Abstract:
An apparatus has an elongate body defining an interior chamber and a gas passage in communication with the interior chamber. The elongate body further includes a base defining a liquid opening and a cap defining an outlet opening. The liquid opening is adapted to receive a liquid disposed in a wellbore. The gas passage is adapted to receive a gas. An outlet of the gas passage is disposed a first distance from the base and an inner conduit is disposed in the interior chamber. The inner conduit includes a first open end in communication with the liquid opening and a second open end in communication with the interior chamber. The second open end is disposed a second distance from the base.

Description:
BACKGROUND 
       [0001]    Downhole wellbores are utilized to extract methane gas from coal beds below ground. The amount of methane extracted can be increased by reducing the pressure on the coal bed. Typically, this is accomplished by removing water from above the beds. This reduces the pressure and thereby increases the rate at which methane is emitted from the coal. Water may be removed in a number of ways. De-watering pumps may be inserted into the wellbore and the water may be pumped out directly; however, traditional methods reach mechanical limits as the pressures decline. Alternatively, gas such as methane gas may be pumped into the wellbore, where it mixes with the water, to produce a mist or vapor that is then extracted from the wellbore. 
       Gas Lift Assembly and Methods 
       [0002]    The systems and methods described herein may be utilized in wells that typically have been inaccessible to traditional gas lift methods. Such gas lift methods typically require excessive back pressures (and therefore casing packer completion) on the reservoir to operate properly. Indeed, even low water levels in a well bore can exert sufficient hydrostatic head to prevent gas flow. The systems and methods described herein can extract product gas from well with very low pressures at the well reservoir face. 
         [0003]    Additionally, the systems and methods described herein can also be utilized in existing rod well pump applications with minimal modifications to the well. For example, the rods and pump can be removed from the well. Thereafter, the inner string, seal assembly, tool, and other components, can be inserted into the outer string and set in the seating nipple previously occupied by the rod pump. This seals the two strings and the reservoir to properly control gas flow. The configuration allows for a reduction in hydrostatic head and still enables lifting of water, in a mist form, to the surface. Use of the choke helps prevent excessive head in the inner string, and thus, prevents water from entering the tool. 
         [0004]    In one aspect, the technology relates to an apparatus having an elongate body defining an interior chamber and a gas passage in communication with the interior chamber, the elongate body further includes a base defining a liquid opening and a cap defining an outlet opening, wherein the liquid opening is adapted to receive a liquid disposed in a wellbore, and wherein the gas passage is adapted to receive a gas, wherein an outlet of the gas passage is disposed a first distance from the base; and an inner conduit disposed in the interior chamber, and wherein the inner conduit includes: a first open end in communication with the liquid opening; and a second open end in communication with the interior chamber, wherein the second open end is disposed a second distance from the base. 
         [0005]    In another aspect, the technology relates to an apparatus which includes an elongate body defining an interior chamber, a liquid inlet, a gas inlet in communication with the interior chamber, and a liquid-gas outlet in communication with the interior chamber; and an inner conduit disposed within the elongate body and in communication with liquid inlet, wherein the inner conduit defines a liquid outlet in communication with the interior chamber, and wherein the gas inlet is disposed a first distance from the liquid inlet and wherein the liquid outlet is disposed a second distance from the liquid inlet, wherein the second distance is less than the first distance. 
         [0006]    In yet another aspect, the technology relates to a method which includes pressurizing, with a working gas, an outer string of a downhole wellbore so as to expel water from the outer string; pressurizing, with the working gas, an inner string of a downhole wellbore so as to expel water from the inner string and a tool disposed therein; reducing pressure in the inner string, wherein reducing pressure in the inner string allows water to enter the tool from an inlet; causing the working gas to flow from the outer string through the tool, wherein the flow of working gas and water, in combination, produce an upward flow of mist in the inner string; and collecting the mist from the inner string. 
         [0007]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The same number represents the same element or same type of element in all drawings. 
           [0009]      FIG. 1A and 1B  depict a schematic side sectional view of a wellbore. 
           [0010]      FIG. 2  depicts a schematic side sectional view of one embodiment of a gas lift assembly. 
           [0011]      FIG. 3  depicts a schematic side sectional view of the gas lift assembly of  FIG. 2  disposed in a wellbore. 
           [0012]      FIG. 4  depicts a schematic side sectional view of another embodiment of a gas lift assembly disposed in a wellbore. 
           [0013]      FIG. 5  depicts a schematic side sectional view of another embodiment of a gas lift assembly disposed in a wellbore. 
