Abstract:
In accordance with the teachings of the present invention, a method is provided for preventing formation of sludge in a subsurface cavity having particulate laden fluid disposed therein. The method includes positioning a downhole device having a fluid agitator into the fluid of the subsurface cavity and agitating the fluid using the fluid agitator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. application Ser. No. 10/188,159, filed Jul. 1, 2002 now abandoned, by Joseph A. Zupanick, entitled “Cavity Positioning Tool and Method” which is a continuation of U.S. patent application Ser. No. 09/632,273 filed Aug. 3, 2000 by Joseph A. Zupanick, entitled “Cavity Positioning Tool and Method”, now U.S. Pat. No. 6,412,556. 

   TECHNICAL FIELD OF INVENTION 
   This invention relates generally to the field of downhole cavity tools and more particularly to a cavity positioning tool and method. 
   BACKGROUND OF THE INVENTION 
   Subsurface resources such as oil, gas, and water are typically recovered by drilling a bore hole from the surface to a subterranean reservoir or zone that contains the resources. The bore hole allows oil, gas, and water to flow to the surface under its own pressure. For low pressure or depleted zones, rod pumps are often used to lift the fluids to the surface. 
   To facilitate drilling and production operations, cavities are often formed in the production zone. The cavity allows the well bore to be more readily intersected during drilling operations and collects fluids during production operations. The collection of fluids allows pumps to be operated intermittently when the cavity is full, which reduces wear on the pump. 
   Short extensions called a “rat hole” are often formed at the bottom of the cavity to collect cuttings and other drilling debris. As the subsurface liquids collect in the well bore, the heavier debris falls to the bottom of the rat hole and is thereby both centralized and collected out of the cavity. To avoid being clogged with debris, inlets for rod and other downhole pumps should be positioned within the cavity above the rat hole. In addition, the pump inlet should be positioned fairly low in the cavity to avoid vapor lock (i.e., below the fluid waterline). Traditional methods of positioning the pump inlets, however, are often inaccurate and inefficient, leading to clogging or vapor lock and increased maintenance and operation costs for the well. 
   SUMMARY OF THE INVENTION 
   In accordance with the teachings of the present invention, a method is provided for preventing formation of sludge in a subsurface cavity having particulate laden fluid disposed therein. The method includes positioning a downhole device having a fluid agitator into the fluid of the subsurface cavity and agitating the fluid using the fluid agitator. 
   In accordance with one embodiment of the present invention, a method is provided for preventing formation of sludge in a subsurface cavity. The method includes positioning an inlet of a pump via a well bore into a cavity formed underground, the cavity including fluid and a plurality of particles in the fluid. The method further includes agitating the fluid and removing the fluid. 
   In accordance with another aspect of the present invention, a method is provided for removing particulate laden fluid from a subterranean zone. The method includes lowering an inlet of a pump through a well bore into a cavity formed in a subterranean zone, the cavity extending radially from the well bore. The method also includes radially extending within the cavity a plurality of arms coupled to the pump inlet and positioning the inlet in the cavity by resting the arms on a floor of the cavity. The method further includes collecting particulate laden fluid in the cavity, rotating the arms about a longitudinal axis of the pump, and removing the particulate laden fluid with the pump. 
   Important technical advantages of the invention include providing an improved cavity positioning tool and method. In particular, the tool includes arms that are retractable for lowering through a well bore to a cavity and extendable in the cavity to position a device within or at a set relation to the cavity. In one embodiment, the arms are extended by centrifugal force and automatically retract in the absence of centrifugal force. As a result, the tool has a minimum of parts and is highly durable. 
   Another technical advantage of the present invention includes providing a method and system for positioning a pump inlet in a cavity. In particular, the pump inlet is positioned in a lower portion of the cavity by extending arms that rest on the cavity floor above a rat hole. This position of the pump inlet significantly reduces clogging of the pump inlets and prevents the pump from inadvertently entering the rat hole. Additionally, this position minimizes vapor lock. 
