METHODS AND ARRANGEMENTS FOR CASING A BOREHOLE

Abstract: Strip steel (4) is wound around a mandrel (5), which may be a standard length of drill pipe, to form an axially elongate resilient helix (6) in a condition of torsional stress. The stressed helix is clamped onto the mandrel with releasable clamps (8). The clamps are released when the helix is at a desired depth so that the stored energy tends to unwind the helix to increase its diameter, whereby the helix bears against the borehole wall to provide a self-supporting casing. The clamped helices may pass down through the diameter of previously cased borehole lengths and it is thus possible to form a self-supporting cased borehole of non-reducing diameter.

Description: METHODS AND ARRANGEMENTS FOR CASING A BOREHOLE TECHNICAL FIELD This invention relates to the casing of boreholes. Boreholes are drilled for water, oil, exploratory 5 and other purposes. A drilling mud or slurry is normally- pumped down the drill pipe to carry cuttings away from the • drill bit and up to the surface in the annular space between the drill pipe and the borehole wall. The hydrostatic pres¬ sure of the mud also serves to counteract pressures produced 10. by gas or fluids released as earth formations are penetrated by the borehole. The presence of the mud and its movement can however lead to damage to 'the borehole or to formations through which the borehole has been drilled with possible consequent collapse of the borehole. 15 BACKGROUND OF THE INVENTION It is customary in the art to protect the borehole, e.g. from such mud damage, by suspending a casing string within the borehole and introducing a cement slurry into the annular space between the string and the borehole wall 20 to set therein. Such casings are typically formed of a string of strong, thick-walled, heavy steel pipes, each about 13 metres in length, secured together end to end as they are lowered in sequence by the drilling derrick into the borehole. 25 To avert the possibility of borehole damage or col¬ lapse the decision often has to be taken to insert a casing string before the borehole has attained its desired terminal O;.TPI depth. Once a first length of borehole has been so cased drilling proceeds with a drill pipe and bit lowered withi the first casing string. The next decision to case a secon lower length of the borehole necessarily then has to b carried out with a casing string of smaller diameter tha the first length, so that it may be passed through and sus pended therein. Successive casing strings must each b of smaller diameter than their predecessor, and so the bore hole diameter is progressively reduced with increasing depth There is a practical minimum casing diameter and thus given borehole, cased in a number of stages, attains a prac tical maximum depth which may not be the desired termina depth. That is particularly likely where the borehole pene trates an unexpected type of formation leading to collapse, or otherwise becomes liable to damage,, and an extra, unplan ned stage of casing has to be inserted. In order to minimize the risk of not attaining th desired terminal depth it is known to commence drillin with a wide initial borehole diameter. This course ha considerable disadvantages, in drilling time and cost, an the very substantial expense of installing large diamete blow-out prevention and other well head equipment.. Borehole particularly for gas and oil, must be capped with such equip ment to withstand possible high downhole pressures. A further disadvantage of the prior art casing metho is that each successive stage of casing string extends dow the borehole right from the head of the borehole, and more¬ over each steel pipe section has to support the cumulative weight of all the sections dependent therefrom as it is lowered by the derrick. The casing sections therefore gener¬ ally each have much greater tensile strength, thickness and therefore weight than is necessary for the actual task of protecting the borehole wall from damage. Particularly in offshore drilling operations, provision of a platform and a derrick of sufficient capacity to support and handle the heavy pipe sections necessary for casing a deep well, which must all be available on the platform prior to drilling ay impose a heavy economic burden on the particular opera¬ tion. An object of the present invention is to obviate or at least mitigate some of the above described disadvant- ages of conventional casing techniques. SUMMARY OF THE INVENTION According to the present invention there is provided a method of casing a borehole comprising disposing in the borehole a strip of material in the form of an axially elong- ate helix, and causing the diameter of the disposed helix to increase. The invention also provides a method of casing a borehole with steel casing material comprising winding strip steel material around a mandrel to form an axially elongate resilient helix in a condition of torsional stress, releas- ably securing said stressed helix around said mandrel with securing means, disposing said secured stressed helix and mandrel at a desired depth in a borehole, .releasing said securing means so that the stored energy of said torsional stress tends to unwind the helix with a consequent reduction in its axial length and an increase in its diameter whereby the helix bears against the wall of the borehole to provide a self-supporting casing therefor, and withdrawing the mandrel. In another aspect the invention provides a borehole having a self-supporting casing comprising " a. strip of mater¬ ial in the form of an axially elongate helix bearing against the wall of the borehole under the influence of stored torsional stress energy in the material of the helix. In a further aspect the invention provides an arrange¬ ment for use in casing a borehole comprising a strip of material wound around a mandrel to form a helix in a condi¬ tion of torsional stress, and securing means releasably securing said stressed helix around said mandrel. