Patent Publication Number: US-9422794-B2

Title: System for lining a wellbore

Description:
PRIORITY CLAIM 
     The present application which is a 371 application of PCT/EP2012/051461, filed Jan. 30, 2012, claims priority from European application 11152987.1, filed Feb. 2, 2011. 
     The present invention relates to a system for lining a wellbore, the system comprising an expandable tubular element arranged in the wellbore. The wellbore is, for example, a wellbore for the production of hydrocarbon fluid. 
     During conventional wellbore drilling, sections of the wellbore are drilled and provided with a casing or a liner in subsequent steps. In each step, a drill string is lowered through the casings already installed in the wellbore, and a new wellbore section is drilled below the installed casings or liners. In view of this procedure, each casing that is to be installed in a newly drilled wellbore section must pass through a previously installed casing. Therefore the new casing has a smaller outer diameter than the inner diameter of the previous casing. As a consequence, the diameter of the wellbore available for the production of hydrocarbon fluid decreases with depth. For relatively deep wells this consequence can lead to impractically small diameters. 
     In conventional wellbore terminology the word “casing” refers to a tubular member extending from surface into the wellbore, and the word “liner” refers to a tubular member extending from a downhole location into the wellbore. However in the context of this description, references to “casing” and “liner” are made without such implied difference. 
     It has been proposed to overcome the problem of stepwise smaller inner diameters of wellbore casing by using a system whereby an expandable tubular element is lowered into the wellbore and thereafter radially expanded to a larger diameter using an expander which is pulled, pushed or pumped through the tubular element. 
     US-2004/0231860-A1 discloses such system whereby an end portion of an expandable tubular element is first expanded against the wellbore wall so as to anchor the end portion to the wellbore wall. An inflatable packer suspended on a deployment string is used to expand the end portion. Thereafter the deployment string is retrieved to surface, and a working string provided with an expander is lowered into the wellbore to expand the remainder of the tubular element. 
     It is a drawback of the known system that separate strings need to be run into the wellbore to anchor the end portion of the tubular element to the wellbore wall and thereafter to expand the remainder of the tubular element with the expander. Moreover during expansion with the expander, the expansion forces are relatively high since the expander moves away from the anchored end portion so that the tubular element is expanded under axial tensile forces. 
     U.S. Pat. No. 3,162,245 discloses a method and an apparatus for setting a metallic liner inside a casing in a well. The apparatus is used on a wireline. Upon igniting a propellant, the gases from the propellant press hydraulically-actuated slips against the casing wall. At the same time, the gas pressure is applied to a hydraulic cylinder and piston where it acts to force an expander cone through a corrugated tube expanding the tube out against the casing. When the cone reaches a rod, pressure on the rod actuates a firing mechanism which detonates a booster charge to destroy a fangible cylinder as well as said rod. 
     Disadvantages of the apparatus of U.S. Pat. No. 3,162,245 include the once-only use thereof, due to the destruction of the cylinder and rod. Debris will remain in the wellbore, possibly causing obstruction. Additionally, the apparatus is designed for use on a wire line, and all forces for expanding the corrugated tube are dealt with in a closed-loop system within the piston-cylinder assembly of the apparatus. The slips are not included in said loop and are unsuitable to exert expansion forces in axial direction to the casing. 
     It is an object of the invention to provide an improved system for lining a wellbore, which overcomes the drawback of the prior art. 
     In accordance with the invention there is provided a system for lining a wellbore, the system comprising an expandable tubular element arranged in the wellbore, the tubular element having a first end part and a second end part whereby the second end part extends into a tubular wall located in the wellbore, an expander arranged to radially expand the tubular element by movement of the expander through the tubular element in a direction from the first end part to the second end part, said direction defining an expansion direction, the system further comprising an anchor arranged to anchor said second end part to the tubular wall in a manner that the anchor substantially prevents movement of said second end part in the expansion direction and allows movement of said second end part in the direction opposite to the expansion direction. 
     The anchor provides the necessary reaction force to counter the expansion forces exerted to the tubular element by the expander, therefore there is no need for a separate string to first expand an end portion of the tubular element against the wellbore wall to provide the necessary reaction force. At the same time, the anchor compensates for axial shortening of the tubular element during the expansion process by allowing the second end part to move in the direction opposite to the expansion direction. Furthermore, the expansion forces are relatively low since the tubular element is expanded under axial compression by virtue of the expander being moved towards the anchor. 
     Suitably the anchor is provided with an anchor body and at least one anchor member arranged to grip said tubular wall upon a selected movement of the anchor body in the expansion direction, and wherein the anchor member is arranged to release said tubular wall upon a selected movement of the anchor body in the direction opposite to the expansion direction. For example, the anchor can be provided with a plurality of said anchor members mutually spaced in circumferential direction of the anchor. 
     To allow easy lowering of the anchor into the wellbore, it is preferred that each anchor member is movable between a radially extended position in which the anchor member is extended against said tubular wall and a radially retracted position in which the anchor member is retracted from said tubular wall. 
     Each anchor member is preferably controlled from surface by an elongate string extending from surface to the anchor, wherein the elongate string is arranged to cooperate with the anchor so as to move each anchor member between the extended position and the retracted position thereof. 
