Three-dimensional well system for accessing subterranean zones

A drainage system for accessing multiple subterranean zones from the surface includes an entry well extending from the surface. The system also includes two or more exterior drainage wells extending from the entry well through the subterranean zones. The exterior drainage wells each extend outwardly and downwardly from the entry well for a first selected distance and then extend downwardly in a substantially vertical orientation for a second selected distance.

BACKGROUND OF THE INVENTION

Subterranean deposits of coal often contain substantial quantities of entrained methane gas. Limited production and use of methane gas from coal deposits has occurred for many years. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem in producing methane gas from coal seams is that while coal seams may extend over large areas of up to several thousand acres, the coal seams are not very thick, varying from a few inches to several meters thick. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits may not be amenable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well in a coal seam is produced, further production is limited in volume. Additionally, coal seams are often associated with subterranean water, which typically must be drained from the coal seam in order to produce the methane.

SUMMARY OF THE INVENTION

The present invention provides a three-dimensional well system for accessing subterranean zones that substantially eliminates or reduces the disadvantages and problems associated with previous systems and methods. In particular, certain embodiments of the present invention provide a three-dimensional well system for accessing subterranean zones for efficiently producing and removing entrained methane gas and water from multiple coal seams.

In accordance with one embodiment of the present invention, a drainage system for accessing multiple subterranean zones from the surface includes an entry well extending from the surface. The system also includes two or more exterior drainage wells extending from the entry well through the subterranean zones. The exterior drainage wells each extend outwardly and downwardly from the entry well for a first selected distance and then extend downwardly in a substantially vertical orientation for a second selected distance.

Embodiments of the present invention may provide one or more technical advantages. These technical advantages may include providing a system and method for efficiently accessing one or more subterranean zones from the surface. Such embodiments provide for uniform drainage of fluids or other materials from these subterranean zones using a single surface well. Furthermore, embodiments of the present invention may be useful for extracting fluids from multiple thin sub-surface layers (whose thickness makes formation of a horizontal drainage well and/or pattern in the layers inefficient or impossible). Fluids may also be injected into one or more subterranean zones using embodiments of the present invention.

Other technical advantages of the present invention will be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates an example three-dimensional drainage system10for accessing multiple subterranean zones20a-20d(hereinafter collectively referred to as subterranean zones20) from the surface. In the embodiment described below, subterranean zones20are coal seams; however, it will be understood that other subterranean formations can be similarly accessed using drainage system10. Furthermore, although drainage system10is described as being used to remove and/or produce water, hydrocarbons and other fluids from zones20, system10may also be used to treat minerals in zones20prior to mining operations, to inject or introduce fluids, gases, or other substances into zones20, or for any other suitable purposes.

Drainage system10includes an entry well30and multiple drainage wells40. Entry well30extends from a surface towards subterranean zones20, and drainage wells40extend from near the terminus of entry well30through one or more of the subterranean zones20. Drainage wells40may alternatively extend from any other suitable portion of entry well30or may extend directly from the surface. Entry well30is illustrated as being substantially vertical; however, it should be understood that entry well30may be formed at any suitable angle relative to the surface.

One or more of the drainage wells40extend outwardly and downwardly from entry well30to form a three-dimensional drainage pattern that may be used to extract fluids from subterranean zones20. Although the term “drainage well” is used, it should also be understood that these wells40may also be used to inject fluids into subterranean zones20. One or more “exterior” drainage wells40are initially drilled at an angle away from entry well30(or the surface) to obtain a desired spacing of wells40for efficient drainage of fluids from zones20. For example, wells40may be spaced apart from one another such that they are uniformly spaced. After extending at an angle away from entry well30to obtain the desired spacing, wells40may extend substantially downward to a desired depth. A “central” drainage well40may also extend directly downwardly from entry well30. Wells40may pass through zones20at any appropriate points along the length of each well40.

