Abstract:
A firing head assembly has a sealed chamber containing a piston, a firing pin, and an impact detonator. The firing head assembly and a perforating charge are installed within a sub and the sub is secured into a string of conduit being lowered into a wellbore. After cementing the conduit, the operator drills out the cement in the conduit, disintegrating the chamber and exposing the sealed chamber to the fluid pressure of the drilling fluid in the conduit. The drilling fluid pressure causes the piston to drive the firing pin against the detonator, which detonates the perforating charge. The operator then pumps down a logging tool to survey the well. Fluid in the conduit below the pump-down head can flow out the displacement perforation into the earth formation while the logging tool is moving downward.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to provisional application 60/771,593, filed Feb. 8, 2006. 

   FIELD OF THE INVENTION 
   This invention relates in general to oil and gas well drilling and in particular to a method of completing a horizontal well that enables a wireline well tool to be pumped down a liner. 
   BACKGROUND OF THE INVENTION 
   Highly deviated or horizontal wells are commonly drilled for oil and gas production. As used herein, the term “horizontal” refers to not only wells with truly horizontal sections, but also to wells that are highly deviated. In one type of horizontal well completion, the operator installs and cements a casing or liner that extends to the total depth of the well. Normally, the term “casing” refers to conduit that extends back to the surface wellhead, and “liner” refers to conduit that has its upper end supported near the lower end of a first string of casing. These terms will be used interchangeably herein to refer to a conduit in a well that is cemented in place, whether its upper end extends to the surface or just to the lower end of a first string of casing. 
   After cementing the casing, the operator perforates through the casing into the producing formation. The operator may then perform other operations, such as hydraulic fracturing or dispensing acid or other chemicals into the producing formation. Normally, the operator installs a string of production tubing in the casing for the production flow. 
   Even though wells may be fairly close to each other, producing formations often vary in characteristics from one well to another, such as thickness, depth, porosity, water content, permeability and the like. Consequently, it is useful to have a survey or log made of the well before it is cased to provide the characteristics of the producing formation. In highly deviated and horizontal wells, logging can be made while drilling using measuring while drilling techniques. 
   After cementing, it is also useful for the operator to perform another survey of the well. Because of the casing, the cased-hole log differs from an open-hole survey. By using tools such as ones that measure natural gamma rays emitted by earth formations, the operator will be able to discern the same formations previously noted during the open-hole survey. The operator uses this information to determine precisely where to perforate. Even without an open-hole log, a cased-hole survey provides important information to the operator. 
   In a vertical or even a moderately deviated well, the operator can run a cased-hole log before perforating by lowering a surveying instrument on a wireline into the casing and making the survey either while running-in or retrieving. Logging a cased horizontal well presents a problem, because gravity won&#39;t pull the tool down. One approach has been to mount to the instrument a tractor with motor-driven wheels or tracks. Generally, these logging procedures are expensive and slow. Also, high voltages are typically required, which can be detrimental to the wireline. 
   Surveying instruments have been pumped down wells in the prior art. An annular piston is mounted to the instrument assembly for sealingly engaging the conduit. This type of operation requires a flow path for displaced fluid below the piston as the instrument moves downward. In the prior art, the flow path typically comprises an open annulus surrounding the conduit containing the instrument. In a cased horizontal well, there is no open annulus surrounding the casing and no place for displaced fluid. Consequently, pump-down logging is normally not performed on horizontal wells. 
   SUMMARY 
   In this invention, the operator runs and cements a conduit, such as a liner or casing in a wellbore. The operator then forms one or more displacement perforations through the conduit and surrounding cement and into an earth formation. He then pumps down a wireline logging tool with a pump-down head. The downward movement of the pump-down head causes some of the fluid below the pump-down head to be displaced out through the displacement perforation into the formation. While the logging tool is in the conduit, the operator performs a survey of the well. 
   Preferably, the operator forms the displacement perforation with a firing head assembly comprising a sealed chamber containing a piston, a firing pin, and an impact detonator. The firing head assembly is mounted within a sub and the impact detonator is linked to a perforating charge. The operator secures the sub to the string of conduit as it is being lowered into the wellbore. 
   After cementing, the operator lowers a drill bit into the conduit and drills out cement left in the sub and in the lower portion of the conduit. The drill bit ruptures the sealed chamber of the firing head assembly, which exposes the sealed chamber to drilling fluid pressure. The fluid pressure causes the piston to drive the firing pin against the detonator, thereby detonating the perforating charge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view illustrating a well with a liner having a displacement sub in accordance with this invention, the liner being shown after cementing but before displacement perforations have been made. 
       FIG. 2  is an enlarged sectional view of the displacement sub of  FIG. 1 , shown removed from the liner. 
       FIG. 3  is a further enlarged sectional view of the firing head assembly of the displacement sub of  FIG. 2 . 
       FIG. 4  is a sectional view of the displacement sub of  FIG. 1 , taken along the line  4 - 4  of  FIG. 2 , and shown prior to cementing. 