     Suitably each anchor member is movable to the extended position by an activating parameter selected from hydraulic pressure in the elongate string, a sequence of rotations and/or translations of the elongate string, and a combination of hydraulic pressure in the elongate string and a sequence of rotations and/or translations of the elongate string. The elongate string can be, for example, a drill string. 
     In an exemplary embodiment the drill string (or other elongate string) passes through a central passage of the anchor body, whereby the drill string is provided with a mandrel arranged in the central passage. The mandrel is temporarily connected to the anchor body by one or more shear pins which are arranged to break by the action of hydraulic pressure in the bore of the elongate string. Thus, upon failure of the shear pins, the anchor body becomes disconnected from the drill string. At the same time, the hydraulic pressure induces each anchor member to be moved to its radially extended position. 
     In an alternative embodiment the mandrel is provided with at least one pin, whereby each pin can move through a corresponding J-lock shaped groove provided at the inner surface of the anchor body, comparable to the mechanism in a ball point. During running-in of the assembly into the wellbore, the pins carry the anchor by means of the J-lock shaped grooves. Once the assembly is at target depth, a sequence of drill string rotation(s) and translation(s) enables each pin to pass through the corresponding groove and release the anchor body from the mandrel. To activate the anchor members, the top of the anchor body is provided with friction blocks which drag along the surrounding tubular wall when the anchor moves relative to the surrounding wall. Thus, when the anchor is moved upwards by the tubular element which is to be expanded, the drag force between the friction blocks and the surrounding wall causes each anchor member to be pushed radially outward into engagement with the surrounding wall. 
     In a preferred embodiment the elongate string is provided with a release sub and the anchor is provided with a release device, the release sub and the release device being arranged to cooperate with each other so as to induce the anchor member to move to the retracted position upon pulling of the release sub against the release device. 
     To ensure that the expander is properly positioned before being pulled into the tubular element, the system preferably includes a centraliser for centralising the expander relative to the tubular element, the centraliser extending into said first end part of the tubular element and being releasably connected thereto. Suitably the centraliser is adapted to be released from the first end part of the tubular element upon pulling of the expander through the tubular element in the expansion direction. 
     In practice there will be an annular space between the tubular element and the wall of the wellbore, which can be filled with cement to seal against the rock formation and to affix the tubular element in the wellbore after expansion. In order to prevent flow-back of fluidic cement into the tubular element during expansion of the tubular element, it is preferred that the tubular element is provided with sealing means for sealing the annular space, the sealing means including a foldable wall section of the tubular element, the foldable wall section having a reduced bending stiffness relative to a remainder wall section of the tubular element and being deformable from an unfolded mode to a folded mode by application of a compressive folding force to the tubular element, wherein the foldable wall section when in the folded mode comprises at least one annular fold extending radially outward into said annular space. By virtue of the foldable wall section, the tubular element can be lowered into the wellbore with the foldable wall section in the unfolded mode. Thereafter the foldable wall section can be deformed to the folded mode. Thus, the sealing means does not form an obstruction during the lowering process and therefore there is a reduced risk of the tubular element becoming stuck during the lowering process. 
     In a preferred embodiment said wall section of reduced bending stiffness comprises a wall section of reduced thickness relative to said remainder wall section. For example, the wall section of reduced thickness in the folded mode thereof comprises a plurality of folds in a concertina shape. 
     In order to initiate folding of the section of reduced wall thickness at a predetermined location and/or to reduce the magnitude of the folding force during an initial stage of the folding process, it is preferred that the section of reduced wall thickness is provided with a relatively small annular groove extending in circumferential direction along at least one of the inner surface and the outer surface of the section of reduced wall thickness. 
     Also, the wall section of reduced bending stiffness can comprise a plurality of annular grooves formed in the tubular element, wherein each fold has an upper leg extending between a first annular groove and a second annular groove, and a lower leg extending between the second annular groove and a third annular groove. 
     An expansion force needs to be applied to the expander in order to move the expander through the tubular element during radial expansion of the tubular element. It is preferred that the reduced bending stiffness of the foldable wall section is selected such that the magnitude of said folding force is lower than the magnitude of the expansion force. It is thereby achieved that the foldable wall section is deformed into the folded mode by the compressive force exerted by the expander before the expander starts expanding the tubular element. This is advantageous because each fold thus formed is further expanded as the expander passes through the fold. As a result, the folded wall section has a relatively large expansion ratio. 
     In an attractive embodiment of the system of the invention, said first end part is a lower end part of the tubular element, and said second end part is an upper end part of the tubular element. 
     The anchor is suitably referred to as “top anchor”. To ensure that the first end part of the tubular element remains at a selected depth during the expansion process, and thereby provides a reference point for a next tubular element to be installed in the wellbore, it is preferred that the first end part is provided with a bottom anchor adapted to anchor the first end part to the wall of the wellbore as a result of radial expansion of said first end part by the expander. With the first end part anchored to the wellbore wall by the bottom anchor, axial shortening of the tubular element due to the expansion process is accommodated by the top anchor which allows movement of the second end part of the tubular element in the direction opposite to the expansion direction. 
