FIELD OF INVENTION

The present invention relates to the transport of a fluid, such as cement, through a downhole tool or assembly.

BACKGROUND TO INVENTION

In the oil and gas exploration and production industry, well boreholes are drilled from surface in order to access subsurface hydrocarbon-bearing formations. A tubular string, such as a completion string, may be run into the borehole and the annulus between the outside of the string filled with a fluid, such as cement.

Cementing operations may be required at various instances during the life cycle of a well, for example to assist in securing and supporting the string in the borehole during its initial completion, to prevent uncontrolled migration of fluid in the annulus, to provide isolation of one or more formation zone during production, and/or during remedial or workover operations.

Completion strings beneficially provide the ability to perform a number of different operations in the borehole, the various completion string tools utilising a variety of activation mechanisms and systems.

However, since many completion string tools extend into or require access to an axial throughbore or flow passage of the string, these tools and associated equipment represent an obstruction to the passage of cement. As such, the ability to direct cement to the desired location may be impaired or prevented.

Moreover, there is a significant drive to improve the effectiveness and reliability of tools which are deployed and operated in a downhole environment, for example to ensure that the tools operate at maximum efficiency, have minimum risk of failure or imprecise operation, can be flexible according to operator requirements, and minimise any necessary remedial action, associated time delays and costs. Thus, it is important also that substantially all of the cement is directed through the string, such that the subsequent efficient operation of the tools and downhole equipment is not impaired.

SUMMARY OF INVENTION

Aspects of the present invention relate to apparatus and methods for use in transporting a fluid, such as cement, through a downhole tool or assembly. In particular, but not exclusively, embodiments of the invention relate to methods and apparatus for transporting cement through a downhole tool used in the treatment, for example fracturing, of a subterranean formation.

According to a first aspect of the present invention there is provided a downhole cementing apparatus deployable through a downhole tool with a volume of cement, comprising:

a body; and

a sealing arrangement mounted on the body and configured to define a varying point of sealing contact axially along the sealing arrangement with an inner surface of a downhole tool as the cementing apparatus is deployed through said tool.

Providing a varying point of sealing contact axially along the sealing arrangement may permit a seal to be provided at all times during passage of the apparatus through the downhole tool.

The sealing arrangement may comprise a unitary seal member or structure, wherein said seal member or structure defines a plurality of seal regions of features configured to establish sealing contact with an inner surface of a downhole tool.

The sealing arrangement may comprise at least one seal member for sealingly engaging the inner surface of the downhole tool. At least one seal member may be operable to swab or wipe the inside of the downhole tool during deployment through said tool. The seal members may be spaced so that at least one seal member may provide sealing contact with the inner surface of the downhole tool during deployment through said tool.

The sealing arrangement may comprise a plurality of seal members arranged along the body. The seal members may be spaced so that the seal member employed to provide the sealing contact varies as the apparatus is deployed through the downhole tool. For example, the seal members may be spaced so that where one seal member is adjacent to the portion of the downhole tool which may otherwise result in fluid bypass, at least one other of the seal members is positioned adjacent to a portion of the downhole tool which permits a sealing contact to be provided between the apparatus and the downhole tool, thus ensuring that a sealing contact is provided at all times during passage of the apparatus through the downhole tool. When used in a cementing operation, for example, ensuring that a sealing contact is provided/ maintained may ensure that the cement is efficiently driven through the downhole tool and so eliminate or at least mitigate the possibility that cement will set in the tool and so impair the cementing operation and/or subsequent operation of downhole tools and equipment.

The sealing contact may be formed by a continuous circumferential seal between the seal member and the downhole tool. In use, the apparatus may be configured so that the sealing contact isolates a downstream portion of the downhole tool from an upstream portion of the downhole tool.

The apparatus may isolate fluid located downstream of the sealing contact from the upstream portion of the downhole tool.

In particular embodiments, the downstream fluid may comprise cement to be urged through the downhole tool. Additionally or alternatively, the downstream fluid may comprise a driving fluid, such as glycol, water, drilling mud or other suitable driving fluid. Additionally or alternatively, the downstream fluid may comprise an inhibitor such as sugar grease or the like. Beneficially, providing sugar grease may prevent or inhibit adherence of cement to the downhole tool or other downhole equipment. Thus, an apparatus according to the present invention may be disposed behind a column of fluid and operable to drive the fluid through the downhole tool.

