Abstract:
An isolation sub for use in a perforating system that includes a pressure activated firing head. The isolation sub is set between the pressure source that is used to initiate the firing head. A pressure regulator in the sub is responsive to fluctuations in pressure difference between the pressure source and wellbore and isolates the firing head when the pressure difference is at or approaches a designated pressure difference that could initiate the firing head. The pressure regulator includes a spring loaded piston that seals the firing head from the source pressure before the pressure difference activates the firing head.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to a method and system for perforating a wellbore. More specifically, the present invention relates to a sub for regulating pressure for actuating a differential pressure firing head. 
         [0003]    2. Description of Prior Art 
         [0004]    Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically lined with a string of casing and cement is generally pumped into the annular space between the wellbore wall and the casing. Reasons for cementing the casing against the wellbore wall includes retaining the casing in the wellbore and hydraulically isolating various earth formations penetrated by the wellbore. Sometimes an inner casing string is included that is circumscribed by the casing. Without the perforations oil/gas from the formation surrounding the wellbore cannot make its way to production tubing inserted into the wellbore within the casing. 
         [0005]    Perforating systems typically include one or more perforating guns connected together in series to form a perforating gun string, which can sometimes surpass a thousand feet of perforating length. The gun strings are usually lowered into a wellbore on a wireline or tubing, where the individual perforating guns are generally coupled together by connector subs. Included with the perforating gun are shaped charges that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a jet that perforates the casing and the cement and creates a perforation that extends into the surrounding formation. Each shaped charge is typically attached to a detonation cord that runs axially within each of the guns. Firing heads are usually included with the perforating systems for initiating detonation of the detonation cord. Currently known firing heads may respond to command signals sent via a wireline, telemetry, or from a differential between firing head and wellbore pressure. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention includes methods and devices for isolating pressure from a portion of a perforating system. In one example described herein is an isolation sub for use with a perforating system that includes a body having a passage formed axially therethrough and a lateral port connecting the passage and outer surface of the body. An inlet end of the body is adapted for connection to a pressure source and in fluid communication with an inlet to the passage and an exit end of the body is adapted for connection to a firing head and in fluid communication with an exit of the passage. A pressure regulator is included in the passage that is made up of a valve body axially moveable in the passage having an upper end in selective sealing engagement with a downward facing seat in the passage and a lower end in selective sealing engagement with an upward facing seat in the passage. Thus when fluid flows into the passage an amount of which exits the passage through the port in which pressure is dissipated to create a pressure differential between the passage and outer surface of the body, the lower end of the valve body moves into sealing engagement with the upward facing seat and defines a flow barrier in the passage between the inlet and exit ends of the body. A bypass line is optionally included that is axially formed through the body and having an end connected to the passage at a location between the inlet and the port and another end connected to the passage between the port and the upward facing seat. In an example embodiment, a sleeve is coaxially retained in the passage with a shear pin above the port and that is selectively moveable to adjacent the port for blocking flow between the passage and the port. Alternatively, when the sleeve is adjacent the port, fluid is bypassed to the exit of the passage for providing pressure to a firing head. Optionally, a spring is included for biasing the valve body against the downward facing seat. In an alternate embodiment, the downward facing seat is adjacent to the port. Optionally, the upward facing seat is part of a lower sleeve that threadingly couples with a bore provided on the lower end, wherein the lower seat has an axial passage, an annular groove on an upper portion that extends radially outward from an upper end of the axial passage and that is in fluid communication with the passage between the port and inlet end. 
         [0007]    Also included herein is a method of using pressure to actuate a firing head disposed in a wellbore. In an example embodiment the method includes providing a flow of pressurized fluid through a conduit to the firing head, diverting the flow from the passage into the wellbore and blocking pressure communication of the flow to the firing head when a pressure difference between the passage and wellbore exceeds a designated value. The designated value may be substantially the same as a pressure difference applied across the firing head for activating the firing head. In an example embodiment, the method further includes blocking flow to the wellbore from the passage and increasing pressure to the firing head to activate the firing head. Optionally, pressure communication of the flow to the firing head can be unblocked when the pressure difference is less than the designated value. 