    
    
     
       The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawings in which: 
         FIG. 1  schematically shows, in longitudinal section, an embodiment of the system for lining a wellbore according to the invention, whereby an expandable tubular element extends in the wellbore; 
         FIG. 2  schematically shows a detail of a top anchor of the embodiment of  FIG. 1 ; 
         FIG. 3  schematically shows a first embodiment of a lower wall portion of the tubular element; 
         FIG. 4  schematically shows a second embodiment of a lower wall portion of the tubular element; 
         FIG. 5  schematically shows a third embodiment of a lower wall portion of the tubular element; 
         FIG. 6  schematically shows a fourth embodiment of a lower wall portion of the tubular element; 
         FIG. 7  schematically shows the fourth embodiment after folding of the lower wall portion; 
         FIG. 8  schematically shows a fifth embodiment of a lower wall portion of the tubular element; 
         FIG. 9  schematically shows the fifth embodiment after folding of the lower wall portion; 
         FIG. 10  schematically shows a detail of a bottom anchor of the embodiment of  FIG. 1 ; 
         FIG. 11  schematically shows the bottom anchor during radial expansion of the tubular element; 
         FIG. 12  schematically shows a perspective view of the bottom anchor; 
         FIG. 13  schematically shows the embodiment of  FIG. 1  after cement has been pumped into the wellbore and the top anchor has been extended against a casing in the wellbore; 
         FIG. 14  schematically shows the embodiment of  FIG. 1  during radial expansion of the tubular element; and 
         FIG. 15  shows an alternative embodiment of the system of the invention. 
     
    
    
     In the detailed description hereinafter, like reference numerals relate to like components. 
     Referring to  FIG. 1  there is shown a wellbore  1  extending into an earth formation  2 . The wellbore  1  is provided with a casing  3  or similar tubular element which has been cemented in the wellbore  1 . An open hole section  4  of the wellbore  1  extends below the casing  3 . Reference numeral  5  indicates the wall of open wellbore section  4 . An expandable tubular element in the form of expandable liner  6  is suspended in the open wellbore section  4 . An annular space  7  is formed between the expandable liner  6  and the wellbore wall  5 . 
     The liner  6  has a first or downhole end part  16  and a second or uphole end part  8 . The second end part  8  extends into the casing  3 . Throughout this specification, an upper end may indicate an uphole end, whereas lower end may be used to indicate the downhole end on any of the described features. 
     A drill string  10  extends from a drilling rig, or workover rig, at surface (not shown) into the wellbore  1  and passes through the interior space of liner  6 . The drill string  10  is at its downhole end provided with a conical expander  12  adapted to radially expand the liner  6 . The rig is adapted to pull the drill string  10  with the expander  12  connected thereto towards surface through the liner  6 . Towards surface herein may imply in upward direction as well as partly horizontal direction. The drill string  10  is further provided with an on/off sub  11  which allows the drill string  10  to be disconnected from the expander  12  if required. 
     The diameter of the expander  12  is such that the expander  12  will expand the upper end  8  of the liner  6  forcedly against the inner surface of the casing  3  so that a tight connection is achieved between the upper end  8  of the liner  6  and the casing  3 . The drill string  10  and the expander  12  have a common central bore  13  which provides fluid communication between a pumping facility at surface (not shown) and the open wellbore section  4 . The central bore  13  is provided with a dart catcher  14  (or ball catcher) for receiving a dart (or a ball) that may be pumped through the central bore  13  of the drill string  10 . 
     As shown in  FIG. 1 , the expander  12  is positioned below the liner  6  before expansion of the liner is started. The expander  12  is at its upper end provided with a centraliser  15  for centralising the expander  12  relative to the liner  6 . The centraliser  15  extends into a second end part  16  of the liner  6 . Said second end  16  is a downhole or lower end. The centralizer is connected to the liner  6  by a releasable connection (not shown), for example one or more shear pins. The releasable connection automatically disconnects when the drill string  10  pulls expander  12  upwards through the liner  6 . Thus before expansion of the liner  6  commences, liner  6  is supported in the wellbore  1  by the drill string  10 . Herein the weight of the liner  6  is transferred via the expander  12  to the drill string  10 . Furthermore, the drill string  10  is provided with a release sub  18  arranged a short distance above the centraliser  15 . The function of the release sub  18  will be explained hereinafter. 
     The upper end of the liner  6  is provided with a top anchor  20  comprising an anchor body  22  and a plurality of anchor members  24  mutually spaced along the circumference of anchor body  22 . The top anchor  20  is releasably connected to the liner  6  by arms  26  extending from the anchor body  22  into the liner  6  and clamped to the inner surface of the liner  6 . 
       FIG. 2  shows a detail of the top anchor  20 , indicating one of the anchor members  24 , the other anchor members being similar in design and functionality. The anchor member  24  has a serrated outer surface forming teeth  28 , and a slanted inner surface  30  resting against a corresponding slanted surface  32  of a support element  34 . The slanted surface  30  and the corresponding slanted surface  32  are complementary in shape. The anchor member  24  and the support element  34  are arranged in a chamber  36  of the anchor body  22 , whereby both the anchor member  24  and the support element  34  are radially movable in chamber  36  between a retracted position and an extended position. The anchor member  24 , when in the extended position, extends radially outward from chamber  36  and engages the inner surface of the liner  6 . In the retracted position, the anchor member  24  is free from the inner surface of the liner  6 . To move the anchor member  24  and the support element  34  between their respective retracted and extended positions, a hydraulic actuator  38  is provided in the chamber  36 , the hydraulic actuator  38  being in fluid communication with the central bore  13  of the drill string  10  at a location above the dart catcher  14  so as to allow the hydraulic actuator  38  to be controlled by fluid pressure in the central bore of the drill string  10  when the central bore  13  is blocked by a dart (or ball) received in the catcher  14 . The top anchor  20  is further provided with a release device (not shown) arranged to induce the support element  34  and the anchor member  24  to move to their respective retracted position when the release sub  18  of the drill string  10  is pulled against the release device of the top anchor  20 . 
