Selective Electric Reservoir Heating

Abstract This study was designed to investigate the feasibility of using an electric current for the selective heating of portions of an oil reservoir that are normally bypassed by injected fluids. The process is accompanied by water injection and is expected to increase the amount of oil that can be recovered by waretflood. One technique that was studied is designed to heat the unswept area in a pattern flood. Another process that was investigated is in/ended to heat selectively the low-permeability zones that are not normally contacted by injected fluids. The results of this study suggest that both of these processes are feasible. Further investigation will be required for economic evaluation and for design of electrical systems. Introduction The flow of electricity through an oil reservoir will heat the reservoir and thereby reduce oil viscosity. If electric heating is employed in a waterflood, some of the heated oil will be displaced to production wells, where it can be recovered. Because the cost of heating a reservoir with steam would usually be less than the cost of heating with electricity, the latter oil recovery technique has been used only occasionally.(1) In spite of the relatively high cost of electric reservoir heating, the process does afford a few unique advantages. Itprovides a means for heating deep reservoirs that cannot be economically heated by steam, and it may permit oil recovery from reservoirs that could not otherwise be adequately heated because of low permeability or pressure limitations. However, the authors believe that the most useful application of the process will probably be in heating portions of an oil reservoir that cannot be readily contacted by injected fluids. As described below, selective heating can be accomplished under appropriate reservoir conditions by proper location of the electrode wells and by injection of water with suitable salinity. U.S. and Canadian patent applications are pending on these techniques. Previous Research A review of the petroleum literature indicated that little has been published that directly pertains to electric heating of an oil reservoir, using the reservoir rock as a resistance heating element. Several patents related to the process were noted. However,(2–7) and most of the phenomena that would be expected to occur during electric heating have been investigated. Mathematical descriptions of heat flow in the presence of fluid motion have been available for many years.(8) More recently, the electric resistivity of hydrocarbon reservoirs has been investigated by workers interested in the interpretation of well resistivity logs.(9) The effect of temperature on water viscosity is well known,(10) and similar data are available for some hydrocarbons and hydrocarbon mixtures.(11,12) Empirical formulas for the heat capacities of water and reservoir hydrocarbons are also available(13). Thermal conductivities of reservoir fluids have been investigated,(14) heat capacities of reservoir rocks have been measured(15) and studies have been made of the thermal conductivity of porous systems.(l6,18) Changes in rock permeability due to heating by steam injection have also been measured,(19) and some data are available on the effect of temperature on relative permeability (20–22).