System and method for performing a perforation operation

A technique facilitates performance of a perforating operation in a wellbore. The technique comprises positioning a perforating gun assembly downhole in a wellbore via coiled tubing. The perforating gun assembly has a plurality of individually controllable perforating gun sections which may be selectively fired at different well zones. An optical fiber is deployed along the coiled tubing to deliver control signals to the perforating gun assembly. The control signals enable sequential firing of the individually controllable perforating gun sections at the desired locations, e.g. well zones, along the wellbore.

BACKGROUND

In many well applications, perforation operations are performed to create perforations which extend into the surrounding formation. Perforating guns are deployed downhole and carry charges which are detonated and fired to create radially extending perforations. Coiled tubing is sometimes employed in perforating operations to push gun strings down highly deviated wellbores, e.g. horizontal and extended reach wellbores. Additionally, a telemetry system is employed to carry control signals to the gun string for initiation of detonation and creation of the perforations at a desired well zone.

SUMMARY

In general, a system and methodology are provided for performing a perforating operation in a wellbore with a lighter and more dependable coiled tubing system. The technique comprises positioning a perforating gun assembly downhole in a wellbore via coiled tubing. The perforating gun assembly has a plurality of individually controllable perforating gun sections which may be selectively fired at different well zones. An optical fiber is deployed along the coiled tubing to deliver control signals to the perforating gun assembly while limiting the weight of the overall system. The control signals enable sequential firing of the individually controllable perforating gun sections at the desired locations, e.g. well zones, along the wellbore.

DETAILED DESCRIPTION

The present disclosure generally relates to a system and methodology for performing perforating operations along a wellbore. According to an embodiment, coiled tubing is employed to position a perforating gun assembly downhole in a wellbore at a desired, initial zone to be perforated. The perforating gun assembly has a plurality of individually controllable perforating gun sections which may be selectively fired at different well zones. A surface control system may be used to supply signals downhole, and those control signals are then processed downhole to selectively fire the individual perforating gun sections. The selective control over individual gun sections enables sequential perforation of desired well zones, including non-contiguous well zones. In this embodiment, an optical fiber is deployed along the coiled tubing to reduce weight and to deliver the control signals to the perforating gun assembly.

The system and methodology may be designed to provide a multi-fire perforation system which minimizes the number of trips into the well while perforating well zones, such as non-contiguous well zones. The system and methodology also provide a repeatable, reliable approach to initiating gun detonation in a manner which is impervious to the changing wellbore environment. According to an embodiment, the system utilizes addressable switch technology and is processor controlled, e.g. microprocessor controlled, in response to control signals originating from equipment located at the surface. Communication and telemetry may be established through the optical fiber, e.g. a fiber optic tether, installed along the coiled tubing, e.g. within a fluid flow path of the coiled tubing.

Referring generally toFIG. 1, an embodiment of a perforation system20is illustrated. In this embodiment, perforation system20comprises a coiled tubing perforating assembly22having a bottom hole assembly24which includes a perforating gun assembly26. The bottom hole assembly24, including the perforating gun assembly26, is connected to coiled tubing28. The coiled tubing28may be coiled on appropriate coiled tubing surface equipment29. Additionally, perforating gun assembly26comprises a plurality of individually controllable perforating gun sections30which may each be individually detonated and fired at a desired location along a wellbore32. The perforating guns, e.g. gun sections30, may be individually controlled such that adjacent gun sections30or non-adjacent gun sections30may be sequentially fired.

In the example illustrated, wellbore32has been drilled as a deviated wellbore having a deviated, e.g. horizontal, section34. The deviated section34extends through a plurality of well zones36which may include non-contiguous well zones. The perforating gun assembly26is deployed downhole into the wellbore32to an initial well zone36, e.g. the well zone36closest to the toe of the wellbore32. Once at the desired well zone, the appropriate individually controllable perforating gun section30may be detonated and fired to create radially extending perforations38into the surrounding formation40. Subsequently, the perforating gun assembly26may be moved via the coiled tubing28to the next desired well zone36and the detonation and firing process may be repeated via another individually controllable perforating gun section30to create perforations38at the next well zone36. This process may be repeated until the desired well zones are perforated.

