ure 4. The void space of the chimney will be about 20 to 30 percent, and the radius of the chimney approximately equal to the cavity radius. Generally the height of the chimney will be four to five times the cavity radius, varying with the rock type.

The bulk permeability of the rock surrounding the chimney is increased through fractures created by the shock wave, and by movement on pre-existing planes of weakness, primarily joints and bedding planes. Measurements made in granite indicate that increases in permeability of up to one darcy occur for horizontal distances of at least 3 cavity radii from the shot point. Above the shot point, fractures would be expected to extend for distances of 6 to 8 cavity radii and below for about 11⁄2 radii, (19)

(18)

Safety and Product Contamination

Safety considerations involved with effects at the time of detonation are:

1. Ground motion produced by the explosion.

2. Possibility of accidental release of the radioactive gases from the explosion to the atmosphere.

3. The possibility of radioactivity from the explosion entering the ground water system.

These "operational" safety considerations are discussed on page 18 and in Appendix D, page 58.

Of lesser concern are potential problems due to radioactivity entering the shale oil and exhaust gases. Radiation from the produced crude oil and the retort gases in the recovery plant may require precautions for worker protection. It is conceivable that exhaust gases from the retorts would contain low levels of gaseous radioactive material and some entrained solids and liquids. Release of such gases would be controlled by use of a monitoring stack comparable to systems routinely used at power reactor plants. Entrained material in the gas system would be removed by filtration.

Laboratory scale experiments are being carried out (Appendix B, page 49) to investigate the possibility of radioactive material being carried through the recovery process and appearing in the shale oil products. These studies will provide a better basis for predicting the behavior of the various radionuclides in the processing cycle. On the basis of preliminary results, it appears possible that contamination of oil could take place through the exchange of tritium (formed by a fusion explosion) with hydrocarbon vapors and water during the blast, and also during the retorting operation. This effect can be minimized by draining all liquids from the chimney, prior to retorting, and disposing of the early runs of produced oil. Any disposal of radioactive wastes would be in compliance with appropriate AEC regulations.

Retorting Chimney

Since 1958 many investigators (20.21.22.23) have considered the feasibility of using nuclear explosives to fracture oil shale to be followed by recovery of the oil from the fractured mass of shale by retorting it in place. Many of the proposals for in place retorting are modifications and extensions of a conventional batch retorting process. In such a process, a combustion zone is initiated at the top of the retort and moved downward through the bed of shale at a predetermined rate. Control of the rate of movement of the zone is achieved by manipulation of the flow rate of recycle gas and air. As the hot gases generated by combustion of some of the organic matter in the shale move downward through the shale bed they heat the shale to retorting temperatures (about 700 F) and carry with them the liquid and gaseous products that are released from the shale. The process

stream leaves the retort near the bottom and is cooled so that the liquid products may be removed from it. Other proposals were modifications of in situ thermal techniques used in recovering petroleum." (2) One technique consists of igniting the oil around an injection well in a reservoir and driving the combustion zone through the reservoir toward producing wells with compressed air, with or without recycle gas. Combustion produces hot gases which force the oil and water to producing wells. Another technique, known as reverse combustion, consists of moving the burning zone through the formation countercurrently to air flow. This technique offers advantages when the oil in the reservoir has a relatively high pour point because the oil passes through the heated portion of the reservoir as it travels to the production well. Other concepts would inject hot gases, either inert or reactive, into the formation.

The success of the in situ retorting following nuclear fracturing will depend largely upon:

1. The degree to which mass permeability can be created by the nuclear explosion.

2. The average size of the shale pieces resulting from collapse of the chimney.

3. Ability to control the combustion front and maintain a uniform rate of advance.

The characteristics of the broken mass of shale produced by the explosion cannot be predicted in detail, but are expected to be similar to those of the rubble which has been produced by explosions in other rock types. Particle size distribution studies have been made for oil shale from mine roof falls (24) and from nuclear chimneys in other rock types. (25) It is expected that most of the pieces in a nuclear chimney in oil shale would be less than 4 feet in maximum dimension. Studies also show that the bulk permeability of the rubble would be higher and that the bulk porosity would be about 20 to 30 percent. (19)

If the average size of the shale pieces in the rubble column tends to be too large to furnish sufficient carbon as fuel for the process, recycle gas or a small part of the oil produced may be used to furnish the additional energy required. (2) An alternate method might be to use an inert gas heated in a surface installation as the heat transfer medium for the recovery operation. Details of the recovery process, will be developed after results achieved by the nuclear explosion have been thoroughly evaluated.

Retorting Fractures

The recovery of oil from the fractured zone surrounding the nuclear chimney as well as from the rubble in the chimney itself will be investigated. The substantial difference in permeability between the chimney and surrounding fractured zones, as well as variations in permeability at different locations within the fractured zone, will be a major factor in selecting conditions for a recovery method; for example, in choosing between hori

zontal and vertical passage of a heating gas. Hence, such choices will have to await detailed evaluation of the results of an experimental shot. It may be possible to modify techniques developed for the fractured zone around a single nuclear chimney to make them suitable for recovering the oil from the fractured zones between chimneys of a multiple-shot operation.

