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This application claims the benefit of U.S. Provisional Application No. 60/404,787, filed on Aug. 21, 2002. 

   FIELD OF THE INVENTION 
   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 of directional and horizontal wellbores. 
   BACKGROUND OF THE INVENTION 
   Conventional directional and horizontal 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. 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. 
   Underbalanced directional and horizontal 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 technique s typically use a commingled stream of liquid and gas such as nitrogen or carbon dioxide as the drilling fluid. 
   Even when using underbalanced directional or horizontal 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 wall. Some damage to the formation may still occur due to the continued contact of the drilling cuttings and fluid with the formation. Often, some of the drill cuttings are left in the deviated and horizontal sections of the wellbore in underbalanced drilled wells. As well, underbalanced drilling is very expensive for wells with low or moderate production rates. 
   Formation damage is becoming a serious problem for exploration and production of unconventional petroleum resources. 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 directional or horizontal drilling technology. 
   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 
   The present invention allows for the drilling of directional and horizontal wells into hydrocarbon formations with less damage and in a safe and economical manner. The present invention works particularly well in low and under pressure hydrocarbon formations. Existing underbalanced technologies may be too expensive and prolonged exposure of the wellbore walls to fluids and drill cuttings can damage the formation. Further, with existing underbalanced technologies, there is a higher risk that not all of the drill cuttings are returned to the surface. 
   The present invention has a number of advantages over conventional directional and horizontal drilling, namely;
         1. it reduces drilling damage to the formation;   2. it reduces the accumulation of drill cuttings along the directional or horizontal section of a wellbore;   3. drill cuttings and other materials are returned from the formation through the inner pipe or annulus of the concentric drill string, thus these materials are not pushed between the outside of the drill string and the wellbore wall; and   4. it reduces the chance of a drill string becoming stuck due to the availability of three annuluses to circulate through when using a concentric drill string.       

   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 and the build section of the horizontal well completed. The casing is cemented in the 90 degree built section. The drill rig then changes to a concentric drill string, a downhole blowout preventor is added to the bottomhole assembly and the concentric drill string is then tripped back into the wellbore. 
   The present invention is also useful for well stimulation. Hydraulic fracturing has been one of the most common methods of well stimulation in the oil and gas industry. This method of stimulation is not as effective in low and under pressure reservoirs. Five types of reservoir damage can occur in low and under pressure reservoirs when hydraulic fracturing is used, namely
         1. the pore throats in the rock plug up due to the movement of secondary clays;   2. fracturing gel, fracturing sand and fracturing acid compounds remain in the reservoir;   3. swelling of smectitic clays;   4. chemical additives cause precipitation of minerals and compounds in the reservoir; and   5. improper clean out of wellbore to remove materials from deviated section of the wellbore can cause serious damage to producing reservoirs.       

   Accessing natural fractures is one of the most important parts of completing any well in the oil and gas industry, and this is critical to the success of a low or under pressure well. Studies conducted by the United States Department of Energy showed that In a blanket gas reservoir on average a vertical drilled well encounters one fracture, a deviated drilled well encounters fifty-two fractures and a horizontally drilled well thirty-seven fractures. 
   Use of the reverse circulation drilling method and apparatus for forming directional and horizontal wells provides the necessary stimulation of the well without the damage caused by hydraulic fracturing. 
   Thus, the present invention allows low and under pressure formations or reservoirs to receive the necessary well stimulation without damage that is usually encountered using hydraulic fracturing. 
   A method for drilling a directional or horizontal wellbore in a hydrocarbon formation is provided herein, comprising the steps of:
         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;   connecting a bottomhole assembly comprising a directional drilling means to the concentric drill string;   delivering drilling medium through one of said annulus or inner pipe for operating the directional drilling means to form said directional or horizontal wellbore and removing exhaust drilling medium by extracting said exhaust drilling medium through said other of said annulus or inner pipe.       

   In a preferred embodiment, the drilling medium is delivered through the annulus and drill cuttings, exhaust drilling medium and hydrocarbons are removed through the inner tube. 
   In a further preferred embodiment, the drilling medium is delivered through the inner tube and exhaust drilling medium is removed through the annulus. Any drill cuttings and hydrocarbons will also be removed through the annulus. 
   The method for drilling a directional or horizontal wellbore can further comprise the step of preventing any flow of hydrocarbons from the inner pipe or the annulus or both to the surface of the wellbore when the need arises by providing a downhole flow control means positioned near the directional drilling means. 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. 
   In another preferred embodiment, the method for drilling a directional or horizontal 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. 
