Patent Publication Number: US-2016245050-A1

Title: Methods for drilling and producing from coal bed formations and associated apparatus

Description:
TECHNICAL FIELD 
     The invention relates to methods and apparatus for drilling and production from hydrocarbon formations. In particular, but not exclusively, the invention relates to methods and apparatus for drilling and producing so-called coal bed methane (or coal-seam gas), although aspects of the invention may apply to conventional reservoirs also. 
     BACKGROUND 
     Significant innovation and technological development has occurred in recent years in relation to the development of wells in the oil and gas industry. The ability to drill the most effective wellbore in the most appropriate position for the optimum field development can be important to the commercial success, or failure, of the well. 
     In addition, as the “easy-find” resources become less common, the development of more unconventional reservoirs, such as those in hostile environments or complex geological formations, become increasingly attractive commercially. However, for those resources, the key to any commercial success is to ensure that the cost of establishing the well does not exceed the expected revenue that will be generated from the well. 
     Coal bed formations or reservoirs are unlike conventional gas reservoirs in that gases associated with coal bed formations are absorbed with the solid coal matrix (coal macerals), and can be desorbed, or liberated, typically from cleats or other natural fractures in the coal bed, when depressurised. Under such conditions, gases such as methane can be produced. Generally, methane (or other such compounds) are desorbed from the surface of the coal bed. In other words, the greater the access to the surface, the greater the ability to produce hydrocarbons from such formations. Typically, when coal bed methane formations are initially depressurised, water is produced. Processing and managing such water production, while at the same time maintaining the viability of the well, can be costly. 
     There remains a need to reduce costs and increase recovery rates from such formations if they are to continue to provide a commercially viable source of fossil fuel. 
     This background serves only to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. 
     SUMMARY 
     The invention elates to methods and apparatus for drilling and producing from hydrocarbon formations, such as so-called coal bed methane (or coal-seam gas), formations, so as to reduce costs and increase recovery rates. 
     In some aspects of the invention, there is provided a method for drilling coal bed methane formations, comprising drilling at least one wellbore in a coal bed methane formation using a drill string having an inner string and an outer string, the inner string and outer string defining a passageway therebetween (e.g. a fluid passageway). 
     The method may comprise providing a drilling medium to drill apparatus for use in drilling the formation. That drilling medium may be provided through one of the inner or outer strings. The method may comprise returning a combined medium comprising drilling medium, drilling cuttings, produced water and/or gas from the formation to surface via the other of the inner and outer string. 
     The combined medium may comprise at least drilling medium and drilling cuttings. The method may comprise providing a fluid, such as a gas, as the drilling medium. The method may comprise provided air as the drilling fluid. The method may comprise compressing the gas and providing the compressed gas to the inner or outer annulus. The drilling medium (e.g. gas) may be inert, or substantially inert, to the formation. The method may comprise additionally producing to surface gas inflow to an annulus provided between drill string and the formation. 
     The method may comprise separating the combined medium into drilling medium, drilling cuttings, produced water and gas. The method may comprise separating the combined medium in order to retrieve produced gas from the combined medium. 
     The method may comprise drilling using a drill bit. The method may comprise drilling using a hammer. The method may comprise drilling the wellbore through the formation without unduly affecting a near wellbore formation (e.g. without causing damage, or significant damage, to the wellbore). In other words, the method may reduce the extent to which the formation (or near wellbore) is damaged during the drilling process, when compared with conventional drilling techniques. 
     The method may comprise providing a drilling medium to drill apparatus such that such that drilling medium, when returned to surface, may have been in contact only with the formation at a specific location (e.g. at, or around, the drill bit, and not along the entire length of the wellbore). 
     The method may comprise controlling the flow in one or both the inner string and an outer string. The method may comprise controlling flow so as to prevent undesired flow of hydrocarbons, or other fluids or gases, from uncontrollably reaching the surface (e.g. when drilling through overpressured zones in the formation). The method may comprise providing at least one least one shut-off valve (e.g. blowout preventer) in order to control flow. The method may comprise providing flow controls downhole, for example, at near the drill bit of the drilling apparatus. In some examples, one, or some of the flow control device(s) are used to regulate the flow of fluids or gases from the formation to the surface. 
     The method may comprise, while drilling, isolating one or more waterzones or acquirers from the wellbore. The method may comprise, while drilling, cementing and isolating the one or more waterzone from the wellbore. The method may comprise isolation sections of the wellbore, for example, for testing. The method may comprise isolating (e.g. for cementing, isolation or testing) using one or more inflatable packers, which may be provided at or near the drilling apparatus. 
