US3316020A

US3316020A - In situ retorting method employed in oil shale

Info

Publication number: US3316020A
Application number: US412951A
Authority: US
Inventors: Eric V Bergstrom
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1964-11-23
Filing date: 1964-11-23
Publication date: 1967-04-25
Anticipated expiration: 1984-04-25

Description

April 25, 1967 E. v. BERGSTROM 3,316,020

IN SITU RETORTING METHOD EMPLOYED IN OIL SHALE Filed Nov. 23, 1964 2 Sheets-Sheet 1 ERIC wV. BER G S TROM INVENTOR.

BY WW ATTORNEY April 25, 1967 E. v. BERGSTROM 2 Sheets-Sheet 2- Filed NOV. 23, 1964 3 F 4 4 4 R i a 5 2M WW m M O R T S G R E B V C Du E INVENTOR.

BY LMX ATTORNEY United States Patent C) 3,316,020 IN SITU RETORTING METHOD EMPLOYED IN OIL SHALE Eric V. Bergstrom, Byram, Conn., assignor to Mobil Gil Corporation, a corporation of New York Filed Nov. 23, 1964, 821'. No. 412,951 14 Claims. (Cl. 299-22) This invention relates to a method for the in situ retorting of oil shale and the recovering of the resultant shale oil products. More particularly, it relates to such method practiced on an in place fragmentized oil shale.

One method for recovering shale oil products from oil shales employs surface retorts of suitable design for retorting oil shale. The oil shale is mined, usually from only the richest stratum, from subterranean deposits, crushed to a proper size for retorting, and transported to the surface. Normally, about percent of the mined oil shale is discarded as having a size not suitable for retorting. In the conventional mining procedure, known as the room and pillar method, extraction of oil shale from the earth at the lowest cost requires the use of relatively large mechanical devices with rooms and other openings correspondingly sized. In one such room, it was observed that a portion of the overlying oil shale parted from the roof and fell downwardly in the form of a fragmentized oil shale or debris into the room. This roof failure in conventional mining procedures can be very undesirable. As one result of this roof failure, a natural arch had formed over the room excavated in the oil shale and the ends of the arch were supported by solid oil shale on each side of the room.

Subsequent investigation of oil a superimposed roof of oil shale shows that when shale over an excavation is subjected to tensile stress too great for its strength as a beam, it breaks and falls. The beam stress is transferred to the arch spanning the excavation. The break of the superimposed roof strata is always approximately vertical regardless of the dip of the bedding plane and does not occur above solid oil shale, i.e., beyond the vertical plane extending from the edge of the excavation. As a result of such roof failure, an arch of natural equilibrium develops in the oil shale over the excavation and the pressures of the superimposed strata are transmitted via the arch to the solid shale surrounding the excavation which supports its ends. Extension horizontally of the excavation over which the superimposed roof of rigid strata resides causes successive falls of the superimposed strata until equilibrium in an arch again is formed. Successive superimposed roofs of strata will break and form similar new arches when the end supports of the subtended arch ar disrupted. Successive roof falls of this type can be made to occur until the void provided by the excavation is filled with debris created in forming each arch of natural equilibrium in the superimposed roof of strata since there is a volu- -metric increase in the debris, or the roof falls break through to the earths surface.

The present invention employs the roof failure effects heretofore found so undesirable in certain conventional mining techniques in a method for retorting in situ fragmentized oil shale. More particularly, such fragmentized oil shale is produced as the debris from one or more roof failures deliberately caused by providing an excavation extending horizontally over an area greater than the superimposed roof of strata can span and extending vertically to provide a void for receiving debris from at least one roof failure. arch spanning the excavation and supported at its ends on solid oil shale each side thereof with the resulting debris from the roof failure received in and preferably substantially filling the void provided by the excavation.

The horizontal and vertical extent of the debris may be adjusted as desired. Thereafter, shale oil products are evolved by introducing a cornbustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products and recovering the shale oil products from the debris.

