System and method for deploying a downhole casing patch

A casing patch and methods for using same are provided. The patch can include a hollow, substantially tubular body. An opening can be formed in the body. A tapered slot can be formed in the body below the opening. A width of the tapered slot proximate the opening can be greater than the width of the tapered slot distal the opening. The tapered slot can be adapted to receive a tapered wedge and expand radially outward as the tapered wedge slides within the tapered slot and away from the opening.

BACKGROUND

The present disclosure relates generally to a system and method for deploying a downhole casing patch.

Oil and gas wells are ordinarily completed by cementing metallic casing strings in the wellbore. During the drilling, completion and production phase, operators may find it necessary to perform remedial work, repair and maintenance to the casing. For example, the casing is commonly perforated using an explosive charge to evaluate various formations. In addition to the intended perforations, unintentional holes or defects may also be created in the casing. This can allow a leak to develop in the casing permitting the loss of well fluids to a low pressure, porous zone outside the casing, or permit an unwanted formation fluid, such as water, to enter the well. Regardless of the specific application, it is often necessary to deploy a patch to a downhole casing to seal the wellbore from the external formation.

Numerous methods have been developed over the years to deploy patches in casing. One method includes coating a longitudinally corrugated liner with a thin layer of epoxy resin (or other cementing material) and a glass fiber cloth prior to deployment in the wellbore. The coated liner is run into the wellbore (to the damaged area) on a tubing string and then expanded against the casing by forcing an expander device (e.g., a cone) through the liner. While this methodology has been commercially utilized, application of the epoxy resin can be problematic. For example, engagement of the coated liner with the wellbore wall (especially in deviated wells) can cause a loss of the epoxy resin and fiber materials during deployment. Such loss tends to result in an inadequate seal between the patch and the casing. Moreover, the cure cycle of the epoxy begins when mixing is complete. As such, any delay during deployment of the patch can result in premature curing of the epoxy.

Another method includes a metallic tubular that is hydraulically or mechanically expanded into contact with the casing to create a mechanical seal that relies on the contact stress between the expanded tubular and the casing. The metallic tubular is made of a highly compliant material to improve the contact resistance and therefore better seal the damaged section. This tends to require large pressures to expand the tubular and a tubular patch fabricated from an expensive alloy to obtain an effective seal.

Swage style patches are also known in the art and make use of hydraulically or mechanically deformable swages to seal the upper and lower ends of the patch. A conventional threaded tubular patch is deployed between and coupled with the swages. The damaged section is thereby straddled and isolated by the swages and tubular. While swage style patches provide an effective seal, they also tend to create a restriction in the wellbore, since the tubular patch is not expanded.

Epoxy only patches are also known in the art and make use of an epoxy resin that is pumped downhole to the damaged section. After curing, the wellbore is re-drilled to remove any excess epoxy. While such patches are sometimes effective, they rely only on the properties of the epoxy for their strength. As such, the epoxy-only patch is typically ineffective at high pressures.

There remains a need in the art, therefore, for new casing patches and methods for deploying patches in a subterranean cased wellbore.

SUMMARY

Systems and methods for repairing a casing in a wellbore are provided. The system can include a hollow, substantially tubular body. An opening can be formed in the body. A tapered slot can be formed in the body below the opening. A width of the tapered slot proximate the opening can be greater than the width of the tapered slot distal the opening. The tapered slot can be adapted to receive a tapered wedge and to expand radially outward as the tapered wedge slides within the tapered slot and away from the opening.

The method can include running a patch into a wellbore. The patch can include a hollow, substantially tubular body. An opening can be formed in the body, and a tapered slot can be formed in the body below the opening. A width of the tapered slot proximate the opening can be greater than the width of the tapered slot distal the opening. At least a portion of the patch can be anchored to an inner surface of the casing with an anchoring tool disposed at least partially within the patch. The patch can be expanded radially outward with an expansion tool after the portion of the patch has been anchored to the inner surface of the casing.

DETAILED DESCRIPTION

FIG. 1depicts a cased wellbore40having a tool string200disposed therein, according to one or more embodiments. The wellbore40can be disposed proximate a subterranean oil or gas formation. The wellbore40can be at least partially cased with one or more casing strings or casings50. The casing50can include a defect52(e.g., a perforation, crack, and/or hole) that requires patching. Accordingly, a tool string200can be lowered from a rig20and into the wellbore40. The tool string200can include a substantially tubular patch configured to repair or seal the defect52in the casing50.