           [0014]      FIGS. 6A-6C  depict a method of removing water from a downhole wellbore. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present disclosure is directed generally to systems and methods that are utilized to extract methane gas product from a downhole wellbore. In general, an extraction tool is inserted into a wellbore and pressurized with a compressed working gas, to remove water present in the wellbore. The water is then entrained within the compressed working gas being injected around the tool. This entrainment produces a vapor, mist, or other generally lighter mixture of water and working gas that allows the water to be extracted from the wellbore. The tool allows the water table to drawn down to and maintained at the lowest reservoir level, thereby reducing the pressure on the coal bed and increasing the rate at which the coal bed generates gas. 
         [0016]    Certain terminology used herein describes the relative relationships between pressures, flow rates, etc., as well as the states of the various fluids that are moved through the wellbore and tool. For example, use of the term “high pressure” in one portion of the wellbore does not necessarily mean that the pressure in that portion is at a certain measured threshold in excess of ambient. Instead, use of the term is meant to describe a condition where the pressure in one portion of the wellbore is higher than a pressure in another portion of the wellbore. In another example, the term “mist” or “vapor” is used to describe a mixture of water and working gas that is extracted from the wellbore utilizing the tools described herein. These terms are used for convenience to describe a condition where water is entrained within a working gas being injected upwards into the extraction tool and implies a state where a plurality of discrete, small volumes of water are separated from each other by a volume of working gas, such that the water can be lifted or otherwise extracted out of the wellbore by the pressure of the working gas. It is not necessarily utilized to mean a change in state of the water due to temperatures, pressure, or molecular changes, although such definitions are not excluded from the terms “mist” or “vapor” or similar terms used herein. 
         [0017]      FIG. 1A and 1B  depict a schematic side sectional view of a wellbore  100 .  FIG. 1A  depicts an upper portion of the wellbore  100 , while  FIG. 1B  depicts a lower portion thereof. The upper and lower portions are joined at line X-X, and can be any desired or required length. These two figures are described simultaneously. The wellbore  100  is drilled and lined with a casing  102 . A lower portion  104  of the casing  102  is perforated or screened so as to allow introduction of water W and natural gas into an interior of the casing  102 . An outer pipe or conduit, often referred to as an outer string  106  is inserted into the casing  102 . A number of components are fixed to a bottom portion of the outer string  106  such that they extend into the water W below the casing perforations. These include a perforated or otherwise open tailpipe  108  to allow passage of the water W, as does a seating nipple  114  that is disposed in the outer string  106 . In certain embodiments, the seating nipple  104  can be disposed in the outer string  106  approximately 30 feet above the end thereof. A screen  110  used to filter the water W during extraction operations and seal assembly  112  are secured to an inner pipe or conduit, referred to as an inner string  118 , is inserted into the outer string  106 . A no-go  116  is disposed about the inner string  118  and rests on the seating nipple  114  so as to prevent the inner string  118  from dropping further into the outer string  106 . A gas injection tool  200  (embodiments of which are described below) can be integral with, or inserted into, the inner string  118 . 
         [0018]    A number of valved conduits are connected to the various internal volumes of the wellbore  100 . For example, a product valve  120  controls removal of a product P, such as methane gas, from an interior of the casing  102 . A working gas valve  122  controls the injection of a working gas G into the outer string  106 . An isolation valve  124  controls extraction of a mist M (formed of the working gas G and the water W) from the inner string  118 . Of course, other components may be installed on the various lines proximate the various valves  120 ,  122 ,  124  so as to control and monitor the various flows therein. Such components can include, for example, pressure regulators, temperature, pressure, and flow sensors, automatic emergency shut-off valves, and so on, as known in the art. Methods of utilizing the working gas G so as to remove the mist M (containing the water W) are described herein. 