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, _ by way of example, with reference to the accompanying dra ings, in which: ^ Figure 1 shows schematically a section through borehole having a conventional casing system therein; Figure 2 shows an elevation of a portion of an arrang ment for casing a borehole in accordance with the invention; Figure 3 shows an elevation of a portion of anothe arrangement for casing a borehole in accordance with th invention; and Figures 4A and 4B show diagrammatically a casin according to the invention respectively before and afte torsional stressing and clamping. , DETAILED DESCRIPTION OF THE EMBODIMENTS . . Referring to the drawings, Figure 1 shows a borehol 1 having a conventional casing system 2 comprising fou strings 2a, 2b, 2c and 2d. The borehole is first drille at a wide diameter until the first decision to case is taken Casing 2a, e.g. 150 metres of 47 centimetre diameter pipe is inserted in the borehole and cement slurry 3 forced dow the drill pipe thereby displacing drilling mud up the annulu between the casing and the borehole wall. The cement i in turn forced by further drilling mud into the annulu where it is allowed to set to seal the annulus and suppor " the casing. Drilling is then resumed down through casing strin 2a until the next decision to case is taken. Casing strin 2b is then inserted and cemented in position. This procedur continues with successive concentric casing strings 2c an 2d. If drilling proceeds according -to plan, string 2b ma typically be 900 metres of 35 centimetre diameter pipe casing 2c may typically be 2,100 metres of 25 centir.etr diameter pipe, and casing 2d may typically be 4,500 metre of 18 centimetre diameter pipe to attain the desired termina depth of 4,500 metres. .However, drilling down through eart formations is beset with many uncertainties and it may prov necessary to take any one or more of the casing decision much earlier than planned, leading to the likelihood o Ώ:.;? the borehole not being able in practice to reach the desired terminal depth. When this occurs, the borehole may have to be abandoned and although it is highly desirable to re¬ cover the strings 2a-2d, this is not always practicable. This and the other consequent and related disadvantages of prior art casing techniques have been described above. Figure 2 shows an arrangement for casing a borehole by a method according to the present invention. Thin flat strip steel 4, or other suitable material, is wound around a mandrel 5 to form an axially elongate substantially con¬ stant diameter helix 6. Adjacent turns of the helix prefer¬ ably substantially abut one another or leave a small helical gap 7. The mandrel 5 may be a thin-walled light gauge alum¬ inium tube for example. The strip 4 may be spring steel so that the helix is in a condition of torsional stress. Alternatively a non-spring strip steel may be employed to wind a substantially unstressed first helix of a natural diameter, e.g. on a first mandrel of larger diameter, the first helix is then subject to heat treatment resiliently to set the helix at that preferred natural diameter, and the treated first helix then coiled up on the mandrel 5 to provide , the helix 6 in the desired condition of torsional stress below its natural diameter. The stressed helix 6 is secured onto the mandrel 5 by means of releasable securing means for example in the form of circumferential clamps 8 or shear bolts, to the man¬ drel. A clamp 8 may take the form of a steel band 9 with ends bolted together by an explosive bolt 10. One clamp 8 is provided at each end of the stressed helix and one or more other clamps 8 may be provided between the ends. Figure 4A shows diagrammatically the strip 4 in a helix resiliently set to a natural diameter, and Figure 4B shows the same helix in its condition of torsional stress and clamped onto the mandrel 5 by means of clamps 8. Typic- ally, the outside diameter of the stressed helix 6 is 20 centimetres, the natural diameter being about 30 centimetres, and the natural length being correspondingly shorter than the stressed length in substantially the same ratio. It will be appreciated that if the clamps 8 ar released, the stored energy of the torsional stress tend to unwind the helix with a consequent reduction in its axia length and an increase in its diameter. " In the case o the resiliently set helix, the undamped helix will rever to the Figure 4A condition from the Figure 4B