FIG. 2depicts an illustrative method100for patching the defect52in the casing50, according to one or more embodiments. The method100is described with reference to the tool string200depicted in FIGS.1and3A-3C. A tubular patch300can be disposed on, in, and/or around the tool string200and positioned in the casing50proximate the defect52, as shown at102. The patch300can be anchored to an inner surface of the casing50, for example, using an anchoring tool240, as shown at104.

Once anchored, the patch300can be expanded into contact with the inner surface of the casing50. For example, an expansion tool350can be traversed or pulled in the uphole direction through the patch300, as shown at108. As used herein, the term “uphole” refers to a direction that is toward the surface and/or the rig20, or a position that is closer to the surface and/or the rig20than another position. The term “downhole” refers to a direction that is away from the surface and/or the rig20, or a position that is within the cased wellbore40, i.e., below the Earth's surface.

In one or more embodiments, a sealant or adhesive material, such as an epoxy resin for example, can be used to provide a better seal or adherence between the patch300and the casing50. The sealant or adhesive material can be applied to the patch300before the patch300is lowered into the wellbore50. Alternatively, the sealant or adhesive material can be applied to the patch300after it has been located in the wellbore50. For example, the sealant or adhesive material can be injected between an outer surface of the patch300and the inner surface of the casing50using an injection tool270, as shown at106. In the case of an epoxy resin, the epoxy resin can be mixed with a hardener downhole to form the adhesive mixture after the patch300is located in the casing50. The epoxy resin and the hardener can be mixed together simultaneously with or subsequent to the patch300being anchored to the inner surface of the casing50.

FIGS. 3depicts a cross-sectional view of an illustrative tool string200(FIG. 1) and patch300, according to one or more embodiments. Portions of the illustrative tool string and patch shown inFIG. 3are also illustrated in the cross-sectional views ofFIGS. 3A-1to3C-2. The tool string200can include a ball seat assembly220, anchoring assembly240, expansion assembly350, and optionally an injection assembly270. The expansion tool350can be disposed above and threadably coupled to the ball seat assembly220. Then anchoring assembly240can be disposed above and threadably coupled to the expansion tool350. If needed, the injection tool270can be disposed above and threadably coupled to the anchoring tool240. As used herein, the terms “couple,” “coupled,” “connect,” “connection,” “connected,” “in connection with,” and “connecting” refer to “in direct connection with” or “in connection with via another element or member.” The terms “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; “upstream” and “downstream”; “above” and “below”; and other like terms as used herein refer to relative positions to one another and are not intended to denote a particular direction or spatial orientation.

FIG. 4depicts a cross-sectional view of an illustrative ball seat assembly or tool220, according to one or more embodiments. The ball seat assembly220can include a housing or body222having threaded ends. A ball seat224can be disposed in the housing222and secured in place with one or more shear screws226. In at least one embodiment, the ball seat224can be shaped and sized to accommodate a ball or other sealing mechanism228. For example, the ball seat224can be curved or frustoconical and have an aperture227formed therethrough. The ball or sealing mechanism228can provide a seal against the aperture227to prevent fluid flow in at least one direction through the assembly220. The ball or sealing mechanism228can be made of any suitable material. In one or more embodiment, the ball or sealing mechanism228can be a steel ball, a thermoplastic ball, a dart, or the like.

After the patch300has been positioned proximate the defect52in the casing50(e.g., step102in method100), the ball228can be dropped from the surface and engage the ball seat224to prevent fluid flow in at least one direction therethrough. In deviated wellbores, gravitational force alone may not be sufficient to move the ball228from the surface and into engagement with ball seat224. As such, in deviated wellbores, a liquid, such as a drilling fluid, can be introduced or injected into the wellbore40to force the ball228deeper into the wellbore40(e.g., along the horizontal section of the wellbore40) and into contact with the ball seat224.

