Improved Heavy Oil Recovery by Low Rate Waterflooding
Mai, A. and Kantzas, A.
SPE/PS/CHOA 117648, presented at the 2008 SPE International thermal Operations and Heavy Oil Symposium held in Calgary, Alberta, Canada, October 20-23, 2008.
This work details the results of ambient temperature waterfloods in unconsolidated sand containing heavy oil (11°API and viscosity of 11,500 mPa·s at 23°C). Experiments are performed at rates ranging from 0.4 – 0.02 m/day, in sand with permeability of under 1 D to 9 D. By varying both the injection rate and the permeability of the sand, the relative influence of viscous and capillary forces can be determined. It is shown that by properly controlling the waterflood, significant heavy oil can still be recovered after water breakthrough has already occurred.
The application of this work is for the significant heavy oil resource in Canada that exists in reservoirs that are too thin for thermal operations. A fraction of oil may initially be recovered through primary production, however at its conclusion a significant amount of oil still remains in the reservoir. Waterflooding is a simple process that has potential for recovery of additional oil, in reservoirs where more expensive options will not be possible.
Due to the adverse mobility ratio between water and oil, water breakthrough occurs early in a waterflood, with a significant amount of the oil still being continuous at this time. The key to understanding the displacement of heavy oil by water is to consider the oil recovery after breakthrough. This oil is produced at high water cuts, and under negligible pressure gradients. Oil is therefore recovered by water imbibition into the water-wet sand. By varying the injection rates and the permeability of the sand, the importance of these capillary forces to oil recovery has been quantified.
Capillary forces are generally deemed to be insignificant in heavy oil waterfloods due to the high oil viscosity. As such, concepts of viscous fingering and mobility ratio dominate discussions of heavy oil waterflood responses. In this work, it is shown that not only are capillary forces actually still important in heavy oil reservoirs, but in fact they are a significant mechanism responsible for oil recovery after water breakthrough. With this understood, heavy oil waterfloods can be properly designed to maximize flood efficiency and oil recovery.