WO2005052304A1

WO2005052304A1 - Method for drilling and lining a wellbore

Info

Publication number: WO2005052304A1
Application number: PCT/GB2004/004706
Authority: WO
Inventors: Philip Head; Paul George Lurie
Original assignee: Bp Exploration Operating Company Limited; Xl Technology Limited
Priority date: 2003-11-14
Filing date: 2004-11-08
Publication date: 2005-06-09

Abstract

A method of drilling and lining a wellbore comprises drilling and casing a first wellbore section (1) and then drilling and lining a second wellbore section (15) using a drill string (5) having a drill bit (6) connected directly or indirectly to the lower end thereof, which method comprises: (A) disposing around the drill string (5) above the drill bit (6) (i) a radially extendible cutting member (10), (ii) a radially expandable expansion member (11) and (iii) an expandable liner string (13), (B) drilling the second wellbore section (15) from the first wellbore section (1), (C) cutting a first section of the expandable liner string (16) using the radially extendible cutting member (10), (D) positioning the first section of the expandable liner string (16) so that it extends from within the first wellbore section (1) into the second wellbore section (15), (E) expanding the first section of the expandable liner string (16) using the radially expandable expansion member (11) such that the upper portion of the first section of the expandable liner string forms a sealed connection (17) with the lower portion of the casing of the first wellbore section and the lower portion of the first section of the expandable liner forms a liner for the second wellbore section.

Description

METHOD FOR DRILLING AND LINING A WELLBORE The present invention relates to drilling a wellbore and to lining the wellbore without having to remove the drilling assembly from the wellbore. More particularly, the invention relates to a system of drilling a wellbore and of completing the well through the expansion of tubulars. More particularly still, the invention relates to the drilling of a wellbore and of lining the wellbore by expansion of one tubular into another to provide a sealed connection therebetween without having to pull the drilling assembly from the wellbore. Wellbores are typically formed by drilling and thereafter lining a borehole with steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain sections of the wellbore adj acent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well and the annulus between the outside of the casing and the borehole wall is filled with cement to permanently set the casing in the wellbore. As the wellbore is drilled to a new depth, additional strings of pipe are run into the well to that depth whereby the upper portion of the string of pipe (hereinafter referred to as "liner string"), is overlapping the lower portion of the casing. The liner string is then fixed or hung in the wellbore, usually by some mechanical slip means well known in the art. Emerging technology permits wellbore tubulars to be expanded in situ. In addition to simply enlarging a tubular, the technology permits the physical attachment of a smaller tubular to a larger tubular by increasing the outer diameter of a smaller tubular by means of a radial force applied from within the smaller tubular. The expansion can be accomplished by a mandrel or a cone-shaped member urged through the tubular that is to be expanded or by any other known expander tool. By utilizing an expander tool, the upper end of a liner can be expanded into the surrounding casing. In this manner, the conventional slip assembly and its related setting tools are eliminated. In one example, the liner is run into the wellbore on a run- in string with the expander tool disposed in the liner and connected thereto by a temporary connection. As the assembly reaches a predetermined depth whereby the top of the liner is adjacent a lower section of the casing, the expander tool is actuated and then, through rotational and/or axial movement of the actuated expander tool within the liner, the liner wall is expanded past its elastic limits and into contact with the wall of the casing. Rotation of the expander tool is performed by rotating the run-in string or by utilizing a mud motor in the run-in string to transfer hydraulic power to rotational movement. The present invention relates to a method of drilling a wellbore and of periodically lining the wellbore using sections of expandable liner wherein one section of liner is expanded into another section of liner thereby providing a sealed connection therebetween. Generally, the drilling operation is interrupted while the lining is effected. Thus, according to the present invention, a method of drilling and lining a wellbore comprises drilling and casing a first wellbore section and then drilling and lining a second wellbore section using a drill string having a drill bit connected directly or indirectly to the lower end thereof, which method comprises: (A) disposing around the drill string, above the drill bit, (i) a radially extendible cutting member (ii) a radially expandable expansion member and (iii) an expandable liner string, (B) drilling the second wellbore section from the first wellbore section, (C) cutting a first section of the expandable liner string using the radially extendible cutting member, (D) positioning the first section of the expandable liner string so that it extends from within the first wellbore section into the second wellbore section, (E) expanding the first section of the expandable liner string using the radially expandable expansion member such that the upper portion of the first section of the expandable liner string forms a sealed connection with the lower portion of the casing of the first wellbore section and the lower portion of the first section of the expandable liner forms a liner for the second wellbore section. Preferably, the expandable liner string is positioned substantially concentrically with respect to the drill string. The radially extendible cutting member and radially expandable expansion member are conveniently part of an assembly that may in addition comprise means for gripping the interior of the expandable liner string. The assembly may also comprise means for moving the expandable liner string longitudinally with respect to the assembly. Preferably, the assembly, comprising the radially extendible cutting member and radially expandable expansion member, is disposed around the drill string in the annulus between the expandable liner string and the drill string. In this specification, the term "drill string" includes any inner string that is connected directly or indirectly to the drill bit. In an embodiment of the invention, the method of drilling a wellbore, the wellbore comprising a first wellbore section and a second wellbore section, comprises:

(A) drilling the first wellbore section and running a casing from the surface into the first wellbore section;

(B) drilling the second wellbore section from the first wellbore section and lining the second wellbore section with an expandable liner characterised in that the second wellbore section is drilled using an apparatus comprising a drill string having a drill bit connected either directly or indirectly to the lower end thereof, an expandable liner string arranged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit and an assembly disposed about the drill string in the annulus formed between the expandable liner string and the drill string, the assembly comprising from the top downwards, an upper gripping member, a means for moving the expandable liner string in a downwards direction with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member is moveable in a longitudinal direction with respect to the drill string and wherein the second wellbore section is lined by:

(i) interrupting drilling of the second wellbore section when the expandable liner string extends below the casing of the first wellbore section and the radially extendible cutting member of the assembly is positioned within the lower portion of the casing; (ϋ) actuating the radially extendible cutting member to cut a first section of liner from the expandable liner string with the expandable liner string gripped by the upper and lower gripping means whereby the first section of liner that is cut from the expandable liner string is gripped by the lower gripping member and the upper portion of the first section of liner overlaps the lower portion of the casing of the first wellbore section;

(iii) expanding the upper portion of the first section of liner into the lower portion of the casing of the first wellbore section to form a sealed connection therebetween and expanding the lower portion of the first section of liner to form a liner for the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner; (iv) radially retracting the cutting member and expansion member and actuating the means for moving the liner string downwardly with respect to the assembly so that the expandable liner string moves downwardly through the wellbore until the radially extendible cutting member and radially expandable expansion member of the assembly are positioned in the annulus formed between the expandable liner string and the drill string;

(v) recommencing the drilling operation to extend the second wellbore section; (vi) interrupting drilling of the second wellbore section when the expandable liner string extends below the expanded liner of the second wellbore section and the radially extendible cutting member of the assembly is positioned within the lower portion of the expanded liner; (vii) actuating the radially extendible cutting member to cut a further section of liner from the expandable liner string with the expandable liner string gripped by the upper and lower gripping means whereby the further section of liner that is cut from the expandable liner string is gripped by the lower gripping member and the upper portion of the further section of liner overlaps the lower portion of the expanded liner of the second wellbore section; and (viii) expanding the upper portion of the further section of liner into the lower portion of the expanded liner of the second wellbore section to form a sealed connection therebetween and expanding the lower portion of the further section of liner to extend the liner of the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member with respect to the drill string such that the expansion member moves through the further section of expandable liner. If necessary, the drilling operation may be recommenced and as the second wellbore section is drilled to a greater depth, the drilling operation may be interrupted one or more times to allow yet further sections of expandable liner to be cut from the liner string and the upper portions thereof to be expanded into the lower portion of the expanded liner to form a sealed connection therebetween and to further extend the expanded liner of the second wellbore section. Although the method of the present invention is described above in relation to a substantially vertical well, it will be apparent to the person skilled in the art that the method of the present invention may also be used to drill and to line a side track or lateral well. Even in such wells, the wellbore is generally drilled to increasing depth and the terms "above" and "below" should be understood in this context, e.g. above the drill bit means behind the bit in the drilling direction. Similarly, a person skilled in the art would understand that although it would be preferable for the expandable liner string and drill string to have the same axis, the term "concentric" is not used this strict mathematical sense and a degree of eccentricity can be tolerated, depending on the relative sizes of the equipment and borehole. An advantage of the method of the present invention is that the assembly that is disposed about the drill string may be used: (a) to deliver an expandable liner string to the second wellbore section, (b) to cut a section of liner from the expandable liner string in situ in the wellbore, and (c) to expand the upper portion of the cut section of liner into the lower portion of the casing of the first wellbore section to provide a sealed connection therebetween and to expand the lower portion of the cut section of liner to form a liner for the second wellbore section without having to pull the apparatus from the wellbore. A further interval of the second wellbore section may then be drilled and a further section of liner may be cut from the liner string. The upper portion of this further section of liner may then be expanded into the lower portion of the expanded liner to provide a sealed connection therebetween and the lower portion of the further section of liner may be expanded to extend the liner of the second wellbore section, also without having to pull the apparatus from the wellbore. This procedure may be repeated until the entire second wellbore section has been drilled and lined with sections of expandable liner having sealed connections therebetween. In a preferred embodiment of the present invention there is provided a. method of drilling a wellbore, the wellbore comprising a first wellbore section and a second wellbore section, the method comprising:

(B) drilling the second wellbore section from the first wellbore section and lining the second wellbore section using an apparatus comprising a drill string having a drill bit attached either directly or indirectly to the lower end thereof, an expandable liner string arranged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit, an assembly disposed about the drill string in the annulus formed between the drill string and the expandable liner string, the assembly comprising from the top downwards, an upper gripping member, a means for moving the expandable liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member is moveable in a longitudinal direction with respect to the drill string and wherein the second wellbore section is drilled by the steps of:

(a) lowering the apparatus into the first wellbore section by (i) lowering the expandable liner string into the first wellbore section with the expandable liner string supported by slips at the surface and (ii) lowering the drill string with the assembly disposed about the drill string through the expandable liner string to the bottom of the first wellbore section;

(b) arranging the expandable liner string with respect to the cutting member of the assembly such that a first section of the expandable liner string extends below the cutting member and actuating the upper and/or lower gripping members to grip the inner wall of the expandable liner string;

(c) commencing drilling of the second wellbore section by rotating the drill string to rotate the drill bit;

(d) interrupting the drilling of the second wellbore section when an upper portion of the first section of liner that extends below the radially extendible cutting member overlaps the lower portion of the casing of the first wellbore section;

(e) actuating the radially extendible cutting member of the assembly to cut the first section of liner that extends below the radially extendible cutting member from the expandable liner string with the upper and lower gripping members of the assembly gripping the inner wall of the expandable liner string so that the first section of liner that is cut from the expandable liner string is gripped by the lower gripping member; (f) expanding the upper portion of the first section of liner into the casing of the first wellbore section to form a sealed connection therebetween and expanding the lower portion of the first section of liner to form a liner for the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner; (g) radially retracting the cutting member and expansion member;

(h) actuating the means for moving the expandable liner string downwardly with respect to the assembly such that a further section of liner extends below the radially extendible cutting member of the assembly; (i) re-commencing drilling of the second section of the wellbore by rotating the drill string to rotate the drill bit;

(j) interrupting drilling of the second section of the wellbore when an upper portion of the further section of liner that extends below the cutting member overlaps the lower portion of the expanded liner of the second wellbore section; (k) actuating the radially extendible cutting member of the assembly to cut the further section of liner from the liner string with the upper and lower gripping members gripping the inner wall of the expandable liner string so that the further section of liner that is cut from the expandable liner string is gripped by the lower gripping member; (1) expanding the upper portion of the further section of liner into the lower portion of the expanded liner of the second wellbore section to form a sealed connection therebetween and expanding the lower portion of the further section of liner to extend the liner of the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner; (m) optionally, repeating steps (g) to (1). By casing is meant a string of pipe, in particular, steel pipe, which is run from the surface into the upper part of a wellbore. The term casing as used herein also encompasses a "conductor", i.e. a short string of large diameter pipe that is used to keep the upper part of a wellbore open. Generally, a conductor is drilled or pushed into a wellbore to a depth of 100 to 300 feet (30 to 90 m). It is envisaged that a conductor may be drilled or pushed to the bottom of the first wellbore section. Alternatively, a conductor may be pushed into the upper part of the first wellbore section and a casing may then be run from the surface to the bottom of the first wellbore section. Thus, the upper portion of the first section of liner that is cut from the expandable liner string may be expanded either into the lower portion of a conductor or into the lower portion of a casing that has been run into the first wellbore section through the conductor. The apparatus that is used to drill and line the second wellbore section is lowered into the wellbore through the casing of the first wellbore section. Accordingly, the diameter of the drill bit is restricted by the diameter of the casing. It is envisaged that the method of the present invention may be used to line a second wellbore section having an internal diameter less than the internal diameter of the first wellbore section such that the internal diameter of the expanded liner for the second wellbore section is also less than the internal diameter of the casing of the first wellbore section. However, it is desirable for the wellbore to have a substantially uniform internal diameter along , the entire length thereof (hereinafter "monobore well"). Accordingly, it is preferred that the internal diameter of the expanded liner for the second wellbore section is substantially the same as the internal diameter of the casing of the first wellbore section. Thus, in a preferred aspect of the present invention, the drill bit of the apparatus is provided with a "hole-opener". By "hole-opener" is meant a device used to enlarge the size of a borehole, having cutting surfaces, for example, teeth, arranged on its outside circumference to cut the formation as it rotates. Thus, the hole-opener may be used to increase the diameter of the second wellbore section. Preferably, the diameter of the second wellbore section may be increased until it is substantially the same as that of the first wellbore section. Suitably, the hole-opener may enlarge the second wellbore section during drilling of the second wellbore section, i.e. as the hole-opener is moved downwardly through the wellbore. Alternatively, the hole-opener may be actuated to enlarge the second wellbore section by interrupting the drilling step and moving the apparatus upwardly in the wellbore through the open hole until the hole-opener lies immediately below the casing of the first wellbore section (i.e. prior to expansion of the first section of liner) or immediately below the expanded liner of the second wellbore section. The apparatus is then subsequently moved downwardly in the wellbore until the drill bit engages with the bottom of the wellbore. Suitable hole-openers include conventional under reamers and eccentric drill bits. Preferably, an under reamer is located above the drill bit, preferably, immediately above the drill bit. Suitably, the under reamer has radially extendible cutting surfaces that may be extended into contact with the wellbore wall to ream the wellbore. The internal diameter of the second wellbore section may be enlarged in such a manner that it is of substantially uniform diameter along the entire length thereof. However, as the sealed connections between the lengths of liner are formed by expanding an upper portion of a section of liner into the lower portion of a previously expanded section of liner, these sealed connections are of double thickness.

