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Title:
POSITIONING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2017/065974
Kind Code:
A1
Abstract:
A system (100) for positioning a working tool (21) in a wellbore (1). The positioning system (100) comprises a casing element (110) with a marker (111, 112, 113) provided on an inner surface, the marker (111, 112, 113) having a distinct diameter different from the inner diameter of the casing element (110). The system further comprises a positioning tool (120) with a latching element (121, 122) adapted to form a latch (115) with the marker (111, 112, 113), and a force detector (130) adapted to detect an axial latching force (F£) applied to a tubing string (20) from the latch (115) when the casing element (110) is located within the wellbore (1) and the force detector (130) is located at a surface (2) outside the wellbore (1).

Inventors:
CLEVEN PETER KRIS (CA)
Application Number:
PCT/US2016/054157
Publication Date:
April 20, 2017
Filing Date:
September 28, 2016
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ANTONSEN ROGER (US)
CLEVEN PETER KRIS (CA)
International Classes:
E21B7/20; E21B23/00; E21B23/01; E21B23/02; E21B23/03
Foreign References:
US5579829A1996-12-03
US20140367114A12014-12-18
US20130008640A12013-01-10
US20120006109A12012-01-12
US4979561A1990-12-25
US4635718A1987-01-13
Attorney, Agent or Firm:
PIERSON, Gene (PLLC415 West 16th St,Suite, Houston TX, US)
Download PDF:
Claims:
Claims

1. A system (100) for positioning a working tool (21) in a wellbore (1),

wherein the positioning system (100) comprises

a casing element (110) with a marker (111, 112, 113) provided on an inner surface, the marker (111, 112, 113) having a distinct diameter different from the inner diameter of the casing element (110),

a positioning tool (120) with a latching element (121, 122) adapted to form a latch (115) with the marker (111, 112, 113) and

a force detector (130) adapted to detect an axial latching force (i¾ applied to a tubing string (20) from the latch (115) when the casing element (110) is located within the wellbore (1) and the force detector (130) is located at a surface (2) outside the wellbore (1).

2. The positioning system (100) according to claim 1, further comprising a spacer (122) configured to adapt the axial distance between the latching element (121) and the working tool (21) to the axial distance between the marker (111, 112, 113) and a desired position (12) for the working tool (21) in the wellbore (1).

3. The positioning system (100) according to claim 1 or 2, further comprising centering means (124) configured to keep the positioning tool (120) at a distance from the inner wall of the casing element (110).

4. The positioning system (100) according to any preceding claim, wherein the axial extension of the latch (115) varies along the wellbore.

5. The positioning system (100) according to any preceding claim, wherein the latch (115) comprises a roller ball (135) engaging walls that are inclined (a) with respect to a radial plane (R).

6. The positioning system (100) according to any preceding claim, wherein the latch (115) comprises a radially biasing spring (133, 137).

7. The positioning system (100) according to claim 6, wherein the latch (115) comprises a tensioner (142) configured to adjust the pre-tension of the biasing spring (133, 137).

8. The positioning system (100) according to any preceding claim, wherein the latch (115) comprises a piston area (141) configured to increase the latching force (i¾.

9. The positioning system (100) according to claim 6, wherein the piston area (141) is exposed to a central bore within the positioning tool (120).

10. The positioning system (100) according to claim 7 or 8, wherein the piston area (141) is opposed by a return spring (145).

Description:
Positioning system

BACKGROUND

Field of the invention

[0001] The present application concerns a system for positioning a tool in a wellbore.

Prior and related art

[0002] The present invention may be used in wells in general, i.e. regardless of related field of technology. For example, the invention may be used in a well for exploration, production or injection in the oil and gas industry, similar wells in geothermal applications or a well for producing ground water. The terminology varies slightly between the fields of technology, and sometimes within each field. Thus, the next few paragraphs describe well known procedures to precisely define terms that are central to the present invention.

[0003] Creating a well starts by drilling a borehole through a geological formation, e.g. soil, sand, clay and rock of various kinds. The drilling may involve rotating a drill string with a drill bit on a downhole end while a drilling fluid is pumped down within the central bore of the drill string. The drill bit cuts material from the formation, and the cuttings, e.g. soil, sand or crushed rock, are conveyed by the drill fluid through the annulus between the drill string and the borehole to the surface. As the drill string advances into the borehole, standard lengths of pipe are added to the surface end of the drill string. In the following, the term "joint" means such a pipe or tubing element with a standard externally threaded pin at one end, and a box with complementary internal threads at the opposite end. Further, we will use the term "axial" consistently for the direction along the longitudinal axis of such a tubing element, i.e. never along the axis of some arbitrary guide or cylinder. Similarly, the terms "radial" and "circumferential" should be construed relative to the pertinent tubing element, not relative to some arbitrary cylinder or sphere.