     Further, the anchor member  24  has some axial clearance in the chamber  36  so as to allow anchor member  24  to slide in axial direction a short distance along the slanted surface  32  of support element  34 . As a result of such sliding movement along the slanted surface  32 , the anchor member  24  when in the extended position firmly grips the inner surface of the casing  3  if the anchor body  22  is moved upwards a short distance, and the anchor member  24  releases the inner surface of the casing  3  if the anchor body  22  is moved downwards. In this manner it is achieved that the upper end part  8  of the liner  6  is allowed to move downwards due to axial shortening of the liner during radial expansion, while the top anchor  20  substantially prevents upward movement of upper end part  8  of the liner  6 . 
     In a practical embodiment, a ramp angle α of the slanted surface  32  is in the range of about 5 to 30 degrees, for instance 8 to 20 degrees. An angle β, i.e. the top angle of teeth  28  on the anchor members  24  is in the range of about 60 to 120 degrees. Herein, a top surface of the teeth is substantially perpendicular to the axis of the drill string. A length or height L 1  of the anchor member  24  is for instance in the range of about 0.5 to 3 times the diameter of the expandable casing  6 . The axial clearance L 2 , i.e. a maximum stroke length of the anchor members, is for instance in the order of (diameter host casing  3 −diameter expandable casing  6 )/2/tan(alpha):
 
 L 2=˜(diameter casing 3−diameter liner 6)/2/tan(α).
 
     The length of height L 3  of the chamber  36  is in the order of the length L 1  of the anchor members  24 +the stroke L 2  of the anchor members  24 . 
     Reference is further made to  FIGS. 3-9  showing, in longitudinal section, various embodiments of a foldable wall section  39  of the lower end part  16  of the liner  6 . In each embodiment, reference numeral  40  indicates the central longitudinal axis of the liner  6 . 
     In the first embodiment, shown in  FIG. 3 , an outer annular groove  45  is formed at the outer surface of the lower end part  16 . 
     In the second embodiment, shown in  FIG. 4 , an outer annular groove  46  is formed at the outer surface and two inner annular grooves  47 ,  48  are formed at the inner surface of the lower end part  16 . The inner grooves  47 ,  48  are symmetrically arranged relative the outer groove  46 . 
     In the third embodiment, shown in  FIG. 5 , an inner annular groove  49  is formed at the inner surface and two outer annular grooves  50 ,  51  are formed at the outer surface of the lower end part  16 , the outer grooves  50 ,  51  being symmetrically arranged relative the inner groove  49 . 
     In the fourth embodiment, shown in  FIGS. 6 and 7 , the foldable wall section  39  includes an inner annular groove  52  at the inner surface and two outer annular grooves  53 ,  54  at the outer surface of the lower end part  16 , the outer grooves  53 ,  54  being symmetrically arranged relative to the inner groove  52 . The inner groove  52  tapers in radially outward direction. By virtue of the presence of the annular grooves  52 ,  53 ,  54 , the lower end part  16  of the liner  6  is deformable from an unfolded mode ( FIG. 6 ) to a folded mode ( FIG. 7 ) by application of a selected compressive force to the lower end part  16 . In the folded mode, an annular fold  55  is formed in the lower end part  16  of the liner. The annular fold  55  has an upper leg  55   a  extending between the outer groove  53  and the inner groove  52 , and a lower leg  55   b  extending between the inner groove  52  and the outer groove  54 . Hereinafter the compressive force that needs to be applied to the lower end part  16  to form the annular fold  55 , is referred to as “folding force”. It will be apparent that the magnitude of the folding force depends on the design characteristics of the lower end part  16 , i.e. the material properties of the liner wall, the wall thickness, the depth and width of the annular grooves, and the axial spacing between the grooves. For example, the folding force decreases with decreasing bending stiffness of the wall of the liner  6  or with increasing depth of the grooves  52 ,  53 ,  54 . Also, the folding force increases with increasing axial spacing between the grooves  52 ,  53 ,  54 . It is preferred that these design characteristics are selected such that the folding force is of lower magnitude than the force required to pull the expander  12  through the liner  6  during radial expansion of the liner  6 , for reason explained hereinafter. 
     The first, second and third embodiments of the foldable wall section described hereinbefore with reference to  FIGS. 3-5 , are deformable from an unfolded mode to a folded mode in a manner similar to deformation of the foldable wall section of the fourth embodiment. 
