System and method for caliper calibration

Embodiments of the present disclosure are directed toward a system and method to calibrate calipers of a downhole tool. Specifically, a calibration tool includes a substantially cylindrical calibration ring. The calibration tool also includes a finger coupled to the calibration ring. The finger is configured to engage with the downhole tool such that the calibration ring is mounted to the downhole tool and is coaxial with a tool axis of the downhole tool.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to European Patent Application 14290072.9, filed on Mar. 19, 2014, the entire contents of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates generally to the field of downhole tools and, more particularly, to systems and methods for calibrating calipers on a downhole tool.

A variety of downhole tools may be used to determine the properties of a geological formation surrounding a well. Some downhole tools may include calipers, which provide a measurement of the diameter of the wellbore at various depths. Caliper measurements are used in a variety of operations, including drilling, cementing, and evaluation of the geological formation. During drilling, for example, the caliper data can be used to monitor the wellbore condition (e.g., identifying possible wellbore washout and/or impending wellbore instability), thus allowing the driller to take remedial action. During well completion, the caliper data can be used to accurately evaluate the volume of cement to fill the casing annulus, as well as aiding in the selection of casing points. A reliable caliper may also be useful during logging to adjust formation evaluation measurements for wellbore size and to evaluate the quality of other logging while drilling (LWD) logs. The caliper measurements can also be used to plan services, such as dip meters or formation testers, that may be affected by unfavorable wellbore conditions.

A variety of tools have been developed to accurately calibrate calipers before the downhole tool enters the wellbore. The accuracy of the calibration directly affects the quality of the measurements received from the calipers. For example, an improperly calibrated caliper may return inaccurate data of the wellbore and create challenges for operators during and after drilling operations. In some cases, calipers are calibrated using a calibration tool with a known inner diameter. Such existing calibration tools are now known to have certain drawbacks. For example, existing calibration tools are often subject to gravitational effects that can hinder an accurate centralization of the calibration ring, thereby affecting the accuracy of the calibration may be compromised. It is now recognized that there is a need for more accurate tools for calibrating the calipers on downhole tools.

SUMMARY

In a first embodiment, a calibration tool for calibrating calipers of a downhole tool includes a substantially cylindrical calibration ring and a finger coupled to the calibration ring. The finger is configured to engage with the downhole tool such that the calibration ring is mounted to the downhole tool and is coaxial with a tool axis of the downhole tool.

In a second embodiment, a calibration system includes a downhole tool. The downhole tool is configured to be disposed in a wellbore of a subterranean formation. The downhole tool includes at least one caliper. The caliper is configured to measure a distance between the downhole tool and the formation. The calibration system also includes a calibration tool designed to be disposed about the downhole tool. The calibration tool includes a substantially cylindrical calibration ring disposed around the downhole tool and a finger coupled to the calibration ring. The finger is configured to engage with the downhole tool such that the calibration ring is mounted to the downhole tool and is coaxial with a tool axis of the downhole tool.

In a third embodiment, a method includes disposing a first calibration tool over a downhole tool. The calibration tool includes a substantially cylindrical calibration ring having an inner diameter and a finger coupled to the calibration ring. The method also includes coupling the calibration tool to the downhole tool using the finger of the calibration tool. The finger is configured to engage the downhole tool such that the calibration ring is coaxial with a tool axis of the downhole tool. The method further includes actuating at least one caliper of the drill string to contact an inner circumference of the calibration tool.

DETAILED DESCRIPTION

Present embodiments are directed to a system and method for calibrating calipers on a downhole tool. In certain embodiments, the downhole tool includes a caliper that may be used to measure the diameter of a wellbore. Before being inserted in the wellbore, the calipers may be calibrated using a calibration tool. Rather than calibrate the downhole tool using a pad pressure exerted by the calipers, in presently disclosed embodiments, the calibration tool may enable calibration of the calipers without utilizing pad pressure from the calipers to support the weight of the calibration tool. Moreover, the calibration tool may be aligned with a tool axis of the downhole tool without activation of the calipers. To that end, the calibration tool may include fingers that are designed to engage with the downhole tool. In other words, the fingers may couple to the downhole tool to support and center the calibration tool about the tool axis. After the calibration tool is coupled to the downhole tool, the caliper can be actuated to contact the inner circumference of the calibration tool. The inner circumference of the calibration tool can serve as a reference point for measurements obtained when the downhole tool is lowered into the wellbore. By centering the calibration tool about the downhole tool via the fingers, misalignment due to supporting the calibration tool with caliper pad pressure or gravitational effects may be reduced. Moreover, the calibration tool may be used on downhole tools where the calipers do not exert sufficient pad pressure to support the calibration tool. As a result, the accuracy of caliper calibration may be improved, as compared to caliper calibration performed by calibration tools that use pad pressure.

