Explosive Fracture Studies on Oil Shale

Abstract

Abstract Fracture, fragmentation, and/or enhanced permeability through explosive methods can be difficult to achieve in the often restricted environment demanded of in-situ resource recovery methods. Blasting within confined volumes and the need for uniform fragment size and permeability adds a dimension for which a history of blasting experience is not available. Computer modeling offers a potential tool for rapid evaluation and optimization of the explosive and geometry parameters. This study deals with intermediate-scale explosive blasting tests within confined volumes that have been conducted on blocks of oil shale to evaluate a computational fracture model under realistic loading conditions. The blocks were instrumented with stress gauges in the material to determine the dynamic stress field induced by the explosion. The blocks were recovered, stabilized, and sectioned after testing to measure the extent of damage and fragment-size distribution. Both stress-wave and fragment-size data were compared with numerical computations. Introduction Large deposits of oil shale in the western U.S. have the potential of significantly supplementing this country's waning oil supply. Oil shale exists as a fine-grained sedimentary rock, and the organic constituent, kerogen, is contained in small voids in the rock. Through heating (retorting), the kerogen will decompose, yielding various organic products, including liquid oil. Oil shale is characterized as lean or rich depending on the organic content, which can vary from several millimeters per kilogram to more than 400 mL/kg. About 80 mL/kg is representative of the fairly rich Mahogany zone of the Green River formation in the western U.S. For various technical, economic and environmental reasons, this resource is practically unavailable at present. One process that is being considered to circumvent many of these difficulties requires retorting of the oil shale in place. This method is attractive in that extensive mining is minimized and the environmental concerns of large volumes of spent oil shale resulting from surface retorting would be lessened.A serious technical and economical consideration rests in the controlled breakage or rubbling of the oil shale in preparation for the subsurface retort. Optimization of the retort process requires that the oil shale be broken into fragments of suitable size and that a connected void region be available for fluid flow. Further, it is necessary that fragment size and void region be homogeneously distributed throughout the retort bed to prevent channeling of the burn.It is expected that explosives will provide the major source of energy needed to fragment and distribute the broken oil shale within the retort region. Technical considerations will govern geometry and explosive placement, while the cost of explosives and the difficulty of placement are economic considerations. Optimal rubbling of the oil shale will require consideration of interrelated parameters such as rock properties, explosive properties, charge geometry, blast-hole burden and spacing, decoupling, stemming, type and location of initiators, and sequence of detonation. Clearly, the problem of explosive fragmentation and distribution of the oil shale is a complex, many-variable problem for which previous experience in quarry, mine, and construction blasting will be useful but will not provide all the answers. SPEJ P. 349＾