NO20240455A1 - A well tool, such as a casing plug, with drag blocks - Google Patents

Description

[0001] A WELL TOOL, SUCH AS A CASING PLUG, WITH DRAG BLOCKS

[0002] FIELD OF THE INVENTION

[0004] The present invention relates to well tools for sealing a pipe within a wellbore, and specifically to casing plugs for use in casing pipes or production tubing.

[0006] BACKGROUND OF THE INVENTION

[0008] Well tools, such as casing plugs, can be used in well piping, such as casing pipes or production tubing, for the purpose of temporarily closing the well. A well may be temporarily closed for the purpose of pressure testing the well piping. A casing plug may be run into a well as part of a drill string, in which the casing plug is connected to drill pipes. Casing plugs are typically run into the well in a radially retracted state, in which they generally have a diameter that is smaller than the casing. Subsequently, when in the correct position within the casing pipe, the casing plug can be radially expanded into a radially expanded state, in which parts of the casing plug that are arranged to close the well are in contact with an inner surface of the casing pipe.

[0010] Radially expanding the casing plug may be implemented by rotating an upper part of the casing plug relative to a lower part of the casing plug by means of the drill pipe. To rotate the upper part of the casing plug relative to the lower part of the casing plug, it is useful for the lower part of the casing plug to be held in position within the casing pipe, or at least in the same rotational position within the casing pipe. This can be achieved using drag blocks, which protrude from the casing plug and engage with the casing so that there is friction between the drag block and the casing pipe. The drag blocks hold the lower part of the casing plug stationary, while the drill string above the casing plug can be used to rotate the upper part of the casing plug.

[0012] A casing plug typically includes a mandrel and a housing that surrounds the mandrel. The drag blocks are generally positioned on or partially in the housing. The drag blocks may be connected to the housing by one or more springs, which push or bias the drag blocks outwards towards the casing pipe, so that they protrude further from the casing plug than the rest of the housing. This allows them to be in contact with the casing pipe to allow the relative rotation to take place. The springs may also allow the drag blocks to be pushed inwards towards the housing so that the drag blocks can be pushed past areas of the casing pipe that have a reduced diameter.

[0013] A casing plug with drag blocks can be more difficult to run into a well than a casing plug without drag blocks. This is because the drag blocks may cause increased friction with the casing pipe and because the drag blocks can act as obstacles to fluid flow past the casing plug. The drag blocks create a restriction in fluid flow whose effect is emphasized as the size of the casing becomes smaller deeper into the well. It is an object of the present invention to reduce the difficulty of running a casing plug with drag blocks into a well.

[0014] Furthermore, because the drag blocks are configured to protrude further from the casing plug than other components, at least in a radially retracted state of the casing plug, they are more prone to being damaged than other components. The drag blocks may be damaged during transportation or handling, during deployment or retrieval of the casing plug, or during use of the casing plug. Repairing a casing plug with a damaged drag block can be difficult and time-consuming using current methods, as the casing plug must be sent to a maintenance facility. It is also an object of the present invention to enable efficient repair of a casing plug with a damaged drag block.

[0016] SUMMARY OF INVENTION

[0018] According to a first aspect, there is provided a well tool for sealing a pipe within a wellbore, the pipe having a through-bore with a bore cross-sectional area. The well tool comprises: a mandrel; a housing circumferentially surrounding the mandrel; and at least one drag block for providing friction between the well tool and the pipe, wherein the at least one drag block protrudes from the housing. The well tool has a first region that is free of drag blocks and a second region that includes the at least one drag block. The first region is longitudinally contiguous with the second region. A first difference area represents a difference between the bore cross-sectional area of the pipe and a first external crosssectional area of the well tool in the first region when the well tool is in the through-bore. A second difference area represents a difference between the bore cross-sectional area of the pipe and a second external cross-sectional area of the well tool in the second region when the well tool is in the through-bore. The first difference area is equal to or smaller than the second difference area.

[0020] It may also be said that the second difference area is greater than the first difference area. In other words, when the well tool is in the through-bore, there is a greater area for fluid flow between the drag blocks, the pipe, and the housing than around at least the part of the well tool immediately before or after the drag blocks. Accordingly, fluid flow past the well tool, particularly in the second region, is improved because the second region does not represent a constriction to the flow. Avoiding a constriction may reduce the drag forces experienced by the well tool. This may improve how quickly the well tool can be lowered into the wellbore. This may also improve the forces required to push the well tool into the wellbore. In addition, a reduction in time for the operation of replacing well fluids in the well when the casing plug is located in the well may be achieved.

[0022] By longitudinally contiguous, it is meant that the first region is directly adjacent or borders the second region in along the well tool, and that the first region is contiguous with the second region in a longitudinal direction or along a longitudinal axis of the well tool. In other words, a boundary of the first region is a shared boundary with the second region. As the first and second regions are contiguous, fluid may flow within the pipe past the well tool from one of the first or second regions to the other of the first or second regions as it is lowered into the well, or as fluids are replaced in the well. The first region may border the second region to one side of the second region, or may border the second region to both sides along the well tool.

[0024] An external cross-sectional area is an area of a cross-section of the well tool defined by its outer surface.

