Spreading-fluid Recovery Of Subterranean Oil

Abstract

In a process for producing shale oil by circulating fluid through a cavity that contacts a subterranean oil shale, the cavity is extended laterally into both an injection well and a plurality of surrounding productions wells and the fluid is circulated at a rate causing the velocity within the cavity to decrease with radial distance away from the injection well to a velocity that is too low to transport relatively large particles of mineral solids.

Description

BACKGROUND OF THE INVENTION

The invention relates to producing shale oil components from a subterranean oil shale formation. More particularly, it relates to circulating a hot fluid through an areally extensive cavity that contracts the oil shale so that the pattern of flow controls the extent to which particles of mineral solids are fluid-transported and avoids the plugging of conduits.

Many subterranean oil shale formations comprise nonporous solid earth materials that are impermeable to fluids and contain more inorganic components than organic components. When the organic components are selectively removed by circulating a hot fluid into contact with an oil shale, the materials remaining in the portions of depleted oil shale (along the walls of a cavity or a fracture) may have a bulk volume that is larger than the bulk volume of the original oil shale. In the course of removing portions of the organic components, the remaining inorganic components become heated, thermally expanded, and pushed apart by invading portions of fluid, so that they form a mass of liquid permeated solids that is porous and has acquired a larger bulk volume. Such a volume increase tends to cause the depleted materials to swell into and close fractures, or to spall into and fill up cavities, unless the roofs of the fractures or cavities are hydraulically lifted by the fluid pressures within the fractures or cavities. Since the particle sizes of these materials are normally rather small, the permeability of the filled cavities is greatly reduced. A hydraulic lifting within the fractures or cavities is disadvantageous in creating a possibility of break through of highly pressurized fluid to the surface and/or lifting the surface of earth formation into a mound that may be ecologically undesirable and/or creating the possibility of communicating with some other higher zone, such as an aquifer, or the like actions that would be environmentally very undesirable.

Where a subterranean oil shale contains or is interbedded with water soluble minerals, such as nahcolite, trons, soluble halides, or the like, those materials can be used to form a subterranean cavern or cavity that contacts the oil shale. Such a cavity can be formed by solution-mining, and can be used to provide both a path through which a hot fluid can be circulated to contact oil shale, extract shale oil, and provide space in which some or all of the increased bulk of the depleted oil shale components can be accommodated. However, such a solution-mining-preceded recovery process may have a significant disadvantage with respect to postponing the recovery of oil pending a relatively long and expensive mining operation in order to provide a cavity that is big enough to accommodate a significant portion of the depleted oil shale.

Where a subterranean oil shale contains or is interbedded with heat-sensitive carbonate mateirals, such as nahcolite, dawsonite, or the like, a thermal conversion of those materials can be utilized to reduce the bulk of the depleted materials for the extraction of shale oil. In such a procedure, the hot fluid which is circulated into contact with the oil shale to effect a pyrolysis and/or extraction of shale oil components is an aqueous hot fluid (such as steam and/or hot water) that converts the heat-sensitive carbonate materials to water soluble materials. This dissolves and removes a portion of the inorganic components of the oil shale. It may reduce the bulk volume of the spent oil shale residue to an extent such that the rate of fill-up of a cavity may be less than the rate of material removal and thus may cause the cavity to grow in size rather than become filled in with depleted oil shale materials. However, serious problems may be encountered when such a hot aqueous fluid is circulated through a cavity in contact with a subterranean oil shale. For example, in tests in which steam was flowed into and along the walls of a well so that steam contacted an upper portion of the subterreanean oil shale at a temperature of about 500.degree.F (i.e. a temperature sufficient to cause both a relatively rapid pyrolysis of the oil shale kerogen and a heat-induced decomposition of heat-sensitive carbonate components of the oil shale) and the resultant aqueous and organic liquids were withdrawn from a lower level within the oil shale, the production conduits rather quickly became severely plugged with large and small particles of organic and inorganic solid materials.

SUMMARY OF THE INVENTION

The present invention relates to producing shale oil from a subterranean oil shale. A cavity that contacts the oil shale is extended into and preferably beyond at least one well pattern that contains more production wells than injection wells. A hot fluid that is adapted to interact with the oil shale to yield shale oil is injected into the cavity through the injection well. Fluid that contains shale oil is produced from the cavity through a plurality of production wells. The rates and velocities of the flow of fluid within the cavity are controlled to provide both substantially equal rates of outflow from the plurality of producing wells surrounding each injection well and a flow velocity within the cavity that diminishes with radial distance away from each injection well and becomes too low to transport significantly large particles of solid mineral materials before reaching the surrounding production wells. Shale oil is recovered from the produced fluid.

DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of a portion of the subterranean oil shale containing wells through which the present invention is practiced.

DESCRIPTION OF THE INVENTION

As used herein, the term "cavern" or "cavity" refers to any relatively solids-free opening such as a cave, void, tunnel, borehole, or highly permeable mass of rubble or interconnected fractures, etc. Such a cavern "contacts a oil shale" when a portion of the oil shale forms at least a portion of the wall so that there is fluid communication between the interior of the cavern and the oil shale. The term "heat-sensitive carbonate mineral" refers to a carbonate mineral that decomposes relatively rapidly at a relatively low temperature (such as between about 250.degree.F and 700.degree.F) to yield fluid products such as carbon dioxide and water. Examples of heat sensitive carbonate minerals include nahcolite, dawsonite, trona, and the like, which usually contain carbonate and/or bicarbonate compounds or groups.

The drawing shows the application of the present process to a subterranean oil shale formation, which is interbedded with a horizontally extensive layer of water-soluble mineral, such as nahcolite layer 2.

The oil shale, to which the present process is applied, is preferably one that contains a significant amount of heat-sensitive carbonate. Such an oil shale may contain sections of vertical intervals of as much as several tens of feet thick, which are substantially devoid of heat-sensitive and/or water soluble materials. In such a heterogeneous oil shale formation, in the course of application of the present process, such heat-sensitive or soluble minerals are converted to fluids and/or dissolved and removed so that portions of the oil shale become incompetent and form fractures that provide passageways for the circulation of fluid.

In a preferred embodiment of the present invention, a borehole is extended into a relatively low-lying portion of oil shale that contains or is adjacent to a layer or region that is relatively rich in water-soluble minerals. Water-soluble minerals, which are generally saline materials, are frequently encountered in oil shales in the United States, such as the Green River formation in Colorado, in the form of beds, lenses, nodules, nodes, veins or the like. Such minerals include the alkali metal halide salts such as the sodium or potassium chlorides and/or water soluble heat-sensitive carbonate minerals such as nahcolite, trona, or the like.

Portions of subterranean oil shale that contain selected mineral components such as heat-sensitive carbonate minerals and/or water soluble minerals, can be located by means of known geological investigation procedures and equipment. Such procedures are preferably utilized to locate a portion of an oil shale that contains heat-sensitive carbonate mineral and is adjacent to a portion or layer of water-soluble mineral. The water-soluble mineral is utilized to form a cavern or cavity in contact, for example, along its roof, with the oil shale. The cavern formation can readily be accomplished by means of known techniques, such as solution mining and/or mechanical mining, hydraulic and/or explosive fracturing, slurry mining, or the like, that are currently available

Wells 3 and 4 are drilled and completed in horizontally-spaced locations within the oil shale. The wells are preferably spaced close enough to facilitate their interconnection by forming and propagating fractures through or along the boundaries of the nahcolite layer 2. The nahcolite layer, or at least the upper portion of it, can be solution-mined by circulating an aqueous fluid, which can advantageously be warmed and/or made acidic, between the wells. The mining and/or extensive fracturing forms a cavern or cavity 6 extending to a fluid-communicative contact with the wells 3 and 4 and the oil shale 1. The wells may be initially completed by installing and cementing casing and perforating them only along the sections adjacent to the cavity. As known to those skilled in the art, by reversing the flow direction of the solution mining fluid, varying the injection and production pressures, and the like, the cavity 6 can be provided with a signficant areal extent.

After the cavity 6 has been formed, the injection well 3 and the production wells 4 preferably opened along substantially all of the oil shale interval, for example, by perforating a well casing. As known to those skilled in the art, in certain situation, "barefoot" or open hole completions can be utilized. The injection well 3 is preferably equipped with fluid inflow conduit arrangement, such as tubing string 8 and packer 9, to facilitate an inflow of a hot fluid, such as steam, from a surface location into contact with the oil shale 1. An injection well 3, when opened into a vertical interval of the oil shale, particularly when the oil shale is contacted with a hot fluid, is apt to become partly or completely filled with oil shale chunks or rubble 10.