Accordingly, where the liner is expanded against the wellbore wall, the internal diameter of the wellbore is reduced at the sealed connections. Alternatively, the sealed connections may be expanded against the wellbore wall with the portions of the liner between the sealed connections being expanded to the same internal diameter as the sealed connections resulting in a true monobore well. As will be evident to the person skilled in the art, there will be an annular gap between the wellbore wall and the portions of liner between the sealed connections. This annular gap may be filled by pumping a cement into the wellbore in a conventional manner prior to expanding each section of liner. Thus, the cement flows through the interior of the cut section of liner and into the annular gap between the cut section of liner and the wall of the wellbore. The section of liner is then expanded from the bottom upwards thereby applying pressure on the cement that is present in the annular gap and forcing excess cement to flow upwardly through the annular gap and out from the top of the section of liner into the wellbore. Once the cement has set, a pressure seal is formed between the expanded liner section and the cement. Alternatively, the diameter of the wellbore in the intervals where is its desired to form the sealed connections may be increased to a larger diameter (d_t) than the diameter of the wellbore in. the intervals between the sealed connections (d ) wherein the difference between the diameters d^d₂ is sufficient to accommodate the increased thickness of the sealed connections. Accordingly, both the sealed connections and the portions of liner therebetween may be expanded against the wellbore wall without any restriction in the internal diameter of the wellbore in the vicinity of the sealed connections resulting in a true monobore well. It is envisaged that the expandable liner may be formed of metal so that when a cut section of liner is expanded against the wall of the second wellbore section a metal to rock seal is obtained. Preferably, the expandable liner string is provided with a coating of a resilient material, preferably an elastomeric material, to provide an improved seal with the rock. Preferably, the elastomeric material is resistant to the well environment, i.e. temperature, pressure, well fluids, and the like. Suitably, the elastomeric material is selected from rubber (for example, silicone rubber), polymers of ethylene-propylene diene monomer (EPDM), polytetrafluoroethylene, polyphenylene sulfide, and perfluoroelastomers. Typically, the thickness of the coating of resilient material is in the range 0.05 to 0.2 inches (0.13 to 0.5 cm), for example, about 0.1 inch (0.25 cm). Preferably, the first wellbore section is drilled using a drill string having a drill bit provided with a hole-opener on the lower end thereof. The hole-opener is used to increase the diameter of the lower portion of the first wellbore section, for example, the bottom 0.5 to 5 feet (0.15 to 1.5 m) of the first wellbore section. Suitable hole-openers are as described above. Preferably, a conductor is pushed or drilled into the upper part of the first wellbore section during the drilling of the first wellbore section. Once the drilling of the first wellbore section is complete, a casing is run from the surface to the bottom of first wellbore section in a conventional manner such that the lower end of the casing lies in the reamed lower portion of the first wellbore section. Where the casing is cemented into place in the first wellbore section, the reamed lower portion of the first wellbore section is provided with a crushable spacer, for example, a spacer formed of expandable polystyrene. This crushable spacer allows the sealed connection that is formed between first section of liner and the casing to be expanded against the wall of reamed lower portion of the first wellbore section. Alternatively, the casing of the first wellbore section may be an expandable casing that is expanded against the conductor and against the wall of the wellbore using a conventional expansion tool, for example an expandable mandrel. Suitably, the expanded casing has substantially the same internal diameter along the entire length thereof. In other words, the casing is not expanded against the wall of the reamed lower portion of the first wellbore section thereby allowing the sealed connection that is formed between the first section of expandable liner and the lower portion of the casing to be expanded against the wellbore wall. Preferably, the second wellbore section is then drilled using a drill string having a conventional under reamer tool attached to the lower end thereof above a conventional drill bit and the assembly of the present invention disposed about the drill string. Initially, the diameter of the second wellbore section is less than the internal diameter of the casing of the first wellbore section. The second wellbore section is then reamed to a larger diameter by radially extending the cutters of the under reamer into engagement with the wall of the second wellbore section so that rotation of the drill string causes the cutters to enlarge the diameter of wellbore. This maybe achieved during drilling of the wellbore or by moving the apparatus back through the open hole. Suitably, the second wellbore section may be enlarged until its diameter corresponds to that of the reamed lower portion of the first wellbore section. Thus, the diameter of the second wellbore section may be substantially uniform along the entire length thereof. Alternatively, the second wellbore section may be enlarged such that its diameter alternates between longer intervals having a diameter that corresponds to the diameter of the un-reamed upper part of the first wellbore section and shorter intervals having a diameter that corresponds to that of the reamed lower portion of the first wellbore section. Preferably, the difference between the diameters of the shorter and longer intervals is sufficient to accommodate the sealed connections. Suitably, the shorter intervals that are reamed to a larger diameter to accommodate the sealed connections have a length corresponding to the length of the upper portion of the liner section that is expanded into the lower portion of the expanded liner to form the sealed connection, for example, the shorter intervals may have a length of from 0.5 to 5 feet (0.15 to 1.5 m). Suitably, the longer intervals correspond to the length of the lower portion of the section of liner that is expanded to extend the expanded liner of the second wellbore section. In a first preferred aspect of the method of the present invention, the drill string may extend to the surface with rotation of the drill string effecting rotation of the drill bit. Initially, the expandable liner string extends to the surface with surface slips supporting the external wall of the expandable liner string. The drill string having the assembly disposed about the lower end thereof is then lowered through the expandable liner string until the drill bit emerges from the bottom of the expandable liner string and the assembly is disposed about the drill string in the annulus formed between the expandable liner string and the drill string. Preferably, the lower end of the expandable liner string lies within 100 feet (30 m), preferably, within 5 feet (1.5 m) of the drill bit with the proximity of the lower end of the expandable liner string to the drill bit depending on the longitudinal length of any formation evaluation equipment and of any directional control equipment that is disposed on the drill string above the drill bit. The upper and/or lower gripping members of the assembly are then actuated to grip the inner wall of the expandable liner string. The surface slips are then unset with the drill string bearing the weight of the assembly and the expandable liner string. Lengths of drill pipe may be added to the drill string in a conventional manner until the drill bit is located at the bottom of the first wellbore section. The drill string is then rotated so that the drill bit drills the second wellbore section. Further lengths of drill pipe are added to the drill string as the second wellbore section is being drilled resulting in the expandable liner string moving downwardly through the wellbore. During the drilling steps the expandable liner string is supported on the assembly via the upper gripping member and/or lower gripping member. Preferably, the upper gripping member and/or lower gripping member are provided with bearings, for example, roller bearings, such that the liner string does not rotate with the drill string. Actuation of the radially extendible cutting member causes the section of expandable liner that extends below the radially extendible cutting member to be cut from the bottom of the expandable liner string. Suitably, during the cutting step, the expandable liner string is supported on the assembly above and below the radially extendible cutting member via the upper and lower gripping members respectively such that after completion of the cutting step, the remainder of the expandable liner string and the cut section of liner are likewise supported on the assembly. Typically, the expandable liner string has a length of 90 to 3000 feet (27 to 914 m), with the length of the expandable liner string depending on the planned length of the second wellbore section. Suitably, the length of the section of liner that is cut from the expandable liner string depends on parameters such as the geology of the formation, the pore pressure of the formation, wellbore stability and any loss of drilling fluid to the formation (lost circulation). Thus, if the wellbore is of low stability or if the wellbore is being drilled through a lost circulation zone, it will be necessary to frequently interrupt drilling of the wellbore and to line the wellbore with relatively short sections of expandable liner. However, where the wellbore is stable and no lost circulation zone is encountered, it may be possible to drill a longer interval of the second wellbore section before it becomes necessary to line the wellbore. Preferably, the radially extendible cutting member of the assembly is mpveable in a longitudinal direction with respect to the drill string thereby allowing the length of the section of liner that extends below the cutting member to be adjusted. Suitably, the length of the cut section of liner may be in the range of from 2 to 90 feet (0.6 to 27 m), for example, 25 to 40 feet (7.6 to 12.2 m). Preferably, the upper portion of the cut section of liner that is expanded into the lower portion of the casing of the first wellbore section or into the lower portion of the expanded liner of the second wellbore section to form a sealed connection therebetween is a minor portion of each cut section of liner, for example, the upper 0.5 to 2 feet (0.15 to 0.6 m) of each cut section of liner. In this first aspect of the method of the present invention, it is envisaged that a plurality of sections of liner may be cut from the liner string and expanded to form a liner for the second wellbore section before it is necessary to pull the apparatus from the wellbore to arrange a new expandable