[0026] The well tool may be referred to as a well tool device. The well tool may be a casing plug and the pipe may be a casing pipe. Alternatively, the pipe may be production tubing. The pipe may form part of a well pipe comprising a plurality of further pipes. The further pipes may have different bore cross-sectional areas.

[0028] The bore cross-sectional area may be greater than the first external cross-sectional area. The bore cross-sectional area may be greater than the second external cross-sectional area. The through-bore may have a constant cross-sectional area. The through-bore may have a circular cross-section.

[0030] The bore cross-sectional area is an area of a cross-section of an interior of the pipe. The bore cross-sectional area may be determined based on an internal diameter of the pipe. The internal diameter may be a maximum internal diameter, a minimum internal diameter, or a nominal internal diameter of the pipe, based on expected or predetermined tolerances of the pipe.

[0031] The well tool may have a rating indicating a smallest pipe that it can be used with. The bore cross-sectional area may be the bore-cross sectional area of the pipe indicated by the rating. The first difference area may be equal to or smaller than the second difference area for all pipes with which the well tool can be used. In other words, the first difference area may be equal to or smaller than the second difference area for the smallest pipe and any pipe having a larger internal diameter.

[0033] The pipe may have a predetermined tolerance. The bore cross-sectional area may be the cross-sectional area of the pipe at a maximum internal diameter of the pipe based on the predetermined tolerance. The predetermined tolerance may define the maximum internal diameter. The predetermined tolerance may define a minimum internal diameter. The pipe may have a nominal internal diameter. The bore cross-sectional area may be the crosssectional area of the pipe at its maximum internal diameter. The bore-cross sectional area may be the cross-sectional area of the pipe at its minimum internal diameter. The borecross sectional area may be the cross-sectional area of the pipe at its nominal internal diameter. The first difference area may be equal to or smaller than the second difference area for any bore-cross sectional area of the pipe based on its predetermined tolerance. The first difference area may be equal to or smaller than the second difference area for a plurality of different pipes.

[0035] The first difference area and the second difference area may be determined for when the well tool is within the pipe. The first difference area and the second difference area may be determined for when the well tool is within the pipe and when the drag blocks are in contact with the pipe. In other words, the pipe may be a pipe in which the drag blocks contact an inner surface. An internal diameter of the bore of the pipe may be less than a maximum diameter of the second region and greater than a minimum diameter of the second region. The maximum diameter and minimum diameter of the second region may be determined based on the positions of the drag blocks when they protrude furthest from the housing and when they protrude least from the housing.

[0037] The housing may comprise one or more parts, sections, or mechanisms. The housing may circumferentially surround the mandrel along a length of the mandrel or a portion of the length of the mandrel. The mandrel may define or delimit the through-bore. An inner surface of the mandrel may define or delimit the through-bore. The housing may surround an outer surface of the mandrel.

[0038] The at least one drag block may comprise a plurality of drag blocks. The plurality of drag blocks may be radially spaced or distributed around the housing. The drag blocks may be evenly spaced around the housing.

[0040] The at least one drag block may be positioned within a cavity of the housing. The cavity may be formed in, defined by, or delimited by the housing. The at least one drag block may be moveable relative to the housing. The at least one drag block may be radially moveable relative to the housing. The at least one drag block may be moveable relative to the housing within the cavity. The cavity may prevent longitudinal movement of the at least one drag block relative to the housing. The cavity may prevent rotational movement or the at least one drag block relative to the housing.

[0042] The well tool may comprise one or more springs. The one or more springs may connect the at least one drag block to the housing. The one or more springs may be positioned within the cavity. The one or more springs may be configured to bias the at least one drag block into a radially protruding position, in which the at least one drag block protrudes from the housing. The at least one drag block may be moved towards the housing by applying a force to the drag block which compresses the one or more springs. The force may have a component in the direction of a longitudinal axis of the well tool. Moving the at least one drag block towards the housing reduces how much the at least one drag block protrudes from the housing.

[0044] The at least one drag block defines part of an exterior surface of the well tool in the second region. The housing defines a further part of the exterior surface of the well tool in the second region. The exterior surface of the well tool in the second region has the second external cross-sectional area. Moving the at least one drag block towards the housing into the cavity reduces an amount of the drag block that protrudes from the housing and therefore reduces an amount of the drag block that contributes to the exterior surface of the well tool in the second region. Moving the at least one drag block towards the housing may therefore reduce the second external cross-sectional area.

[0046] The second region may be defined based on a location of the at least one drag block. The second region may extend from a first longitudinal position at which the at least one drag block has a first end to a second longitudinal position at which the at least one drag block has a second end. Where the at least one drag block comprises a plurality of drag blocks, the second region may extend from a first longitudinal position at which one of the plurality of drag blocks has an end to a second longitudinal position at which another of the plurality of drag blocks has an end. The second region may encompass the drag blocks.

[0048] The first and second difference areas are determined for when the well tool is within the through-bore. Accordingly, the first and second external cross-sectional areas are calculated based on a cross-section of the well tool in the first and second regions respectively when the well tool is within the pipe having the bore cross-sectional area. Accordingly, depending on the bore cross-sectional area, the drag block may be compressed towards the housing or may be in a position in which it protrudes furthest from the housing.

[0050] The housing may define the whole of an exterior surface of the well tool in the first region. The exterior surface of the well tool in the first region may have the first external crosssectional area.

