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Figure 1. Distribution of the Green River Formation of Colorado, Utah and Wyoming.

Foundation and are being operated under a research contract with six oil companies: Mobil Oil Corporation, which acts as project manager, Humble Oil and Refining Company, Phillips Petroleum Company, Sinclair Oil and Gas Company, Pan American Petroleum Corporation, and Continental Oil Company. The research program, to cost about seven million dollars, is investigating the gas combustion retorting method, and improved mining techniques. Union Oil Company of California has developed a retorting system'" utilizing a vertical refractory lined vessel through which the shale is moved upward by an unusual charging mechanism referred to as a “rock pump.” The retort is heated by passing air downward to burn the organic matter remaining on the retorted shale. The oil produced in the retorting zone is condensed on the cool, incoming shale, and flows over it to an outlet at the bottom of the retort. During the period 1955-1958, a room-and-pillar mine and a retort processing about 1,200 tons of shale per day were operated at a site on Parachute Creek north of Grand Valley, Colorado. (9) Colony Development Company has conducted operations, also on Parachute Creck, since 1964 in an effort to develop the TOSCO retorting process.

1101 The retort used in this process is a rotary-type kiln utilizing contact with ceramic balls, heated in a separate vessel, to accomplish retorting. The Colony operations included construction of a "semi-works” TOSCO retort, and the attendant opening of a room-and-pillar mine. In addition to the preceding approaches, a number of other methods of mining and surface retorting have been discussed in the literature.

wells could be established through induced and natural fracture systems, that wells could be ignited successfully although high pressures were required to maintain injected rates during the heating period, and that combustion could be established and maintained in the shale bed. More recently Sinclair has been conducting extensive field research at a site on Yellow Creek in Rio Blanco County, Colorado. One of the newer in situ shale oil recovery processes has been patented by Equity Oil Company of Salt Lake City. (12.131 This process employs injection of hot natural gas to retort the shale and it has been successfully field tested in the Piceance Creek Basin. One injection well and four producing wells were drilled into the shale formation. Gas was compressed to about 500 psi, heated to the desired temperature level, and delivered through insulated tubing to the retorting zone. In situ field tests have also been conducted by Mobil Oil Corporation, but little information on their operation has been released. The Bureau of Mines is presently studying two methods for creating permeability. The first uses high-voltage electricity to fracture the shale at predetermined locations approximately parallel to the shale bedding planes. Field tests are being conducted in shale beds near Rock Springs, Wyoming, to determine whether oil shale under pressure of overburden responds the same to the passage of high-voltage electricity as do unrestrained blocks in the laboratory."14)The second approach, under way at the same field location, is a study of the detonation of liquid nitroglycerine, injected into the natural or induced permeable zones, 10 create additional fracturing in oil shale beds. (15) The use of a nuclear explosion to fracture very large quantities of shale, several million tons at one time, is another approach and is the one of principal interest in this report. This technique is primarily applicable to relatively thick shale intervals under substantial overburden. Although minimum specifications for the amounts of shale and overburden required cannot be established definitely until after data are available from one or more experimental nuclear tests in oil shale, the technique should be applicable to a large area of the Piceance Creek Basin. For example, if a 200-foot interval of oil shale averaging 25 gallons of oil per ton under an overburden of 1.000 feet is assumed adequate, an area of about 360 square miles in Colorado alone would be amenable to the technique. This area, which contains intervals of shale from 200 to 2.000 feet thick averaging 25 gallons of oil per ton, would represent on the order of 400 billion barrels

In Situ Treating Because mining. transporting, crushing, and disposal of spent shale make up most of the present cost of producing shale oil, treating shale in place to produce oil is being investigated as a means of reducing the cost of shale oil recovery. This approach has other attractive features. It may be applicable to deposits of various thicknesses, grades, and amounts of overburden, does not disfigure the surface, and eliminates the necessity of disposing of large quantities of spent shale. A prerequisite to in place treatment is creation of adequate permeability in the shale bed. A number of techniques, including fracturing with nuclear explosions, have been suggested to accomplish this. Sinclair Oil and Gas Company began studying the feasibility of in situ retorting of oil shale in 1953.11 From these tests and subsequent ones made during the following year, it was concluded that communication between

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Figure 2. Isopachous map of 25-gallon-per-ton oil shale, Piceance Creek Basin, Colorado.

