Method Of Producing Shale Oil From An Oil Shale Formation

Abstract

A method of producing shale oil from a subterranean oil shale formation by exploding a relatively high energy explosive device therein thereby forming a chimney of rubble within the formation having fractures extending from the chimney into the formation. A plurality of spaced wells are extended into the formation radially outwardly from the chimney and adjacent to at least some of the fractures. Fluid flow paths are formed from the wells through the fractures into the chimney and fluid is circulated from the wells through these fluid flow paths and into the chimney at rates creating a pressure drop from the wells to the chimney. Oil shale-reactive properties are imparted to the circulating fluid whereby the fluid reacts with the oil shale thereby moving solid components thereof into void spaces formed within the chimney thereby increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding the chimney.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing shale oil from a subterranean oil shale formation more 20 particularly, it relates to a method for creating a zone of relatively high permeability within an oil shale formation.

2. Description of the 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.

One of the chief uncertainties with regard to the effects of nuclear explosions within a subterranean oil shale formation is the permeability distribution surrounding the cavity and subsequent chimney produced by a detonation. Evidence from prior explosions suggests that permeability of the fragmented zone may drop very rapidly with distance radially out from the primary rubble zone. A high and uniform permeability is important in order to provide maximum sweep efficiency in any underground hydrocarbon recovery process.

The permeability in the region immediately surrounding the primary rubble zone of an oil shale formation may be increased by surrounding a primary high energy explosive device with a plurality of radially-placed devices of explosive energy, nuclear or nonnuclear. As disclosed in a copending application to Closmann et al. Ser. No. 653,139, filed July 13, 1967, now U.S. Pat. No. 3,448,801, the radially-placed devices are programmed to be detonated by either the main shock wave from the primary device or exploded by other means after the main shock wave has passed. The explosive energy devices are preferably detonated between the time the spherical cavity caused by the explosion of the primary device begins to expand radially outwardly and the time at which a chimney is formed by the collapse of the cavity roof. Heated fluids may then be circulated through the chimney and surrounding rubbled areas by known means so as to increase the volume of the permeable zone swept by the circulating fluid.

While a mass of oil shale is being pyrolyzed by the heated fluids, fluid products are removed from surface portions of the kerogen comprising the oil shale as rapidly as they are formed. But, since the mass of oil shale is impermeable, the fluid which is formed within the mass remains in place until its pressure becomes sufficient to fracture and displace the solid components that block its flow. In the oil shale around a nuclear-detonation chimney, the least resistant direction in which solid components may be moved tends to be toward the chimney where solid materials can be squeezed together or pushed towards a void space formed at the top of the chimney.

SUMMARY OF THE INVENTION

It is an object of this invention to improve the distribution of the permeability in and around a chimney formed within an oil shale formation while shale oil is being produced.

It is a further object of this invention to increase the rate at which the permeability in the outlying areas from a chimney formed within an oil shale formation is increased relative to that within the chimney.

These objects are accomplished by exploding a relatively high energy explosive device within an oil shale formation thereby forming a chimney of rubble within the formation having fractures extending from the chimney through the formation. A plurality of spaced wells are extended into the formation radially outwardly from the chimney and adjacent to at least some of the fractures. Fluid flow paths are formed from the wells through the fractures into the chimney and fluid is circulated through these fluid flow paths and into the chimney at rates creating a pressure drop from the wells to the chimney. Oil shale-reactive properties are imparted to the circulating fluid whereby the fluid reacts with the oil shale thereby moving solid components thereof into void spaces formed within the chimney increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding the chimney.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical cross-sectional view of an oil shale formation prior to detonating a plurality of explosive devices within the formation;

FIG. 2 is a diagrammatic plan view of the cavities formed by detonating the explosive devices within the oil shale formation of FIG. 1; and

FIGS. 3 thorough 5 are vertical cross-sectional views of the teachings of this invention as applied to the final rubble zones created by detonating all of the explosive devices of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows subterranean oil shale formation 11 having a primary explosive device 12 located within the formation 11. Primary explosive device 12 is preferably surrounded by a plurality of explosive devices 13. Devices 13 may be of lesser energy than device 12, if desired. However, optimum results may be obtained by the formation of substantially equal-size chimneys as will be discussed further hereinbelow. The device 12 can be either nuclear or nonnuclear; if a nuclear device is detonated in the subterranean oil shale formation 11, a strong shock wave from the nuclear 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 (i.e., the central 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 and then collapses to form a chimney 15 (FIG. 3). Collapse progresses upwardly until the volume initially in the cavity is distributed between the fragments of the oil shale formation 11. The size of the cylindrical rubble zone (i.e., the "chimney" 15) formed by the collapse of the cavity 14 can be estimated from the depth and explosive yield of the nuclear device and properties of the earth formations.