     The method may comprise drilling a primary wellbore, and drilling one or more secondary wellbores from the primary wellbore, as above (i.e. drilling using drill string having an inner strings and an outer strings). Each secondary wellbore may be drilled in a deviated manner with respect to the primary wellbore. 
     The or each secondary wellbore may be drilled using drilling using drill string having an inner strings and an outer strings, in the manner above. 
     The method may comprise permitting the well to flow during drilling of the primary wellbore. The method may comprise permitting the well to flow during drilling of the or each secondary wellbore. The method may comprise determining the well and reservoir potential during drilling (e.g. determining hydrocarbon content and/or composition from wellbore inflow during drilling). 
     The method may comprise isolating one or more of the secondary wellbores from the primary wellbore during drilling. Isolation may be provided mechanically and/or chemically. 
     The method may comprise acquiring data from drilling in order to determine formation parameters. In some examples, the method may comprise determining the location of natural cleats in the coal bed formation, based on sampled materials from a well in order to determine desired location for secondary wellbores. The method may comprise determining hydrocarbon production, or liberation, at a particular drilling region or location in order to determine the location of cleats (e.g. wherein a determined relative increase in hydrocarbon production indicates the presence of a cleat at that drilling region or location). 
     The method may comprise drilling the well bore such that water and gas can be produced from tubing provided in the wellbore (e.g. the completed wellbore) In some examples, the tubing may be provided by one or the inner or outer drill strings, and that gas can be produced from the other drill string, and/or be permitted to flow up the back side of the well (casing). The method may comprise drilling and then completing the wellbore. 
     In some example, the well may be completed so that the drill string forms part of the completion 
     In some aspects of the invention there is provided a method for producing from coal bed methane formations. 
     The method may comprise producing a first fluid, or fluid product, from a first flow passage within a wellbore. The method may comprise producing a second fluid, or fluid product, from a second flow passage within the wellbore. The production may happen simultaneously. The first fluid passage may be provided by a through bore, tubing, or the like. The second fluid passage may be defined by a second bore (e.g. open wellbore and/or completion, or the like). The second fluid passage may be defined as an annulus between the through bore, or tubing, and the second bore. 
     The through bore may extend beyond a cased or otherwise completed portion of the second bore. The through bore may extend to an open hole section of the second bore. 
     The first fluid may comprise at least water and gas (e.g. may be a multiphase fluid). The first fluid may be principally water. The second fluid may be substantially gas. The first fluid may contain comparatively more water than the second fluid (e.g. by fraction). The second fluid may contain comparatively more gas (e.g. methane) than the first fluid (e.g. by fraction). 
     In further aspects of the invention, there is provided a method for drilling coal bed methane formations, comprising: 
     drilling at least one wellbore in a coal bed methane formation using a drill string having an inner string and an outer string, the inner string and outer string defining a passageway therebetween; 
     providing a drilling medium to drill apparatus for use in drilling the formation, the drilling medium provided through one of the inner or outer strings; and 
     returning a combined medium comprising drilling medium, drilling cuttings, produced water and/or gas from the formation to surface via the other of the inner and outer string. 
     In further aspects of the invention, there is provided drilling apparatus configured to drill coal bed method. The apparatus may comprise a drill string having an inner string and an outer string, the inner string and outer string defining a passageway therebetween. The apparatus may comprise drilling apparatus, configured to receive drilling medium through one of the inner or outer strings. The apparatus may be configured to return a combined medium comprising drilling medium, drilling cuttings, produced water and/or gas from a formation to surface via the other of the inner and outer string. The apparatus may be configured to produce to surface gas inflow to an annulus provided between drill string and the formation. 
     The invention includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. As will be appreciated, features associated with particular recited embodiments relating to systems, may be equally appropriate as features of embodiments relating specifically to methods, and vice versa. 
     It will also be appreciated that one or more embodiments/aspects may be useful for improving well productivity, and/or reducing costs. 
     The above summary is intended to be merely exemplary and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A description is now given, by way of example only, with reference to the accompanying drawings, in which:
           FIG. 1 a    show an example of subterranean hydrocarbon-bearing formation, in particular coal bed methane formation; and  FIG. 1 b    shows an example of wellbore in a section of formation;       

         FIG. 2 a    shows the drilling of a formation according to an embodiment, and  FIG. 2 b    shows completed well; and 
         FIG. 3  shows a drilled formation comprising primary wellbore, and a plurality of secondary wellbore. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The following example is given in relation to what may commonly be considered to be an unconventional reservoir, and in particular a coal bed methane reservoir. Such reservoirs may also be termed coal seam gas reservoirs, or the like. The production from such reservoirs generally is achieved by depressurising a coal seam in order to liberate (or desorb) gas there from. However, it will readily be appreciated that aspects of the invention need not be so limited, and may be applied to many different types of reservoirs. 
