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FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a drilling method and assembly for exploration and production of oil, natural gas, coal bed methane, methane hydrates, and the like. More particularly, the present invention relates to a two string, or dual wall pipe drilling method and apparatus useful for reverse circulation drilling.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional drilling typically uses single wall jointed drill pipe with a drill bit attached at one end. Weighted drilling mud or fluid is pumped through a rotating drill pipe to drive the drill bit to drill a borehole. The drill cuttings and exhausted drilling mud and fluid are returned to the surface up the annulus between the drill pipe and the formation by using mud, fluids, gases or various combinations of each to create enough pressure to transport the cuttings out of the wellbore. Compressed air can also be used to drive a rotary drill bit or air hammer. However, in order to transport the drill cuttings out of the wellbore, the hydrostatic head of the fluid column can often exceed the pressure of the formation being drilled. Therefore, the drilling mud or fluid can invade into the formation, causing significant damage to the formation, which ultimately results in loss of production. In addition, the drill cuttings themselves can cause damage to the formation as a result of the continued contact with the formation and the drill cuttings. Air drilling with a rotary drill bit or air hammer can also damage the formation by exceeding the formation pressure and by forcing the drill cuttings into the formation.  
           [0003]    Underbalanced drilling technology has been developed to reduce the risk of formation damage due to the hydrostatic head of the fluid column, which uses a mud or fluid system that is not weighted. Hence, drill cutting can be removed without having the fluid column hydrostatic head exceed the formation being drilled resulting in less damage to the formation. Underbalanced drilling techniques typically use a commingled stream of liquid and gas such as nitrogen or carbon dioxide as the drilling fluid.  
           [0004]    Nevertheless, even when using underbalanced drilling technology, there still is the possibility of damage to the formation. The drilling fluid and drill cuttings are still being returned to the surface via the annulus between the drill pipe and the formation. Hence, some damage to the formation may still occur due to the continued contact of the drilling cuttings and fluid with the formation. As well, underbalanced drilling is very expensive for wells with low or moderate production rates.  
           [0005]    Formation damage is becoming a serious problem for exploration and production of unconventional petroleum resources. For example, conventional natural gas resources are buoyancy driven deposits with much higher formation pressures. Unconventional natural gas formations such as gas in low permeability or “tight” reservoirs, coal bed methane, and shale gases are not buoyancy driven accumulations and thus have much lower pressures. Therefore, such formations would damage much easier when using conventional oil and gas drilling technology.  
           [0006]    The present invention reduces the amount of pressure which normally results when using air drilling, mud drilling, fluid drilling and underbalanced drilling by using a two string drilling system, thereby greatly reducing formation damage.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention allows for the drilling of hydrocarbon formations in a less damaging, safe and economical manner. The present invention works particularly well in under-pressured hydrocarbon formations where existing underbalanced technologies may be too expensive, or fluids can damage the formation.  
           [0008]    The present invention has a number of advantages over conventional drilling technologies in addition to virtually eliminating drilling damage to the formation. The invention reduces the accumulation of drill cuttings at the bottom of the wellbore; it allows for gas zones to be easily identified; and multi-zones of gas in shallow gas well bores can easily be identified without significant damage during drilling. Finally, the chances of a concentric drill string becoming stuck are greatly reduced due to the availability of three annuluses to circulate through.  
           [0009]    The present invention can be used to drill an entire well or can be used in conjunction with conventional drilling technology. For example, the top portion of a hydrocarbon bearing formation can first be drilled using conventional drill pipe. The drill pipe can then be tripped out of the wellbore and the well casing cemented in place. The remainder of the well can then be drilled using the present two string drilling system.  
           [0010]    A method for drilling a wellbore in a hydrocarbon formation is provided herein, comprising the steps of:  
           [0011]    providing a concentric drill string having an inner pipe, said inner pipe having an inside wall and an outside wall and situated within an outer pipe having an inside wall and an outside wall, said outside wall of said inner pipe and said inside wall of said outer pipe defining an annulus between the pipes;  
           [0012]    connecting a drilling means at the lower end of the concentric drill string;  
           [0013]    delivering drilling medium through one of said annulus or inner pipe for operating the drilling means to form a borehole and removing said drilling medium by extracting said drilling medium through said other of said annulus or inner pipe.  
           [0014]    In a preferred embodiment, the drilling medium is delivered through the annulus and removed through the inner tube. Any drill cuttings, drilling medium and hydrocarbons will also be removed through the inner tube.  
           [0015]    In a further preferred embodiment, the drilling medium is delivered through the inner tube and removed through the annulus. Any drill cuttings, drilling medium and hydrocarbons will also be removed through the annulus.  