   Still another technical advantage of the present invention includes providing an improved method for supporting a pump string extended from the surface to a subterranean zone. In particular, a pump string is supported from the floor of the cavity. This allows well head maintenance and other surface operations to be performed without pulling out or otherwise supporting the string from the surface. 
   Still another technical advantage of the present invention includes providing an improved method for removing solid-laden fluids from a coal seam or other subterranean zone. In particular, a pump inlet is coupled to a cavity positioning device with extending arms that rest on a cavity floor above a rat hole. The arms are rotated slowly to agitate the liquid in the cavity, thereby suspending debris to allow removal within the liquid and lowering the tendency of particulate matter to coalesce. Thus, the debris and particulate matter is less likely to form clumps of larger particles, which reduces clogging of the pump inlets. 
   Other advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A–B  are diagrams illustrating side views of a cavity positioning tool in accordance with one embodiment of the present invention; 
       FIGS. 2A–C  are a series of diagrams illustrating operation of the tool of  FIG. 1  in accordance with one embodiment of the present invention; and, 
       FIGS. 3A–B  are a series of diagrams illustrating operation of the tool of  FIG. 1 , in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A–B  illustrate a cavity positioning tool  10  in accordance with one embodiment of the present invention. In this embodiment, tool  10  is adapted to position a pump inlet in a subsurface cavity. It will be understood that tool  10  may be adapted to position other suitable devices within or in relation to a cavity. For example, motors, controllers, and valves may be positioned in or relative to a cavity with the tool  10 . Tool  10  is constructed of steel or other suitable metals or materials, such that are resistant to damage in the downhole environment. 
   Referring to  FIG. 1A , the tool  10  comprises a head piece  12  and a plurality of blunt arms  14 . As described in more detail below, the arms are coupled to the head piece  12  and operable to be radially extended outward from a first position of substantial alignment with a longitudinal axis associated with the head piece  12  to a second extended position. In the illustrated embodiment, the blunt arms  14  are coupled to head piece  12  by pivot assembly  16 . It will be understood that blunt arms  14  may by slidably or otherwise suitably coupled to head piece  12 . 
   The head piece  12  is configured at one end to receive a downhole string  20 . Head piece  12  may be threaded to receive a downhole string, or may include clamps, interlocking pieces, or be otherwise suitably configured to attach to, engage, or mate with downhole string  20 . Head piece  12  may be an integrated piece or a combination of components. For example, head piece  12  may include a downhole motor for rotating the head piece  12 , such as a bottom part of the head piece  12 , relative to the downhole string. 
   The downhole string  20  is a drill string, pump string, pipe, wireline, or other suitable downhole device that can be used to dispose the tool  10  within a cavity and extend the blunt arms  14 . In the illustrated embodiment, the downhole string  20  is a pump string  22  with an inlet  24  coupled directly to the tool  10 . The pump string  22  may be a sucker or other rod or multistage pump, a downhole pump with piping to the surface, or other suitable pumping system. 
   The blunt arms  14  are rounded, dull, or otherwise shaped so as to prevent substantial cutting of or damage to the cavity. In the illustrated embodiment, blunt arms  14  are cylindrical in shape with an elongated body and having a circular cross-section. 
   The blunt arms  14  may be end-weighted by adding weight to the ends distal to the head piece  12 , or may comprise a hollow portion proximate to the head pin such that the ends of the blunt arms  14  are thereby made heavier than the rest of the blunt arms  14 . The blunt arms  14  are sized to fit within a cavity when in an extended position and to exceed a diameter of a rat hole, bore hole, or other extensions, if any, below the cavity. 
   The pivot assembly  16  rotatably connects the blunt arms  14  to the head piece  12 . In one embodiment, the pivot assembly  16  allows the blunt arms  14  to radially extend and retract in response to rotational energy applied to the tool  10 . In this embodiment, pivot assembly  16  may be a clovis-and-pin type assembly. 
   As illustrated, blunt arms  14  hang freely down, in substantial alignment with the longitudinal axis of head piece  12 . Blunt arms  14  are in substantial alignment when the blunt arms  14  hang freely down, within a few degrees of the longitudinal axis and/or fit down and through a well bore. As described in more detail below, in response to rotation of head piece  12 , blunt arms  14  are radially extended towards a perpendicular position relative to head piece  12 . The blunt arms  14  are automatically retracted when head piece  12  ceases to rotate by force of gravity or other suitable mechanism. It will be understood that the blunt arms  14  may be slidably or otherwise suitably connected to the head piece  12 . 