[0004] When the borehole has the required depth, the drill string is retrieved and a steel tubing known as a casing is inserted into the borehole to prevent sand or rock from entering and blocking the borehole. The casing is usually made by adding joints to the surface end in the same manner as the drill string was extended, i.e. by means of standard pins and boxes.

[0005] In some applications, i.e. relatively shallow wells in all of the above industries, one drilling operation and one casing operation suffices. In deeper wells, e.g. most wells in the oil and gas industry, the first casing is cemented to the formation. When the cement has cured, a drill bit with a slightly smaller diameter is inserted through the casing. The drill bit is connected to a drill string, and may be driven by rotating the drill string as described above. Alternatively, the drill bit may be rotated by a mud motor driven by the flow of drilling fluid through the central bore. When a new section is drilled, the drill string is retrieved, and a new casing with smaller outer diameter than the previous casing's inner diameter is inserted into the borehole and cemented to the formation. The sequence of drilling and casing is repeated with decreasing outer diameters to create a telescopic structure of casing. In some applications, the last section may remain uncased. The term "wellbore" as used herein means a partly or fully cased borehole, regardless of whether the casing is cemented to the formation or not. For example, a valve attached to a production string may be inserted through the casing to an uncased part of the wellbore.

[0006] In the oil and gas industry, there is a convention that a casing comprises tubing elements with outer diameter of 114.3 mm (4 ½ inches) and above. This convention does not necessarily apply to the present invention, e.g. in geothermal applications.

[0007] The drill bit may be steered in any direction relative to the Earth's crust by applying a suitable lateral force. An example is so-called "horizontal drilling" to provide a borehole that follows a layer containing hydrocarbons.

[0008] The terms "up" and "down" have different meanings in a vertical and a horizontal wellbore, and the terms "upstream" and "downstream" have different meanings during injection and production. In the following description and claims, we will use the term "uphole" for the direction toward the surface, regardless of any inclination relative to the vertical defined by gravity. As the term is used herein, "uphole" does not mean "at the surface". Similarly, in this description and the claims, "downhole" means the direction away from the surface, not "within the well(bore)".

[0009] Using the definitions above, the wellbore may comprise one or more casing sections. Each casing section comprises several joints with a standard outer diameter. If there are several casing sections, their outer diameters decrease in the downhole direction.

[0010] In any well, the casing merely isolates a fluid within from the geological formation around it. To complete a well, further operations and equipment are usually required. For example, a ground water well might comprise a production string with a pump and a valve to convey the ground water from a certain position along the casing to a location at the surface. Similarly, a production string with a throttle valve might be used in a geothermal application. In both examples, the production string with associated equipment may be retrieved from the wellbore for maintenance, while the casing remains in place and prevents soil, sand etc. from entering the well. In both examples, there is a need to position a valve at the location it had before the maintenance. [0011] In a third example, fetched from the oil and gas industry, a perforation gun, e.g. connected to a wireline or a tubing string, creates perforations in the casing at geological layers containing hydrocarbons. After the perforation gun is retrieved to the surface, there may be a need to treat the surrounding formation, e.g. by hydraulic fracturing or acid stimulation, to enhance production. In general, treating a well involves inserting a tubing string with an injection valve into the wellbore and pump an injectant into the formation. In particular, radial openings of an injection valve should preferably align with the previously created perforation, and a section isolated by packers uphole and downhole from the perforation should be as short as possible. Such treatment is often repeated several times during the lifetime of a well. During production, similar radial openings in a production valve should preferably align with the perforation. Thus, there is a need to locate different working tools, here represented by the perforation gun, injection and production valves and packers, at a certain location several times during the lifetime of a well.

[0012] As a tubing string with a working tool may take slightly different paths through the wellbore each time it is used, measuring the inserted length of tubing string is inaccurate. The uncertainty increases with distance from the surface, and may easily become impractically large some kilometers along the wellbore. Similar problems are encountered during logging, e.g. using a wireline or slickline tool. Thus, today many or most tool strings and tools for cased wells in the oil and gas industry include casing-collar locators for depth control.