           [0016]    The method for drilling a wellbore can further comprise the step of providing a downhole flow control means positioned near the drilling means for preventing any flow of hydrocarbons from the inner pipe or the annulus or both to the surface when the need arises. Typically, the flow control means will operate to shut down the flow from both the inner pipe and the annulus when joints of concentric drill string are being added or removed.  
           [0017]    In another preferred embodiment, the method for drilling a wellbore can further comprise the step of providing a surface flow control means for preventing any flow of hydrocarbons from the space between the outside wall of the outer pipe and the walls of the wellbore. This as well is important when adding or removing joints of concentric drill string.  
           [0018]    In one preferred embodiment, the drilling means is a rotary drill bit or reciprocating air hammer and the drilling medium is compressed air. In another preferred embodiment the drilling means is a rotary drill bit, which uses a rotary table or top drive drilling system, and the drilling medium is drilling mud, drilling fluid, gases or various combinations of each.  
           [0019]    The present invention further provides an apparatus for drilling a wellbore in hydrocarbon formations, comprising:  
           [0020]    a concentric drill string having an inner pipe having an inside wall and an outside wall and an outer pipe having an inside wall and an outside wall, said outside wall of said inner pipe and said inside wall of said outer pipe defining an annulus between the pipes;  
           [0021]    a drilling means at the lower end of said concentric drill string; and  
           [0022]    a drilling medium delivery means for delivering drilling medium through one of said annulus or inner pipe for operating the drilling means to form a borehole and for removing said drilling medium through said other of said annulus or inner tube.  
           [0023]    The drilling medium can be air, drilling mud, drilling fluids, gases or various combinations of each.  
           [0024]    In a preferred embodiment, the apparatus further comprises a downhole flow control means positioned near the drilling means for preventing flow of hydrocarbons from the inner pipe or the annulus or both to the surface of the wellbore.  
           [0025]    In a further preferred embodiment, the apparatus further comprises a surface flow control means for preventing any flow of hydrocarbons from the space between the outside wall of the outer pipe and the walls of the wellbore.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a vertical cross-section of a section of concentric drill string.  
         [0027]    [0027]FIG. 2 is a vertical cross-section of a section of concentric drill string and drilling means thereto attached.  
         [0028]    [0028]FIG. 3 is a general view showing a partial cross-section of the apparatus and method of the present as it is located in a drilling operation.  
         [0029]    [0029]FIG. 4 is a perspective of a surface flow control means.  
         [0030]    [0030]FIG. 5 is a vertical cross-section of one embodiment of a downhole flow control means.  
         [0031]    [0031]FIGS. 6 a  and  6   b  show a vertical cross-section of the top portion and bottom portion, respectively, of another embodiment of a downhole flow control means in the open position.  
         [0032]    [0032]FIGS. 7 a  and  7   b  show a vertical cross-section of the top portion and bottom portion, respectively, of the downhole flow control means shown in  6   a  and  6   b  in the closed position.  
         [0033]    [0033]FIG. 8 is a perspective of the plurality of flow through slots of the downhole flow control means shown in  6   a  and  6   b  in the open position.  
         [0034]    [0034]FIG. 9 is a perspective of the plurality of flow through slots of the downhole flow control means shown in  7   a  and  7   b  in the closed position.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Apparatus and methods of operation of that apparatus are disclosed herein in the preferred embodiments of the invention that allow for drilling a wellbore in hydrocarbon formations. From these preferred embodiments, a person skilled in the art can understand how this reverse circulation drilling process can be used safely in the oil and gas industry.  
         [0036]    [0036]FIG. 1 is a vertical cross-section of a section of concentric drill string  4 . Concentric drill string  4  comprises an inner pipe  6  having an inside wall  8  and an outside wall  10  and an outer pipe  12  having an inside wall  14  and an outside wall  16 . The diameter of inner pipe  6  and outer pipe  12  can vary; in one embodiment of the invention, the outer diameter of the outer pipe  12  is 4½ inches and the outer diameter of the inner pipe  6  is 2½ inches. Joints of concentric drill string  4  are attached one to another by means such as threading means  42  to form a continuous drill string.  
         [0037]    Concentric drill string annulus  20  is formed between the outside wall  10  of the inner pipe  6  and the inside wall  14  of the outer pipe  12 . Drilling medium  76 , for example, drilling mud, drilling fluid, compressed air or commingled mixtures of drilling mud, fluids and gases such as nitrogen and carbon dioxide, is pumped down concentric drill string annulus  20  and removed through the inner pipe. Drill cuttings  38  are removed through the inner pipe along with the exhausted drilling medium.  
