Abstract:
A system includes first and second wells. The first well has a first tube that extends from a first well head to a first end disposed within a coal seam. The second well is disposed at a distance from the first well and includes a second tube that extends from a second well head to a second end disposed within the coal seam. A pump is coupled to the first well and is configured to supply the first tube with pressurized fluid that includes nutrients for methanogenesis. At least a portion of the pressurized fluid introduced into the first tube of the first well is received within the second tube of the second well by way of the coal seam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/374,796, filed Aug. 18, 2010, the entirety of which is herein incorporated by reference. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    The disclosed systems and methods relate to the production of methane gas. More specifically, the disclosed systems and methods relate to the injection of nutrients, which may include metabolic amendments, and/or microorganisms for microbially-enhanced coal bed natural gas (e.g., methane or “coal bed methane”) recovery. 
       BACKGROUND 
       [0003]    Many coal seams around the world either have produced or are capable of producing biogenic methane. Biogenic methane was created initially through a process known as a methanogenesis, which is a naturally occurring process that has been in existence for millions of years. 
         [0004]    Recently, laboratory studies have duplicated the methanogenesis process and have created new biogenic gas in relatively short time periods, in some instances, as few as twenty (20) days. After completion of these laboratory studies, field pilot studies were initiated in an attempt to duplicate the findings in previous lab studies. Field pilot programs have replicated laboratory studies in that new biogenic methane was produced in several coal bed methane wells that prior to the study were completely void of gas. However, these field pilot programs have not resulted in a wide-distribution of the nutrients and/or microbes. 
       SUMMARY 
       [0005]    In some embodiments, a system includes first and second wells. The first well has a first tube that extends from a first well head to a first end disposed within a coal seam. The second well is disposed at a distance from the first well and includes a second tube that extends from a second well head to a second end disposed within the coal seam. A pump is coupled to the first well and is configured to supply the first tube with pressurized fluid that includes nutrients for methanogenesis. At least a portion of the pressurized fluid introduced into the first tube of the first well is received within the second tube of the second well by way of the coal seam. 
         [0006]    In some embodiments, a method includes injecting a fluid having a first nutrient concentration into a coal seam through a first well under a first pressure and extracting a second fluid having a second nutrient concentration from the coal seam through a second well disposed apart from the first well. The second nutrient concentration is less than a first nutrient concentration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a flow diagram of one example of a method of increasing methanogenesis in a coal bed. 
           [0008]      FIG. 2A  4 illustrates one example of a downhole arrangement of an injection well including a flow directing nozzle. 
           [0009]      FIG. 2B  illustrates one example of a downhole arrangement of a circulating well. 
           [0010]      FIG. 2C  illustrates one example of a well-site arrangement used for injecting nutrient-enriched fluid under pressure. 
           [0011]      FIG. 2D  illustrates one example of a well site arrangement for a daily/repetitive nutrient injection and circulation method. 
           [0012]      FIG. 3A  illustrates one example of a pattern of injection and circulating wells in a site for producing coal bed methane. 
           [0013]      FIG. 3B  illustrates another example of a pattern of injection and circulating wells in a site for producing coal bed methane. 
           [0014]      FIG. 3C  illustrates another example of a pattern of injection and circulating wells in a site for producing coal bed methane. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The disclosed systems and methods advantageously enable distribution of nutrients throughout a coal bed to provide a maximum exposure of the microorganisms to coal pore space and surface area of coal beds. The exposure of the microorganisms to the coal pore space and surface area is maximized by forcing nutrient rich water through the pore space itself thereby enabling the full potential of a methanogenesis process to convert the soluble or free carbon to methane. The system also advantageously establishes routes for the resultant methane to flow and be extracted. 
         [0016]    For example, the system includes one or more pressurized pumps disposed adjacent to an injection well. The pumps inject nutrient-rich water into the injection well. The water is circulated through the coal bed by the injection pump and one or more circulation pumps disposed at a distance from the injection pump. The setup and operation of such systems may be performed in accordance with an improved method. 
