Experimental investigation of near-miscible water-alternating-gas (WAG) injection performance in water-wet and mixed-wet systems

Laboratory data on WAG injection for non-water-wet systems are very limited especially for near-miscible (very low IFT) gas-oil systems, which represents injection scenarios involving high-pressure hydrocarbon gas and CO2 injection. Simulation of these processes requires three-phase relative permeability (kr) data. Most of the existing three phase relative permeability correlations have been developed for water-wet conditions. However, majority of oil reservoirs are believed to be mixed-wet and hence, prediction of the performance of WAG injection in these reservoirs is associated with significant uncertainties. Reliable simulation of WAG injection, therefore, requires improved relative permeability and hysteresis models verified with reliable measured data.

In this paper, we, first, report the results of a comprehensive series of coreflood experiments carried out in a core under natural water-wet conditions. These included water injection, gas injection and also WAG injection. Then, to investigate the impact of wettability on the performance of the above injection strategies, the wettability of the same core was changed to mixed-wet (by aging the core in an appropriate crude oil) and a similar set of experiments were performed. WAG experiments under both wettability conditions started with water injection (I) followed by gas injection (D) and this cyclic injection of water and gas was repeated (IDIDID).

The results show that in both the water-wet and mixed-wet cores, the performance of WAG injection is better than water injection and gas injection alone. Changing the rock wettability form water-wet to mixed-wet significantly improve the performance of water injection. Under both wettability conditions (water-wet and mixed-wet), the breakthrough (BT) of the gas during gas injection happens sooner than the BT of water in water injection. Ultimate oil recovery by gas injection is considerably higher than that obtained by water injection in the water-wet system, while in the mixed-wet system gas injection recovered considerably less oil.