STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

Technical Field of the Invention

The present disclosure relates to the field of hydrocarbon recovery operations. More specifically, the invention relates to a tandem sub used to mechanically and electrically connect detonation tools in a perforating gun assembly. Further still, the invention relates to an assembly residing within a tandem sub for initiating an explosive charge for a perforating gun, and further, to a detonation assembly that protects the electronics located inside of the tandem sub from wellbore fluid and debris produced by the detonation of charges from an associated perforating gun.

Discussion of the Background

In the drilling of an oil and gas well, a near-vertical wellbore is formed through the earth using a drill bit urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation penetrated by the wellbore.

A cementing operation is conducted in order to fill or “squeeze” the annular volume with cement along part or all of the length of the wellbore. The combination of cement and casing strengthens the wellbore and facilitates the zonal isolation, and subsequent completion, of hydrocarbon-producing pay zones behind the casing.

In connection with the completion of the wellbore, several strings of casing having progressively smaller outer diameters will be cemented into the wellbore. These will include a string of surface casing, one or more strings of intermediate casing, and finally a production casing. The process of drilling and then cementing progressively smaller strings of casing is repeated until the well has reached total depth. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface.

Within the last two decades, advances in drilling technology have enabled oil and gas operators to “kick-off” and steer wellbore trajectories from a vertical orientation to a horizontal orientation. The horizontal “leg” of each of these wellbores now often exceeds a length of one mile, and sometimes two or even three miles. This significantly multiplies the wellbore exposure to a target hydrocarbon-bearing formation. The horizontal leg will typically include the production casing.

FIG. 1is a side, cross-sectional view of a wellbore100, in one embodiment. The wellbore100has been completed horizontally, that is, it has a horizontal leg156. The wellbore100defines a bore10that has been drilled from an earth surface105into a subsurface110. The wellbore100is formed using any known drilling mechanism, but preferably using a land-based rig or an offshore drilling rig operating on a platform.

The wellbore100is completed with a first string of casing120, sometimes referred to as surface casing. The wellbore100is further completed with a second string of casing130, typically referred to as an intermediate casing. In deeper wells, that is, wells completed below 7,500 feet, at least two intermediate strings of casing will be used. InFIG. 1, a second intermediate string of casing is shown at140.

The wellbore100is finally completed with a string of production casing150. In the view ofFIG. 1, the production casing150extends from the surface105down to a subsurface formation, or “pay zone”115. The wellbore100is completed horizontally, meaning that a horizontal “leg”156is provided. The production casing150extends across the horizontal leg156.

It is observed that the annular region around the surface casing120is filled with cement125. The cement (or cement matrix)125serves to isolate the wellbore100from fresh water zones and potentially porous formations around the casing string120.

The annular regions around the intermediate casing strings130,140are also filled with cement135,145. Similarly, the annular region around the production casing150is filled with cement155. However, the cement135,145,155is optionally only placed behind the respective casing strings130,140,150up to the lowest joint of the immediately surrounding casing string. Thus, a non-cemented annular area132is typically preserved above the cement matrix135, a non-cemented annular area142may optionally be preserved above the cement matrix135, and a non-cemented annular area152is frequently preserved above the cement matrix155.

The horizontal leg156of the wellbore100includes a heel153and a toe154. In this instance, the toe154defines the end (or “TD”) of the wellbore100. In order to enhance the recovery of hydrocarbons, particularly in low-permeability formations115, the casing150along the horizontal section156undergoes a process of perforating and fracturing (or in some cases perforating and acidizing). Due to the very long lengths of new horizontal wells, the perforating and formation treatment process is typically carried out in stages.

In one method, a perforating gun assembly200is pumped down towards the end of the horizontal leg156at the end of a wireline240. The perforating gun assembly200will include a series of perforating guns (shown at210inFIG. 2), with each gun having sets of charges ready for detonation. The charges associated with one of the perforating guns are detonated and perforations (not shown) are “shot” into the casing150. Those of ordinary skill in the art will understand that a perforating gun has explosive charges, typically shaped, hollow or projectile charges, which are ignited to create holes in the casing (and, if present, the surrounding cement)150and to pass at least a few inches and possibly several feet into the formation115. The perforations create fluid communication with the surrounding formation115so that hydrocarbon fluids can flow into the casing150.