         [0019]      FIG. 2  depicts a schematic side sectional view of one embodiment of a gas lift assembly or tool  200 . The tool  200  is a generally elongate device that includes an elongate body  202  defining an interior chamber  204 . The body  202  includes a base  206  that defines a liquid inlet  208  for allowing entry of water W when the tool  200  is inserted into a wellbore. A choke  210  having a diameter less than that of the liquid inlet  208  may be disposed proximate thereto so as to limit the flow of water W into the tool  200 . An inner conduit  212  is disposed within the elongate body  202  and defines a liquid inlet  214  and a liquid outlet  216 . The liquid inlet  214  of the inner conduit  212  is in fluidic communication with the liquid inlet  208  defined by the base  206 . The elongate body  202  may include an outer wall  218  that defines a gas passage or gas inlet  220  therethrough. Each of the liquid outlet  216  and the gas passage  220  are spaced from the liquid inlet  208  in the base  206 . In the depicted embodiment, the liquid outlet  216  is disposed a distance D 1  from the base  206 , while the gas passage  220  is disposed a distance D 2  from the base  206 . In the depicted embodiment, the distance D 2  is less than the distance D 1 . This helps ensure proper mixing of gas G and water W so as to form a mist M for extraction from the inner string  118 . A no-go  222  connects the elongate body  202  to a cap  224 . The cap  224  includes an enlarged head  226  that functions as a retrieval neck, such as a configuration often referred to as a fishing neck, which allows the tool  200  to be lowered into the inner string  118  with a wireline unit during operation, and later removed as required or desired. The cap  224  defines a mist outlet  228  through which the mist M of gas G and water W exits during extraction operations. The depicted tool  200  also includes one or more seal assemblies  230  that substantially surround the elongate body  202 . The seal assemblies  230  allow for connection to one or more elements, not shown, that can aid in insertion of the tool  200  into the inner string  118 . 
         [0020]      FIG. 3  depicts a schematic side sectional view of the gas lift assembly  200  of  FIG. 2  disposed in a wellbore  100 . A casing  102 , such as that described above in  FIGS. 1A and 1B  is not depicted in whole for clarity. Additional elements of the wellbore  100  are described above with regard to  FIGS. 1A and 1B  and are therefore not necessarily described further. A number of components of the tool  200  are described above with regard to  FIG. 2  and are therefore not necessarily described further. The tool  200  is disposed within an inner string  118  that is, in turn, disposed within an outer string  106 . The depicted tool  200  also includes two bore supports  232 , which help maintain alignment of the elongate body  202  within the inner string  118  along an axis A. In certain embodiments, the surfaces of the bore supports  232  may be polished or otherwise smooth to provide pressure containment between the seal assemblies  230 . The two bore supports  232  also create the chamber where gas can enter through the outer string to the tool  200  gaining access to the gas inlet  220 . The tool  200  is lowered into the inner string  118  until a desired depth is achieved. The inner string  118  defines one or more string gas passages  118   a , which allows passage of the working gas G from the outer string  106  into the inner string  118 . Notably, the string gas passages  118   a  are disposed a distance D 3  from the no-go  116 . With this known distance D 3 , the tool  200  may be inserted such that the distance D 3  is less than both of distances D 2  and D 1 , as described above. Thus, when working gas G is injected downward into the outer string  106 , the direction of the gas G turns upwards as it enters the inner string  118  via the gas passages  118   a . Once within the inner string  118 , the working gas G travels upwards as it enters the elongate body  202  of the tool  200  through gas inlet  220 . The gas G is still travelling upward as it makes contact with and mists the water W that is flowing out from the liquid outlet  216  of the inner conduit  212 . By controlling the flow rate of gas, this upward flow of working gas G and water W efficiently mixes the gas G and water W such that a fine mist M is produced. This mist M is easily removed from the tool  200  via the mist outlet  228 , thereby dewatering the well. Long term operation of the dewatering tool, then, allows the water table near the well to be lowered and maintained substantially at the depth of the tool. 
         [0021]      FIG. 4  depicts a schematic side sectional view of another embodiment of a gas lift assembly  300  disposed in a wellbore  100 . A number of the components of the wellbore  100  are described above and are therefore not necessarily described further. Certain components of the gas lift tool  300  are already described above with regard to the tool  200  depicted in  FIG. 2 . These components are numbered similarly to the components of  FIG. 2  (e.g., choke  210 ,  310 ; interior chamber  204 ,  304 ; etc.) and are not necessarily described further. The tool  300  in this embodiment is integrated into the inner string  118  and may comprise a bottom-most portion of the inner string  118  that is inserted into the outer string  106 . The tool  300  includes a manifold  350  that defines a plurality of passages, as described in more detail below. The manifold  350  may be secured in the inner string  118  above a choke  310 . At least one gas passage or inlet  320  penetrates the manifold  350  and the inner string  118 . Multiple gas passages  320  are joined within the manifold  350  so as to allow passage of a working gas G out of a single gas outlet  352  that is axially disposed within the manifold  350  and inner string  118 . Water W enters one or more conduits  312  formed in the manifold  350  at a liquid inlet  308  and exits the conduits  312  at a liquid outlet  316 . Here, a plurality of conduits  312  are formed about a circumference of the manifold  350 , but other locations within the tool  300  are contemplated. In the depicted embodiment, the gas outlet  352  may be a distance D 4  above the liquid outlets  316 . The water W and working gas G produce a mist M in the interior chamber  304  and may pass through a throat  354  having a reduced diameter, relative to the interior chamber  304 . This mist M then is discharged from, drawn out of, or otherwise expelled from the inner string  118 . Again, as with the embodiment of  FIG. 3 , changing the direction of the working gas G to an upward flow prior to mixing with the water W helps efficiently produce the mist M. 