[0013] A casing-collar locator essentially detects distortions in a magnetic field when the tool passes a metallic collar arranged on the casing. More precisely, like-faced magnets may be placed on opposite sides of a sensing coil. A current is induced in the sensing coil when it passes the thicker metal at the casing-collar. The resulting current or voltage signal is amplified to a so-called "collar kick", which may be sent to the surface or stored locally in the tool. Either way, the distance from a casing-collar is much smaller than from the surface, so the casing-collar locator significantly improves the accuracy of depth measurements and positioning. However, casing-collar locators comprise a downhole amplifier and possibly other components that must be protected from the temperature, pressure and potentially corrosive fluids within the well. In addition, the components must be fast and precise to locate the casing collar precisely. Thus, a casing-collar locator tends to be relatively expensive and precise or less expensive and less accurate, so there is a need for a more accurate and less expensive positioning tool.

[0014] US patent no. 9097079 discloses a fracturing port locator and isolation tool with dragging blocks that enter an annular groove caused by a shifted sliding sleeve, and thereby increases the dragging resistance. The tool comprises spring biased lugs, e.g. arranged on collet fingers, that expand radially into the annular groove created by a shifted sliding sleeve. The lugs may be arranged around the circumference of the tool with spaces between them to allow an axial flow.

[0015] The objective of the present invention is to provide a positioning system that is accurate, reliable and inexpensive to manufacture and operate, and that retains the benefits of prior art.

SUMMARY OF THE INVENTION

[0016] The objective is achieved by a positioning system according to claim 1.

[0017] More particularly, the invention provides a system for positioning a working tool in a wellbore. The positioning system comprises a casing element with a marker provided on an inner surface, the marker having a distinct diameter different from the inner diameter of the casing element. The system further comprises a positioning tool with a latching element adapted to form a latch with the marker and a force detector adapted to detect an axial latching force applied to a tubing string from the latch when the casing element is located within the wellbore and the force detector is located at a surface outside the wellbore.

[0018] The casing element preferably has length and threads similar to those of a standard joint in a casing section to facilitate inclusion in the casing section and installation in the wellbore. The latch may be any of four combinations of a radially movable element and a static element, of which one protrudes from a surface and the other has a complementary recess. As expected by the skilled person, the radially movable element may be activated by a spring force and/or a pressure.

[0019] The positioning system preferably comprises a spacer configured to adapt the axial distance between the latching element and the working tool to the axial distance between the marker and a desired position for the working tool in the wellbore. The spacer would be connected to the positioning tool, for example to ensure that radial ports in a valve align with a perforation made in a previous operation by a perforation gun with different length, and the latching element and marker form the same latch in both operations.

[0020] The positioning system also preferably comprises centering means configured to keep the positioning tool at a distance from the inner wall of the casing element. More specifically, the purpose of the centering means is to prevent the latching element from inadvertently engaging the marker. The centering means may be, for example, a leaf spring or a set of wheels or roller balls. [0021] The axial extension of the latch may vary along the wellbore. For example, the axial lengths of annular grooves in the casing members could increase in the downhole direction such that a protruding latching member with a certain axial extension would pass a number of markers before it enters the first groove with sufficient length. Alternatively, protruding markers could have decreasing lengths in the downhole direction to achieve the same effect.

[0022] Preferably, the latch comprises a roller ball engaging walls that are inclined with respect to a radial plane. The purpose is to reduce the effect of friction between a solid lug and the inclined wall, in particular those caused by a well fluid lubricating steel surfaces, as the lubrication would depend on the well fluid and possibly local temperature and pressure. Alternatively, the engaging lug may be coated with a suitable material such as PTFE to reduce the friction and thereby the effects of the friction.

[0023] In preferred embodiments, the latch comprises a radially biasing spring. As "radial" in the present description and claims refers to the pertinent casing element, the implied spring force works to press the latching member toward the marker or vice versa. Thus, the biasing spring provides a radial spring force to ensure that the latching member and marker latches when aligned. In principle, a pressure exerted on a suitable piston area to move the piston radially would achieve the same effect.

[0024] However, pre-tensioning a spring is probably more accurate and convenient than controlling a pressure accurately to provide a suitable radial biasing force. Hence, the latch preferably comprises a tensioner configured to adjust the pre-tension of the biasing spring.

[0025] In preferred embodiments, the latch also comprises a piston area configured to increase the latching force, i.e. the axial force that can be detected at the surface. The latching force is proportional to a radial force pressing the latching element against the marker or vice versa. Thus, an increased pressure on a piston area may increase a radial force that, multi- plied by a constant, provides an increased latching force. This may be used to vary the latching force along the wellbore by varying the pressure, and/or to ensure that the position- ning tool is securely latched to the casing member when the pressure increases. In principle, the varying pressure might be the static pressure along the wellbore acting against a low pressure chamber.