         [0038]    [0038]FIG. 2 is a vertical cross-section of the bottom portion of concentric drill string  4  showing drilling apparatus  2  attached to concentric drill string  4  by threading means  42 . Drilling apparatus  2  as shown in this embodiment is a reciprocating rock drill operated by compressed air  36  traveling down concentric drill string annulus  20 . The reciprocating rock drill comprises a wearing drill bit  22 . Wearing drill bit  22  is connected to a reciprocating piston  24  moving within piston casing  26 . Venturi  34 , positioned between the reciprocating piston  24  and the inner pipe, directs and accelerates exhaust air from the reciprocating piston  24  to the inner pipe  6 . The compressed air  36  is of sufficient velocity to pick up and carry all drill cuttings  38  to the surface of the well bore through the inner pipe  6 .  
         [0039]    Shroud  28  is located between the piston casing  26  and the formation  30  in relatively air tight and frictional engagement with the inner wellbore wall  32 . Shroud  28  prevents compressed air  36  and drill cuttings from escaping up the formation annulus  40  between the outside wall  16  of the outer pipe  12  of the concentric drill string  4  and the inner wellbore wall  32 .  
         [0040]    In another embodiment of the present invention, compressed air can be pumped down the inner pipe  6  and the drill cuttings and exhaust compressed air carried to the surface of the well bore through concentric drill string annulus  20 .  
         [0041]    Reverse circulation drilling of the present invention can also use drilling mud or drilling fluids as well as air to power a rotary drill bit to cut the rock in the well bore. Powerful mud pumps push mud or fluids down concentric drill string annulus  20 . Drill cuttings, drilling mud and fluids travel up the inner pipe  6  to surface of the wellbore where they are put into a mud tank or pit. In the alternative, drilling mud or drilling fluids can be pumped down the inner pipe  6  and the drilling mud or drilling fluids and drill cuttings travel up the concentric drill string annulus  20  to the surface of the wellbore.  
         [0042]    [0042]FIG. 3 shows a preferred embodiment of the present method and apparatus for safely drilling a natural gas well or any well containing hydrocarbons using the concentric drilling string method. Drilling rg  46  comprises air compressor  48  which pumps compressed air down the concentric drill string annulus  20  of concentric drill string  4 . Drilling apparatus comprises air hammer  50  which is operated as described above to cut the rock in well bore  52 . As air hammer  50  cuts through the rock in well bore  52 , exhaust compressed air, drill cuttings and hydrocarbons from formation bearing zones are carried up inner pipe  6  as shown in FIGS. 1 and 2. Discharge line  54  carries the exhaust compressed air, drill cuttings and hydrocarbons produced from the well bore to blewie line  56 . A suction type compressor (not shown) may be hooked up at the surface of the well bore to assist in lifting the drilling medium, drill cutting and hydrocarbons up the inner pipe.  
         [0043]    Drill cuttings are deposited in pit  58 . Hydrocarbons produced through blewie line  56  are flared through flare stack  60  by means of propane torch  62  to atmosphere. Propane torch  62  is kept lit at all times during the drilling operations to ensure that all hydrocarbons are kept at least 100 feet away from the drilling rig floor  64 .  
         [0044]    A surface flow control means or surface annular blowout preventor  66  is used to prevent hydrocarbons from escaping from the formation annulus between the inner well bore wall and the outside wall of the outer pipe of the concentric drill string during certain operations such  88  tripping concentric drill string in or out of the well bore. An example of a suitable surface annular blowout preventor  66  is shown in FIG. 4. Other surface blowout preventors that can be used are taught in U.S. Pat. Nos. 5,044,602, 5,333,832 and 5,617,917, incorporated herein by reference.  
         [0045]    It is preferable that the surface annular blowout preventor contain a circular rubber packing element (not shown) made of neoprene synthetic rubber or other suitable material that will allow the surface annular blowout preventor to seal around the shape of an object used downhole, for example, drill pipe, air hammer, drill bits, and other such drilling and logging tools.  
         [0046]    Surface annular blowout preventor  66  is not equipped to control hydrocarbons flowing up the inside of concentric drill string  4 , however. Therefore, a second downhole flow control means or blowout preventor  68  Is used to prevent hydrocarbons from coming up inner pipe  6  and concentric drill string annulus  20 . For example, when concentric drill string  4  is tripped out of the well bore, downhole flow control means  68  should be in the closed position to ensure maximum safety. This allows for the safe removal of all joints of concentric drill string from the well bore without hydrocarbons being present on the drill rig floor  64 . The downhole flow control means  68  is preferably attached at or near the drilling apparatus for maximum effectiveness.  