     In the fifth embodiment, shown in  FIGS. 8 and 9 , the foldable wall section  39  is formed by a section of reduced wall thickness  56  where the wall is recessed at both the inner surface and the outer surface. By virtue of the recessed wall section  56 , the lower end part  16  of the liner  6  is deformable from an unfolded mode ( FIG. 8 ) to a folded mode ( FIG. 9 ) by application of a selected compressive force to the lower end part  16  of the liner  6 , which compressive force is again referred to as “folding force”. In the folded mode, a plurality of annular folds is formed in the lower end part  16  of the liner. The present example shows two annular folds  57 ,  58  in a concertina shape, however more annular folds can be formed in similar manner. The magnitude of the folding force depends on the design characteristics of the lower end part  16 , i.e. the material properties of the liner wall, the wall thickness of the recessed section  56  of the liner  6 , and the axial length of the recessed section  56 . For example, the folding force decreases with decreasing bending stiffness of the recessed section  56  or with decreasing wall thickness of the recessed section  56 . It is preferred that these design characteristics are selected such that the folding force is of lower magnitude than the force required to pull the expander  12  through the liner  6  during radial expansion of the liner  6 , for reason explained hereinafter. 
     Referring further to  FIGS. 10-12 , the lower end part  16  of liner  6  is provided with bottom anchors  59 , each bottom anchor  59  being adapted to engage the wellbore wall  5  as a result of radial expansion of the lower end part  16  so that the lower end part  16  becomes anchored to the wellbore wall  5 . In  FIG. 1 , three such bottom anchors  59  are indicated. However any other suitable number of bottom anchors  59  can be applied. 
     Each bottom anchor  59  comprises an anchor arm  60  and a wedge member  62 , both mounted on the outer surface of the lower end part  16  of liner  6  and vertically displaced from each other. The anchor arm  60  is provided with annular grooves  63   a ,  63   b ,  63   c  forming plastic hinges allowing radially outward bending of the anchor arm. Although three annular grooves are shown, any other number of grooves can be applied in accordance with circumstances. Furthermore, the anchor arm  60  has a fixed end  64  affixed to the outside of liner  6 , for example by welding or other suitable means, and a free end  65  extending toward wedge member  62 . The free end  65 , also referred to as “tip”, is not affixed to the outside of liner  6  so that all of anchor arm  60  except fixed end  64  is free to move relative to liner  6 . The anchor arm  60  may be constructed such that its inner diameter is the same as or greater than the unexpanded outside diameter of liner  6 . 
     Similarly, wedge member  62  includes a fixed end  66  affixed to liner  6 , for example by welding or other suitable means. The free other end of the wedge member  62  extends toward the anchor arm  60  and defines a brace  68  having a length L B . Brace  68  is not affixed to the outside of liner  6  and is free to move relative to the liner  6 . At the free end, wedge member  62  includes a ramp  70  extending toward the anchor arm  60  and touching, or nearly touching, the free end  65  of the anchor arm  60 . The ramp  70  may be constructed with any desired surface angle and may be integral with or a separate piece from brace  68 . The thickness of each wedge member  62  and anchor arm  60  is a matter of design, but is limited by the maximum allowable diameter of the system prior to expansion. 
     Anchor arm  60  and wedge member  62  can each have either an annular and/or a segmented construction. In a segmented construction, anchor arm  60  and/or wedge member  62  may comprise longitudinal strips, rods, or plates. As shown in  FIG. 12 , the anchor arm  60  and the wedge member  62  each comprise for instance eight strips  72 ,  74  respectively. The strips  72 ,  74  extend around the outer circumference of the liner  6 . Optionally, the strips of the anchor arm  60  and/or the wedge member  62  include a segmented section, comprising strips or fingers  76  of smaller width than the strips. The anchor arm and the wedge member may include any number of strips  72 ,  74  and/or corresponding fingers  76  suitable in relation to the size of the liner  6 . 
     Hereinafter normal operation of the system of  FIG. 1  is explained whereby it is assumed that the lower end part  16  of the liner  6  is provided with the fourth embodiment of the foldable wall section (shown in  FIGS. 6 and 7 ). Normal operation of the system, if provided with the other embodiments of the foldable wall section, is similar to normal operation of the system provided with the fourth embodiment. Further it is assumed that the open wellbore section  4  has already been drilled using a conventional drill string (not shown) which has been removed from the wellbore  1 . 
     During normal operation, the assembly formed by the drill string  10 , the expander  12 , the centraliser  15 , the expandable liner  6  and the top anchor  20  is lowered on the drill string  10  into the wellbore until the major part of the liner  6  is positioned in the open wellbore section  4  whereby only the upper end part  8  of the liner extends into the casing  3  (as shown in  FIG. 1 ). The anchor members  24  of the top anchor  20  are in the retracted position during the lowering operation. 
     Referring further to  FIG. 13 , in a next step a slurry of cement is pumped from surface via the central bore  13  of the drill string  10  and the expander  12  into the open wellbore section  4 . The cement slurry flows into the annular space  7  between the liner  6  and the wellbore wall  5  so as to form a body of cement  80  which is still in fluidic state. Thereafter a dart (not shown) is pumped using a stream of fluid, for example drilling fluid, through the central bore  13 . When the dart enters the dart catcher  14 , any further passage of fluid through the central bore  13  is blocked. As a result a pressure pulse is generated in the stream of fluid, which induces the actuators  38  to move the respective anchor members  24  to their extended position so that the anchor members  24  become engaged with the inner surface of the liner  6 . The fluid pressure in the stream of fluid is then temporarily further increased to release the dart from the dart catcher  14  and thereby to restore the hydraulic connection between the open hole section  4  and the drilling rig at surface. 