It is therefore an object of the present invention to provide a method for retorting in situ fragmentized oil shale and recovering the resultant shale oil products by employing roof failure effects. Another object is to provide oil shale in a fragmentized form and amount suitable for in situ retorting with a minimum of mining costs. Another object is to provide fragmentized oil shale with sulficient permeability to air to permit efficient retorting by in situ combustion and recovering of the resultant shale oil products. Another object is to provide fragmentizecl oil shale in which the amount of oil shale extracted to produce an excavation capable of producing roof failure is about that amount of oil shale discarded in conventional mining and retorting procedures heretofore practiced.

These and other objects will be more apparent when considered with the following detailed description of the present invention, the attached drawings, and the appended claims. The drawings illustrate oil shale structures facilitating the practice of the method of this invention.

In the drawings like elements bear like reference numerals in all views and wherein:

FIGURE 1 is a diagrammatic horizontal section, of an oil shale initial steps of the present method;

FIGURE 2 is a vertical section taken along line 2-2 of FIGURE 1;

FIGURE 3 is a partial horizontal section taken along line 3-3 of FIGURE 2;

FIGURE 4 is a vertical section taken along line 4-4 of FIGURE 3 after the practice of additional steps in the structure shown in FIGURE 2; and

FIGURE 5 shows the structure of FIGURE 2 after practice of the basic steps of the present method.

Referring now to FIGURE 1, there is shown an oil, shale deposit 10 in which may be practiced the present method for the recovery of shale oil products. The oil shale deposit 10 maybe subdivided for convenience into operational divisions 11 through 26 separated from one another by barrier walls of oil shale. The divisions may be of any suitable size and configuration. For example, assuming the deposit 10 to have an area of 1 square mile in the horizontal and having a suitable thickness, then each plan view, partially in deposit after practice of of the divisions may be about of a square mile in area with a square configuration. If desired, the divisions may be processed in any order but preferably in progression serially through the steps of this method. An access entrance, such as shaft 28, is provided from the earths surface into the deposit 10. Radiating horizontally from the shaft 28 may be one or

more galleries

29, 30, 31, and 32. Drifts may extend from the galleries for access to each of the divisions 11 through 26, such as drift 33 extending from the gallery 29 into

divisions

12 and 15. This arrangement of openings provides access. to within all the areas of the deposit 10 wherein desired excavations are to be made for creating roof failures.

The induced roof failure occurs forming an More particularly, referring to the division -11, a plurality of parallel passageways 34 through 40, as shown in FIGURE 2, are extended in the horizontal from the gallery 29. The passageways preferably are formed below the richest stratum of oil shale. For example, these passageways may be formed about 350 feet below the earths surface in massive oil shale where the host rock may contain only 15 barrels of shale oil products per ton of rock. The passageways 34 through 40 do not extend into the adjacent divisions .12 and 15 but terminate at'

arrier walls

41, 42, 43, and 44 which contain fluids withl the division 11 and for other purposes to be apparent ereinafter. The barrier walls 41 to 44 may be of any uitable thickness, for example 150 feet. The remaining ivisions may be provided with similar passageways exending horizontally from the remaining galleries and rifts.

Referring to FIGURE 2, a plurality of boreholes are trovided, extending vertically into the oil shale deposit .0 from the earths surface along the horizontal perimeer of the division 11, two of which are shown and desigrated by the

numerals

45 and 46. The passageways 34 hrough 40 are separated by intervening walls 48 to 53 and paced from one another a distance greater than the :uperimposed roof of strata can span in the deposit 1%) vithout an intervening wall in place between the passagevays. For example, the passageways may be feet wide and feet high at 80-foot centers with intervening Walls of 60 foot thickness, or if desired, twice as many passageways may be mined out at 40-foot centers with BO-fQot-thick intervening walls. The mining of these passageways may be by any suitable mining technique. The volume of the oil shale removed in forming these passageways should be such that a portion may be horizontally displaced throughout each of the intervening walls into the adjacent passageways. Removing a'portion throughout the intervening walls between passage ways produces an excavation extending horizontally between passageways. As a result, the span of the superimposed roof of strata is greater than can be sustained so that roof failure occurs forming a natural arch of equilibrium. This arch spans the excavation and is supported at its ends on each side of the excavation. The excavation also extends vertically to provide a void (from the passageways and displaced portions) for receiving the debris from at least one roof failure. Generally, the material removed in mining using the passageways 34 through 40 will not be more than 5 percent of the total oil shale converted into debris by the subsequent roof failures. This use of the passageway is an advantage in that the amount of oil shale to be removed by mining can be in the same range as the fines discarded in conventional mining techniques providing oil shale for surface retorting.