       FIG. 1 a    shows an example of a subterranean coal bed formation  100  (e.g. comprising methane), which extends beneath a surface  110 . Within the formation  100  there may be present a number of naturally-occurring fractures or so-called cleats  120 . Those cleats  120 , depending on the geology, are spaced throughout the formation. The naturally-occurring cleats  120  have been greatly simplified for ease of understanding. 
     For production to be successful, it is helpful for gas to be able to be liberated, or desorbed, from the surface of the coal formation. Then, it can be useful to optimise the ability with which liberated hydrocarbons can be transported to the wellbore for production to the surface. The number of available cleats and access to those cleats is imperative to ensure optimum organic carbon recovery. 
     Typically, when the existence of such a formation  100  is suspected, further analytical data is accumulated, such as lithography assessments, structural geological characteristics, as well as potential drilling data from the local area, and other such data. This cumulative data can be used to estimate a suitable location for a well, and approximate the potential recovery from any such drilled well. 
     During such drilling operations it is common for the formation  100  (e.g. that formation extending around a wellbore) to become damaged, for example due to egress of drilling fluids or the like, into the near-wellbore formation (i.e. the formation surrounding the wellbore). Further, in order to attempt to increase gas production, then the drilled well will be fractured, for example hydraulically. 
     In some cases, the location of the wellbore may be selected such that, when fractured hydraulically, production from the formation  100  can be increased, or at least initially increased. In other words, where the formation  100  is a coal bed methane reservoir, the location of the wellbores are selected with an a priori understanding that the well will be fractured subsequently. 
     By way of an example,  FIG. 1 b    shows a section of the formation  100  through which a wellbore  130  has been drilled. Here, the section of the wellbore  130  shown can be considered to have been drilled horizontally. 
     While initial data collection, analysis, and subsequent fracturing may indeed increase the initial production from the formation  100 , the use of fracturing fluid may significantly increases the overall costs of drilling the well in the formation  100 . Further, wells that have been developed by initially fracturing the formation can require some amount of re-fracturing later in the lifecycle of the well. Without being bound by theory, this may be due to the formations propensity to return to original natural state, after fracturing is complete. Any such re-fracturing of the well again increases costs. 
     Development of such coal bed methane wells using existing techniques, and relying on fracturing to maximise potential production, can be ineffective and overly costly. There remains a desire to reduce costs and increase recovery rates from such formations if they are to continue to provide a commercially viable source of fossil fuel. 
     Consider now  FIG. 2 a   , which shown of wellbore  150  that has been drilled to a coal-bed methane formation  100 . Within the wellbore  150 , there is provided a drilling apparatus comprising a drill string  200  having an inner string  210  and an outer string  220 , such that the inner string  210  and outer string  220  define a passageway  230  therebetween (e.g. a fluid passageway). In this example, the inner drill string  210  can be considered to be concentrically positioned within the outer drill string  220  (e.g. so that an annulus is provided between each string), and are configured to rotate together. 
     Here, this drilling apparatus further comprises a drill bit  240 , although in other examples a hammer, or the like, may be used. 
     As is represented on  FIG. 2 a   , a drilling medium  250  is provided to the drill bit  240  for use in drilling the formation  100 . In the example shown, the drilling medium  250  is provided to the drill bit  240  via the outer drill string  220  (e.g. via the passageway  230  provided between the inner string  210  and outer string  220 ). Of course, in other examples, the drilling medium  250  may be provided via the alternate string  210 ,  220 ). Here, compressed air is used as the drilling fluid  250 , which is provided to the drilling apparatus via a compressor  260 . Such compressed gas does not unduly affect the formation  100 . 
     The apparatus is further configured such that returning fluids and solids are returned to the surface via the other string (in this case the inner string  210 ). Here, and as the formation  100  is penetrated, a combined medium  270  comprising drilling medium, together with drilling cuttings, produced water and/or gas from the formation is produced to surface  110  via the other string  210 . At surface  110 , a separator  280  can be used to separate the recovered combined medium into drilling medium, drilling cuttings, produced water and gas, and so permits the retrieval of produced gas from the combined medium when drilling. 
