Shale Oil Production

Abstract

A nuclear device is positioned in a shale oil formation and detonated to produce a chimney of fractured oil shale with lean shale at the bottom and rich shale near the top of the chimney and producing shale oil by retorting with a heated gas, preferably at a low temperature for a prolonged period followed with retorting at a higher temperature to yield shale oil.

Description

BACKGROUND OF THE INVENTION

This invention relates to improvements in recovery of oil from subsurface oil shale and similar formations. In accordance with one aspect, this invention relates to an improvement for fracturing an oil shale formation with a nuclear device to form a chimney and retorting of the mass of fractured oil shale in the chimney with a heated gas. In accordance with another aspect, this invention relates to a method for strategically locating a nuclear device in an oil shale formation in order to produce a usable nuclear chimney which can be heated at a lower temperature in order to maintain more permeability without compaction of the shale causing plugging. In accordance with a further aspect, this invention relates to the determining of where to locate a nuclear device in order to produce a nuclear chimney comprising a mass of fractured oil shale with lean shale at the bottom and rich shale near the top of the chimney, followed by producing shale oil from the chimney by heating stepwise at different temperatures.

Despite the widespread occurrence of oil shale throughout much of the world, the large scale recovery of shale oil from such deposits has not been widely practiced. The barriers of geology, technology and economics have heretofore effectively prevented more than token use of this source of oil. Geologically, many of the potentially most productive shales are covered by deep overburdens of earth and rock and, except in a few instances of outcroppings or surface valleys, are inaccessible for commercial recovery. Technologically, oil shale occurs as a relatively compact, impermeable rock which by present practice must be crushed or fractured by mechanical means before oil can be recovered by retorting the fragments; because of this impermeability, in situ retorting of oil shale has not met with success. From the economic standpoint, shale mining by open pit methods involves problems of overburden disposal, transportation to the refinery, crushing and grinding and disposal of spent shale. Similarly, underground mining by gallery techniques and subsequent crushing and heating in special retorts is hardly suitable when considering current day liquid fuel requirements.

Control of the tremendous energy of nuclear devices for peacetime uses has of late become a subject of considerable interest. With the knowledge that such energy in the form of thermal nuclear explosives should be available for a fraction of a mil per kilowatt hour equivalent, numerous applications involving underground explosions have been proposed. Further, it has now been realized that ultrahigh energy explosions can be used in mining operations to break up formations in the oil industry to increase or stimulate productivity by heating or raising the pressure of a reservoir and in landscaping or earth moving techniques such as digging canals, making harbors or removing troublesome obstacles.

The present invention is primarily directed to the production of oil utilizing an underground explosion chamber in a bituminous deposit suitable for the explosion of a high energy explosive charge. A nuclear explosion within a bed of shale deep in the earth produces a huge chimney containing a mass of shale rubble which has high permeability and is amenable to production by contacting the shale with hot gases. The nuclear chimney may have a diameter of 600 feet and a height of about 1,400 feet. In heating oil shale with hot gases, one of the problems encountered is that of plastic flow which greatly reduces or completely eliminates permeability, thereby hindering or terminating the pyrolysis operation. This invention is concerned with the strategic location of a nuclear device prior to detonation to form a chimney with lean shale near the bottom and rich shale near the top and subsequent heating with hot gases with the reduction or prevention of plastic flow during the heating of the oil shale, with such gases.

Accordingly, it is an object of this invention to provide a method for producing a usable nuclear chimney in a bituminous formation.

Another object of this invention is to provide a process for producing oil from an oil shale by pyrolysis with hot gases which avoids or substantially diminishes plastic flow of the shale.

Another object is to provide a process for producing oil from shale rubble in a nuclear chimney by effecting pyrolysis with hot gas while avoiding substantial plastic flow of the shale.

A further object of this invention is to provide a process for heating a nuclear chimney at a lower temperature in order to maintain more permeability without compaction of the shale causing plugging.

Other objects and aspects as well as the several advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure and the appended claims.

SUMMARY OF THE INVENTION

In accordance with a broad aspect of the invention, a nuclear device is located in an oil shale formation in such a manner that upon detonation of the nuclear device a nuclear chimney comprising a mass of fractured oil shale is produced wherein the crumbled shale is lean shale at the bottom and rich near the top of the chimney.