       FIG. 5  is a sectional view of the displacement sub of  FIG. 1 , taken along the line  4 - 4  of  FIG. 2 , and shown after cementing. 
       FIG. 6  is a sectional view similar to  FIG. 1 , but showing a drill string drilling through the interior of the displacement sub after cementing. 
       FIG. 7  is an enlarged sectional view of the displacement sub as shown in  FIG. 6 , after it has been drilled through and the displacement perforations made. 
       FIG. 8  is a sectional view of the displacement sub as shown in  FIG. 7 , taken along the line  8 - 8  of  FIG. 7 . 
       FIG. 9  is a sectional view of a logging instrument being pumped down the liner of  FIG. 1 . 
       FIG. 10  is a sectional view of the well of  FIG. 1  after final perforating and installation of production tubing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the well has a vertical section with a conventional string of casing  11  that is cemented in place. The operator has drilled an open hole section  13  below casing  11  open hole section  13  having a substantially horizontal portion that may extend thousands of feet. In the embodiment of  FIG. 1 , a string of drill pipe  15  is shown extending into the casing  11 . A setting tool  17  is located on the lower end of drill pipe  15 . Setting tool  17  is connected to a tieback extension  19 , which in turn is connected to a packer  21 . Packer  21  is connected to a liner hanger  23 . A liner  25  is secured to liner hanger  23  for securing the upper end of liner  25  to the inner diameter of casing  11 . Liner  25  is a string of casing smaller in diameter than the casing  11 . Rather than having its upper end near the lower end of casing  11 , liner  25  could have its upper end at the surface. Liner  25  is shown in the process of being installed with its upper end a short distance above the lower end of casing  11  and its lower end near the bottom of the well. Setting tool  17 , tieback extension  19 , packer  21  and hanger  23  are conventional components used to set liner  25 . 
   Liner  25  has a landing collar  27  at its lower end for receiving a conventional cement plug (not shown). A displacement sub  29  constructed in accordance with this invention is secured to the lower end of landing collar  27 . An extension member  31 , which may be a section of the same pipe as liner  25 , extends below displacement sub  29 . A conventional cement set shoe  33  is secured to the lower end of extension member  31 . 
   After running liner  25 , the operator pumps cement down liner  25 , landing collar  27 , displacement sub  29 , extension member  31  and cement shoe  33 . Cement  35  flows out cement shoe  33  and back up the annulus in open hole  13  surrounding liner  25 , as illustrated in  FIG. 1 . After dispensing the desired amount of cement, the operator pumps down a conventional drillable plug (not shown), which lands in landing collar  27 . Cement  35  will cure not only in the annulus surrounding landing collar  27 , extension member  31 , displacement sub  29 , and liner  25 , but also within extension member  31  and displacement sub  29 . Immediately after pumping cement  35 , the operator sets packer  21  and retrieves the string of drill pipe  15  and setting tool  17 . 
   Referring to  FIG. 2 , displacement sub  29  has a tubular steel housing  37  of substantially the same diameter as liner  25 . Housing  37  contains a body  39  of a drillable material, such as aluminum, brass or composite. Body  39  is a cylindrical member that is sealingly secured within housing  37 . Body  39  has a flow port  41  extending from its upper end to its lower end for fluid circulation prior to cementing and also for cement  35  flow. As shown in  FIG. 4 , flow port  41  may be crescent-shaped, and it is offset from the longitudinal axis of body  39 . Prior to pumping the cement through displacement sub  29 , flow port  41  is open. As shown in  FIG. 5 , after pumping cement  35 , the cement will cure within and block flow port  41 . 
   Referring to  FIG. 3 , a firing head assembly  43  is secured by threads into the upper end of body  39 . Firing head assembly  43  is also of drillable materials and is offset from the axis of body  39 . Firing head assembly  43  has a housing  45  made up of a number of tubular sections secured and sealed together as shown in  FIG. 3 . A bore  47  is located within an upper portion of firing head housing  45 . Firing head housing  45  has a cap  49  that encloses the upper end of bore  47 . A piston  51  is carried within bore  47  for movement from the initial position shown in  FIG. 3  to a lower position (not shown). Piston  51  is initially spaced with its upper end below cap  49 . A chamber  53  at atmospheric pressure is located between the upper end of piston  51  and cap  49 . Piston  51  sealingly engages bore  47  and is held in the initial position by shear pins  55 . Piston  51  has a downward extending rod with a sharp firing pin  57  fixed to its lower end. 
   A percussive detonator  59  is located within firing head housing  45  a short distance below firing pin  57 . Detonator  59  is connected to detonating cord  61 , which leads to one or more shaped or perforating charges  63  (only one shown in  FIGS. 2 and 3 ). Detonator  59 , detonating cord  61  and shaped charges  63  are conventional components used in perforating operations. The number of shaped charges  63  can vary. 
