Method of producing shale oil from a subterranean oil shale formation

BACKGROUND OF THE INVENTION

Field of the invention

This invention relates to a method for recovering shale oil from a normally impermeable oil shale formation normally subject to vertical fracturing and having at least one layer of water-soluble · minerals disposed therein.

Description of the prior art

It is known that it is extremely difficult to recover liquifiable components from deposits of various subterranean impermeable formations such as oil shale, coral beds, deposits of cinnabar, etc. under conditions in which the deposits are normally present in these formations. Various proposals have been made, such as described in a U.S. Patent No. 3,284,281, to recover oil from oil shale. Therein shale oil is produced from an oil shale formation through fractures interconnecting wells. However, under the conditions described therein, the fractures tend to close as their walls become heated and thermally expanded and oil recovery is stopped. Under conditions described, it is generally necessary to resort to a repeating sequence of fracturing, heating and expanding and refracturing operations until flow patterns are finally formed that will remain open while the desired components are being liquified.

In certain situations, particularly at relatively shallow depths, a heating procedure can be utilized to cause the swelling tendencies of the earth formations to create horizontal stresses that exceed the vertical stresses. By such a procedure, pairs of wells can be interconnected by means of a horizontal fracture that can be kept open by hydraulically lifting or compressing the overlying earth formations. Such a procedure, in which the heating j_saccomplished by injecting a liquid while maintaining a specified rate of fluid flow and temperature increase, is described in a copending application to Matthews et al., ?ί;Λο\⁵⁷⁸’^{533, filed 121 i966}> ^n0W Patent 3,455,391.

However, normally, horizontal fractures cannot feasibly be formed by thermally increasing the horizontal stresses. Generally, heating oil shale formations causes the vertical stresses to increase at a rate comparable to that at which the horizontal stresses are increased and this prevents the formation of a horizontal fracture. Although communication between wells might be established by repetitively heating and fracturing at successively higher pressures and temperatures, as proposed in the aforementioned U.S. patent, this would result in excessively high operating costs and the production and maintenance of higher pressures and temperatures than are actually required.

When the regional tectonics are such that vertical fractures form and propagate along generally parallel paths when fluid is injected into adjacent-wells at about the normal subsurface temperature at pressures and rates sufficient to form and extend the fractures, the use of the successively higher pressures and temperatures is apt to form a succession of numerous differently nrirnMd vertical fractures at pressures which do not become high enough to produce a horizontal fracture. The pressure necessary to form a horizontal fracture is generally about equal to or slightly less than one p.s.i. per foot of depth. When, for example, a vertical fracture opening into a first well has been extended for a significant dictan™. and at least one vertical fracture has been opened and then thermally closed in a second well, the formation and extension of a subsequent fracture from the second well is apt to cause the fractures to intersect so that fluid can flow from one well to the other, however, if the fluid pumped through such intersecting fractures is heated i at or above the temperature at which· the last fracture was formed, the walls of the fracture will swell and the flow path will close.

Recent geological investigations have disclosed the presence of substantial amounts of water-soluble minerals (such as alkali or a lens of halite and nahcolite) , in ceriain subsurface oil shale formations such as in the Green River formation in the Colorado area ofrthe United States? Such minerals have been mined commercially in the past by solution-mining by pumping a liquid solvent, e.g., aqueous acidic solutions, into the vein or bed in which the mineral is fqund and returning the saturated mined solution preferable as . a mineral-containing solution to the surface of the earth and extracting the minerals from the solution by any suitable means.

For example, it has recently been found that layers of nahcolite, each two to three feet deep, underlie a large portion of the Green River Formation, of the Piceance Creek Basin in Colorado. In addition to the value of nahcolite as a mineral, these layers provide a means of contacting a significant portion of the oil shale deposit.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of recovering shale oil from a normally impermeable oil shale formation by extending generally vertical fractures ³ · , tween wells extending into water-soluble mineral layers posed in the formation.

It is a further object of this invention· to provide a methof interconnecting wells extending into normally imrmeable oil shale formations by forming generally veral fractures between such wells.

It is a still further object of this invention to interconct wells extending into oil shale formations by means generally vertical fractures which remain open so that lid flow remains unrestricted between such wells. j These objects are preferably attained by forming genally vertical fractures extending along generally parallel iths from at least a pair of well boreholes extending into normally impermeable subterranean oil shale formain. Hot fluid is injected through at least one of the well ireholes until flow into at least one of the fractures erein is thermally closed by the swelling-shut of the alls of the fracture. Fluid in at least one borehole in hich at least one fracture has been thermally closed is ressurized until at least one new fracture is formed and le steps of injecting hot fluid and the pressurizing of fluid re repeated at successively higher temperatures and presires until the resultant fractures form a channel interannecting the well boreholes, through which channel uid can flow from one borehole to another. The walls of le fracture interconnecting the boreholes are leached at controlled temperature until channels provided between le well boreholes are capable of remaining open while lot fluid is circulated between the broeholes. This is acomplished by injecting a leaching fluid into one of the oreholes opening into the well-interconnecting fracture hannel and producing fluid from another of the boreholes ipening into the fracture channel, with the fluid being njected at a temperature below that at which the last racture was opened into the well borehole into which the :irculaing fluid is injected but above that at which the ast fracture was thermally closed within the well borerole into which the circulating fluid is injected so that most of the injected fluid is conveyed through the channels interconnecting the boreholes.