         [0017]      FIG. 1  is a flow diagram of one example of a method  100  of setting up and operating an improved circulation system. As shown in  FIG. 1 , a test hole is drilled from the ground surface through the bottom of the coal seam at block  102 . At block  104 , the test hole is used to assess characteristics of the coal seam. For example, the structure of an overburden layer can be analyzed and the depths of the overburden, the coal seam, and the layers that separate multiple coal seams can be determined. Additionally, the porosity, elemental constitutions, and heat value may also be determined. Coal samples may be used to set up microcosms to determine the feasibility and potential of producing biogenic natural gas in a laboratory. The depth of the coal, slope of the coal seam, and the structure of the coal may also be analyzed and determined at block  104 . Formation/ground water may be collected at block  104  to determine the chemical and biological characteristics and be used in laboratory tests. The water depth, formation water recharge rate, and amount of water available may also be determined at block  104 . 
         [0018]    At block  106 , an injection well is drilled in a site. The size and depth of the injection well may be based on the characteristics of the coal as determined at block  104 .  FIG. 2A  is a cross-sectional view of one example of an injection well  200 . As shown in  FIG. 2A , injection well  200  includes a well head  202  coupled to a casing  204  sized and configured to house a tubing  206 . Casing  204  supports and protects tubing  206  from surrounding rock or earth  10  and extends from well head  202  to coal seam  12 . Tubing  206  is coupled to and extends from well head  202  and nozzle  208 , which is embedded within coal seam  12 , and is configured to receive and transport fluid  14 . Nozzle  208  may includes one or more apertures  210  configured to expel fluid in various directions. 
         [0019]    Injection well  200  may be coupled to an injection pump  250  as illustrated in the embodiment shown in  FIG. 2B . As shown in  FIG. 2B , pump  250  may be disposed between well head  202  and a water/nutrient source tank  260 . A conduit  270  is coupled to tank  260  and to tubing  306  such that nutrient-rich fluid  14  may be transferred from tank  260  through injection well  200  into coal seam  12 . In some embodiments, pump  250  is configured to deliver between approximately 500 and 6,000 gallons of nutrient-rich fluid into the coal seam  10  in conjunction with or followed by a volume of non-nutrient rich fluid between approximately 5,000 and 15,000 gallons. One skilled in the art will understand that the other amounts of nutrient-rich water and non-nutrient rich water may be adjusted based on the size of the coal seam  12 , i.e., greater or less than the identified ranges. The clean water flush (i.e., non-nutrient rich water flush) pushes the initial nutrient-rich water out of the fractures in cleats and into the pore space of the coal. Upon the completion of injection, the injection well may be used as a recovery and/or circulation well. 
         [0020]    A second well, which may be a recovery and/or circulation well, is drilled at a distance from the injection well at block  108 . As will be understood by one skilled in the art, the distance at which the injection well is positioned from the test well may be based on the initial permeability assessment of the coal performed at block  104 . 
         [0021]    An example of such a recovery/circulation well  220  is illustrated in  FIG. 2C . As shown in  FIG. 4 , recovery/circulation well  220  includes a well head  222  coupled to a casing  224 . Casing  224  extends between well head  222  and coal seam  10  and is configured to receive and protect tubing  226 . Tubing  226  is coupled to an intake nozzle  228 , which is configured to receive fluid  14  from coal seam  12 . Nozzle  228  may include one or more apertures through which the fluid is received. 
         [0022]    In some embodiments, such as the embodiment illustrated in  FIG. 2D , injection well  200  and recovery/circulation well  220  are coupled together by a conduit  270  such that fluid  14  is recycled and reused. As shown in  FIG. 2D , well head  222  of recovery/circulation well  220  is coupled to well head  202  of injection well  200  by conduit  270 . An injection pump  250  is disposed along the length of conduit  270  and is configured to force pressurized fluid  14  into well head  202  and extract fluid from well head  222 . The extracted fluid  14  received from well head  222  may be passed through a nutrient injection system  280  along conduit  270  to increase a concentration of nutrients for methanogenesis and other microbial pathways including, but not limited to, fermentation, facultative oxidation, and acetogenesis by the time it is injected into coal seam  12  by injection well  200 . 
         [0023]    The nutrient injection may be implemented by gravimetric and/or low pressure (e.g., approximately less than or equal to 50 psi). Injection system  280  may include a mixing system comprising one or more tanks used for mixing nutrients with other chemical amendments or with a tracer. The one or more mixing tanks are filled with water from well  222  and/or from make-up water from another formation water source. The nutrients and other chemical amendments or tracer is mixed in the one or more mixing tanks while being purged with an inert gas such as, for example nitrogen or argon. Mixing is conducted by impellers, pumps, gas diffusion, or any combination of methods as will be understood by one skilled in the art. The mixture from the mixing tanks are injected in-line with conduit  270  into well  202 . 