FIG. 1illustrates a drilling system10used to drill a well through subsurface (e.g., subterranean) formations12. A drilling rig14at the surface16is used to rotate a drill string18that includes a drill bit20at its lower end. As the drill bit20is rotated, a “mud” pump22is used to pump drilling fluid, referred to as “mud” or “drilling mud,” downward through the drill string18in the direction of an arrow24to the drill bit20. The mud, which is used to cool and lubricate the drill bit20, exits the drill string18through ports (not shown) in the drill bit20. The mud then carries drill cuttings away from the bottom of the wellbore26as it flows back to the surface16, as shown by arrows28, through an annulus30between the drill string18and the formation12. At the surface16, the return mud is filtered and conveyed back to a mud pit32for reuse.

As illustrated inFIG. 1, the lower end of the drill string18includes a bottom-hole assembly (“BHA”)34that includes the drill bit20, as well as a plurality of drill collars36,38that may include various instruments and downhole tools40such as sample-while-drilling (“SWD”) tools that include sensors, telemetry equipment, pumps, sample chambers, and so forth. For example, the drill collars36,38may include logging-while-drilling (“LWD”) modules42and/or measurement-while-drilling (“MWD”) modules44. The LWD modules42ofFIG. 1are each housed in a special type of drill collar36,38, and each contain any number of logging tools and/or fluid sampling devices. The LWD modules42include capabilities for measuring, processing and/or storing information, as well as for communicating with the MWD modules44and/or directly with the surface equipment such as a logging and control computer. While the downhole tools40are generally illustrated as part of a drill string18inFIG. 1, in other embodiments the downhole tool40may be used in the context of wireline, tractor, coiled tubing, or any other desirable downhole exploration systems.

Present embodiments of the downhole tools40may also include calipers46used to measure a distance between the downhole tool40and the subsurface formation12. For example, the calipers46may include pads that extend out from the downhole tool40to contact a wellbore wall48of the wellbore26. The calipers46may include electronics to continually obtain and transmit information about the diameter of the wellbore26based on the distance of the extended caliper pads.

As described in detail below, the calipers46of the downhole tool40may be calibrated with a calibration system. The calibration system may include a calibration tool that engages with and is supported by the downhole tool40to calibrate the calipers46before the downhole tool40is lowered into the wellbore26. The calibration tool may provide reference points for known circumferences used to determine an accurate measurement of a position of the calipers46. That is, the electronics may associate a detected position of the calipers46with a certain diameter and/or radius of the wellbore26. Based on the calibration of the calipers46, an equation (e.g., linear, quadratic, etc.) may be computed to determine an accurate diameter of the wellbore26based on the detected position of the calipers46relative to the downhole tool40. In this manner, a relatively accurate wellbore26profile may be achieved by using the calipers46after they are calibrated.

Present embodiments include a calibration system50for calibrating the calipers46as illustrated inFIG. 2. As shown, the calibration system50includes at least one calibration tool52disposed about the downhole tool40. The downhole tool40generally extends along a tool axis54, and the calibration tool52is centered about the tool axis54. That is, a calibration tool axis56of the calibration tool52and the tool axis54of the downhole tool40are aligned. In this way, the calibration tool52and the downhole tool40are coaxial when the calibration tool52engages with the downhole tool40. The calibration tool52is positioned about the downhole tool40such that the calipers46are also disposed within the calibration tool52. In the illustrated embodiment, the downhole tool40has two sets57of calipers46. Each set57may include any desirable number of calipers46, which may be arranged circumferentially about the tool axis54. For example, the two sets57may include eight calipers46(just four shown). While the depicted embodiment shows a total of four calipers46(two in each set57), it is understood that the downhole tool40may have any number of calipers46depending on specifications of the drilling operation. Moreover, the different sets57of calipers46may not be aligned with one another along the downhole tool40in some embodiments. For example, the two sets57of calipers46may be offset from one another circumferentially about the tool axis54(e.g., 45 degrees).