   In one preferred embodiment, the directional drilling means comprises a drill bit or a reciprocating air hammer and a bent sub or housing for positioning the drill bit and air hammer in the proper direction, and the drilling medium is compressed air, 
   The bottomhole assembly can further comprise a downhole data collection and transmission means such as a measurements-while-drilling (MWD) tool for providing formation pressure and temperature and wellbore trajectory, a shock sub for reducing the amount of vibration received by the MWD tool, a drill collar and an interchange means for directing exhaust drilling medium through the annulus or the inner pipe. 
   In another preferred embodiment, the directional drilling means is a rotary drill bit, which uses a rotary table or top drive drilling system and a bent sub or housing, and the drilling medium is drilling mud, drilling fluid, gases or various combinations of each. 
   The bottomhole assembly can further comprise one or more of the following downhole tools: a MWD tool, a logging-while-drilling (LWD) tool, a downhole blowout preventor and interchange means for adapting the various tools to dual wall drill pipe. Where drilling conditions require, stabilizers, drill collars and jarring devices can also be added to the bottomhole assembly, as well as other drilling tools to meet various drilling requirements which are known in the art. 
   The present invention further provides an apparatus for drilling a directional or horizontal wellbore in hydrocarbon formations, comprising:
         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;   a bottomhole assembly comprising a directional drilling means operably connected to the lower end of said concentric drill string; and   a drilling medium delivery means for delivering drilling medium through one of said annulus or inner pipe for operating the directional drilling means to form said directional or horizontal wellbore and for removing exhaust drilling medium through said other of said annulus or inner tube.       

   The drilling medium can be air, drilling mud, drilling fluids, gases or various combinations of each. 
   In a preferred embodiment, the bottomhole assembly further comprises one or more tools selected from the group consisting of a downhole data collection and transmission means, a shock sub, a drill collar, and an interchange means. 
   In a preferred embodiment, the downhole data collection and transmission means comprises a measurement-while-drilling tool or a logging-while-drilling tool or both. 
   In a preferred embodiment, the apparatus further comprises a downhole flow control means positioned near the directional drilling means for preventing flow of hydrocarbons from the inner pipe or the annulus or both to the surface of the wellbore. 
   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 
       FIG. 1  is a vertical cross-section of a section of concentric drill string. 
       FIG. 2   a  is a schematic illustration of one embodiment of a bottomhole assembly of the present invention for directional and horizontal drilling. 
       FIG. 2   b  is a schematic illustration of another embodiment of a bottomhole assembly having an interchange means for directional and horizontal drilling. 
       FIG. 3   a  is a schematic of a bottomhole assembly for drilling directional and horizontal wells with an air hammer. 
       FIG. 3   b  is a vertical cross-section of an air hammer used with concentric drill string. 
       FIG. 4   a  is an illustration of a wellbore being drilled through subterranean formations in accordance with the present invention using compressed air as the drilling medium. 
       FIG. 4   b  is an illustration of a wellbore being drill through subterranean formations in accordance with the present invention using drilling fluids as the drilling medium. 
       FIG. 5  is a perspective of a surface flow control means. 
       FIG. 6  is a vertical cross-section of one embodiment of a downhole flow control means. 
       FIGS. 7   a  and  7   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. 
       FIGS. 8   a  and  8   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 dosed position. 
       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 open position. 
       FIG. 10  is a perspective of the plurality of flow through slots of the downhole flow control means shown in  8   a  and  8   b  in the closed position. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Apparatus and methods of operation of that apparatus are disclosed herein in the preferred embodiments of the invention that allow for drilling a directional or horizontal wellbore in hydrocarbon formations. From these preferred embodiments, a person skilled in the art can understand how this reverse circulation directional and horizontal drilling process can be used safely in the oil and gas industry. 
     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. The bottomhole assembly is attached to the concentric drill string  4  by threading means  42 . As discussed in more detail below, bottomhole assembly comprises a variety of specialty tools and components which are also attached one to the other by comparable threading means. 
   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  104 . 
     FIG. 2   a  is a schematic illustration of a bottomhole assembly  2  attached to concentric drill string  4  by threading means  42 . In this embodiment, all bottomhole tools which comprise the bottomhole assembly  2  have been adapted for use with concentric drill string and reverse circulation drilling. For example, an outer casing can be provided for encasing existing drilling tools for single wall drill string, thereby providing an annulus between the outer wall of the drilling tool and the inner wall of the outer casing. 
   Bottomhole assembly  2  as shown in this embodiment is operated by compressed air  36  traveling down concentric drill string annulus  20 . Bottomhole assembly  2  comprises a directional drilling means having a wearing drill bit  22 . Wearing drill bit  22  is connected to bent sub  5 , which positions wearing drill bit  22  in the desired direction. Bent sub  5  is connected to air motor  24 , which rotates drill bit  22 . In another embodiment, a drill bit with a bent sub  5  can be used. It is understood that a bent housing can also be used which houses the air motor for positioning of the wearing drill bit. 