Any suitable means for deploying the apparatus through the downhole tool may be used. For example, the apparatus may be deployed or urged through the downhole tool by a fluid. In particular embodiments, the apparatus may be deployed or urged through the downhole tool by a fluid pressure, for example a fluid pressure acting across the sealing contact. The apparatus may be configured for deployment through the downhole tool by an upstream fluid. The upstream fluid may comprise a driving fluid, such as glycol, water, drilling mud or other suitable driving fluid. Additionally or alternatively, the upstream fluid may comprise an inhibitor such as sugar grease or the like. Alternatively or additionally, the upstream fluid may comprise cement or the like. Thus, in some instances an apparatus according to the present invention may be disposed ahead of a column of fluid, the apparatus being deployed by said fluid.

The apparatus may alternatively or additionally comprise or be coupled to mechanical means for deploying the apparatus through the downhole tool. For example, the apparatus may be coupled to a work string or the like for urging the apparatus through the downhole tool.

The apparatus, and more particularly at least one of the seal members, may be configured to swab or wipe the inside of the downhole tool. For example, the apparatus may be configured to remove any cement deposits from the downhole tool as the apparatus is deployed therethrough.

The seal members may be of any suitable form or construction.

The radial extent of the seal members may be configured such that at a first axial location in the downhole tool one of the seal members cooperates with the downhole tool to provide a sealing contact and at a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact.

The radial extent of the seal member may be configured such that at a first axial location in the downhole tool a first seal member cooperates with the downhole tool to provide a sealing contact, and at a second axial location within the downhole tool the first seal member may not provide sealing contact and a second seal member cooperates with the downhole tool to provide a sealing contact.

In some embodiments, two or more of the seal members may extend radially to the same diameter. In particular embodiments, one or more of the seal members may extend to a different radial extent than at least one other of the seal members. The radial extent of each seal member may be selected according to the downhole tool.

In particular embodiments, one or more of the seal members may be configured to define a cup shape. One or more of the seal members may be configured to define a disc shape.

In particular embodiments, the apparatus may comprise five seal members, although it will be recognised that the apparatus may alternatively comprise two seal members, three seal members, four seal members or more than five seal members.

The seal members may be disposed on a mandrel. Any suitable means for disposing the seal members on the mandrel may be provided. One or more of the seal members may be integrally formed with the mandrel. One or more of the seal members may be secured to the mandrel. One or more of the seal members may be configured to slip over the mandrel. One or more of the seal member may be moulded onto the mandrel. One or more of the seal members may engage the mandrel by an interference fit, may be shrink fitted onto the mandrel.

The mandrel may comprise a unitary component. The mandrel may comprise at least one module.

Alternatively, and in particular embodiments, the mandrel may comprise a plurality of modules. In particular embodiments, a module may be associated with each of the seal members. For example, each of the seal members may be mounted or otherwise provided on its own module. Alternatively, a module may be associated with a plurality of the seal members. At least one module may support at least one seal member. In embodiments where the apparatus comprises seal members of different type or form, for example but not exclusively of different shape or radial extent, a module may be associated with each seal member type. The modules and/or the seal members of each type may comprise an identifier. Providing an identifier may facilitate ready identification of each module and/or seal members or of at least one module or seal member, for assembly or inventory purposes for example.

The apparatus may comprise one or more spacer configured to provide the axial spacing between the seal members. The spacer may comprise a module of the mandrel.

A connection arrangement may be provided for coupling the mandrel to adjacent components and/or for coupling modules of the mandrel together. The connection arrangement may be provided for coupling at least two modules of the mandrel together. The mandrel may comprise at least one spacer for providing an axial spacing between at least two seal members.

The connection arrangement may comprise a female connector.

The connection arrangement may comprise a male connector. In particular embodiments, each module may comprise a male connector and a female connector.

The connection arrangement may comprise a threaded connection, push fit connector or the like.

The downhole tool may be of any suitable form or construction.

The downhole tool may for example comprise a tool for use in treating a subterranean formation. The downhole tool may comprise a tool for use is hydraulic fracturing. Hydraulic fracturing, commonly known as 'Tracking", may involve the injection of fluid into the formation to propagate fractures in the formation rock and increase flow of hydrocarbons into the borehole for extraction. In use, one or more fracturing tools may be run into the borehole and located adjacent to the formation.