         [0047]    One embodiment of downhole flow control means  68  is shown in greater detail in FIG. 5. This figure shows downhole flow control means  68  in the open position, where drilling medium  76  can flow down concentric drill string annulus  20  and in communication with flow path  78 . Drilling medium  76  is allowed to continue through flow control means  68  and communicate with and power the air hammer. Exhausted compressed air, drill cuttings and hydrocarbons can flow freely from the reverse circulation of the air hammer up flow path  80 . Exhausted compressed air, drill cuttings and hydrocarbons then flow through ports  82  which allow for communication with the inner pipe  6  through flow path  84 .  
         [0048]    When desired, flow paths  78  and  80  can be closed by axially moving inner pipe a downward relative to outer pipe  12 , or conversely moving outer pipe  12  upward relative to inner pipe  6 . Inner pipe  6  can be locked into place relative to outer string  12 . A friction ring  86  on surface  88  aligns with recess  90  on surface  92  to lock the inner pipe  6  and outer pipe  12  together until opened again by reversing the movement. When in the closed position, surface  92  is forced against surface  88  to close off flow path  80 . Similarly, surface  94  is forced against surface  96  to seal off flow path  78 . Applying axial tension between the two pipes reverses the procedure, and restores flow through flow path  78  and  80 .  
         [0049]    An optional feature of flow control means  68  is to provide a plurality of offsetting ports  98  and  100  which are offset while the downhole flow control means is open, but are aligned when the downhole flow control means is in the closed position. The alignment of the plurality of ports  98  and  100  provide a direct flow path between flow paths  78  and  80 . This feature would allow for continued circulation through the inner pipe  6  and the concentric drill string annulus  20  for the purpose of continuous removal of drill cutting from the concentric drill string while the downhole flow control means  68  is in the closed position.  
         [0050]    It should be noted that while downhole flow control means  68  has been described in the context of air drilling, this downhole flow control means can also be used when drilling with drilling mud, drilling fluids, gas or various mixtures of the three. However, when the drilling medium used is drilling mud or drilling fluid, an alternate downhole flow control means can be used which only shuts down flow through the inner pipe  6 . This is because the hydrocarbons would likely not be able to escape through the drilling mud or drilling fluid remaining in concentric drill string annulus  20 . One embodiment of such a downhole flow control means is shown in FIGS. 6 a  and  6   b , FIGS. 7 a  and  7   b , FIG. 8 and FIG. 9. This flow control means is further described in more detail in U.S. patent application Ser. No. 10/321087, incorporated herein by reference.  
         [0051]    [0051]FIGS. 6 a  and  6   b  show the downhole flow control means  680  in the open position, where exhausted compressed air, drilling mud or fluids, drill cuttings and hydrocarbons can flow freely up the concentric drill string attached thereto to the surface of the well bore. FIGS. 7 a  and  7   b  show the downhole flow control means  680  in the closed position. To place the downhole flow control means  680  in the closed position, the concentric drill string must be resting solidly on the bottom of the well bore. The entire concentric drill string is rotated three quarters of one turn to the left. The mechanical turning to left direction closes a plurality of flow through slots  102 , shown in FIG. 8 in the open position. The closed position of the downhole flow control means  480  is shown in FIG. 9 where the plurality of flow through slots  102  is in the closed position. To open the downhole flow control means  480 , the downhole flow control means  480  is place solidly on the bottom of the well bore and the entire concentric drill string  480  is rotated back to the right, three quarters of one turn. This will restore the plurality of flow through slots  102  to the open position.  
         [0052]    It often occurs during drilling operations that a “kick” or overpressure situation occurs down in the well bore. If this occurs, both the surface annular blowout preventor  66  and the downhole flow control means  68  would be put into the closed position. Diverter line  70  and manifold choke system  72  would be used to reduce the pressure in the well bore. If this fails to reduce the pressure in the well bore then drilling mud or fluid could be pumped down the kill line  74  to regain control of the well.  
         [0053]    While various embodiments in accordance with the present invention have been shown and described, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications as known to those skilled in the art, and therefore the present invention is not to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

Summary:
Method and apparatus for drilling a well bore in a hydrocarbon formation using concentric drill string having an inner pipe and an outer pipe defining an annulus there between. A drilling means such as an air hammer or a rotary drill bit and driving system is provide at the lower end of the concentric drill string and drilling medium is delivered through the annulus or inner pipe for operating the drilling means to form a borehole. Drilling medium, drilling cutting and hydrocarbon are removed from the well bore by extracting the drilling medium, drilling cutting and hydrocarbon through the other of the annulus or inner pipe.