Self-Potential Profiling - A New Technique For Determination Of Heat Movement In A Thermal Oil Recovery Flood

Abstract

Abstract Self-potential profiling, a technique long used to locate mineral deposits, has recently been found by various investigators to be an excellent tool for locating and delineating buried geothermal reservoirs in clastic and pyroclastic environments. Laboratory experiments indicate that application of heat to the surface of various core samples result in an instantaneous generation of electric current, due to thermoelectric coupling effects which are related to conductivity contrasts within the inhomogeneous media. These currents can be detected over geothermal deposits by profiling across the boundary between normal and abnormally high heat flow. Field tests at the Slocum Oil Field, Texas, show that heat fronts can also be established by self-potential profiling over an artificial geothermal reservoir—a profiling over an artificial geothermal reservoir—a thermal oil recovery flood. Tests have been run over shallow reservoirs in which both steam and fire floods are in progress, with significant results. Successive profiles appear to indicate that the temperature distribution within the reservoir, as well as changes in location of the heat front with time, can also be determined by this comparatively simple technique. Introduction Self-potential (SP) voltages result from natural earth currents that are relatively stable in time, in both direction and magnitude. If two metal stakes are placed in conductive ground and connected to a sensitive voltmeter, an electric voltage is found to exist between them. Such voltages normally range up to a few millivolts and are known as "background potentials"; they are the spontaneous voltages due to potentials"; they are the spontaneous voltages due to such factors as variations in concentrations of the electrolytes in the ground, or small streaming potentials due to flow of groundwater through the pores potentials due to flow of groundwater through the pores of rocks. A host of other factors can give rise to natural or artificial background potentials; these include bioelectric activity of plant roots, the topographic effect of the normal atmospheric electric gradient, and such phenomena as corrosion of buried pipelines and of the metal stakes used as electrodes. pipelines and of the metal stakes used as electrodes. Accordingly, non-polarizable electrodes are always used for these measurements. The method is best employed in areas where the ground offers a good conductive layer, and is extremely unreliable in areas of bad surface conduction such as dry crystalline rock, or frozen ground. The mapping of natural electric potentials in the ground has been studied for many years, and has been used extensively for locating anomalies associated with metallic ore deposits. Voltages ranging from a few tens of millivolts to a volt have been noted over sulphide bodies, notably those containing pyrite, chalocopyrite and pytrhotite. Graphite, pyrolusite, magnetite and anthracite coal often result in measurements of over one volt. Due to the sometimes mineralized surface conditions, SP mapping of ore bodies has been considered only partially reliable. However, in recent years, SP surveys have begun to be used in sedimentary environments for other purposes and with more consistent results. Nourbehecht, for example, in an exhaustive theoretical study of self-potential, discussed the results of a self-potential anomaly caused by a nuclear explosion in Nevada; Corwin, Morrison and others have reported self-potential anomalies as precursors to earthquakes in California and China, presumably due to streaming potentials generated by the flow of water into a dilatent zone.