Once the ball228is located within the seat224, hydraulic pressure can build within the internal bore of the tool string200. Once the internal hydraulic pressure reaches a predetermined level, the anchoring tool240and/or the injection tool270can actuate, as described in more detail below. Upon completion of the anchoring and injection steps (e.g., steps104and106in method100), the hydraulic pressure in the internal bore of the tool string200can be increased until the shear screw226shears or breaks, allowing the ball seat224to drop and reestablishing fluid flow through ball seat assembly220.

Considering the patch300in more detail,FIG. 5Adepicts a cross-sectional view of a portion of the patch300, according to one or more embodiments. The patch300can include a substantially tubular, thin-walled (i.e., hollow) body302disposed around a portion of the tool string200. The patch300can be made from a metal. In at least one embodiment, the patch300can be made from steel or stainless steel. The patch300can have a length ranging from a low of about 1 m, about 2 m, or about 3 m to a high of about 6 m, about 8 m, about 10 m, or more. The patch300can have a cross-sectional length, e.g., diameter, ranging from a low of about 10 cm, about 15 cm, or about 20 cm to a high of about 30 cm, about 40 cm, about 50 cm, or more.

The patch300can have one or more expansion relief windows or openings (one is shown306) formed therein. The opening306can reduce the stress on the patch body302when the patch300is expanded radially outward, e.g, during anchoring step104. The opening306can be substantially rectangular having a width measured along the circumference of the patch body302and a length measured in the axial direction. A ratio of the width of the opening306to the diameter of the patch body302can range from a low of about 0.1:1, about 0.2:1, or about 0.4:1 to a high of about 0.6:1, about 0.8:1, about 1:1, or more. For example, the width of the opening306can range from a low of about 5 cm, about 10 cm, or about 15 cm to a high of about 20 cm, about 30 cm, about 40 cm, or more. A ratio of the length of the opening306to the diameter of the patch body302can range from a low of about 0.5:1, about 1:1, about 2:1, or about 3:1 to a high of about 4:1, about 6:1, about 8:1, about 10:1, or more. For example, the length of the opening306can range from a low of about 20 cm, about 40 cm, about 60 cm, about 80 cm, or about 1 m to a high of about 1.2 m, about 1.4 m, about 1.6 m, about 1.8 m, about 2 m, or more.

A tapered (V-shaped) slot308can be formed in the patch300proximate a lower end of the opening306. As shown, the tapered slot308can be in communication with the opening306. A width of the tapered slot308, as measured along the circumference of the patch body302, proximate the opening306can be greater than the width of the tapered slot308distal the opening306. For example, the sides of the tapered slot308can be oriented at an angle with respect to a longitudinal center line through the patch body302ranging from a low of about 1°, about 2°, about 4°, about 6°, about 8°, or about 10° to a high of about 15°, about 20°, about 25°, about 30°, about 35°, about 40°, about 45°, or more. The tapered slot308can be adapted to receive a tapered wedge320, as described in more detail below.

The patch body302can also include one or more protrusions or upsets305formed below the opening306and/or proximate the tapered slot308. The upsets305can extend radially outward from the patch body302to increase the contact stress or force between the patch body302and the inner surface of the casing50(FIG. 1). For example, each upset305can extend radially outward beyond the outer surface of the patch body302by about 0.5 mm, about 1 mm, about 2 mm, about 3 mm, about 4 mm, about 5 mm, about 6 mm, about 8 mm, about 10 mm, or more. Each upset305can have a height or axial length ranging from about 1 mm, about 2 mm, about 5 mm, or about 1 cm to about 2 cm, about 4 cm, about 6 cm, about 8 cm, about 10 cm, or more. When two or more upsets305are used, the axial spacing between the upsets305can range from about 1 mm, about 2 mm, about 5 mm, or about 1 cm to about 2 cm, about 4 cm, about 6 cm, about 8 cm, about 10 cm, or more. By reducing the surface area in contact with the inner surface of the casing50, the upsets305can increase the contact stress or force between the patch body302and the inner surface of the casing50. Accordingly, the upsets305can improve the anchoring ability of the patch body302within the casing50.

A ring or web312can be formed proximate the lower end of the tapered slot308. The ring312can be a portion of the patch body302that extends, at least partially, around the circumference of the body302. As such, the ring312can prevent the patch body302from prematurely expanding during deployment in the wellbore40. The ring312can have a height or axial length ranging from a low of about 0.5 cm, about 1 cm, or about 2 cm to a high of about 4 cm, about 6 cm, about 8 cm, or more.