liner string concentrically about the drill string. In a second preferred aspect of the method of the present invention, the expandable liner string extends to the surface with rotation of the expandable liner string effecting rotation of the drill string and hence rotation of the drill bit. Initially, the external wall of the expandable liner string is supported by surface slips. The drill string having the assembly disposed thereabout may be lowered through the expandable liner string until the drill bit has emerged from the bottom of the expandable liner string, as described above for the first preferred aspect of the method of the present invention. Again, the lower end of the expandable liner string lies within 100 feet (30 m), preferably, within 5 feet (1.5 m) of the drill bit with the proximity of the lower end of the expandable liner string to the drill bit depending on the longitudinal length of any formation evaluation equipment and of any directional control equipment that is disposed on the drill string above the drill bit. The surface slips are then unset with the expandable liner string bearing the weight of the assembly and the drill string. Lengths of expandable liner pipe are added to the expandable liner string at the surface until the drill bit is at bottom of the first wellbore section. The expandable liner string is then rotated to rotate the drill string which, in turn, rotates the drill bit such that the drill bit drills the second wellbore section. Further lengths of expandable liner pipe are added to the expandable liner string as the second wellbore section is drilled resulting in the drill string moving downwardly through the wellbore. In this second preferred aspect of the present invention, the drill string is supported on (hangs from) the expandable liner string via the upper gripping member and/or the lower gripping member of the assembly that is disposed about the drill string. Bearings are omitted from the upper and/or lower gripping member such that the drill string rotates with the liner string. Suitably, during each of the cutting steps, the assembly is supported on the expandable liner string above and below the radially extendible cutting member via the upper and lower gripping members respectively such that after completion of each cutting step, the cut section of liner is supported by the lower gripping member of assembly while the drill string is supported on the remainder of the expandable liner string via the upper gripping member of the assembly. Suitably, the drill string employed in this second preferred aspect of the present invention has a length of between 30 and 500 feet (9 and 152 m). Preferably, the cutting member is moveable in a longitudinal direction with respect to the drill string from a position near the top of the drill string to a position close to the lower end of the drill string. Thus, where the cutting member is arranged near the top of the drill string, a relatively long section of liner may be cut from the expandable liner string. Where the cutting member is moved to a lower position with respect to the drill string, shorter sections of liner may be cut from the expandable liner string. Preferably, the sections of liner that are cut from the expandable liner string have a length of from 30 to 500 feet (9 and 152 m). As discussed above, the desired length of each liner section will be dependent upon the conditions encountered in the wellbore. Thus, where the wellbore is unstable or a lost circulation zone is encountered, the open hole interval should be kept to a minimum by regularly interrupting drilling of the second wellbore section to seal off unstable formation zones and lost circulation zones with relatively short sections of expandable liner. An advantage of this second aspect of the method of the present invention is that lengths of liner pipe are added to the liner string at the surface. Thus, unlike the first aspect of the method of the present invention, there is no requirement to pull the apparatus from the wellbore to arrange a new expandable liner string concentrically about the drill string. Yet another advantage of this second aspect of the present invention is that the second wellbore section can be drilled through formation zones of increasing pore pressure with the zones being continuously sealed off. This allows the weight of the drilling mud to be continuously increased without having to pull the drilling apparatus from the wellbore to seal off the wellbore. Preferably, the radially extendible cutting member of the assembly is provided with a radially extendible blade. Thus, in the first preferred aspect of the method of the present invention, rotation of the drill string causes the blade to cut transversely through the expandable liner string. In the second preferred aspect of the method of the present invention, the radially extendible cutting member is arranged on bearings such that the cutting member does not rotate with the liner string. Accordingly, rotation of the expandable liner string causes the blade to cut transversely through the expandable liner string. As discussed above, the radially expandable expansion member of the assembly is moveable in a longitudinal direction with respect to the drill string. Preferably, the radially expandable expansion member is moveable with respect to the drill string between an upper position where the radially expandable expansion member is adjacent the radially extendible cutting member and a lower position where the radially expandable expansion member is aligned with the lower end of the section of liner that is to be cut from the expandable liner string. In particular, the radially expandable expansion member is moveable between a position adjacent the radially extendible cutting member to a position adjacent the lower gripping member. Preferably, the assembly that is disposed about the drill string has an elongate body, preferably, a tubular body with the drill string passing through the body. Suitably, the body is connected either directly or indirectly to the drill string. Where the assembly comprises an elongate body, the radially expandable expansion member is preferably capable of moving relative to the elongate body (and hence with respect to the drill string) between a position immediately below the radially extendible cutting member and a position at ornear the bottom of the elongate body. Where the radially expandable expansion member initially lies immediately below the radially extendible cutting member, the radially expandable expansion member will form the sealed connection prior to moving downwardly with respect to the elongate body to expand the lower portion of the liner section. Where the radially expandable expansion member initially lies at the bottom of the elongate body, the radially expandable expansion member will expand the lower portion of the section of liner by moving upwardly with respect to the elongate body prior to expanding the upper portion of the section of liner to form the sealed connection. A preferred radially expandable expansion member is a tool provided with radially extendible balls or rollers, for example, hydraulically actuated radially extendible balls or rollers. In the first preferred aspect of the method of the present invention, rotation of the drill string will cause the radially expandable expansion member to rotate so that the balls or rollers expand the liner section. In the second preferred aspect of the method of the present invention, rotation of the liner string will cause the drill string to rotate and hence the radially expandable expansion member to rotate so that the rollers expand the liner section. It is envisaged that the radially expandable expansion member may also function as the lower gripping member. Thus, prior to actuation of the cutting member, the rollers of the radially expandable expansion member may be partially expanded to grip the inner wall of the section of expandable liner string that extends below the radially extendible cutting member. Preferably, the rollers of the radially expandable expansion member are hydraulically expanded. In a third preferred aspect of the method of the present invention, both the expandable liner string and the drill string extend to the surface. Drilling mud can be circulated down through the drill string and back up through the annulus between the drill string and expandable liner string or vice versa. The drill string may have one or more sections that are not capable of transmitting sufficient torque from the surface to rotate the drill bit. In such an arrangement, the drill bit can be driven by a down hole motor. The lower end of the expandable liner string can be supported on an assembly that is connected to the bottom of the drill string. Suitably, the assembly comprises a tubular housing having a conventional drill bit on the lower end thereof and a first conventional hole-opener, for example, an underreamer preferably located immediately above the drill bit for reaming the second wellbore section. Suitably, a stop member is provided on the tubular housing for engaging the lower end of the expandable liner string. Thus, prior to cutting a section of liner from the expandable liner string, the portion of the tubular housing above the stop member lies within the expandable liner string while the portion of the tubular housing below the stop member extends below the expandable liner string. A second conventional hole-opener, for example, an underreamer may be provided on the housing of the assembly below the stop member at a distance from the first hole-opener. This second hole-opener is used to enlarge intervals of the wellbore to a larger diameter to accommodate the sealed connections that are formed between the sections of expandable liner. Suitably, the first and second underreamers are separated by a longitudinal distance that corresponds to the desired longitudinal distance between the sealed connections of the expanded liner. The assembly additionally comprises a radially expandable expansion member, a radially extendible cutting member and a gripping member which are located on the tubular housing above the stop member. Preferably, the radially extendible cutting member is located immediately below the radially expandable expansion member and the gripping member is, in turn, located below the cutting member. However, it is envisaged that the stop member may serve to support the cut section of liner in the desired location in the wellbore in which case the gripping member may be omitted. In this third embodiment of the method of the present invention, drilling fluid may be passed to the assembly and hence to ports in the drill bit and hole-openers through the annulus that is formed between the drill string and the expandable liner string and the entrained cuttings stream may be returned to the surface through the interior of the drill string (hereinafter referred to as "reverse circulation mode"). Alternatively, it is envisaged that the drilling fluid may flow from the surface to the assembly through the interior of the drill string and the entrained cuttings stream may flow back to the surface through the annulus formed between the expandable