   As drill bit  22  cuts formation rock, exhausted air and drill cuttings are carried to the surface through 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 wellbore through the inner pipe  6 . 
   A shroud  28  may be located between drill bit  22  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  38  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 . 
   The bottomhole assembly  2  further comprises a downhole telemetry measurement and transmission device, commonly referred to in the industry as a measurements-while-drilling (MWD) tool  31 , which is used in directional and horizontal drilling to evaluate a number of physical properties such as, but not limited to, pressure, temperature, and wellbore trajectory in three-dimensional space. The MWD tool  31  transmits the drilling associated parameters to the surface by mud pulse, electromagnetic transmission or the like. These signals are received by a data receiving device which is commercially available and necessary with the use of MWD tool  31 . An optional tool, called logging-while-drilling (LWD) tool (not shown), which measures formation parameters such as resistivity, porosity, sonic, velocity and gamma can also be part of the bottomhole assembly  2 . Shock sub  7  is placed between air motor  24  and MWD tool  31  to reduce the amount of vibration MDW tool  31  receives from the drilling operation. Downhole assembly  2  further comprises a downhole blowout preventor or flow control means  68  to prevent hydrocarbons from coming up inner pipe  6  and concentric drill string annulus  20 , should the need arise. 
     FIG. 2   b  is a schematic illustration of a preferred embodiment which uses conventional drilling tools used with single walled drill pipe. In this embodiment, bottomhole assembly  22  comprises an interchange means  106  for diverting drill cuttings  38  from the formation annulus  40  into the inner pipe  6 . Interchange means  106  comprises vertical slot  107  to let drill cuttings  38  escape through the center of inner pipe  6 . Interchange means  106  further comprises wings or shroud  108  which prevents drill cuttings  38  from continuing up the formation annulus to the surface of the wellbore. Generally, if the wellbore being drilled is 6¼ inches in diameter, the outer diameter (OD) of the interchange means  106  would be 5½ inches, which would include the wings or shroud  108 . 
     FIG. 3   a  is a schematic of a bottomhole assembly for drilling directional and horizontal wells with an air hammer. Bottomhole assembly  202  comprises reciprocating air hammer  222 , said reciprocating air hammer shown in more detail in  FIG. 3   b . The bottomhole assembly  202  is attached to concentric drill string  4  by threading means  42 . Bottomhole assembly  2  further comprises bent sub  205  which positions air hammer  222  in the desired direction at a small angle offset from the axis of the concentric drill pipe. Shock sub  7  helps reduce the impact from the reciprocating air hammer  222  on MWD tool  31 . 
   MWD tool  31  provides a number of evaluations of physical properties such as, but not limited to, pressure, temperature and wellbore trajectory in three-dimensional space. A LWD tool (not shown), which measures formation parameters such as resistivity, porosity, sonic, velocity and gamma, may also form part of the bottomhole assembly  2 . 
     FIG. 3   b  is a vertical cross-section of reciprocating air hammer  222  which is operated by compressed air  36  traveling down concentric drill string annulus  20 . The reciprocating air hammer  222  comprises a wearing drill bit  122 . Wearing drill bit  122  is connected to a reciprocating piston  24  within piston casing  26 . Venturi  34 , positioned between the reciprocating piston  24  and the inner pipe  6 , 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 wellbore through the inner pipe  6 . If required, a suction compressor at the surface can be attached to inner pipe  6  to assist in the discharge of the drill cuttings  38 . 
   A shroud  28  may be 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 . 
   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 wellbore through concentric drill string annulus  20 . 
     FIG. 4   a  shows a preferred embodiment of the present method and apparatus for safely drilling a directional or horizontal natural gas well or any well containing hydrocarbons using concentric drill string and compressed air as the drilling medium. Drilling rig  46  comprises air compressor  48  which pumps compressed air down the concentric drill string annulus of concentric drill string  4 . Downhole assembly comprises a directional drilling means having a drill bit  22 , bent sub  5  and air motor  24 , and drill bit  22  operates to cut into the rock in wellbore  52 . Downhole assembly further comprises shock sub  7 , MWD tool  31 , and downhole flow control means  68 . 
   As drill bit  22  cuts through the rock, exhaust compressed air, drill cutting and hydrocarbons from formation bearing zones are carried up the inner pipe  6  of concentric drill string  4  as shown in more detail in  FIG. 1 . Discharge line  54  carries the exhaust compressed air, drill cuttings and hydrocarbons produced from the wellbore to blewie line  56 . A suction type compressor (not shown) may be hooked up at the surface of the wellbore to assist in lifting the drilling medium, drill cutting and hydrocarbons up the inner pipe. 
