Abstract:
A flange assembly and method for securing the downhole tubing string and packer to a blowout preventer is disclosed. The application is particularly useful for high pressure fluid applications downhole such as hydraulic fracking.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to oil field operations and in particular, down hole hydraulic fracturing or oftentimes referred to as fracking. 
       BACKGROUND ART 
       [0002]    Fracking is used to increase production from underground hydrocarbon bearing zones. With the recent development of horizontal drilling, wellbores can now penetrate horizontally within a hydrocarbon bearing zone. Fracking, in simplest terms, is a procedure where fluid is pumped into the wellbore at a rate sufficient to increase pressure down hole to exceed the pressure gradient of the targeted strata. The rock cracks and the fracture fluid continues further into the rock, extending the crack. Typically, a proppant is included with the injected fluid such as grains of sand, ceramic, or other particulates which prevent the fractures from closing when injection is terminated. The strata attempts to return to its original condition but the injected particles prevent complete closure and thus provide pathways for hydrocarbon within the strata to migrate to the wellbore. 
         [0003]    In preparation of a down hole fracking operation, a string of high pressure tubing capable of withstanding high pressures, typically at least 7,000 psi, is lowered into a well directly above the strata to be hydraulically fractured by pumping fluid through the tubing string. The lower portion of the tubing string is anchored into position using a packer. Depending on the packer used, it is set in neutral, tension or compression. For example, if the packer is set in compression, the tubing string is rotated causing the seals of the packer to engage the adjacent casing wall and thereafter the tubing string is lowered so that weight is applied to the packer and the seals firmly engage the casing wall. This requires the tubing string to be lowered possibly several feet at the surface and the actual length required is dependent upon how deep the packer is positioned. 
         [0004]    Because of the high pressures associated with fracking, a blow-out-preventer (BOP) is located at the top of the well, typically bolted to the casing flange which is located at the surface end of the casing. The BOP seals the annular region between the casing and the tubing to prevent gasses or fluids from escaping the well from this annular region. 
         [0005]    A typical fracking operation will involve multiple diesel trucks powering positive displacement pumps. The discharge from these pumps typically connect at a manifold and exit the manifold via a single line connected to the tubing at the top of the BOP. 
         [0006]    Once the packer is set, typical procedure is to apply a backpressure in the annular region between casing and tubing so it is against the topside of the packer. This backpressure serves as further insurance against packer failure. However, if the downhole packer fails for any reason during the fracking operation, the high down hole pressure can act upon the bottom side of the packer and cause the packer and tubing string to be violently displaced upward out of the well before surface pumps can be shut down. Any sudden and unexpected movement of the tubing string upward can cause a serious safety problem not only for the topside equipment but also for nearby personnel. 
       SUMMARY OF INVENTION 
       [0007]    My invention provides an added level of protection for on-site service personnel during a fracking operation. 
         [0008]    The invention is a method and the supportive equipment used for securing the tubing string to the BOP so in the event of a packer failure, the tubing string cannot violently displace out of the hole before the fracking pumps can be shut down. 
         [0009]    The invention provides that the tubing string is secured to the BOP to prevent sudden vertical movement of the tubing string and comprises: a) a threaded flange which is secured to the BOP; and, b) a mandrel threadably secured to the threaded flange and which operatively connects to the down hole tubing string and the surface pumps. 
         [0010]    The appearance of the threaded flange can be generally described as a modified weld neck flange. The threaded flange comprises a main body having a plurality of bolt holes passing through the main body and arranged circumferentially and spaced for alignment with corresponding holes on the top flange of a BOP. Preferably, the bottom surface of the threaded flange has an annular groove for receiving a ring gasket which will be disposed between the flange groove and a corresponding groove found on the top flange of the BOP. 
         [0011]    Extending outward from the top surface of the main body of the threaded flange is a neck portion integral with the main body. The neck portion may be either cylindrical or tapered. 
         [0012]    The threaded flange further comprises a central aperture having a first inside diameter passing through the main body and a second inside diameter passing through the neck portion. The first inside diameter is larger than the second inside diameter. The neck portion further comprises a set of female or internal threads for connection to a mandrel which is described below. The central aperture is sufficiently wide to accommodate tubing collars to pass through. 
         [0013]    A mandrel is provided having a first end and a second end with a set of female threads at each end. The set of female threads located at the first end are for operatively connecting to a down hole tubing string and the set of female threads located at the second end are for operatively connecting to surface pumps. The mandrel further comprises a set of male or external threads located at the second end for connection to the set of female threads located within the threaded flange. The mandrel is sized to be slidably receivable within the central aperture of the main body, by inserting the second end of the mandrel into the aperture of the main body so that the male threads of the mandrel can threadably engage the female threads of the neck portion. The outside diameter of the mandrel is therefore larger than the outside diameter of tubing collars used for the tubing string particularly proximal to the BOP. 
