Method For Producing Shale Oil From A Subterranean Oil Shale Formation

Abstract

A method for producing shale oil from a subterranean oil shale formation wherein a chimney of fragmented oil shale is formed in the formation by exploding a relatively high energy explosive device therein, the chimney having a substantially void space formed at the top thereof. A liquid is flowed through the voids formed between the oil shale fragments, the liquid being adapted to selectively bypass small voids and plug larger voids formed between the fragments at least in the substantially vertical central portion of the chimney. Hydrocarbons at substantially the top of the chimney are ignited and a combustion supporting fluid is flowed into the chimney at substantially the top thereof, thereby advancing a combustion front down the chimney to substantially the bottom thereof. The fluid flow path of the fluid supporting the combustion tends to be substantially confined to the vertical outlying portions of the chimney and the untreated small voids within the chimney until the heat from the combustion front thermally mobilizes the liquid plugging the larger voids thus decomposing the plugging liquid thereby pyrolyzing substantially all of the fragmented oil shale along a substantially horizontal level within the chimney without the combustion front bypassing the portions of the fragmented oil shale adjacent to the small voids as the combustion front proceeds down the chimney.

Description

The invention relates to an improved method for producing shale oil from a subterranean oil shale formation, and more particularly, to producing oil from fragmented oil shale within an oil shale formation.

2. Description of Prior Art

The use of contained nuclear explosions has been proposed in subterranean oil shale formations in an attempt to break up the oil shale formation so that shale oil can be recovered from the rubbled zone by known techniques, such as in situ retorting.

Experience has shown that when a relatively high energy device, such as a nuclear bomb, is exploded within a subterranean earth formation, an almost spherical cavity filled with hot gases is formed. This cavity expands until the pressure within the cavity equals that of the overburden. On cooling, the roof of the cavity collapses since, generally, it cannot support itself, and a so-called "chimney" develops. Chimney growth ceases when the rock pile substantially fills the cavity, or, a stable arch develops. In both cases, a substantially void space is formed below the overburden and above the rubble contained within the chimney. Surrounding the chimney is a fractured zone which results from the shock of the nuclear explosion.

However, in any chimney of rubble or fragmented oil shale formed by the explosion of a relatively high energy device, the occurrence of large blocks of rock or oil shale indicates large void volumes distributed throughout the chimney. In an in situ flow process for recovering shale oil from such a chimney of rubble, these voids result in significant bypassing of injected and produced fluids, leaving large portions of the rock untreated.

It is an object of this invention to provide an improved method for recovering shale oil from a fragmented zone within a subterranean oil shale formation.

It is a further object of this invention to provide a method for recovering shale oil from an oil shale formation by advancing a combustion front uniformly down a chimney or rubble formed within the formation.

These objects are preferably carried out by exploding a relatively high energy explosive device within a subterranean oil shale formation thereby forming a chimney of oil shale fragments therein, the chimney having a substantially void space formed at the top thereof. A liquid is flowed through the voids formed between the oil shale fragments, the liquid being adapted to selectively bypass small voids and plug larger voids formed between the fragments at least in the substantially vertical central portion of the chimney. Hydrocarbons at substantially the top of the chimney are ignited and a combustion supporting fluid is flowed into the chimney at substantially the top thereof thereby advancing a combustion front down the chimney to substantially the bottom thereof. The fluid flow path of the combustion supporting fluid tends to be substantially confined to the vertical outlying portions of the chimney and the untreated small voids within the chimney until the heat from the combustion front thermally mobilizes the liquid plugging the larger voids thus decomposing the plugging liquid thereby pyrolyzing substantially all of the fragmented oil shale along a substantially horizontal level within the chimney without the combustion front bypassing the portions of the fragmented oil shale adjacent to the small voids as the combustion front proceeds down the chimney.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical cross-sectional view of an oil shale formation prior to detonating a relatively high energy explosive device therein;

FIG. 2 is a vertical cross-sectional view of the oil shale formation of FIG. 1 after the explosive device has been detonated;

FIG. 3 is a vertical cross-sectional view of the final rubble zone created by the detonation of the explosive device of FIG. 1;

FIG. 4 is a vertical cross-sectional view of the treatment of the rubble zone of FIG. 3 in accordance with the teaching of this invention;

FIG. 5 is a vertical cross-sectional view of single-well recovery of shale oil from the treated rubble zone of FIG. 4;

FIG. 6 is a vertical cross-sectional view of dual-well recovery of shale oil from the treated rubble of FIG. 4; and

