Patent Abstract:
A method for unseating a threaded connection of wellbore tubing within the wellbore. The method utilizes a back-off tool which consists of a tubular metal housing, a shaped charge and HNS detonating cord within the housing, and an explosive material attached to the housing. The back-off tool is detonated near the threaded connection, creating a shockwave that strikes the threaded connection with sufficient force to unseat the connection.

Full Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to oil and gas production. More specifically, the present invention relates to a tool that creates a shockwave in a wellbore to “back-off” threads engaged in a threaded couplings within a tubular string. 
     2. Description of Prior Art 
     Typically, tubulars are connected together by threaded couplings to form a string that is suspended and cemented in a wellbore to create a casing for the wellbore. From time to time, the casing string may need to be removed from the wellbore and the threaded couplings are decoupled at surface. In some instances while removing the casing it may become wedged within the wellbore; further complicating string removal, while still downhole, one of the threaded couplings may resist detachment under an applied torque to become immovable. The immovable coupling is sometimes unseated by directing a shockwave at the coupling site to break loose the threaded connection. 
     A typical prior art tool used to create this shockwave consists of multiple strands of detonator cord wrapped around a shot rod in a rope-like fashion and wrapped with friction tape. Generally this tool employs a detonation cord having HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), which can withstand operating temperatures of 400 degrees Fahrenheit for only about an hour. While a detonating cord having HNS (1,3,5-Trinitro-2-[2-(2,4,6-trinitrophenyl)ethenyl]benzene) can operate at temperatures above those limiting use of HMX detonator cord, HNS detonating cord cannot side detonate and thus is not utilized in the above described prior art tool. Also, operating pressure of typical prior art is limited to 20,000 psi due to the use of exposed (to wellbore fluids) interface between detonator and detonating cord. 
     SUMMARY OF THE INVENTION 
     The present disclosure involves a method of unseating a threaded connection that connects sections of wellbore tubing. In an example the method uses a tool that includes a housing, a shaped charged located inside the housing, an HNS detonating cord and an energetic material attached to the steel housing. The tool is placed near the threaded connection, where it is detonated, creating a shockwave that contacts the threaded connection with sufficient force to unseat the threaded connection. 
     Also disclosed is a method of an operation in a wellbore that includes inserting an amount of reactive material within a string of wellbore tubular segments, where a threaded connection joins upper and lower adjacent tubular segments. A shockwave is generated by initiating the reactive material that unseats the threaded connection by directing the shockwave towards the threaded connection. The upper tubular segment is rotated thereby eliminating the threaded connection and the upper tubular segment is removed from the wellbore. In an example, the reactive material is initiated by a jet from a shaped charge that terminates proximate an outer surface of the reactive material. In one alternative embodiment, the reactive material includes a high explosive, wherein initiating the high explosive causes the high explosive to detonate. Optionally, the reactive material is a low explosive, wherein initiating the low explosive causes the low explosive to deflagrate. In another alternative, the reactive material includes a combustible material, wherein initiating the combustible material causes the combustible material to combust. Alternatively, initiating the reactive material includes using a detonation cord having HNS to detonate a shaped charge thereby forming a jet, and directing the jet at the reactive material. The pressure can be at least about 30,000 pounds per square inch within the string of tubular segments. At least a portion of the HNS detonating cord can be maintained at a temperature of at least about 480° F. and for a time up to about 1 hour. 
     Also disclosed herein is an embodiment of a back off tool for use in a downhole tubular. In one example the back off tool includes a body selectively suspended in the downhole tubular by attachment to a deployment member. A reactive material is included adjacent the body for generating a shockwave to unseat an immovable threaded connection between adjacent tubular segments. An initiator is provided in selective communication with the deployment member and in selective initiating communication with the reactive material. In one example, the initiator is a shaped charge that forms a jet to initiate a reaction in the reactive material. Alternatively, a detonating cord having HNS can be included with the back off tool. In an example embodiment, the body and the reactive material each include an axis, and the reactive material is disposed adjacent an end of the body and positioned so that the axis of the reactive material is substantially parallel with the axis of the body. Alternatively, the reactive material can be a high explosive. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side sectional view of an embodiment of a back-off tool in accordance with the present disclosure. 
         FIG. 2  is a partial cutaway side view of a back-off operation. 
         FIG. 3  is a partial cutaway side view of a shockwave striking the threaded coupling. 
         FIG. 4  is a partial cutaway side view of a wellbore as the upper casing section is removed. 
         FIG. 5  depicts in a side sectional view an alternate embodiment of a back-off tool in accordance with the present disclosure. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the improvements herein described are therefore to be limited only by the scope of the appended claims. 
       FIG. 1  depicts, in a cross-sectional view, an embodiment of a portion of a back off tool  20  that can be used in high pressure and high temperature applications. In the example of  FIG. 1 , the back off tool  20  includes an annular gun tube  22  shown containing a shaped charge  24  and oriented orthogonal to an axis A X  of the gun tube  22 . The shaped charge  24  is shown having an open end set within an opening  25  formed through a side wall of the gun tube  22 . In the example of  FIG. 1 , the gun tube  22  is enclosed in a tubular housing  26  that, in an example embodiment, may be formed from steel. A detonating cord  28  is further included with the embodiment of the back off tool  20  of  FIG. 1 . The detonating cord  28 , which in an example embodiment may be an HNS detonating cord, is shown extending along the gun tube  22  and routed so that its path runs adjacent an end of the shaped charge  24 . A sleeve  30  is shown encasing the outer surface of the tubular housing  26 . The sleeve  30  may be formed from an energetic material that when initiated reacts and generates a shockwave. Materials for the sleeve  30  can include any material capable of generating a shockwave, examples include an oxidizer, a propellant, a high explosive, e.g. HMX, RMX, HNS, a low explosive, a combustible material, and combinations thereof. 