         [0008]    An example embodiment of an isolation sub for use with a subterranean perforating system is included herein. In one example the isolation sub includes a body having an axial passage, a port extending radially outward from the axial passage to an outer surface of the body, an inlet end in pressure communication with the axial passage and in selective attachment to a pressure source, an exit end in pressure communication with the axial passage and selectively connected to a firing head, and a pressure regulation means in the passage. In this example the pressure regulation means limits a pressure differential between a portion of the firing head and ambient to the body to a designated amount. In an optional embodiment, the isolation sub further includes a bypass line that is in pressure communication with the inlet end and with the passage adjacent the pressure regulation means. The pressure regulation means can include a piston that is axially urged against a seat to form a pressure barrier between the passage and the firing head when pressure in a fluid flowing from the passage through the port is decreased by an amount that is substantially the same as the designated amount. In one alternate embodiment, the piston has an upstream end that is biased into sealing engagement with a downstream facing seat so that all fluid flowing into the passage is forced through the port. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1A  is a side sectional view of an example embodiment of an isolation sub in accordance with the present invention. 
           [0011]      FIG. 1B  is a side sectional view of the isolation sub of  FIG. 1A  isolating pressure communication to a firing head in accordance with the present invention. 
           [0012]      FIGS. 2A and 2B  are side sectional views of the isolation sub of  FIG. 1A  allowing pressure communication to a firing head in accordance with the present invention. 
           [0013]      FIG. 3  is a side partial sectional view of an example embodiment of a perforating system having the isolation sub of  FIG. 1  or  2  and disposed in a wellbore in accordance with the present invention. 
           [0014]      FIGS. 4A and 4B  are side sectional views of an alternate example embodiment of an isolation sub in accordance with the present invention. 
       
    
    
       [0015]    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 
       [0016]    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. 
         [0017]    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. 
         [0018]      FIGS. 1A and 1B  illustrate in side sectional view an example embodiment of an isolation sub  20  used to selectively isolate pressure from a pressure activated firing head  22 . In the example of  FIG. 1A , the isolation sub  20  is shown having an elongate body  24  with a circular outer surface. Formed within an inlet end of the body  24  is a box fitting  26  whose outer periphery is generally conically shaped and threaded for connection to a lower end of a conduit (not shown) for delivering pressurized fluid to the sub  20 . The fitting  26  is in pressure communication with a passage  28  that extends axially through the body  24 . The passage  28  has an upper end  30 , which is also conically shaped, and provides a transition from the lower radius passage  28  to the larger radius fitting  26 . 
         [0019]    An annular sleeve  32  is shown coaxially inserted within the passage  28 , an upper edge of the sleeve  32  is located at about where the upper end  30  terminates. In the example of  FIG. 1A , the sleeve  32  is held in place by a shear pin  34  that extends radially inward through the body  24  via a slot  36 . An end of the pin  34  inserts into a recess  37  shown circumscribing the outer surface of the sleeve  32 . A port  38  is shown outlined that also extends radially outward from the passage  28  into an outer surface of the body  24 . O-ring seals  39  are shown around the sleeve  32  and disposed axially apart at opposite sides of the recess  37  for providing a pressure seal between the sleeve  32  and wall of the passage  28 . 
         [0020]    A check valve assembly  40  is further illustrated in the example of  FIG. 1A  and set within the passage  28  downstream of the sleeve  32 . The check valve assembly  40  includes a valve body  42  that has a generally frusto-conically shaped upper end  44  that terminates in a rounded tip. An outer surface of the conically shaped portion of the end  44  is depicted in sealing engagement with an opposingly conically shaped seat  46  that is downward facing within the passage  28 . On an end of the valve body  42  opposite its upper end  44  is a spring  48  that coaxially circumscribes a portion of the valve body  42  for biasing the valve body  42  into sealing engagement with the seat  46 . A shoulder  50  is defined on the valve body  42  at a location where the valve body outer surface transitions radially inward. Past the shoulder  50  and away from the upper end  44  is a lower end  52  having a radius that is less than the mid-portion of the valve body  42  between the upper and lower ends  44 ,  52 . 