         [0022]      FIG. 5  depicts a schematic side sectional view of another embodiment of a gas lift assembly  400  disposed in a wellbore  100 . A number of the components of the wellbore  100  are described and are therefore not necessarily described further. Certain components of the gas lift tool  400  are already described above with regard to the tools depicted in  FIGS. 2 and 3 . These components are numbered similarly to the components of  FIGS. 2 and 3  (e.g., choke  210 ,  310 ,  410 ; interior chamber  204 ,  304 ,  404 ; etc.) and are not necessarily described further. The tool  400  in this embodiment is integrated into the inner string  118  and may comprise a bottom-most portion of the inner string  118  that is inserted into the outer string  106 . A support  460  holds the inner conduit  412  in place and may align the conduit  412  within the inner string  118  along axis A. A liquid inlet  408  of the inner conduit  412  allows water W to enter the conduit  412 . One or more gas passages  420  penetrate the inner string  118  and are disposed a distance D 5  from the liquid inlet  408 . As with other embodiments described herein, gas passages  420  change the downward flow of the working gas G into an upward flow as it enters the inner string  118 . A choke  410  is disposed at an outlet  416  of the inner conduit  412  at a distance D 6  from the liquid inlet  408 . The water W and gas G mix in an interior chamber  404  and form a mist M that is discharged from the inner string  118  at the top of the well. 
         [0023]      FIGS. 6A-6C  depict a method  500  of removing water from a downhole wellbore  100 . Although the method  500  is depicted in parallel with a wellbore  100  configuration utilizing the tool  200  described above, a person of skill in the art would recognize the modifications required to utilize tool  300 , tool  400 , or other tool configurations, so as to perform the method  500 . The method  500  begins with insertion of a tool  200  into the wellbore  100 , defined by the casing  102 , operation  502 . In the depicted embodiment, the tool  200  is lowered L into the inner string  118 , so as to be disposed below a level of water W in the wellbore  100 . In other embodiments, the tool may be integral with the inner string  118 , such that when the inner string  118  is inserted into the outer string  106 , the tool is also inserted. As can be seen in the figure corresponding to operation  502 , water W is disposed in the wellbore  100  and enters the casing  102  at least through open portions  104   a  of the casing  102 . Once disposed at the desired depth, operation  504 , the tool  200  is also filled with water W from the wellbore  100 , as depicted in the corresponding figure. 
         [0024]    In operation  506 , a working gas G is used to pressurize the outer string  106  of the wellbore  100 , so as to expel water W from the outer string  106 . In operation  508 , working gas G is also used to pressurize the inner string  118  in operation  506 , so as to expel water W from both the inner string  118  and the tool  200  disposed therein. In general, operations  506  and  508  are performed substantially simultaneously, while water W continues to fill the space between the casing  102  and outer string  106 . 
         [0025]    The purpose of operations  506  and  508  is to drive the water from the tool before the gas lift is initiated. Generally, it may be desirable that water W is expelled to a level below that of the string gas passages  118   a  in the wall of the inner string  118 . As can be seen in the same figure, water W is substantially expelled from the tool  200 , so as to only be present at the liquid inlet  208  thereof. 
         [0026]    The method  500  continues at operation  510 , where working gas G pressure on the inner string  118  is reduced. This allows water W, under pressure from the surrounding water table, to enter the tool  200  and flow up the inner conduit  212  thereof. Additionally, the working gas G flows from the outer string  106 , through the string gas passages  118   a  and into the inner string  118 . The working gas G continues to flow upwards within the inner string  118  and then into the tool  200  via the tool gas passages  220 . In an alternative embodiment, the working gas G pressure within the inner string  118  may be maintained and the working gas G pressure in the outer string  106  may be increased to as to have the same effect. The interaction of the upwardly-flowing working gas G and water W in the interior chamber  204  produces a mist M that is expelled from the tool  200  and collected at the surface. Here, the water may be separated from the mist M. 