Experimental Investigations

In order to gather information necessary to design the nuclear retort, a series of investigations are being carried out at the Bureau of Mines Petroleum Research Center at Laramie. An experimental aboveground retort, with a capacity of about ten tons of shale, was put into operation in January of 1965 (Figure 5). The retort was designed to study some variables considered important in retorting a nuclear chimney. Results have shown that yields of oil as high as 80% of Fischer Assay can be obtained by retorting mine run oil shale containing pieces as large as 20 inches in two dimensions. Other work indicates that oil-recovery efficiency is a function not only of the operating variables such as air rate, recycle gas rate, retorting temperature, etc., but also of the maximum particle size. Particle size also determines the quantity of residual carbon that is available for fuel for the combustion phase of the process. Air can contact only the carbon at or fairly close to the surface of the shale pieces during the time they are in the combustion zone because of the low permeability of oil shale. The amount of carbon available for combustion, therefore, is determined largely by the surface area or particle size of the shale mass.

Investigations by the BuMines on the strength of retorted oil shale demonstrate that there is a rapid loss of compressive strength on retorting of higher grade shale. To investigate the effect that this would have on a column of broken oil shale under retorting conditions, a dynamic retort was constructed and put in operation in June, 1967. The retort is designed to maintain a constant load in a column of broken shale during retorting. and measure changes in volume of the rock column and its permeability as retorting progresses.

The future commercial development of the oil shale resources in the Piceance Basin will probably require not only recovery from isolated chimneys such as that contemplated for the Bronco experiment, but from a regular development pattern of multiple chimneys to optimize oil recovery and conserve the total resource. (22) Data are necessary from single explosion experiments to refine the technical and economic calculations for these concepts, and allow selection of the optimum process.

One proposed concept envisions the interconnection of a number of collapsed chimneys, forming one large fragmented rubble zone which could be treated as a single retort or plant. It was suggested that several such plants could be created in a consecutive manner with pillars of relatively undisturbed shale between, as shown in Figure 6. A second concept envisions overall development of the resource, such as shown in Figure 7. In this pattern, instead of interconnection of the actual chimneys. the fractured zones surrounding the chimneys would intersect providing flowpaths for fluids over the entire developed area. Both concepts have advantages and disadvantages, the answers to which are largely a matter of conjecture with current technology. Answers can only come with experimental determination of a nuclear explosion environment in oil shale and solution of attendant retort operating problems.

Data to be obtained from the proposed Project Bronco experiment will help to supply answers to such current unknowns as:

1. Cavity characteristics (radius, height, bulking). 2. Fracture characteristics (density, extent, block size and orientation).

3. Chimney rubble size and bulk permeability.

4. Optimum treatment methods and shale oil recovery factors.

5. Operating pressure in both chimney and fracture

zones.

6. Retorting temperatures.

7. Air injection and recycle gas requirements.

8. Effects of possible channeling of fluids.

9. Heat transfer characteristics, and methods.

10. Behavior of hot shale under pressure of the column of broken oil shale.

11. Physical characteristics of spent shale. Appropriate economic factors applied to these data will help to determine the optimum distance between explosions for commercial basin development.

On the basis of information available today, it is not possible to accurately predict the cost of producing shale oil by the nuclear method on a commercial scale. It is possible, however, to postulate a set of reasonable assumptions and calculate profitable recovery of shale oil It is also possible, with another set of equally logical assumptions, to calculate that shale oil recovery is uneconomic in today's market.

There are three primary technical assumptions required in calculating the recovery of shale oil from a nuclear chimney. They are:

1. The size of the chimney and, hence, the volume of oil shale available for subsequent in situ treatment. Explosions in water bearing rocks indicate that the cavity size, and therefore the amount of broken rock in the chimney is related to the water content of the rock. (19.31) It is expected that the kerogen bearing oil shale would behave similarly. On the other hand, experience with massive, relatively elastic rocks suggests that a smaller chimney would be formed.

2. The percent recovery of oil from the broken oil shale in the chimney. The uncertainty is primarily due to the wide range of rubble size contained in the chimney. It is assumed that retorting of the rubble will give an oil recovery of 50 to 70 percent of Fischer Assay.

3. The pressure and air (or gas treatment) rate at which the recovery operations will take place. Questions on the effect of the hydrology of the basin on the nuclear chimney and attendant recovery operations must be answered to define these quantities. The chimney may exist approximately as a tight closed chamber such that low pressure surface retorting conditions may be used. Alternatively, if, the hydrology is such that the chimney treatment must be operated at higher pressure, or at a hydrostatic head of 1.000 psi or above, the total investment and operating costs are substantially increased.