US20030047313A1

US20030047313A1 - Drillable core perforating gun and method of utilizing the same

Info

Publication number: US20030047313A1
Application number: US09/954,814
Authority: US
Inventors: Clyde Wehunt
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-09-12
Filing date: 2001-09-12
Publication date: 2003-03-13

Abstract

The present invention includes a perforating gun and method of using the same to perforate casing in a wellbore. The perforating gun includes a hollow carrier having a core. Mounted within the core are components including at least one charge and at least one plug for sealing the charge within the core. Preferably, a pair of end plugs are used to seal the ends of the carrier about the charge or charges. A string of tubing is used to position the perforating gun adjacent the casing to be perforated. The charge or charges are detonated creating perforation tunnels in the casing and surrounding wellbore. A drill is then used to drill out the core of the carrier to provide ready access through the carrier for other tools and fluid flow. The carrier is ideally of a stronger material than the core components and enhances drillability of the core by providing guidance as the drill removes the core components.

Description

FIELD OF INVENTION

This invention relates to perforating guns, to drillable tools used in completing wellbores and to methods of employing such perforating guns and drillable tools.

BACKGROUND OF THE INVENTION

A perforating gun is a downhole tool that uses either bullets, or more commonly, shaped explosive charges. The charges shoot high-velocity gases to blow holes, referred to as perforations, in the casing or liner, cement and adjacent producing formation of a well. A bullet perforating tool has several barrels from which are fired high-velocity projectiles called bullets. The bullets may be fired simultaneously or individually. Usually, the bullet perforating gun is fired electronically from a well surface.

During the process of drilling and constructing wells, a fluid system commonly known as mud is used to provide various desirable qualities, including lubrication, transport of drill cuttings, hole stability, and control of fluid loss to the adjacent subterranean formation. The physical property that describes the capability of a porous subterranean formation to conduct fluid is called permeability. A mud system used during construction of a well normally reduces the permeability of the formation adjacent to the wellbore. In addition, the wellbore is sometimes lined with a casing. Perforating guns are used to create perforation tunnels through the casing and the damaged or localized region of reduced permeability in the surrounding producing formation.

Perforating guns generally include an outer carrier supporting internally arranged explosive charges. The explosive charges are shaped and directed to create perforation tunnels of a predetermined pattern. After this perforation operation, the portions of the perforating gun remaining in the wellbore must be disposed of to accommodate passage of other tools and fluid flow.

Currently, there are numerous options for disposing of perforating equipment following detonation. In a first option, the perforating equipment or guns are conveyed by temporary tubing, and retrieved after detonation. The formation pressure should be overbalanced by fluid pressure to ease removal of the gun from the wellbore. Overbalanced means that the pressure within a drilling string and casing is greater than the pressure in the surrounding production formation. This option of inserting, firing, and then retrieving the gun is commonly known as the “shoot and pull” method. However, the step of retrieving the gun is time consuming and thus expensive during well drilling and completion.

In a second option, guns are conveyed by permanent tubing, and dropped into the well after detonation. To prevent the obstruction of the perforated interval in the casing following this process, additional well depth is required so the guns can fall into an unused portion of the wellbore. The additional length of a wellbore drilled for this purpose is called a rathole. Drilling a rathole can be very expensive when perforating long intervals. This option is commonly known as the “shoot and drop” method.

In a third option, guns are conveyed by permanent tubing, and left hanging in place adjacent to the perforated interval. This method blocks access to the perforated interval for wellbore maintenance or performance data acquisition. This option is commonly known as the “shoot and hang” method.

In all three of the above methods, hollow carriers are typically used which swell and deform when explosives contained therein are detonated. The guns are designed small enough in diameter so that, even in their swelled state, the guns can be moved within the casing or wellbore after the explosives detonate. This is considered desirable for future well utility, even when the shoot and hang method is being used.

In a fourth option, guns are conveyed by permanent tubing, and substantial portions of the guns are constructed of a material that fragments after detonation. Most of the resulting debris falls into the rathole. However, a smaller amount of rathole is required because the debris is in small pieces and it packs more tightly into the available space. This option is commonly known as the “disintegrating gun” method. One disadvantage of this method is that a substantial portion of the gun debris fragments can flow to the surface and plug surface equipment. Most of the larger debris fragments originate from a composite hollow carrier section of the equipment.

In a fifth option, guns are conveyed by tubing or drill pipe, and the metallic portions of the gun are made of aluminum. One disadvantage of this method is that most of the gun, including the carrier, is made of aluminum, substantially increasing the cost of the gun's manufacture. The aluminum is relatively soft allowing the carrier to be completely drilled away and discarded. Another disadvantage is that the gun is typically made of a smaller diameter and is thus not designed to be left in the wellbore, and the smaller gun size limits the size of the perforating charges. This option is commonly known as the drillable gun option.