As is illustrated in the example system10ofFIG. 1, each well40extends downward from the surface and through multiple subterranean zones20. In particular embodiments, zones20contain fluids under pressure, and these fluids tend to flow from their respective zone20into a well40passing through such a zone20. A fluid may then flow down a well40and collect at the bottom of the well40. The fluid may then be pumped to the surface. In addition or alternatively, depending on the type of fluid and the pressure in the formation, a fluid may flow from a zone20to a well40, and then upwardly to the surface. For example, coal seams20containing water and methane gas may be drained using wells40. In such a case, the water may drain from a coal seam20and flow to the bottom of wells40and be pumped to the surface. While this water is being pumped, methane gas may flow from the coal seam20into wells40and then upwardly to the surface. As is the case with many coal seams, once a sufficient amount of water has been drained from a coal seam20, the amount of methane gas flowing to the surface may increase significantly.

In certain types of subterranean zones20, such as a zones20having low permeability, fluid is only able to effectively travel a short distance to a well40. For example, in a low permeability coal seam20, it may take a long period of time for water in the coal seam20to travel through the seam20to a single well drilled into the coal seam20from the surface. Therefore, it may also take a long time for the seam20to be sufficiently drained of water to produce methane gas efficiently (or such production may never happen). Therefore, it is desirable to drill multiple wells into a coal seam20, so that water or other fluids in a particular portion of a coal seam or other zone20are relatively near to at least one well. In the past, this has meant drilling multiple vertical wells that each extend from a different surface location; however, this is generally an expensive and environmentally unfriendly process. System10eliminates the need to drill multiple wells from the surface, while still providing uniform access to zones20using multiple drainage wells40. Furthermore, system10provides more uniform coverage and more efficient extraction (or injection) of fluids than hydraulic fracturing, which has been used with limited success in the past to increase the drainage area of a well bore.

Typically, the greater the surface area of a well40that comes in contact with a zone20, the greater the ability of fluids to flow from the zone20into the well40. One way to increase the surface area of each well40that is drilled into and/or through a zone20is to create an enlarged cavity45from the well40in contact with the zone20. By increasing this surface area, the number of gas-conveying cleats or other fluid-conveying structures in a zone20that are intersected by a well40is increased. Therefore, each well40may have one or more associated cavities45at or near the intersection of the well40with a subterranean zone20. Cavities45may be created using an underreaming tool or using any other suitable techniques.

In the example system10, each well40is enlarged to form a cavity45where each well40intersects a zone20. However, in other embodiments, some or all of wells40may not have cavities at one or more zones20. For example, in a particular embodiment, a cavity45may only be formed at the bottom of each well40. In such a location, a cavity45may also serve as a collection point or sump for fluids, such as water, which have drained down a well40from zones20located above the cavity45. In such embodiments, a pump inlet may be positioned in the cavity45at the bottom of each well40to collect the accumulated fluids. As an example only, a Moyno pump may be used.

In addition to or instead of cavities45, hydraulic fracturing or “fracing” of zones20may be used to increase fluid flow from zones20into wells40. Hydraulic fracturing is used to create small cracks in a subsurface geologic formation, such as a subterranean zone20, to allow fluids to move through the formation to a well40.

As described above, system10may be used to extract fluids from multiple subterranean zones20. These subterranean zones20may be separated by one or more layers50of materials that do not include hydrocarbons or other materials that are desired to be extracted and/or that prevent the flow of such hydrocarbons or other materials between subterranean zones20. Therefore, it is often necessary to drill a well to (or through) a subterranean zone20in order to extract fluids from that zone20. As described above, this may be done using multiple vertical surface wells. However, as described above, this requires extensive surface operations.

The extraction of fluids may also be performed using a horizontal well and/or drainage pattern drilled through a zone20and connected to a surface well to extract the fluids collected in the horizontal well and/or drainage pattern. However, although such a drainage pattern can be very effective, it is expensive to drill. Therefore, it may not be economical or possible to drill such a pattern in each of multiple subterranean zones20, especially when zones20are relatively thin.