liner string and the drill string (hereinafter referred to as "normal circulation mode"). Suitably, the assembly is provided with ports that are in fluid communication with the wellbore and with the interior of the drill string. Preferably, these ports are located at or near the top of the assembly. An advantage of the reverse circulation mode of this third aspect of the method of the present invention is that there is less tendency for the cuttings to disentrain from the drilling fluid when they are carried to the surface through the interior of the drill string i.e. the cuttings are carried more efficiently to the surface. An advantage of both the normal and reverse circulation modes is that the absence of cuttings in the annulus formed between the expanded wellbore liner and the expandable liner string means that there is reduced friction when the expandable liner string is moved downwardly through the wellbore during the drilling of the second wellbore section. If necessary, a fluid may be pumped from the surface down the annulus between the expanded wellbore liner and the expandable liner string to ensure that this annulus is kept substantially free of drill cuttings. A further advantage of this third preferred aspect of the method of the present invention is that there is a reduced pressure drop compared with embodiments of the method of the present invention where the entrained cuttings stream flows to the surface through the annulus formed between the expanded liner of the wellbore and the expandable liner string. This reduced pressure drop allows the drilling fluid that is pumped from the surface to be passed, under pressure, through at least one hydraulically powered motor located within the housing of the assembly. This hydraulically powered motor may be used to operate a generator that produces electricity for operating the moving parts of the assembly (i.e. the drill bit, hole-openers, radially extendible cutting member, radially expandable expansion member and the gripping member). Accordingly, the drill bit and hole-opener(s) are electrically driven so that there is no requirement to rotate the drill string and expandable liner. Preferably, the assembly is provided with a first hydraulically powered generator for producing electricity to drive the cutting and expansion members, and a second hydraulically powered motor for rotating the drill bit and hole-openers. Preferably, a reel of fibre optic or electrical cable is disposed about the upper part of the assembly in the annulus formed between the assembly and the expandable liner string. The fibre optic or electrical cable extends from the reel to the top of the wellbore where it is fixed in position at the upper end of the expandable liner string. Suitably, signals are transmitted from the surface to the assembly along the fibre optic or electrical cable with the signals being transmitted through the liner string to the cable via a telemetry link, for example, an electromagnetic or wireless telemetry link. Owing to the downhole generation of electricity, the radially expandable expansion member is preferably an electrically actuated tool provided with radially extendible balls or rollers. However, rotation of the cutting member and expansion tool may also be achieved by rotation of the drill string. Yet a further advantage of this third aspect of the method of the present invention is that the upper gripping member may be omitted from the assembly. Thus, the assembly may be provided with a single gripping member for gripping the cut section of expandable liner while the remainder of the liner string is supported at the surface. Suitably, the gripping member of the assembly lies immediately above the stop member and is radially expanded to grip the bottom of the expandable liner string prior to actuation of the radially extendible cutting member. During the addition of expandable liner pipe and drill pipe to the expandable liner string and drill string respectively, the expandable liner string and drill string are supported at the surface via surface gripping means, for example, surface slips. Accordingly, in this third embodiment of the method of the present invention, the upper gripping member is located at the surface. Suitably, the radially expandable expansion member, radially extendible cutting member and gripping member are moveable in a longitudinal direction relative to the tubular housing of the assembly. Suitably, the radially extendible cutting member may lie immediately below the radially expandable expansion member and the gripping member lies immediately above the stop member. Thus, after a section of expandable liner has been cut from the expandable liner string, the radially expandable expansion member is actuated and the radially expandable expansion member and radially extendible cutting member are moved downwardly through the liner string (relative to the tubular housing) towards the lower gripping member thereby initially forming a sealed connection between the upper portion of the liner string and the expanded wellbore liner and subsequently expanding the lower portion of the cut section of liner against the wellbore wall to extend the liner. Suitably, the expansion member and cutting member may be moved relative to the tubular housing by means of a piston or a screw thread. It is also envisaged that the radially expandable expansion member may lie immediately above the stop member and may be expanded into engagement with the bottom of the expandable liner string prior to cutting a section of liner from the expandable liner string. Thus, if the radially expandable expansion tool has rollers aligned in a vertical direction, the radially expandable expansion tool may also acts as the gripping member. The radially expandable expansion member may then be further radially expanded and moved upwardly through the cut section of liner to expand the lower portion of the liner against the wellbore wall prior to expanding the upper portion of the liner into the lower portion of the wellbore liner to form a sealed connection therebetween. However, it is preferred to form the sealed connection prior to expanding the lower portion of the cut section of liner. Following expansion of the cut section of liner, the drill string is pulled from the wellbore relative to the expandable liner string (which remains stationary in the wellbore) until the stop member of the assembly engages the bottom of the expandable liner string. The expansion member and cutting member are then returned to their original positions and lengths of expandable liner pipe and drill pipe are added, at the surface, to the expandable liner string and the drill string respectively. Suitably, the length of the expandable liner pipe added to the expandable liner string is longer than the length of the drill pipe added to the drill string so as to allow a further section of liner to be cut from the liner string. The expandable liner string and the drill string are then both lowered through the wellbore until the drill bit engages with the bottom of the wellbore. The second wellbore section may then be extended and lined by repeating the steps outlined above. In yet a further aspect of the present invention there is provided an apparatus for drilling and lining a wellbore the apparatus comprising: a drill string having a drill bit attached to the lower end thereof, an expandable liner string arranged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit, an assembly disposed about the drill string in the annulus formed between the drill string and the expandable liner string, the assembly comprising from the top downwards an upper gripping member, a means for moving the liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member arranged on the assembly below the radially extendible cutting member, and a lower gripping member wherein the radially expandable expansion member and optionally the radially extendible cutting member are moveable in a longitudinal direction with respect to the drill string and wherein the inner wall of the expandable liner string is gripped by the upper and/or lower gripping members of the assembly. If necessary, the drill string may be provided with a fluid by-pass for the drilling mud to avoid excess pressures building up in the vicinity of the drill bit. In yet a further aspect of the present invention there is provided an assembly for delivering a liner to a wellbore, the assembly being disposable around a drill string and comprising from the top downwards: a first gripping member, a means for moving the liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member and optionally the radially extendible cutting member are moveable in a longitudinal direction with respect to the drill string. Suitable radially extendible cutting and radially expandable expansion members are described above. Preferably, the means for moving the radially expandable expansion member in a longitudinal direction with respect to the drill string comprises a piston. Preferably, the means for moving the radially extendible cutting member in a longitudinal direction with respect to the drill string also comprises a piston. Preferably, the upper gripping member of the assembly comprises a first (upper) slip and a second (lower) slip that are connected either directly or indirectly by a piston. The piston of the gripping means comprises a cylinder that fits within a tube in which it moves up and down. When the cylinder of the piston is in its upper position, the first slip may be set to grip the inner wall of the expandable liner string and the second slip is unset. The cylinder is then moved to its lower position within the tube of the piston causing the liner string to move downwardly through the wellbore. The second slip is then set and the first slip unset. The cylinder is then moved back to its upper position within the tube and the first slip is set and the second slip unset. This operation may be repeated a plurality of times to move the liner string downwardly through the wellbore with respect to the assembly. Thus, the upper and lower slips and the piston together comprise the means for moving the liner string downwardly with respect to the assembly while the upper and/or lower slips also comprise the upper gripping member. Suitably, the radially extendible cutting member is locked to the second (lower) slip. Preferably, the radially extendible cutting member is locked to the second lower slip via a piston so that the cutting member is moveable in longitudinal direction with respect to the drill string. Preferably, the radially expandable expansion member is connected either directly or indirectly to the cutting member via a piston such that the radially expandable expansion member is moveable between an upper position where the radially expandable expansion member is adjacent the cutting member and a lower position where the radially expandable expansion member lies at the bottom of the section of liner that is to be cut from the expandable liner string. Preferably, the pistons of the assembly are hydraulically powered and hydraulically actuated. However, it is also contemplated that the pistons may be electrically powered. For example, the assembly may