   The pivot assembly  16  may include stops  18  to control extension of blunt arms  14 . Stops  18  may be configured to allow blunt arms  14  to extend ninety degrees to a perpendicular position, may limit the extension of blunt arms  14  to a lesser range, or permit a range greater than ninety degrees. Stops  18  may be integral or adjustable. Controlling the stops  18 , and the extension of blunt arms  14  thereby, controls the resting place of the pump string  22  relative to the floor of the cavity. 
     FIGS. 2A–C  are a series of drawings illustrating the operation of tool  10 . Referring to  FIG. 2A , a pump string is positioned in a cavity for a degasification operation in connection with a coal seam prior to mining operations. In this embodiment, a well bore  30  is drilled from the surface  35  into a coal seam  40 . A cavity  32  is formed within the coal seam  40 . A rat hole  34  is drilled at the bottom of cavity  32 . The rat hole  34  has a diameter  37 . In a preferred embodiment, the blunt arms  14  have a length such that when extended, the distance from the distal end of one blunt arm  14  to the distal end of another blunt arm  14  exceeds the diameter  37 . It will be noted that in this instance, as well as throughout this description, use of the word “each” includes all of any particular subset. A drainage pattern  45  is drilled from a radiused bore  46  and extends into the coal seam  40  and connects to cavity  32 . The well bore  30  may have a diameter between seven and ten inches, the cavity a diameter between seven and nine feet, and the rat hole a diameter between seven and ten inches. Further information regarding the dual wells and drainage pattern is described in co-owned U.S. patent application Ser. No. 09/444,029, entitled “Method and System for Accessing Subterranean Deposits from the Surface,” which is hereby incorporated by reference. 
   The pump string  20  is positioned by coupling an inlet to the coupling means  12  of the positioning tool  10 . Next, the tool  10  on the pump string  20  is lowered through the well bore  30 . While tool  10  is lowered through well bore  30 , the blunt arms  14  remain in the retracted position with the blunt arms  14  hanging down in substantial alignment with the longitudinal axis of pump string  20 . Blunt arms  14  are lowered until proximate to the cavity  32 . Estimating the position of the cavity may be accomplished by comparing the known approximate depth of the cavity  32  to the length of pump string  20  in hand or deployed, or other suitable methods. 
   Referring to  FIG. 2B , after the tool is positioned proximate to the cavity  32 , blunt arms  14  are extended by rotating the head piece  12 . In the illustrated embodiment, head piece  12 , is rotated by rotating the pump string  20 , for example, in the direction of arrow  38 . As pump string  20  is rotated, the blunt arms  14  are extended radially outward from pump string  20  in opposite directions, traveling generally as indicated by arrow  50 . One skilled in the art will recognize that other methods are available to extend blunt arms  14  radially outward from pump string  20 . For example, mechanical means such as a wire connected to blunt arms  14  might be used to extend blunt arms  14  radially outward from pump string  20 . The blunt arms  14  are extended until they contact the stops  18 . 
   Referring to  FIG. 2C , once the blunt arms  14  are extended, or while being extended, the pump string  20  is lowered further into well bore  30 . Pump string  20  is lowered until blunt arms  14  make contact with the floor  33  of cavity  32 . When resting on the cavity floor  33 , pump inlets  24  are at a known position within the cavity  32 . By adjusting the spacing between the pump inlets  24  and the blunt arms  14  of the tool  10 , the distance between the pump inlets  24  and the cavity floor  33  can be modified. This adjustment may be made in a variety of ways, including adding spacers to the head piece  12 . Additionally, by changing the maximum angle of the blunt arms  14 , the distance between the pump inlets  24  and the cavity floor  33  can be modified. Adjusting the maximum angle of the blunt arms  14  can be accomplished in a variety of ways, including adjusting the stops  18  to restrict the radial extension of the blunt arms  14 . Therefore, the present invention provides for more definite location of the pump inlets  24  within cavity  32 , by use of positioning tool  10 . 