After perforating, the operator will fracture (or otherwise stimulate) the formation115through the perforations (not shown). This is done by pumping treatment fluids into the formation115at a pressure above a formation parting pressure. After the fracturing operation is complete, the wireline240will be raised and the perforating gun assembly200will be positioned at a new location (or “depth”) along the horizontal wellbore156. A plug (such as plug112) is set below the perforating gun assembly200using a setting tool160, and new shots are fired in order to create a new set of perforations. Thereafter, treatment fluid is again pumping into the wellbore100and into the formation115at a pressure above the formation parting pressure. In this way, a second set (or “cluster”) of fractures is formed away from the wellbore156.

The process of setting a plug, perforating the casing, and fracturing the formation is repeated in multiple stages until the wellbore has been completed, that is, it is ready for production. A string of production tubing (not shown) is then placed in the wellbore to provide a conduit for production fluids to flow up to the surface105.

In order to provide perforations for the multiple stages without having to pull the perforating gun200after every detonation, the perforating gun assembly200employs multiple guns in series.FIG. 2is a side view of an illustrative perforating gun assembly200, or at least a portion of an assembly. The perforating gun assembly200comprises a string of individual perforating guns210.

Each perforating gun210represents various components. These typically include a “gun barrel”212which serves as an outer tubular housing. An uppermost gun barrel212is supported by an electric wire (or “e-line”)240that extends from the surface105and delivers electrical energy down to the tool string200. Each perforating gun210also includes an explosive initiator, or “detonator” (shown at594inFIG. 25C). The detonator is typically a small aluminum housing having a resistor inside. The detonator receives electrical energy from the surface105and through the e-line240, which heats the resistor.

The detonator is surrounded by a sensitive explosive material. When current is run through the detonator, a small explosion is set off by the electrically heated resistor. This small explosion sets off an adjacent detonating cord (shown at595inFIG. 25C).

The detonating cord contains an explosive compound that is detonated by the detonator. The detonating cord initiates one or more shots, typically referred to as “shaped charges.” The shaped charges (shown at520inFIG. 5) are held in an inner tube (shown at500inFIG. 5), referred to as a carrier tube, for security and discharge through openings215in the selected gun barrel212. The detonating cord propagates an explosion down its length to each of the shaped charges along the carrier tube.

The perforating gun assembly200may include short centralizer subs220. In addition, tandem subs225are used to connect the gun barrel housings212end-to-end. Each tandem sub225comprises a metal threaded connector placed between the gun barrels210. Typically, the gun barrels210will have female-by-female threaded ends while the tandem sub225has opposing male threaded ends.

The perforating gun assembly200with its long string of gun barrels (the housings212of the perforating guns210) is carefully assembled at the surface105, and then lowered into the wellbore10at the end of the e-line240. The e-line240extends upward to a control interface (not shown) located at the surface105. An insulated connection member230connects the e-line240to the uppermost perforating gun210. Once the assembly200is in place within a wellbore, an operator of the control interface sends electrical signals to the perforating gun assembly200for detonating the shaped charges520and for creating perforations into the casing150.

After the casing150has been perforated and at least one plug112has been set, the setting tool120and the perforating gun assembly200are taken out of the wellbore100and a ball (not shown) is dropped into the wellbore100to close the plug112. When the plug112is closed, a fluid (e.g., water, water and sand, fracturing fluid, etc.) is pumped by a pumping system down the wellbore (typically through coiled tubing) for fracturing purposes.

As noted, the above operations may be repeated multiple times for perforating and/or fracturing the casing150at multiple locations, corresponding to different stages of the well. Multiple plugs may be used for isolating the respective stages from each other during the perforating phase and/or fracturing phase. When all stages are completed, the plugs are drilled out and the wellbore100is cleaned using a circulating tool.