     Referring further to  FIG. 14 , in a next step an upward pulling force is applied to the drill string  10  so that the assembly formed by the drill string  10 , the expander  12 , the centraliser  15 , the expandable liner  6  and the top anchor  20  moves upwards an incremental distance. While the anchor body  22  moves upwards, the anchor members  24  have a tendency of remaining stationary due to friction between the anchor members  24  and the inner surface of the liner  6 . As a result the anchor members  24  slide downwards relative to the support elements  34  whereby the anchor members  24  are forced radially outward into a gripping engagement with the inner surface of the casing  3 . In this manner the top anchor  20  is activated and prevents any further upward movement of the liner  6  in the wellbore  1 . 
     The upward pulling force applied from surface to the drill string  10  is then further increased until the compressive force exerted by the expander  12  to the lower end part  16  of the liner  6  reaches the magnitude of the folding force. Upon reaching the folding force, the foldable wall section of the lower end part  16  moves from the unfolded mode to the folded mode whereby the annular fold  55  is formed. The fold  55  extends radially outward from the remainder of the liner  6  and into the annular space  7 . The fold  55  thus formed may locally contact the wellbore wall  5 , however that is a not yet a requirement. 
     After the fold  55  has been formed, the upward pulling force applied to the drill string  10  is further increased until the upward force exerted to the expander  12  reaches the magnitude of the expansion force which is the force required to pull the expander  12  through the liner  6  during expansion of the liner  6 . The expander  12  is thereby pulled into the lower end part  16  of the liner  6  and starts expanding the liner  6 . The centraliser  15  becomes automatically disconnected from the liner  6  by virtue of the upward movement of the expander  12 . If, for example, shear pins are used to connect the centraliser  15  to the liner  6 , such shear pins shear-off upon upward movement of the expander. 
     As a result of radial expansion of the lower end part  16  of the liner  6 , the fold  55  is radially expanded and is thereby compressed against the wellbore wall  5 . In this manner the expanded annular fold  55  forms a sealing member that seals an upper portion  90  of the annular space  7  above the fold  55  from a lower portion  92  of the annular space below the fold  55 . Since the fold  55  is formed at the lower end part  16  of the liner, which is near the wellbore bottom, the lower portion  92  of the annular space is of minor volume relative to the upper portion  90 . By virtue of the fold  55  forming a sealing member, no substantial flow-back of fluidic cement  80  from the upper portion  90  of the annular space  7  into the lower portion  92  occurs during further expansion of the liner  6 . 
     The expansion process then proceeds by pulling the expander  12  further upwards through the liner  6 . The liner  6  is subject to axial shortening due to the expansion process. Therefore, as the expander  12  passes through the lower end part  16  of the liner, at each bottom anchor  59  the axial distance between the fixed end  64  of the anchor arm  60  and the fixed end  66  of the wedge member  62  decreases. As a result, the free end  65  of the anchor arm slides onto the ramp  70  and toward the borehole wall  5 , thereby overlapping the ramp  70  and extending radially outward from the liner  6 . Preferably the length of the anchor arm  60  is selected such that the free end  65  thereof engages the borehole wall  5  by the time that the expander  12  passes the ramp  70 . 
     The expander  12  subsequently progresses beyond the ramp  70 , and the liner  6  continues to expand and shorten at the position of the expander. Due to the shortening, fixed end  64  of wedge member  62  moves toward anchor arm  60 , and as a result ramp  70  is pushed against anchor arm  60 . If the radial force on the free end of anchor arm  60 , which is induced by shortening of the liner  6  due to expansion thereof, is greater than the local resistance or strength of the formation, the tip of the anchor arm  60  at the free end thereof will penetrate further into the formation. 
     However, if said radial force is smaller than or equal to the local resistance or strength of the formation, the tip  65  of the anchor arm  60  will be unable to penetrate further into the formation. In that case, anchor arm  60  will be held in place by the formation and ramp  70  will in turn be held in place by anchor arm  60 . With the brace  68  of wedge member  62  unable to slide further along the outside of liner  6 , no further shortening can occur. The final distance between fixed end  66  of wedge member  62  and fixed end  64  of anchor arm  60  is reached once the expansion device has moved past the fixed end  66  of the wedge member  62 . If the free end of the wedge member  62 , which comprises the ramp  70 , is held in place by the anchor arm, the maximum load that is applied to the wall of the liner  6  is about equal to the so-called fixed-fixed load. The fixed-fixed load is the local load that is applied to the liner wall when the expander  12  moves between two points at which the liner is fixed, such that the liner cannot shorten between the two points. As the fixed-fixed load can be determined beforehand, for instance during lab tests, the anchor arm  60  of the invention can be designed such that the radial force exerted on the formation does not exceed the maximum allowable radial load applied to the wall of the liner  6 . Thus, the anchor arm of the present invention ensures that the liner wall can be sufficiently strong to withstand the maximum radial force during expansion, so that the wall will remain substantially circular (in cross-section) when the anchor arm engages the formation. This embodiment allows the liner  6  to be designed so as to avoid collapse, even in the event that the formation is too hard to receive the anchor arm  60 , as the maximum load on the liner wall will not exceed the fixed-fixed load, which can be calculated or at least determined empirically. In this manner it is prevented that collapse, rupture, or similar damage to the liner wall occurs during the expansion process. As indicated above, if the expandable liner  6  were damaged, the entire downhole section could be rendered useless and would then have to be removed, at considerable costs. The expandable liner arrangement of the present invention thus greatly improves reliability in this respect. 