Fluid may then be directed through ports in a sidewall of the fracturing tool and injected into the formation. In some instances, a number of fracturing tools may be located at different axially spaced positions in a tubular string and configured to facilitate fracturing of multiple and/or selected formations. The downhole tool may comprise a tool housing defining a central bore and including a fluid port. The fluid port may be configured to permit fluid communication between the central bore and a location external to the housing. The fluid port may extend in any suitable direction. The fluid port may extend generally perpendicularly relative to the central bore. In some embodiments the fluid port may extend generally obliquely relative to the central bore. The fluid port may extend in varying directions, for example portions of the fluid port may extend at least one of perpendicularly, parallel and obliquely relative to the central bore.

A valve member, such as a valve sleeve may be mounted within the housing. The valve member may be moveable, for example axially moveable, from a closed position in which the fluid port is blocked to an open position in which the fluid port is opened. The fluid port may be opened to provide fluid communication between the central bore of the tool and an external downhole location, such as an annulus, a surrounding formation or the like. The fluid port may be arranged to accommodate one or both of outflow and inflow.

The downhole tool may comprise a catching apparatus, such as provided in accordance with any other aspect. The catching apparatus may be mounted within the housing, for example on a downhole side of the valve member.

The downhole tool may comprise an indexing mechanism mounted within the housing. The indexing mechanism may be located on an uphole side of the valve member. The indexing mechanism may be arranged to be moved axially along the housing towards an actuation site. Upon reaching the actuation site the indexing mechanism may initiate actuation, for example movement, of at least one of the valve member and the catching apparatus.

The downhole tool may define a downhole fracturing tool.

The downhole tool may comprise a tool as disclosed in WO 201 1/117601 and/or WO 2011/1 17602, which are incorporated herein by reference.

In use, the apparatus may be configured for deployment through an axial flow passage or throughbore of the downhole tool.

The axial flow passage of the downhole tool may comprise or define a varying profile. The axial flow passage may comprise a first portion of a first diameter and a second portion of a second, larger, diameter. The second portion may comprise a recess. In particular embodiments, the profile may comprise an indexing profile.

Providing an apparatus according to embodiments of the present invention may permit a sealing contact to be provided and maintained in an axial flow passage having such a varying profile, since the seal members may be spaced so that where one seal member is adjacent to the profile portion which may otherwise result in fluid bypass, at least one other of the seal members is positioned adjacent to a portion of the downhole tool which permits a sealing contact to be provided between the apparatus and the downhole tool, thus ensuring that a sealing contact is provided at all times during passage of the apparatus through the downhole tool.

Alternatively or additionally, a restriction may be provided in the downhole tool and the apparatus may be configured for deployment through the restriction. The restriction may comprise part of the downhole tool disposed in or extendable into the axial flow passage. Alternatively, or additionally, the restriction may comprise a tool disposed within the axial flow passage. The restriction may define a flow passage other than the axial flow passage which may, for example, define a leak path. In particular embodiments, the restriction may comprise a sleeve, such as a collet sleeve.

Providing an apparatus according to embodiments of the present invention may permit a sealing contact to be provided and maintained in an axial flow passage having such a restriction, since the seal members may be spaced so that where one seal member is adjacent to the profile portion which may otherwise result in fluid bypass, at least one other of the seal members is positioned adjacent to a portion of the downhole tool which permits a sealing contact to be provided between the apparatus and the downhole tool, thus ensuring that a sealing contact is provided at all times during passage of the apparatus through the downhole tool.

An inhibitor may be provided to prevent or mitigate adherence/ setting of the fluid, such as cement, to the apparatus and/or the downhole tool. The inhibitor may comprise sugar grease or the like. Sugar grease has been found to be particularly effective in preventing adherence of cement to downhole tools and equipment. The inhibitor may be disposed on the downhole tool. The inhibitor may be disposed on the apparatus. The inhibitor may be provided in the form of a coating.

The downhole tool may be coupled to or form part of a tubular string, such as a completion string.

A single downhole tool may be provided. In particular embodiments, a plurality of downhole tools may be provided. In embodiments where the downhole tool comprises a tool for treating, for example fracturing, a subterranean formation may comprise up to 30 downhole tools, for example up to 150 downhole tools. A single apparatus may be provided. In particular embodiments, a plurality of downhole tools may be provided. In embodiments where a plurality of the downhole tools are provided, one apparatus may be provided for each downhole tool.