         [0014]    Preferably, at least one annular groove is provided in the outside wall of the mandrel for positioning of an O-ring which provides a seal to the inner wall of the threaded flange and permits all types of packers to be utilized down hole, including tension, compression or neutral set packers. 
         [0015]    Also preferably, the outside diameter of the mandrel is substantially uniform from the first end to the second end and having a pre-determined diameter. The crest of the male threads is equivalent to the outside diameter of the mandrel. 
         [0016]    Thus, the mandrel is operably connected on one side to the tubing string below and on the other side operably connected to the fracking pumps. The male set of threads on the mandrel is threadably connected to the threaded flange which is bolted to the BOP after the packer is set. 
         [0017]    By securing the tubing string to the BOP, the seals of the BOP are not subjected to failure as a result of a sudden movement by the tubing string. This eliminates any chance of gasses or fluids escaping the well as a result of BOP failure during high pressure fracking operations; thus protecting the health of the nearby personnel as well as the surrounding environment. 
         [0018]    The method of use is as follows and is considered an improvement in flowline assembly for delivering fracking fluid from surface pumps to the wellbore. In combination with a hydraulic fracturing operation for a well having a hydrocarbon bearing zone and casing string extending up to a surface casing flange, a blowout preventer connected to the casing flange and a tubing string and packer positioned at a pre-determined depth within the casing string for the delivery of fluid from surface pumps connected to the tubing string at high pressure, the method for securing the tubing string to the blowout preventer comprising the steps of: 
         [0019]    providing the flange and mandrel as described above; operatively connecting the first end of the mandrel to the down hole tubing string; positioning the threaded flange upon the mandrel where the lower surface of the main body of the threaded flange is facing the blowout preventer; 
         [0020]    setting the packer at the pre-determined depth; 
         [0021]    rotating the threaded flange to engage the male threads of said mandrel; 
         [0022]    aligning the bolt holes on the threaded flange to the bolt holes on the blowout preventer; 
         [0023]    securing the threaded flange to the blowout preventer; and, 
         [0024]    operatively connecting the second end of the mandrel to the surface pumps. 
         [0025]    A tubing string having a downhole packer is provided for setting the packer at a pre-determined depth. The top of the tubing string is operably connected to the mandrel either by direct threaded connection or indirectly using tubing extensions well known in the art. Above the mandrel is typically a short length of tubing for operably connecting to the discharge from the pump trucks or manifold. However, there can be any number of configurations for the flowline from the pumps to the second end of the mandrel. What is to be appreciated is that the mandrel, at the second end, is operatively connected to the pump discharge line and will be threadably connected to the threaded flange once the packer is set. This configuration allows for the tubing string to travel vertically up or down to set either a tension or compression packer and subsequently have the threaded flange rotated to engage the mandrel and finally be bolted to the top flange of the BOP. With the flange resting on top of the mandrel before the packer is set, securement to the BOP is quick; requiring only that threaded flange be rotated into engagement with the mandrel and then bolted to the BOP. This can be accomplished by a single worker and require approximately 20 minutes. 
         [0026]    Before the packer is set, the threaded flange does not threadably engage the mandrel although tubing extends not only below but above the threaded flange. Since the inside diameter across the neck portion is sufficiently large to allow tubing collars to pass, this allows the tubing string to be not only raised but lowered as well. 
         [0027]    After the packer is set, the male threaded portion of the mandrel should be located above the top flange of the adjacent BOP at a height to permit threadable engagement with the threaded flange by rotating the threaded flange about the mandrel and alignment of the bolt holes with corresponding bolt holes on the BOP. Bolts are then used to secure the threaded flange to the BOP. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is an exploded perspective view of the flange assembly connected to a blowout preventer. 
           [0029]      FIG. 2  is a side view of the mandrel and connections upstream and downstream. 
           [0030]      FIG. 3  is a side view indicating the positioning of threaded flange above the mandrel. 
           [0031]      FIG. 4  is a cross-sectional view of the flange assembly and connections upstream and downstream. 
           [0032]      FIG. 5  is a side view of threaded flange resting upon the mandrel. 
           [0033]      FIG. 6  illustrates the rotation of threaded flange to connect to the mandrel. 
           [0034]      FIG. 7  is a cross-sectional view of the final attachment of the flange assembly to the blow out preventer. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    The figures are provided for illustration purposes and are not necessarily drawn to scale. 
         [0036]      FIG. 1  illustrates an exploded view of my flange assembly  10 . The assembly comprises a threaded flange  12 , a mandrel  14 , and threaded connections  16  and  18  for operable connections to the surface pumps designated by P and the tubing string TS respectively. In a preferred embodiment, flange assembly  12  can function up to a maximum pressure of at least 8,500 psi. 
