Abstract:
A method for extracting accumulated material from a well bore includes pressurizing gas recovered from the well bore and disposing an extraction string in communication with a sump. The sump is disposed to receive liquid from the well bore. The method further includes sealing the sump and injecting at least a portion of the pressurized gas into the sump such that at least some of the liquid in the sump is driven upward into the extraction string.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to the recovery of subterranean deposits, and more particularly to a method and system for cleaning a well bore.  
       BACKGROUND  
       [0002]     Subterranean zones that contain valuable deposits frequently include other materials, such as entrained water or solids, that are considered extraneous. Since such materials can interfere with the production of the valuable deposits, it may be desirable or necessary to have some way to remove extraneous materials from the production well bore. One method for handling extraneous, co-produced materials is to form a “sump” or “rat hole.” The sump is a well bore drilled below the production well bore such that extraneous materials are allowed to fall into the sump and to collect therein. Sumps may be drilled vertically or obliquely from an existing well bore.  
         [0003]     As materials are collected within the sump, the sump may become nearly or completely filled. In such instances, it is desirable to remove some of the collected material in order to provide sufficient capacity for new material to be collected in the sump. For example, a pump may be lowered into the sump, and water may be pumped to the surface. Such techniques permit the sump to be used to facilitate production after the capacity of the sump would ordinarily have been exhausted. Therefore, it is advantageous to have efficient and versatile methods for removing collected material from a sump. Furthermore, collected materials with a high solid content may present additional challenges for the removal process. For example, the solid phase material may obstruct the flow of collected material through pumps and potentially damage pump mechanisms. In another example, the relatively low liquid content of such collected materials may prove insufficient liquid flow to adequately lubricate and/or cool various types of pumping mechanisms. Consequently, it would be useful to have a technique for extracting collected material that can effectively remove materials with a high solid content as well.  
       SUMMARY  
       [0004]     In a particular implementation, a method for extracting accumulated material from a well bore includes pressurizing gas recovered from the well bore and disposing an extraction string in communication with a sump. The sump is disposed to receive liquid from the well bore. The method further includes sealing the sump and injecting at least a portion of the pressurized gas into the sump such that at least some of the liquid in the sump is driven upward into the extraction string. In another implementation, 1. A system includes a compressor, a sump, a seal, a gas injection string, and an extraction string. The compressor pressurizes gas recovered from a well bore. The sump disposed receives liquid from the well bore. The seal seals the sump so that the sump is substantially airtight when sealed. The gas injection string is coupled to the compressor, and it injects at least a portion of the pressurized gas into the sump. The extraction string disposed within the sump such that at least some of the liquid in the sump is driven upward into the extraction string when the pressurized gas is injected into the sump.  
         [0005]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0006]      FIG. 1  illustrates a system for extracting liquid from a well bore in accordance with an implementation of the present invention;  
         [0007]      FIG. 2  illustrates a cross-sectional view of a working string in the system of  FIG. 1 ;  
         [0008]      FIG. 3  illustrates a downhole portion of a system for extracting liquid from a well bore; and  
         [0009]      FIG. 4  illustrates a method for extracting liquid from a well bore in accordance with another implementation of the present invention. 
     
    
       [0010]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0011]      FIG. 1  depicts a system  100  for cleaning a well bore  102 . In the depicted implementation, system  100  includes a working string  104  and a compressor  106  with a low pressure line  108  and a high pressure line  110 . System  100  also includes valves  112 A and  112 B coupled to high pressure line  110  that permit pressurized gas to be supplied to other parts of system  100 . Overall, system  100  uses pressurized gas to remove undesired materials from well bore  102 .  
         [0012]     In the depicted embodiment, well bore  102  is an articulated well bore extending into a subterranean zone  114 , such as a coal seam, in which there are subterranean deposits of natural gas, such as, for example, methane. An articulated well bore, such as the one depicted in  FIG. 1 , includes a first portion that is vertical, a second portion that is oriented within a plane of a subterranean zone, and a curved portion that connects the first and second portions. It should be understood that the described techniques are applicable to other types of well bores, and the articulated well bore is only one example. Well bore  102  may be reinforced using a tubular casing  103 , which is any rigid material affixed (such as, for example, by cementing) within well bore  102 . Although the described implementation describes a gas well, it should be understood that the described methods are also applicable to recover of a variety of materials from a subterranean zone, including natural gas, crude oil, associated solution gas, formation water, injected water, natural gas liquids, and numerous other subterranean minerals and solids. Within subterranean zone  114 , there may be liquids and/or solids that could collect within the horizontal portion of well bore  102 . The accumulation of such liquids and/or solids may interfere with the production of natural gas from well bore  102 . Accordingly, there is a sump  116  drilled below the horizontal portion of well bore  102 , allowing such liquids and solids to drain by gravity or reservoir pressure into sump  116 . Sump  116  may be drilled using any suitable drilling technique, including any of the numerous well-known techniques for directional drilling. Although sump  116  is depicted as being drawn at an angle from well bore  102 , it should be understood that the described techniques are equally applicable to a sump that is drilled vertically.  