According to a second aspect of the present invention there is provided a method comprising:

providing a body;

mounting a sealing arrangement on the body;

configuring the sealing arrangement such that where the cementing apparatus is deployed through a downhole tool the sealing arrangement defines a varying point of sealing contact axially along the sealing arrangement with an inner surface of a downhole tool.

The sealing arrangement may comprise a plurality of axially spaced seal members. At a first axial location within the downhole tool, one of the seal members is located to cooperate with the downhole tool to provide a sealing contact. At a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact.

The method may comprise urging a fluid, such as cement or the like, through the downhole tool. The method may comprise or form part of a cementing operation in a borehole, such as a well borehole.

The method may comprise treating, for example fracturing, a subterranean formation.

According to third aspect of the present invention there is provided a downhole method comprising:

deploying an apparatus through a downhole tool, wherein the apparatus comprises a sealing arrangement;

establishing a point of sealing contact between the sealing arrangement and an inner surface of the downhole tool; and

varying the point of sealing contact axially along the sealing arrangement during deployment through the downhole tool.

The method may comprise providing the varying point of sealing contact between a plurality of axially spaced seal members within the sealing arrangement.

At a first axial location within the downhole tool, a first seal member may cooperate with the downhole tool to provide a first point of sealing contact. At a second axial location within the downhole tool the first point of sealing contact may be removed, and a second seal member may cooperate with the downhole tool to provide a second point of sealing contact.

The method may comprise pumping the apparatus through the downhole tool.

The method may comprise passing a volume of cement through the downhole tool on one axial side of the apparatus.

The method may comprise:

deploying first and second apparatuses through the downhole tool, wherein each apparatus may include a respective sealing arrangement;

deploying a volume of cement intermediate the first and second apparatuses; establishing a point of sealing contact between the respective sealing arrangements and an inner surface of the downhole tool; and

varying the respective points of sealing contact axially along the respective sealing arrangements during deployment through the downhole tool.

The method may comprise or form part of a downhole cementing operation. The method may comprise treating a subterranean formation via the downhole tool.

The downhole tool may comprise a tool for use in fracturing a subterranean formation.

According to a fourth aspect of the present invention there is provided:

a downhole tool; and

an apparatus for deployment through the downhole tool, the apparatus comprising a plurality of axially spaced seal members, wherein the axial spacing of the seal members is configured such that at a first axial location in the downhole tool one of the seal members cooperates with the downhole tool to provide a sealing contact and at a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact.

According to a fifth aspect of the present invention there is provided an apparatus for deployment through a downhole tool, the apparatus comprising a plurality of axially spaced seal members, wherein the axial spacing of the seal members is configured such that at a first axial location in the downhole tool one of the seal members cooperates with the downhole tool to provide a sealing contact and at a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact. According to a sixth aspect of the present invention there is provided a downhole system, comprising:

a downhole tool; and

an apparatus for deployment through the downhole tool, wherein the apparatus comprises a sealing arrangement configured to define a varying point of sealing contact axially along the sealing arrangement with an inner surface of the downhole tool during deployment through said tool.

The apparatus may comprise a plurality of axially spaced seal members, wherein the axial spacing of the seal members may be configured such that at a first axial location in the downhole tool one of the seal members cooperates with the downhole tool to provide a sealing contact and at a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact.

The downhole tool may comprise a tool for use in treating a subterranean formation, a tool for use in hydraulic fracturing, a tool housing defining a central bore and including a fluid port, a valve sleeve, a catching apparatus and/or an indexing mechanism, or the like.

According to a seventh aspect of the present invention there is provided a method comprising deploying an apparatus having a plurality of axially spaced seal members through a downhole tool, wherein at a first axial location within the downhole tool one of the seal members is located to cooperate with the downhole tool to provide a sealing contact and at a second axial location within the downhole tool said seal member permits passage of fluid and at least one other of the seal members cooperates with the downhole tool to provide a sealing contact.

An eighth aspect of the present invention relates to use of an inhibitor to prevent adherence of cement to downhole equipment.

The downhole equipment may comprise an apparatus according to any previous aspect.