[0026] However, in embodiments with such a piston area, the piston area is preferably exposed to a central bore within the positioning tool, as this allows adjusting the latching force by adjusting the bore pressure within the tubing string. The piston area exposed to the bore pressure is preferably larger than a piston area exposed to the annulus between the tubing string 20 and the casing 10, such that no air filled or other low pressure chamber is needed.

[0027] The piston area is also preferably opposed by a return spring. That is, a pressure force applied to the piston area and the spring force from the return spring work in opposite directions. Thereby, the spring force increases as the pressure increases to displace the piston area, and the piston area is pushed back by the return spring when the pressure decreases. The radially biasing spring may double as the return spring through an inclined surface, e.g. on the tensioner.

[0028] Further features and benefits may appear from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will be explained with reference to exemplary embodiments and the accompanying drawings, in which:

Fig. 1 illustrates main components in a system according to the invention;

Fig. 2 illustrates embodiments with a spring biased member on a positioning tool;

Fig- 3 illustrates embodiments with a spring biased member on a casing element;

Figs. 4a-c illustrate a first embodiment of the system in three states of operation;

Fig. 5 illustrates forces acting in a latch; and

Figs. 6a-c illustrate a second embodiment of the system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0030] The drawings are schematic and not necessarily to scale. Numerous details known to the skilled person are omitted from the drawings and following description. Furthermore, in the claims, the terms "a", "an" and "the" means "at least one" and "one" means exactly one, whereas terms such as "several" and "at least one" may be used in the following detailed description for ease of understanding.

[0031] Fig. 1 illustrates a system 100 according to the invention. Reference numerals above 100 refer to components of the system, whereas reference numerals below 100 are provided to explain the context of the system 100 in use, and are not part of the invention as such.

[0032] More particularly, Fig. 1 depicts a wellbore 1 extending vertically and horizontally through a geological formation. The wellbore 1 may be located onshore or offshore, and the surface 2 may thus be a sea surface or dry land. The wellbore 1 is lined with a casing 10, which comprises casing joints with different outer diameters and standard lengths, pins and boxes as explained in the introduction. [0033] In use, one or more casing elements 110 according to the invention are included in the casing 10 at predefined locations. Casing elements 110 with different outer diameters may be provided in the system, e.g. for use in different sections in a "telescopic" casing as described in the introduction, or for use in different wells with different casing diameters.

[0034] Regardless of outer diameter, each casing element 110 in the system 100 has a marker 1 11 , 1 12, 113 on an inner surface, and may be of any length. Preferably, the casing element 110 has the standard length and standard threaded pins and boxes as an adjacent casing joint to facilitate inclusion in the pertinent section of casing 10 and subsequent installation in the wellbore.

[0035] A tubing string 20 extends from a rig 3 on the surface 2 to a position identified by a marker 1 11 on the inner surface of the casing element 1 10. The downhole end of the tubing string 20 comprises a positioning tool 120 with a latching element (121, Fig. 2) adapted to form a latch 115 with the marker 1 11. Reference numeral 21 illustrate an interchangeable tool included in the tubing string 20. Using the third example in the introduction, reference numeral 21 may represent a perforation gun and an injection valve, such that Fig. 1 illustrates an injection valve 21 located at a perforation 12 previously made by a perforation gun 21. Using the same positioning tool 120 and latch 115 for the perforation gun and injection valve ensures precise positioning. In the future, the inj ection valve 21 may be repeatedly located within some decimeters of the same location for repeated treatment, e.g. hydraulic fracturing or acid stimulation. Between treatments, a production valve 21 may be accurately positioned at the perforation 12 by aligning the latching element 121 on the positioning tool 120 with the marker 1 11 to create the latch 115.

[0036] In Fig. 1, the interchangeable working tool 21 and the positioning tool 120 are shown with slightly greater diameter than the tubing string 20 for illustration. In a real embodiment, one or both of the tools 21 and 120 will often have the same diameter as the tubing string 20. Furthermore, the positioning tool 120 might be located uphole from the interchangeable working tool 21, and a suitable spacer (122, Figs. 2 and 3) may be arranged between the positioning tool 120 and the working tool 21 to adapt the length of the tools assembly 21, 120 to an axial distance in the casing 10. For example, the length from the latching element 121 on the positioning tool 120 to radial openings in a sliding sleeve valve 21 should match the distance from the marker 1 11 to the perforation 12 made by the perforation gun 21.