[0052] The first difference area may represent an area between the first region and the pipe for fluid flow. The second difference area may represent an area between the second region and the pipe for fluid flow.

[0054] The first difference area may be defined as a cross-sectional area of a channel between the first region and the pipe in use. The second difference area may be defined as a crosssectional area of at least one channel between the second region and the pipe. Where the second region includes a plurality of drag blocks, a plurality of channels may be defined or delimited between the second region and the pipe, the channels being between adjacent drag blocks of the plurality of drag blocks.

[0056] The first difference area may be at least 5% smaller than the second difference area. The first difference area may be at least 10% smaller than the second difference area. The first difference area may be at least 15% smaller than the second difference area. The first difference area may be at least 20% smaller than the second difference area. The first difference area may be at least 50% smaller than the second difference area.

[0058] The percentage differences above may be determined as the change between the second difference area and the first difference area, divided by the average of the first difference area and the second difference area, and multiplied by 100. A formula for percentage difference may be:

[0061] where Δ is percentage difference, A1 is the first difference area, and A2 is the second difference area.

[0063] The at least one drag block may taper radially inwardly towards the first region and the housing tapers radially outwardly towards the first region. Including tapers in the at least one drag block and the housing may smooth the transition from the first region to the second region. Accordingly, there may be less of a sudden change in shape between the two regions, thereby smoothing the flow of fluid and reducing turbulent flow. Reducing turbulent flow may reduce drag on the well tool. Providing a taper may also reduce changes in pressure and flow rate around the tool, which may also reduce drag and may reduce fatigue or damage to the tool.

[0065] The second region may taper towards the first region. The at least one drag block may include a taper at each end. A taper in the housing may comprise a gradual change in diameter along a length of the housing.

[0067] The well tool may be configurable in a radially retracted state and a radially expanded state. The first external cross-sectional area may be an external cross-sectional area of the first region when the well tool is in the radially retracted state.

[0069] The radially retracted state may be referred to as a run state. The radially expanded state may be referred to as a set state. In the radially retracted state, the well tool can be run into the well. In the radially expanded state, the well tool is set in place within the pipe by expansion of one or more mechanisms. For example, said mechanisms may be one or more seals or one or more anchors. The well tool may be transitioned from the radially retracted state to the radially expanded state by relative movement between the mandrel and the housing.

[0071] The first external cross-sectional area may be an external cross-sectional area of the first region when the well tool is configured for a particular size of casing pipe and/or for a particular internal diameter of a casing pipe. The first external cross-sectional area may be an external cross-sectional area of the first region when the well tool is configured for use in a smallest size of casing pipe for which it is intended.

[0072] The first external cross-sectional area may be constant along the first region. In other words, the first region may extend longitudinally along the well tool, and any crosssectional area of the tool in the first region may be the same. The first region may have a constant diameter along its length.

[0074] The first external cross-sectional area may comprise an average external cross-sectional area across the first region. Having a second region whose difference area between its external cross-section and the pipe cross-section is substantially similar to an average cross-sectional area of another region of the well tool is useful to further smooth fluid flow.

[0075] The well tool may consist of the first region and the second region. The second region may encompass a region of the well tool including the at least one drag block. The remainder of the well tool may be the first region.

[0077] The well tool may have a distal end and a proximal end, the distal end being configured to enter the pipe before the proximal end. The first region may be longitudinally closer to the distal end than the second region. In other words, the first region may comprise a region arranged to be further downwell than the second region. The first region may be a portion of the well tool where a diameter of the housing is at its greatest. The first region may be a portion of the well tool in which the housing comprises a drag block cover.

[0079] The first region may be a region that is free of radially-expandable features, such as a sealing device, sealing element, or anchoring device. The first region may be free of any other features on its outer surface, i.e. it may comprise only the housing.

[0081] According to another aspect, there is provided a well tool for sealing a pipe within a wellbore, wherein the well tool comprises: a mandrel; a housing circumferentially surrounding the mandrel; at least one drag block for providing friction between the well tool and the pipe, wherein the at least one drag block protrudes from the housing. The well tool has a first region that is free of drag blocks and a second region that includes the at least one drag block. The first region is contiguous with the second region. A first external cross-sectional area of well tool in the first region is greater than a second external crosssectional area of the well tool in the second region.

[0083] According to another aspect, there is provided a well tool for sealing a pipe within a wellbore, the pipe having a through-bore. The well tool comprises: a mandrel; a housing circumferentially surrounding the mandrel; and at least one drag block for providing friction between the well tool and the pipe, wherein the at least one drag block protrudes from the housing. The well tool has a first region that is free of drag blocks and a second region that includes the at least one drag block. The first region is longitudinally contiguous with the second region. The housing has a first diameter in the first region and a second diameter in the second region. The second diameter is less than the first diameter.

[0085] The well tools in these aspects may have the features of the well tool of the first aspect.

[0086] According to a yet further aspect, there is also provided a well tool for sealing a pipe within a wellbore, the well tool comprising: a mandrel; a housing circumferentially surrounding the mandrel; a drag block for providing friction between the well tool and the pipe, wherein the drag block is provided within a cavity of the housing; and a fixing system for fixing the drag block to the housing, wherein the fixing system comprises a cuff that circumferentially surrounds the housing and is moveable relative to the housing to at least a first position and a second position, wherein, in the first position, the cuff prevents removal of the drag block from the cavity, and, in the second position, the cuff permits removal of the drag block from the cavity.