More particularly by way of example, the portions displaced from the intervening walls 48 to 53 are likened to slots extending horizontally therethrough. Slots can be provided between adjacent passageways by any suitable method. A preferred method will be described in reference to FIGURE 3. From adjacent passageways, a plurality of parallel crossopenings 54 are provided into each intervening wall. These crossopenings 54 need not extend completely through each intervening wall but preferably do so. Any suitable means for providing the crossopenings 54 may be utilized. For example, a conventional rock drill may be utilized to provide crossopenings 54 in the form of shot holes into the intervening walls 48 to 53 between adjacent passageways. The crossopenings 54 are spaced in the intervening wall a suitable distance that when packed with explosives with subsequent detonation, a portion throughout each intervening wall will be horizontally displaced into the adjacent passageways.

Preferably, the crossopenings 54 are disposed in the horizontal at an angle other than normal to the passageways 34 to 40. Preferably, this angle is 45' so that detonation of exposives within the crossopenings 54 throws the surrounding rock horizontally and about equal in amount into the adjacent passageways. Any exposive may be used. The type and the amount of explosive to be used are selected to produce a sufficient force to rupture the intervening wall about the crossopenings 54 and displace a portion therethrough into the adjacent passageways leaving an opening resembling a slot. As is well known, judicious placing of the explosive along the length of each crossopening 54 may be practiced to assert more or less exposive force locally in the intervening wall as is desired.

Preferably, the explosive amounts are adjusted in the crossopenings 54 so that upon detonation, as can best be seen in FIGURE 4, the disrupted portion forms a chevronshaped opening which advantageously increases the vertical shear upon falling strata. For example, such opening may be 2 feet high at the center and 5 feet high adjacent each passageway. Upon detonating the exposive charges disposed in the crossopenings 54, in the intervening

walls

50 and 51 between

adjacent passageways

36, 37, and 38, there is provided an excavation extending horizontally over an area which for descriptive purposes is greater than the superimposed roof of strata can span whereby roof failure occurs.

Referring to FIGURE 2, the detonation of the explosive charges in the crossopenings 54 provides openings through the intervening

walls

50 and 51 interconnecting the

passageways

36, 37, and 38. The created roof failure produces an arch 56 as is shown by a chain line centrally in the figure. The debris resulting from the roof failure is received in the void provided by the excavation extending horizontally between the

passageways

36 and 38. The excavation can be extended horizontally to the remaining passageways 3'5 and 39 and to

passageways

34 and 40 through the intervening

walls

49 and 52 and 48 and 53 in the same

manner forming arches

57 and 58 until the excavation covers the desired horizontal area in the division 11 of oil shale. At-this point, the superimposed roof of oil shale reaches equilibrium in the arch 58 with the debris from the roof falls received therebelow in the void provided by the excavation extending between

passageways

34 and 40. The arch 58 is supported at its end on solid oil shale each side of the excavation.

Whatever the horizontal extent of the arch 58, the excavation extending between

passageways

34 and 40 must also extend vertically to provide a void for receiving the debris from the roof failures. Also, if the arches are to be extended upwardly, then the void must accommodate the debris created by the upward extensions of such arches. It is to be noted that the debris created upon roof failure undergoes a volumetric increase over the volume of solid oil shale from which it is formed. Thus, the void provided by the excavation must there-fore receive at least this volumetric increase in the debris formed by at least one roof failure.