System10, on the other hand, only requires a single surface location and can be used to economically extract fluids from multiple zones20, even when those zones20are relatively thin. For example, although some coal formations may comprise a substantially solid layer of coal that is fifty to one hundred feet thick (and which might be good candidates for a horizontal drainage pattern), other coal formations may be made up of many thin (such as a foot thick) layers or seams of coal spaced apart from one another. While it may not be economical to drill a horizontal drainage pattern in each of these thin layers, system10provides an efficient way to extract fluids from these layers. Although system10may not have the same amount of well surface area contact with a particular coal seam20as a horizontal drainage pattern, the use of multiple wells40drilled to or through a particular seam20(and possibly the use of cavities45) provides sufficient contact with a seam20to enable sufficient extraction of fluid. Furthermore, it should be noted that system10may also be effective to extract fluids from thicker coal seams or other zones20as well.

FIG. 2illustrates another example three-dimensional drainage system110for accessing multiple subterranean zones20from the surface. System110is similar to system10described above in conjunction with FIG.1. Thus, system110includes an entry well130, drainage wells140formed through subterranean zones20, and cavities145. However, unlike system10, the exterior drainage wells140of system110do not terminate individually (like wells40), but instead have a lower portion142that extends toward the central drainage well140and intersects a sump cavity160located in or below the deepest subterranean zone20being accessed. Therefore, fluids draining from zones20will drain to a common point for pumping to the surface. Thus, fluids only need to be pumped from sump cavity160, instead of from the bottom of each drainage well40of system10. Sump cavity160may be created using an underreaming tool or using any other suitable techniques.

FIG. 3illustrates a cross-section diagram of example three-dimensional drainage system110, taken along line3—3as indicated in FIG.2. This figure illustrates in further detail the intersection of drainage wells140with sump cavity160. Furthermore, this figure illustrates a guide tube bundle200that may be used to aid in the drilling of drainage wells140(or drainage wells40), as described below.

FIG. 4illustrates entry well130with a guide tube bundle200and an associated casing210installed in entry well130. Guide tube bundle200may be positioned near the bottom of entry well130and used to direct a drill string in one of several particular orientations for the drilling of drainage wells140. Guide tube bundle200comprises a set of twisted guide tubes220(which may be joint casings) and a casing collar230, as illustrated, and is attached to casing210. As described below, the twisting of joint casings220may be used to guide a drill string to a desired orientation. Although three guide tubes220are shown in the example embodiment, any appropriate number may be used. In particular embodiments, there is one guide tube220that corresponds to each drainage well40to be drilled.

Casing210may be any fresh water casing or other casing suitable for use in down-hole operations. Casing210and guide tube bundle200are inserted into entry well130, and a cement retainer240is poured or otherwise installed around the casing inside entry well130. Cement retainer240may be any mixture or substance otherwise suitable to maintain casing210in the desired position with respect to entry well130.

FIG. 5illustrates entry well130and guide tube bundle200as drainage wells140are about to be drilled. A drill string300is positioned to enter one of the guide tubes220of guide tube bundle200. Drill string300may be successively directed into each guide tube220to drill a corresponding drainage well40from each guide tube220. In order to keep drill string300relatively centered in entry well130, a stabilizer310may be employed. Stabilizer310may be a ring and fin type stabilizer or any other stabilizer suitable to keep drill string300relatively centered. To keep stabilizer310at a desired depth in entry well130, a stop ring320may be employed. Stop ring320may be constructed of rubber, metal, or any other suitable material. Drill string300may be inserted randomly into any of a plurality of guide tubes220, or drill string300may be directed into a selected guide tube220.

FIG. 6illustrates entry well130and guide tube bundle200as a drainage well140is being drilled. As is illustrated, the end of each guide tube220is oriented such that a drill string300inserted in the guide tube220will be directed by the guide tube in a direction off the vertical. This direction of orientation for each tube220may be configured to be the desired initial direction of each drainage well140from entry well130. Once each drainage well140has been drilled a sufficient distance from entry well130in the direction dictated by the guide tube220, directional drilling techniques may then be used to change the direction of each drainage well140to a substantially vertical direction or any other desired direction.