         [0021]    Further shown in the passage  28  is an annular sleeve  54  that is threadingly mounted within the passage  28 . The sleeve  54  is set on a side of the valve body  42  opposite the sleeve  32  and also includes an annulus  56  whose radius is less than the radius of the lower end  52  of the valve body  42 . An upward facing seat  57  is shown provided on the sleeve  56  and on a side facing the valve body  42 . As will be described in more detail below, the contours of the lower end  52  and seat  57  are correspondingly shaped so that when engaged they form a pressure barrier. An axial bypass line  58  is shown axially formed through the sub body  24  and extending from the upper end  30  into a recess  60  in the sub body  24  that circumscribes the lower end  52  of the valve body  42 . A port  62  is formed through the sub body  24  and extends radially outward from the passage  28  to the outer surface of the sub body  24  so that the passage  28  is in fluid communication with outside of the body  24 . The port  62  is located such that axial movement of the valve body  42  does not block flow from the passage  28  and through the port  62 . 
         [0022]    A lower end of the body  24  is conically shaped and threaded to define a pin portion  64  for threaded engagement into a box portion  68  formed on an upper end of the firing head  22 . The firing head  22  also includes an axial passage  70  whose upper end expands radially outward and shown in pressure communication with the annulus  56  in the sleeve  54 . The passage  70  has a frusto-conically shaped upper end adjacent the box portion  68  and a substantially circular mid portion. The mid portion transitions radially outward to provide a housing for a piston assembly for the firing head  22 . The piston assembly includes a firing pin  72  partially circumscribed by a sleeve  73 . The firing pin  72  is held in place with a shear pin  74  whose opposing ends are set in a mounting block  75 . A lower end of the firing pin  72  is shaped into a chiseled tip and shown spaced above a primer  76  set within the firing head  22 . A threaded receptacle  78  is formed in the lower end of the firing head  22  and threaded for attachment to a perforating gun (not shown). 
         [0023]    Still referring to  FIG. 1A , a port  80  is shown formed through a sidewall of the body  68  of the firing head  22  and into fluid communication with an annular gallery chamber  82  that circumscribes a portion of the pin  72 . Set radially inward from the gallery chamber  82  is an inner port  84  laterally through the sleeve  73 . The inner port  84  provides pressure communication from the chamber  82  to an annular recess  88  that is formed in a space between the sleeve  73  and pin  72 . The annular recess  88  is also in fluid communication with a lower chamber  90  that defines the open space between the lower tip end of the pin  72  and primer  76 . Thus, the combination of the ports  80 ,  84 , gallery chamber  82 , and annular recess  88  allow open fluid communication with the outside of the firing head  22 . Thus, when enough pressure differential exists between the passage  70  and lower chamber  90  to generate a force on the upper end of the pin  72  to shear the shear pin  74 ; the pin  72  is propelled downward and its pointed tip propelled into contact against the primer  76  for creating a detonation to initiate detonation of shaped charges and perforating guns (not shown). 
         [0024]    Fluid flow exiting the port  62  may create a sufficient pressure differential between the passage  70  and chamber  90  to actuate the firing head  22 . In one example a surge of flow through the passage  28  that then exits the port  62  can create a pressure differential between the passage and the space ambient to the firing head  22 . Ultimately, the surge flow rate may be large enough so that the ensuing pressure differential activates the firing head  22 . Referring now to  FIG. 1B , the check valve assembly is responsive to pressure increases caused by increasing flow rate and closes to isolate the firing head  22  from a pressure source that can cause it to activate. The pressure differential between the passage  28  and passage  70  provides a resultant force F that downwardly urges the valve body  42  so that its lower end  52  is forced into sealing engagement with the seat  57 . Engaging the valve body  42  with the seat  57  blocks supply pressure in the box fitting  26  and bypass  58  from the firing pin  72 . Thus, as long as surging flow through passage  28  and exit port  62  produces a pressure differential that could propel the firing pin  72  against the primer  76 ; the force F will retain the valve body  42  in the sealing position. When the flow excursion has ceased thereby equalizing pressure between the passage  28  and passage  70 , the spring  48  may then urge the valve body  42  into its position illustrated in  FIG. 1A . 