additionally comprise downhole batteries or a downhole electric generator comprising a turbine with the turbine driven by the flow of drilling mud through the wellbore. Suitably, the assembly is provided with pressure sensors that actuate the hydraulically or electrically powered pistons. Thus, the pressure sensors may actuate the pistons in response to specific drilling mud pressure sequences or specific sequences of pulses of drilling mud. Where the drill string extends to the surface, the first and second slips may be supported on bearings about the drill string, for example, roller bearings, such that the first and second slips do not rotate with the drill string and hence the liner string does not rotate with the drill string. Where the liner string extends to the surface, but the drill string does not, the bearings are omitted from the first and second slips such that rotation of the liner string effects rotation of the drill string and hence rotation of the drill bit. The first and second slips and the piston that connects these slips can also be used to lower the assembly and hence the drill string through the liner string to the bottom of the first wellbore section, as described below with reference to the Figures. Suitably, the lower gripping member is radially expandable. Where the drill string extends to the surface, it is preferred that this lower radially expandable gripping member is not actuated until immediately prior to the cutting step in which case there is no requirement for any bearings. However, where the lower radially expandable gripping member is used to support the expandable liner string during the drilling steps, it is prefeπed that this gripping member is provided with bearings. As discussed above, where the radially expandable expansion member is capable of being partially expanded to grip the inner wall of the liner, the lower gripping member may be omitted. The invention will now be described with reference to Figures 1 to 19. Figures 1 and 2 are schematic cross-sectional drawings of a drill string and associated equipment in a first wellbore section. Figures 3 to 8 are schematic cross-sectional drawings of apparatus used to drill and line a second wellbore section according to the invention in which the drill string, but not the expandable liner string, extends to the surface. Figures 9 to 12 are schematic cross-sectional drawings of apparatus used to drill and line a second wellbore section according to the invention in which the expandable liner string, but not the drill string extends to the surface. Figure 13 is a schematic cross-sectional drawing of another embodiment of apparatus used to drill and line a second wellbore section according to the invention in which both the expandable liner string and drill string extend to the surface. Figures 14 to 19 are schematic cross-sectional drawings illustrating the operation of the apparatus of Figure 13. In Figure 1 is shown a first wellbore section 1 that has been drilled through a subterranean formation 2 using a drill string 5 having a conventional under reamer tool (not shown) attached to the lower end thereof above a conventional drill bit 6 wherein the under reamer tool has been used to ream the wall of the lower portion 3 of the first wellbore section 1 to a larger diameter. A casing 4 has^' been run into the bottom of the first wellbore section 1. An assembly according to the present invention is shown disposed about the lower end of a drill string 5 having a conventional under reamer tool (not shown) attached to the lower end thereof above a conventional drill bit 6. The assembly comprises a first slip 7 and a second slip 8 connected by a piston 9, a radially expandable cutting member 10 that is locked to the second slip 8, a radially expandable expansion member 11 that is connected to the radially extendible cutting member 10 via a piston (not shown) and a radially expandable lower slip 12. The first and second slips 7, 8 are supported on bearings, for example roller bearings (not shown) such that first and second slips 7,8 do not rotate with the drill string 5. The second slip 8 is shown gripping the internal wall of an expandable liner string 13 that is arranged concentrically about the drill string 5 while a further support or slip at the surface 14 ("surface slip means") is shown gripping the external wall of the liner string 13. In Figure 2, the liner string 13 remains gripped by the surface slip means 14 while the second slip 8 is unset and the drill string 5 having the assembly disposed thereabout is lowered through the liner string 13 until the drill bit 6 emerges from the bottom of the expandable liner string. Alternatively, the first and second slips 7,8 and the piston 9 may be actuated to "walk" the assembly and hence the drill string 5 downwardly through the expandable liner string 13. Thus, the second slip 8 grips the expandable liner string 13 while the piston 9 moves the first slip 7 downwardly with respect to the liner string 13. The first slip 7 is then set to grip the expandable liner string 13 and the second slip 8 is unset and the piston 9 is actuated to move the second slip 8 downwardly through the expandable liner string 13. This procedure is repeated a plurality of times until the drill bit 6 and under reamer have emerged from the expandable liner string 13 and the assembly is positioned at the bottom of the expandable liner string 13. When the assembly is positioned at the bottom of the expandable liner string 13, the first and/or second upper slips 7,8 are set to grip the internal wall of the expandable liner string 13 and the surface slip means 14 is unset so that expandable liner string 13 is supported on the assembly and the assembly is in turn supported on the drill string 5. Lengths of drill pipe may now be added to the drill string 5 in a conventional manner with the surface slip means 14 being used to grip the drill string 5 during addition of each length of drill pipe. In Figure 3, the drill bit 6 has been actuated to drill a second wellbore section 15. The under reamer (not shown) has also been actuated to ream the second wellbore section to a larger diameter, for example, to the diameter of the under reamed portion of the first wellbore section 3. Lengths of drill pipe have been added to the drill string 5 until a section of the liner string 16 having a length of, for example, 90 to 3000 feet has been conveyed into the second wellbore section and the cutter of the radially extendible cutting member 10 lies within the lower portion of the cased first wellbore section 1, for example, at a distance of 2 to 90 feet from the lower end of the casing. Suitably, the radially expandable expansion member 11 initially lies immediately below the cutting member 10. At least one of the first and second slips 7,8 remains set and the lower radially expandable slip 12 is also set. The radially extendible cutting member 10 is then actuated such that the cutting blade of the cutting member is extended radially outwardly to cut through the liner. The remaining liner string is then supported by the first and/or second slips 7,8 while the cut liner section 16 is supported by the lower radially expandable slip 12. In Figure 4, the expansion member 11 has been actuated to expand the upper portion of the cut section of expandable liner 16 against the casing 4 of the first wellbore section to form a sealed connection 17 therebetween. Suitably, the sealed connection 17 is expanded against the wall of the reamed lower portion 3 of first wellbore section. Typically the wellbore wall may be reamed to a diameter that is capable of accommodating the sealed connection such that the internal diameter of the sealed connection is substantially the same as the internal diameter of the casing above the sealed connection 17. In Figure 5, the expansion member 11 has been moved downwardly with respect to the drill string through the cut section of liner 16 using piston 18 so as to expand the cut section of liner 16 against the wall of the second wellbore section. After completion of this expansion operation the expansion member 11 lies adjacent the lower radially expandable slip 12. Suitably, the lower end of the expanded section of liner 16 lies within a reamed part 19 of the second wellbore section 15 having a diameter that corresponds to that of the reamed lower portion 3 of the first wellbore section. Advantageously, the entire second wellbore section may be reamed to a diameter that is substantially the same as that of the reamed lower portion of the first wellbore section in which case the liner is expanded until the internal diameter of the liner is substantially the same as the inner diameter of the casing of the first wellbore section. In Figures 6 to 8 the first and second slips 7,8 and piston 9 are actuated so as to "walk" the expandable liner string 13 downwardly through the wellbore so that the cutting member 10 and expansion member 11 lie in the annulus formed between the expandable liner string 13 and the drill string 5. As illustrated in Figure 8, the expansion member 11 is radially retracted so that it can enter the expandable liner string 13 while the piston 18 has been actuated to move the expansion member to a position immediately below the cutting member 10. The expandable liner string 13 is "walked" downwardly through the wellbore until the lower end of the expandable liner string 13 is again gripped by the lower radially expandable gripping member 12. The steps outlined in Figures 1 to 8 are then repeated as the second wellbore section is drilled to greater depth such that the second wellbore section is lined with further lengths of expandable liner having sealed connections therebetween without having to pull the apparatus from the wellbore. In Figure 9 is shown a first wellbore section 20 that has been drilled through a subterranean formation 21 using a drill string 24 having a conventional under reamer tool (not shown) attached to the lower end thereof above a conventional drill bit 25 wherein the under reamer tool has been used to ream the wall of the lower portion of the first wellbore section 23 to a larger diameter. A casing 22 has been run into the first wellbore section 20 to a position where the lower end of the casing lies within a reamed lower portion 23 of the first wellbore section. The assembly of the present invention is shown disposed about the drill string 24 having a conventional under reamer tool (not shown) attached to the lower end thereof above a conventional drill bit 25. The drill string 24 does not extend to the surface. The assembly of the present invention comprises a first slip 26, a second slip 27, a piston 28, a radially extendible cutting member 29, a radially extendible expansion member 30 and a radially expandable lower slip 31. At least one of the first and second slips 26, 27 grips the internal wall of an expandable liner string 32 that is arranged concentrically about the drill string 24. This expandable liner string 32 extends to the surface. A second wellbore section 33 has been drilled by rotating the expandable liner string 32 which effects rotation of the drill string 24 and hence rotation of the drill bit 25 and undeπeamer (not shown). The under