         [0013]     During gas production, gas produced from well bore  102  travels into a phase separation vessel  130 , where the gas is allowed to flow upward while any entrained liquids and/or solids drop from suspension within phase separation vessel  130 , so that phase separation vessel  130  also acts as a storage vessel  130  for entrained liquids and/or solids. Such entrained liquids and/or solids may include, for example, subterranean water from a coal seam. A floater  132  or other similar level indicator may be used to indicate when the liquid level in storage vessel  130  reaches a predetermined level. When the predetermined level is reached, drain  134  may be opened to drain accumulated liquids and solids from storage vessel  130 . The gas, minus any removed liquids and solids, is then provided to low pressure line  108  of compressor  106 . Compressor  106  pressurizes the gas and sends the pressurized gas out of high pressure line  110 , which carries the pressurized gas to a sales or storage facility.  
         [0014]     At the same time, subterranean liquids and/or solids within well bore  102  flow to sump  116 , where they are collected. As liquids and/or solids accumulate within sump  116 , sump  116  may eventually become filled to a level at which it becomes desirable to extract the accumulated material from the sump and produce them at the surface. In previous systems, a pump, such as an electric submersible pump, is placed within sump  116  to pump liquids to the surface through a tube or other conduit. The use of a pump to extract liquids incurs costs to purchase and operate pumps and also introduces technical challenges such as the need for a power and control system for the pump. Additionally, most conventional pumps do not adequately handle high volumes of entrained solids, and they may be damaged if they continue to run in a “pumped off” condition, such as after most of the accumulated material has been extracted Accordingly, it is advantageous to have an alternative technique for extracting liquids and/or solids from sump  116 . Various implementations of the present invention provide such an alternative by using pressurized gas to extract liquid from sump  116 .  
         [0015]     In the depicted implementation, system  100  uses packer  118  to act as a seal for an annular space  126  (illustrated in the cross-sectional view of  FIG. 2 ) between working string  104  and an interior of sump  116 . Packer  118  may be any suitable device adapted to seal sump  116  in a substantially airtight manner. In the depicted implementation, packer  118  is an inflatable device comprising an expandable material, such as an elastomer or numerous other similar materials, that inflates to seal the annular space between working string  104  and sump  116 . Packer  118  is controlled by a control string  120 . Control string  120  is any suitable apparatus for causing packer  118  to seal and unseal sump  116 . In the depicted implementation, control string  120  comprises tubing that couples high pressure line  110  of compressor  106  to packer  118  through valve  112 A, which valve  112 A also includes a vent  113  to the atmosphere. Valve  112 A may be controlled by any suitable method, such as manual operation, electrically-controlled solenoid actuation, or numerous other methods for opening and closing valves. Valve  112 A may thus be opened, closed, and/or vented to cause packer  118  to be inflated or deflated.  
         [0016]     To seal sump  116 , valve  112 A is opened, allowing pressurized gas to flow through control string  120  into packer  118 , thus expanding packer  118  to fill annular space  126 . Once packer  118  is inflated, valve  112 A may be closed to prevent gas from being driven back into high pressure line  110 , such as, for example, by external pressure on packer  118 . To unseal sump  116 , vent  113  of valve  112 A is opened, allowing the pressurized gas in packer  118  to escape into the atmosphere, which in turn deflates packer  118 .  
         [0017]     When sump  116  is sealed, working string  104  is used to inject pressurized gas into sump  116  and to recover gas from sump  116 . In the depicted implementation, working string  104  includes a gas injection string  122  and an extraction string  124 , which surrounds gas injection string  122  to define an annular space  126 , as illustrated in the cross-sectional view of working string  104  shown in  FIG. 2 . Gas injection string  122  comprises tubing or other suitable conduit that couples sump  116  to high pressure line  110  of compressor  106  through valve  112 B, which may be of a similar type to valve  112 A. By opening valve  112 B while sump  116  is sealed, a flow of pressurized gas through gas injection string  122  raises the pressure in sump  116 , which in turn drives liquid into annular space  126 . As the pressure in sump increases  116 , accumulated material from sump  116  is carried to the surface by extraction string  124 , which may be any suitable form of tubing or conduit for producing liquid and/or solid material to the surface. The produced liquids and/or solids are allowed to flow into storage vessel  130 , where they accumulate along with the products dropped from suspension in the produced gas. As noted above, when the accumulated material exceeds a predetermined level, it may be drained from storage vessel  130  in order to prevent storage vessel  130  from overfilling.  