Referring again toFIG. 1, an optical fiber42is deployed along the coiled tubing28to provide control signals which are used to selectively initiate detonation and firing of the desired individually controllable perforating gun sections30, as described in greater detail below. The optical fiber42may comprise an individual fiber or a plurality of fibers and may be in the form of, for example, a fiber optic tether disposed along the coiled tubing. The optical fiber42adds a very limited amount of weight to the overall coiled tubing perforating assembly22, and the lightweight system facilitates greater reach into deviated and extended reach wellbores. As illustrated, the optical fiber42may be deployed along an interior44of coiled tubing28and is therefore deployed in a fluid flow path in the interior44of the coiled tubing28. In many applications, the optical fiber42also may be used to relay data from the bottom hole assembly24to the surface46. For example, real-time feedback may be transmitted uphole along optical fiber42regarding the perforating operation taking place downhole. The feedback also may be used to verify perforating operations via measurements taken from the perforating tool string and transmitted along optical fiber42from the perforating gun assembly26to the surface46.

The optical fiber42may be coupled between surface equipment48, such as a surface control system, and a downhole processor50, such as a microprocessor. In some applications, the downhole processor50is constructed as a control system with a main processor52and a secondary processor54. In the embodiment illustrated, the downhole processor50is located in a perforating head56of perforating gun assembly26. By way of example, the surface control system48may utilize a dongle58or other suitable device to enable the surface control system48to send control signals to processor50via optical fiber42for testing and other purposes. The dongle58may be mated to the bottom hole assembly24such that the perforating gun assembly26may only fire to create the perforations38when the dongle58is in communication or otherwise present in the control system48.

Referring generally toFIG. 2, an example of bottom hole assembly24and perforating gun assembly26is illustrated, although the assembly may comprise additional or other components arranged in a variety of configurations. In the example illustrated, the perforating gun assembly26comprises a telemetry module60powered by suitable power source62, such as a battery. The telemetry module60is coupled with optical fiber42and is powered to receive and/or send signals via optical fiber42. In some applications, the telemetry module60may be incorporated into a pressure, temperature, and casing collar locator (PTC) sensor sub. Regardless of the specific structure, the telemetry module60may be connected to a sensor system64, such as a measurement sensor sub, having a plurality of sensors66. By way of example, sensors66may comprise pressure sensors, temperature sensors and depth correlation sensors, e.g. casing collar locators (CCLs) or gamma ray detectors. The depth correlation sensors66correlate the depth of the perforating gun assembly26and/or individual perforating gun sections30with a reference depth to enable adjustment for placement of the selected, individual perforating gun section30at the desired location in the zone36to be perforated.

The perforating gun assembly26further comprises perforating head56which is connected to individually controlled perforating gun sections30through a protection switch68. In the example illustrated, the perforating head56is coupled to gun sections30through a plurality of protection switches68. The perforating head56also may be coupled to the individually controllable perforating gun sections30via an addressable switch system70which may comprise a plurality of addressable switches72. Examples of an addressable switch system70include the ASFS and Secure systems available from Schlumberger Wireline. System control is achieved using, for example, a computer of surface control system48to communicate with the downhole perforating gun assembly components through optical fiber42which may be deployed in the interior44coiled tubing28. In the example illustrated, the addressable switches72, in combination with perforating head56, may be used to selectively detonate and fire individual perforating gun sections30via detonators74. Each perforating gun section30may comprise a plurality of shaped charges76oriented to create perforations38at a desired well zone36upon detonation and firing.

The perforating head56may have a variety of components and configurations, however an example is illustrated inFIG. 3. In this example, the perforating head56comprises controller or processor50having main processor52and secondary processor54. The perforating head56also comprises a power source78, e.g. a battery pack, a capacitor bank80, and an accelerometer82which may constitute one of the sensors66. Protection switches68also may be part of perforating head56in some embodiments.

The processor50, e.g. processors52and54, may be programmed to perform multiple functions. For example, processor50may be designed to communicate with telemetry module60which, in turn, communicates uphole and/or downhole via optical fiber42to accept commands and to convey information uphole to surface control system48. The processor50also may be designed to communicate in a downhole direction with the addressable switch system70and addressable switches72to enable firing of a specific perforating gun, e.g. a specific perforating gun section30. In some applications, processor50also is employed to control the process of charging up the capacitors in capacitor bank80. For example, the processor50may be designed to exercise control over the flow of electrical power from power source78, e.g. a downhole battery, to the capacitor bank80and then to control release of energy from capacitor bank80to the selected perforating gun section30.