The patch body302can also include a plurality of ports314formed above the expansion opening306through which adhesive may be injected (e.g., during injection step106—FIG. 2). Any number of ports314can be used. The ports314can be circumferentially and/or axially spaced apart around the patch body302. In at least one embodiment, a resilient barrier cup304, e.g., formed of a thin metallic material, can be at least partially disposed about the patch body302and below the injection ports314. The barrier cup304can form a seal with an inner surface of the casing50to prevent injected adhesive from traveling in the downhole direction through the annulus towards the opening306. Rather than a barrier cup304, an extension (see303inFIG. 10B) can be formed in the patch body302below the injection ports314.

FIG. 5Bdepicts a perspective view of an illustrative tapered locking wedge320, according to one or more embodiments. The tapered wedge320can be located or disposed within the tapered slot308and adapted to help anchor the patch300against the casing50, as described in more detail below. The tapered wedge320can be made from a metal, such as a hardened steel alloy and have a radius of curvature to match the patch body302. As such, the tapered wedge320can be adapted to slide axially within the tapered slot308. And as with the tapered slot308, the width of the tapered wedge320can decrease in the downhole direction. For example, the sides of the tapered wedge320can be oriented at an angle with respect to a longitudinal center line through the patch body302ranging from a low of about 1°, about 2°, about 4°, about 6°, about 8°, or about 10° to a high of about 15°, about 20°, about 25°, about 30°, about 35°, about 40°, about 45°, or more. In at least one embodiment, the sides of the tapered wedge320can have a profile adapted to engage the sides of the tapered slot308.

The axially-extending sides of the tapered slot308and the axially-extending sides of the tapered wedge320can each have a helical profile. In other words, when the tapered wedge320is engaged with the tapered slot308, the upper or uphole end of the tapered slot308can be disposed radially outward from the lower or downhole end of the tapered slot308with respect to a longitudinal center line through the body302. Similarly, the upper or uphole end of the tapered wedge320can be disposed radially outward from the lower or downhole end of the tapered wedge320with respect to the longitudinal center line through the body302. Accordingly, the helical profile of the tapered slot308and the tapered wedge320can cause the force between axially extending sides of the tapered slot308and the tapered wedge320to be circumferential.

The axially-extending sides of the tapered wedge320can also include a groove322adapted to receive a protrusion formed in the sides of the tapered slot308, or vice versa. However, as may be appreciated, the axially-extending sides of the tapered slot308and tapered wedge320can be formed in any manner to form a track to prevent the tapered wedge320from becoming disengaged with the tapered slot308as the tapered wedge320slides therein.

The tapered wedge320can further include a plurality of holes324through which the wedge320can be coupled to the anchoring tool240. For example, one or more shear screws253(shown inFIG. 6B) can be disposed through the holes324to couple the tapered wedge320to the anchoring tool240, as described in more detail below.

The tapered wedge320can also include a plurality of wickers or teeth325formed in the outer surface thereof. The wickers325can be adapted to engage (bite) the inner surface of the casing50to prevent axial motion of tapered wedge320in the uphole direction (e.g., during expansion step108inFIG. 2). The wickers325can extend radially outward from the tapered wedge320by about 0.1 mm, about 0.2 mm, about 0.5 mm, or about 1 mm to about 2 mm, about 3 mm, about 4 mm, about 5 mm, or more.

When the patch300is disposed adjacent the defect52in the casing50, the anchoring tool240can move the tapered wedge320downward in the tapered slot308. As the tapered wedge320moves downward, the portion of the patch300, i.e., patch body302, proximate the tapered slot308can expand radially outward and contact the casing50. For example, the upsets305can contact the casing50. The contact between the patch300and the casing50can anchor the patch300in place, thereby substantially preventing axial movement of the patch300with respect to the casing50. Any slippage of the patch300in the uphole direction can drive the tapered wedge320deeper into the tapered slot308, thereby increasing the tangential force that secures the patch300in the casing50. Once a predetermined downward force has been applied to the tapered wedge320(anchoring the patch300in the casing50), the shear screws253can shear or break, releasing or decoupling the patch300and the tapered wedge320from the anchoring tool240. The tool string200(including the expansion tool350) can then be pulled upward toward the surface. As the expansion tool350moves upward through the patch300, it can expand the patch300radially outward and into contact with the casing50, as described in more detail below.

