observed cycle- dependent relative permeabilities taking place during WAG injection. The approach was successfully tested and verified using the measured three-phase relative permeability values obtained from WAG experiment. In line with our laboratory observations, the new model predicts the reduction of the gas mobility during consecutive water and gas injection cycles as well as the increase in oil relative permeability happening in consecutive water injection cycles.

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