A zone of permeability 17 within the fragmented oil shale formation is formed surrounding the "chimney" 15 as can be seen in FIG. 3. The permeability of this zone 17 may be preferably increased by surrounding the primary explosive device which formed the central cavity with a plurality of devices 13. For example, in FIG. 1, a primary nuclear explosive device 12 is surrounded by explosive devices 13, equally spaced from each other and radially spaced from the primary explosive device 12. These devices 13 are preferably on substantially the same horizontal plane as the primary nuclear device (see FIG. 1) and 500 to 1,000 feet from the nearest part of the outer wall of the central cavity 14 produced by the explosion of the high energy nuclear device 12. As discussed above, the devices 13 preferably have an energy yield substantially equal to that of the primary high energy nuclear device 12 and can be either nuclear or nonnuclear.

The explosive devices 13 form cavities 18 (FIG. 2) when detonated, surrounded by fractured zones 19 as can be seen in FIG. 2. The devices 13 may be preset with detonating means adjusted to explode upon arrival of the main shock wave from the explosion of the primary explosive device 12. Alternatively, the devices 13 may be suitably delayed to explode after passage of the main shock wave. Of course, another characteristic of the explosion of the primary explosive device 12 can be utilized to detonate the devices 13, as, for example, changes in temperature or pressure as a result of the explosion of the primary explosive device.

Because of this time delay, either detonating the devices 13 upon arrival of the main shock wave or after the main shock wave has passed but before the central cavity 14 becomes filled with rubble due to the chimney collapse from above, the shock waves from the secondary explosions (that is, the explosions of the devices 13) will cause spalling into the central cavity. The movement of rock towards the central cavity 14 due to the satellite explosions will enhance the permeability in the regions between these explosions and the central cavity 14, by allowing development of a greater void space in this region. This void space, indicated as a zone of increased permeability 17 in the drawings, has a high and uniform permeability in the fragmented oil shale formation 11.

Thus, chimney 15 includes a lower rubble zone 21 and an upper void space 22. Similar "chimneys" formed by the detonation of the devices 13 also include lower rubble zones and upper void spaces. For example, as illustrated in FIG. 3, two such chimneys 23, and 24, formed, for example by devices such as explosive devices 13, form lower rubble zones 25 and 26 and upper void spaces 27 and 28, respectively. A plurality of fractures 29 are formed between the satellite "chimneys" and the central chimney 15 as illustrated in FIG. 3. Fractures 29 are generally substantially horizontally extensive through formation 11; however fractures 9 may also be substantially vertically extensive. A more detailed discussion of the formation of chimneys 15, 23 and 24 appears in the aforementioned copending application to Closmann et al., Ser. No. 653,139, filed July 13, 1967, now U.S. Pat. No. 3,448,801. Alternatively to forming chimneys 25 and 26 as indicated hereinabove, after chimney 15 is formed, fluid flow paths through fractures 29 may be formed by hydraulically or explosively fracturing wells 32 and 33 by fracturing procedures such as those known in the art, so that the latter fractures communicate with fractures 29.

Referring to FIG. 3, in accordance with the teachings of this invention, a producing well borehole 30 is extended from the earth surface 31 into communication with the lower portion of chimney 15. A plurality of outlying injecting well boreholes, such as well boreholes 32 and 33, shown in FIG. 2, are extended from earth surface 31 into communication with the upper portion of chimneys 25 and 26, respectively. Well boreholes 30, 32, and 33 are preferably cased as is well known in the art. The vertical intervals, that is, the "chimneys" or rubbled or fractured regions into which the outlying wells are opened are preferably located at substantially the same depth as the chimney 15.