FIG. 6Adepicts a perspective view of an illustrative anchoring tool240, andFIG. 6Bdepicts a cross-sectional view of the anchoring tool240, according to one or more embodiments. The anchoring tool240can be sized and shaped to be disposed in the interior of patch300, as depicted inFIG. 3. A first “main” piston250and a second “locking” piston260can be disposed around a piston rod246. An upper end portion of the piston rod246can be threadably engaged with an upper mandrel242, which can be coupled to the injection tool270. A lower end portion248of the piston rod246can be coupled to the expansion tool350. The main piston250can also be coupled to a wedge carrier252. The wedge carrier252can be coupled to the tapered wedge320(seeFIG. 5B) via one or more shear screws253.

Hydraulic pressure can be communicated to surfaces254,262of the corresponding main and locking pistons250,260through one or more radial bores247formed in the piston rod246. Prior to hydraulic activation, an outer surface of a dog264can be substantially flush with an outer surface of a cylindrical sleeve244. As the pressure increases, the locking piston260can be urged upward, thereby moving the dog264up a ramp265. Movement of the dog264up the ramp265can cause the dog264to engage the patch body302in the opening306. Such engagement can prevent subsequent axial movement of the patch body302in the downhole direction when the tapered wedge320is driven into the tapered slot308. Increasing hydraulic pressure can also urge the main piston250against a shear screw255. At a predetermined hydraulic pressure, the screw255can break or shear, thereby allowing downhole movement of the main piston250and the wedge carrier252relative to the piston rod246. Such movement of the main piston250can urge the tapered wedge320into the tapered slot308. The wedge carrier252can move in a radial direction as the main piston250urges the tapered wedge320into the tapered slot308. Radial movement of the tapered wedge320can allow it to follow the expansion of patch body302caused by the wedging action.

The anchoring tool240can further include a fixed cone268coupled to the piston rod246. The cone268can be sized and shaped to provide a preliminary expansion (e.g., about 50% of the total expansion) of the patch body302as the tool string200is drawn uphole during expansion step108. Such a preliminary expansion can reduce the force requirements of expansion tool350during the subsequent expansion.

In at least one embodiment, the anchoring tool240can be spring actuated. U.S. Pat. No. 7,428,928 discloses a spring actuated anchoring tool. This application is incorporated herein by reference in its entirety to the extent consistent with the present disclosure.

FIG. 7depicts a cross-sectional view of an illustrative injection tool270, according to one or more embodiments. Portions of the illustrative injection tool270are also illustrated in the cross-sectional views ofFIGS. 7A-1,7A-2, and7B. As shown, the injection tool270can include plurality of moving pistons or tubes. For example, the injection tool270can include a main piston272, inner push tube274, and outer push tube276. Increasing hydraulic pressure can urge the main piston272in the downhole direction and into contact with the inner push tube274and the outer push tube276. The inner push tube274can engage an epoxy resin piston278, and the outer push tube276can engage a hardener piston280, or vice versa. As such, the push tubes274,276can increase the pressure of the epoxy resin and hardener disposed in corresponding chambers282,284.

At a substantially predetermined hydraulic pressure, one or more burst discs285can rupture allowing the epoxy resin and hardener to flow into a static mixing chamber290. The static mixing chamber290can include a number of tortuous elements292that alter the direction of fluid flow which causes the epoxy resin and hardener to intermingle and form an adhesive mixture. The adhesive mixture can exit the mixing chamber290through ports295(and ports314of patch body302ofFIG. 5A) into the annular region between the patch body302and the casing50. A barrier cup298can, at least partially, prevent migration of the mixture between the injection tool270and the patch body302. As described above, a barrier cup304(or extension303) can also be disposed around the patch body302(as depicted inFIG. 5A) to substantially prevent the adhesive mixture from migrating in the downhole direction toward the opening306. The injection tool270can use substantially any suitable formulation of epoxy resin/hardener. For example, the epoxy resin and hardener can be mixed in a two-to-one volume ratio.