     The radial load during expansion on the liner  6  and on the formation depends for instance on one or more of the surface angle of the ramp  70 , the friction between the wedge member  62  and the liner  6 , the friction between the wedge member  62  and the anchor arm  60 , the formation hardness, the distance between the liner wall and the formation during expansion, etc. The surface angle of the ramp is preferably designed such that a maximum radial force is applied, whereas at the same time the radial load remains within the radial collapse load of the liner. 
     As the radial and axial loads on the wall of the tubular element are limited, the present embodiment is suitable for relatively hard formations, such as those, for example, having a strength or hardness of for instance 3000 psi (20 MPa) to 4000 psi (28 MPa) or more. In addition, the radial load on the wall can be limited by limiting the overlap between the anchor arm and the wedge member, and/or by limiting the contact area between the anchor arm and the formation. In a practical embodiment, the surface angle of the ramp  70  is in the range of 30 to 60 degrees, for instance about 45 degrees. 
     In this manner the lower end part  16  of the liner  6  is firmly anchored to the wellbore wall  5  after expansion of the lower end part  16 . Therefore the position of the lower end part  16  in the wellbore  1  does not change anymore during further expansion of the liner, and thereby provides a reference point, for example during installation of a next tubular element in the wellbore at a later stage or during a workover operation in the wellbore. This is advantageous since it obviates the need to determine the position of the lower end part  16  of the liner  6  at such later stage. 
     With the lower end part  16  of the liner firmly anchored to the wellbore wall  5 , the expander  12  is further pulled upwards through the liner  6  so as to radially expand the remaining part of the liner. The upper end of the liner with the top anchor  20  connected thereto moves downwards due to axial shortening of the liner during the expansion process, whereby the anchor members  24  automatically release the inner surface of the casing  3  as explained hereinbefore. As the expander  12  passes through the upper end part  8  of the liner  6 , said upper end part  8  is thereby clad against the casing  3  so as to form a strong and fluid tight connection between the expanded liner  6  and the casing  3 . Optionally the outer surface of the upper end part  8  of the liner can be provided with one or more elastomeric seals to enhance the fluid tightness between the expanded upper end  8  and the casing  3 . 
     At this stage the release sub  18  of the drill string  10  is pulled against the release device of the top anchor  20  so that the anchor members  24  thereby move to their retracted positions. By pulling the drill string  10  further upwards, the expander  12  pushes the arms  26  of the top anchor  20  out of the upper end part  8  of the liner  6 . The drill string  10  with the expander  12 , the centraliser  15  and the top anchor  20  attached thereto, is then retrieved to surface. 
     The body of cement  80  in the annular space  7  is allowed to harden after the expansion process is finalised. By virtue of the fold  55  which forms an annular sealing member, no substantial volume of hardened cement is present in the lower portion  92  of the annular space  7  after the expansion process is completed. Therefore only a minor cement plug, or no cement plug at all, needs to be drilled out if the wellbore  1  is to be drilled deeper. If a next expandable liner is to be installed in the wellbore, the already expanded liner takes the role of the casing. It is then preferred that an expander of slightly smaller diameter or a collapsible expander is used to expand such next liner to allow the expander to be lowered with some clearance through the already expanded liner. 
     The alternative embodiment of the system according to the invention, as shown in  FIG. 15  is similar to the embodiment described hereinbefore with reference to  FIGS. 1-14 , except that the drill string  10  extends below the expander  12  and is there provided with a drilling assembly including a collapsible underreamer  94  and a steerable drilling tool  96  having a pilot drill bit  98 . The underreamer  94 , when in collapsed mode, and the steerable drilling tool  96  are of smaller diameter than the inner diameter of the expanded liner  6  so as to allow the underreamer  94  and the steerable drilling tool  96  to be retrieved to surface through the expanded liner  6 . 
     Normal operation of the alternative embodiment shown in  FIG. 15  is similar to normal operation of the embodiment described hereinbefore with reference to  FIGS. 1-14 , except that the open wellbore section  4  is not drilled using a separate drill string before lowering the liner into the wellbore  1 . Instead, the open wellbore section is drilled using the underreamer  94  and the steerable drilling tool  96 . After drilling with the underreamer  94  and the steerable drilling tool  96 , the liner  6  is expanded in the manner described hereinbefore. It is an advantage of the alternative embodiment that the liner  6  is drilled to target depth and subsequently expanded without requiring an extra round trip. In order to provide adequate flow area for drilling fluid during drilling of wellbore section  4 , it is preferred that the expander  12  is collapsible to a relatively small diameter. 
