Patent Publication Number: US-6991048-B2

Title: Wellbore plug system and method

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to systems and methods for the recovery of subterranean resources and, more particularly, to a wellbore plug system and method. 
   BACKGROUND OF THE INVENTION 
   Subterranean deposits of coal (typically referred to as “coal seams”) often contain substantial quantities of entrained methane gas. Limited production and use of methane gas from coal seams has occurred for many years because substantial obstacles have frustrated extensive development and use of methane gas deposits in coal seams. 
   In recent years, various methods have been used to retrieve methane gas deposits from coal seams. One such method is the use of underbalanced drilling using a dual-string technique. As an example of this method, a fluid such as drilling fluid is circulated down a drill string, while another relatively light fluid such as air or nitrogen is circulated down an annulus formed between an outside surface of a drill string and an inside surface of a casing string. A mixture of these fluids is retrieved from an annulus formed between an outer surface of the casing string and an inside surface of the wellbore after mixing with a gas or other fluid obtained from a lateral wellbore being drilled. The purpose of the lighter fluid is to lighten the weight of the drilling fluid such that the hydrostatic head of the drilling fluid does not force the drilling fluid into the subterranean formation and create detrimental effects. 
   SUMMARY OF THE INVENTION 
   The present invention provides a wellbore sealing system and method that substantially eliminates or reduces the disadvantages and problems associated with previous systems and methods. 
   In accordance with one embodiment of the present invention, a method for drilling wellbores includes drilling a main wellbore, disposing a casing string having a deflecting member at a lower end thereof in the main wellbore, disposing a drill string having a drill bit at a lower end thereof in the casing string, and drilling, with the drill bit, a first lateral wellbore at a first depth in the main wellbore. The method further includes transferring the casing string to a second depth in the main wellbore that is less than the first depth, disposing a first temporary plug in the main wellbore at the second depth to prevent gas from flowing up the main wellbore past the second depth, transferring the casing string to a third depth in the main wellbore that is less than the second depth, and drilling, with the drill bit, a second lateral wellbore at the third depth. 
   Some embodiments of the present invention may provide one or more technical advantages. These technical advantages may include more efficient drilling and production of methane gas and greater reduction in costs and problems associated with other drilling systems and methods. For example, there may be less damage to lateral wellbores because of mud or other fluids entering a lateral wellbore from the drilling of another lateral wellbore. In addition, cuttings are prevented from dropping into lower lateral wellbores while an upper lateral wellbore is being drilled. Another technical advantage includes providing a method for killing a lateral wellbore, while still being able to drill another lateral wellbore. An additional technical advantage is that underbalanced drilling may be performed along with the teachings of one embodiment of the present invention. 
   Other technical advantages of the present invention are readily apparent to one skilled in the art from the figures, descriptions, and claims included herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like numerals represent like parts, in which: 
       FIG. 1  is a cross-sectional view of an example slant well system for production of resources from one or more subterranean zones via one or more lateral wellbores; 
       FIG. 2  illustrates an example system for drilling lateral wellbores according to one embodiment of the present invention; 
       FIG. 3  illustrates another example system for drilling lateral wellbores according to one embodiment of the present invention; and 
       FIG. 4  is a flowchart demonstrating an example method for drilling lateral wellbores according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention and their advantages are best understood by referring now to  FIGS. 1 through 4  of the drawings, in which like numerals refer to like parts. 
     FIG. 1  is a cross-sectional view of an example well system  100  for production of resources from one or more subterranean zones  102  via one or more lateral wellbores  104 . In various embodiments described herein, subterranean zone  102  is a coal seam; however, other subterranean formations may be similarly accessed using well system  100  of the present invention to remove and/or produce water, gas, or other fluids. System  100  may also be used for other suitable operations, such as to treat minerals in subterranean zone  102  prior to mining operations, to inject or introduce fluids, gasses, or other substances into subterranean zone  102 , or for any other appropriate purposes. 