The downhole equipment may comprise a downhole tool according to any previous aspect.

The inhibitor may comprise sugar grease.

According to a ninth aspect of the present invention there is provided a method for inhibiting adherence of cement to a downhole tool of a string, comprising coating a surface of the tool with sugar grease. The method may comprise directing a volume of sugar grease through the string. The downhole tool may comprise a tool for use in treating a subterranean formation, a tool for use in hydraulic fracturing, a tool housing defining a central bore and including a fluid port, a valve sleeve, a catching apparatus, and/or an indexing mechanism, or the like.

The method may comprise directing a volume of cement through the downhole tool. The method may comprise using a downhole apparatus comprising a body and a sealing arrangement mounted on the body to direct sugar grease through the downhole tool.

According to a tenth aspect of the present invention there is provided a downhole cementing method, comprising:

directing a volume of cement through a string comprising a fracturing tool; and

directing the cement into an annulus surrounding at least a portion of the string.

The method may comprise performing a fracturing operation after the cement has been directed into the annulus.

The method may comprise deploying an apparatus according to any other aspect through the string with the volume of cement.

The method may comprise disposing a first apparatus on a downhole side of the volume of cement.

The method may comprise disposing a second apparatus on an uphole side of the cement.

The method may comprise coating surfaces of the fracturing tool with an inhibitor configured to inhibit adherence of cement to the surface.

The inhibitor may comprise sugar grease.

An eleventh aspect of the present invention relates to a downhole apparatus deployable through a downhole tool. The apparatus may be deployable with a volume of a fluid. The apparatus may comprise a body. The apparatus may comprise a sealing arrangement mounted on the body. The sealing arrangement may be configured to define a sealing contact with an inner surface of a downhole tool. The sealing arrangement may be configured to provide a varying point of sealing contact along a length of the sealing arrangement as the apparatus is deployed through said tool. It should be understood that the features defined above in accordance with any aspect of the present invention or below in relation to any specific embodiment of the invention may be utilised, either alone or in combination with any other defined feature, in any other aspect or embodiment of the invention.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 a longitudinal sectional view of a downhole cementing apparatus according to an embodiment of the present invention;

Figure 2 a longitudinal sectional view of the downhole cementing apparatus shown in Figure 1 , shown disposed within a downhole tool;

Figure 3 an enlarged view of a first, uphole, end region of the downhole tool, showing the cementing apparatus at a first axial location;

Figure 4 an enlarged view of a second, downhole, end region of the downhole tool;

Figure 5 a cross sectional view of the apparatus and downhole tool along section A-A shown in Figure 4;

Figure 6 a diagrammatic view of a wellbore system, in which embodiments of the present invention may be utilised;

Figure 7 a diagrammatic view of the wellbore system of Figure 6, shown during a cementing operation;

Figure 8 a diagrammatic view of the wellbore system of Figures 7 and 8, shown after a cementing operation has been completed;

Figure 9 a diagrammatic view of the wellbore system of Figures 7, 8 and 9, shown during a fracturing operation.

DETAILED DESCRIPTION OF DRAWINGS

Referring first to Figures 1 and 2, there is shown a downhole cementing apparatus, in the form of a cement dart 10, according to an embodiment of the present invention. In use, the dart 10 may be employed during a downhole cementing operation, the dart 10 being deployable through a downhole tool 12 (Figure 2) with a volume of cement 14 and configured to provide a varying point of sealing contact 16 between the dart 10 and the downhole tool 12 which permits the cement 14 to be driven through the downhole tool 12. As shown in Figure 1 , the dart 10 comprises a body in the form of a mandrel 18 and a sealing arrangement 20 for providing the sealing contact 16. In the illustrated embodiment, the sealing arrangement 20 comprises a plurality of axially spaced seal members 22a, 22b, 22c, 22d, 22e and, in use, the dart 10 is configured so that as the dart 10 is deployed though the downhole tool 12 the point of sealing contact 16 between the dart 10 and the tool 12 varies axially along the sealing arrangement 20, thus ensuring that a sealing contact between the dart 10 and the tool 12 is maintained at all times during passage of the dart 10 through the tool 12.

