Abstract:
A well system comprising at least one gas well extending from a ground surface into the ground, a water production well extending from a surface into the ground and a water drainage well fluidly connecting the at least one gas well to the water production well.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a well system. In particular, although not exclusively, the invention relates to a Coal Bed Methane (CBM) well system. 
       BACKGROUND TO THE INVENTION 
       [0002]    CBM, also referred to as coalbed gas, coal seam gas or coal-mine methane is a form of natural gas extracted from coal seams. CBM is distinct from other types of gas, as the methane is stored within the coal by a process called adsorption. When a coal seam is put into production, water in the fractured or cleated spaces is pumped off first. This leads to a reduction of pressure enhancing desorption of gas from the coal seam. 
         [0003]    Existing wells for Coal Bed Methane typically have a steel encased hole that is drilled into a coal seam. A submerged pump located towards the bottom of the steel encased hole is used to pump the produced water through tubing in the hole to the surface. A disadvantage with existing CBM wells is that capital equipment costs are quite high due to the number of wells that need to be drilled. Another disadvantage is that tubing for the produced water and the submerged pump can obstruct reservoir surveillance activities. 
         [0004]    It will be clearly understood that any reference herein to background material or information, or to a prior publication, does not constitute an admission that any material, information or publication forms part of the common general knowledge in the art, or is otherwise admissible prior art, whether in Australia or in any other country. 
       OBJECT OF THE INVENTION 
       [0005]    It is an object of the invention to overcome or at least alleviate one or more of the above problems and/or provide the consumer with a useful or commercial choice. 
         [0006]    Other preferred objects of the present invention will become apparent from the following description. 
       DISCLOSURE OF THE INVENTION 
       [0007]    In one form, although it need not be the only or indeed the broadest form, the invention resides in a well system comprising: 
         [0008]    at least one gas well extending from a ground surface into the ground; 
         [0009]    a water production well extending from a surface into the ground; and 
         [0010]    a water drainage well fluidly connecting the at least one gas well to the water production well. 
         [0011]    Preferably the well system is a coal bed methane well system. 
         [0012]    Preferably, the water production well comprises a pump towards a lower end thereof. Preferably water and/or other liquids that flow into the gas well flow through the water drainage well and into the water production well. Preferably the water production well pump pumps the water and/or other liquids in the water production well to the surface. Typically the water production well pump pumps the water and/or other liquids in the water production well to the surface through tubing located within the water production well. 
         [0013]    It will be appreciated that a reference to water also includes other liquids that may be present. For example, water and other liquids that are present in a coal seam. 
         [0014]    A skilled addressee will understand that not all of the water that flows into the gas well will flow through the water drainage well, and that not all the water in the water production well will necessarily be pumped up to the surface. For example, water in the gas well below a point where the water drainage well is fluidly connected to the gas well may not flow through the water drainage well and water below the pump in the water production well may not be pumped to the surface. Additionally, it will be appreciated that some water may be present in the gas that is produced to the surface. 
         [0015]    Typically, the water production well is a vertical water production well. A skilled addressee will understand that a vertical water production well includes a water production well that is substantially vertical. In an alternate embodiment, the water production well may be a deviated water production well. 
         [0016]    In an alternate embodiment, the water production well may be used to also convey gas from an annulus (the void between a casing of the water production well and the tubing located within the water production well) to the surface. 
         [0017]    Preferably, the at least one gas well is a plurality of gas wells. Preferably each of the at least one gas well intersects a coal seam. Typically each of the at least one gas well is cased. For example, each of the at least one gas well is cased in a steel casing. Typically, each casing has perforated portions where it intersects the coal seam(s) to enable gas and water from the coal seam to flow into the at least one gas well. In an alternate embodiment, there may be no casing where each of the at least one gas well intersects the coal seam(s). Once in the gas well, the gas will flow towards the top of the gas well and the water will flow towards the bottom of the gas well. In one embodiment, each of the at least one gas well intersects a plurality of coal seams. Typically, each of the at least one gas well is a vertical gas well. A skilled addressee will understand that a vertical gas well includes a gas well that is substantially vertical. In an alternate embodiment, each of the at least one gas well may be a deviated gas well, or the at least one gas well may be a plurality of gas wells comprising one or more vertical and deviated gas wells. 
         [0018]    Preferably each of the at least one gas well will comprise a wireline entry guide. Typically the wireline entry guide is located towards an upper end of the corresponding gas well. A skilled addressee will understand that a wireline entry guide is a fitting on the end of a tubing string that is shaped to assist with the retrieval of reservoir surveillance tools when pulled upwards into the tubing string from the wellbore below. 
         [0019]    Preferably the water drainage well fluidly connects a lower portion of each of the at least one gas well to a lower portion of the water production well. 
         [0020]    Typically the water drainage well is inclined to assist the flow of water from the at least one gas well to the water production well. For example, the water drainage well may be inclined in a downward direction from where it is fluidly connected to the at least one gas well to where it is fluidly connected to the water production well. It will be appreciated that the water drainage well may be oriented in a substantially horizontal orientation or be inclined upwardly toward the water drainage well, although this is undesirable, due to more water potentially remaining in the at least one gas well, the water can still flow into the water production well due to hydrostatic pressures. 