In a variety of applications, processor50also may be employed to monitor selected tool parameters and to store desired data. Processor50may further be used to control and send data from sensors66, e.g. accelerometer output, temperature, voltage, current, pressure, and/or other sensor data, uphole to surface control system48such as along the optical fiber42. The sensor measurements may be conveyed in real time to provide details about the perforation operation, such as whether the desired perforating gun section has actually fired. If processor50comprises main processor52and secondary processor54, the two processors may be used redundantly to confirm commands. For example, the processors may be programmed to agree that valid commands are sent before initiating detonation of perforating gun sections30.

Although some embodiments may utilize power supplied from a surface location, many applications utilize power supplied from a downhole location to run the downhole electronics and to fire the perforating gun sections30. Power sources62and78may comprise batteries or other suitable power sources used to supply the desired electric power. For example, power source78may comprise a battery coupled to capacitor bank80to charge the capacitors and to create a sufficiently high voltage to detonate the charges76.

Processor50may be used to control the detonation by selectively activating the detonators74. For example, following a command from surface control system48, the processor50may be used to initiate boosting of the battery voltage to a desired perforating voltage level through appropriate electronic circuitry and via charge stored in capacitor bank80. On demand from processor50, the capacitor bank80is discharged and the appropriate addressable switch72is activated to enable supply of sufficiently high voltage to the desired detonator74, thus causing detonation and firing of the gun section30associated with that particular detonator74. In some embodiments, the capacitor bank80includes or cooperates with a voltage drain which bleeds off any undesirable voltage buildup in the capacitor bank80.

In some applications, power may be supplied from the surface46using an appropriate conductor. For example, a conductor may be embedded in or otherwise packaged with the optical fiber42. The level of voltage supplied from the surface in this type of configuration may be far lower than with a conventional setup using a wireline cable to transmit power. The special fiber optic tether comprising the internal conductor would be smaller in size and lighter in weight compared to a wireline cable, thus facilitating deployment of the perforating gun assembly26in deviated wellbores, such as the deviated section34. In such an embodiment, voltage supplied from the surface would be used to charge the downhole capacitor bank80and the system would remain in a low voltage mode until initiation of the capacitor charging process.

In an embodiment, power to charge the capacitor bank80is generated downhole by a suitable power generation system. For example, power source62and/or power source78may be designed as a turbine positioned to extract energy from fluid flow pumped from the surface down through the interior44of coiled tubing28. The power sources62,78also may comprise a downhole photovoltaic cell designed to generate power downhole by converting light to electricity. In this example, laser light is supplied from the surface down through optical fiber42and the laser light is converted into electricity at one or both power sources62,78. This power may then be used to charge capacitor bank80and/or to provide power for other system components.

Depending on the specific application, a variety of detonators74may be employed. For example, Secure detonators available from Schlumberger Wireline may be employed. This latter type of system may utilize an exploding foil initiator (EFI) technology with no primary high explosives used in the detonator, as will be appreciated by those skilled in the art. The electronics may be contained in the detonator package and may be completely expendable so that no separate downhole cartridge is employed.

Additionally, various types of protection switches68may be employed. In some applications, protection switches68may be in the form of addressable arming protection switches which isolate the system and prevent stray voltages from energizing the perforating gun system accidentally. In some applications, the addressable arming protection switches68may be placed at a top of the gun string and the state of the switches may be processor controlled by, for example, processor50. Similarly, a variety of addressable switch systems70and addressable switches72may be employed depending on the parameters of a specific application. The addressable switch firing system may be designed as a microprocessor controlled switch attached to each detonator74in the gun string/assembly26and controlled by processor50. In this example, each addressable switch72has a unique address so that each gun section30is identified prior to firing. The system may be designed so that two way communication is a prerequisite to the detonation and firing of a given gun section30, thus reducing the potential for inadvertent detonation. Additionally, bulkheads may be placed between gun sections30and may use one-wire feedthroughs which enable current flow for the detonation and firing of selected gun sections30.

In some applications, the surface equipment48, e.g. a computer-based surface control system, is equipped with a single point safety switch. This type of switch may be a single keylock safety switch having a properly secured single key which isolates the surface equipment prior to attachment of an explosive device, such as charges76. In the embodiment described herein, the surface control system48comprises an electronic dongle58which prevents inadvertent sending of commands down through optical fiber42, thus reducing or eliminating the risk of inadvertent detonation. During rig-up and assembly of the downhole components, electronic dongle58is disconnected to effectively prevent the downhole perforating gun assembly26from firing, similar to the way that a perforating key is removed from a conventional perforating surface control system. The surface control system48becomes active when the electronic dongle58is connected but not until the gun string assembly26and its associated components are a predetermined distance downhole, e.g. 200 feet into the well. Similarly, the electronic dongle58may be disabled during retrieval when the bottom hole assembly24is at a predetermined depth downhole, thus disabling the surface control system48. Additionally, a timeout feature in the communication link between the surface control system48and the downhole processor50may be used to mitigate the potential for failing to manually disable the communication link between the system48and the downhole processor50.