[0088] It is particularly useful to be able to remove a drag block from a well tool so that a drag block can be repaired. Utilizing the cuff allows for the drag block to be removed as desired in an efficient way. In other words, by utilizing relative movement between the cuff and the housing to enable the removal, there is an efficient mechanism for removal of the drag block that also ensures that the drag block is secure when in use.

[0090] The fixing system may comprise one or more screws or other fixings. The fixing system may comprise one or more springs.

[0092] The cuff may be moveable between the first position and the second position by rotation. Enabling securing and removal of drag blocks using a rotation rather than translation may be beneficial in a well tool, as the well tool may be subjected to forces that cause translation. Providing a rotatable cuff enables greater control over how the drag blocks can be removed.

[0094] The cuff may comprise a cut-out that aligns with the cavity when the cuff is in the second position. The cut-out may align with a portion of the cavity. In the first position, the cutout may be misaligned with the cavity. The cuff may cover a portion of the drag block in the first position. The portion of the drag block may be uncovered in the second position, because the cut-out aligns with the cavity. The cut-out may align with the cavity by an outline of the cut-out conforming with an outline of the cavity.

[0096] The cuff may comprise a ring that circumferentially surrounds the housing.

[0098] The well tool may comprise a further drag block provided within a further cavity of the housing, wherein: the cuff is moveable relative to the housing to a third position that is different to the first position or the second position; in the first position, the cuff prevents removal of the further drag block from the further cavity; and in the second position, the cuff permits removal of the further drag block from the further cavity.

[0100] The cuff may prevent removal of the further drag block from the further cavity in the second position and the cuff prevents removal of the drag block from the cavity in the third position.

[0102] The cuff may permit removal of only one drag block at a time. Because of the springs beneath drag blocks, high forces may be required to return the drag block to its cavity against the biasing action of the springs. Therefore, it is useful that only one drag block is removed at a time. Furthermore, only enabling one drag block to be removed in each position of the cuff prevents too many drag blocks being removed or becoming loose from the housing if the cuff rotates when it should not.

[0104] In the third position, the cut-out may align with the further cavity.

[0106] Accordingly, with these aspects, improvements in relation to drag blocks are provided. The improvements allow improved use of a well tool with drag blocks, including improved running of a well tool into a well, improved use of the well tool in the well, and improved repair of the well tool outside of the well.

[0108] BRIEF DESCRIPTION OF DRAWINGS

[0110] Fig. 1 is a side view of a portion of a casing plug.

[0112] Fig. 2 is a side view of the portion of the casing plug of Fig.1 within a casing pipe.

[0114] Fig. 3 is a cross-section of the portion of the casing plug of Fig.1.

[0116] Fig. 4 is a cross-section of the casing plug within the casing pipe along the line A-A in Fig. 2.

[0117] Fig. 5 is a cross-section of the casing plug within the casing pipe along the line B-B in Fig. 2.

[0119] Fig 6 is a perspective view of the portion of the casing plug in which a cuff for fixing the drag blocks within the housing of the casing plug has been removed and illustrated separately.

[0121] Figs. 7A to 7C are a series of perspective views of the portion of the casing plug illustrating how a drag block may be removed from the housing by rotation of the cuff.

[0123] DETAILED DESCRIPTION

[0125] Fig. 1 shows a portion 15 of a casing plug 10, which is an example of a well tool. The features described herein may also be applied to other well tools. The casing plug 10 is for use in a casing pipe, and is shown in a casing pipe, CP, in Fig.2. The casing pipe CP has a through-bore TB. A cross-section of the casing plug 10 is shown in Fig.3 so that internal details are visible.

[0127] The casing plug 10 extends between a distal end, which is not visible in Figs.1 to 3 but is in the direction of arrow 11, and a proximal end, which is also not visible in Figs. 1 to 3 but is in the direction of arrow 12. The distal end and proximal end will be referred to as the distal end 11 and proximal end 12. The distal end 11 enters the casing before the proximal end 12. Accordingly, in use, within a casing pipe, the distal end 11 of the casing plug 10 is below the proximal end 12. The casing plug 10 has a longitudinal axis, LA.

[0128] Generally, the terms “upper”, “above”, “below” and “lower” are used herein to define parts of the well tool, when the well tool device is used in a well. “Upper” and “above” refer to a position relatively closer to the well opening and “below” and “lower” refer to a position relatively further away from the well opening. These terms apply both when the well has a vertical and horizontal orientation.

[0130] The casing plug 10 has a mandrel 20, which may be referred to as an inner mandrel. The mandrel 20 extends along the casing plug 10, and has an internal through-bore 16, which can be seen in Fig.3. The casing plug 10 also has a housing 30, which may be referred to as an outer housing. The housing 30 circumferentially surrounds the mandrel 20. The housing 30 comprises a plurality of different parts that surround the mandrel 20 to perform different functions.

