During gas injection, bypassing of oil is common because of gravitational, viscous, and/or heterogeneity effects. The oil in the bypassed regions can be recovered through enhanced flow and mass transfer between the bypassed region and the injectant gas. Previously, experiments in our laboratory have been carried out to evaluate the effects of phase behavior and capillary crossflow in near-miscible gasfloods; however, these studies were conducted in the absence of water. In this paper, we evaluate the effects of water saturation on oil bypassing and the rate of mass transfer from the bypassed zones. Injectant gases are first-contact miscible (FCM), multicontact miscible (MCM), or submiscible with the bypassed oil. Gasfloods are conducted in different orientations with different levels of water saturation. Mass-transfer experiments are carried out to isolate and investigate mass-transfer mechanisms. Results indicate that oil recovery from vertical, submiscible gasfloods is not influenced by water-saturation level. Horizontal gasfloods showed evidence of less gravity override in the presence of water. The mass-transfer experiments showed that recovery increases with enrichment and is reduced by the presence of water. Effective diffusion coefficients are estimated as functions of water saturation and enrichment.
Near-miscible gasflood refers to injection of gases that do not quite develop complete miscibility with the oil, but come close. For example, condensing/vaporizing gasdrives and gasfloods at enrichments slightly below minimum miscibility enrichment (MME) or at pressures slightly below minimum miscibility pressure (MMP) are near miscible.Miscible and near-miscible gasfloods are being conducted or considered in many oil reservoirs. Miscible gas injections are also being considered for many fractured oil reservoirs. Bypassing of large quantities of oil can occur during gas injections because of formation heterogeneity, gravity override, and viscous fingering. A significant fraction of this oil can be recovered by subsequent mass transfer from the bypassed regions. It is important to identify factors that affect bypassing and mass transfer and develop processes that minimize bypassing and maximize the subsequent mass transfer.