It can be appreciated that a reliable electrical connection must be made between the gun barrels210in the tool string200through each tandem sub225. Currently, electrical connections are primarily made using a side entrance port on the tandem sub225to manually connect wires. When the charges are fired, the electronics in each carrier tube are lost and the tandem subs are frequently sacrificed.

A need exists for a detonation system wherein the electronic switch is housed within the tandem sub such that the wiring connections may be pre-assembled before the perforating guns are delivered to the field. A need further exists for a detonation system utilizing a tandem sub having a carrier end plate, wherein the end plate seals off the tandem sub from wellbore fluids and debris following detonation of explosive charges in an associated perforating gun. Additionally, a need exists for a detonation system that uses signal transmission pins that extend through an end plate in order to deliver detonation signals, while mechanically and fluidically sealing off an associated tandem sub from wellbore fluids and debris following detonation of explosive charges.

SUMMARY OF THE INVENTION

A detonation system for a perforating gun assembly is provided. The detonation system utilizes an addressable switch that transmits a detonation signal to a detonator in an adjacent perforating gun. The detonator, in turn, ignites an explosive material, creating an explosion that is passed through a detonating cord. The detonating cord then ignites shaped charges along the perforating gun.

The detonation system first includes a tandem sub. The tandem sub defines a short tubular body having a first end and a second opposing end. A circular shoulder may be provided intermediate the first and second ends. The first and second ends comprise male threads that are configured to connect to gun barrels of adjacent perforating guns. The gun barrels are threaded onto the opposing ends of the tandem sub until they reach the intermediate shoulder.

The detonation system also includes a perforating gun. The perforating gun comprises a carrier tube, a plurality of charges residing within the carrier tube, and a gun barrel. The gun barrel serves as a housing for the carrier tube and the plurality of charges. In one aspect, the gun barrel has female threads that connect to male threads at a first end of the tandem sub.

The detonation system additionally includes a switch housing. The switch housing resides within an inner bore of the tandem sub, proximate the first end.

As noted, the detonation system also includes the addressable switch. The addressable switch resides entirely within the switch housing. The addressable switch is configured to receive instruction signals from the surface by means of a signal line. The addressable switch listens for a detonation signal that is associated with that tandem sub.

The detonation system also comprises a bottom end plate. The bottom end plate resides between the carrier tube of the perforating gun and the first end of the tandem sub. The bottom end plate has a first through-opening.

The detonation system additionally comprises a detonator pin. The detonator pin extends through the first through-opening of the bottom end plate. The detonator pin has a proximal end that extends into the carrier tube and that is in electrical communication with a detonator. The detonator pin further has a distal end that extends into the switch housing and is in electrical communication with the addressable switch. The detonator pin is preferably fabricated from an electrically conductive material.

Beneficially, the bottom end plate provides a seal against the first end of the tandem sub to protect the addressable switch from a pressure wave generated by detonation of the plurality of charges in the adjacent carrier tube. Preferably, the carrier tube is upstream from the tandem sub, which means that the bottom end plate is actually above, or upstream from, the tandem sub.

In one aspect, the detonation system further comprises a bulkhead for the detonation pin. The bulkhead resides around an intermediate portion of the detonation pin such that the bulkhead frictionally resides within the through-opening of the bottom end plate. Preferably, the bulkhead for the detonation pin is fabricated from a non-conductive material.

In one aspect, the detonation system further comprises a contact pin. The contact pin is also fabricated from a conductive material and also resides within the inner bore of the tandem sub. The contact pin comprises a contact head that extends into the switch housing from the bottom, a shaft, and a distal end in electrical communication with the signal line. The contact pin is configured to transmit instruction signals from the surface to a next (or downstream) perforating gun by means of the signal line.

Preferably, the detonation system also has a top end plate. The top end plate resides at the second end of the tandem sub, between the tandem sub and a next perforating gun. The top end plate receives the distal end of the contact pin. Note that the top end plate is preferably above a downstream carrier tube associated with the next perforating gun, which means that the top end plate is actually below, or downstream from, the tandem sub.

The detonation system also has a transmission pin. The transmission pin resides within a second through-opening of the bottom end plate, and delivers detonation signals from the electric line to the addressable switch.