reamer (not shown) has been actuated to ream the second wellbore section to a larger diameter, for example, longer intervals having a diameter coπesponding to that of the upper portion of the first wellbore section and shorter intervals 34 having a diameter corresponding to that of the reamed lower portion 23 of the first wellbore section 20. Lengths of liner pipe are added to the expandable liner string at the surface as the second wellbore section is being drilled to a greater depth with surface slip means (not shown) being used to grip the liner string during addition of each length of liner pipe. A section 35 of the expandable liner string 32 extends below the radially extendible cutting member 29 of the assembly. The cutting member 29 is located within the reamed lower portion 23 of the first wellbore section 20, for example, at a distance of 0.5 feet from the lower end of the casing. The radially expandable expansion member 30 lies immediately below the cutting member 29 of the assembly. At least one of the first and second slips 26, 27 remains set and the lower radially expandable slip 30 is also set. The radially extendible cutting member 29 is then actuated such that the cutter of the radially extendible cutting member 29 is extended radially outwardly to cut through the liner. The remaining expandable liner string 32 is then supported by the first and/or second slips 26, 27 while the cut liner section 35 is supported by the lower radially expandable slip 31. In Figure 10, the expansion member 30 has been actuated to expand the upper portion of the cut section 35 of expandable liner against the casing of the first wellbore section to form a sealed connection 36 therebetween. Suitably, the sealed connection 36 is expanded against the wall of the reamed lower portion 23 of first wellbore section. Typically the wellbore wall may be reamed to a diameter that is capable of accommodating the sealed connection such that the internal diameter of the sealed connection is substantially the same as the internal diameter of the casing above the sealed connection. In Figure 11, the expansion member 30 has been moved downwardly relative to the drill string through the cut section 35 of liner using piston 37 so as to expand the lower portion of the liner section. Suitably, the end of the expanded section of liner lies within a reamed part 34 of the second wellbore section having a diameter that corresponds to that of the reamed lower portion of the first wellbore section. Advantageously, the entire new wellbore section may be reamed to a diameter that is substantially the same as that of the reamed lower portion 23 of the first wellbore section in which case the liner may be expanded until the internal diameter of the liner is substantially the same as the inner diameter of the casing of the first wellbore section. In Figure 12, further sections of liner 38 have been added to the expandable liner string 32 at the surface such that the drill bit 25 drills a further portion of the second wellbore section 33. The drilling step is stopped when the radially extendible cutting member 29 of the assembly lies immediately above the radially expandable expansion member 30. The cutter of the radially extendible cutting member 29 and the radially expandable expansion member 30 are then retracted and the first and second upper slips 26, 27 and the piston 28 are actuated to move the expandable liner string 32 downwardly with respect to the drill string such that the radially extendible cutting member 29 and the radially expandable expansion member 30 lie in the annulus formed between the liner string and the drill string and a further section of liner extends below the radially extendible cutting member 29. The piston 37 has also been actuated to move the radially expandable expansion member 30 to a position adjacent the radially extendible cutting member 29. The steps outlined in Figures 9 to 12 are then repeated such that the second wellbore section is lined with further sections of expandable liner having sealed connections therebetween without having to pull the apparatus from the wellbore. In Figure 13 is shown a wellbore that has been drilled through a subterranean formation. A first wellbore section 40 is drilled a relatively short distance into the ground and a casing is run into the first wellbore section. Thereafter, a second wellbore section 42 is drilled from the first wellbore section using an apparatus comprising an expandable liner string 43 arranged concentrically about a drill or inner string 44 and an assembly attached to the lower end of the drill string wherein the expandable liner string 43 is supported on the assembly. The assembly comprises a drill bit 45, a conventional underreamer 46 which is used to ream the second wellbore section to a larger diameter and a further underreamer (not shown) which is used to increase the diameter of the section wellbore section at locations where it is desired to form a sealed joint between a cut section of expandable liner and the expanded liner 58. In addition, the drilling assembly comprises a radially extendible cutting member 47 for cutting a section of liner from the liner string, a radially expandable expansion member 48 for expanding the cut section of liner, and a gripping member (nor shown) for gripping the cut section of liner. Both the expandable liner string 43 and drill string 44 extend to the surface. A drilling fluid is passed to the drill bit and underreamers through the annulus formed between the drill string and expandable liner string and an entrained cuttings stream is returned to the surface via the interior of the drill string 44. However, it is also envisaged that the drilling fluid may flow to the assembly through the interior of the drill sfring and the entrained cuttings stream may flow back to the surface through the annulus formed between the drill string and the expandable liner string. At least a portion of the drilling fluid is passed through at least one hydraulically powered motor that operates an electrical generator (not shown), which is used to generate electricity to power the moving parts of the drilling assembly including the drill bit and undeπeamers. Accordingly, in this embodiment of the present invention, the drill bit is electrically powered so that that there is no requirement to rotate the drill string and/or the expandable liner string. The drilling fluid could also be passed through a first hydraulically powered motor that operates a first electrical generator that produces electricity to power some of the system components, and through a second hydraulically powered motor that operates a second electrical generator that produces electricity to drive the drill bit and hole-openers. Alternatively or additionally, the components of the assembly could be made to operate by rotation of both the inner drill string and outer liner string. The operation of the apparatus of the embodiment of the present invention illustrated in Figure 13 will now be described in detail with reference to Figures 14 to 19. Figure 14 shows the expandable liner string 43 the lower end of which engages a stop member 50 of the assembly. A gripping member 54 (slips or expandable packer) lies immediately below the radially extendible cutting member 47. The assembly is connected to the lower end of the drill string 44. A reel 51 for a fibre optic or electrical cable is aπanged concentrically about the upper end of the assembly in the annulus formed between the expandable liner string 43 and the upper part of the assembly. The fibre optic or electrical cable 52 extends to at or near the top of the expandable liner string. An electromagnetic or wireless telemetry link is provided to pass signals through the liner string so that signals can be transmitted from the surface (or vice versa) along the fibre optic or electric cable 52 to the assembly. Suitably, the cable 52 is fixed in position at or near the top of the liner string, and is drawn from the reel 51 when expandable liner pipe and drill pipe are added to the liner string and drill string respectively at the surface. The further underreamer 49 has been actuated to underream a portion of the wellbore 53 to a larger diameter so as to accommodate the joints that are to be formed between the cut sections of liner and the expanded liner 58. In Figure 15, the second wellbore section has been drilled to a greater depth and the further undeπeamer 49 has been radially extended to undeπeam a further portion 53 of the wellbore to a larger diameter to accommodate a future sealed joint. The gripping member 54 has been moved downwardly relative to a tubular portion 57 of the housing of the assembly until the gripping member 54 lies immediately above the stop 50. The gripping member 54 is then radially extended to grip the lower end of expandable liner string 43. A section of expandable liner 55 is then cut from the expandable liner string 43 using the radially extendible cutter member 47. The remainder of the expandable liner string 43 is supported at the surface. In Figure 16, the radially expandable expansion member, (preferably, an electrically operated rotatable expansion member comprising radially extendible balls or rollers) is acutated and initially forms a sealed connection 59 between the cut section of liner and the expanded liner 58 in a portion 53 of the wellbore that has been reamed to a larger diameter. The expansion member 48 is then moved downwardly relative to the tubular portion 57 of the housing, while rotating the expansion member 48, thereby expanding the cut section of liner 55 against the wellbore wall. Suitably, the expansion member 48 is moved downwardly relative to the tubular portion 57 of the housing of the assembly by means of a screw thread or piston (not shown), until the expander tool lies immediately above the radially extendible cutting member 47 and the gripping member 54. As shown in Figure 17, the expansion member 48 is then radially retracted and the drill string 44 is pulled upwardly relative to the expandable liner string 43 until the stop 50 of the assembly engages the lower end of the expandable liner string 43. In Figure 18, the radially extendible cutting member 47 and the radially expandable expansion member 48 are moved upwardly relative to the tubular portion 57 of the housing of the assembly until they lie in the same position as in Figure 14. A length of drill pipe is then added to the drill string at the surface. A length of expandable liner pipe is also added to the liner string at the surface. The length of the liner pipe is about twice the length of the drill pipe so as to allow a further section of expendable liner pipe to be cut from the bottom of the liner string 43. As illustrated in Figure 19, the expandable liner string 43 and the drill string 44 are both lowered into the wellbore until the drill bit 45 of the assembly engages with the bottom of the second wellbore section. The steps outlined in Figures 14 to 19 are then repeated such that the second ^• wellbore section is extended and lined with further sections of expandable liner having sealed connections therebetween, shown as 41 in Figure 13, without having to pull the assembly from the wellbore.

