Recovery of heavy oil (herein defined as bitumen and oil with a viscosity of greater than 100 mPa.s) from the extensive tar sand deposits in Alberta, Saskatchewan and other parts of Canada is hampered by its viscosity, which renders it partially or completely immobile under reservoir conditions. For example, the heavy oil in Lloydminster reservoirs has limited mobility, with a viscosity of several thousand mPa.s, whereas the bitumen in the Cold Lake reservoir is almost completely immobile, with a viscosity in the order of 40,000-100,000 mPa.s.
Currently, oil production from viscous deposits which are too deep to be mined from the surface is generally achieved by heating the formation with hot fluids or steam to reduce the viscosity of the heavy oil so that it is mobilized toward production wells. For example, one thermal method, known as "huff and puff", relies on steam injected into a formation through a producer well, which is then temporarily sealed to allow the heat to "soak" and reduce the viscosity of the bitumen in the vicinity of the well. Mobilized bitumen is then produced from the well, along with steam and hot water until production wanes, and the cycle is repeated. Another thermal method, known as steam assisted gravity drainage (SAGD), provides for steam injection and oil production to be carried out through separate wells. The optimal configuration is an injector well which is substantially parallel to, and situated above a producer well, which lies horizontally near the bottom of a formation. Thermal communication between the two wells is established, and as oil is mobilized and produced, a steam chamber or chest develops. Oil at the surface of the enlarging chest is constantly mobilized by contact with steam and drains under the influence of gravity. Under this scheme, production can be carried out continuously, rather than cyclically.
All thermal methods have the limitation that steam and heat are lost to the formation. In reservoirs where the deposits are relatively thin, in the order of 8 meters, loss of heat to overburden and underburden makes thermal recovery particularly uneconomical. Another problem is loss of heat and steam through fractures in the formation, or to underlying aquifers.
Because of the difficulties encountered in attempting to produce tar sands formations with thermal processes, the use of solvents, rather than heat, as a means to mobilize heavy oils has been proposed. Hydrocarbon solvents such as ethane, propane and butane are partially miscible in oil, and when dissolved in oil, reduce its viscosity. A number of references have suggested mixing of solvents to achieve miscibility with heavy petroleum under reservoir conditions.
In a method known as the VAPEX method, hydrocarbon solvents, rather than steam, are used in a process analogous to SAGD, which utilizes paired horizontal wells. An hydrocarbon such as heated propane in vapor form, (or propane in liquid form in conjunction with hot water) is injected into the reservoir through an injector well. Propane vapor condenses on the gas/oil interface, dissolves in the bitumen and decreases its viscosity, causing the bitumen-oil mixture to drain down to the producer well. The propane vapors form a chest, analogous to the steam chest of SAGD.
The pressure and temperature conditions in the reservoir must be such that the propane is primarily in vapor, rather than liquid form so that a vapor chest will develop. Ideally, the conditions in the reservoir should be just below the vapor liquid line. A serious drawback of the VAPEX method is that temperature and pressure conditions in a reservoir are seldom at the dew point of known solvents. Therefore, it is necessary to adjust the pressure and/or temperature in the system to create reservoir conditions under which the particular solvent is effective. However, this is not feasible in all reservoirs. Increasing the pressure could lead to fluid loss into thief zones. Reducing the pressure could cause an influx of water.
A recently described process called "Butex" relies on the use of an inert "carrier gas" such as nitrogen to vaporize a hydrocarbon solvent such as butane or propane in the reservoir.
In order to make the use of hydrocarbon solvents to reduce oil viscosity generally feasible and economical under field conditions, there is a need for solvents which:
are predominantly in the vapor phase at reservoir conditions, and can be used without the need to adjust the pressure or temperature conditions in the reservoir; PA1 have high solubility in reservoir oil at reservoir conditions; and PA1 are readily obtainable at reasonable cost. PA1 1. The mixture should exist partially, preferably predominantly, in the vapor phase at reservoir conditions, in order to fill the chest cavity and minimize solvent inventory, but some liquid is desirable because liquid is more aggressive as a solvent than vapor. PA1 2. The mixture should have a high solubility in the reservoir oil, preferably being capable of dissolving at least 10 weight percent in the reservoir oil at reservoir conditions. PA1 3. The resultant oil/solvent mixture should have a low viscosity, preferably below 100 mPa.s for efficient gravity drainage.