In accordance with another embodiment of the invention, a mass of fractured or broken oil shale in a nuclear chimney having lean shale at the bottom and rich shale near the top is produced by being preheated with a heating gas at a temperature not in excess of 650.degree. F. for a prolonged period of time so as to maintain permeability of the shale without compaction of the shale and causing plugging, followed by heating at a retorting temperature in excess of 700.degree. F.

In accordance with a preferred embodiment of the invention, the nuclear chimney comprising a mass of fractured oil shale with lean shale at the bottom and rich shale near the top of the chimney is produced by passing a heating gas through the mass of fractured oil shale at a temperature so as to preheat the shale at a temperature in the range of 500.degree.-575.degree. F. for a period of time of at least 30 days, and then increasing the temperature of the fractured shale at a rate of 1/2.degree. to 2.degree. F. per day to a temperature in the range of 600.degree.-650.degree. F., and continuing the heating within this range for a period of 3 to 70 days and then rapidly heating the formation to a temperature of 700.degree.-800.degree. F., the final retorting temperature to produce the oil shale.

In accordance with another preferred embodiment, the heating gas is introduced into the top of the chimney and the oil is received principally from the bottom thereof.

The heating gas is preferably combustion gases. The heating can, however, be carried out also with hot inert gases and/or hot recycle produced gases, depending upon availability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 graphically illustrates shale richness distribution as a function of formation depth in an oil shale formation treated according to the present invention; and

FIG. 2 graphically shows the temperature history of a fragmented oil shale bed treated in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, one of the major technical operability uncertainties regarding in situ retorting of oil shale contained in a nuclear chimney is shale compaction during heating and the resulting reduction in permeability. In accordance to the invention, this reduction of permeability is controlled by selection of the chimney location so that the bottom of the fragmented shale is lean and the thick sections of the rich shale are near the top, and producing of the shale oil is preferably effected by prolonged heating of the shale at temperatures below the rapid retorting temperature and then subsequent recovery of the remaining oil by heating to a higher temperature.

Bituminous deposits containing oil shale can be produced in accordance with the method of the present invention. The process is suitable for rock formations known as "oil shale" which contain a combination of organic and inorganic sediments which have become hardened into impermeable rock. Suitable shales have a compressive strength in the range of 5,000 to 30,000 p.s.i. The organic portion laid down in layers is a solid amorphous material generally known as kerogen which can be converted to oil under the application of heat. The oil recovered is a black, viscous, waxy substance which will not flow below about 85.degree. or 90.degree. F.

The method of the present invention is employed with bituminous deposits lying in the range of from 100 to 20,000 feet below the surface of the earth. The minimum ground cover required is that necessary to insure complete containment of the explosion. This depends upon the energy yield of the explosive utilized. For nuclear devices, the minimum depth in feet is approximately equal to in the range of 250-450 times the cube root of the size of the device in kilotons. Thus, the explosion from a 1-kiloton nuclear bomb is completely contained if the device is exploded 250-450 feet below the nearest surface point. The maximum depth is limited only by the economic considerations involved in penetrating very deep-lying formations with conventional drilling equipment.

The method of the present invention is carried out utilizing a thermal nuclear device such as the hydrogen or atomic bomb. Suitable thermal nuclear devices are now available for underground explosion; therefore, it is to be understood that the present discovery involves merely the use of a nuclear device in a novel and useful method of exploding oil deposits and that the fabrication and manufacture of hydrogen and atomic bombs form no part of this invention.

To employ the invention, it is preferable to drill an access well into the formation and then performing a geological survey of the oil shale along the length of said access well to determine the location of a rich shale section and an underlying lean shale section. In the Green River shale in the Piceance Creek basin there is excellent lateral continuity of the shale strata therefore some definition of the shale beds already exists. In addition the richness of the shale can be obtained by two methods, (a) core drilling of the entire shale section and analysis of the recovered core, and (b) logging of the drilled hole.

Upon determining location of a rich shale section and an underlying lean shale section, a nuclear explosive device is then placed into the formation in such a manner that upon detonation a nuclear chimney is produced with lean shale at the bottom and rich shale near the top of the chimney.

In a specific embodiment of the process, oil is retorted from a shale seam which is 900 feet in thickness and carries an overburden of 1,650 feet of rock. The shale seam contains an average of 27.3 gallons of oil per ton (Fischer assay) in the top half of the seam and 23.1 gallons of oil per ton (Fischer assay) in the bottom half of the seam. A 100 kiloton nuclear device is disposed in a well near the lower innerface of the shale seam and the underlying rock. Upon detonation of the device, a chimney having a radius of 180 feet and 900 feet height is formed. The crumbled shale formation is provided with inlet means for introducing hot retorting gas into the upper portion of the cavity and recovering means for recovery from the bottom of the mass of crumbled shale and bringing to the earth's surface gases and liquids. An inert gas such as nitrogen or combustion gases or hot recycle produced gases are introduced through the inlet means into the cavity and heat the formation as described above to produce shale oil. Gases and condensed liquids are withdrawn and transported to the surface via the recovery means.