Fluid flow paths are then formed from the outlying well boreholes 32 and 33 to chimney 30 through the fractures extending out from chimneys 25 and 26 into communication with interconnecting fractures 29. These flow paths are preferably enlarged by circulating acidizing fluids from well boreholes 32 and 33 through fractures 29 and into chimney 30. Another method of forming or enlarging such fluid flow paths from the outlying wells to the central chimney 15 is to fracture the oil shale formation by flowing an electrical current between electrodes that contact the oil shale. A more detailed description of this process for fracturing an oil shale is given in an article by Melton and Cross, Journal of Petroleum Technology, Jan., 1968, pp. 37--41, which is incorporated herein by reference. The electrical energy may be applied prior to or during the initial circulation of fluid from the outlying wells to central chimney 15 as will be explained further hereinbelow.

In operation, fluid is injected into the satellite chimneys 25 and 26 through well boreholes 32 and 33, through fractures 29 and into the rubble zone 21 of chimney 15 as indicated by the arrows in FIG. 3. Fluids are then produced from central chimney 15 through producing well borehole 30.

A preferred method for producing shale oil from the oil shale formation 11 of FIG. 3 is to inject a combustion-supporting gas, such as air or oxygen, into the satellite well boreholes after the hydrocarbons in the formation have been raised to ignition temperature. This may be accomplished by various means well known in the art, such as by lowering suitable heaters down well boreholes 32 and 33. A combustion zone is thus formed which gradually moves through the intervening rock between chimneys 25, 26, and 15 by means of fractures 29 into central chimney 15. As this rock is heated, it expands, releasing gas and other products and effectively provides additional flow paths for the injected fluid. At the same time, as the rock nearest the satellite chimneys expands, it expands or moves towards the central rubble chimney 15 thus tending to relieve some of the thermal stress generated by the hot fluids. This method makes the porosity distribution of oil shale formation 11 more uniform by developing some porosity adjacent the outside of chimneys 25 and 26 where the rock is first heated and by exerting pressure due to thermal expansion on the central rubble zone (i.e., chimney 15) thus tending to reduce the porosity of central chimney 15.

As an alternative to air or oxygen, the injected fluid may be a heated gas, liquid, or steam. If steam is used, thermal expansion of the rock takes place. After the rock is heated, combustion may again be carried out. The displaced fluids are produced from the bottom of the central chimney 15 to which they drain and out of production well borehole 30. As it becomes desirable to treat more of the upper regions of the rock, the production well borehole 30 may be shut off at the bottom and perforated at progressively higher places within the central chimney. This is illustrated in FIG. 4 where the lower end of the well borehole 30 is packed off, such as by a wireline-set or a tubing-set packer 34, and a perforating device 35 is lowered into well borehole 30 by means of cable 36. The casing of well borehole 30 is then perforated by device 35 as is well known in the art thus forming a plurality of perforations 37 which may be progressively moved up well borehole 30 as the central chimney 15 is produced.

Alternatively to injecting a fluid such as disclosed hereinabove, acid may be injected from the outlying chimneys through fractures 29 and into central chimney 15. The acid flows through fractures 29, leaching out part of the rock and developing some heating. This acid is produced from the central chimney 15. In some cases, fine suspended material (e.g., produced by decomposition of the oil shale during combustion or acidizing) may be carried from the inlets of this flow system (e.g., chimneys 25 and 26 and/or fractures communicating with wells 32 and 33) and deposited near the central chimney 15. This action makes the overall flow path more uniform. This step may be then followed by hot fluid injection or a combustion process such as previously discussed hereinabove.

In both cases, that is, the circulation of a fluid such as a gas or an acid, when the oil shale-reactive properties of a fluid comprise or include a temperature sufficient to pyrolyze kerogen in the oil shale and the fluid is flowing through interconnected fractures between chimneys at a rate providing a pressure gradient along the flow path, pyrolysis-induced fracturing tends to enhance the movement of solids and fluids in the direction of the lowest pressure. Within a nuclear detonation chimney, such as, for example, chimney 15, the permeability increases with increases in height and becomes substantially infinite in the void at the top. Since the fractures that are formed by a nuclear detonation are initiated by a radially expanded bubble centered in the lower portion of the region that becomes a chimney, the density of radially extending fractures is less at depths near the top of the chimney. Conventional equipment and techniques, such as heaters, pumps, a separator and a heat exchanger, may be used for pressurizing, heating, injecting, producing, and separating components of the fluid circulated through the oil shale formation 11. The production of the fluid may be aided by downhole pumping means, not shown, or restricted to the extent necessary to maintain the selected pressure within the oil shale formation 11.

