Effect of Wettability on Oil Recovery by Near-Miscible Gas Injection

Summary Oil can become bypassed during gas injection as a result of gravitational, viscous, and heterogeneity effects. Mass transfer from the bypassed region to the flowing gas is dependent upon pressure-driven, gravity-driven, and capillary-driven crossflows as well as diffusion and dispersion. The focus of this study is on the influence that wettability has on bypassing and mass transfer. Experimental results reveal comparatively less bypassing occurs in a strongly oil-wet sandstone than in a water-wet sandstone for gravity-dominated, secondary gas floods. Mass transfer under oil-wet conditions is enhanced, as a result of oil-wetting film connectivity, over that of water-wet conditions, where water shielding is significant. Introduction As gas flooding becomes a more viable means of enhanced oil recovery, the ability to quantify and simulate bypassing and mass transfer becomes increasingly important. Bypassing in gas injection processes may occur as a result of gravity override, viscous fingering, or heterogeneities in the reservoir, such as low permeability layers or a fracture-matrix network. Mass-transfer mechanisms, such as pressure-driven, gravity-driven, capillary-driven, and diffusion/dispersion crossflows are studied on the laboratory scale before being scaled up for incorporation into reservoir simulations. The laboratory studies reveal influences that govern the extent that each mechanism contributes to overall mass transfer. The enrichment of the injected gas has been discovered, through simulation and experiment, to play a key role in overall gas flood performance.1–6 Pande2 proposed, using 1D numerical simulation, that secondary and tertiary hydrocarbon gas floods, at or below minimum miscibility pressure or enrichment (MMP or MME), may perform as well as enriched gas floods. Shyeh-Yung1 demonstrated that tertiary gasflood recoveries below MMP do not decrease as severely as predicted by slim-tube tests for CO2 and Shyeh-Yung and Stadler5 and Grigg et al.7 showed that gasflood Sorm increases almost linearly as hydrocarbon gas enrichment decreases. The injection methodology has been shown to affect ultimate oil recovery.7,8 The experiments of Jackson et al.7 demonstrated that the optimum miscible WAG ratio in a water-wet bead pack under tertiary conditions was 0:1 (continuous gas injection) and 1:1 for a miscible flood in an oil-wet bead pack. Laboratory studies have also revealed the influence of mass-transfer zone orientation and water saturation on gasflood oil recovery.9–11 Burger et al.9,10 have found that mass transfer increases with solvent enrichment and that horizontal mass transfer provides the most efficient oil recovery as a result of gravity-driven crossflow. The inverted, or positive gravity orientation, exhibits countercurrent gravity-driven crossflow that inhibits mass transfer somewhat. The vertical, or negative gravity orientation, yielded the lowest recovery, as diffusion was the only significant mass-transfer mechanism for their particular fluid system. Wylie and Mohanty11 have investigated the effect of water saturation on bypassing and mass transfer, concluding that mass transfer is decreased in the presence of water, but that capillary forces become more dominant as enrichment decreases. Less bypassing, due to gravity override, was observed in horizontal gasflood experiments in the presence of water; however, it was conjectured that bypassing was still present as a result of fluid redistribution and water shielding. With the exception of Jackson et al.7 these studies were performed under strongly water-wet or at restored mixed-wet conditions. The extent that media wettability influences gasflood bypassing and the subsequent mass transfer is largely unexplored. Recent research has examined wettability alteration and its influence on waterflood oil recoveries.12,13 Buckley et al.12 concluded that high pH, low ionic strength, monovalent salt solutions typically induce more water-wet conditions on silica surfaces or cores, aged with asphaltic crude oils, while lower pH solutions led to less water wettability. Their results showed optimum waterflood oil recoveries from Clashach cores under mixed-wet conditions with a slightly positive Amott index. Tang and Morrow13 investigated the influences that temperature, salinity, and oil composition have on wettability and waterflood oil recovery from cores aged in crude oil. They discovered wettability to shift toward more water-wet conditions and waterflood oil recovery to increase with a decrease in the salinity of the connate or invading brine. Waterflood oil recoveries also increased as the displacement temperature increased. Basu and Sharma14 provided evidence suggesting that mixed wettability results from the capillarity-induced, destabilization of brine films on the rock surface.