[0037] When the latching element 121 on the positioning tool 120 engages the marker 11 1 in the casing 100, the resistance increases. More specifically, the axial force applied to the tubing string 20 increases by a latching force that should be easily detectable by a force detector 130 at the surface 2, e.g. in the rig 3. The required latching force depends on the force detector 130 and the distance between the surface 2 and the marker, e.g. markers 1 11 , 112, 113. The latching force must be distinguishable from normal force variations that occur when inserting the string 20 into the wellbore 1 , and could in some applications be in the range 50,000 -100,000 N, roughly corresponding to the weight of 5-10 metric tons. However, the latching force depends on the application, so suitable latching forces below and above this range are anticipated.

[0038] Marker 112 illustrate an annular groove in the inner surface of a second casing element 1 10, and marker 113 illustrate a third alternative with a smaller diameter than the inner diameter of the casing element 1 10. Either way, the marker 1 12, 1 13 has a diameter that is distinct from the inner diameter of the casing element 110.

[0039] The marker 112 in Fig. 1 has a larger width, i.e. axial extension, than the uphole marker 1 11. This illustrates that the axial extension of grooves 1 11 , 1 12 in some embodiments increases in the downhole direction, i.e. with the distance from the surface 2. In these embodiments, a radially biased lug 121 on the positioning tool might pass annular grooves 11 1 in the casing wall, and enter the first annular groove 1 12 with a sufficient width or axial extension. For the same effect, radially biased lugs 1 13 in the casing wall may enter a groove 123 in the outer wall of the positioning tool 120. In these alternative embodiments, the axial extension of the lugs 113 would decrease in the downhole direction. In the claims, these embodiments are combined in the expression "the axial extension of the latch varies along the wellbore".

[0040] Figure 2 illustrates an embodiment with an outwardly radially biased latching element 121 mounted on the positioning tool 120 and a complementary annular groove 1 11 representing the marker in the inner wall of the casing element 1 10. The latching element 121 is shown as a split ring, also known as a C-ring, to illustrate that there are obvious altematives to a lug 121 described with reference to Figs. 4a-c. When the tubing string 20 is pushed further downhole such that the latching element 121 aligns with the marker 1 1 1, the radially biased latching element 121 expands into the groove 1 11. This forms the latch 1 15 (Fig. 1) and provides the required latching force mentioned above. The currently preferred embodiment comprises a radially biased latching element on the positioning tool, because the number of casing elements 110 is likely to be greater the number of positioning tools, and a radially biased element is expected to be more expensive than a groove.

[0041] Figure 3 illustrates an embodiment similar to that shown in Fig. 2, but with an annular groove 123 around the positioning tool 120 representing the latching element 121 and an inwardly biased C-ring 1 13 mounted in the wall of casing element 110 representing the marker. While the embodiment in Fig. 2 is currently preferred, the embodiment in Fig. 3, possibly with a lug and/or collet fingers replacing the C-ring 1 13, is/are not excluded from the scope of the claims.

[0042] In contrast to the embodiment shown in Fig. 1, the positioning tools 120 in Figs. 2 and 3 are connected directly to the tubing string 20 in their uphole ends and to a spacer 122 in their downhole ends. The spacer 122 is connected between the positioning tool 120 and the interchangeable working tool 21 (Fig. 1) to adjust the length of the tools assembly 21, 120 as described previously.

[0043] From the above, it should be understood that the system 100 comprises several casing elements 110, each provided with a marker 1 11 , 1 12, 1 13, at least one positioning tool 120 with a latching element 121 or 123 capable of forming a latch 1 15 with a complementary marker 1 11 , 1 13 and a suitable force detector 130. The force detector 130 is a commercially available device and need no further explanation herein. In addition, the system 100 includes zero or more spacers 122 to adjust the position of a working tool 21 relative to the latch 115 formed at a marker 1 11 , 1 12, 1 13.

[0044] Optional centering means 124, e.g. wheels or leaf springs, are provided to guide the positioning tool 120 along the wellbore 1 , i.e. to maintain a distance from the inner wall of the casing 10 to the tool assembly. More precisely, the purpose of the centering means 124 is to prevent or inhibit the latching elements 121 on the positioning tool 120 from inadvertently engaging the marker 11 1, 112, 113 on one side when moving through the wellbore, as such engagement may cause a false position reading. The centering means 124 may be provided on the positioning tool 120, on a spacer 122 or on a separate sub. The embodiment shown in Fig. 1 comprises centering means 124 at the downhole end of the positioning tool 120 to keep the tool assembly 21, 120 away from the inner surface of the casing element 1 10.