   Referring to  FIG. 1 , well system  100  includes an entry wellbore  105 , a main wellbore  106 , a plurality of lateral wellbores  104 , a cavity  108  associated with main wellbore  106 , and a rat hole  110  associated with main wellbore  106 . Entry wellbore  105  extends from a surface  12  towards subterranean zones  102 . Entry wellbore  105  is illustrated in  FIG. 1  as being substantially vertical; however, entry wellbore  105  may be formed at any suitable angle relative to surface  12  to accommodate, for example, surface  12  geometries and/or subterranean zone  102  geometries. 
   Main wellbore  106  extends from the terminus of entry wellbore  105  toward subterranean zones  102 , although main wellbore may alternatively extend from any other suitable portion of entry wellbore  105 . Where there are multiple subterranean zones  102  at varying depths, as illustrated in  FIG. 1 , main wellbore  106  extends through the subterranean zone  102  closest to surface  12  into and potentially through the deepest subterranean zone  102 . There may be one or any number of main wellbores  106 . As illustrated, main wellbore  106  is a slant well and, as such, is formed to angle away from entry wellbore  105  at an angle designated α, which may be any suitable angle. Main wellbore  106  may also include cavity  108  and/or rat hole  110  located at a terminus thereof. Main wellbore  106  may include one, both, or neither cavity  108  and rat hole  110 . 
   Lateral wellbores  104  extend from main wellbore  106  into an associated subterranean zone  102 . Lateral wellbores  104  are shown in  FIG. 1  to be substantially horizontal; however, lateral wellbores  104  may be formed in other suitable directions off of main wellbore  106  and may have a curvature associated therewith. Any suitable systems and/or methods may be used to drill lateral wellbores  104 ; however, example systems for drilling lateral wellbores  104  according to various embodiments of the present invention are described below in conjunction with  FIGS. 2 and 3 . 
     FIG. 2  illustrates an example system  200  for drilling lateral wellbores  104  according to one embodiment of the present invention. As illustrated, system  200  includes a drill string  201  having a drill bit  202 , a casing string  204 , a deflecting member  206  having a deflecting surface  208  coupled to a lower end of casing string  204 , and one or more temporary plugs  210  disposed within main wellbore  106 . 
   Drill string  201  may be any suitable drill string having any suitable length and diameter and any suitable drill bit  202  for the purpose of drilling lateral wellbores  104 . Drill string  201  is typically a hollow conduit for allowing drilling fluids to flow therethrough. Drill bit  202  may be driven through the use of any suitable motor powered by the drilling fluid or otherwise powered and may have any suitable configuration. To direct drill string  201  and drill bit  202  for the purpose of drilling lateral wellbore  104 , deflecting surface  208  of deflecting member  206  is utilized. 
   Casing string  204  may be any suitable casing string having any suitable diameter that is to be inserted into main wellbore  106 . Casing string  204  may be adapted to rotate within main wellbore  106  as illustrated by arrow  216 . Although arrow  216  is illustrating a counterclockwise direction, casing string may also be rotated in a clockwise direction. An inner annulus  212  is formed between the inner surface of casing string  204  and the outer surface of drill string  201 . An outer annulus  214  is also formed between an outside surface of casing string  204  and the surface of main wellbore  106 . Inner annulus  212 , outer annulus  214 , and drill string  201  may be used to perform underbalanced drilling. As one example of underbalanced drilling, a first fluid may be circulated down drill string  201 , such as drilling mud or other suitable drilling fluids. A second fluid is circulated down inner annulus  212 , such as air, nitrogen, or other relatively light fluid. Both first and second fluids may be retrieved from outer annulus  214  after mixing with a gas or other fluid produced from lateral wellbore  104 . The purpose of the second fluid is to lighten the weight of the first fluid such that the hydrostatic head of the first fluid does not force first fluid into the subterranean formation. As a variation, the second fluid may be circulated down outer annulus  214  and the mixture of the first and second fluids along with the gas from lateral wellbore  104  may be retrieved via inner annulus  212 . 