[0131] The casing plug 10 is configured to be run into a well as part of a drill pipe, which may also be referred to as a drill string. Within the well, the casing plug 10 will be within the through-bore TB of the casing pipe CP, as shown in Fig. 2. A casing pipe may also be referred to as a casing. The purpose of the casing plug 10 is to enable the well to be temporarily closed, to enable pressure testing of the casing pipe and the full casing that the casing pipe forms part of.

[0133] The well may be temporarily closed by sealing the through-bore TB within the casing pipe using a sealing device (not shown). As used herein, the term “sealing device” is referring to a part of the well tool which is radially expanded into contact with the inner surface of the well at the desired location of the well. The function of the sealing device is to prevent or reduce longitudinal fluid flow at the desired location in the well, i.e. to prevent or reduce fluid from flowing between a location above the well tool device and a location below the well tool device. Radial expansion of the sealing device is achieved by transferring the casing plug from a radially retracted state to a radially expanded state. The casing plug 10 is run into the well in the radially retracted state and then radially expanded.

[0135] It is commonly known that a sealing device such as the sealing device referred to above is only capable of performing their function in their radially expanded state in the well. Moreover, it should be noted that a well tool is rated to a pressure level or pressure interval and/or a temperature level or temperature interval.

[0137] As used herein, the terms “radially retracted state” and “run state” are used interchangeably for the state in which the well tool is lowered to a desired location in the well. The terms “radially expanded state” and “set state” are used interchangeably for the state in which the well tool is engaged with the inner surface of the well at the desired location in the well.

[0139] Radial expansion can be achieved by rotating an upper part of the casing plug 10 relative to a lower part of the casing plug 10 by means of the drill pipe. Rotation of different parts of the casing plug 10 may cause relative longitudinal movement between the mandrel 20 and the housing 30.

[0141] To rotate the upper part of the casing plug 10 relative to the lower part of the casing plug 10, the casing plug 10 includes a plurality of drag blocks 40. The drag blocks 40 protrude from the casing plug 10 and engage with the casing pipe or production tubing so that there is friction between the drag blocks 40 and the casing pipe. The drag blocks 40 are configured to engage with the casing pipe at an outwardly-facing surface 43 of the drag blocks 40. The drag blocks 40 are radially spaced around the casing plug 10. An example of the drag blocks 40 engaging a casing pipe, CP, is shown in Fig.2.

[0143] The drag blocks 40 are evenly spaced around the circumference of the casing plug 10. The drag blocks 40 each include a plurality of friction pads 44 on said outwardly-facing surface 43 for engaging the casing pipe, CP. Friction between the friction pads 44 of the drag blocks 40 and the casing pipe, CP, hold the lower part of the casing plug 10 stationary, while the upper part of the casing plug 10 is rotated relative to the lower part.

[0145] In Figs. 1 to 3, some of the drag blocks 40 are longitudinally displaced relative to others of the drag blocks 40. Specifically, within the second region 32, each drag block 40 is displaced longitudinally relative to its adjacent drag blocks 40, such that a first set of drag blocks has a first longitudinal displacement and a second set of drag blocks has a second longitudinal displacement. The first and second sets of drag blocks are interspersed. This results in the end of the drag blocks of the first set being below the end of the drag blocks of the second set, longitudinally. In Figs. 2 and 3, a first drag block is labelled 40-1, a second drag block is labelled 40-2, a third drag block is labelled 40-3, and a fourth drag block is labelled 40-4. The first and third drag blocks 40-1, 40-3 form part of the first set of drag blocks, and the second and fourth drag blocks 40-2, 40-4 form part of the second set of drag blocks.

[0147] The housing comprises a drag block housing 34 and a nose portion 35, which is best seen in Fig.3. The drag block housing 34 comprises a plurality of cavities 42. Each drag block 40 is provided within a respective cavity 42. The cavities 42 receive the drag blocks therein, and allow radial movement of the drag blocks 40 towards and away from the housing 30. The cavities 42 are shaped to prevent longitudinal or rotational movement of the drag blocks 40. The cavities 42 can therefore be said to define a movement path of the drag blocks 40.

[0149] A fixing system 50 is provided as part of the casing plug 10 for fixing the drag block 40 to the housing 30 and for further limiting movement of the drag blocks 40 relative to the housing 30 and within the cavities 42. The fixing system 50 prevents movement of the drag blocks 40 radially outwardly further than a predetermined limit.

[0150] The fixing system 50 comprises a plurality of springs 45. Each drag block 40 is associated with a set of springs 45. In this example, as can be seen in Fig. 3, each set of springs 45 comprises five springs. The springs 45 of each set connect their associated drag block 40 to the housing 30. The springs 45 therefore define the movement of the drag block 40 relative to the housing, and may both limit the radially inward, towards the housing 30 and the longitudinal axis, LA, and radially outward, away from the housing 30 and the longitudinal axis, LA, movement of the drag block.

[0152] The springs 45 also push or bias their respective drag block 40 outwards towards the casing pipe, in use, so that the drag block 40 protrudes beyond the housing 30. The springs 45 allow for radial movement of the drag block 40 relative to the housing 30 and within the cavity 42. When not in the casing pipe, the springs 45 bias the drag blocks 40 into a position in which they protrude furthest from the housing 30. Within the casing pipe, CP, an example of which is found in Fig.2, the drag blocks 40 may be displaced radially inwardly into the cavities 42 and towards the housing 30.

