Heat Transfer Fundamentals for Electro-thermal Heating of Oil Reservoirs

Abstract Electro-thermal methods are being used for extraction of bitumen from the oil sands. Several processes have been tested or are being proposed. Shell has proposed the use of electro-thermal methods in the carbonates and have tested a process at their Shell Peace River operation. E-T Energy is using an electro-thermal process in the Athabasca Oil Sands. Other institutions and companies, for example, the Alberta Research Council has also developed electro-thermal approaches for bitumen recovery. The heat transfer mechanisms, either from horizontal or vertical wells, associated with the electro-thermal approach distinguishes the various methods. In some of the approaches, heat transfer by conduction is the dominant method of transferring heat to the reservoir. In other methods, heat is generated within the reservoir electrically and transferred conductively, and in other processes convection is a key heat transfer mechanism in combination with the others. The purpose of this paper is to present a model for radial heat transfer that can be used to compare different electro-thermal heating methods. The model compares the resulting temperature distribution, time to achieve a heated volume at some distance away from the wellbore, and the power density in the reservoir between the different electro-thermal methods. Also, insight into design issues, such as well spacing and input power requirements, as well as practical matters related to efficiency, near wellbore heating, and water vaporization are presented. Introduction Oil reservoirs are a mixture of sand, bitumen and water. In Alberta, most of the oil is heavy or bitumen (from the oil sands) and cannot be produced easily from the reservoir. Electro-thermal methods are presently being considered for mobilizing bitumen from the oil sands. Bitumen is defined as oil that is less than 10 API and will not flow to a well in its naturally occurring state. Steam assisted gravity drainage (SAGD) is a promising in-situ thermal recovery method, having the advantages of lower energy requirements and higher recovery factors over other steam injection methods. However, about two thirds of the total deposit is too deep for surface mining and too shallow for steam injection [4] as depicted in Figure 1. These shallow resources may be well suited for electro-thermal processes. Also, electro-thermal methods have the potential to produce bitumen from oil sands that are at the mineable depths [2]. All in-situ thermal recovery methods as applied in oil sand deposits have the common objective of accelerating the hydrocarbon recovery process. Raising the temperature of the host formation reduces the bitumen viscosity allowing the near solid material at original temperature to flow as a liquid. These effects assist in sweeping the bitumen to be recovered from the formation when driving agents are externally injected or when autogenous processes, such as gravity drainage come into play. Conduction Methods that use electro-thermal energy to increase the temperature of the wellbore without current flow in the reservoir have been also been developed. This is the first configuration shown in Figure 2. Electric heater elements are placed within the wellbore and are operated at very high temperatures.

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