The mechanism of oil recovery by water-alternating-gas injection at near-miscible conditions in mixed wet systems

The modelling of WAG processes at the pore scale in the "near-miscible" regime is still not fully understood, where "near-miscible" implies at low gas/oil interfacial tension (IFT, s _go) near the transition from immiscible to miscible conditions. Micromodel experiments under near-miscible conditions have been performed previously at Heriot-Watt U. (HWU) and results from these show clear differences from the immiscible flooding cycles. In particular, there is significant oil production through "thick" films after the breakthrough of the gas finger and, in repeated gas floods, the gas finger tends to re-establish rather than to redistribute the phases as in the immiscible floods. Here, we address some of the major issues in modelling near-miscible WAG and a new mechanism is proposed based on the interfacial physics of the process. At near-miscible conditions, mass transfer between phases occurs and the oil and gas hydrocarbon phases approach each other in properties, which leads to both the swelling and extraction of oil. The importance of both viscous and gravity forces may increase and it is also thought that water blocking (shielding) leads to bypassing of oil, indicating that gas-water and oil-water capillary forces remain important. The flow of oil through "thick" films and layers becomes more important, possibly as a result of a "wetting transition" or gas-oil contact angle change. To explain these processes, a consistent model is proposed for the IFT and contact angles (which must also change) as the three-phase system goes from immiscible to near-miscible conditions. A linear model is assumed for the variation of the gas-water and gas-oil IFTs, s _gw and s _ow, as functions of the gas-oil IFT, s _go, consistent with measurements. Along with some further linear assumptions on the solid-fluid IFTs, expressions are presented for the varying (cosines of) gas-water and oil-water contact angles, cos?_gw and cos ?_ow. Surprisingly, cos?_go is predicted to be constant down to fully miscible conditions (s _go = 0). Indeed, accurate measurements of the two-phase cos?_go, confirm this trend, but only down to a finite value of s _go, below which the values of cos? _go rapidly increases to 1 (? _go ? 0). This behaviour has been incorporated in our model. The consequences of the various IFT and contact angle models are then worked through, using our previously developed theory of film and layer formation for three-phase configurations in angular pores. We demonstrate how the formation of these thick conducting layers is affected as the system goes from immiscible to miscible conditions. By incorporating the more realistic behaviour with cos? _go approaching 1 as the system goes miscible, much thicker and more conductive oil layers are predicted, very like those observed in the HWU micromodel experiments. This may not be the only explanation for the changes in oil recovery and WAG flooding behaviour in near-miscible systems, but we believe that it is an important and novel component of the mechanism. Additionally, it is shown that oil wetting films or spreading layers are always more likely to occur and increase in thickness when the pores change from strongly water-wet to weakly water-wet to oil-wet.