Improved Heavy Oil Recovery by Low Rate Waterflooding

Abstract

Abstract 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. Introduction The Canadian deposits of heavy oil and bitumen are some of the largest in the world. With a rising worldwide demand for oil, accompanied by steadily declining conventional oil reserves, the Canadian oil sands will help Canada to remain an important energy source in future years. Approximately 82% of this oil resource is located in reservoirs where the oil must be recovered through in-situ methods 1, thus issues of flow of high oil viscosity oil are prominent in all recovery options. Heavy oil is a special subset of the oil sands, which is characterized by viscosity ranging from approximately 50 - 50,000 mPa·s, and low API gravity. This oil often contains some solution gas at initial reservoir conditions, and has some limited mobility at the reservoir temperature and pressure. In Alberta, 2% (5.7 billion m3) of the oil sand resource base is considered to be heavy oil 1. In addition, Saskatchewan contains significant heavy oil deposits, and many of the lower viscosity oil reservoirs classified as bitumen also still have some limited oil mobility. Therefore, the recovery of this heavy oil is an important reservoir engineering problem.