         [0021]    Preferably the water drainage well is lined or cased. For example, a steel, glass reinforced epoxy casing or the like. 
         [0022]    Preferably the water drainage well is open to a lower portion of at least one of the at least one gas well. More preferably, the water drainage well intersects each of the at least one gas well. If the water drainage well is lined, the liner (e.g. casing) of the water drainage well may be perforated in proximity to the lower portion of the corresponding gas well such that the corresponding gas well is fluidly connected to the water drainage well. 
         [0023]    Preferably the water drainage well is open to a lower portion of the water production well. 
         [0024]    In yet a further form the invention relates to a method of forming a well system comprising the steps of: 
         [0025]    providing at least one gas well extending from a ground surface into the ground; 
         [0026]    drilling a water production well from a surface into the ground; 
         [0027]    drilling a water drainage well such that the water drainage well fluidly connects a lower portion of each of the at least one gas well to a lower portion of the water production well; and 
         [0028]    providing a pump in the lower portion of the water production well. 
         [0029]    Preferably the well system is a coal bed methane well system. 
         [0030]    In an alternate embodiment, the step of providing at least one gas well extending from a ground surface into the ground includes the step of removing a pump and associated tubing from each of the at least one gas well. 
         [0031]    Preferably the step of drilling a water drainage well such that the water drainage well fluidly connects a lower portion of each of the at least one gas well to a lower portion of the water production well involves drilling the water drainage well from a lower portion of one of the at least one gas well into a lower portion of the water production well. 
         [0032]    Further features of the invention will become apparent from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein: 
           [0034]      FIG. 1  shows a schematic view of a well system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    With reference to  FIG. 1 , there is shown a CBM well system  100 . The CBM well system  100  has gas wells  110 ,  112  that extend from a surface  12  into the ground and into coal seams  14 . A water production well  114  extends from the surface  12  into the ground. 
         [0036]    A water drainage well  116  fluidly connects the gas wells  110 , 112  to the water production well such that water (water flow indicated by arrows  18 ) can flow from the gas wells  110 , 112  through the water drainage well  116  and into the water production well  114 . In the water production well  114 , water is pumped by a pump  118  through tubing  120  to the surface  12 . 
         [0037]    Gas wells  110 , 112  are drilled from the surface  12  through coal seam  14 . The gas wells each have a casing  122 . The casings have perforations  124  adjacent the coal such that the gas (gas flow indicated by arrows  16 ) and the water in the coal seam can flow into the gas wells  110 , 112 . 
         [0038]    Gas wells  110 , 112  have wireline entry guides  126  fitting on the ends of tubing strings  128 . The wireline entry guides  126  assist with the retrieval of reservoir surveillance tools (not shown) when they are pulled upwards towards the well head  130 . 
         [0039]    The water drainage well  116  is drilled from a lower portion of the gas well  110  and into the water production well  114 . The water drainage well  116  also intersects the gas well  112 . The casing  122  of gas well  112  does not extend to the bottom of gas well  112 , instead it stops just above the water drainage well  116 . In this manner, the drilling of the water drainage well is not impeded by the casing  122  of gas well  112 . The water drainage well  116  has a liner  132  that is attached to the lower portion of the gas well  110 . The liner  132  has perforations  134  where the water drainage well  116  intersects the gas well  112  such that water can flow from the gas well  112  into the water drainage well  116 . 
         [0040]    The water production well  114  is drilled from the surface  12  into the ground. The water production well  114  has a casing  136 . The casing  136  is open to the water drainage well  116  such that water can flow from the water drainage well  116  into a lower portion of the water production well  114  where the pump  118  is located, which pumps the water through the tubing  120  to the well head  138  on the surface  12 . 
         [0041]    As can be seen from  FIG. 1 , because water flows through the water drainage well  116  and into the water production well  114 , the gas wells  110 , 112  do not have water pumps disposed therein. A benefit of this is that reservoir surveillance activities are not obstructed in the gas wells. Additional benefits include a reduction in capital expenditure and operating costs, an improvement in vertical flow performance of the gas wells due to a larger effective cross section for gas flow, improved operational times due to less pumps to maintain and operate, and an improved pressure drawdown on the coal reservoir resulting in higher production rates and lower abandonment pressures. 
         [0042]    Although it is counterintuitive, as the present invention requires more wells to be drilled than the prior art, there are advantages, as mentioned above, in having a water drainage well and a water production well. 
         [0043]    The foregoing embodiments are illustrative only of the principles of the invention, and various modifications and changes will readily occur to those skilled in the art. The invention is capable of being practiced and carried out in various ways and in other embodiments. It is also to be understood that the terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0044]    In the present specification and claims (if any), the word “comprising” and its derivatives including “comprises” and “comprise” include each of the stated integers but does not exclude the inclusion of one or more further integers unless the context of use indicates otherwise.