Electrical Heating

Abstract The purpose of this paper is to present the modifications to a multi-component, thermal reservoir simulator to incorporate electrical heating equations. The results of the simulator are compared with actual field data and a semi-analytical model. The visco-skin is described and the use of electrical heating to remove the visco-skin and increase the productivity of the well is demonstrated. The visco-skin concept is a physical phenomenon which explains the rapid productivity increase of some wells undergoing electrical heating. It is important to include the visco-skin concept in the discussion of electrical heating since it develops in the near wellbore, as does the temperature distribution from electrical heating. The electric field equations have been solved using a 3D finite differencing technique coupled to a multi-purpose reservoir simulation program. The resulting program can be used forpredicting the fluid rates from a well undergoing electrical resistance heating,calculating the temperature distribution in the reservoir and on the electrode,obtaining the voltage-current relationship for designing power supplies,establishing operational criteria such as input power requirements as a function of flow rate and reservoir heterogeneity, andwell completion design. Introduction Electrical Resistance Heating Electrical heating is a thermal process which can be applied to a well to increase its productivity. The productivity increase is substantial and comes about because of the removal of thermal adaptable skin effects (visco-skin for example) and the reduction of the oil viscosity in the vicinity of the wellbore. Salient features of the process are:It is a continuous, not a cyclic process. Electrical heating occurs simultaneously with production of fluids.Low frequency power (not microwave frequency) is used.All the downhole equipment can be contained within a single wellbore. FIGURE 1: Electrical healing single wellbore configuration (oil production equipment not shown). (Available in full paper) Figure 1 describes how the process works. The specific system described here is similar to Gill, 1969; Spencer, 1988; and Rice, 1992. The essential components of an electrical heating system are:power supply,power delivery system,electrode assembly, andground return. The variable frequency (2 to 60 Hz), power supply (Isted, 1992), is capable of delivering up to 300 kW of power. The power delivery system may consist of tubing, cables or a combination of both. The electrode assembly consists of bare casing pipe with FIGURE 2: Visco-skin concept. (Available in full paper) fiberglass electrical isolation joints attached to the ends. The length of the electrode and location in the reservoir is a matter of engineering design. The current return or ground can be the casing string above the fiberglass insulation. Current leaves the power supply and is conducted down the power delivery system to the electrode assembly. The electrode is in electrical contact with the reservoir formation. From the electrode, the current is forced to flow through the reservoir and return to the power supply up the casing.