In some embodiments, the perforating gun assembly26is designed to provide shot firing event confirmation. Depending on the construction of the perforating gun assembly26, the addressable switch72associated with a given controllable gun section30may be destroyed when the gun section30is fired. The inability to communicate with the processor50may be used as an indication of firing. In addition, however, the indication may be augmented due to the occurrence of a shock load upon firing and the sensing of this shock load by suitable sensors66located in the perforating gun assembly. Accelerometer82also may be used as a suitable sensor66to detect the shock load. The lack of communication from the addressable switch72and the sensing of the shock load by a suitable sensor, e.g. accelerometer82, provide a positive confirmation of downhole detonation. However, other sensors also may be used to confirm or to augment confirmation of firing. For example, downhole pressure sensors66and/or downhole temperature sensors66also may be used to confirm a successful perforation operation at a given well zone36. In some applications, fluid channels extending into the reservoir/formation due to the perforation operation enable an influx of fluids into the wellbore. The inflow of fluids creates a change in pressure and/or temperature conditions downhole which may be detected by suitable sensors66as an indication of a successful perforation operation.

During a perforating application, bottom hole assembly components are assembled at the surface as illustrated in, for example,FIG. 2. Prior to connection of the individually controllable perforating gun sections30, a surface function test may be performed on the system. In some applications, the surface function test is performed with a tester module84connected to the perforating gun assembly26, e.g. to the bottom of the perforating gun assembly26. The tester module84may be formed as a separate module; incorporated into the processor module50; or combined with another suitable component of the perforating gun assembly26. During the surface function testing, a “pairing” of the electronic dongle58of surface control system48and the downhole electronic tester module84is performed. The test pairing ensures that the downhole tester module84responds to commands validated through the electronic dongle58.

The module84also may be designed as an addressable switch gun simulator able to mimic the presence of addressable switches72connected to the perforating head56. By simulating a series of switches, the software and hardware of the system may be checked without involving explosives. Once pairing has been completed, the surface test also may involve tearing out a comprehensive system function check of the entire process cycle for perforating. According to an embodiment, the system function check may comprise establishing communication with the individual addressable switches72, initiating the charging of the capacitor bank80to the appropriate voltage level, and applying voltage to a selected detonator to simulate firing of a gun section30. Successful completion of the procedure provides an indication that the system is functioning properly.

Other equipment also may be used during the surface test procedure. For example, an addressable switch tester and a personal data assistant controller may be employed to further facilitate testing of the addressable switch system70prior to deployment of the perforating gun assembly26downhole into wellbore32but after the perforating gun assembly has been assembled. Such testing may be performed prior to operatively connecting the perforating head56.

The perforation system20provides an improved, coiled tubing-based system for selectively perforating desired zones of wells, such as oil and gas wells. Selective perforating implies performing multiple detonations during a single run downhole. However, the system also may be employed for single fire perforation applications.

Referring generally to the flowchart ofFIG. 4, an example of a perforating application is illustrated. In this example, the perforating gun assembly26is assembled and coupled with coiled tubing28and optical fiber42, as indicated by block86. The perforating gun assembly26is then conveyed downhole into wellbore32and moved along deviated section34, as indicated by block88. An initiation signal is then sent downhole from surface control system48along optical fiber42to the perforating tool string, e.g. perforating gun assembly26, to initiate a perforating operation with a selected perforating gun section30, as indicated by block90. The processor50may then be used to transmit a confirmation signal uphole along optical fiber42to surface control system48to confirm receipt of the initiation signal, as indicated by block92. The perforating operation is then performed at a given well zone36by firing the appropriate gun section30, as indicated by block94. Upon completion of the perforation operation, the coiled tubing28is moved which, in turn, moves the perforating gun assembly to the next perforation location, as indicated by block96. The perforation procedure is then repeated at this next location and at each subsequent location until the overall perforation operation is completed, as indicated by block98.

