This invention relates to an improved method for using electricity to stimulate production of viscous hydrocarbons from a subsurface formation. More particulary, electrical current is applied in a broaden horizontal plate-like manner to a subsurface hydrocarbon-bearing formation through electrode wells completed in relatively high permeability, large volume aquafers containing mobile water. The aquafers overlay and underlay oil-bearing formation.
For many years, it has been known that large deposits of relatively shallow, viscous oil are present in subterranean formations. Normally, the viscous oil is produced through a vertical production well. The well productivity is nearly inversely proportional to the viscosity of the oil. It has been proposed to use electrical current to add heat to a subsurface pay zone containing tar sands or viscous oil to render the viscous hydrocarbon more flowable. Electrodes are connected to an electrical power source and positioned at spaced apart points in contact with the earth. Currents up to 1,800 amperes are passed between the electrodes. This heats oil in the formation. Electrical power utilizes energy from various sources. This energy is expended for and in a sense is replaced by viscous oil. Therefore, the relative success of electric heating depends on the amount of oil produced per unit of electrical power supplied. Unfortunately, most of the heat occurs adjacent to the electrode wells and heat transfer outward into the formation by conduction is slow. Moreover, the power efficiency of electrical generation is only about one-third. Brine injection and fractures have been suggested for decreasing electrode resistance and increasing electrode radius. Moreover, it has been proposed to use the electrode wells as water injection wells in well patterns based in part on the number of phases of the electricity used to apply heat to the formation. Such patterns have been used in conventional ways and their efficiency, therefore, is less than it could be. Moreover, when the electrode wells are completed into the producing formation, injection of fluids into or production of fluids from the hydrocarbon-bearing formation through the electrode wells interferes with normal gravity drainage, other drive mechanisms, and/or injection programs. It, therefore, would be advantageous to provide a method for stimulating the viscous oil production that does not interfere with optimizing the way that the hydrocarbon formation is produced.
In U.S. Pat. No. 3,862,622, it has been proposed to place electrodes in upper and lower layers situated immediately above and below a subsurface hydrocarbon-bearing formation. The upper and lower layers have a lower electrical resistivity than that of the formation to be produced. The resistivity of a subsurface layer is dependent upon a number of factors. For example, many subsurface layers are made up of hydrated minerals or contain what is called connate water. Connate water and water in salts is not mobile water. A layer may have lower resistivity than an oil bearing formation without containing mobile water. U.S. Pat. No. 3,862,622, therefore, sets forth several electrode completion techniques wherein water from the surface of the earth is used. For example, voids or cavities are filled with an electrolyte such as aqueous sodium chloride solution to increase the effective radius of the electrode. The voids or cavities can be created by hydraulic fracturing and propped with propping agents or by considerably enlarging the diameter of the bottom portion of the well which is left uncased.
By computer modelling experiments, it has been demonstrated that the effectiveness of these prior electrodes with limited water volumes for electrical heating of viscous oil-bearing formation is seriously limited. In such confined, limited water volume electrodes, it is difficult to resolve the adverse effects of boiling fluids adjacent the electrode. Vaporization and pressure buildup adjacent the electrode limits and adversely affects the amount of electric current that can be flowed through a subsurface formation.