The oil-shale-contacting hot fluid used in this invention can comprise aqueous or nonaqueous fluids such as superheated, dry, or wet, steam or substantially any gas, vapor or liquid, such as carbon dioxide, phenols, hydrocarbons, alcohols, halogenated hydrocarbons, acids, or the like, or substantially any hot aqueous liquid solution, such as an aqueous acid or base or solution of neutral salt. The inflowing hot fluid can be heated by means of surface located and/or downhole located heaters, such as steam generators, water heaters or the like. The heating can be effected or supplemented by an in situ combustion within the oil shale formation. The inflowing fluid is preferably (at least at some location within the cavity) a hot aqueous fluid having a temperature of from about 250.degree.F to a temperature sufficient to cause a relatively rapid oil shale pyrolysis (e.g., from about 600.degree.-1,000.degree.F), with sufficient aqueous liquid being present (or being formed by steam condensation) to dissolve a significant portion of inorganic solid material.

The production wells 4 are each equipped with an arrangement for returning the circulating fluid to a surface location, such as tubing 11 and packer 12. The fluid-return, or production conduit, is preferably arranged to provide an inlet above, but in fluid communication with, the cavity 6.

Where a single pattern of wells is opened into a cavity, the well pattern can advantageously be one such as a five spot (Or six, or seven, or nine spot) with the injection well in the center and the producing wells relatively uniformly displaced radially around the injection well. Where a cavity is extended into and around a plurality of patterns of wells, the patterns are preferably arranged so that the fluid injected through each injection well is produced through a plurality of producing wells, i.e., so that the cavity interconnects the wells of a well pattern in which the ratio of production wells to injections wells is greater than 1 (and thus the well patterns are preferably a series serious of seven spot or nine spot patterns).

The flow directions and velocities of fluids within the wells and the cavity are illustrated by the arrows in the drawing. In the injection well 3 the velocity of the inflowing fluid is relatively high. However, as fluid moves radially through cavity 6 the flow velocity diminishes and, by the time it reaches any of the production wells, the velocity is relatively low. In accordance with this invention, the rates and velocities are controlled so that in an intermediate zone between the injection and production wells the flow velocity diminishes to one at which significantly sized particles of mineral solids are dropped out of the flowing fluids to form a layer such as layer 7 along the bottom of the cavity. The so-deposited solids are repetitively swept by incoming portions of the hot fluid and subsequently become substantially completely depleted of shale oil. Plugging is avoided since the flow within the cavity is substantially radially outward from the injection well with each increment of flowing fluid moving towards larger and larger volumes of free space within the cavity. Any localized plugs thus tend to be temporarily by passed and subsequently depleted as their exposed surfaces are swept by the incoming portions of hot fluid.

As indicated by the dashed line 13 around the borehole of the injection well 3, with the time, the extraction of organic and inorganic components from the borehole walls causes the walls to move generally radially outward. As known to those skilled in the art periodic expansions of a cavity and/or revisions of the patterns of flow within the wells in a cavity can be employed to vary or enhance the rate or extend of oil recovery as long as the well patterns are arranged to include at least one injection well and the fluid injected through it is produced from a plurality of surrounding production wells.

The rates and/or amount of carbonate and/or other heat sensitive or water soluble minerals removed by the hot fluid can be controlled, for example by alternating slugs of aqueous and nonaqueous fluids to vary the rates of extraction of water soluble minerals.

Claims

What is claimed is:

1. A process of producing shale oil from a subterranean oil shale which comprises:

extending a subterranean cavity that contacts the oil shale generally radially throughout at least one well pattern that contains more production wells than injection wells;

injecting a hot fluid that is adapted to interact with the oil shale to yield shale oil into the cavity through at least one injection well;

producing fluid inclusive of shale oil from the cavity through a plurality of production wells;

coordinating the rates and velocities of said injections and production of fluid to maintain both substantially equal rates of outflow from a plurality of production wells around each injection well, and a pattern of flow velocities within the cavity that diminish with radial distance away from each injection well and become too low to transport significantly large particles of solid material into the production wells; and

recovering shale oil from the produced fluid.

2. The process of claim 1 in which the injected fluid is flowed into contact with oil shale located above the cavity and is flowed down along a generally vertical section of exposed oil shale.

3. The process of claim 1 in which an initial well pattern is expanded by forming a generally radial extension of the cavity and opening additional wells in the so-expanded cavity with said wells being located in an arrangement that provides a plurality of injection wells which are each surrounded by a plurality of production wells.

4. The process of claim 1 in which the injected fluid includes at least one slug each of a hot aqueous fluid and a hot nonaqueous fluid.

5. The process of claim 4 in which the oil shale being treated contains a heat sensitive mineral component and the rate of its extraction is controlled by adjusting the volumes of said alternating slugs of injected fluid.