Another procedural example is illustrated in the flowchart ofFIG. 5. In this example, the bottom hole assembly (BHA)24is assembled and attached to a bottom end of coiled tubing28, as indicated by block100. In some embodiments, this initial assembly of bottom hole assembly24does not include attaching the perforating gun sections30. Once attached to coiled tubing28, system function tests may be performed using, for example, testing module84, as indicated by block102. After successful testing, the remainder of the perforating gun assembly26may be assembled and combined into the bottom hole assembly24. For example, the gun sections30, detonators74, and addressable switches72may be assembled, as indicated by block104. The addressable switches72are then tested with, for example, an addressable switch tester as discussed above and as indicated by block106.

Following testing, makeup of the bottom hole assembly24is completed and the perforating gun assembly26is deployed into wellbore32to an initial perforation interval, as indicated by block108. In many applications, the perforation sequence involves detonation at a lower or distant well zone36with subsequent detonations and perforation procedures being performed along the wellbore32moving the bottom hole assembly24in a direction toward surface46. Once at the initial perforation interval, the depth of the appropriate gun section30is correlated with a reference so that appropriate adjustments may be made, as indicated by block110.

A control signal may then be sent from surface control system48to processor50, and processor50controls the charging of capacitor bank80, as indicated by block112. Electric power from the capacitors in the capacitor bank80may then be used to detonate and fire the selected, e.g. lowest, perforating gun section30by sending the appropriate signal to the corresponding addressable switch72, as indicated by block114. Successful firing of the gun section30is then confirmed by, for example, suitable sensor66, as described above and as indicated by block116. In some embodiments, the addressable switches72may be employed in both receiving and sending initiation and confirmation signals, respectively.

After the initial perforations38are formed at the desired well zone36, the perforating gun assembly26is moved via coiled tubing28to the next perforating interval, as indicated by block118. The depth of the next sequential gun section30is then adjusted and correlated with a reference, as indicated by block120. After adjusting the gun section30to the desired depth, the appropriate gun section30is detonated and fired to create perforations38in the subsequent well zone36, as indicated by block122. The successful firing is again confirmed, as indicated by block124. This movement, placement, firing, and confirmation process is repeated for each of the intervals to be perforated, as indicated by block126. Once the desired intervals are perforated, the bottom hole assembly24is pulled back to the surface and the perforating gun sections are un-deployed from the well, as indicated by block128. At this stage, the bottom hole assembly24may be disassembled or otherwise processed for a subsequent perforating operation.

During the perforating procedure, the capacitor bank80may be charged back up should the voltage drop below the predetermined voltage used for detonation. Additionally, various other processes may be combined with or used in place of portions of the procedures described above. For example, the activation/de-activation of the protection switches68, electronic dongle58, testing module84, and/or other components may be performed prior to and/or during the overall perforation procedure.

In many oil and gas well applications, the perforation techniques described herein may be employed to provide a selective, reliable and repeatable firing of perforating guns to provide perforations at various locations along a wellbore. By using optical fiber42and fiber optic-based telemetry, the weight of the overall coiled tubing system is reduced. The lighter weight system is particularly helpful in long, extended reach wells, where additional weight may result in compromises with respect to depth penetration capability.

The perforation system20also may be powered from downhole locations by, for example, batteries or other power sources. Such systems may utilize relatively low voltages with virtually no elevated voltages present at the surface. The higher voltage for detonation is selectively created downhole by controlled charging of the capacitor bank80. Except for the possible, short duration surface system test, the voltages of the capacitor bank80are held at a low level until the perforating operation is ready to be performed downhole. Various protection switches and other devices also may be employed to provide high system dependability and fail-safe functionality. Additionally, the downhole processor, e.g. microprocessor, further ensures a high level of reliability. The redundancy of a second processor54also may be used to provide an additional stop-gap that ensures very dependable functioning of the overall perforation system.

As described herein, the systems, devices and procedures used to perform perforating operations may have a variety of configurations and may be designed for use in a variety of environments. For example, the number and arrangement of perforating gun sections may vary depending on the well zones to be perforated. Additionally, the surface control systems and downhole control systems may utilize a variety of microprocessors or other types of processors for sending and/or receiving signals. The fiber optic telemetry system may utilize individual fibers, multiple fibers, combinations of fibers and conductors, various fiber optic tethers, and other types of optical fiber communication lines. Several types of equipment also may be employed for transmitting and receiving the optical signals. The arrangement of perforating gun assembly components, bottom hole assembly components, coiled tubing components, and other components of the overall perforation system may be modified, interchanged, and/or supplemented according to the parameters of a given perforation operation and environment.