Claims

Claims: 1. A method of drilling and lining a wellbore comprises drilling and casing a first wellbore section and then drilling and lining a second wellbore section using a drill string having a drill bit connected directly or indirectly to the lower end thereof, which method comprises: (A) disposing around the drill string above the drill bit (i) a radially extendible cutting member (ii) a radially expandable expansion member and (iii) an expandable liner string (B) drilling the second wellbore section from the first wellbore section (C) cutting a first section of the expandable liner string using the radially extendible cutting member ' (D) positioning the first section of the expandable liner string so that it extends from within the first wellbore section into the second wellbore section (E) expanding the first section of the expandable liner string using the radially expandable expansion member such that the upper portion of the first section of the expandable liner string forms a sealed connection with the lower portion of the casing of the first wellbore section and the lower portion of the first section of the expandable liner forms a liner for the second wellbore section.

2. A method as claimed in claim 1 in which the radially extendible cutting member and radially expandable expansion member are part of an assembly that also comprises means for gripping the interior of the expandable liner string.

3. A method as claimed in claim 2 in which the assembly also comprises means for moving the expandable liner string longitudinally with respect to the assembly.

4. A method as claimed in either of claims 2 or 3 in which the assembly, comprising the radially extendible cutting member and radially expandable expansion member, is disposed around the drill string in the annulus between the expandable liner string and the drill string.

5. A method as claimed in any one of claims 2 to 4 in which the drill string extends to the surface and the gripping means of the assembly grip the inners wall of the expandable liner string such that the drill string bears the weight of the assembly and expandable liner string.

6. A method as claimed in claim 5 in which the gripping means is mounted on bearings such that the expandable liner string does not rotate with the drill string.

7. A method as claimed in any one of claims 2 to 4 in which the expandable liner string extends to the surface, the drill string and assembly are supported on the expandable liner string and rotation of the expandable liner string effects rotation of the drill string.

8. A method as claimed in any one of claims 2 to 4 in which both the expandable liner string and the drill string extend to the surface and drilling mud is circulated down through the drill string and back up through the annulus between the drill string and expandable liner string or vice versa.

9. A method as claimed in claim 8 in which the drill string comprises a length of inner string that is not capable of transmitting sufficient torque from the surface to rotate the drill bit and the drill bit is driven by a downhole motor.

10. A method of drilling a wellbore, the wellbore comprising a first wellbore section and a second wellbore section, the method comprising:

(A) drilling the first wellbore section and running a casing from the surface into the first wellbore section; (B) drilling the second wellbore section from the first wellbore section and lining the second wellbore section with an expandable liner characterised in that the second wellbore section is drilled using an apparatus comprising a drill string having a drill bit connected either directly or indirectly to the lower end thereof, an expandable liner string aπanged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit and an assembly disposed about the drill string in the annulus formed between the expandable liner string and the drill string, the assembly comprising from the top downwards, an upper gripping member, a means for moving the expandable liner string in a downwards direction with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member is moveable in a longitudinal direction with respect to the drill string and wherein the second wellbore section is lined by: (i) interrupting drilling of the second wellbore section when the expandable liner string extends below the casing of the first wellbore section and the radially extendible cutting member of the assembly is positioned within the lower portion of the casing; (ii) actuating the radially extendible cutting member to cut a first section of liner from the expandable liner string with the expandable liner string gripped by the upper and lower gripping means whereby the first section of liner that is cut from the expandable liner string is gripped by the lower gripping member and the upper portion of the first section of liner overlaps the lower portion of the casing of the first wellbore section; (iii) expanding the upper portion of the first section of liner into the lower portion of the casing of the first wellbore section to form a sealed connection therebetween and expanding the lower portion of the first section of liner to form a liner for the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner; (iv) radially retracting the cutting member and expansion member and actuating the means for moving the liner string downwardly with respect to the assembly so that the expandable liner string moves downwardly through the wellbore until the radially extendible cutting member and radially expandable expansion member of the assembly are positioned in the. annulus formed between the expandable liner string and the drill string; (v) recommencing the drilling operation to extend the second wellbore section; (vi) interrupting drilling of the second wellbore section when the expandable liner string extends below the expanded liner of the second wellbore section and the radially extendible cutting member of the assembly is positioned within the lower portion of the expanded liner; (vii) actuating the radially extendible cutting member to cut a further section of liner from the expandable liner string with the expandable liner string gripped by the upper and lower gripping means whereby the further section of liner that is cut from the expandable liner string is gripped by the lower gripping member and the upper portion of the further section of liner overlaps the lower portion of the expanded liner of the second wellbore section; and

(viii) expanding the upper portion of the further section of liner into the lower portion of the expanded liner of the second wellbore section to form a sealed connection therebetween and expanding the lower portion of the further section of liner to extend the liner of the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member with respect to the drill string such that the expansion member moves through the further section of expandable liner.

11. A method of drilling a wellbore, the wellbore comprising a first wellbore section and a second wellbore section, the method comprising:

(B) drilling the second wellbore section from the first wellbore section and lining the second wellbore section using an apparatus comprising a drill string having a drill bit attached either directly or indirectly to the lower end thereof, an expandable liner string arranged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit, an assembly disposed about the drill string in the annulus formed between the drill string and the expandable liner string, the assembly comprising from the top downwards, an upper gripping member, a means for moving the expandable liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member is moveable in a longitudinal direction with respect to the drill string and wherein the second wellbore section is drilled by the steps of: (a) lowering the apparatus into the first wellbore section by (i) lowering the expandable liner string into the first wellbore section with the expandable liner string supported by slips at the surface and (ii) lowering the drill string with the assembly disposed about the drill string through the expandable liner string to the bottom of the first wellbore section; (b) aπanging the expandable liner string with respect to the cutting member of the assembly such that a first section of the expandable liner string extends below the cutting member and actuating the upper and/or lower gripping members to grip the inner wall of the expandable liner string; (c) commencing drilling of the second wellbore section by rotating the drill string to rotate the drill bit;

(e) actuating the radially extendible cutting member of the assembly to cut the first section of liner that extends below the radially extendible cutting member from the expandable liner string with the upper and lower gripping members of the assembly gripping the inner wall of the expandable liner string so that the first section of liner that is cut from the expandable liner string is gripped by the lower gripping member;

(f) expanding the upper portion of the first section of liner into the casing of the first wellbore section to form a sealed connection therebetween and expanding the lower portion of the first section of liner to form a liner for the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner;

(g) radially retracting the cutting member and expansion member;

(h) actuating the means for moving the expandable liner string downwardly with respect to the assembly such that a further section of liner extends below the radially extendible cutting member of the assembly;

(i) re-commencing drilling of the second section of the wellbore by rotating the drill string to rotate the drill bit;

(j) interrupting drilling of the second section of the wellbore when an upper portion of the further section of liner that extends below the cutting member overlaps the lower portion of the expanded liner of the second wellbore section;

(k) actuating the radially extendible cutting member of the assembly to cut the further section of liner from the liner string with the upper and lower gripping members gripping the inner wall of the expandable liner string so that the further section of liner that is cut from the expandable liner string is gripped by the lower gripping member; (1) expanding the upper portion of the further section of liner into the lower portion of the expanded liner of the second wellbore section to form a sealed connection therebetween and expanding the lower portion of the further section of liner to extend the liner of the second wellbore section by radially expanding the radially expandable expansion member and moving the expansion member in a longitudinal direction with respect to the drill string such that the expansion member moves through the first section of liner;

(m) optionally, repeating steps (g) to (1).

12. An apparatus for drilling and lining a wellbore the apparatus comprising: a drill string having a drill bit attached to the lower end thereof, an expandable liner string aπanged concentrically about the drill string with the lower end of the expandable liner string positioned above the drill bit, an assembly disposed about the drill string in the annulus formed between the drill string and the expandable liner string, the assembly comprising from the top downwards an upper gripping member, a means for moving the liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member aπanged on the assembly below the radially extendible cutting member, and a lower gripping member wherein the radially expandable expansion member and optionally the cutting member are moveable in a longitudinal direction with respect to the drill string and wherein the inner wall of the liner string is gripped by the upper and or lower gripping members of the assembly.

13 An assembly for delivering a liner to a wellbore, the assembly being disposable around a drill string and comprising a means for moving the assembly longitudinally with respect to the drill string, means for gripping or supporting the expandable liner string, a radially expandable expansion member and a radially extendible cutting member.

14. An assembly for delivering a liner to a wellbore as claimed in claim 13 in which the means for gripping the expandable liner string comprises an upper gripping means and a lower gripping means.

15. An assemble for delivering a liner to a wellbore as claimed in claim 14 in which the radially expandable expansion means is also the lower gripping means.

16. An assembly for delivering a liner to a wellbore, the assembly being disposable around a drill string and comprising from the top downwards: a first gripping member, a means for moving the liner string downwardly with respect to the assembly, a radially extendible cutting member, a radially expandable expansion member and a lower gripping member wherein the radially expandable expansion member and optionally the radially extendible cutting member are moveable in a longitudinal direction with respect to the drill string.