Three-phase relative permeability and hysteresis effect during WAG process in mixed wet and low IFT systems

Abstract

There is significant uncertainty associated with the selection of three-phase relative permeability in particular for the numerical simulations of water-alternating-gas (WAG) injection. Generally, three-phase relative permeabilities are calculated from empirical correlations, which are based on two-phase relative permeabilities. Water alternating gas injection involves drainage and imbibition processes taking place sequentially. Therefore, accurate prediction of the relative permeability ( k r) functions and their hysteresis effects applicable to WAG are extremely complex. The problem of 3-phase k r selection becomes even more difficult for three-phase systems involving mixed-wet rocks and low gas–oil IFT (interfacial tension) fluids (nearly-miscible fluids). We use the results of coreflood experiments carried out on two different cores (one water-wet and one mixed-wet) using a low IFT gas–oil system to generate two-phase and three-phase relative permeability data by using an in-house three-phase coreflood simulator. The results show that water and gas three-phase k r values depend on two independent saturations, contrary to the inherent assumption in most of the existing empirical k r models. Three-phase water, oil and gas k r values were all significantly lower than their corresponding two-phase values. This reduction in k r was more profound for the gas where the three-phase k rg was an order of magnitude lower than its corresponding two-phase k rg. Using the laboratory derived three-phase k r data, the performance of some of the existing three-phase k r models was evaluated by comparing the results of the WAG experiments with predictions made by a commercial reservoir simulator. In general, considerable discrepancies between the measured and the predicted k r values were observed. Some models performed better than others but no single model was capable of matching all experimental results adequately.