         [0024]    Both injection wells  200  and circulating wells  220  are capable of producing new biogenic gas generated from the circulation methodology. For example, each of the injection wells  200  and recovery/circulation wells  220  may be tied to a gathering system for transfer to a sales facility. 
         [0025]    Referring again to  FIG. 1 , a tracer fluid is injected into the injection well and removed (e.g., pumped out) from the second well at block  110 . The tracer fluid injected through the injection well may have known characteristics as well as be injected at a known rate. Examples of such tracer fluid include, but are not limited to, sodium bromide and potassium bromide. The tracer may pumped into the injection well in water in which the concentration of the tracer is between approximately 20-1,000 mg per liter of water. The tracer may be removed from the second well using a pump operating a second rate that may be different from a first rate at which the injection pump pumps the tracer fluid into the injection well. 
         [0026]    At block  112 , the fluid  14  removed from the second well is analyze to determine the amount of tracer recovered such that the fluid connection between the injection well and the second well may be determined. For example, if fifty percent or more of the injected tracer is recovered from the second well, then it may be determined that a sufficient fluid connection between the injection well and the second well has been established. One skilled in the art will understand that other threshold values may be used other than fifty percent. 
         [0027]    The rates at which the tracer fluid is pumped into the injection well and pumped out of the test well may be measured to provide a real-time measurement of the permeability of the coal seam at block  114 . The real-time permeability measurement of the coal may be used to adjust the pumping parameters of the injection pump and the extraction pump. 
         [0028]    At block  116 , one or more additional wells may be drilled at distances from the injection well. As will be understood by one skilled in the art, the one or more additional wells may be drilled at distances based on the real-time permeability of the coal over an area in which the coal is to be used to produce methane. The one or more wells may include one or more injection wells  200  and/or one or more recovery/circulation wells  220 . As described above, injection wells  200  may be coupled to a tank  260  or to an output of a recovery/circulation well  220  through a conduit  270  and pump  250 . 
         [0029]      FIG. 3A  illustrates one example of a site  300 A in which a plurality of wells are drilled to extract methane from coal. Site  300 A may be divided into a number of subdivisions in which the number of subdivisions  302  is based on the permeability of the coal. For example, site  300  may have an area of approximately 40 acres and each subdivision  302  has an area of approximately 2.5 acres. In some embodiments, each of the subdivisions  302  has an approximately equal area to form a grid, although one skilled in the art will understand that area  300  may be divided into subdivisions  302  having differing areas and do not form a grid. 
         [0030]    One or more wells  200 ,  220  may be disposed in each of the subdivisions  302 . For example, subdivisions  302 - 6 ,  302 - 7 ,  302 - 10 , and  302 - 11  each include an injection well  200  associated with a corresponding injection pump  250 . Each of the subdivisions  302  in which an injection pump  200  and injection well  250  are not disposed, i.e., subdivisions  302 - 1 : 302 - 5 ,  302 - 8 ,  302 - 9 , and  302 - 12 : 302 : 16 , may be configured with a respective recovery/circulation well  220  and corresponding pump  250 . 
         [0031]    In embodiments in which the well heads  202  of injection wells  200  are coupled to the well heads  222  of recovery/circulation wells  220 , such as the embodiment illustrated in  FIG. 2B , conduits  270  may extend from one subdivision  302  to an adjacent subdivision. For example, injection well  200 - 1  in subdivision  302 - 6  may received fluid from recovery/circulation well  220 - 2  in subdivision  302 - 2  as identified by the arrow  304 . Similarly, injection well  200 - 2  in subdivision  302 - 7  may receive fluid from recovery/circulation well  220 - 6  in subdivision  302 - 8  as identified by arrow  304  extending between the two wells. 