FIG. 8depicts a cross-sectional view of an illustrative expansion tool350, according to one or more embodiments. Portions of the illustrative expansion tool350are also illustrated in the cross-sectional views ofFIGS. 8A-1,8A-2, and8B. The expansion tool350can include a mandrel352disposed within a collet cone354, a collet356, and a spring subassembly360. The spring subassembly360can include a Belleville spring stack362disposed between upper and lower washers364and365. The spring stack362can be biased (i.e., compressed) to provide a predetermined axial force urging the collet356in the uphole direction. The collet356can include a plurality of circumferentially spaced fingers that ride up on the collet cone354into contact with a shoulder355of the cone354.

During the expansion step108(FIG. 2), both the shoulder355of the collet cone354and the collet356can provide additional expansion of the patch body302. The collet cone354and collet356can be sized and shaped so as to mechanically expand the patch300radially outward and into contact (or near contact) with the inner surface of the casing50as the tool string200is drawn uphole. Maximum expansion can be provided when the collet356is urged in the uphole direction into contact with the shoulder355. The spring stack362can provide a compliant mechanism that allows the collet356to move axially downhole (at a predetermined force) and the fingers to move radially inward should the expansion tool350encounter irregularities in the installed casing50(e.g., debris or a casing collar). Such axial and radial motion is intended to minimize the likelihood of the tool350becoming stuck in the casing50during the expansion step.

FIG. 9depicts a cross-sectional view of another illustrative expansion tool370, according to one or more embodiments. Portions of the illustrative expansion tool370are also illustrated in the cross-sectional views ofFIGS. 9A-1,9A-2, and8B. The expansion tool370can include a plurality of circumferentially spaced flex segments372disposed between an upper retainer374and a lower retainer375and around a flex segment cone376. The spring stack362can be biased to provide an axial force that urges the flex segments372in the uphole direction such that they ride up on the cone376and expand the patch body302as the tool string200is drawn uphole. The spring stack362can also provide a compliant mechanism that allows the flex segments372to move axially downhole and radially inward should the expansion tool370encounter irregularities in the installed casing50. The flex segments372and flex segment cone376can be sized and shaped so as to mechanically expand the patch300into contact (or near contact) with the inner surface of the casing50.

FIGS. 10A and 10Bdepict cross-sectional views of an illustrative bulger assembly400before and after actuation, according to one or more embodiments. The bulger assembly400can replace the upper mandrel242in the anchoring tool240(seeFIG. 6B) and form or create a seal between the patch body302and the inner surface of the casing50. The bulger assembly400can include uphole and downhole body portions405,410. An axial piston420can be disposed between the body portions405,410and engage a bulger element425. The bulger element425can be fabricated from a resilient material, such as a nitrile rubber, suitable for use in the downhole environment. First and second extrusion rings427,428can be disposed about the bulger element425. The extrusion rings427,428can have an L-shaped cross-section and be fabricated from a low yield, highly ductile material such as brass. The bulger assembly400can be disposed at substantially any suitable location axially between injection port314and opening306of the patch300.

During operation, hydraulic pressure can be communicated to the surface408of axial piston420through one or more radial bores407formed in body portion405. As the pressure increases, the axial piston420can be urged uphole, thereby compressing the element425between the extrusion rings427,428. The element425can buckle radially outward into contact with the patch body302, thereby deforming the patch body302radially outward into the inner surface of the casing, forming an extension303, as best illustrated inFIG. 10B. The extrusion rings427,428can also deform outward into contact with the patch body302and substantially prevent axial extrusion of the element425into the annular region on the inside of the patch body302. The diameter of the patch body302in the extension region can be increased by about 1 cm, about 2 cm, about 3 cm, about 4 cm, about 5 cm, or more. Further, the extension303can have a height or axial length ranging from a low of about 1 cm, about 2 cm, about 3 cm, about 4 cm, about 5 cm, or more. The extension303can sealingly engage the inner surface of the casing50to substantially prevent injected epoxy from migrating in the downhole direction through the annulus towards expansion opening306.

Various terms have been defined above. To the extent a term used in a claim is not defined above, it should be given the broadest definition those in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Furthermore, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions in which such incorporation is permitted.

While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the invention can be devised without departing from the basic scope thereof. Accordingly, such other and further embodiments are intended to be included in the scope of this disclosure.