FIG. 7 is a vertical cross-sectional view of an alternate treatment of the rubble zone of FIG. 3 in accordance with the teachings of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a subterranean oil shale formation 11 having a relatively high energy explosive device 12 located therein. Explosive device 12 may be nuclear or nonnuclear. When a relatively high energy explosive device, such as a nuclear bomb, is detonated within an oil shale formation, a strong shock wave from the explosive device begins to move radially outwardly, vaporizing, melting, crushing, cracking, and displacing the oil shale formation 11. After the shock wave has passed, the high-pressure vaporized material expands, and a generally spherical cavity, such as the cavity 14 in FIG. 2, is formed which continues to grow until the internal pressure is balanced by the lithostatic pressure. The cavity 14 persists for a variable time depending on the composition of the oil shale formation 11, then collapses to form a chimney 15 (FIG. 3). Collapse progresses upwardly until the volume initially in cavity 14 is distributed between the fragments of the oil shale of formation 11. The size of the cylindrical rubble zone (i.e., the "Chimney" 15) formed by the collapse of the cavity 14 may be estimated from the depth and explosive yield of the explosive device 13 and the properties of the formations 11 and 16. A substantially void space 13 is formed at the top of chimney 15.

A zone of limited permeability 17 within the fragmented oil shale formation 17 is also formed surrounding chimney 15 as seen in FIG. 3. The permeability of this zone 17 may be preferably increased by surrounding the explosive device 12 with a plurality of explosive devices of lesser energy and subsequently detonating the lesser energy devices in the manner discussed in my copending application Ser. No. 735,684, filed Jun. 10, 1968.

After forming chimney 15, it may be desirable to extend a well borehole 18 to a point adjacent to the bottom of the chimney 15. Fluids which are apt to be encountered within such a zone (i.e., at the bottom of chimney 15) are liquid and/or gaseous petroleum products and/or steam and/or water. Particularly where petroleum fluid is encountered, it may be desirable to produce substantially all the liquid phase present at the bottom of chimney 15 so that most of the fluid remaining in the fragmented zone or chimney 15 is gaseous petroleum or air.

Referring now to FIGS. 4 and 5, the invention disclosed herein is illustrated as preferably applied to such a fragmented zone in which most of the fluid remaining in the chimney 15 is mainly a gas. The same well borehole 18, preferably cased at casing 19, cemented therein, if desirable at cementing 20, may be used to inject a liquid down tubing string 22 into the oil shale fragments 21 disposed at the bottom of chimney 15. The liquid is preferably pumpable and adapted to solidify in situ. Thus, each portion of the inflowed liquid is allowed to solidify, between the series of such injections, in order to selectively plug the central portion of the chimney. Preferably then, a first injection is made at the bottom of chimney 15 with the liquid allowed to solidify, then subsequent injections are made upwardly within chimney 15 to substantially the top thereof by selectively opening casing 19 as is well known in the art.

The injected liquid is one which tends to flow into the larger voids and channels in the central portion of the chimney 15 and may be a foaming and thermosetting resin. Such materials, by foaming in situ, increase the pressure gradient necessary for flow through such large void spaces and channels.

After the selective plugging of preferably a substantial portion of the vertical central portion of chimney 15, as indicated by a solidified or treated zone 23 of relatively low permeability as in FIG. 5, a tubing string 22 is packed off as at packers 24 and 25 below perforations 26 near the top of the chimney and above the bottom of tubing string 22, respectively. Packer 24 is preferably removed or unseated to provide a path of fluid communication with perforations 26 within the treated zone 23.

After igniting the hydrocarbons present at the top of chimney 15, by any suitable means, such as by downhole heating means, a combustion front 27 is initiated and advanced downwardly towards a production point near the bottom of chimney 15. This may be accomplished by circulating a heated combustion supporting fluid down casing 19, through perforations 26 and into the fragmented oil shale 21 within chimney 15. The initial flow paths of the heated fluid are confined mainly to the outlying portions of chimney 15, that is, the untreated zone 28 of relatively high permeability as indicated by the direction of the major portion of the arrows in FIG. 5 and also to the untreated smaller voids within the chimney 15, until the heat from the combustion front thermally mobilizes the plugging material that was formed within the larger voids in treated zone 23.

By the time the plugging material decomposes, the relatively slow advance of combustion front 27, and the resultant gradual heating of all the rocks within the remaining fragmented zone of chimney 15, initiates the pyrolysis of the kerogen in the larger oil shale fragments. The overall effect is a pyrolysis of substantially all the fragmented oil shale material without a bypassing of the portions of fragmented oil shale material adjacent to smaller voids through which the flow resistance is significantly larger than that within the larger voids.