   Referring to  FIG. 2 , an optional dye pack housing  65  is secured by threads to the lower end of body  39 . Dye pack housing  65  is also of drillable material and has a sealed chamber that contains a dye. When exposed to well bore fluid, the dye will discolor the fluid circulating back to the surface to indicate that displacement sub  29  has been drilled through. 
   Referring to  FIG. 6 , after cement  35  is cured and the operator has removed setting tool  17  ( FIG. 1 ), the operator runs back into the well with a drill bit  67  on the lower end of drill pipe  15 . Drill bit  67  will drill the cement plug (not shown) in collar  27 , and then began drilling components of displacement sub  29 . During drilling, the operator pumps drilling fluid through drill pipe  15 , which discharges from drill bit  67  and flows back up the annulus between drill pipe  15  and liner  25 . Once drill bit  67  drills through cap  49  ( FIG. 3 ), the pressure of the drilling fluid will be applied to chamber  53 , which was previously at atmospheric pressure. The drilling fluid pressure causes shear pins  55  to shear, pushing piston  51  and firing pin  57  downward. Firing pin  57  strikes and ignites detonator  59 , which in turn ignites detonating cord  61  and shaped charges  63 . The explosion creates perforations  69  through cement  35  and into the earth formation as illustrated in  FIGS. 7 and 8 . 
   After firing, the operator continues drilling firing head assembly  43  ( FIG. 3 ) and body  39  ( FIG. 2 ). When drill bit  67  reaches dye pack assembly  65 , the dye is released. The fluid being pumped down drill string  15  causes dye  66  to color the drilling fluid returning to the surface, indicating to the operator that he has now drilled through displacement sub  29 . Tile operator stops drilling substantially at this point, leaving cement  35  within extension member  31  and cement shoe  33 . The operator then retrieves drill pipe  15  and drill bit  67  ( FIG. 6 ). 
   Referring to  FIG. 9 , the operator may now perform wireline services in the well, using a wireline tool  73 . Wireline tool  73  may be any type of conventional wireline service equipment, such as a gamma ray wireline tool, a cement bond wireline tool, perforating equipment or a plug or packer setting tool. Wireline tool  73  may be attached to a pump-down head  71  to facilitate pumping down liner  25 . Pump-down head  71  is piston-like member that fits closely within tile inner diameter of liner  25 . Because of their large diameter, some wireline tools  73 , such as a bridge plug, may not need an additional pump down head  71 . Pump down head  71  is located at the lower end of wireline tool  73 , which is connected to an electrical cable  77  that leads to the surface. 
   At the surface, a blowout preventer  79  will close the well in the event of an emergency. Blowout preventer  79  may include wireline rams that close around electrical cable  77  as well as shear rams that will cut it. A manifold  81  is secured to blowout preventer  79  for pumping fluid, typically water, into casing  11  and liner  25  to force pump-down head  71  downward. A lubricator  83  seals around electrical cable  77  as it moves. Electrical cable  77  is dispensed by a winch  85  at the surface. A logging unit  87  supplies electrical power to electrical cable  77  and receives signals indicating parameters of the earth formations and cement  35 . 
   As illustrated in  FIG. 9 , fluid  89  is located below pump-down head  71 . As pump-down head  71  moves downward, it displaces some of the fluid  89 , which flows into displacement perforations  69 . The exterior of pump-down head  71  does not form a tight seal with the inner diameter of liner  25 ; rather a small clearance will exist for some of the fluid  89  to flow around pump-down head  71  as it moves downward. However, without displacement perforations  69 , it would not be feasible to pump wireline tool  73  to the lower end of liner  25 . Preferably, the operator continues pumping down pump-down head  71  until it reaches the lower end of displacement sub  29 . 
   Subsequently, the operator will retrieve pump-down head  71  and tool  73  by winding electrical cable  77  back onto winch  85 . The operator may perform the log while retrieving tool  73 , or while pumping tool  73  down, or both. The operator then may complete the well by running production tubing and perforating in a variety of conventional manners. 
   Referring to  FIG. 10 , in one completion method, the operator perforates to form production perforations  93  above displacement perforations  69 . The production perforations  93  could be made in several ways, one of which could be pumping down through liner  25  a pump-down perforating gun on wireline, with displaced fluid flowing out displacement perforations  69 . A bridge plug  91  could then be set above the displacement perforations  69  to isolate them from production perforations. The operator may then run a string of production tubing  95  and set a packer  97  in liner  25  above production perforations  93 . Tubing  95  is suspended conventionally from a wellhead assembly  99  for conveying well fluid to the surface. 
   Alternately, the operator could first set bridge plug  91 , then run tubing  95 , then pump down a perforating gun through tubing  95  with displaced fluid flowing back up the tubing annulus within liner  25  before setting packer  97 . The operator could also make the production perforations with a tubing conveyed perforating gun. 
   The invention has significant advantages. By forming a displacement perforation into the formation, the operator can use a pump-down logging tool, with displacement fluid flowing into the formation. Forming the displacement perforation while drilling out the cement avoids an additional trip just to make the displacement perforation. This method avoids the need for a tractor, thus saving time and expense. 
   While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.