Numerical Simulation Results for the Electrical Heating of Athabasca Oil-Sand Formations

Abstract

Summary Electrical preheating has been proposed as a method of overcoming many of the problems inherent in using a steamdrive in very viscous oil reservoirs-such as the Athabasca oil sands of Alberta, Canada. A numerical simulator was developed to study the process of electrically heating oil reservoirs consisting of several layers with different electrical resistivities. This simulator was used to study the effects of electrode placement on the final temperature contours resulting from electrically heating realistic reservoirs. Three cases illustrating the development of a hot-oil communication path low in the formation are described. Introduction The recovery of oil from heavy-oil and oil-sand deposits is hindered primarily by the high viscosity of the reservoir oil. For example, the intrinsic viscosity of the bitumen in the Athabasca oil-sand deposit has been estimated to be more than 1000 Pa.s [1× 10 6 cp]. The application of heat is the simplest and most efficient way of lowering bitumen viscosity. The methods of heating the reservoir oil include well-known fluid-injection methods-such as cyclic steam stimulation, steam-flooding, and fire-flooding-and the newer techniques of heating the reservoir with electromagnetic energy. Steam injection and fireflooding techniques are now applied commercially to heavy-oil deposits, but they are still technically difficult and usually uneconomical in very viscous oil-sand deposits. All fluid-injection methods in oil- sand deposits are faced with (1) very low initial injectivity, (2) difficulty in establishing communication paths between wells, (3) poor control of injected fluid movement, (4) reservoir inhomogeneity, (5) steam override, and (6) a very poor mobility ratio leading to a poor sweep efficiency. Electromagnetic heating techniques, which heat the reservoir by means of the ohm losses of electric current flowing in the connate water and the oil of the reservoir, can overcome many of these difficulties. Because no fluid is injected, low initial injectivity is not a problem. Heating occurs both near the well and deep into the problem. Heating occurs both near the well and deep into the formation. In addition, with proper attention to the reservoir lithology and electromagnetic properties, the reservoir may be heated relatively uniformly. Production may occur during or immediately after electromagnetic heating if the formation pressure is sufficient, or electromagnetic heating may be used to preheat the formation for a steamflood or fireflood. The disadvantages of electromagnetic heating should be weighed against the advantages. First, electromagnetic energy is more costly than the same amount of steam energy. The additional cost of the electricity for the same amount of steam energy, however, is usually a small fraction of the total cost of the recovery scheme, and the electrical energy may be directed more efficiently into the formation. Second, electrically insulating all or part of the tubing, casing, and wellhead is necessary to protect the operating personnel and to prevent short circuits. The well completion technology for electromagnetic heating has been developed for some of the simpler heating procedures, but much work remains to be done in this area. procedures, but much work remains to be done in this area. The three types of electromagnetic heating usually considered are radio frequency heating, induction heating, and low-frequency conduction heating. This paper discusses only low-frequency (60-Hz [60-cycle/sec]) electrical heating and reports on our construction and use of the MEGAERA numerical simulator. Several simulation cases are presented to illustrate the simulator's flexibility for studying the electrical heating of the Athabasca oil- sand deposits and to show how the oil-sand formation can be heated almost uniformly with the more conductive adjacent formations as extended electrodes. These results are an improvement over the previously reported electrical heating predictions because actual multilayered reservoir lithologies and properties of the Athabasca oil-sand deposit were used. The simulator and results are described after a short review of the low-frequency electrical heating literature. Previous Work Previous Work Refs. 2 through 5 report on the selective electrical reservoir heating process. SPERE p. 76