Thus, when such a flow path is opened between a pair of wells, the path may be kept open by lowering the temperature of the fluid pumped through the path to a temperature that is less than that at which the last fracture was formed but greater than that a which the next preceding fracture was thermally closed. In addition, if the effective permeability of the flow path between the well boreholes is then increased by circulating through it a fluid that removes solid components from the walls of the channels, the flow path is converted to one through which the flow of fluid from one well borehole to the other remains efficient at whatever temperature is subsequently imparted to the fluid. .

The attainment of a significant increase in the effective permeability of such a flow path may be detected by an increase in the rate of flow in response to the injection pressure that was initially required to displace fluid from one well borehole to another. If the temperature of the circulating fluid is too high relative to the rate which permeability is being increased, this may be detected by a reduction in the flow rate. If the temperature is too low relative to the rate at which the permeability is being increased, this may be detected by a decrease in the rate of outflow from the production well borehole without a corresponding decrease in rate of inflow into the injection well borehole. Where the temperature is too high, the thermal closing of the last-opened fracture throttles the flow and where the ^temperatures is too low, a thermal opening of previously closed fractures diverts the flow of the injected fluid.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE 1 is a vertical sectional view of a subterranean oil shale formation in accordance with the teaching of my invention;

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FIGURE 2 is a top plan diagrammatic view of a preferred arrangement of well boreholes in accordance with the teachings of my invention. ,., .,

FIGURE 3 is a vertical sectional view of a well ’’orehole extending into the oil shale formation of FIGURE 1;

FIGURES 4 through 6 are vertical sectional views or preferred arrangements for producing shale oil from the well borehole formation of FIGURES 2; and

FIGURE 7 is a vertical sectional view of an alternate method of treating the oil shale formation of FIGURES 4 through 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGURE 1, an oil shale formation 11 is shown having at least one bed or layer of water-soluble minerals such as nahcolite layers 12 and 12 disposed therein. The distance D between such layers, for example, may be on the order of 300 feet or so. Such j^ay*re 12 and 13 may, again for example, be on the order of two to three feet thick and underly a large portion Of oil shale formation 11.

Layers 12 and 13 may be utilized in order to form intercommunication between at least a pair of well boreholes extending into a subterranean earth formation. For example, as illustrated in FIGURE 2, a pattern of five well boreholes, such as production well boreholes 14 through 17 and centrally located injection well borehole 18, may be drilled by means well known in the art into shi’l formation 11 to points communicating with layers 12 and 13 (FIGURE 3). Injection well borehole 18 may be fractured hydraulically so as to develop a generally vertical fracture 19 (FIGURE 2). .

Such fractures may be formed by any known technique such as by applying a fluid pressure above the breakdown pressure of oil shale formation 11. Well borehole 14 may, then be hydraulically fractured so as to develop a generally vertical fracture 20 generally parallel to fracture 19. Hot fluid, such as hot water, steam, or hot gas is next injected down well borehole 14 causing the fracture 20 to close and a new generally vertical fracture to develop in some other direction, as for example, fracture 21. This procedure may be continued in well borehole 14 untjl a generally vertical fracture, such as fracture 22, is formed which communicates with the fracture 19 .extending from central injection well borehole 18 thus inter-connecting well boreholes 14 and 18. These steps are carried out at each well borehole 15, 16 and 17 until all the wqll boreholes develop generally vertical fractures communicating with the centrally located fracture 19 extending from well borehole 18, as for example, generally vertical fractures 23, 24 and 25, respectively.

Next, the water-soluble mineral layers 12 and 13 are partially dissolved or leached out successively at the four comers of the well borehole pattern of FIGURE 2 leaving substantially void spaces therein. This may be accomplished by lowering a flexible tubing 26 down tubing string 27 disposed in well borehole 14, for example, (FIGURE 3). Well borehole 14 may be cased as at casing 28 with casing 28 cemented therein as is well known in the art. The water-soluble minerals in layer 12 are then dissolved by jet action from an aqueous leaching or solution-mining liquid. The aqueous liquid may comprise water and/or steam or aqueous solutions of acid or acidforming materials and is circulated at pressures either above or below the overburden pressure. The circulating aqueous liquid dissolves the water-soluble minerals in layer 12 and mineral by-products thereof are recovered from the fluid flowing out of well borehole 14 by conventional evaporation and/or precipitation procedures. Layer 13 may be treated in the same manner with well boreholes 15, 16 and 17 also treated accordingly.

As an alternative to tubing 26, the vertical fracture 20 already developed in well borehole 14 may be used to inject a water-soluble mineral fluid into layers 12 and 13.

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The leached void spaces or zones (indicated by the outlined areas of FIGURE 2) of layers 12 and 13 tend to be relatively narrow channels, however, following more or less the line of the fractures. These channels are widened by a continuation of the leaching action, and the rate of . the widening can be enhanced by an intermittent flow ⁰and/or alternately reversed-direction flow of the leaching fluid.