The mandrel 18 comprises a number of separate modules 24a, 24b, 24c, 24d, 24e, 24f and 24g. In the illustrated embodiment, the modules 24a, 24b, 24c, 24d and 24e comprise seal member modules, each having a seal member 22a, 22b, 22c, 22d, 22e disposed or formed thereon while the modules 24f and 24g comprise spacer modules. Each of the modules 24a to 24g has a male connector 26a, 26b, 26c, 26d, 26e, 26f and a corresponding female connector 28a, 28b, 28c, 28d, 28e, 28f and 28g for coupling the modules 24a, 24b, 24c, 24d, 24e, 24f and 24g together and which permits the dart 10 to be constructed with the spacing required to provide the required sealing contact 16.

The axial spacing of the seal members 22a, 22b, 22c, 22d, 22e is configured so that the point of sealing contact 16 between the dart 10 and the axial flow passage 14 of the tool 12 is maintained by at least one of the seal members 22a, 22b, 22c, 22d, 22e at all times during passage of the dart 10 through the downhole tool 12. Providing and maintaining the seal 16 ensures that the cement 14 is driven through the downhole tool 12, even where one or more of the seal members 22a, 22b, 22c, 22d, 22e is disposed at an axial location in the downhole tool 12 which may permit fluid passage around the respective seal member 22a, 22b, 22c, 22d, 22e.

In the illustrated embodiment, each seal member 22a to 22e comprises a cup-shaped seal element 30a, 30b, 30c, 30d, 30e moulded or otherwise disposed on its respective mandrel portion 24a to 24e. The seal elements 30a to 30e each define a cup angle 0a, 0b, 0c, 0d, 0e with respect to the mandrel 18 and define a maximum diameter Da, Db, Dc, Dd, De at their distal ends 32a, 32b, 32c, 32d, 32e. The cup angles 0a, 0b, 0c, 0d, 0e and the diameters Da, Db, Dc, Dd, De may be configured so that the point of sealing contact 16 between the dart 10 and the axial flow passage 14 of the tool 12 is maintained by at least one of the seal members 22a, 22b, 22c, 22d, 22e at all times during passage of the dart 10 through the downhole tool 12. Referring now in particular to Figures 2 to 4, the downhole tool 12 comprises a housing 34 which defines a central bore 36 and extends between an uphole connector 38 and a downhole connector 40. The connectors 38, 40 facilitate connection of the tool 12 to an uphole string component S1 and a downhole string component S2 (shown schematically in Figure 1).

Fluid ports 42 are provided radially through a wall of the housing 34 and, when opened, the ports 42 facilitate outflow of a fluid from the central bore 36 of the housing 34. A valve member in the form of a sleeve 44 is moveable axially along the housing 34 from a closed position in which the sleeve 44 blocks or closes the ports 42, to an open position. Movement of the sleeve 44 towards its open position is achieved by an associated actuator portion 46.

A catching sleeve 48 is located downhole of the valve sleeve 44 and is moveable from a free configuration in which an object, such as a ball (not shown) may freely pass, to a catching configuration in which an object, such as a ball, may be caught. In use, the catching sleeve 48 may function to catch an object and establish diversion of any fluid from the central bore 36 outwardly through the fluid ports 42 when open. Further, the catching sleeve 48 is operated to move to its catching configuration by movement of the valve sleeve 44 towards its open configuration.

The actuator portion 46 of the tool 12 defines an indexing profile 50 provided on the inner surface of the housing 34.

The indexing profile 50 includes a plurality of axially spaced annular recesses 52a, 52b, 52c, 52d, 52e, 52f, 52g, 52h formed in the inner surface of the housing 34. An indexing sleeve 54 is mounted within the housing 34 and is configured to cooperate with the indexing profile 50 to be driven in a number of discrete linear movement steps through the housing 34 by passage of a corresponding number of actuation objects, such as balls. The indexing sleeve 54 is driven in discrete movement steps until reaching an actuation site within the tool 12, where the indexing sleeve 54 engages and moves the valve sleeve 44 in a downhole direction to open the ports 42.