In some embodiments, the calipers46may be actuated (e.g., opened, moved radially out from the downhole tool, moved away from the downhole tool) into an open position58. In the open position58, pads60of the calipers46are brought into contact with the inner circumference of the calibration tool52. In contrast, the pads60do not contact the inner circumference of the calibration tool52when in a closed position62. It is understood that while the depicted embodiment illustrates one set57of four calipers46(just two shown) in the open position58, the two sets57of four calipers46may be actuated such that they are in the open position58at the same time. Furthermore, in some embodiments, each of the calipers46in a set57may be actuated into the open position58at the same time. Moreover, in some embodiments, certain calipers46in the set57may be actuated into the open position58at the same time while the other calipers46in the set57are not actuated into the open position58. In other embodiments, however, each of the calipers46in a set57may be individually actuated into the open position58. For example, one of the calipers46may be actuated into the open position58while the other calipers46are not. That is, the calipers46may be separately actuated into the open position58, as well as actuated into the open position58together.

As previously mentioned, the calibration system50may be used to calibrate the calipers46before the downhole tool40is placed in the wellbore26. That is, a first calibration tool64may be placed about the downhole tool40prior to the actuation of the calipers46. Once the first calibration tool64is in place, the calipers46may be moved to the open position58to contact the inner circumference of the first calibration tool64. The position of the calipers46along the known inner circumference of the first calibration tool64may be used as a reference point. In some embodiments, a second calibration tool66may be placed about the same set57of calipers46and the calipers46may be moved to the open position58to contact the inner circumference of the second calibration tool66. This second position of the calipers46along the known inner circumference of the second calibration tool66may be taken as a second reference point. The two reference points may then be used to develop a relationship (e.g., linear equations, quadratic equations, gain, offset, etc.) between the position of the calipers46and the inner circumference the calipers46are contacting. In this manner, the calipers46may be calibrated relative to the known inner circumferences of the first calibration tool64and the second calibration tool66before being lowered into the wellbore26. In the depicted embodiment, the inner diameter of the first calibration tool64is different than the inner diameter of the second calibration tool66. It should be noted that any desirable number of reference points may be taken using 2, 3, 4, 5, or more calibration tools52.

Having now described how the calibration tool52may be used to calibrate the calipers46of the downhole tool40, a more detailed discussion of the calibration tool52is provided.FIG. 3is a perspective view an embodiment of the calibration tool52engaging with the downhole tool40. The calibration tool52includes a substantially cylindrical calibration ring68. That is, the calibration ring68is cylindrically shaped within machining tolerances. In other embodiments, the calibration ring68may have any desirable shape that, when axially centered about the tool axis54, is the same distance from the downhole tool40in the directions of the calipers46. The calibration ring68has an elongated body with a first end70and a second end72. The calibration ring68may be at least long enough to cover the pads60of the calipers46. In other words, the calibration ring68may be long enough to engage the full length of the pads60when the calipers46are actuated into the open position58. Additionally, the calibration ring68may have apertures74disposed along the elongated body of the calibration ring68. These apertures74may enable an operator working with the calibration ring68to ensure proper alignment of the pads60with an inner circumference76of the calibration ring68. For example, the operator may be able to look through the apertures74and see the calipers46within the calibration tool52. By being able to see the calipers46, the operator may be able to position the calibration tool52in a manner that ensures the pads60of the calipers46contact the calibration ring68when moved to the open position58. In addition, the apertures may reduce the overall weight of the calibration tool52.

In the illustrated embodiment, the calibration tool52is equipped with fingers78coupled to the calibration ring68at the first end70and the second end72. While the embodiment shows a plurality of four fingers78on the first end70, it is understood that more or fewer fingers78may be used. For example, the calibration tool52may have six fingers78(e.g., three on the first end70and three on the second end72). In the illustrated embodiment, the fingers78on the first end70and the second end72are substantially aligned with each other circumferentially about the tool axis54. However, in other embodiments, the fingers78on the first end70may not be aligned with the fingers78on the second end72. For example, other tools disposed on or coupled with the downhole tool40may inhibit circumferential alignment of each of the fingers78. The fingers78are coupled to the calibration tool52by finger mounts80. The finger mounts80may be mounting brackets rigidly coupled to the calibration ring68. As will be appreciated, there may be at least as many finger mounts80as there are fingers78.

