Mechanisms of Heavy Oil Recovery by Low Rate Waterflooding

Abstract At the conclusion of primary heavy oil production, significant volumes of oil still remain in the reservoir under depleted reservoir pressure. Waterfloods are often consideredfor additional oil recovery. It is accepted that conventional oil waterflooding theory is not applicable for heavy oil. However, there is a lack of understanding of how waterfloods should perform in these reservoirs, particularly after water breakthrough. In this study, waterfloods were performed at multiple rates in cores containing heavy oil and connate water. In some cores oil was initially free of solution gas, and waterfloods were a primary recovery process. In other cores, waterfloods were performed after primary production. Experiments were performed in linear systems for a high viscosity oil (11,500 mPa?s), at different injection rates. The influence of viscous and capillary forces is studied in primary vs. secondary recovery systems. A common misconception is that capillary forces are negligible in heavy oil; however this work shows that these forces are significant, and that water imbibition after water breakthrough can lead to improved oil recovery in both primary and secondary waterfloods. Introduction The Canadian deposits of heavy oil and bitumen are some of the largest in the world. Recent estimates by the AEUB1 suggest that this resource could exceed 270 billion m3 in Alberta alone, with a significant portion of this oil located in reservoirs where expensive thermal operations will not be economic for recovering the oil. Heavy oil is a special class of this unconventional oil, which has viscosity ranging from 50 – 50,000 mPa?s (cP), and low API gravity. Heavy oil reservoirs are often found in high porosity, high permeability, unconsolidated sand deposits. Permeability of the sand averages in the range of 3 D2, but oil does not flow easily due to its high viscosity. At the initial reservoir temperature andressure, the oil may contain dissolved solution gas, thus a fraction of the oil can be recovered using the energy from heavy oil solution gas drive. Primary production can recover around 5% of the oil in place1, leaving significant oil volumes in the reservoir for potential secondary recovery. Waterflooding is a common technique for secondary oil recovery in conventional oil reservoirs. In heavy oil systems, the extremely high oil viscosities lead to adverse mobility ratio conditions, thus water will tend to "finger" through the oil, and recoveries are expected to be extremely low3,4. Despite the poor recoveries predicted theoretically, there have been numerous reports of heavy oil waterfloods performed in the literature5-8. All of these studies reported poor sweep efficiencies and overall recovery. However it is significant that in all cases some oil was recovered despite the highly adverse mobility ratios in the waterfloods. Laboratory studies of waterflooding in heavy oil systems also demonstrate that some oil can be recovered by controlled injection of water. Although heavy oil waterflood responses cannot be readily predicted through theory5, waterfloods have still been carried out in heavy oil reservoirs in Alberta and Saskatchewan for the past 50 years8,9.

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