   According to the teachings of the present invention, each temporary plug  210  is adapted to plug main wellbore  106  such that a gas or other fluid existing in main wellbore  106  below temporary plug  210  is prevented from flowing upward past temporary plug  210 . In addition, any drilling fluid or cuttings are prevented from flowing down main wellbore  106  past temporary plug  210 . In one embodiment of the invention, this allows the drilling of a lateral wellbore  104   a  in a subterranean zone  102   a  at a first depth  216  and then the drilling of a lateral wellbore  104   b  in a subterranean zone  102   b  at a third depth  218 , while ensuring that any gas or other fluid obtained from lateral wellbore  104   a  at first depth  216  does not flow past a temporary plug  210   a  existing at a second depth  217  and interfere with the drilling of lateral wellbore  104   b  at third depth  218 . 
   In one embodiment, temporary plugs  210  are formed from a bentonite clay; however, temporary plugs  210  may be formed from a polymer or other suitable viscous material. In addition, any suitable type of accelerator and/or catalyst may be added to the material that forms temporary plugs  210  in order to speed the curing time of temporary plugs  210  to a suitable time period. Temporary plugs  210  may be other suitable plugs, such as mechanical plugs, drill plugs, and cement plugs. Each temporary plug  210  may have any suitable length within main wellbore  106 . Any suitable system or method may be used to install temporary plugs  210  in main wellbore  106 ; however, in one embodiment, casing string  204  is utilized to deliver the material down to the desired depth. 
   In operation of one embodiment of system  200  of  FIG. 2 , main wellbore  106  is drilled via any suitable method. Casing string  204  having deflecting member  206  attached thereto is inserted into main wellbore  106 . Once at a desired depth, such as first depth  216 , drill string  201  having drill bit  202  is inserted within casing string  204  so that lateral wellbore  104   a  may be drilled at first depth  216 . After drilling lateral wellbore  104   a , drill bit  202  is retracted from lateral wellbore  104   a  and casing string  204  is then raised to second depth  217  so that temporary plug  210   a  may be disposed within main wellbore  106  at second depth  217 . The disposing of temporary plug  210   a  in main wellbore  106  prevents any gas or other fluid produced from lateral wellbore  104   a  from flowing up main wellbore  106  from a depth below temporary plug  210   a  past second depth  217 . As mentioned previously, this allows successive lateral wellbores  104  to be drilled at successively higher depths while ensuring that any gas or other fluid from a lower lateral wellbore  104  does not cause detrimental effects. 
   After disposing temporary plug  210   a , casing string  204  is transferred to third depth  218  where lateral wellbore  104   b  is drilled with drill bit  202 . After drilling lateral wellbore  104   b , drill bit  202  is retracted from lateral wellbore  104   b  and casing string  204  is then raised to a fourth depth  219  where a temporary plug  210   b  is disposed within main wellbore  106 . Temporary plug  210   b  prevents any gas or other fluid from lateral wellbore  104   b  from flowing up to a depth in main wellbore  106  higher than fourth depth  219 . Other lateral wellbores  104 , such as a lateral wellbore  104   c , may be drilled at higher depths according to a similar procedure as described above. 
   When the gas or other fluid from all drilled lateral wellbores  104  are desired to be accessed, then each temporary plug  210  that has been disposed within main wellbore  106  may be removed from main wellbore  106  using any suitable procedure, such as drilling. Alternatively, temporary plugs  210  may be removed by their dissolving over a period of time if temporary plugs  210  are formed from a material suitable to dissolve over a period of time. Another example of the use of temporary plugs  210  is shown below in conjunction with FIG.  3 . 
