BRONCO OIL SHALE STUDY (PARTS I AND II, AND APPENDICES A, B, C, D)

"he study was prepared cooperatively as part of the  AECs Plowshare Program by the San Francisco Operations  Office of the U. S. Atomic Energy Commission,  the U. S. Bureau of Mines, the University of California's  Lawrence Radiation Laboratory and the CER Geonuclear  Corporation, the latter acting for approximately  a score of oil companies.  Part I of the study examines the feasibility of using  deeply buried underground nuclear explosions to break  oil shale deposits for in situ retorting, and recommends  that a nuclear explosion experiment be designed to test  the concept. Oil shale is a fine grained, calcareous rock containing kerogen, a solid hydrocarbon. When kerogen is heated to temperatures over 700° F, it rapidly decomposes to produce a liquid oil similar to petroleum. Oil shale is widely distributed throughout the world. and constitutes a major hydrocarbon resource. However, most of the world's known resources. over two trillion bbls, are located in Colorado, Utah, and Wyoming. In some places the thickness of oil shale yielding 25 gal/ton reaches 2.000 feet. It is this higher grade shale that is being considered for utilization. Associated with the oil hale in these deposits are the minerals nahcolite and dawsonite,  which are potential sources of soda ash and aluminum.  Efforts to develop a commercial oil shale industry date  back to the mid-1800's. Conventional methods involved  mining the oil shale and heating it in a retort to extract  the oil. Recently interest has developed in methods to  retort the oil shale underground. The nuclear concept  involves firing a deeply buried. totally contained nuclear  explosive to fracture the shale, which would then be  retorted in place. A number of methods of retorting  the broken oil shale, and associated fracture zones are  described. If successful, the utilization of nuclear explosives for this application would eliminate the necessity of mining and bringing to the surface huge quantities of shale for surface treatment and subsequent disposal of the retorted rock, increase the nation's available oil supply by allowing the economic development of vast resources of oil shale that are currently beyond the scope of any recovery technique, and permit large-scale operations with a minimum disturbance of the natural landscape. A location in the Piceancc Creek Basin in western Colorado has been investigated as a site for further studies and field investigation. The report recommends that safety and engineering field work to determine whether the location is suitable for a field test proceed simultaneously with design of a field experiment.  Part II of the study describes Project Bronco, a proposed  50-kiloton nuclear explosion experiment. The detonation  will fragment and fracture a deep, thick oil shale deposit which will subsequently be retorted in place. Bronco will provide information related to: the technical and economic feasibility of the basic concept, a predictive capability for the physical effects of nuclear explosions, and the distribution of radioactivity and its behavior during retorting.  Although the Bronco experimental design is based on  a potential site in the Piceance Creek Basin, a pre-shot  investigation will determine whether the nominated site  will meet the technical and safety criteria for a first nuclear  explosion in oil shale. Following site confirmation, holes will be drilled for fracture studies, emplacing the explosive, and for shock wave measurements. The explosion is expected to produce a chimney 230 feet across and 520 feet high (measured up from the shot point), containing over one million ton of fragmented oil shale. Fractures may extend as far as 460 feet laterally beyond the chimney edge. Posthot drilling will reveal the size and shape of the chimney, the extent of fracturing, and the distribution of heat and radioactivity. The final design of the in situ retorting experiment will depend on results of the post-shot exploration and on laboratory research currently under way. Due to this uncertainty of retorting design, no cost estimate is included in this report. Tentatively, mixtures of air and recycle gas will be injected via drill holes to the chimney top. Drill holes to the  chimney bottom will remove offgas, oil mist, and liquid oil. During retorting, measurement will be made of temperatures in the chimney. Samples of gas and oil will be analyzed for physical characteristics, chemical composition, and radioactive content, if any. Additional data on retorting efficiency will be obtained in post-retorting drill holes. It is tentatively planned to follow the chimney retorting with an experimental outward moving treatment in a 45 sector of the fractured region outside the nuclear chimney."

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