Promising Progress in Field Application of Reservoir Electrical Heating Methods

Abstract

Abstract Electrical Heating of heavy oil reservoirs has been successfully applied in near wellbore regions. Well stimulations through downhole resistive, dielectric, or induction electric heating systems when applied in suitable reservoirs can triple flow rates for a small additional operating cost. In such cases high-energy conversion ratios such as 10–15 bbls. Of oil for every bbl of oil consumed at the power plant can be achieved. Applications of electric heating can particularly be beneficial in situations where steam cannot be used due to either depth, formation incompatibility, low incipient injectivity, excessive heat losses or existence of thief zones. Additionally, for reservoirs already having a high temperature in the 60–80 degree C and higher range, only a small amount of electric heating-stimulation is enough to increase oil production by an order of magnitude. Some Orinoco reservoirs of Venezuela fall into this category. Heavy oil reservoirs, overlain by heat sensitive permafrost, may also find electrical heating a viable thermal stimulation technique. Electrical heating process has also been adapted to apply in preheat phase of other complimentary processes like VAPEX. The process also has been investigated for hot water flood in deep reservoirs. This paper proposes to use downhole electric systems for stimulating for both vertical and horizontal wells in such reservoirs. Using CMG's thermal simulator-STARS (ref. 7), results of several worldwide investigations are included. A summary of field applications is also presented. In addition, incremental oil recovery vs. energy consumption is tabulated. Introduction Electrical heating tools and subsequent applications can be broadly divided into three different categories based on frequency of electrical current used by the tool (ref 5).Low frequency currents are used in Resistive/Ohmic heating andHigh frequency currents are used in Microwave heating methods.The Induction tools have the ability to use a wide range of low to medium frequency currents depending on heat requirements and desired temperature. Electric current from low frequency resistivetools penetrates deeper into the reservoir than from the high frequency RF tools at temperatures below the vaporization point of water, though the temperature of the affected zone may be higher with RF tools. The insitu water provides an ionic conduction path in the resistive heating systems permitting the use of less costly low frequency energy supplies. Induction tools on the other hand produce electromagnetic fields, which induce eddy currents and hysteresis-losses in the casing or liner resulting in heat generation. Thus heated casing or liner provides heat for the near well bore section of the reservoir. These tools are very efficient and Induction heating technology has found its place in a large number of industrial applications. For heavy oil reservoir insitu heating applications, both Resistive and Induction tools have been more widely used as opposed to RF tools. This paper discusses only Formation-resistive and Induction heating methods. Low Frequency Heating: In this method, low frequency current, using ionic conduction mechanism, is made to travel through interstitial water present in the reservoir matrix system. Electrical energy is converted into heat energy through associated ohmic losses in the formation. The overall effect of the heat generation is to reduce the pressure drop near the wellbore by decreasing oil viscosity and improving oil mobility. The bulk electrical conductivity in formation can be obtained from the Archie and Humble's relation (ref.2) given below. It demonstrates how for interstitial water is essential.Equation 1 The temperature dependence of the water resistivity is given by:Equation 2 Where Rw is in ohm-meters and T is in degrees Kelvin.