         [0037]    As can best be viewed looking at both  FIG. 1  and  FIG. 6 , threaded flange  12  comprises a main body  20  having a plurality of bolt holes H passing through the main body and arranged circumferentially and spaced for alignment with corresponding holes H′ located on the top flange of a blowout preventer BOP. Preferably, the bottom surface of the threaded flange has an annular groove G for receiving an API ring gasket R/46 R which will be disposed between annular groove G and a corresponding groove G′ found on the top flange of the BOP. Threaded flange  12  is rated API Series 6-1500 flange which is bolted down with 12 1½ inch bolts  40  and nuts  42  to the top of the BOP. 
         [0038]    Extending outward from the top surface of main body  20  is a neck  22  integral with the main body. 
         [0039]    As shown in  FIG. 4 , threaded flange  12  further comprises a central aperture A having a first inside diameter passing through main body  20  and a second inside diameter passing through neck portion  22 . The first inside diameter is larger than the second inside diameter. Neck portion  22  further comprises a set of female square threads  24  for connection to mandrel  14  which is described below. The central aperture is sufficiently wide to accommodate the outside diameter of tubing collars to pass through. 
         [0040]    Mandrel  14  comprises a first end and a second end with a set of 8 round N-80 female threads at each end and an outside diameter extending from the first end to the second end. The set of female threads located at the first end are for operatively connecting to a down hole tubing string and the set of female threads  30  located at the second end are for operatively connecting to surface pumps. Mandrel  14  further comprises a set of male square threads  34  located at the second end for connection to the set of female threads  24  located within threaded flange  12 . The crest of male threads  34  is equivalent to the outside diameter. A pair of annular grooves  36  are present in the outer wall of mandrel  14  for placement of respective O-rings  38  which permits use of the mandrel with all types of packers to be utilized down hole, including tension-compression or neutral set packers. Mandrel  14  is sized to be slidably receivable within the central aperture, by inserting the second end of mandrel  14  into the aperture of main body  20  so that male threads  34  can threadably engage female threads  24  as shown in  FIG. 4 . 
         [0041]    Having described the individual parts comprising my flange assembly, the method for assembly will be discussed. 
         [0042]    Hydraulic fracturing of a well is a procedure well known. Generally, my invention is an improvement over the current flowline configuration for delivering fracking fluid from surface pumps to the wellbore because it prevents upward movement of a tubing string in the event of a packer failure. For a hydraulic fracturing operation of a hydrocarbon bearing zone and casing string extending up to a surface casing flange, a blowout preventer connected to the casing flange and a tubing string and packer positioned at a pre-determined depth within the casing string for the delivery of fluid from surface pumps connected to the tubing string at high pressure, the method for securing the tubing string to the blowout preventer will now be explained. 
         [0043]    Because the desired depth for setting a packer may not position the top collar of the tubing string at the optimum position for setting my flange assembly  10  to the BOP, an extension  16  can be used for operably connecting my flange assembly to the down hole tubing string. Extension  16  can comprise one or more tubular parts threaded to one another for connection to the down hole tubing string TS illustrated in  FIG. 2 . As is well known to those having skill in the oilfield services industry, the top collar of the tubing string is held in position using slips (not shown) above the BOP. When the packer is down hole at or near its intended depth, mandrel  14  is operably attached to the tubing string. Extension  16 , mandrel  14  and extension  18  are threadably connected in series to the top of the tubing string as illustrated generally in  FIG. 3  along with an O-ring R set upon the top surface of the BOP. In a most preferred embodiment, the proximal portion of extension  16  to mandrel  14  is spot welded. 
         [0044]    Threaded flange  12  is then placed over extension  18  and lowered represented by motion L in  FIG. 3  until it rests upon the upper edge of males threads  34  of mandrel  14  as shown in  FIG. 5  and  FIG. 6 . The packer is set at a pre-determined depth and final movement of the tubing string is made; the direction of which depends upon the type of packer utilized down hole. The final position of the tubing string has the lower surface of mandrel  14  on substantially the same plane as the top surface flange of the surrounding BOP. Next, threaded flange  12  is rotated as represented by X in  FIG. 6  into threadable engagement with male threads  34 . Rotation continues until the bottom surface of threaded flange  12  contacts the top flange surface of the BOP and bolt holes H align with respective holes on the BOP. Threaded flange  12  is thereafter secured to the BOP using bolts  40  and nuts  42  as illustrated in  FIG. 7 . After flange assembly  10  is secured to the BOP, extension  18  is operably connected to the surface pumping equipment (not shown).