[0154] The fixing system 50 also comprises a drag block cover 33 and a cuff 60. The drag block cover 33 and cuff 60 form part of the housing 30 and circumferentially surround the drag block housing 34. The drag block cover 33 and cuff 60 each comprise a lip 36, 61 that extends over the cavities 42. The lips 36, 61 are configured to limit radially-outward movement of the drag blocks 40 so that the springs 45 do not push them out of the cavities 42 prior to use within a casing pipe. The drag blocks 40 include a protruding section 46 for engaging the casing pipe and lip-engaging portions 47 at either end for engaging the lips 36, 61. The protruding section 46 includes a taper 41 towards the lip-engaging portions 47.

[0155] In other casing plugs, a region where the drag blocks protrude from the housing has a similar diameter to the surrounding regions. Accordingly, the drag blocks may represent a barrier to fluid flow past this region. This is because the cross-sectional area of the casing plug 10 as a whole increases around the drag blocks compared to the regions above or below the drag blocks. This causes a reduction in the area for fluid flow past the casing plug which is the area between the casing plug and the casing pipe. This may make it difficult to run the casing plug into the well, especially in smaller sizes of casing pipes.

[0156] Instead, in the present casing plug 10, the casing plug 10 is configured so that the area allowed for fluid flow past the drag blocks 40 is greater than the area allowed for fluid flow past a region above or below the drag blocks.

[0157] As can be seen in Figs.1 to 3, the casing plug 10 has a first region 31 and a second region 32. The first region 31 has a constant cross-sectional area along its length, comprising a portion of the drag block cover 33 having a constant diameter. The first region 31 is a region that is free of drag blocks, meaning that there are no drag blocks present or protruding beyond the housing 30 in the first region 31. In other examples, the first region 31 may extend further along the casing plug 10 and may encompass other features of the casing plug 10. The casing plug 10 also has a second region 32, in which there are drag blocks 40. In other words, the second region 32 is a region in which drag blocks protrude from the housing. The second region 32 may be defined between two longitudinal positions on the casing plug 10, beginning where a first drag block 40-1 begins to protrude from the housing 30 and ending where a second drag block 40-2 stops protruding from the housing 30. In this example, the second region 32 comprises a portion of the casing plug 10 where the drag block housing 34 is partially exposed and forms part of an outer surface of the casing plug 10. The first region 31 and second region 32 are longitudinally or axially contiguous because the first region 31 and the second region 32 share a border.

[0159] Although the drag blocks 40 and the drag block cover 33 may partially overlap at the lip 36 and lip-engaging portion 47, it is intended that the first region 31 being a region in which no drag blocks are present means that the drag blocks do not protrude from the outer housing within this region or in which the drag blocks do not form part of an outer surface of the casing plug 10, and therefore do not contribute to its cross-sectional area. Correspondingly, the second region 32 can therefore be described as a region in which the drag blocks form part of an outer surface of the casing plug 10 and therefore contribute to its cross-sectional area.

[0161] The second region 32 and first region 31 are configured so that a difference in crosssectional area between the casing pipe, CP, and the second region 32 is greater than a difference in cross-sectional area between the casing pipe, CP, and the first region 31. This means that there is a greater fluid flow area past the drag blocks than the region below the drag blocks. To demonstrate this, Figs. 4 and 5 illustrate cross-sections along the lines A-A and B-B respectively, which are indicated in Fig. 2. These cross-sections are accompanied by associated cross-sectional areas to demonstrate the differences.

[0163] Fig. 4 shows the cross-section A-A in the centre, with a representation of the crosssectional area ACP of the through-bore TB of the casing pipe CP to the left, and a representation of the cross-sectional area A32 of the casing plug 10 in the second region 32 to the right. Fig. 5 shows the cross-section B-B to the left with a representation of the cross-sectional area A31 of the casing plug 10 in the first region 31 to the left. As the casing pipe CP remains the same in Fig.5, the cross-sectional area of the casing pipe remains the same as the one, ACP, shown in Fig.4.

[0165] Casing pipes may be specified based on an external diameter and may have an internal bore diameter that is variable according to a tolerance.

[0167] As can be seen in Figs.4 and 5, there are areas A2, A1 between the casing pipe CP and the casing plug 10 through which fluid can flow. The area between the casing plug 10 and the casing pipe CP in the first region 31 is referred to as A1, and can be calculated as the difference between the cross-sectional area ACP of the casing pipe CP and the crosssectional area A31 of the casing plug 10 in the first region 31. The area between the casing plug 10 and the casing pipe CP in the second region 32 is referred to as A2, and can be calculated as the difference between the cross-sectional area ACP of the casing pipe CP and the cross-sectional area A32 of the casing plug 10 in the second region 32. The area A1 may be referred to as a first difference area, and the area A2 may be referred to as a second difference area.

[0169] As can be seen in Fig. 5, the first difference area is formed as an annular region between the drag block cover 33 and the internal surface SCP of the casing pipe CP, while the casing plug 10 is in the through-bore TB. As can be seen in Fig. 4, the second difference area is formed as a plurality of channels 49 that are each bordered by two drag blocks 40, the housing 30, and the internal surface SCP of the casing pipe CP, while the casing plug 10 is in the through-bore TB.