In all five of the above methods, the explosive charges are contained within the hollow carrier. The charges are held in place and aligned in a preferred orientation by attachment to a metallic charge strip supported within the hollow carrier. After detonation, the metallic strip remains in the hollow carrier or falls into the rathole. The above methods all utilize pipe to convey the guns which allows long sections to be perforated simultaneously. Furthermore, this perforating operation can be done with substantially lower pressure inside the wellbore than in the formation adjacent to the wellbore. Consequently, mud does not flow into the surrounding production formation while the perforating is taking place due to the underbalanced condition.

In a sixth option, explosive charges are attached to the casing when it is installed in the well. These charges and a detonating cord that connects the charges are subject to mechanical damage during installation of the casing. Also, the charges and detonating cord are exposed to high temperatures within the wellbore. This exposure can cause damage if the charges and detonating cord are exposed to the high temperatures for excessively long periods of time during the casing installation process.

In a seventh option, the guns are conveyed through the permanent tubing by a solid or stranded cable. This method is known as the “wireline” method. Guns conveyed in this manner can either be dropped into a rathole or retrieved from the well. If retrieved, the guns must be retrieved in sections that are shorter than the available pressure control equipment (commonly called a lubricator), or special connectors must be utilized between the sections of guns, or the formation pressure must be overbalanced to allow safe removal. Dropping the guns or overbalancing the formation pressure have the same disadvantages previously described. Recovery of the guns in sections through the lubricator is a time-consuming and expensive process when perforating long intervals. With this method, the length of the gun assembly is substantially limited by the strength of the solid or stranded cable. Also, the charges are sometimes, but not always, contained within a hollow carrier.

In an eighth option, guns are conveyed through permanent tubing by a continuous reel of flexible tubing, commonly known as “coiled tubing”. This tubing is smaller than the permanent tubing that will remain in the well, and it can be run through the permanent tubing in a concentric fashion. Guns conveyed in this manner can either be dropped into rathole or retrieved from the well. If retrieved, they must be retrieved in sections that are shorter than the lubricator, or special connectors must be utilized between the sections, or the formation pressure must be overbalanced to allow safe removal. Dropping the guns or overbalancing the formation pressure have the same disadvantages previously described. Recovery of the guns in sections through the lubricator is a time-consuming and expensive process when perforating long intervals. This method is known as the “coiled-tubing” method.

There are also additional permutations that combine various features of some of the different methods described above. These conventional methods of perforating casing and formation adjacent the casing have numerous shortcomings. Therefore, there is a need for perforating apparatus and methods which have distinct advantages for cost and well performance as compared to conventional perforating guns and methods.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a perforating gun which perforates casing in a wellbore and which has a permanent carrier with a core, containing charges, which may be drilled out after detonation to remove all internal components and to provide a relatively smooth, clean core through which tools and fluid can readily pass.

A further object is to provide a method of perforating a casing in a wellbore of a production well which uses the aforementioned perforating gun with drillable internal components mounted within the core of a permanent carrier, the carrier being positioned and ideally permanently disposed with the casing.

This invention includes a perforating gun for perforating casing in a wellbore. The perforating gun comprises a carrier, at least one charge and at least one plug. The carrier has a generally hollow interior forming a core and is adapted to be mounted on a string of tubing. Ideally, there are a number of charges mounted on a carrier strip which is sealed in the hollow interior of the carrier between one or more plugs.

Internal components disposed within the core are designed to be readily drilled out from the core of the carrier. For example, the plugs may be made of aluminum or a plastic. To assist in the boring out of the core, the internal components are ideally made of a softer material than the carrier. Preferably, the carrier is made of a strong, rigid material, such as steel, and is permanently located within the perforated casing. The carrier enhances the drillability of the core while leaving the carrier relatively intact and with a relatively smooth inner surface through which tools and fluid may pass.