         [0032]    One skilled in the art will understand that wells  200 ,  220 , and pumps  250  may be configured in other patterns with respect to subdivisions. For example,  FIG. 3B  illustrates another embodiment of a site  300 B divided into a plurality of subdivisions  302 , but each subdivision  302  does not include a respective well  200 ,  220 . As shown in  FIG. 3B , subdivisions  302 - 1 ,  302 - 3 ,  302 - 6 ,  302 - 8 ,  302 - 9 ,  302 - 11 ,  302 - 13 , and  302 - 16  do not include an injection well  200  nor a recovery/circulation well  220 . Each pair of injection wells  200  and recovery/circulation well  220  is disposed in non-vertically and horizontally aligned subdivisions. For example, an injection well  200 - 1  is disposed in subdivision  302 - 4  and is fluid communication with recovery/circulation well  220 - 1 , which is disposed in subdivision  220 - 1 , and injection well  200 - 2  is disposed in subdivision  302 - 7  and is coupled to recovery/circulation well  220 - 2  disposed in subdivision  302 - 4 . Injection wells  200 - 3 ,  200 - 4  disposed in subdivisions  302 - 10 ,  302 - 12  are respectively coupled to recovery/circulation wells  222 - 3 ,  220 - 4  disposed in subdivisions  301 - 13 ,  302 - 16 . 
         [0033]      FIG. 3C  illustrates another embodiment in which a single injection well  200  disposed at an approximate center of site  300 C and coupled to one or more recovery/circulation wells  220 . As shown in  FIG. 3C , recovery/circulation wells  220 - 1 ,  220 - 2 ,  220 - 3 , and  220 - 4  are disposed in the corner subdivisions  302 - 1 ,  302 - 4 ,  302 - 13 , and  302 - 16  of site  300 C. Circulation wells  220  are each coupled to injection well  200  via injection pump  250 . 
         [0034]    Referring again to  FIG. 1 , fluid is injected into a coal seam at injection wells  200  at block  118 . Injection wells  200  and pumps  250  are configured to inject nutrient-rich fluid, e.g., water, into coal seams  14  under pressure. In some embodiments, the nutrient-rich fluid is injected into coal seams  14  under a pressure of up to and including 100 psi. One skilled in the art will understand that less pressure or greater pressure may be used to inject nutrient-rich fluid into coal seams  14  via injection wells  200 . The amount of nutrient-rich fluid injected into a coal seam may also vary based on an area of the site and size of the coal seam. For example, approximately 500 and 6,000 gallons of nutrient-rich fluid may be injected at an injection well  200  in a 40 acre site. 
         [0035]    At block  120 , a non-nutrient enriched fluid may be injected into the coal seam via the injection well(s)  200 . For example, approximately 5,000 to 15,000 gallons of non-nutrient enriched fluid may be injected into the coal seam  14  through injection well(s)  200  in a 40 acre site. One skilled in the art will understand that other amounts of non-nutrient enhanced fluid may be injected based on the size of the coal seam, i.e., greater or less than the identified range. The non-nutrient enriched fluid flush pushes the initial nutrient-rich fluid out of the fractures in cleats and into the pore space of the coal. 
         [0036]    At block  122 , recovery/circulating wells  220  are turned to move the nutrients away from the injection wells  200  to spread the nutrients throughout the entire coal seam  14 . In some embodiments, circulating pumps  220  are configured to move fluid in a range from 5 gallons per minute to 200 gallons per minute depending on the size of the site and the number of injection wells  200  and/or recovery/circulation wells  220  disposed in the site. One skilled in the art will understand that circulating pumps  220  may be configured to move fluid with other flow rates. 
         [0037]    As described above, the nutrient-depleted fluid extracted from recovery/circulating wells  220  may pass through a nutrient injection system ( 280  in  FIG. 2D ) and then pumped into injection wells  200  by injection pumps  250 . In some embodiments, the process is repeated on a daily basis running 24 hours per day until such time as the optimal reservoir saturation is achieved. The saturation point will be reported in field monitoring systems and can be read via a supervisory control and data acquisition (“SCADA”) system in the field or at other offices. In some embodiments, soluble carbon sources, such as carbon dioxide, may be injected by an injection well  200  to increase methanogenesis. The disclosed systems and methods advantageously increase methanogenesis by increasing the amount of nutrients in the coal seam. 
         [0038]    Although the systems and methods have been described in terms of exemplary embodiments, they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the systems and methods, which may be made by those skilled in the art without departing from the scope and range of equivalents of the systems and methods.