It should be noted that although the use of a guide tube bundle200is described, this is merely an example and any suitable technique may be used to drill drainage wells140(or drainage wells40). For example, a whipstock may alternatively be used to drill each drainage well140from entry well130, and such a technique is included within the scope of the present invention. If a whipstock is used, entry well130may be of a smaller diameter than illustrated since a guide tube bundle does not need to be accommodated in entry well130.FIG. 7illustrates the drilling of a first drainage well140from entry well130using a drill string300and a whipstock330.

FIG. 8illustrates an example method of drilling and producing fluids or other resources using three-dimensional drainage system110. The method begins at step350where entry well130is drilled. At step355, a central drainage well140is drilled downward from entry well130using a drill string. At step360, a sump cavity160is formed near the bottom of central drainage well140and a cavity145is formed at the intersection of central drainage well140and each subterranean zone20. At step365, a guide tube bundle200is installed into entry well130.

At step370, a drill string300is inserted through entry well130and one of the guide tubes220in the guide tube bundle200. The drill string300is then used to drill an exterior drainage well140at step375(note that the exterior drainage well140may have a different diameter than central drainage well140). As described above, once the exterior drainage well140has been drilled an appropriate distance from entry well130, drill string130may be maneuvered to drill drainage well140downward in a substantially vertical orientation through one or more subterranean zones20(although well140may pass through one or more subterranean zones20while non-vertical). Furthermore, in particular embodiments, wells140(or40) may extend outward at an angle to the vertical. At step380, drill string300is maneuvered such that exterior drainage well140turns towards central drainage well140and intersects sump cavity160. Furthermore, a cavity145may be formed at the intersection of the exterior drainage well140and each subterranean zone20at step382.

At decisional step385, a determination is made whether additional exterior drainage wells140are desired. If additional drainage wells140are desired, the process returns to step370and repeats through step380for each additional drainage well140. For each drainage well140, drill string300is inserted into a different guide tube220so as to orient the drainage well140in a different direction than those already drilled. If no additional drainage wells140are desired, the process continues to step390, where production equipment is installed. For example, if fluids are expected to drain from subterranean zones20to sump cavity160, a pump may be installed in sump cavity160to raise the fluid to the surface. In addition or alternatively, equipment may be installed to collect gases rising up drainage wells140from subterranean zones20. At step395, the production equipment is used to produce fluids from subterranean zones20, and the method ends.

Although the steps have been described in a certain order, it will be understood that they may be performed in any other appropriate order. Furthermore, one or more steps may be omitted, or additional steps performed, as appropriate.

FIG. 9illustrates a nested configuration of multiple example three-dimensional drainage systems410. Each drainage system410comprises seven drainage wells440arranged in a hexagonal arrangement (with one of the seven wells440being a central drainage well410drilled directly downward from an entry well430). Since drainage wells440are located subsurface, their outermost portion (that which is substantially vertical) is indicated with an “x” in FIG.9. As an example only, each system410may be formed having a dimension d1of 1200 feet and a dimension d2of 800 feet. However, any other suitable dimensions may be used and this is merely an example.

As is illustrated, multiple systems410may be positioned in relationship to one another to maximize the drainage area of a subterranean formation covered by systems410. Due to the number and orientation of drainage wells440in each system410, each system410covers a roughly hexagonal drainage area. Accordingly, system410may be aligned or “nested”, as illustrated, such that systems410form a roughly honeycomb-type alignment and provide uniform drainage of a subterranean formation.

Although “hexagonal” systems410are illustrated, may other appropriate shapes of three-dimensional drainage systems may be formed and nested. For example, systems10and110form a square or rectangular shape that may be nested with other systems10or110. Alternatively, any other polygonal shapes may be formed with any suitable number (even or odd) of drainage wells.