     The material for the sleeve  30  can detonate, deflagrate, combust, or a combination thereof. In an example, the definition of detonation describes a reaction that can propagate through the material being detonated at the sound speed of the material. In a further example, detonation describes a reaction or decomposition of an explosive that, typically in response to a shock wave or heat, forms a high pressure/temperature wave. Example velocities of the high pressure/temperature wave can range from 1000 m/s to in excess of 9000 m/s. In an example, the definition of deflagration describes a rapid autocombustion of a material, such as an explosive. Generally, explosives that detonate are referred to as high explosives and explosives that deflagrate are referred to as low explosives. In an example, combustion describes an exothermic reaction of a material that can produce an oxide. 
     In one example of operation, and as provided in  FIGS. 2-4 , a detonation wave is initiated in the detonating cord  28  that transfers a shock wave to and detonates the shaped charge  24 . As will be discussed in further detail below, in one example embodiment of the back off tool  20 , a jet (not shown) formed from detonation of the shaped charge  24  penetrates the housing  26  and the sleeve  30  reacting the sleeve  30 , which provides the necessary shockwave for the back-off operation. In an example embodiment, the jet does not extend past the sleeve  30 , or extends slightly past. 
     Referring now to  FIG. 2 , shown in a side sectional view is an embodiment of the back off tool  20 . In the embodiment of  FIG. 2 , the back off tool  20  is suspended by a wireline  32  shown being reeled from and controlled by a surface truck  33 . Alternatively, the wireline  32  can be threaded through a wellhead assembly (not shown) disposed on the surface. The back off tool  20  and wireline  32  are inserted within a string of wellbore casing  34  that line a wellbore  35 . The casing string is made up of segments of casing  34 , each segment having threaded ends that threadingly couple together to form a threaded connection  36 . More specifically in the example of  FIG. 2 , the back off tool  20  is suspended adjacent a threaded connection  36  that is immovable. For the purposes of discussion herein, and as described above, a threaded connection  36  that is immovable describes a threaded connection  36  that resists decoupling. 
     In the example embodiment of  FIG. 3  shown in side partial sectional view is an example embodiment where the shaped charge  24  in the back off tool  20  has been detonated that in turn initiates detonation of the sleeve  30 . When the sleeve  30  is detonated it creates a shockwave  38  that propagates through the threaded connection  36 , as shown in  FIG. 3 . The force of the shockwave  38  can remove stresses in the threaded connection  36  joining upper and lower segments of casing  34   U ,  34   L  thereby allowing the threaded connection  36  to back-off as torque is applied to the upper segment of casing  34   U . Thus continued application of torque to the upper segment casing  34   U  rotates the upper segment of casing  34   U  decoupling upper and lower threads  37   U ,  37   L  to eliminate the threaded connection  36  that couples the upper and lower segments of casing  34   U ,  34   L . As shown in side sectional view in  FIG. 4 , once decoupled, the upper segment of casing  34   U  can be detached from the lower segment of casing  34   L  and removed from the wellbore  35 . In an optional embodiment, the back off tool  20  includes more than one sleeve  30  so that a shock wave can be generated at a first depth, the back off tool  20  raised or lowered to a second depth, and another shock wave generated by initiating the more than one sleeve. 
     An alternate embodiment of a portion of a back off tool  20 A is shown in a side sectional view in  FIG. 5 . The back off tool  20 A of  FIG. 5  includes a shaped charge  24 A suspended from a length of detonating cord  28 A shown disposed inside a generally cylindrically shaped housing  26 A. Disposed adjacent to a lower end  39  of the housing  26 A is a substantially cylindrically shaped amount of reactive material  40  oriented generally coaxial with the housing  26 A. In an example embodiment, the reactive material  40  includes the same or similar material of the sleeve  30  as described above. The shaped charge  24 A of  FIG. 5  is oriented so that when detonated any jet resulting from the shaped charge  24 A is directed towards the lower end  39  and reactive material  40 , rather than a side radial wall as illustrated in the example of  FIG. 1 . In the example embodiment of  FIG. 5 , an axis A H  of the housing  26 A is shown to be substantially coaxial with an A EM  of the reactive material  40 . Embodiments exist as well where the axes A H , A EM  are substantially parallel. Optionally, the reactive material  40  may be encased in a jacket  42  for protecting the reactive material  40  during the trip downhole. Operation of the back off tool  20 A of  FIG. 5  is similar to the operation described above; that is, the back off tool  20 A is inserted into a tubular string and the reactive material  40  is reacted, such as by detonating the shaped charge  24 A. An ensuing shock wave, not shown, transfers energy to an immovable threaded connection so that the connection can be decoupled. 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the back off tool  20  and its alternate embodiments can be disposed in other downhole tubulars, such as production tubing strings, caissons, risers, and the like. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Technology Classification (CPC): 4