         [0027]    This circulation of working gas G continues. In operation  512 , a flow rate of the water W entering the tool  200  or leaving the well may be monitored during the injection of the working gas G. This flow rate may be used as a basis to adjust the working gas circulation flow rate or adjust a differential pressure between the outer string  106  pressure and the inner string  118  pressure, as in operation  514 . As this injection of working gas G continues, mist M continues to be produced, which removes water W from the wellbore  100 , such that the level of water W outside the casing  102  drops below the level of the open portion  104   a  thereof, as depicted in the figure accompanying operations  512  and  514 . In certain embodiments, the working gas G flow may be balanced against the water W flow so as to remove substantially all or all of the water W from the tool  200  as a mist M. Once the pressure on a nearby coal bed is reduced due to the removal of water W from the wellbore  100  as a mist M, methane gas product P is extracted, passively or actively, via the space between the casing  102  and the outer string  106 , as depicted in operation  516  and the accompanying figure. 
         [0028]    Injection rates for the working gas G may be determined by the Coleman method “Critical Flow for Water Removal”. Based on wellhead pressures ranges from 5 to 50 psig, the required injection rates would range from 50 to 100 MCFD (thousand standard cubic feet/day, sometimes also shown as MSCFD) in order to lift water from the inner tubing string. The Coleman method “Critical Flow for Water Removal” is: 
         [0000]    
       
         
           
             
               
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       Additionally, 
       [0029]    
       
         
           
             
               
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         [0000]    In the above equations: 
         [0030]    T tf =Flowing tubing temperature, ° R 
         [0031]    q c =Critical flow rate, MCFD 
         [0032]    p=Wellhead pressure, psi 
         [0033]    v c =Critical velocity of water, ft/sec 
         [0034]    A=Cross sectional area, ft 2    
         [0035]    Z=Compressibility factor 
         [0036]    A pressure/head differential from the wellbore/casing into the inner string is utilized so that the water W flows into the tool. The injected working gas G can carry the mist M to the surface. If the combined back pressure/head in the inner string is greater than the head pressure at the equivalent depth in the casing, no fluid will enter the tool, as depicted in the equation below: 
         [0000]      ( P   w )&gt;( P   t ) 
         [0000]    Where the inner string combined pressure at the tool (P t )=Surface Pressure+Working Gas Head+Water Head+Friction. The wellbore combined pressure (P w )=Surface Pressure+Working Gas Head+Water Head+Friction. For ease of application, certain assumptions may be made. For example, the Surface Pressure is assumed to be the same for both P t  and P w . Additionally, the Working Gas Head is considered to be negligible (e.g., less than 5 psig). Friction in the casing is also assumed to be negligible, given the large diameter of the casing. 
         [0037]    Thus, for P w  to be greater than P t , Water Head in the wellbore must be greater than the Water Head in the inner string plus Friction in the inner string. In an embodiment, a ⅛″ choke is placed below the tool to regulate water flow into the inner string and keep the water head to a manageable level in the inner string (e.g., 0.25 to 3 GPM calculated). With typical tubing/inner string depth of 3000 ft., it takes less than about 2 minutes to clear the inner string of water. Additionally, the greater the working gas injection rate, the faster the inner string is cleared of water and the greater the reduction in water volume/head. An increase in working gas injection rate, however, increases friction. Conversely, when the working gas injection rate is lowered, the friction falls. This reduction in working gas injection rate, however, increases water head and allows more water to enter the inner string. 
         [0038]    In one embodiment, a starting point for working gas injection is about 100 MCFD and it normally takes up to 30 minutes or more before mist M is seen at the surface. Water rates in the mist M range from 1 to 8 barrel of water per day (which indicates that the differential pressure of the wellbore to inner string is less than 1 psi). Wellbores utilizing the tools described herein may require 20 to 60 psig injection pressure at 100 MCFD injection rate with approximately 5 psig at the surface. In certain embodiments, inner string depths of about 1000 ft. are injected at about 20 psig, while inner string depths greater than 3500 ft. may require about 60 psig injection pressure. In other embodiments, wellbores may require more pressure, e.g., approximately 100 psig to inject 100 MCFD, depending on the configuration of the tool utilized. Because most of the pressure drop happens across the nozzle, the depth of inner string does not affect the injection pressure as much as the tool. 
         [0039]    This disclosure described some embodiments of the present technology with reference to the accompanying drawings, in which only some of the possible embodiments were shown. Other aspects may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments were provided so that this disclosure was thorough and complete and fully conveyed the scope of the possible embodiments to those skilled in the art. 
         [0040]    Although specific embodiments were described herein, the scope of the technology is not limited to those specific embodiments. One skilled in the art will recognize other embodiments or improvements that are within the scope and spirit of the present technology. Therefore, the specific structure, acts, or media are disclosed only as illustrative embodiments. The scope of the technology is defined by the following claims and any equivalents therein.