The mass of fractured or broken oil shale ordinarily will have a Fischer assay value in the range of 15-50 gallons per ton.

The fractured shale in a preferred embodiment is first preheated to a temperature of 500.degree.-575.degree. F. for a period of time of at least 30 days. Generally the heating will be continued at this temperature for a period of time ranging from 100 to 1,000 days.

The temperature of the preheated fractured shale is then preferably increased to a temperature in the range of 600.degree.-650.degree. F. at a rate of 1/2 to 2.degree. F. per day, and the heating in this temperature range is continued for a period of 3 to 70 days. Maintaining the shale temperature in these low temperature ranges hardens the shale, retaining more of the fragmented shale bed permeability.

After the formation is heated to 600.degree.-650.degree. F. for a prolonged period of time, it is heated rapidly to 700.degree.-800.degree. F., the final retorting temperature. The heating during the final retorting temperature is continued generally until the formation is substantially depleted of shale oil.

In heating the oil shale rubble with hot gases, it is feasible to utilize combustion gases, inert gases and/or hot recycle produced gases. It is preferred to inject a hot gas into the top of the chimney and move the heat front downwardly through the rubble.

EXAMPLE

In order to illustrate the operation of the invention, the effective permeability at 800.degree. F. has been calculated for four specific cases. These calculations were performed using data which are summarized in table 1. The average particle size in the 900-foot high nuclear chimney used in each case was assumed to be one foot. In all four cases the shale represented in FIG. 1 was used. The four cases calculated are stated below.

Case I--A fragmented shale column 900 feet high located between 1,400 and 2,300 feet below the surface in the well shown in FIG. 1. FIG. 1 shows the position of the shale column as position number 1. The shale richness distribution is also shown in FIG. 1. This fragmented shale column is heated to 800.degree. F. by the injection of hot gases into the top of the chimney to recover the shale oil. The resulting retorted fragmented shale bed permeability is estimated to be 30 Darcy.

II-- A fragmented shale column 900 feet high located between 1,650 and 2,550 feet below the surface in the well shown in FIG. 1. FIG. 1 shows this position of the shale column as position number 2. Again the shale richness distribution is shown in FIG. 1. This fragmented shale column is heated to 800.degree. F. by the injection of hot gases into the top of the chimney to recover the shale oil. The resulting permeability of the retorted fragmented shale bed is estimated to be 120 Darcy.

Case III--This case is identical to case I except that the shale is heated to 600.degree. F. for 350 hours before the shale temperature is increased to 800.degree. F. This low temperature heating results in a permeability of the retorted fragmented shale bed of 150 Darcy.

Case IV--This case is identical to case II except that the shale is heated to 600.degree. F. for 350 hours before the shale temperature is increased to 800.degree. F. This low temperature heating results in a permeability of the retorted fragmented shale bed of 540 Darcy.

It can be seen that by placing the fragmented shale column in such a position that the richer shale sections are nearer the top (case II compared with case I) that the permeability of the retorted shale bed was increased from 30 to 120 Darcy. It can also be seen that by preheating the shale to 600.degree. F. for 350 hours the retorted shale bed permeability was increased from 30 to 150 Darcy (comparison of case III with case I) and from 120 to 540 Darcy (comparison of case IV to case II) for chimney positions numbers 1 and 2, respectively.

It should especially be noted that a combination of chimney placement and preheating increased the retorted shale bed permeability from 30 to 540 Darcy (comparison of case IV to case I).