     In some examples, the flow in one or both the inner string and an outer string may be controlled so as to prevent undesired flow of hydrocarbons, or other fluids or gases, from uncontrollably reaching the surface (e.g. when drilling through overpressured zones in the formation). As will be appreciated, the apparatus may comprise at least one least one shut-off valve (e.g. blowout preventer) in order to control flow in a manner known. Flow controls may be downhole, for example, at near the drill bit  240  of the drilling apparatus. 
     However, in some examples, the apparatus may be configured to isolate, while drilling, one or more waterzones  290  or acquirers from the wellbore  150 . In some example, the apparatus may be configured to cement  295  so as to isolate the one or more waterzones  290  from the wellbore  150 . Isolated sections of the wellbore  150  may be use, for example, for testing. Isolation of specific section of the wellbore may be achieved using one or more inflatable packers, for example, which may be provided at or near the drilling apparatus. Flow of cement, or other mediums, may be provided through one string, and returned via the other. 
     Providing the drilling medium  250  to drill apparatus in this manner, and isolating waterzones, or the like, means that the drilling medium  250  or concrete, etc. may have been in contact only with the formation at a specific locations (e.g. at, or around, the drill bit, and not along the entire length of the wellbore). In such a manner, the  150  wellbore through the formation can be drilled without unduly affecting a near wellbore formation (e.g. without causing damage, or significant damage, to the wellbore  150 ). In other words, the drilling apparatus and method can be used to reduce the extent to which the formation  100  (or near wellbore) is damaged during the drilling process, when compared with conventional drilling techniques. As such, not fracturing of the formation maybe required, and so the damage to the formation  100  can be limited even further. 
     As a consequence, upon completion, and as is shown in  FIG. 2 b   , central tubing  310  can be positioned within the completed wellbore. This may in some examples be used to decrease water pressure at the formation  100  (e.g. by pumping via the tubing  310 , or selecting a particular drawdown pressure), thus permitting production of both gas and water  370  from the formation  100  using the tubing  310 . This gas and water  370  can be provided to the separator  280 , in a similar manner as when drilling, in order to extract the methane (and possibly other hydrocarbons) from the composition. However, in addition, and due to the fact that wellbore has not been significantly damaged during the drilling operation, liberated methane  375  (e.g. and other gases) may be permitted to flow from the formation  100  using the annulus passing between any completion (and/or open wellbore) and the tubing  310 . In such a manner, the extent to which methane is produced using the tubing is reduced, and the complexity of components and wear and tear of the components at the separator is reduced, when compared to only using the tubing  310 . In addition, methane (or other hydrocarbons) may be able to be flowed straight to pipeline, as is shown in  FIG. 2 b   , without any (or at least any significant) processing. 
     In such a way, a first fluid (e.g. principally water and gas) can be produced from a first flow passage within a wellbore. Further, a second fluid (e.g. principally gas) can be produced from a second flow passage within the wellbore. This may occur simultaneously. In some examples, the tubing  310  (e.g. central through bore) may extend beyond a cased or otherwise completed portion of the wellbore. In some particular examples, the tubing may extend to an open hole section of the wellbore. In those cases, liberated methane, or the like, may be permitting to pass, via the annulus provided between the tubing  310  and outer casing or the like, to surface. 
     While in the above example, a single wellbore  150  has been described, it will readily be appreciated that further wellbore may be drilled using the above methodology. In some examples, data may be acquired during drilling (e.g. logging and/or measurement while drilling) in order to determine formation parameters. In some examples, the method may comprise determining the location of natural cleats in the coal bed formation, based on sampled materials from a well in order to determine desired location for secondary wellbores. This may be achieved additionally or alternatively by observing cuttings or the like from the combined medium  270 , which can be used to identify the precise location of the origins of those cuttings, products, etc. 
     For example, and with particular reference to  FIG. 3 , the drilling apparatus may permit drilling a primary wellbore  300  using, as above for example the drilling apparatus. During such drilling, one or more secondary wellbores  310  may be drilled from the primary wellbore  310 . Each secondary wellbore  310  may be drilled in order to intersect one or more natural cleats  120  in the formation  100 . As with the primary wellbore  300 , the or each secondary wellbore  310  can be drilled using the same drilling methodology. 
     During such process, the well may be permitted to flow during drilling of the primary wellbore  300 . In addition, the well may be permitted to flow during drilling of the or each secondary wellbore  310 . During drilling or each secondary wellbore  310 , well and reservoir potential may be determined (e.g. determining hydrocarbon content and/or composition from wellbore inflow during drilling) in order to optimise well design, and ultimately production. 
     In some examples, isolating one or more of the secondary wellbore  310  from the primary wellbore  300  during drilling may be desired. In such cases, isolation may be provided mechanically and/or chemically. 
     The applicant discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.