[0045] The latch 1 15 may be any of four combinations of a biased element and a static element, of which one protrudes from a surface and the other has a complementary recess. However, the physics involved is similar in each combination, so only one combination need a detailed description. Thus, the following detailed description regards embodiments wherein the marker 11 1 is an annular groove in the casing member 110 and the latching element 121 is a complementary biased and protruding element on the positioning tool 120.

[0046] Figs. 4a-c shows an embodiment of the positioning tool 120 in different states. More specifically, Figs. 4a-c are sections through the casing element 1 10 and the positioning tool 120 on one side of a central cylinder axis along the positioning tool 120, and illustrate an embodiment with configurable resistance or latching force.

[0047] Fig. 4a illustrates the positioning tool 120 in a run-in state, in which a radially biased lug 121 approaches an annular groove 1 11 in a casing element 1 10. An optional casing collar 114 provides a radial extension or depth to the groove 1 11. Techniques for making and handling the casing collar 114 are well known in the art, e.g. from the electromagnetic casing- collar locators mentioned in the introduction.

[0048] The positioning tool 120 comprises a housing 130, a mandrel 150 fixed to the housing 130 and an optional piston 140 arranged radially between the housing 130 and the mandrel 150. The piston 140 may move axially relative to the housing 130 and mandrel 150. The centering means 124 maintains a distance between the positioning tool 120 and the inner wall of the casing element 1 10, and is shown as a wheel attached to the housing 130.

[0049] The housing 130 comprises a radial guide 131 for the lug 121. As shown, the guide 131 comprises a radial bore 132 with a biasing spring 133 arranged between the lug 121 and a support ball 134. The radial bore 132 has a section with extended diameter that limits the axial motion of the lug 121 by means of a collar, e.g. an external C-ring, on the lug 121. The extended diameter of the radial bore 132 is exaggerated to illustrate that a surface on the lug

121 may provide a piston area for a bore pressure. In such an embodiment, there would be a seal between the lug 121 and the part of the bore 132 with extended diameter, such that a bore pressure, i.e. below the lug 121 in Figs. 4a greater than an ambient wellbore pressure, i.e. above the lug 121 in Figs. 4a-c, would exert a net force toward the casing element 110. The resulting force would be a radial force component as the term "radial" is used herein.

[0050] The support ball 134 is configured to move within the radial bore 132, and engages an inclined surface 143 on a tensioner 142 such that an axial motion of the tensioner 142 causes a varying radial compression of the biasing spring 133.

[0051] For illustrative purposes, the guide 131 as shown comprises a slit, and the tensioner 142 is shown as a blade able to move axially within the slit. Numerous alternatives to achieve an adjustable biasing force will be apparent to the skilled person. For example, the guide 131 with the lug 121 may be simplified, and a frusto-conical tensioner 142 may replace the blade shown in Figs. 4a-c. As noted, the piston 140 is optional. A frusto-conical tensioner 142 with threads engaging complementary threads on the mandrel 150 is described with reference to Fig. 6c, and would allow an operator to set a fixed bias on the lugs 121 before the positioning tool 120 is inserted into the wellbore 1. [0052] In the example shown in Figs. 4a-c, the tensioner 142 in the form of a blade is attached to the piston 140. The piston 140 has a piston area 141 exposed to a central bore through the positioning tool 120, i.e. the region below the mandrel 150 in Figs. 4a-c. A retum spring 145 arranged axially between the piston 140 and a stopper 151 on the mandrel 150 opposes a pressure force exerted on the piston area 141. Thus, a varying pressure in the central bore causes the piston 140 to move axially with respect to the housing 130, and hence the tensioner 142 to move relative to the radial guide 131.

[0053] Fig. 4b is not intended to show an actual state, but to illustrate that the biasing spring 133 expands, and thereby provides a smaller biasing force, when the lug 121 enters the groove 1 11 in the casing element 1 10. For this, the entire positioning tool 120, including the piston 140, has moved an axial distance a within the casing element 110 from the position shown in Fig. 4a, such that the support ball 134 is in the same radial position in Fig. 4b as in Fig. 4a. In reality, the piston 140 with tensioner 142 would probably be displaced axially with respect to the housing 130 due to variations in the bore pressure within the central bore.

[0054] The radial groove 11 1 has walls that are inclined in the axial direction. In Fig. 4b this is illustrated by an angle of attack a formed between a radial plane R and a sidewall of the annular groove 11 1. The axial pull or push required to retract the lug 121 from the groove 1 11 depend on the angle of attack a, the radial force applied to the lug and perhaps the friction between the lug 121 and groove 1 11 as further explained with reference to Fig. 5.