Hot fluid, such as hot water, steam or hot gas, is next injected down the corner well boreholes 14 through 17 to flow through fractures 22 through 25 and out of injection well borehole 18. Alternatively, such fluid may be either injected down well borehole 18 to flow through the fractures and out of the comer well boreholes or a combination of both procedures.

Such hot fluid flows through the interconnecting fractures and void spaces, causing the oil shale contacted therein to be heated and to expand into the void spaces developed in the water-soluble mineral layers 12 and 13. If the fractures tend to close due to thermal expansion, .,θ they may be partially leached out with hot solvent, acid or other reacting fluid which removes both organic matter and mineral content. In this manner, the channels formed by the fractures and void spaces may be maintained permeable long enough to permit significant heating and .,thermal expansion of the oil shale into the void spaces formed by the leaching layers 12 and 13. In addition, spalling may take place into the void spaces due to expansion and/or pyrolysis of the oil shale near the free spaces of the void spaces. The thermal expansion of the ._j()oil shale permits the development of many horizontal fissures 29 (FIGURE 4) along the bedding planes of oil shale formation 11. As can be seen in FIGURE 4, hot fluid is injected down tubing string 27 disposed in well borehole 14 and flows through perforations 30 therein, through the vertical fracture 22 formed between well borehole 14 and 18, through perforations 31 and out of well borehole 18. All the well boreholes may include conventional equipment for injecting, heating, and separating such fluids as is well known in the art. Well borehole 18 may also be cased at casing 32. with casing 32 cemented therein as is well known in the art. The vertical arrows within the vertically fractured portion 22 of oil shale formation 11 indicate the spalling and expansion of the oil shale into the void spaces formed by the leaching of layers 12 and 13. 45

Upon the development of a significant number of horizontal fissures 29, heating phase may be replaced by a production phase in which hot recovery fluid may be injected, or an underground combustion process conducted, as is well known in the art. If bypassing at the void space 50 is too great, the injected reacting fluids may be injected over only limited intervals away from the permeable void channels. Alternatively, as illustrated in FIGURE 5 wherein like elements equal like parts of FIGURE 4, the reacting fluids may be injected through perforations 55 33 in well borehole 14 and into the bottom layer 13 and allowed to move upwardly through the oil shale to be produced at the top of the oil shale out through perforations 34 in well borehole 18. Note that well boreholes 14 and 18 are packed off at packers 35 and 36, respectively. Also, 60 the reacting fluid may be introduced at the top and produced at the bottom of oil shale formation 22 as illustrated in FIGURE 6. Thus, the reacting fluid is injected down tubing string 27 above packer 37 and out perforations 30, adjacent void layer 12, down through oil shale formation 65 11 and back into production well borehole 18 through perforations 31. Finally, both top and bottom voids 12 and 13 may be partially filled by a material such as coarse propping sand by any means well known in the art. This is illustrated in FIGURE 7 wherein coarse material 38 is 70 shown partially filling void spaces 12 and 13 thus reducing the tendency of injected fluids to flow through the void spaces. This increases the relative tendency of the injected fluids to flow through the fissures (such as fissures 29) and fractures (such as fracture 22) developed in oil shale 75

201 formation 11 between the well boreholes as illustrated in FIGURES 5 and 6.

The composition of the reaction fluid is preferably adjusted to the extent required to circulate fluid that removes solid materials from the walls of the interconnecting channels without a significant reduction in the average rate of flow between the well boreholes so that the fluid flowing through the interconnecting channels, as for example fractures 19 and 22 and leached-out layers 12 and 13, is increased relative to fluid flowing between well boreholes 14 and 18. Solid-material removing components may be incorporated into the reacting fluid being circulated through the interconnecting channels without interrupting the flow to an extent that permits the channels to close and reseal. Such components may comprise hot benzene, steam, or other solvent, or nitric acid, of a lower temperature than the hot fracturing fluid. Nitric acid has the advantage of reacting with the organic matter as well as the carbonate present in the subterranean earth formation. The injection of such a reacting fluid leaches out part of the kerogen adjoining the faces of the interconnecting channels. The injection at a lower temperature and at substantially the same injection pressure permits the channels to open slightly for better passage of the fluids. The temperature of the solid-material-removing fluid may be increased as the permeability of the interconnecting channels, fractures 19 and 22 and layers. 12 and 13, for example, is increased until the circulating fluid becomes hot enough to liquify the oil shale components of the subterranean oil shale formation 11.

Following the hot solvent injection as discussed hereinabove, acid may be injected to react with part of the rock matrix along the fracture walls. This acid injection renders the channels even more permeable.

After all the steps discussed hereinabove are carried out, an underground combustion process, as discussed hereinabove and is well known in the art, which develops considerably higher temperatures, may be undertaken. The steps of leaching out part of the kerogen and the rock generally make closure of the fracture paths during combustion very unlikely. In this manner, it is possible to treat a substantial part of the formation by underground combustion.