Carbon Dioxide Injection Into Bottom-Water, Undersaturated Viscous Oil Reservoirs
Authors: Stright D.H. , Aziz K. 
Published: December 2007

This paper describes an improved oil-recovery process that uses carbon dioxide in viscous, undersaturated oil reservoirs where the oil zone is underlain by bottom water. The results of a single-well pilot test are presented and an analysis of the potential of the process based on presented and an analysis of the potential of the process based on mathematical model studies is discussed.

Introduction
Improved oil recovery recently has received much attention. However, the new recovery methods being proposed are generally less widely applicable than proposed are generally less widely applicable than the standard recovery technique of waterflooding. For most reservoirs, the recovery scheme must be "tailor-made" to optimize recovery. This is especially true of the Lower Cretaceous sandstone reservoirs of southeastern Alberta. These consolidated sandstone reservoirs are characterized by (1) high permeabilities (500 to 15,000 md), (2) high porosities (20 to 30 percent), (3) medium oil viscosities (3 to 100 cp), (4) low saturation pressure (50 to 800 psi), (5) low solution gas-oil ratios (30 to 250 scf/STB), and (6) bottom water and varying degrees of natural water influx. The presence of bottom water in conjunction with high-viscosity oil results in relatively low oil recoveries since recovery in these reservoirs is controlled by water coning and is inversely proportional to oil viscosity (Fig. 1). This situation precludes the use of most currently available improved oil-recovery schemes. It has been shown that for a given oil viscosity, recovery under water-coning conditions can be improved by increasing total fluid rate to extend economic limits; however, for viscous oil reservoirs, more dramatic recovery increases should be possible by decreasing oil viscosity to increase oil rate. Applying heat to the reservoir is one of the most widely used methods for reducing viscosity. Steam injection has been successful in reservoirs with associated water zones where the water volume is small relative to the oil volume. An alternative to thermal methods is the reduction of oil viscosity by chemical means. By contacting he undersaturated oil with a suitable gas, viscosity reduction of the same order of magnitude as possible with thermal methods can be achieved. Suitable gases include CO2, methane, ethane, or propane, either as pure components or as mixtures.

CO2 Recovery Schemes
Most of the current CO2 recovery schemes are contended to be miscible. (A review of miscible CO2 flooding was recently presented by Holm.) Miscible CO2 displacement is preferred to immiscible CO2 displacement because the miscible process can potentially displace 100 percent of the oil in place. However, for undersaturated crude oil reservoirs, the benefits if immiscible flooding can be significant because of high CO2 solubility in the oil. The benefits of oil carbonation have been known for some time. There are seven mechanisms in the CO2 displacement process believed to contribute to improved recovery:oil-viscosity reduction,oil swelling,energy supplied to the reservoir,vaporization of crude oil,blowdown recovery,stimulation effects, andinterfacial-tension effects. With the exception of interfacial-tension effects, these mechanisms all have been discussed extensively in the literature.
JPT
p. 1248