[0055] Fig. 4c shows the tensioner 142 axially displaced a distance x relative to the housing 130 and mandrel 150. This contracts the retum spring 145 and provides an increased spring force on the piston assembly 140, 141, 142. It follows that the pressure exerted on the piston area 141 has increased from a circulation pressure implied by the run-in state illustrated in Fig. 4a.

[0056] In addition, the inclination β of the surface 143 to a central axis (not shown; parallel to the outer housing 130 in Fig. 4c) has caused the support ball 134 to move radially outward by a distance y = x-tan β relative to its radial position in Figs. 4a and 4b. This radial motion of the support ball 134 causes a contraction of the biasing spring 133 and an associated increase in the radial force applied to the lug 121.

[0057] Fig. 5 illustrates forces acting on a lug 121 in an annular grove 1 11 with inclined walls. More precisely, a side wall of the groove 11 1 forms an angle a with the radial plane R as shown in Fig. 4b. A radial force or bias FR is applied to the lug 121 in the radial direction into the groove 1 11. An axial force FA is applied to the lug 121 in the axial direction, i.e. parallel to the longitudinal axis through the casing element 1 10. FR and FA are decomposed in components parallel to and perpendicular to the inclined sidewall, respectively.

[0058] The lug 121 does not move transverse to the sidewall, so there must be a normal force with magnitude (FA- cos a + i¾-sin a) directed opposite the components of FR and FA in the transverse direction.

[0059] In the parallel direction, FA provides the only component pointing out of the groove 1 1 1, i.e. FA-sin a. This component must overcome the component FR-COS a from the radial bias and, in a general case, an additional friction force FF proportional to the normal force, i.e. FF = //-(T -COS a + «-sin a) where μ is a static coefficient of friction. Thus, the condition for moving the lug 121 out of the groove 1 1 1 is: i -sin a > FR-COS a + ,«-( < cos a + i¾-sin a) (1)

[0060] Setting FR = 0 in equation (1), yields tan a > μ regardless of axial force FA. This sets a minimum angle of attack a, which may be illustrated by a = 0: No practical axial force moves a lug past a truly radial steel wall.

[0061] Dividing all terms in equation (1) with cos a and rearranging yields:

F > F L - - L ^ ^ . F A - C F (2)

tana -

[0062] We note that tan a must be truly greater than μ to avoid a zero denominator, and that the latching force FL is proportional to the radial force FR. Thus, the proportionality constant C is easily determined by calculation using equation (2) if a and the static coefficient of friction μ are known, or by measuring the ratio FI/FR directly. This value C scales the latching force FA over a wide range of radial forces FR.

[0063] The static friction, i.e. μ, between steel surfaces may vary over a large range depending on the lubrication provided by the well fluid, which in turn may depend on temperature and pressure. Thus, it would be advantageous to reduce the dependency on friction. This may be achieved by reducing the friction to an insignificant level. Setting μ = 0 in equation (2) yields:

(3) [0064] The coefficient of friction may be reduced by coating the lug 121 with a suitable material, e.g. PTFE.

[0065] Figs. 6a and 6b illustrate an embodiment where a roller ball 135 replaces the lug 121 to reduce the static friction. More precisely, Fig. 6a is a cross section along the plane A-A in Fig. 6c, and shows two of several fingers 137 distributed around the housing 130. Each finger 137 may be resilient, and provide a radial spring force on the roller ball 135 toward the annular groove 111 similar to the spring force from the radial spring 133 on the lug 121 in the previous example. Alternatively, the fingers 137 may be rigid, hinged to the mandrel 150 and biased by a separate spring 133 as the one shown in Figs. 4a-c. The housing 130 has one or more longitudinal openings 138 such that the fingers 137 can move radially with respect to the housing 130.

[0066] Fig. 6b illustrates a possible arrangement for mounting the roller balls 135, in which each ball 135 is retained on a distal end of its associated finger 137 by two holders 136. Each holder 136 is essentially a plate with a first bore to receive part of the ball 135 and a second bore to receive a hinge element 139, e.g. a bolt, which couples the two holders 136 to the finger 137. A stopper (not shown) between the holder 136 and finger 137 is required to prevent the holder 136 from pivoting freely about the hinge element 139. This is perhaps best seen in Fig. 6c: If the holder 136 was allowed to pivot freely about the distal end of the finger 137, the roller ball 135 would not engage the walls with inclination a properly when an axial force is applied, and hence no proper latching force would be provided.