To secure the fingers78, the finger mounts80include several holes in the illustrated embodiment. For example, each of the fingers78may be rotatably coupled to the corresponding finger mount80at a finger coupling hole82. The finger coupling hole82may extend through the finger mount80and be aligned with a corresponding mounting hole84of the finger78when the finger78is in a finger slot86. The finger coupling hole82has a finger axis88running through it. The finger axis88is substantially perpendicular (e.g., plus or minus approximately five degrees) to the tool axis54. A bolt, pin, or other fastener may be inserted through the finger coupling hole82and the mounting hole84to couple the finger78to the finger mount80.

In some embodiments, the fingers78are rotatably coupled to the finger mounts80such that they are able to rotate about the finger axis88. In some embodiments, the fingers78may be configured to rotate approximately 270 degrees about the finger axis88. That is, the fingers78may rotate about the finger axis88between a position where the fingers78contact the elongated body of the calibration ring68and a position where the fingers contact a face90of the first end70or the second end72. However, in some embodiments stopping mechanisms may be included in the finger mounts80to hinder rotation of the fingers78past a designated point.

Rotation of the fingers78about the finger axis88enables the fingers78to transition between a first position92and a second position94. In the first position92, the fingers78are engaged with the downhole tool40. In other words, the fingers78extend radially inward toward the tool axis54and are substantially perpendicular (e.g., within approximately 10 degrees) to the tool axis54when in the first position92. Additionally, the finger78may include a latching hole96that is aligned with a first position coupling hole98of the finger mount80when the finger78is in the first position92. As shown in the depicted embodiment, the first position coupling hole98is positioned radially inward toward the tool axis54relative to the finger coupling hole82, and is configured to align with the latching hole96when the finger78is in the first position92. The spacing of the first position coupling hole98may be designed to ensure that the finger78is not secured into the first position92prematurely. That is, the first position coupling hole98may be positioned on the finger mount80to prevent alignment with the latching hole96when the finger78is not in the first position92. A finger locking mechanism100(e.g., a pin, bolt, or other fastener configured to secure an object in place) may be inserted through the latching hole96and first position coupling hole98to secure the finger78to the finger mount80when the finger78is in the first position92. However, other mechanisms may be used to secure the finger78to the finger mount80. For example, the finger mount80may have a slot or notch formed in the side that aligns with a peg on the finger78when the finger78is in the first position92.

Returning to the rotation of the fingers78about the finger axis88, the finger78is in the second position94when the fingers78are not engaged with the downhole tool40. In the second position94, the fingers78may be substantially parallel (e.g., plus or minus approximately 5 degrees) to the tool axis54. In some embodiments, the second position94may be any position where the fingers78do not extend inward toward the tool axis54. Moreover, a second position coupling hole102of the finger mount80may align with the latching hole96of the finger78when the finger78is in the second position94. As shown in the depicted embodiment, the second position coupling hole102is laterally disposed from the finger coupling hole82in a direction substantially parallel to the tool axis54. The spacing of the second position coupling hole102may be designed to ensure that the finger78is not secured into the second position94prematurely. That is, the second position coupling hole102may be positioned on the finger mount80to prevent alignment with the latching hole96when the finger78is not in the second position94. The finger locking mechanism100may be inserted through the latching hole96and second position coupling hole102to secure the finger78to the finger mount80when the finger78is in the second position94. However, other mechanisms may be used to secure the finger78to the finger mount80. For example, the finger mount80may have a slot or notch formed in the side that aligns with a peg on the finger78when the finger78is in the second position94. In some embodiments, the second position94may be used when storing the calibration tool52, moving the calibration tool52between different measurement positions, and/or inserting the calibration tool52over an end of the downhole tool40.

As previously mentioned, the fingers78are configured to engage the downhole tool40when in the first position92. To that end, the downhole tool40may include an engagement feature104configured to receive the finger78. In other words, the finger78and the downhole tool40are configured to be coupled together via the engagement feature104. In some embodiments, the engagement feature104may utilize a finger locking mechanism (e.g., similar to the finger locking mechanism100described above) to secure the finger78to the engagement feature104. For example, in the disclosed embodiment, the finger78includes a locking hole106that aligns with a securing hole108of the engagement feature104when the finger78is in the first position92. As mentioned above, the finger locking mechanism100may be inserted through the locking hole106and the securing hole108to couple the finger78to the engagement feature104. However, in other embodiments, the structure of the engagement feature104may secure the finger78in place. For example, the engagement feature104may include a groove, a slot, or indentation configured to receive a complementary feature of the finger78. Or, the engagement feature104may include a compressible stop that may be compressed into the downhole tool40while the finger78is moving into the first position92and returned to its original position to secure the finger78into the first position92once the finger78passes over the compressible stop. It is understood that a variety of techniques may be used to securely enable the finger78to engage with the engagement feature104. By securing the fingers78to the engagement feature104, the calibration ring68may be fully supported by the fingers78. That is, the weight of the calibration tool52may be supported by the fingers78and not rely on a pad pressure from the calipers46. Additionally, by engaging with the downhole tool40, the calibration tool52can be aligned along the tool axis54when using fingers78with equal lengths.

