Thermal recovery methods, as applied in heavy oil and oil sand deposits, and in environmental remediation, have the common objective of accelerating the hydrocarbon recovery process. Raising the temperature of the host formation reduces the oil and bitumen viscosity, and, in environmental remediation, increases vapour pressure. These effects assist in sweeping the substances to be recovered from the formation when driving agents are externally injected or when autogenous processes come into play.
Transferring electromagnetic energy to the deposit is proving to be an effective means of supplying the necessary heat. In this electro-thermal process electromagnetic energy is converted to heat in situ using a system of wellbore electrodes from which currents flow through the formation. By proper choice of electrode location and spacing, considerable control can be exerted over the path taken by the currents and, hence, over the temperature profiles that will develop in the formation. Electro-thermal processes are mostly free of problems related to very low initial formation injectivity, poor heat transfer, and the difficulty of controlling the movement of injected fluids and gases, which have plagued other thermally stimulated recovery processes.
Most electro-thermal processes are carried out at power frequencies where the formation acts as a resistive heating element between the various electrodes. At this low frequency, current flow in the formation is primarily via ionic conduction through the water-saturated portion of the interconnected pore spaces in the reservoir. Due to the inherent geometry of current flow emanating from an electrode, current densities and heating rates are highest near the electrodes. Care must be taken lest the water in the immediate vicinity of the electrodes vapourizes and the continuous water path between electrodes is broken. Hence, power frequency heating is generally appropriate when the desired temperatures to be achieved in the formation are lower than the in situ steam temperature.
A variation of power frequency heating, termed inductive heating, is achieved by placing the primary winding of a current transformer inside the casing at the bottom of the wellbore. The section of casing adjacent to the transformer acts as a single turn secondary winding. Large induced currents resistively heat the steel of the casing, and heat is transferred to the formation by thermal conduction. By increasing the frequency to multiples of the power frequency, the rate of heating can be proportionately increased. At greater frequencies however, the electrical transmission losses and capital cost of equipment will also increase.
In a further variation of power frequency heating, the casing, or a section thereof, is resistively heated by the flow of large currents in the casing itself. The adjacent formation is heated by thermal conduction from the casing. This approach shows promise in the heating of long horizontal wells, with applications, for instance, in processes related to steam assisted gravity drainage or in surface mining of tar sands.
All formation heating at power frequency has the inherent advantage of the ready availability of 60 Hz power and the associated apparatus, such as transformers and measurement equipment. Since heating at power frequency is a viable approach to formation heating, a need to consider in situ electromagnetic heating at frequencies much higher than 60 Hz is not immediately evident. As the moisture content of the formation is decreased, however, whether this is due to little water content in the formation to begin with, or because water has been driven off by heating above the steam point, higher frequencies become necessary.
The rate, in W/m3, at which electrical energy is converted to heat within unit volume of formation, is given by σE2. Here, σ is the effective electrical conductivity in S/m, and E is the rms electric field intensity in V/m. The effective electrical conductivity increases as the square of the water content by weight. It is made charges oscillating in the applied electric field. This second term is directly proportional to the number of oscillations per unit time, and, therefore, increases linearly with frequency. The dependence of effective electrical conductivity on water content and frequency is illustrated in Figure 1, which shows averaged data obtained from many measurements on reconstituted samples of typical Athabasca oil sands(1). It follows from the foregoing that use of higher frequencies allows heating of those formations that have low electrical conductivities at 60 Hz, without the need for excessively large electric field intensities.
While formation heating at high frequency is technically feasible, there are several factors that must be considered. These include the relatively high cost of the capital equipment required to convert 60 Hz power to high frequency, electrical losses in the frequency conversion, and the need to match the electrical impedance of the formation to the output impedance of the source. Also, as frequency is increased, there is a decrease in wavelength and the distance to which electromagnetic energy penetrates the formation, which may lead to non-uniform formation heating. This, generally, is undesirable. The behaviour of these two parameters as a function of frequency is shown in Figure 1 for two typical oil sand formations having different moisture contents. It is seen that the depth of penetration is reduced to the order of 20 cm in oil sand “A” as the microwave region is approached at 109 Hz. Hence, while microwave heating finds wide applications in other areas, such as the cooking of popcorn, it is unsuitable or severely limited for in situ heating applications.