Today’s techniques to recover heavy oil are mining, when possible, or steam injection. For steam injection, once the oil is heated, its viscosity is reduced; it becomes mobile and can be extracted. In some cases, when steam injection isn’t efficient, other methods should be developed, one of them is based on electrical technology.
One solution studied by Total to reduce viscosity when steam injection isn’t possible or efficient, is to use Radio Frequency heating. The electromagnetic waves have the capability to heat immediately deeper than steam. To improve efficiency of this process, we developed a downhole Radio Frequency generator composed of triodes. An antenna is added to form a complete assembly. Here, we will present the advantages of this original solution patented by Total.
The optimization of this device concerns the impedance adaptation between generator and the antenna. But the impedance of this antenna depends also on the electrical characteristics of the reservoir. The reservoir’s impedance is mainly determined by the water in liquid phase. One critical parameter is the complex electric permittivity. This parameter impacts directly on the conversion from the electrical energy to heat, the dissipated power in the reservoir being related to the complex part of the permittivity.
In this paper, we will show a solution to keep an optimum conversion ratio from high frequency electrical energy to heat. As the impedance varies with time and reservoir heating, the optimum radiating frequency of our system has to vary too. The choice of the central frequency could also depend on the presence of iron oxide in shale layer in the hydrocarbon reservoir. In this paper, we will choose a frequency around 10 MHz and we will show different spatial repartitions of the applied electrical field and the corresponding heated zone. The frequency band is a compromise between fast heating and depth of penetration: the lower the frequency, the deeper the electromagnetic waves penetrate in the reservoir.