[0171] The housing 30 and drag blocks 40 in the second region 32 are configured so that the second difference area A2 is greater than or equal to the first difference area A1. In other words, there is more area for fluid to flow past the second region 32 than past the first region 31. The first difference area A2 may be greater than or equal to the first difference area A1 regardless of the internal diameter of the casing pipe CP.

[0173] In order to smooth the transition between A31 and A32, the second region 32 includes tapering towards the first region. The drag block housing 34 tapers towards the drag block cover 33. The drag blocks 40 include a taper 41 at each end towards the drag block housing 34.

[0175] Although the first region 31 is described in relation to the drag block cover 33 in this example, in other examples the first region 31 may include more of the casing plug 10. Additionally, the cross-sectional areas for the first region 31, such as the first difference area, may be determined as an average across the length of the first region 31, especially where parts of the first region 31 differ in diameter.

[0177] The differences in the first region 31 and the second region 32 may be characterized in other ways. For example, it can be said that the cross-sectional area A32 of the second region 32 is less than the cross-sectional area A31 of the first region 31. Alternatively, it may described that a diameter, D2, of the housing 30 in the second region 32 is smaller than a diameter, D1, of the housing 30 in the first region 31.

[0179] As an example, the casing plug 10 may be used in a casing pipe, CP, of size 95/8” and having a wall thickness of 47#. The following table provides details of the ACP, A31, A1, A32, and A2 for such a casing plug and casing pipe, as well as a percentage difference between A1 and A2.

[0184] Figs. 6 and 7A to 7C concern a further feature of the casing plug 10. Enabling efficient removal and replacement of drag blocks 40 is useful. Conventionally, a casing plug 10 may have to be returned to a specialist facility to enable replacement of a drag block. In the present invention, the drag blocks 40 can be removed and replaced elsewhere, such as on-site, avoiding delays in use of the casing plug 10.

[0186] As described above, the casing plug 10 includes a cuff 60. The cuff 60, like the drag block cover 33, provides a limit for radial movement of the drag blocks away from the housing 30. The drag block cover 33 may be fixed to the drag block housing 34, meaning that the drag blocks 40 are slotted underneath it. The cuff 60, however, is configured to permit movement, and said movement can be used to allow removal of a drag block 40 from its cavity 42.

[0188] Fig. 6 shows the cuff 60 separately from the casing plug 10. As part of the casing plug 10, the cuff 60 circumferentially surrounds the housing 30, and specifically the drag block housing 34. As can be seen in Fig. 6, the cuff 60 comprises a ring 63. A cut-out 62 is defined in the ring 63. The ring 63 has a flange 64 having a smaller internal diameter than the rest of the ring 63. The flange 64 engages the housing 30 and prevents movement of the cuff 60 along the drag block housing 34. An edge of the ring 63 forms the lip 61.

[0189] The cut-out 62 allows selective removal of a drag block from its cavity 42. The cuff 60 is moveable relative to the housing 30 to at least a first position and a second position, wherein, in the first position, the cuff 60 prevents removal of a drag block, such as drag block 40-3 from its cavity 42 , and, in the second position, the cuff 60 permits removal of the drag block 40-3 from its cavity 42. In the first position, the cut-out 62 is misaligned with the drag block 40-3, whereas in the second position, the cut-out 62 is aligned with the drag block 40-3. Specifically, the cut-out 62 aligns with the edge of the drag block 40-3 and therefore uncovers the lip-engaging portion 47 of the drag block 40-3. When the cutout 62 is aligned with the drag block 40-3, the cuff 60 no longer presents a barrier to the movement of the drag block and it can be removed from the cavity 42. The cuff 60 permits removal of a single drag block at a time because it has only a single cut-out.

[0191] To move the cuff 60 to its second position it is rotated about the housing 30 from its first position. This process is shown in Figs.7A to 7C.

[0193] In Fig.7A, the portion 15 of the casing plug 10 is shown with the cuff 60 in its first position. As can be seen in this figure, the cut-out 62 is positioned between two drag blocks 40-2, 40-3. Accordingly, all of the drag blocks 40 are prevented from being removed from their respective cavities 42 by the cuff 60.

[0195] In Fig. 7B, the cuff 60 has been rotated around the housing 30 to its second position, in which the cut-out 62 is aligned with the edge of the drag block 40-3, and so has uncovered its lip-engaging portion 47. The drag block 40-3 is therefore able to be removed from its cavity 42.

[0197] In Fig. 7C, the portion 15 is shown with the drag block 40-3 removed, so that the cavity 42 contains no drag block. A new drag block or a repaired drag block can be replaced in the cavity 42 and the cuff 60 can be rotated to the first position again or to a further position where the cut-out 62 is between two drag blocks.

[0199] The cuff 60 can be rotated to remove further drag blocks. It can be rotated to further positions in which the cut-out is aligned with the drag blocks to permit their removal.