A method is also described wherein the above perforating gun may be used to perforate a casing in a wellbore. The carrier of the perforating gun is then drilled out to provide a relative smooth interior such that tools and fluid may readily pass there through. The gun is ideally mounted on tubing which is permanently mounted with the casing and need not removed prior to beginning production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a drillable core hollow carrier perforating gun, which is made in accordance with the present invention, the gun being mounted on tubing; and [0021]
FIGS. [0022] 2A-C illustrate a schematic sectional view of the perforating gun shown in FIG. 1, along with a hollow carrier gun section and a ballistic charge end section.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 A-C schematically illustrate a drillable core hollow [0023] carrier perforating gun 10. In FIG. 1, gun 10 is suspended from a string of tubing or drill pipe 12. String of tubing 12 includes a landing nipple 16, and tubing 20 which may contain auxiliary equipment. The auxiliary equipment (not shown) includes conventional components such as a shock sub, a firing head or other desired tools. The shock sub is used as a dampening device to dampen the compression wave transmitted to the tubing 12 by the increase in pressure below the end of the perforating gun 10, subsequent to the firing of perforating gun 10. The firing head initiates the burning of a primer cord. Conventional methods of activating the firing may be used such as sending a pressure waved down mud disposed in tubing 12. Alternatively, a wire could be deployed in tubing 12 to carry an electrical impulse to activate firing the firing head.
A casing [0024] 22 is set in the bottom of a wellbore by use of an anchor 24. A packer 26 cooperates with tubing 12 to seal the annulus between the casing 22 and the tubing 12 and to isolate it from the pressure inside the tubing 12 and below the packer 26.
[0025] Gun 10 is shown in greater detail in FIGS. 2 A-C. Gun 10 includes an outer hollow carrier 40 and end caps 42. Hollow carrier 40 is preferably cylindrical and has an inner diameter or core 44. Disposed within core 44 is a charge strip 46 which supports a plurality of charges 48. A detonating cord 50 is attached to charge strip 46. A ballistic charge transfer end section 51 is located at the bottom of carrier 40. The charges 48 are ideally arranged to create perforation tunnels 23 (FIG. 1) in carrier 40 and surrounding casing 22 in a plurality of radial directions with the perforation tunnels 23 also being ideally longitudinally spaced apart. Prior to detonation, it is preferred that perforating gun 10 be filled with a high pressure gas. Otherwise, pressure waves might cause a longitudinal split in hollow carrier 40 if the compressibility of the fluid in gun 10 were otherwise too low.
All the internal components disposed within [0026] core 44 are specifically designed to be readily drilled out of carrier 40 after detonation. That is, they are either frangible or else made of a soft or low strength material. Ideally, the carrier 40 is made of a higher strength or rigid material such as steel. Thus, a drill, such as a hydraulically powered drill, may be introduced into the upper end of core 44 and remnants of components within core 44 drilled out. The drilled out or cleaned out core 44 will then allow for easy access of any tools to be subsequently used. Further, with the core 44 drilled out, fluid may readily pass through carrier 40.
End caps [0027] 42 may be made of soft aluminum which is readily drilled through. Alternatively, materials such as ceramics or plastics may be used to form end caps 42 and the ballistic charge transfer end section. While not shown, one skilled in art will appreciate some type of seal device, such as O-rings may be used to create the seal between core 44 and end caps 42. It is also envisioned that charges 48 could be encapsulated in a single end cap or plug made of plastic such as polyurethane or polyethylene or the like. Thus, only a single plug would be needed for each gun section. Alternatively, a series of the encapsulated charges 48 could be used. A common characteristic of whatever components may be placed in core 44 is that these components be designed such that they can be readily removed when core 44 is drilled out.
By providing the strong, outer [0028] non-drillable carrier 40 and the softer drillable internal components, the drilling operation will result in a core 44 that is relatively smooth despite the swelling or deformation caused by the detonation of charges 48. Accordingly, fluid or other tools passing through core 44 are not likely to get hung up. Of course, how smooth the interior core 44 becomes depends on the relative diameter of the drill bit and the diameter of core 44.
As seen in FIG. 1, it is desirable to have a [0029] carrier 40 which has a large outer diameter than that of the tubing 12 to which gun 10 is mounted. This permits the inner diameter of the carrier 40 to be at least as large as the inner diameter of the tubing 12. Consequently, inner core 44, after drilling, will not impede the passage of tools or fluid also passing through tubing 12.
Other advantages the present invention provides over previous perforating guns, including the use of a carrier which is completely drilled away such that there is no remaining hanging portion, include: [0030]
(1) there is no need to drill excessive rathole for dropping the guns; [0031]
(2) if coiled tubing is used to drill out the core, it can be done in an underbalanced condition which results in minimal damage to the producing formation, and there is no need to kill the well; [0032]
(3) there is no requirement that the hollow carrier be moved after perforating, so less clearance is required, and larger diameter hollow carriers can be used within the same size casing than with previously known perforating guns; and [0033]
(4) because the [0034] hollow carrier 40 is fixed in place, it is easier to drill out the components within the core 44 than it would be if the entire carrier 40 was free to move as in the fifth option of the prior art described above wherein the carrier is drilled away.
In the preferred implementation, one or more of [0035] guns 10 are conveyed by the tubing 12 and are preferably permanently located adjacent to the interval to be perforated in casing 22. Charges 48 are detonated with perforations tunnels 23 being formed in casing 22. A coiled tubing and drill assembly (not shown) is run down through tubing 12. Then end caps 42 and the other remaining internal components within hollow carrier 40 are drilled out. Hollow carrier 40 remains affixed in place within casing 22. This is similar to the approach used in the shoot and hang method. However, in the present invention, the carrier 40 is drilled out with the carrier 40 serving as a guide for the drilling operation which removes any remaining internal components in core 44. The drilling operation also serves to “smooth” out the inner diameter or core of carrier 40, if the diameter of the core 44 is similar in size to that of the tubing 12 to which the gun 10 is mounted. Because gun 10 is mounted on tubing 12, many guns 10 may be mounted in series such that hundreds or even thousands of feet of casing 22 can be perforated at a single time. Use of wireline perforating guns often restricts the length of casing 12 which can be perforated in a single operation. Tubing 12 and carrier 40 can be left in place after the perforating operation.
After the drilling operation, the [0036] charges 48 and other internal components do not obstruct the perforated interval from access by maintenance or other devices for three reasons. First, the internal components of the equipment above and at the connections within the hollow carrier 40 are constructed of materials that are easily drilled up with a bit and hydraulic motor (not shown) conveyed by concentric tubing, hence the name, “drillable core”. Second, the charge strip 46 is made of materials that fragment upon explosive detonation or are easily drilled up with a bit. Third, perforating guns 10 does not obstruct the perforated interval because the hollow carrier 40 is made larger in diameter than those used with methods that employ dropped or discarded carriers. Consequently, there is no requirement that the carrier 40 be retrieved or dropped at some future time. It is also within the scope of this invention that a plurality of guns 10 may be joined together to perforate an extended length of casing 22. Again, all of their internal cores 44 may be readily drilled away.
In an alternative implementation of this method, guns could be dropped into the well, but still include easily drillable components above and within the hollow carrier. In both cases, wireline- or concentric-tubing conveyed tools can be used to access the full perforated interval after the drillable components have been removed from the gun. However, this second implementation is less preferred for three reasons. The hollow carrier must remain small enough to allow it to fall, the drilling operation is easier if the carrier or carriers remain attached, and some alignment problems can occur when trying to enter the detached hollow carrier with maintenance devices. [0037]
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention. [0038]