of oil in place. (The recovery technique may not be applicable at some locations in the area due to extensive concentrations of saline minerals, excessive quantities of water, land status problems, or other factors.) At present there are insufficient data on the oil shales in Utah and Wyoming to predict the extent of applicability of the technique in these states.

mendous volume of oil rich shale in the central part of the Piceance Creek Basin (Figure 1). Although there is no current recovery of these minerals in the basin, interest is being expressed on their potential as raw materials for producing aluminum and soda ash. Limited research to date indicates that the minerals might be extracted from a nuclear chimney by in place aqueous leaching methods. (16) Laboratory investigations of methods of extracting nahcolite and dawsonite from oil shale are being conducted by the Bureau of Mines.

Dawsonite and Nahcolite Deposits of nahcolite (Na HCO3) and dawsonite (Naal (CO:) (OH)) are distributed through a tre

FRACTURING OIL SHALE WITH

NUCLEAR EXPLOSIVES

zone has been dissipated and the chimney rubble and adjacent rock have been heated to temperatures of 10 to 200 degrees F.

Cavity and Chimney Formation Upon detonation, the energy of a nuclear explosive is developed in microseconds, vaporizing the adjacent rock, and farther out, melting and crushing the rock. The expanding gases thrust the surrounding rock radially outward, creating, in fractions of a second, a spherical cavity within the earth, filled with vaporized and melted rock (Figure 3). The radius of the cavity is a function of the energy yield of the explosive and, to a lesser extent, the rock characteristics, and the depth of burial. The melted rock that initially lines the walls of the cavity collects in a pool at the bottom of the cavity prior to cavity collapse. Most of the solid radioactive fission products are trapped in this melt, which solidifies into a refractory slag that effectively immobilizes the entrapped radionuclides. After a period of time ranging from seconds to hours, the roof of the cavity collapses, and a cylindrical column (chimney) of broken rock develops upward as the cavity fills with rock falling from the roof. The volume of the cavity is translated into interstitial space between the fallen rock fragments, with a void space at the top.

Post-shot Environment As an example of the environmen that might exist in the shale after a nuclear blast, we can consider the case of a 50 kt shot at the base of a 1,000-foot-thick oil shake section at a depth of 3,000 feet. On the basis of experiments in other types of rock, it is postulated that the effect of this shot would be to create a chimney 230 feet in diameter and 520 feet high. Initially, the interstices of the permeable chimney would be filled with water vapor and other gases from the vaporized and melted oil shale that surrounded the explosive. The lower hemisphere of the cavity would contajn most of the radioactive fission products of the explosion entrapped in the refractory slag, but any vol atile species, those with volatile or gaseous precursors and any species present in organic-metallic compounds would be dispersed throughout the chimney. The hydrogen compounds: water, hydrocarbon gases, and liquid oil, would probably be somewhat radioactive due to the exchange of tritium with hydrogen in the early, high temperature environment of the cavity.

Temperature In deeply buried detonations, 95% of the energy released by the explosion (101 calories per kiloton) remains in the chimney area as residual thermal energy. '17) Initially the bulk of this heat is in the melt, but within a few months the heat is distributed throughout the mass of broken rock by conduction to the rock that has fallen into the melt pool and by refluxing of water and other fluids through the chimncy zone. The net result is that within a few months, the high temperature of the melt

Fragmentation and Fracturing The maximum particle size in the chimney will generally be determined by the joint spacing in the rock. It will grade downward, in a fully unsorted fashion, to sand size grains. The chimney is thus a highly permeable mass of broken and displaced rock surrounded with relatively unbroken rock on all sides as shown in Fig.

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A typical sequence of events when a nuclear explosion is detonated underground. Different geological formations would cause variations in the general outcome.

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1. During the first few micro-seconds the explosion creates a spherical cavity filled with hot gases at extremely high pressures.

2. The high pressure forces the cavity to expand. When the pressure inside the cavity is equal to that of the overburden, expansion ceases.

3. As the cavity cools, some of the gases liquefy

and the molten rock runs to the bottom. Within a few seconds the cavity roof begins to collapse.

4. Falling rock from the roof creates the chimney of broken rock, which is typical of underground explosions. As the chimney rises to a point where the roof becomes self supporting, its growth ceases. Surrounding the chimney is a broad, high fractured area which results from the shock of the nuclear explosion.

Figure 3. Sequence of events in an underground nuclear detonation.

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