         [0025]      FIGS. 2A and 2B  illustrate in side partial sectional view an example of how the firing head  22  may be actuated to initiate detonation of perforating guns. More specifically, shown in  FIG. 2A , a spherical ball B has been dropped from surface and allowed to make its way with fluid in the supply conduit into the box fitting  26 . The ball B is shown landed in an upper seat of the sleeve  32  and configured so that when seated a pressure differential is created when additional pressure is supplied onto the upper end of the ball B. The ball B therefore blocks flow through the passage  28  and through the port  62 . Thus, additional flow of fluid combined with pressure pressurizes the bypass line  58  and passage  70 . As the flow within the box fitting  26 , bypass  58 , and passage  70  is isolated from the outside of the firing head  22  by the inclusion of the ball B, pressure in the passage  70  will rise over that of the lower chamber  90  as additional fluid is forced into the box fitting  26 . Ultimately, the pressure will exceed a designated pressure and the resulting force on the head of the pin  72  will fracture the shear pin  74 A allowing the pin  72  to slide axially within the sleeve  73  and against the primer  76 . 
         [0026]    Optionally, after initiation of the firing head  22  pressure may continue to be supplied to the box fitting  26  until sufficient force is applied to the shear pin  34 A and the sleeve  32 , thereby causing that shear pin  34 A to be severed and allow the sleeve  32  to slide axially within the passage  28 , thereby providing fluid communication from within the firing head  22 , bypass  58 , and box fitting  26  to outside of the isolation sub  20 . One advantage of moving the sleeve  32  as illustrated in  FIG. 2B  is that fluid pressures within the perforating system can be vented to the ambient pressures and not store excess pressures within sections of the perforating string. 
         [0027]      FIG. 3  provides a side partial sectional view of an example of a perforating system  94  deployed within a wellbore  96  that is shown intersecting formation  98 . In the example of  FIG. 3 , the perforating system  94  includes perforating guns  100  connected end to end by connectors  102 . Once assembled in a string, the perforating system  94  can be deployed within the wellbore  96  on tubing  104  shown threaded through a wellhead assembly  106 . Each of the perforating guns  100  of the example of  FIG. 3  include shaped charges  108  that detonate in response to activating the firing head as described above. When disposed in the wellbore  96  an annulus  110  is defined in the annular space between the string  94  and inner surface of the walls of the wellbore  96 . In an example, it is the pressure in the annulus  110  that defines the pressure outside of the isolation sub  20  and firing head  22  as described above. 
         [0028]      FIGS. 4A and 4B  illustrate in side sectional view one alternate embodiment of an isolation sub  20 A coupled with a firing head  22 A. In the example of  FIG. 4A  a check valve assembly  40 A is made up of a valve body  42 A, that like the valve body  42  has an upper end  44 A with conically shaped sides for sealing engagement with a downward facing seat in the body  24 A of the isolation sub  20 A. The body  24 A of  FIG. 4A  includes multiple ports  62 A that extend radially outward through the body  24 A and proximate to the upper end  44 A of the valve body  40 A. Moreover, the valve body  40 A has a bore  112  formed axially within the body and obliquely provided ports  114  that extend from the conically shaped portion of the upper end  44 A into communication with the axial bore  112 . As illustrated in  FIG. 4B , the valve assembly  40 A operates strictly on differential pressures between the passage  28 A and passage  70  in the firing head  22 A. A spring  48 A is included for biasing the piston body  42 A against the downward facing seat  57 A. With sufficient pressure, as illustrated in  FIG. 4B , flow from the passage  28 A downwardly urges the piston body  42 A and away from the seat  57 A so that fluid can enter into the ports  114 , into the bore  112  and force the pin  72  against the primer  76 . An equalization port  116  is shown extending through the body  68 A of the firing head  22 A for providing a conduit between the passage  70  and ambient to the firing head  22 A. Strategically sizing the equalization port  116  in relation to the cross sectional area of the passage  28 A and volume of the passage  70  allows sufficient pressurization to occur in the passage  70  to fracture the shear pin  74  although some amount of fluid may escape the passage  70  through the port  116 . Over time pressure from the passage  70  can vent through the port  116 . 
         [0029]    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. 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.