Although in the above example the shale was preheated at 600.degree. F. to increase the retorted shale bed permeability, other preheat temperatures and other times can be used. The low temperature (500.degree. to 650.degree. F.) history of the shale is a determining factor in maintaining a high bed permeability. There are several methods that can be used to heat the shale to a low temperature and then continue the retorting at a temperature in excess of 700.degree. and in general about 800.degree. F. One method which uses the injection of hot inert gases and recycle produced gases is outlined below:

A typical operation to achieve the required low temperature heating of the shale would be to inject hot gases into the top of the nuclear chimney and withdraw the gases and generated oil from the chimney bottom. The temperature of the injected gases is increased over a several-day span to a temperature in the 500.degree.-575.degree. F. range. Thereafter, the temperature is increased very slowly (perhaps, for example, 1/2.degree. to 2.degree. F. per day) to a temperature in the range of 600.degree.-650.degree. F. Then the temperature is raised relatively rapidly to the final retorting temperature of at least 700.degree. and generally to about 800.degree. F. This temperature history is represented graphically in FIG. 2. The result of a temperature history such as the above is that the shale throughout the chimney is subjected to a low temperature history without the necessity of an accurate knowledge of the magnitude of the heat losses to the chimney wall. As the heat is carried down the chimney by the injected and created gases, the shale will be heated at a lower rate, thus subjecting the lower shales to a longer effective low temperature history.

It appears that the critical temperature span for subjecting the shale to an effective low temperature history is from about 500.degree. to 650.degree. F. The data in table I illustrate that prolonged low temperature heating of the shale at 650.degree. F. had only a minor influence upon the retorted shale bed permeability. The time required to achieve an appreciable permeability benefit at a temperature of 500.degree. F. would be too long for practical application; therefore, temperatures below 500.degree. F. are thus excluded. Indeed, the critical temperature range is probably within 550.degree. to 625.degree. F. However, in practice the shale could be heated slowly over a broader temperature range (such as 500.degree. to 650.degree. F.) to insure a sufficient low temperature history even in the presence of natural variations in heating rates due to the heat lost to the chimney wall and variations in the gas flow due to variations in the shale bed permeability. ##SPC1##

By a comparison of runs 1 and 5, 12 and 13, it can be seen that prolonged heating at 600.degree. F. (a temperature below the rapid pyrolysis temperature range) is effective in maintaining a higher permeability. A comparison of runs 12 and 14 shows that a temperature of 650.degree. F. had little effect upon the final permeability at 800.degree. F. Heating preferably is achieved by the use of hot inert gases and the use of recycle produced gases.

Claims

I claim:

1. An improved process of recovering shale oil from a subsurface oil shale formation containing hydrocarbons not naturally flowable into a well bore traversing said formation with a nuclear explosive device which comprises:

a. forming a nuclear chimney in said formation by detonating a strategically located nuclear explosive device so that the nuclear chimney formed following detonation contains a mass of fractured and broken oil shale with lean shale at the bottom of the chimney and rich shale near the top of the chimney,

b. passing a heated gas through said mass of fractured and broken oil shale at a temperature such that said mass is preheated to a temperature not in excess of 650.degree. F. and maintaining the heating of said mass for a prolonged period of time of at least 30 days sufficient to bake the oil shale to reduce or substantially prevent plastic flow during heating of the oil shale and thereby maintain permeability of the shale without compaction of the shale causing plugging,

c. retorting said preheated fractured shale by elevating the temperature of same to above 700.degree. F., the retort temperature of said fractured shale, so as to pyrolyze and produce oil therefrom, and

d. recovering produced oil from said formation.

2. A process according to claim 1 for forming said nuclear chimney in step (a) which comprises the additional steps of:

1. drilling an access well into the formation,

2. performing a geological survey of the oil shale along the length of said access well to determine the location of a rich shale section and an underlying lean shale section,

3. Disposing a nuclear device in said formation and positioning same in said formation so that upon subsequent detonation of said device the fragmented shale forming the nuclear produced chimney is lean at the bottom and the thick sections of rich shale are near the top of the chimney, and

4. detonating the nuclear device to create a cavity in said formation, which cavity at least partially fills with collapsing oil shale to form said chimney of fractured and crumbled shale with lean shale at the bottom and rich shale near the top of said chimney.

3. A process according to claim 1 wherein

1. said fractured shale is produced by first preheating same to a temperature of 500.degree.-575.degree. F. for a period of time of at least 30 days,

2. the temperature of the preheated fractured shale is increased to 600.degree.-650.degree. F. at a rate of 1/2.degree. to 2.degree. F. per day and said heating is continued within the latter temperature range for 3 to 70 days, and

3. said preheated fractured shale is then rapidly heated to 700.degree.-800.degree. F., the final retorting temperature, and continued within this temperature range until said fractured shale is essentially produced.

4. A process according to claim 1 wherein said heating gas is introduced into the top of the chimney and said oil is recovered principally from the bottom thereof.

5. A process according to claim 1 wherein said gas comprises essentially combustion gases.