In the illustrated embodiment, the indexing sleeve 50 includes a tubular wall structure 56 which defines a central bore 58 corresponding with the central bore 36 of the housing 34. The central bore 58 is sized to permit an actuation object, such as a ball, to pass therethrough. The indexing sleeve 54 also includes first and second circumferential arrays of engagement members 60, 62 which are arranged such that the array of first engagement members 60 are axially spaced apart from the array of second engagement members 62. The engagement members are arranged within slots 64, 66 formed through the wall structure 56. In use, the arrays of engagement members 60, 62 cooperate with the indexing profile 50 of the housing 34 to be sequentially engaged by a passing object, such as a ball, to drive the indexing sleeve 54 one discrete linear movement step. More specifically, the first and second arrays of engagement members 60, 62 are arranged to be moved radially within their associated slots 64, 66 such that each array of engagement members 60, 62 is moved in an alternating or out of phase manner relative to the other array of engagement members 60, 62 by cooperation with the indexing profile 50 during movement of the indexing sleeve 54 through the housing 34. Such alternating radial movement alternately moves the first and second arrays of engagement members 60, 62 radially inwardly and into the central bore of the indexing sleeve 54, to thus be sequentially engaged by a passing actuation object. In this way, a passing object may engage the engagement members 62,64 of one of the first and second arrays to move the indexing sleeve 54 a portion of a discrete movement step, and then subsequently engage the engagement members 62, 64 of the other one of the first and second arrays to complete the discrete movement step of the indexing sleeve 54.

The engagement members 62, 64 are mounted on the distal end of respective collet fingers 68 which are secured at their proximal ends to the tubular wall structure 56. The collet fingers 68 are resiliently deformable to facilitate radial movement of the engagement members 62, 64 by cooperation with the indexing profile 50.

In the illustrated embodiment the collet fingers 68 are unstressed when the engagement members 62, 64 are positioned radially outwardly and thus removed from the central bore. As such, the collet fingers 68 must be positively deformed by appropriate cooperation between the engagement members 62, 64 and the indexing profile 50 to move the engagement members 62, 64 radially inwardly into the central bore to permit engagement by an actuation object, such as a ball. In such an arrangement, the collet fingers 66 may function to bias the engagement members 62, 64 in a direction to be moved radially outwardly from the central bore. Each slot of the indexing sleeve 54 accommodates two respective engagement members 62, 64. Further, the slots are defined between respective elongate ribs. Each rib includes a spline feature or key which are received in corresponding longitudinally extending slots or key-ways (not shown) formed in the housing 34. Engagement between the keys and the longitudinal slots or key-ways may function to rotationally lock the indexing sleeve 54 relative to the housing 34, while still permitting movement of the indexing sleeve 54 linearly through the housing 34. Such an arrangement may facilitate milling of the indexing sleeve 54, if ever required.

It will be recognised that the downhole tool 12 may thus include a profile, such as the indexing profile 50, or a restriction through which a dart must pass, such as the catching sleeve 48 or indexing sleeve 50, but which may prevent the creation of a continuous circumferential seal. In the case of a restriction, for example, fluid leakage may be permitted around the restriction and so prevent the creation of a continuous circumferential seal. Alternatively or additionally, the form of the restriction, which may for example comprise a collet sleeve or the sleeve, may prevent the creation of a continuous circumferential seal.

In embodiments of the present invention, however, the dart 10 is configured to provide a varying point of sealing contact 16 axially along the sealing arrangement 20 which permits a sealing contact 16 to be provided at all times during passage of the dart 10 through the downhole tool 12.

Deployment of the dart 10 through the downhole tool 12 will now be described.

In Figure 3, the dart 10 is shown located within the axial flow passage 14 of the downhole tool 12 at a first axial location. At this first location, the sealing contact 16 is provided by the first seal member 22a which is disposed adjacent to and seals against the valve sleeve 44 of the downhole tool 12.

However, as the dart 10 progresses in the direction of arrow 90, seal member 22a will become aligned with ports 92 defined in the valve sleeve (which are used, eventually, to become aligned with ports 42). Accordingly, seal member 22a will loose its sealing function. However, at this point sealing member 22b will become aligned with cylindrical surface 94 on the tool 12, and as such will now establish sealing contact.

As the dart 10 continues to progress, the particular seal member providing a sealing function may vary, and as such permitting at least one sealing contact to be achieved at all times during passage of the dart 10 through the downhole tool 12. It should be understood that the embodiments described herein are merely exemplary and that various modifications may be made thereto without departing from the scope of the invention.