When oil shale pyrolyzing fluid is circulated along a path extending through fractures, from the outlying wells to a nuclear detonation chimney, in the initial stages and at depth near the top of the central chimney, the permeability is the least, the pressure gradient is the highest and the resistance to solid-material displacement toward the central chimney is the least. As fractures are formed by the pyrolysis of the oil shale, they tend to form first in the regions which are contacted by the hottest portion of the fluid, and these regions are located near the outlying wells. The largest fractures tend to form at depths near the top of the central chimney where the resistance to the movement of solid material is the least. In addition, the relatively high permeability within the central chimney tends to decrease as solids move into the central chimney. This results in both the creation of additional permeability in regions surrounding the central chimney and an increase in the permeability in the surrounding regions relative to that within the central chimney. The creation of additional permeability in regions surrounding the central chimney increases the amount of permeable oil shale material that is available for depletion and the increase in permeability in the surrounding regions increases the uniformity of the depletion.

The fluid being circulated through central chimney 15 is preferably injected into all the satellite chimneys or intervals into which outlying wells have been opened and, at least initially, produced from near the bottom of central chimney 15. The fluid circulation may advantageously be initiated by circulating air or relatively cool liquid to sweep out any shale oil released by the nuclear detonation. When oil shale-reactive properties imparted to the circulating liquid comprise or include a temperature sufficient to pyrolyze the oil shale, the method of this invention provides a unique advantage over processes in which production wells are extended through the chimney, or through the immediately adjacent relatively highly fractured zone, to provide conduits arranged for a downward advance of a combustion front. In the process, when the advance of a heat front towards the production well borehole 30 subjects the borehole 30 to a high temperature, the production well borehole conduit or conduits, i.e., the well casing or tubing string, may be shortened to terminate in a relatively cool zone near the top of central chimney 15. After an extended and relatively uniform permeability distribution has been obtained by circulating fluid from the outlying wells to central chimney 15, the flow direction may be reversed, with central chimney 15 now operating as a very large diameter central injection well as illustrated in FIG. 5. Such a flow reversal allows the pyrolysis products to be produced from the tops of the intervals (for example, chimneys 23 and 24) into which the outlying wells are opened. This capability of the present process to avoid heat damage to the production well conduits provides material improvement in the economy of the shale oil-production process.

As discussed hereinabove, the oil shale-reactive properties imparted to the circulating fluid may advantageously comprise or include acidizing properties in respect to mineral components, such as the carbonates, in the oil shale. In addition to acids that are commonly used in well acidization treatments, acids suitable for use in the process of this invention comprise those derived from sulfur, such as sulfuric acid; sulfurous acid, etc., and/or their anhydrides, such as oleum, sulfur trioxide, sulfur dioxide and the like, and nitric acid and acids derived from the oxides of nitrogen and the like. The sulfur-derived acids are not generally used in well acidization treatments because of the tendency of the resultant aqueous solutions of such acids to precipitate polyvalent metal sulfates, sulfites, etc. In the initial stages of the present process, such precipitates tend to be deposited within the large voids in the central chimney 15, where the flow rate drop relative to those in the smaller void spaces in the fractures 29 leading central chimney 15. Thus, in the process of this invention, such a initial dissolving of solid materials and subsequent precipitation of solid materials is advantageous since it increases the rate at which the permeability in the outlying regions is increased relative to that within the central chimney 15.

Thus, the specific arrangement of injection and production locations and fluid pressure gradients, together with the fracturing pattern of a nuclear detonation and the behavior of a mass of oil shale undergoing pyrolysis, improves the distribution of the permeability of the oil shale formation in and around a nuclear detonation-created chimney while shale oil is being produced.