     In exemplary embodiments, the foldable wall section of the wall of the expandable tubular element may have a thickness of about 50% or less than the thickness remainder of the tubular element, for instance about 40% or less. The length of the foldable wall section is for instance in the range of about 50 to 500 mm, for instance in the range of about 75 to 150 mm. The expansion ratio of the tubular element, being the ratio of the pipe diameter of the expanded pipe relative to the pipe diameter of the pipe before expansion, may be in the range of 5 to 25%, for instance about 10 to 20%. The expansion ratio of the foldable wall section, being the ratio of the outer diameter of the foldable wall section after expansion relative to the outer diameter of the foldable wall section before expansion, may be in the range of 30% to 60%, for instance about 40 to 55%. After expansion, the folded section may seal against an enclosed wall (such as the wellbore wall), providing a fluid tightness of more than 50 bar, or for instance more than about 150 bar. Herein, fluid tightness provides zonal isolation between annular areas above and below the folded section respectively. The folding force required to expand and fold the foldable section is for instance in the range of about 250 to 1000 kN, for instance 400 to 700 kN. Tubular elements may be substantially made of solid steel. 
     A number of tests have been performed on pipe samples having a foldable wall section to test the forming of annular folds under compressive loading and subsequent radial expansion of the folds thus formed, as described hereinafter. 
     Test 1 
     The test samples have a foldable wall section in accordance with the fifth embodiment described hereinbefore ( FIGS. 8 and 9 ). Furthermore, the test samples have the following characteristics: 
     manufacturer: V&amp;M 
     material: S355J2H 
     outer diameter: 139.7 mm 
     wall thickness: 10 mm 
     yield strength: 388 MPa 
     tensile strength: 549 MPa 
     production method: seamless 
     heat treatment: normalized 
     The pipe sample has a section with a reduced thickness of 3.5 mm, which section has a length of 100 mm. To ensure proper centralisation of the machining and a uniform wall thickness in the reduced section area, the wall has been recessed both at the inner surface and the outer surface. Furthermore a small annular groove is provided at the inner surface of the section of reduced wall thickness to initiate the folding action and lower the required compressive folding force. The pipe samples were internally lubricated with Malleus STC1 lubricant prior to expansion. The expander used for expanding the samples is a Sverker21 material with an outer diameter of 140.2 mm. The expansion ratio, being the ratio of the increase in pipe diameter to the diameter before expansion, with the expander is 17%. 
     A compressive load was applied by the expander to the sample to cause the foldable wall section to fold into a concertina shape. The test showed that the required force to initiate the folding is about 450 kN. The applied load caused iterative formation of wrinkles on the sample, evolving to a folded section. The folded section has a lower axial stiffness and collapse resistance than the remainder of the sample, leading to a significant drop of the axial load during the formation of each fold. The outer diameter of the fold thus formed was 170.4 mm. This corresponds to an equivalent expansion ratio of 37%. The load applied to the expander was then increased to pull the expander through the pipe sample to radially expand the sample. The outer diameter of the fold after being expanded was 185.1 mm which corresponds to an equivalent expansion ratio of about 50%. The tests showed that the average expansion load, i.e. the force required to move the expander through the sample, is about 520 kN with a peak load of 650 kN during expansion of the fold. 
     Test 2 
     The test samples have a foldable wall section in accordance with the fifth embodiment described hereinbefore ( FIGS. 8 and 9 ). Furthermore, the test samples have the following characteristics: 
     manufacturer: V&amp;M 
     material: S355J2H 
     outer diameter: 139.7 mm 
     wall thickness: 10 mm 
     yield strength: 388 MPa 
     tensile strength: 549 MPa 
     production method: seamless 
     heat treatment: normalized 
     The pipe sample has a section with a reduced thickness of 3.5 mm, which section has a length of 100 mm. To ensure proper centralisation of the machining and a uniform wall thickness in the reduced section area, the wall has been recessed both at the inner surface and the outer surface. Furthermore a small annular groove is provided at the inner surface of the section of reduced wall thickness to initiate the folding action and lower the required compressive folding force. The pipe samples were internally lubricated with Malleus STC1 lubricant prior to expansion. The expander used for expanding the samples is a Sverker21 material with an outer diameter of 140.2 mm. The expansion ratio, being the ratio of the increase in pipe diameter to the diameter before expansion, with the expander is 17%. The sample has been placed and expanded inside a S355J2H steel pipe with an internal diameter of 174.7 mm and 9.5 mm wall thickness. 
     A compressive load was applied by the expander to the sample to cause the foldable wall section to fold into a concertina shape. The test showed that the required force to initiate the folding is about 450 kN. The applied load caused iterative formation of wrinkles on the sample, evolving to a folded section. The folded section has a lower axial stiffness and collapse resistance than the remainder of the sample, leading to a significant drop of the axial load during the formation of each fold. The load applied to the expander was then increased to pull the expander through the pipe sample to radially expand the sample. The outer diameter of the fold after being expanded was in contact with the internal diameter of the outer pipe which corresponds to an equivalent expansion ratio of about 41%. The tests showed that the average expansion load, i.e. the force required to move the expander through the sample, is about 520 kN with a peak load of 850 kN during expansion of the fold. The annular space between the inner and outer pipe has been subjected to water pressure. The pressure test revealed a pressure tightness of about 200 bar. 
     The present invention is not limited to the above described embodiments thereof, wherein many modifications are conceivable within the scope of the appended claims. Features of respective embodiments may for instance be combined.