Thus, as illustrated in FIG. 5, at the top of chimney 15, between void space 13 and combustion of front 27, a zone A is formed depleted of oil and plugging material. A partially depleted zone B is formed between combustion front 27 and the bottom of chimney 15. The preferred path of hot combustion products and entrained oil shale is indicated at 29.

Numerous types of pumpable liquids may be used to selectively permeate and temporarily plug the larger voids between oil shale fragments 21 within chimney 15. Suitable materials include fluid mixtures containing the components of polyurethane, ureaformaldehyde, melamine formaldehyde, and the like types of foaming resin formulations. As the foam begins to form, the gas entrained within the liquid tends to divert the foams from the small voids and keeps them within the larger voids where they remain until the liquid components solidify. In a gas-filled fragmented zone, the relatively higher density of such a foam causes it to form a layer along the bottom of the gas-filled zone.

The plugging liquid may also be a liquid resin containing filler particles of sizes such that flow through smaller pores and channels is inhibited. Such formulations may include solutions of the components of resin, such as epoxy resins, phenol-formaldehyde resins, and the like resin formulations containing particles like shredded rubber, walnut shells, wood fibers, etc., of the types used as conventional lost-circulation controlling materials in working wells.

It may also be desirable to inject a fluid adapted to wet preferentially the oil shale material. Such preferentially wetting formulations may comprise aqueous surfactants which tend to contact the smaller pore spaces and block them off during a subsequent injection of resin. The presence of the liquid surfactant phase on the walls of the oil shale fragments adjacent to the smaller void spaces inhibits the wetting of the oil shale fragments by the resin at least for a time and to an extent sufficient to divert the resin into the larger channels and voids. The presence of the liquid surfactant phase may, in some cases, be useful in causing reaction of the injected resinous fluid. Such a wetting fluid may be injected into the chimney 15 down the annulus formed between casing 19 and tubing string 22 as discussed hereinabove with respect to FIG. 5.

The combustion-supporting fluid adapted to be injected into chimney 15 may be heated prior to circulation by means of a heating device 30. In other words, the fluid is pumped by means of a pump or compressor 31 through heating device 30 and into the annulus formed between tubing string 22 and casing 19. The fluid then flows through perforations 26 and into the zone a of chimney 15. Oil shale pyrolysis products are removed at the bottom of chimney 15 up tubing string 22, through heat exchanger 32 and into separator 33 where the oil and gas components are separated as is well known in the art. At least some makeup gas or preferably air is added at, for example, pump 31.

Referring now to FIG. 6, a preferred arrangement for producing shale oil from chimney 15 utilizing at least one production well and one injection well is shown. Here, like numerals refer to like parts of FIG. 5. The fluid from heater 30 is injected into injection well 34, cased and casing 35, through tubing string 36. The fluid exits past packer 37 and enters the void space 13 of chimney 15. Oil shale pyrolysis products are produced up the casing 38 of production well 39 and into heat exchanger 32.

Where the fragmented zone within chimney 15 is filled with a relatively dense liquid, such as water, by using a relatively low-density formulation, such as a solution of melamine-formaldehyde resin components containing shredded rubber, the formation of treated zone 23 may be accomplished by injecting the foaming formulation near the top as illustrated in FIG. 7, while producing dense liquid from near the bottom of the chimney. The setting time of the resin components should be adjusted so that the treated layer extends down through the chimney to near the bottom before the foaming formulation becomes immobile. Thus, tubing 22 is packed in casing 19 by means of packers 19a as is well known in the art. Shale oil is then produced from the treated chimney 15 up tubing 22 in the manner discussed hereinabove with respect to FIGS. 5 and 6. The injected formulation passes from casing 19 out perforations 19b and into the top of chimney 15. Alternatively, if the chimney of rubble 15 is liquid filled initially, then the flow of injected resinous fluid may be controlled by adjusting its density to be below of the filling liquid, such as water. The chimney 15 may then be filled upwardly in a series of steps, beginning at the bottom, as disclosed hereinabove with respect to FIG. 4.

Because of the large rubble volume to be so treated in chimney 15, the foaming resin formulation may be injected at a number of vertical positions from the same well (i.e., either well 18 or 34) by either selectively opening well 18 at different vertical positions in chimney 15 or by extending well 34 downwardly into selective vertical positions in chimney 15.

