Effects of Mobile Water on Multiple-Contact Miscible Gas Displacements

Miscible gas flooding using an alternate gas/water injection process (AGWIP) is presently being applied for enhanced oil recovery (EOR) in several waterflooded reservoirs. A mobile-water saturation in the vicinity of the miscible displacement front can occur in this process. To design field applications of miscible gas floods process. To design field applications of miscible gas floods properly, it is necessary to understand the effects of properly, it is necessary to understand the effects of water saturations above the connate saturation on the oil-displacement efficiency. Previous research on AGWIP has involved water-wet long-core flow tests using an injected solvent that is first-contact miscible with the inplace oil. Miscible floods employing CO2, enriched gas, methane, and flue gases, however, are rarely first-contact miscible with reservoir oils; the oil miscibility is normally achieved by a multiple-contact mechanism. This paper discusses the effects of mobile water on multiple-contact miscible displacements under water- and oil-wet conditions. Tests were conducted in 8-ft (244-cm) water- and oil-wet Berea cores in which CO2 and water were injected both separately and simultaneously to displace a reservoir oil. The data presented focus on effects of water in the oil-moving zone (OMZ) where the CO2 is generating miscibility with the oil and mobilizing residual oil to waterflooding. Special emphasis is placed on understanding the effect of mobile-water saturation on the oil-displacement efficiency and the component transfer between phases necessary to develop miscibility in the CO2/reservoir-oil system. This study demonstrates that reservoir wettability is a key factor in the performance of AGWIP. Gas/water injection can, under certain conditions, have adverse effects on characteristics of the OMZ. These effects are in part caused by the water trapping portions of the oil and part caused by the water trapping portions of the oil and solvent. It was observed that mobile water did not change the mass transfer process by which miscibility develops in a multiple-contact miscible displacement. Introduction Miscible gas flooding has been and will be used as a commercial EOR process. In most reservoir applications the injected gas has a lower viscosity than the reservoir oil being displaced. This leads to an inherently unfavorable gas/oil mobility ratio. AGWIP has been used to control mobility. To improve sweep of the injected miscible gas, and to utilize this relatively expensive fluid more effectively. In many field applications of this process, volumes of miscible gas and water are injected process, volumes of miscible gas and water are injected alternately into the reservoir until the desired cumulative slug volume of miscible gas has been injected. The AGWIP process may lead to a high mobile-water saturation in the reservoir, particularly in waterflooded reservoirs. Several authors have discussed the effects of this mobile water on the first-contact miscible oil-displacement process. These studies have shown that the in-place oil can be shielded from the injected solvent by the mobile water in water-wet porous media. The ability of the injected solvent to displace residual oil in laboratory systems was detrimentally affected by high mobile-water saturations. In simulated oil-wet porous media, this solvent trapping was either much less severe or nonexistent. Simulated oil-wet conditions were obtained in a water-wet core by displacing a glycerin/water solution by simultaneous injection of water and oil. SPEJ P. 447