   Once the pump  22  is positioned within cavity  32  by tool  10 , fluids that drain from the drainage pattern  45  into the cavity  32  are pumped to the surface with the pump string  20 . Fluids may be continuously or intermittently pumped as needed to remove the fluids from the cavity  32 . Additionally, gas is diffused from the coal seam  40  and is continuously connected at the surface  35  as it passes through well bore  30 . 
   When fluid and gas removal operations are complete, the tool  10  may be removed from its position within cavity  32 . In reverse operation, pump string  20  is raised until blunt arms  14  are no longer in contact with the floor  33  of cavity  32 . Blunt arms  14  are moved from an extended position to one of substantial alignment with pump string  20 . If the blunt arms  14  were extended by centrifugal force, the blunt arms  14  will return to the first position of substantial alignment with pump string  20  upon being raised from the cavity floor. Once the blunt arms  14  have been returned to a position of substantial alignment with pump string  20 , pump string  20  may be raised through and out of well bore  30 . 
     FIGS. 3A–B  are a series of drawings illustrating operation of tool  10  during production of fluid and gas from the cavity  32 . Referring to  FIG. 3A , the pump string  20  is positioned in the cavity  32  for degasification operation of the coal seam  40  as previously described. The pump inlets  24  are positioned within the cavity  32  such that the pump inlets  24  are above rat hole  34 , but below the waterline of the fluids collected in cavity  32 . 
   As fluids are collected in the cavity  32 , particulate matter and other debris such as drilling cuttings and coal fines are also collected in the cavity  32 . Operation of the downhole pump  22  causes the suspended particulate matter and other debris to move through different locations within the body of fluid in cavity  32 . As the settling of particulate matter and other debris proceeds, the amount of particulate matter and other debris suspended in the fluid changes. Accordingly, different locations within the fluid body, or phases, have different concentrations of particulate matter and other debris. The heavier debris settles to the floor of cavity  32  and may eventually settle in rat hole  34 . 
   The relative size of the particulate matter and other debris changes across the different phases of the fluid body. The smallest particulate matter and other debris remains close to the surface in Phase III, as shown in  FIG. 3A . As the particulate matter and other debris coalesces or clumps together, the composite matter begins to settle through the phases and may eventually fill the rat hole  34  and form a solid layer of sludge on the floor of cavity  32 . Eventually, the depth of the sludge layer and size of the composite matter is such that the pump inlets  24  become clogged, causing production delays and added expense. 
   Referring to  FIG. 3B , the blunt arms  14  are rotated in the cavity  32  about the longitudinal axis of pump string  20  by rotating the pump string  20  at the surface or by other suitable means. In one embodiment, the pump string is rotated at the surface by a tubing rotator, at approximately one rotation per day. 
   Rotating the blunt arms  14  agitates the fluid collected within the cavity  32 . In the absence of agitation the particulate matter and other debris may coalesce or clump together forming larger composite matter that would eventually clog the pump inlets  24 . With rotation of the blunt arms  14 , however, solids remain suspended in the fluid and are removed with the fluid. In addition, the distribution of the remaining particulate matter is pushed away from the pump inlets  24 , towards the sidewalls of cavity  32 . 
   As illustrated in  FIG. 3B , rotation of the blunt arms  14  causes the levels or phases decrease in area. Furthermore, rotation causes the shape of the phases to become more sharply sloping from the sidewalls of cavity  32  towards the floor of cavity  32 . The change in shape of the phases prevents particulate matter from clumping in the liquid in the near vicinity of the pump inlets  24 . Thus, rotation of the blunt arms  14  decreases the concentration of large particulate matter and other debris surrounding the pump inlets  24 , and thereby greatly reduces clogging of the pump inlets  24 , and the increased costs associated therewith. 
   Although the present invention has been described in detail, it should be understood that various changes, alterations, substitutions, and modifications may be made to the teachings herein without departing from the spirit and scope of the present invention, which is solely defined by the appended claims.