The arch 58 may be formed without the debris from the previous roof failures filling the void provided by the excavation extending between

passageways

34 and 40. Thus, an open space may reside beneath the last-formed arch. The injection of fluids for retorting the resultant debris would tend to by-pass at least in part portions of the debris via such open space. To reduce the effects of this undesired space and for other reasons, the arch 58 spanning the excavation is extended upwardly as

arches

62, 63, and 64 in vertical alignment with the horizontal perimeter of the excavation until a resulting debris 61, as shown in FIGURE 5, substantially fills the void provided by the excavation. Any convenient means for obtaining this result can be used.

One preferred means is by disrupting the end supports of each arch at successively lesser depths in the oil shale deposits 10 and about a horizontal boundary in vertical alignment with the horizontal perimeter of the excavation until the resultant debris substantially fills all of the mentioned void. Any suitable means for destroying the arch end supports according to the stated arrangements may be used. Preferably, the described boreholes, including

boreholes

45 and 46, are used for these purposes. Explosive charges are disposed at successively lesser depths or levels in these boreholes. These charges are detonated to cause roof failure producing

new arches

62, 63, and 64 of natural equilibrium at each level.

Preferably, the explosive charges are disposed vertically from one another at suitable spacing that the resulting debris has a desired thickness and vertical breakage. For example, oil shale naturally contains fractures commonly oriented in vertical planes at a spacing within 6 to 10 feet. The explosive charges may be placed at successive levels spaced at about the same dimensions as the desired thickness for the debris 61. Thus, the vertical spacings at which the arch end supports are disrupted are expected to determine the thickness of the debris 61; whereas, the length of this debris will be less usually than the vertical fracture spacing since additional vertical fractures are formed upon the roof failure in each piece of oil shale which falls. Preferably, by these arrangements the debris 61 has rough cubical dimensions of about one foot on each side. Obviously other arrangements of these dimensions can be used.

The explosive charges within the boreholes are preferably detonated simultaneously at each level to cause roof failure and successively upwardly from one level to the next to establish successive arches 62, '63, and 64 until all of the void is filled with the resultant debris 61.

With the formation of the arch 64, the oil shale deposit 10 has a structure shown in FIGURE 5. The void provided by the excavation is filled with the debris 61 in substantially full containment. The debris 61 is, of course, oil shale which has an increased volume upon roof failure determined by the vertical dimension of the excavations void. The debris 61 generally needs to be about 2 to percent greater in volume than originally. With such increases in volume the debris 61 has suitable fractures which provide a permeability to gases suflicient for in situ retorting by any in situ combustion or heating procedure.

Any suitable structure may retorting the debris be used for purposes of 61 and recovering the resultant shale oil products. For example,

conduits

66 and 67 may be provided in sealed relationship within the

boreholes

45 and 46 to extend into the uppermost and lowermost portions and at the opposite horizontal extremities of the debris '61, respectively. A source 68 of combustionsupporting fluid is connected to the conduit 66. The conduit 67 preferably extends to the lower extremities of the former passageway 40 and is connected at the surface to means for recovering shale oil products. Such means may include a pump 69 for passing the shale oil products to an oil-gas separator 71 from which the liquid portions of the shale oil products are passed to suitable surface storage such as oil tank 72. The gaseous portion of the shale oil products is taken from the gas-oil separator 71 through a conduit 73 and thence conveyed to any suitable utilization. The gallery 29 may need to be sealed if undesired losses of shale oil products therethrough occur.

' More particularly, a combustion-supporting fluid, such as air, mixtures of air and combustible materials, and inert diluents and mixtures thereof, are transmitted from the source 68 via the conduit 66 into one extremity of the debris 61. At this time the conditions 'within the debris 61, including the rate of injection of the combustion-supporting fluid and the ignition of the debris adjacent the entrance point of such fluid, are arranged for in situ combustion of the fragmented oil shale. It will be apparent that the heating of the fragmentized oil shale from the resulting in situ combustion is continued for a suitable period to evolve shale oil products. This procedure is then continued until the debris 61 has been retorted to substantial completeness. Simultaneously or separately, the shale oil products are recovered from the other extremity of the debris 61. For example, the shale oil products will collect in the former passage 40 from whence they are recovered by the previously described recovery"rnearis. It 'is to be understood that a though a very simple procedure for evolving and recovering shale oil products has been described, in actual practice variations of the described procedure can and will be made to conform with any particular situation encountered. Also, other means for retorting the debris to evolve or recover shale oil products may be utilized.