As described above, each finger78is configured to engage with an engagement feature104of the downhole tool40. In some embodiments, each of the engagement features104of the downhole tool40may correspond interchangeably with each finger78of the calibration tool52. However, in other embodiments, specific fingers78of the calibration tool52may align with specific engagement features104of the downhole tool40, in order to ensure proper placement of the calibration tool52. Moreover, the engagement features104may be designed to fit fingers78of multiple calibration tools52having different inner diameters. For example, the fingers78of the first calibration tool64and the fingers78of the second calibration tool66illustrated inFIG. 2may be designed to engage with the same engagement features104. Like the fingers78, the engagement features104are arranged circumferentially about the downhole tool40. The spacing of the engagement features104may be dependent on a variety of design conditions. For example, the engagement features104may be arranged to avoid interference with other equipment on or coupled to the downhole tool40. Additionally, the engagement features104may be installed at specific locations to enable the pads to contact the inner circumference76of the calibration tool52. As will be appreciated, the locations of the engagement features104may correspond to the locations of the fingers78.

The fingers78enable the alignment of the tool axis54with the calibration tool axis56without using pad pressure from the calipers46to support the weight of the calibration tool52. That is, the fingers78are able to support the calibration ring68in alignment with the tool axis53even when the calipers46are not actuated into the open position58. Turning toFIG. 4, a front view of an embodiment of the calibration tool52is shown. As previously mentioned, the fingers78are coupled to the calibration ring68by finger mounts80. In the depicted embodiment, the fingers78are circumferentially spaced equally about the calibration ring68. However, the fingers78may not be equally spaced in other embodiments. For example, more fingers78may be placed on the upper half110of the calibration ring68to support the weight of the calibration tool52when the calibration tool52is mounted to the downhole tool40.

A process flow diagram of an embodiment of a method112for calibrating calipers46on the downhole tool40is shown inFIG. 5. The method112includes disposing (block114) the first calibration tool64over the downhole tool40. The method112also includes coupling (block116) the first calibration tool64to the downhole tool40. This may involve engaging one or more fingers78with the engagement features104to couple the first calibration tool64to the downhole tool40. The first calibration tool64and downhole tool40are coaxial about the tool axis54when the finger78engages with the engagement feature104. In other words, the first calibration tool64is aligned with the tool axis54of the downhole tool40when the calibration ring68is mounted to the downhole tool40. In addition, the method112includes actuating (block118) the caliper46to contact an inner circumference of the first calibration tool64. As a result, the position of the caliper46relative to the downhole tool40may be known for a given circumference.

To obtain another set of calibration data for an additional calibration tool52, the first calibration tool64may be removed (block120) from the downhole tool40. Then, the second calibration tool66may be disposed (block122) over the downhole tool40. Similarly to the first calibration tool64, the second calibration tool66may be coupled (block124) to the downhole tool40using one or more fingers78. The fingers78may engage with the downhole tool40and coaxially align the downhole tool40and the second calibration tool66. In addition, the method112may include actuating (block126) the caliper46to contact an inner circumference of the second calibration tool66. The second reference point obtained may be combined with the reference point obtained utilizing the method112to compute and extrapolate a calibration equation for future downhole measurements.

As discussed in detail above, the disclosed embodiments include the calibration system50for calibrating calipers46on the downhole tool40. Specifically, the calibration system50includes the calibration tool52, which is configured to engage with the downhole tool40via fingers78. In this manner, the calibration tool52and the downhole tool40may be aligned about the tool axis54with the fingers78supporting the weight of the calibration tool52. Thereafter, the calipers46of the downhole tool40can be actuated to contact the inner diameter of the calibration tool52. The position of the calipers46can be recorded with the known inner circumference76, generating a reference point for future measurements when the downhole tool40is lowered into the wellbore. A number of calibration tools52with different inner circumferences76may be utilized to generate a function of caliper position compared to diameter.