   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 . 
   In another preferred embodiment using compressed air as the drilling medium, the downhole assembly comprises a bent sub, a reciprocating air hammer and a MWD tool, as shown in  FIG. 3   a . The air hammer cuts through rock in the wellbore, exhaust compressed air, drill cuttings, and hydrocarbons from formation bearing zones are carried up the inner pipe  6  as shown in  FIG. 1 . Discharge line  54  carries the exhaust compressed air, drill cuttings and hydrocarbons produced from the wellbore to blewie line  56 . A suction type compressor (not shown) may be hooked up at the surface of the wellbore to assist in lifting the drilling medium, drill cutting and hydrocarbons up the inner pipe. 
   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 . 
     FIG. 4   b  shows a preferred embodiment of the present invention for safely drilling a directional or horizontal natural gas well or any well containing hydrocarbons where the drilling medium is drilling fluids. Drilling rig  46  comprises drilling fluid pump system  49  which pumps drilling fluid down the concentric drill string annulus of concentric drill string  4 . Downhole assembly comprises drill bit  50 , a bent housing mud motor  55 , and MWD tool  53 , the latter two of which are used to power and direct drill bit  50 . As drill bit  50  cuts through the formation rock in wellbore  52 , returned drilling fluids, drilling cuttings and hydrocarbons from the formation bearing zones are carried up the inner pipe of concentric drill string  4 . 
   Drill cuttings are deposited in pit  58 . Hydrocarbons produced through blewie line  56  are pumped into tank  65  or 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 . 
   Shroud  57  may be placed around drill bit  50  to prevent drilling fluids 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  as shown in  FIG. 3   a.    
   In  FIG. 4   a , the directional drilling means is rotated by rotary table  57  as is understood in the art. In  FIG. 4   b , the directional drilling means is rotated by top drive  59  as is understood in the art. 
   It is a preferred feature of the present invention that a surface flow control means or surface annular blowout preventor  66  be provided to prevent hydrocarbons from escaping from the formation annulus between the inner wellbore wall and the outside wall of the outer pipe of the concentric drill string during certain operations such as tripping concentric drill string in or out of the wellbore. An example of a suitable surface annular blowout preventor  66  is shown in  FIG. 5 . 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. 
   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. 
   Surface annular blowout preventor  66  is not equipped to control hydrocarbons flowing up the inside of concentric drill string  4 , however. Therefore, preferably 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 wellbore, 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 wellbore 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. 
   One embodiment of downhole flow control means  68  is shown in greater detail in  FIG. 6 . 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 ultimately communicate with and power the directional drilling means of the bottomhole assembly. Exhausted drilling medium, drill cuttings and hydrocarbons can flow freely from bottomhole assembly up flow path  80 . 
   Exhausted drilling medium, drill cuttings and hydrocarbons then flow through ports  82  which allow for communication with the inner pipe  6  through flow path  84 . 
   When desired, flow paths  78  and  80  can be closed by axially moving inner pipe  6  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 . 
   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 now 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. 
   The downhole flow control means can also be used when drilling with air, drilling mud, drilling fluids, gases or various combinations of each. 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. 7   a  and  7   b ,  FIGS. 8   a  and  8   b ,  FIG. 9  and  FIG. 10 . This flow control means is further described in more detail in U.S. patent application, Ser. No. 10/321,087, incorporated herein by reference. 
     FIGS. 7   a  and  7   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 wellbore.  FIGS. 8   a  and  8   b  show the downhole flow control means  680  in the dosed 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 wellbore. 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. 9  in the open position. The closed position of the downhole flow control means  680  is shown in  FIG. 10  where the plurality of flow through slots  102  is in the closed position. 
   To open the downhole flow control means  680 , the downhole flow control means  680  is place solidly on the bottom of the wellbore and the entire concentric drill string  680  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. 
   It often occurs during drilling operations that a “kick” or overpressure situation occurs down in the wellbore. 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 wellbore. If this fails to reduce the pressure in the wellbore then drilling mud or fluid could be pumped down the kill line  74  to regain control of the well. 
   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 directional or horizontal wellbore in a hydrocarbon formation using concentric drill string having an inner pipe and an outer pipe defining an annulus there between. A bottomhole assembly comprising a directional 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 directional drilling means to form a borehole. Exhaust drilling medium, drilling cutting and hydrocarbon are removed from the wellbore by extracting the exhaust drilling medium, drilling cutting and hydrocarbon through the other of the annulus or inner pipe.