It may be necessary in certain instances to provide an impervious seal over the earths surface above the debris 61 during the injection of the combustion-supporting fluid and the recovery of the shale oil products. One reason for such seal is the possible undesired loss of fluids to the surface of the earth by openings created or enlarged during the formation of the arch 64. Any type of seal may be used. For example, the seal may be provided by coating the ear-ths surface with asphalt mixed with a soil or other earthen substances. Other types of openings through which undesired loss of fluids can occur may be likewise sealed against fluid flow.

From the foregoing it will be apparent that there has been provided a method for retorting oil shale in situ Well suited to satisfy all of the stated objects of this invention. Various changes and alternatives may be made by those skilled in the art to the present method without departing from the spirit of the invention. It is intended that deviations of such nature from the present disclosure be included within the scope of the appended claims and that these claims recite the only limitations to be applied to the present invention.

What is claimed is:

1. A method for retorting oil shale in situ, comprising the steps:

(a) providing in the horizontal parallel passageways with an intervening wall between adjacent passageways in the oil shale at a spacing between passageways greater than the superimposed roof of strata can span without an intervening wall in place between said passageway,

(b) displacing horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving the volumetric increase of debris created upon at least one roof failure whereby roof failure occurs forming in the superimposed roof of strata an arch spanning the excavation and supported at its ends on solid oil shale each. side of the excavation with the resulting debris received in the void provided by the excavation, and

(c) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

2. A method for retorting oil shale in situ, comprising the steps:

(a) providing in the horizontal parallel passageways with an intervening wall between adjacent passageways in the oil shale at a spacing between passageways greater than the superimposed roof of strata can span without an intervening wall in place between said passageways,

(b) displacing horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming in the superimposed roof of strata an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the resulting debris received in the void provided by the excavation,

(c) extending the arch spanning the excavation upwardly in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

(d) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

3. A method for retorting oil shale in situ, comprisng the steps:

(b) displacing horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the resulting debris received in the void provided by the excavation,

(c) disrupting the end supports of each arch causing roof failure at lesser depths in the oil shale and in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

4. A method for retorting oil shale in situ, comprising the steps:

(c) placing explosive charges at the end support of each arch at various levels in the oil shale and in 'vertical alignment with the horizontal perimeter of the excavation,

(d) detonating the explosive charges at each of the levels causing roof failure until the resulting debris substantially fills the void provided by the excavation, and

(e) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

5. A method for retorting oil shale in situ, comprising the steps:

(b) driving a plurality of parallel crossopenings into each intervening wall between adjacent passageways,

(c) placing explosive charges in the crossopenings in each intervening wall,

(d) detonating the explosive charges to displace horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving the volumetric increase of debris created upon at least one roof failure whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the resulting debric received in the void provided by the excavation, and

6. The method of claim 5 wherein the crossopenings are disposed horizontally at an angle less than degrees to the passageways.

7. The method of claim 5 wherein the crossopenings are disposed horizontally at an angle of about 45 degrees relative to the passageways.

8. A method for retorting oil shale in situ, comprising the steps:

(c) placing explosive charges in the crossopenings in each intervening wall,

((1) detonating the explosive charges to displace horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and sup ported at its ends on solid oil shale each side of the excavation with the resulting debris received in the void provided by the excavation,

(e) extending the arch spanning the excavation upwardly in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

(f) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

9. A method for retorting oil shale in situ, comprising the steps: 7

(a) providing in the horizontal parallel passageways with an intervening wall between adjacent passageways in the oil shale at a horizontal spacing between passageways greater than the superimposed roof of strata can span without an intervening wall in place between said passageways,

(c) placing explosive charges in the crossopenings 1n each intervening wall,

(d) detonating the explosive charges to displace horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the resulting debris received in the void provided by the excavation,

