Mechanisms of heavy oil recovery by waterflooding

Waterflooding is a common technology applied in heavy oil reservoirs after primary production, but the actual recovery mechanisms are still poorly understood. Due to the adverse mobility ratio between oil and water, injected water will tend to finger through the oil, leaving large portions of the reservoir unswept. Breakthrough of water will occur early in the life of a waterflood, followed by further oil production at high water cuts. Viscous instability theory can be used to describe the oil recovered up to breakthrough, but the mechanisms leading to oil recovery at later times have been largely unexplored. In this work, a suite of experiments was performed to investigate the relative significance of viscous and capillary forces on heavy oil waterfloods after water breakthrough.;Experiments performed in dead oil systems varied the oil viscosity, water injection and sand permeability. At lower injection rates and in higher permeability sands, optimal oil recovery was obtained. Normalized oil production rates could also be sustained at low water injection rates, and rates became relatively insensitive to the pressure gradient across the core. This showed that viscous forces are not responsible for oil production after water breakthrough. Similar trends were observed in systems where waterflooding was performed at the conclusion of primary production, indicating that the same recovery mechanisms were present in dead oil core floods and in cores after primary production.;The response of the cores to low rate waterflooding indicated that capillary forces are an important contributor to heavy oil recovery. This was further investigated through CT imaging of fast versus slow waterfloods, and through an NMR study of fluids redistribution under static and flowing conditions. It was concluded that heavy oil recovery after breakthrough is mainly due to water imbibition and film thickening in water wet sands. This effect can be manifested in reduced apparent relative permeability to water at low injection rates. An empirical correlation was also developed, which can make predictions of oil recovery for a wide range of conditions in linear, water wet systems.