     FIG. 3  illustrates another example system  300  for drilling lateral wellbores  104  according to one embodiment in the present invention. System  300  is similar to system  200  described above; however, a difference is that one or more temporary plugs  310  are disposed within each lateral wellbore  104  instead of being disposed within main wellbore  106 . Accordingly, when lateral wellbore  104   a  is drilled at first depth  216 , then a temporary plug  310   a  is disposed within lateral wellbore  104   a  at a location adjacent to main wellbore  106  to prevent any gas or other liquid from lateral wellbore  104   a  from flowing into main wellbore  106 . Casing string  204  and drill bit  202  may then be raised to third depth  218  so that lateral wellbore  104   b  may be drilled. After drilling lateral wellbore  104   b , a temporary plug  310   b  is installed in lateral wellbore  104   b  at a location adjacent to main wellbore  106 . This prevents any gas or other fluid from flowing from lateral wellbore  104   b  into main wellbore  106   b . Successively higher lateral wellbores  104  may be drilled at successively higher depths using similar procedures. Temporary plugs  310  may be installed using any suitable method; however, in one embodiment, the material that forms temporary plugs  310  is pumped down drill string  201 . The material that forms temporary plugs  310  may be the same as those described above in conjunction with temporary plugs  210 . When gas or other fluid from all lateral wellbores  104  that have been drilled is desired, each temporary plug  310  may be removed using any suitable technique, such as those described above. 
     FIG. 4  is a flow chart demonstrating an example method of drilling lateral wellbores  104  according to one embodiment of the present invention. The method begins at step  400  where main wellbore  106  is drilled. Casing string  204  is disposed in main wellbore  106  at step  402 . Casing string  204  has deflecting member  206  at a lower end thereof. At step  404 , drill string  201  is disposed in casing string  204 . Drill string  201  has drill bit  202  at a lower end thereof. At step  406 , a first lateral wellbore  104   a  is drilled from main wellbore  106  at first depth  216 . Deflecting surface  208  of deflecting member  206  is utilized to direct drill string  201  in the desired drilling direction. 
   At step  408 , casing string  204  is transferred to second depth  217  in main wellbore  106  that is higher than first depth  216 . At step  410 , a first temporary plug  210  is disposed within main wellbore  106  at second depth  217  to prevent gas or other fluid from flowing up main wellbore  106  past second depth  217 . To facilitate the disposing of first temporary plug  210 , drill bit  202  is extracted away from second depth  217 . In some embodiments, drill string  201  and drill bit  202  may be completely removed from casing string  204  before disposing first temporary plug  210 . As an alternative to disposing first temporary plug  210  in main wellbore  106 , first temporary plug  210  may be disposed in lateral wellbore  104   a  at first depth  216 . After disposing first temporary plug  210 , casing string  204  is transferred, at step  412 , to third depth  218  in main wellbore  106  that is higher than second depth  217 . 
   At step  414 , a second lateral wellbore  104   a  is drilled from main wellbore  106  at third depth  218  with drill bit  202 . Because first temporary plug  210  is disposed in main wellbore  106  at second depth  217 , second lateral wellbore  104   b  may be drilled with the assurance that temporary plug  210  will prevent any gas from flowing upward to and past second lateral wellbore  104   b.    
   At step  416 , casing string  204  and drill bit  202  are extracted away from third depth  218 . First temporary plug  210  may then be removed, at step  418 , so that gas or other fluid may be obtained from lateral wellbores  104   a  and  104   b . In the alternative embodiment where plug  210  is disposed in lateral wellbore  104   a , casing string  204  and drill bit  202  do not have to be extracted away from third depth  218 . 
   Although only two lateral wellbores  104   a  and  104   b  are drilled in the above described method, other successive lateral wellbores  104  may be drilled at successively higher depths in accordance with the above method. In this case, there would be a respective temporary plug  210  disposed within main wellbore  106  at a depth just above the depth of the respective lateral wellbore  104 , except there does not need to be a temporary plug  210  for the shallowest lateral wellbore  104 . In lieu of a slant well system, the described example method may be used with other suitable well systems. 
   Although the present invention is described with several embodiments, various changes and modifications may be suggested to one skilled in the art. The present invention intends to encompass such changes and modifications as they fall within the scope of the appended claims.