Finally, the detonation system comprises a ground post. The ground post has a proximal end extending into the switch housing, and a distal end threaded onto the bottom end plate.

In the detonation system, the addressable switch is configured to monitor instruction signals received through the signal line and transmission pin. When an instruction signal is received to detonate charges in the adjacent carrier tube, that is, the gun barrel, the addressable switch sends a detonation signal through the detonation pin and to the detonator. Preferably, the perforating gun having the adjacent carrier tube is upstream of the tandem sub. However, in the detonation system the gun barrel may be downstream of the tandem sub.

In operation, the detonation system is part of the perforating gun assembly. The perforating gun assembly is run into a wellbore at the end of an electric line. More typically, the perforating gun assembly is pumped into the horizontal portion of the wellbore. The ground post and the contact pin are in electrical communication with the e-line, with the e-line extending from the perforating gun assembly up to the surface. When a signal is sent through the e-line, it is carried through the perforating gun assembly by means of the signal line and the contact pins residing within the string of perforating guns and tandem subs.

The addressable switches filter instruction signals from the operator at the surface. When an addressable switch receives a signal associated with its tandem sub and perforating gun, the addressable switch will send a detonation signal through the detonation pin and to the detonator. The detonator, in turn, ignites the explosive material that passes through the detonating cord and on to the charges along the carrier tube.

In addition to the detonation system, a tandem sub for a perforating gun assembly is also provided herein. The tandem sub comprises a first end and an opposing second end. The first end represents a male connector and is threadedly connected to a first perforating gun. Similarly, the second end represents a male connector and is threadedly connected to a second perforating gun.

The first end abuts a first end plate while the second end abuts a second end plate. An inner bore extends between the first end of the tandem sub and the second end.

A switch housing resides within the inner bore of the tandem sub proximate the first end. An addressable switch resides within the switch housing. The addressable switch is configured to receive instruction signals from an operator at the surface via a signal line.

The tandem sub includes a detonation pin and a separate signal transmission pin. The detonation pin has a proximal end that extends into an adjacent carrier tube and is in electrical communication with a detonator. The detonation pin also has a distal end that extends into the switch housing and is in electrical communication with the addressable switch. Similarly, the transmission pin has a proximal end that extends into the switch housing, and a distal end that is in electrical communication with a signal line coming in from the carrier tube.

The tandem sub includes a receptacle. The receptacle is positioned within the inner bore of the tandem sub proximate the second end. The receptacle is dimensioned to closely receive a bulkhead, wherein the bulkhead comprises:a tubular body having a first end, a second end and a bore extending there between;an electrical contact pin having a shaft extending through the bore of the bulkhead body and having a first end and a second end, wherein the shaft frictionally resides within the bore, and wherein the electrical contact pin transmits current from the first end to the second end; anda contact head located at the second end of the electrical contact pin outside of the bulkhead body and extending into the switch housing.

The contact pin is fabricated substantially from a conductive material. The contact head transmits instruction signals from the electric line (such as by means of a ground post) to a next perforating gun.

In one aspect, the first end plate comprises a first through-opening and a second through-opening. The first through-opening receives the detonation pin while the second through-opening receives the signal transmission pin. The signal transmission pin and the contact pin are in electrical communication with the e-line, with the e-line extending from the perforating gun assembly up to the surface.

The addressable switch filters instruction signals from the operator at the surface. When the addressable switch receives a signal associated with its tandem sub and adjacent perforating gun, the addressable switch will send a detonation signal through the detonation pin and back up to the detonator. As noted above, the detonator defines a small aluminum housing having a resistor inside. The resister is surrounded by a sensitive explosive material. When current is run through the detonator, a small explosion is set off by the electrically heated resistor. This small explosion ignites an explosive material placed within the detonating cord. As the explosive material is ignited, the detonating cord delivers the explosion to shaped charges along the first perforating gun.

Beneficially, the bottom end plate provides a seal against the first end of the tandem sub to protect the addressable switch from a pressure wave generated by detonation of charges in the upstream gun barrel.

