Fracturing Oil Shale With Electricity

Abstract

Abstract Laboratory studies to evaluate the use of electricity for fracturing various grades of Colorado oil shale were started in 1964. Breakdown voltages varied greatly from one grade of shale to another, ranging from a minimum of 300 v to a maximum of 15,000 v. With few exceptions, the shale samples were readily fractured at relatively low current levels. On the basis of encouraging results obtained in the laboratory, experiments were expanded to field tests to study electrical characteristics of oil shale when subjected to overburden pressure. These tests generally confirmed the laboratory experiments. Fracturing was accomplished at all electrode spacings used. The spacings varied from 3 to 129 ft. Based upon results obtained in these field experiments, additional fracturing tests were conducted in four shallow wells drilled in an oil shale formation. Some new permeable zones were created between wells. The induced permeability was improved by detonating two conventional nitroglycerin wellbore shots in one of the wells. Introduction Before an in situ retorting process can function, it is necessary to develop techniques to increase the permeability of the oil shale formation. Induced fracturing, the best method of increasing the effective permeability of oil-shale deposits, may be accomplished by hydraulic pressure, high explosives, high-voltage electricity or combinations of two or more of these. This article discusses the laboratory experiments and preliminary field tests being made to study high-voltage electricity as a method of fracturing oil shale. Fracturing is the first phase of the electrocarbonization process and is the phase with which this investigation is primarily concerned. Several investigators have studied the electrocarbonization process as applied in coal, petroleum and oil shale formations, but none has reported making field tests in Green River oil shale. Laboratory Experiments Preliminary Studies Samples used for the preliminary tests to evaluate electrical properties of various types of oil shale ranged in size from 2 to 175 lb and represented a broad spectrum of Colorado oil shales ranging from 1.5 to 58.6 gal of oil per ton of shale. The first tests determined the breakdown voltage required to initiate a current flow of 5 milliamp through the sample. In all cases, 1/4-in. stainless steel tubes spaced 4 in. apart were used as the electrodes. Breakdown voltages varied greatly from one sample to another, ranging from 300 to 15,000 V. Fracturing Studies Establishment of Air Communication After it was definitely established that electrical fracturing could be accomplished, other factors related to an in situ retorting process were investigated. The first of these was the establishment of air communication between two or more holes drilled at different locations in the shale block. Experiments were conducted on several shale blocks in which three holes were drilled in a line and spaced 2 in. apart, with 1/4-in. perforated steel tubes cemented into the holes. In most cases, interconnecting fractures created by the electrical current permitted substantial amounts of air to flow between all three holes. In these experiments the entire length of the steel tubes served as the electrodes; under these conditions the fractures occurred at random along the low-resistance paths taken by the electric current. Controlled Fracturing Fracturing at random is not a satisfactory solution to the problem. For an in situ retorting operation to be successful, a technique is required to create a system of horizontal fractures vertically spaced at preselected intervals. Accurate control of positioning the horizontal fractures is essential because the combustion process may require a vertical spacing between fractures of as little as 1 or 2 ft In Feb., 1965, experiments were initiated to investigate the feasibility of creating a controlled system of horizontal fractures. Electrodes used in these experiments were designed to concentrate the current at a small, well defined contact point. The first experiments were made with slabs of oil shale weighing 150 to 300 lb. Horizontal fractures were induced at preselected levels vertically spaced at 3-in. intervals (Fig. 1). Based on these results the experiments were expanded to involve oil shale slabs weighing several tons. The slab shown in Fig. 2 assayed at 55.7 gal of oil per ton. The top of the slab was first leveled off by inducing a fracture at the base of the peak. Two 1-in. diameter holes spaced 4 ft apart were drilled to a depth of 36 in. JPT P. 37ˆ