Claims

What is claimed is:

1. A perforating gun for perforating casing in a wellbore, the perforating gun comprising:

a carrier having a generally hollow interior forming a core;

at least one charge mounted within the hollow interior; and

a plug made of a soft, drillable material which seals at least a portion of the at least one charge within the carrier;

wherein the carrier remains intact after the at least charge has detonated and may serve as a guide to a drill to drill out the at least one plug and at least one charge from the core.

2. The perforating gun of claim 1 wherein:

a pair of plugs cooperate to seal the at least one charge in the carrier.

3. The perforating gun of claim 1 wherein:

there are a plurality of charges disposed in the carrier.

4. The perforating gun of claim 1 further comprising:

a charge strip for positioning the at least one charge within the carrier.

5. The perforating gun of claim 1 wherein:

the carrier is made of a material having a higher tensile strength than the at least one plug.

6. A method of perforating a casing in a well core, the method comprising the steps of:

mounting a perforating gun on tubing and positioning the perforating gun in a well-bore adjacent to a casing to be perforated, the gun having a hollow carrier defining a core and having at least one charge stored in the core and having at least one plug which seals the charge within the carrier;

detonating the charge to perforate the adjacent casing; and

drilling out the core of the carrier to provide a drilled out passageway through which other tools and fluid may readily pass.

7. The method of claim 6 wherein:

the drilling out leaves the core of carrier relatively smooth such that other tools may pass there through without hanging up on the drilled out core.

8. The method of claim 6 wherein:

the plug is made of material which is softer than the carrier.

9. The method of claim 6 wherein:

the perforating gun includes a charge strip upon which the at least one charge is mounted; and

the step of drilling out the core removes the charge strip from the carrier.

10. The method of claim 6 wherein:

the perforating gun is mounted upon a string of hollow tubing and is placed adjacent the casing to be perforated; and

the core of the carrier, after the core is drilled out, is at least as large as the interior of the hollow tubing to thereby provide uninhibited passage of tools through the hollow tubing and the drilled out core of the carrier.

11. The method of claim 10 wherein:

the carrier is generally cylindrical and the outer diameter of the carrier is larger than the outer periphery of the tubing.

12. The method of claim 6 wherein:

at least a hundred feet of casing are perforated in a single detonation of the gun.

13. The method of claim 6 wherein:

at a thousand feet of casing are perforated in a single detonation of the gun.