[0067] Clearances shown between the roller ball 135 and holder 136 in Fig. 6b are intended to allow the roller balls 135 to rotate in any direction and to allow a small particle to pass.

[0068] In principle, the fingers 137 and associated roller balls 135 could be closely spaced as indicated by the dotted finger and roller ball. For example, the angular displacement of adjacent fingers 137 could be 10° as indicated in Fig. 6b, yielding 36 roller balls and 72 holders distributed along the circumference. A latching force of 10 metric tons would then exert a weight of 140 kg, i.e. less than 1.4 kN, on each holder 136. Accordingly, the roller balls 135 and holders 136 might be made of relatively inexpensive materials and be designed for regular replacement.

[0069] However, a currently preferred embodiment comprises far less than 36 roller balls, e.g. six as implied by Fig. 6a, to allow an axial fluid flow between the fingers 137 and to reduce the number of movable parts. Finding a suitable balance between a large number of inexpensive parts and fewer, potentially more expensive, parts is a design issue left to the skilled person.

[0070] Fig. 6c shows the casing element 110 with its annular groove 11 1, the housing 130, piston 140 and mandrel 150 as in the embodiment shown in Figs. 4a-c. In contrast to the previous embodiment, the tensioner 142 is not attached to the piston 140, but engages threads 146 on the mandrel 150 such that a relative rotation causes an axial displacement of the tensioner 142 with respect to the mandrel 150. Thus, the tensioner 142 can be rotated on the mandrel 150 to provide a preset angle or bias to the finger 137. For example, an operator at the surface 2 may adjust the angle of the fingers 137 such that the roller balls 135 provide the optional centering means 124 described above. Alternatively, the tensioner 142 may be rotated on the mandrel 150 to permit the roller balls 135 to retract fully into the housing 130.

[0071] The piston 140 produces a radial force component on the spring 137 depending on the bore pressure as explained with reference to Figs. 4a-c. A return spring 145 similar to that shown in Figs. 4a-c may be provided in a real embodiment, but is omitted from Fig. 6c for simplicity. The radial force applied to the roller ball 135 as function of the axial positions of the piston 140 and the tensioner 142 on the mandrel 150 may be measured directly and/or calculated using known formulas for a spanned beam 137. Such measuring and/or calculation is/are within the abilities of the skilled person, and thus need no further explanation.

[0072] Another obvious embodiment would be to mount roller balls 135 and holders 136 on the lug 121 in Figs. 4a-c.

[0073] In Fig. 6a, the openings 138 are shown as discrete longitudinal slits in the housing 130 for illustration. Fig. 6c shows that the mandrel 150 connects the uphole and downhole parts of the positioning tool. Implementing the opening 138 by connecting separate uphole and downhole parts of the housing 130 to the mandrel 150 might be less expensive and more practical than providing several discrete slits 138 in the housing 130 as shown in Fig. 6a.

[0074] Continuing the example with inexpensive and easily replaceable roller balls 135 and holders 136, we note that the distal end of finger 137 may move a considerably longer radial distance than the proximal end at the right hand side of Fig. 6c. Hence, the fingers 137 may be permanently attached to a collet (not shown). If a collet with fingers 137 is significantly more expensive than the roller balls 135 and holders 136, the fingers 137 may alternatively be separated to facilitate the replacement. After replacement, the separate fingers may be aligned, and a wire string inserted through holes near their proximal ends. Then the ends of the wire string might be connected to form a continuous ring for ease of handling when the fingers 137 are arranged around the mandrel 150 and clamped in place, e.g. between the edge of opening 138 and the tensioner 142 as shown in Fig. 6c. In contrast, replacing the hinge elements 139 at the distal ends of the fingers 137 with a continuous wire string would be impractical. For example, a flexible wire string may fit the large diameter of the annular groove 11 1, but would be unable to retain the holders 136 within the housing 130 in a retracted state.

[0075] Summarized, preferred embodiments provide a radial force FR with a spring component and a pressure component. A minimum spring component can be preset by tensioning a spring 133, 137 by a tensioner 142. The radial pressure component can be used to provide a variable radial force, and thereby a pressure dependent latching force FL according to equation (2) or (3). Thus, the latching force FL may be adjusted along the wellbore if desired. Alternatively, the adjustable latching force might just ensure that the latch 1 15 is properly set after a suitable increase in bore pressure, for example at an injection pressure substantially higher than a circulation pressure applied during run-in.

[0076] While the invention has been explained by means of examples and certain embodiments, the scope of the invention is defined by the accompanying claims.