Alternatively, two or more wells may be drilled to communicate with different levels within chimney 15. A limited amount of the foaming resin formulation may be then injected into each of these wells to treat a specified region of the chimney 15.

One advantage of injecting the foaming resin formulation through a central well is that, by carefully regulating the quantity of such injected materials, the outer portions of the chimney of rubble 21, i.e., zone 28, remain substantially untreated. Injected fluids then tend to flow preferentially near the walls of the chimney 15 but not beyond and improve the overall sweep efficiency of the flow process.

Claims

I claim:

1. In a method for producing shale oil from a subterranean formation comprising the steps of: placing a relatively high energy explosive device within the formation; exploding the relatively high energy explosive device within the oil shale formation, thereby forming a cavity within the oil shale formation having a roof beneath the overburden which subsequently collapses to form a chimney of fragmented oil shale within the oil shale formation, said chimney having a substantially void space formed adjacent to the top thereof; flowing a liquid through voids formed between said oil shale fragments in said chimney, said liquid being adapted to bypass small voids and selectively plug larger voids formed between said oil shale fragments at least in the substantially vertical central portion of said chimney; igniting hydrocarbons at substantially the top of said chimney; and flowing a combustion-supporting fluid through said chimney at substantially the top thereof thereby advancing a combustion front down said chimney to substantially the bottom thereof, said fluid flow path of said combustion-supporting fluid tending to be substantially confined to the vertical outlying portions of said chimney and the untreated small voids within said chimney until the heat from said combustion front thermally mobilizes the liquid plugging said larger voids thus decomposing said plugging liquid thereby pyrolyzing substantially all of the fragmented oil shale along a substantially horizontal level within said chimney without said combustion front bypassing the portions of said fragmented oil shale adjacent to said small voids as said combustion front proceeds down said chimney.

2. The method of claim 1 including the step of recovering shale oil displaced from said combustion front.

3. The method of claim 1 including: the step of extending at least a central well from a surface location to a first point adjacent to a substantially vertical central portion of said chimney; and subsequently flowing said liquid and said combustion-supporting fluid through said well and into said chimney.

4. The method of claim 3 wherein the step of flowing a liquid through said voids includes the step of flowing said liquid from a plurality of vertical positions within said central well into said chimney.

5. The method of claim 1 wherein the step of flowing said liquid includes flowing a liquid containing dispersed material which tends to cause the inflowing liquid to bypass said small voids and flow through said larger voids.

6. The method of claim 1 wherein the step of flowing said liquid includes flowing a liquid having a density differing from the formation fluid being displaced from within the chimney in a manner such that gravity tends to segregate the inflowing liquid toward the nearest vertical extremity of the chimney.

7. The method of claim 1 wherein the step of flowing said liquid includes flowing a liquid which is capable of substantially solidifying in situ thereby materially reducing the permeability of the larger voids in which it is present.

8. The method of claim 7 wherein the step of flowing a liquid capable of substantially solidifying in situ includes flowing a liquid capable of being thermally converted from a substantial solid to a mobile fluid at a temperature between about 400.degree. F. and 1,200.degree. F.

9. The method of claim 8 wherein the step of flowing a combustion-supporting fluid includes the step of flowing a heated fluid at a temperature exceeding the thermal conversion temperature of the substantially solidified liquid formed in situ within said larger voids.

10. The method of claim 8 including the step of terminating the inflowing of liquid when a layer of the liquid extends over a significantly large proportion of the central cross-sectional area of the chimney and allowing said inflowing liquid to solidify in situ prior to igniting said hydrocarbons.

11. The method of claim 10 including the steps of repeating the steps of flowing said liquid and terminating the inflowing of said liquid from a first point within said chimney to an additional point within said chimney closer than than said first point towards the center of said chimney.

12. The method of claim 11 wherein said first point is a point substantially adjacent to the bottom vertical central portion of said chimney and said repeated steps move upwardly within said chimney along said vertical central portion thereof.

13. The method of claim 11 wherein said first point is a point substantially adjacent to the top vertical central portion of said chimney and said repeated steps move downwardly within said chimney along said vertical central portion thereof.

14. The method of claim 1 including the step producing substantially all of the liquids present at the bottom of said chimney after forming said chimney and prior to flowing a liquid through said voids so that most of the fluid remaining in said chimney is a gas.

15. The method of claim 1 wherein the step of flowing a liquid through said voids includes the step of flowing a foaming thermosetting resin formulation through said voids.

16. The method of claim 1 including the step of injecting a fluid adapted to wet preferentially oil shale fragments adjacent said small voids prior to flowing said liquid through said voids.