The outlying chimneys may be formed either in the manner disclosed hereinabove as disclosed in the aforementioned copending application to Closmann et al. In either case, it is preferable that the outlying chimneys be located at substantially the same depth as the central chimney. Optimum results are obtained when the outlying chimneys are substantially equal to the height of the central chimney, such as when the outlying chimneys are formed by the use of explosive devices of relatively similar explosive energy as that used to form the central chimney.

Claims

I claim:

1. A method of producing shale oil from a subterranean oil shale 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 rubble within the oil shale formation and extends fractures from the chimney through the oil shale formation;

extending a plurality of spaced wells into said oil shale formation at locations radially outwardly from said chimney and adjacent to at least some of said fractures;

forming fluid flow paths from said wells through said interconnecting fractures to said chimney;

circulating fluid into and them from said wells and through said interconnecting fractures into said chimney and out of said chimney at rates creating a pressure drop from the wells to said chimney;

imparting oil shale-reactive properties to said circulating fluid whereby said fluid reacts with said oil shale thereby moving solid components thereof into void spaces formed within said chimney thereby increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding said chimney;

forming a plurality of fractured regions adjacent at least some of said fractures from said chimney, said fractured regions being located at substantially the same depth of said first mentioned chimney;

subsequently extending fractures from said plurality of spaced wells into communication with said fractures from said chimney;

said plurality of fractured regions being formed by placing a plurality of devices of substantially lesser explosive energy than said relatively high energy explosive device within the formation; and

spacing the plurality of devices such a distance from the relatively high energy device that the exploding of the plurality of devices causes fractures from said spaced wells to extend into communication with fractures formed by said high energy explosive device.

2. A method of producing shale oil from a subterranean oil shale 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 rubble within the oil shale formation and extends fractures from the chimney through the oil shale formation;

extending a plurality of spaced wells into said oil shale formation at locations radially outwardly from said chimney and adjacent to at least some of said fractures;

forming fluid flow paths from said wells through said interconnecting fractures to said chimney;

circulating fluid into and then from said wells and through said interconnecting fractures into said chimney and out of said chimney at rates creating a pressure drop from the wells to said chimney;

imparting oil shale-reactive properties to said circulating fluid whereby said fluid reacts with said oil shale thereby moving solid components thereof into void spaced formed within said chimney thereby increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding said chimney;

extending a well into said chimney adjacent substantially the lower portion thereof;

circulating said fluid from said plurality of wells through said fractures, into said chimney and out of said well; and

producing fluid from said well at progressively higher places within said chimney.

3. A method of producing shale oil from a subterranean oil shale 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 rubble within the oil shale formation and and extends fractures from the chimney through the oil shale formation.

extending a plurality of spaced wells into said oil shale formation at locations radially outwardly from said chimney and adjacent to at least some of said fractures;

forming fluid flow paths from said wells through said interconnecting fractures to said chimney by electrically fracturing the portion of the oil shale formation between the chimney and said plurality of spaced wells;

circulating fluid into and then from said wells and through said interconnecting fractures into said chimney and out of said chimney at rates creating a pressure drop from the wells to said chimney; and

imparting oil shale-reactive properties to said circulating fluid whereby said fluid reacts with said oil shale thereby moving solid components thereof into void spaces formed within said chimney thereby increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding said chimney.

4. A method of producing shale oil from a subterranean oil shale 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 rubble within the oil shale formation and extends fractures from the chimney through the oil shale formation;

extending a plurality of spaced wells into said oil shale formation at locations radially outwardly from said chimney and adjacent to at least some of said fractures;

forming fluid flow paths from said wells through said interconnecting fractures to said chimney;

circulating fluid into and then from said wells and through said interconnecting fractures into said chimney and out of said chimney at rates creating a pressure drop from the wells to said chimney;

imparting oil shale-reactive properties to said circulating fluid whereby said fluid reacts with said oil shale thereby moving solid components thereof into void spaces formed within said chimney thereby increasing the permeability of the oil shale formation relative to the permeability of the chimney in regions surrounding said chimney;

stopping the circulating of fluid from said wells through said fractures and out of said chimney; and

circulating fluid from said chimney through said fractures and out of said wells while imparting oil shale-reactive properties to said fluid circulating from said chimney and out of said wells.