(e) disrupting the end supports of each arch causing roof failure at various lesser depths in the oil shale in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

10. A method for retorting oil shale in situ, comprising i the steps:

(c) placing explosive charges in the crossopenings in each intervening wall,

(e) placing explosive -charges at the end supports of each arch at various levels in the oil shale and in vertical alignment with the horizontal perimeter of the excavation,

(f) detonating the explosive charges at each of the levels causing roof failure until the resulting debris substantially fills the void provided by the excavation, and

(g) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

11. A method for retorting oil shale in situ, comprising the steps:

(a) providing in the horizontal parallel passageways with an intervening wall between adjacent passageways in the oil shale at a spacing between passageways .greater than the superimposed roof of strata can span Without an intervening wall in place between said passageways,

(b) displacing horizontally a portion throughout each intervening wall into the adjacent passageways to provide an excavation extending horizontally between passageways and vertically to produce a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the debris received in the void provided by the excavation,

(c) repeating the preceding steps (a) and (b) until the excavation extends over a desired horizontal area in the oil shale,

(d) extending the arch spanning the excavation upwardly in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

(e) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions effecting in situ combustion to evolve shale oi-l products, and recovering the shale oil products.

12. A method for retorting oil shale in situ, comprising the steps:

(a) providing in the horizontal parallel passageways with intervening walls between adjacent passageways in the oil shale at a spacing between passageways greater than the superimposed roof of strata can span without an intervening wall in place between said passageways,

(b) displacing horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the debris received in the void provided by the excavation,

(c) repeating the preceding steps (a) and b) until the excavation extends over a desired horizontal area in the oil shale,

(d) disrupting the end supports of each arch causing roof failure at 'lesser depths in the oil shale and in vertical alignment with the horizontal perimeter of the excavation until the resulting debris substantially fills the void provided by the excavation, and

(e) introducing a combustion-supporting fluid into the resulting debris in the oil shale under conditions eifecting in situ combustion to evolve shale oil products, and recovering the shale oil products.

13. A method for retorting oil shale in situ, comprising the steps:

(b displacing horizontally a portion throughout each intervening wall into the adjacent passageways to produce an excavation extending horizontally between passageways and vertically to provide a void for receiving debris from subsequent roof failures whereby roof failure occurs forming an arch spanning the excavation and supported at its ends on solid oil shale each side of the excavation with the debris received in the void provided by the excavation,

(d) placing explosive charges at the end supports of each arch at various levels in the oil shale and in vertical alignment with the horizontal perimeter of the excavation,

(e) detonating the explosive charges at each of the levels causing roof failure until the resulting debris substantially fills the void provided by the excavation, and

14. The method of claim 13 wherein the explosive charges are detonated at each level in upward succession.

References Cited by the Examiner UNITED STATES PATENTS ERNEST R. PURSER, Primary Examiner.

Claims

1. A METHOD FOR RETORTING OIL SHALE IN SITU, COMPRISING THE STEPS: (A) PROVIDING IN THE HORIZONTAL PARALLEL PASSAGEWAYS WITH AN INTERVENING WALL BETWEEN ADJACENT PASSAGEWAYS IN THE OIL SHALE AT A SPACING BETWEEN PASSAGEWAYS GREATER THAN THE SUPERIMPOSED ROOF OF STRATA CAN SPAN WITHOUT AN INTERVENING WALL IN PLACE BETWEEN SAID PASSAGEWAY, (B) DISPLACING HORIZONTALLY A PORTION THROUGHOUT EACH INTERVENING WALL INTO THE ADJACENT PASSAGEWAYS TO PRODUCE AN EXCAVATION EXTENDING HORIZONTALLY BETWEEN PASSAGEWAYS AND VERTICALLY TO PROVIDE A VOID FOR RECEIVING THE VOLUMETRIC INCREASE OF DEBRIS CREATED UPON AT LEAST ONE ROOF FAILURE WHEREBY ROOF FAILURE OCCURS FORMING IN THE SUPERIMPOSED ROOF OF STRATA AN ARCH SPANNING THE EXCAVATION AND SUPPORTED