         [0018]     Once the extraction of accumulated material from sump  116  is completed, valve  112 B may be closed to stop the flow of pressurized gas, and packer  118  may be deflated to unseal sump  116  and to permit the pressurized gas in sump  116  to escape. The escaping gas is recovered at the surface along with the rest of the gas produced using well bore  102 . To deflate packer  118 , the gas in packer  118  is vented to the atmosphere through vent  113  of valve  112 A. In an alternative implementation, another valve  112 C may be used to couple control string  120  to a low pressure side of the well system. Such an implementation enables the gas used to inflate packer  118  to be recovered along with the other gas injected into sump  116 . Further, the gas may be introduced into the extraction string  124 , and the sudden entry of gas into extraction string  124  may create a pressure increase that can dislodge debris, such as loose coal or rocks from subterranean zone  114 , that may become caught around the end of working string  104  as liquid enters extraction string  124 .  
         [0019]     A variety of techniques may be used to determine when to extract liquid from sump  116  and when sufficient liquid has been drained from sump  116 . In some implementations, the inflation and deflation of packer  118  and the injection of gas is controlled by control timer  136 . Control timer  136  is set to open and close valve  112 A,  112 B, and/or  112 C so that sump  116  is periodically drained. In other implementations, the determination that sufficient liquid has been drained is based on reading a pressure sensor  128  coupled to packer  118  that measures gas and/or liquid pressure. In such an implementation, control string  120  may include an insulated wire or any of numerous other media for carrying signals from pressure sensor  128  to the surface. In an example of operation, pressure sensor  128  may measure the liquid pressure resulting from accumulated liquid in sump  116 . When the pressure exceeds a certain amount, accumulated material is extracted from sump  116 . In another example, pressure sensor  128  may monitor the gas pressure in sealed sump  116 , and once the gas pressure reaches a predetermined level deemed sufficient to indicate that most of the accumulated material in sump  116  has been driven to the surface, sump  116  may be unsealed. Alternatively, a pressure sensor, which may be located on the surface, may be coupled to the gas injection string  122  to monitor the pressure of a constant, low-volume flow of gas. Rising pressure would then indicate an increase in the level of accumulated material. When the pressure reaches a predetermined threshold level, accumulated material is extracted from sump  116 . The implementations described here are merely examples, and it should be understood that numerous other methods for determining when to extract accumulated material from sump  116  and when to unseal sump  116  may be employed.  
         [0020]      FIG. 3  illustrates an implementation of a downhole portion of working string  104 . In the depicted implementation, sump  116  has been provided with cavity portions  138  extending transversely to the longitudinal axis of sump  116 . Cavity portions  138  increase the capacity of sump  116  to contain liquid. Pressure sensor  128  is a liquid pressure sensor that is placed to measure the liquid level  140  within sump  116  in order to facilitate the determination of when to extract liquid from sump  116 . In the depicted implementation, extraction string  124  includes a flared, end  142 . End  142  may be flared inward in order to prevent larger debris in sump  116  from being pulled into annular space  126  by the flow of liquid and gas into extraction string  124 . This tends to prevent extraction string  124  from becoming obstructed or clogged by such debris.  
         [0021]      FIG. 4  illustrates an example of a method for extracting accumulated material from sump  116  using injection of pressurized gas. At step  402 , valve  112 A coupling packer  118  to high pressure line  110  is opened, inflating packer  118  and sealing sump  116 . Once packer  118  is inflated, valve  112 A to packer  118  may be closed at step  404 . In alternative implementations, valve  112 A may be left open. Valve  112 B coupling gas injection string  122  to high pressure line  110  is opened at step  406 . This causes the pressure in sump to rise, thus driving accumulated liquid and solid material into annular space  126  within extraction string  124  and eventually to the surface. Liquids and/or solids are collected in storage vessel  130  at step  408 . Accumulated material may be drained out of storage vessel  130  to prevent storage vessel  130  from overfilling.  
         [0022]     The removal process continues until the drainage of sump  116  has been completed, as shown at decision step  410 . The determination of when the drainage is completed may be made based on elapsed time, measured changes in pressure, or any other suitable method, including any of those described herein. Once the drainage is completed, valve  112 B is closed at step  412 . The gas within packer  118  is then vented at step  414 , thus unsealing sump  116 . The gas from packer  118  may be vented in any suitable manner, including venting the gas to the atmosphere using valve  112 A or venting the gas back into extraction string  124 .  
         [0023]     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the described techniques may be used to extract any manner of liquids and solids from any type of subterranean well drilled using any suitable technique. In another example, the extraction string may be separated from the gas injection string, so that the extraction string does not enclose the gas injection string. Accordingly, other embodiments are within the scope of the following claims.