[0200] LIST OF REFERENCE NUMBERS 10 casing plug

[0201] 11 distal end of the casing plug 12 proximal end of the casing plug 15 portion

[0202] 16 internal through-bore

[0203] 20 mandrel

[0204] 30 housing

[0205] 31 first region

[0206] 32 second region

[0207] 33 drag block cover

[0208] 34 drag block housing

[0209] 35 nose portion

[0210] 36 lip

[0211] 40 drag blocks

[0212] 40-1 first drag block

[0213] 40-2 second drag block

[0214] 40-3 third drag block

[0215] 40-4 fourth drag block

[0216] 41 taper

[0217] 42 cavities

[0218] 43 outwardly-facing surface

[0219] 44 friction pads

[0220] 45 springs

[0221] 46 protruding section

[0222] 47 lip-engaging portions

[0223] 49 channels

[0224] 50 fixing system

[0225] 60 cuff

[0226] 61 lip

[0227] 62 cut-out

[0228] 63 ring

[0229] 64 flange

[0230] A1 first difference area

[0231] A2 second difference area

[0232] A31 first external cross-sectional area A32 second external cross-sectional area ACP bore cross-sectional area

[0233] CP casing pipe

[0234] ID tolerance

[0235] LA longitudinal axis

[0236] SCP internal surface

[0237] TB through-bore

Claims (15)

1. CLAIMS

1. A well tool (10) for sealing a pipe (CP) within a wellbore, the pipe (CP) having a through-bore (TB) with a bore cross-sectional area (ACP), wherein the well tool (10) comprises:

a mandrel (20);

a housing (30) circumferentially surrounding the mandrel (20); and

at least one drag block (40-1) for providing friction between the well tool (10) and the pipe (CP), wherein the at least one drag block (40-1) protrudes from the housing (30);

wherein:

the well tool (10) has a first region (31) that is free of drag blocks and a second region (32) that includes the at least one drag block (40-1);

the first region (31) is longitudinally contiguous with the second region (32);

a first difference area (A1) represents a difference between the bore crosssectional area (ACP) and a first external cross-sectional area (A31) of the well tool (10) in the first region (31) when the well tool (10) is in the through-bore (TB);

a second difference area (A2) represents a difference between the bore cross-sectional area (ACP) and a second external cross-sectional area (A32) of the well tool (10) in the second region (32) when the well tool (10) is in the through-bore (TB); and

the first difference area (A1) is equal to or smaller than the second difference area (A2).

2. The well tool (10) of claim 1, wherein the first difference area (A1) is at least 10% smaller than the second difference area (A2).

3. The well tool (10) of claim 2, wherein the first difference area (A1) is at least 20% smaller than the second difference area (A2).

4. The well tool (10) of any preceding claim, wherein the pipe (CP) has a predetermined tolerance, and wherein the bore cross-sectional area (ACP) is the crosssectional area of the pipe at a maximum internal diameter of the pipe based on the predetermined tolerance.

5. The well tool (10) of any preceding claim, wherein the at least one drag block (40-1) tapers radially inwardly towards the first region (31) and the housing (30) tapers radially outwardly towards the first region (31).

6. The well tool (10) of any preceding claim, wherein the well tool (10) is configurable in a radially retracted state and a radially expanded state, and wherein the first external cross-sectional area (A31) is an external cross-sectional area of the first region (31) when the well tool (10) is in the radially retracted state.

7. The well tool (10) of any preceding claim, wherein the first external cross-sectional area (A31) is constant along the first region (31).

8. The well tool (10) of any of claims 1 to 6, wherein the first external cross-sectional area (A31) comprises an average external cross-sectional area across the first region (31).

9. The well tool (10) of claim 8, wherein the well tool (10) consists of the first region (31) and the second region (32).

10. The well tool (10) of any of claims 1 to 8, wherein the well tool (10) has a distal end (11) and a proximal end (12), the distal end (11) being configured to enter the pipe (CP) before the proximal end (12), and wherein the first region (31) is longitudinally closer to the distal end (11) than the second region (32).

11. A well tool (10) for sealing a pipe (CP) within a wellbore, the well tool (10) comprising:

a mandrel (20);

a housing (30) circumferentially surrounding the mandrel (20);

a drag block (40-3) for providing friction between the well tool (10) and the pipe (CP), wherein the drag block (40-3) is provided within a cavity (42) of the housing (30); and

a fixing system (50) for fixing the drag block (40-3) to the housing (30), wherein the fixing system (50) comprises a cuff (60) that circumferentially surrounds the housing (30) and is moveable relative to the housing (30) to at least a first position and a second position, wherein, in the first position, the cuff (60) prevents removal of the drag block (40-3) from the cavity (42) , and, in the second position, the cuff (60) permits removal of the drag block (40-3) from the cavity.

12. The well tool (10) of claim 11, wherein the cuff (60) is moveable between the first position and the second position by rotation.

13. The well tool (10) of claim 12, wherein the cuff (60) comprises a cut-out (62) that aligns with the cavity (42) when the cuff (60) is in the second position.

14. The well tool (10) of any of claims 11 to 13, comprising a further drag block (40-4) provided within a further cavity of the housing (30), wherein:

the cuff (60) is moveable relative to the housing to a third position that is different to the first position or the second position;

in the first position, the cuff (60) prevents removal of the further drag block (40-4) from the further cavity; and

in the second position, the cuff (60) permits removal of the further drag block (40-3) from the further cavity.

15. The well tool (10) of claim 14, wherein the cuff (60) prevents removal of the further drag block (40-4) from the further cavity in the second position and the cuff (60) prevents removal of the drag block (40-3) from the cavity in the third position.