For example, it will be recognised that a wellbore system may comprise a plurality of darts 10 and downhole tools 12 of the same or different configuration and a dart 10 may be provided behind a column of cement 14 and used to drive cement 14 through the downhole tool 12 or may be located ahead of a column of cement and used, for example, to deploy an inhibitor in the downhole tool 12 prior to the cementing operation.

Referring now to Figures 6 to 9, there is shown a wellbore system 100 including a drilled borehole 102 which extends from surface 104 and intercepts a subterranean reservoir or formation 106. In the illustrated embodiment, the borehole 102 may comprise a deviated, high angle or horizontal section 108.

The formation 106 may contain hydrocarbons to be produced to surface 104 via the system 100. Alternatively, or additionally, the subterranean formation 106 may define a target for receiving a treatment medium or fluid injected from surface 104 via the system 100, for example for increasing formation pressure to improve production of hydrocarbons from the formation 106 or a neighbouring formation, for sequestration purposes, or the like.

A tubular string 110 extends through the borehole 102, the string 1110 comprising a plurality of fracturing tools 1 12 distributed along its length at a desired interval spacing. One of more of the tools 1 12 may, for example, comprise a tool such as the tool 12 described above.

Figure 6 shows the wellbore system 100 after location of a tubular string 1 10 at a required depth.

A number of operations may be required during the life cycle of the wellbore system 100 which require the ability to direct a fluid into and/or from the formation 106. For example, a cementing operation may be carried out in order to assist in securing and supporting at least part of the string 110 in the borehole 102, to prevent uncontrolled migration of fluid in annulus 114 between the string 1 10 and the borehole 102 and/or in the isolation of particular formation zones prior to perform a fracturing or stimulation operation.

Referring to Figure 7, the cementing operation may involve directing a volume of cement 1 16 through the string 1 10 which is then directed into the annulus 114 and circulated back towards surface to fill the annulus 1 14 or an annulus section. In order to control the cementing process, a first cement apparatus may disposed ahead of the cement 116 and engage a landing collar (not shown) while a second cement apparatus is disposed behind the column of cement 116. The first cement apparatus may comprise a dart 1 18, similar to or identical to the dart 10 described above. The second cement apparatus may also or alternatively comprise a dart 120, similar to or identical to the dart 10 described above. Increased pressure may then be used to rupture or open a fluid passage so that the cement 1 16 may be driven into the annulus 114. In use, the second dart 120 may also act to clean any cement 1 16 as it is driven through the string 1 10 which may otherwise form an obstruction. The cemented borehole is shown in Figure 8.

Alternatively or additionally, the formation 106 may require stimulation or treatment to provide improved production or injection rates to be achieved or restored. Stimulation techniques include hydraulic fracturing which involves injecting a fracturing fluid into the formation at high pressure and/or flow rates to create mechanical fractures within the geology. These fractures may increase the effective near-wellbore permeability and fluid connectivity between the formation and wellbore. The fracturing fluid may carry proppant material, which functions to prop open the fractures when the hydraulic fracturing pressure has been removed. Matrix stimulation provides a similar effect as hydraulic fracturing. This typically involves injecting a chemical such as an acid, for example hydrochloric acid, into the formation to chemically create fractures or wormholes in the geology. Such matrix stimulation may have application in particular geology types, such as in carbonate reservoirs.

As shown in Figure 9, each of the tools 1 12 includes a plurality of circumferentially arranged ports 122, which are initially closed. Further, each tool 112 includes or is associated with a downhole actuator (not shown) which is operable to actuate the tool 112 to open the associated ports 122 to allow injection of a treating fluid, such as a fracturing fluid or acid, from the string 110 into the surrounding formation 106 to create fractures 124. Each tool 112 is operated by actuation objects, such as balls, which are delivered through the string 110 from surface 104.

The tools 112 are capable of being actuated in a desired sequence, thus allowing the formation 106 to be treated along the length of the borehole 102 in stages. Such ability to actuate the tools 112 sequentially may be achieved via the associated downhole actuator. In the illustrated embodiment, the tools 112 are arranged to be actuated in an uphole sequence or direction. This is shown in Figure 9 in which the lowermost illustrated tool 112 has previously been actuated, with an adjacent tool 112 on the uphole side shown in an actuated state with fracturing fluid from the opened ports 122 being directed into the formation 106 in the direction of arrows 126. Once appropriate fracturing has been achieved via tool 112, the next uphole tool 1 12 may then be actuated.