conditio in the absence of any other surrounding constraint. In practice of the present invention the clampe stressed resiliently set helix and mandrel arrangement o Figure 4B is lowered into a borehole and disposed at a des ired depth. The clamps are then released by firing th explosive bolts 10, and the helix tends to unwind. Whe the borehole diameter at that desired position is less tha the natural diameter of Figure 4A, the helix unwinds unti its turnsbear against, the walls of the borehole to provid a self-supporting casing therefor. The mandrels 5 may the be freely withdrawn from the borehole. It will be apprec iated that the force exerted by the helix 6 against th borehole wall is a function of the remaining stored sprin energy, which in turn depends on the original spring charac teristics and on the ratio of the borehole diameter to th natural helix diameter of Figure 4A. It is found " in practic that the expanded diameter helix can provide a self-supporti and surprisingly rigid borehole casing. Where the strip 4 is a strip of spring material woun from a coil onto the mandrel so as to store torsional energy the arrangement of the stressed helix clamped to the mandre is similarly disposed at the desired depth in the borehole The securing means are released and the helix tends to unwin until its turns bear against the walls of the borehole t provide a self-supporting casing. The outside diameter of the clamped helix of Figur 4B, including the clamps, is in practice chosen to be sub stantially less than the outside diameter of the release helix when in position in a borehole at a particular depth, the minimum borehole diameter at that depth being know OMPI from the drill bit diameter used at that depth. Consequently the clamped helix arrangement of Figure 4B may be passed axially through the expanded diameter of a previously posi¬ tioned helix. Thus the technical problem described in rela- tion to Figure 1 is avoided. Successive casing lengths according to the invention may be of the same installed diameter as previously positioned casing lengths higher up in the borehole, and the borehole can therefore attain its planned terminal depth. Moreover the borehole may be of substantially constant diameter over "its entire depth, avoiding the necessity for commencing drilling at a large diameter and with consequent significant reduction in total volume of hole to be drilled and weight of casing to be set for a given well objective. The arrangements and methods according to the inven¬ tion also provide considerable flexibility in the manner in which they are used, unlike the strict succession . -of procedural operations imposed by the conventional casing technique. A number of operational embodiments will now be described. The clamped stressed helices may be manufactured in standard, e.g. ' 13 metre, length and transported to a drilling site, the mandrels being returned for re-use after placement of the helix in a borehole. Alternatively the drilling pipe sections themselves may be employed as the mandrels 5, .a resiliently set unstressed helix being placed over a drill pipe section over its length between its end screw connectors. The helices may then be torsioned arid clamped, the steps of this operation being performed in a factory or on site. In a further alternative, the helix at be formed and stressed by winding the strip material onto the mandrel at the borehole head. The mandrel may be progressively inserted into the borehole as the helix is progressively formed and secured thereon. Again, the drill pipe sections may themselves serve as the mandrel. A series of the clamped stressed helices of Figure 4B may be lowered into a borehole in a sequence to form a string, the growing weight being carried directly on t successive lengths of drill string where these serve the mandrels. Where the mandrels are hollow tubing thes may be threaded together and lowered into the borehol directly from the derrick. As previously explained, us of the present invention permits clamped helices, or a strin thereof, to be lowered down through previously positione such casing lengths higher up the borehole. The clamps of each clamped helix are released whe that helix is in a desired position and may be release together or in succession to control final positioning o the unwinding helix. For example, in the embodiment wher helices are wound on and clamped directly to the pipe sec tions of a drill string, helices may be released in succes sion working progressively up the drill string from th bit to form a growing casing installed progressively down wards from the borehole head or other desired position i the borehole. In that case each clamped helix passes throug the "expanded diameters of all the previously installe helices prior to its own installation immediately belo the adjacent previously installed helix. Alternatively a string of clamped helices are suspended in the borehol and successive lengths released beginning with the uppermos and proceeding successively downwards. It will be recalled that the helices contract i length when released in direct proportion to their expansio in diameter. This may be pre-calculated from the know borehole and drill bit characteristics. Thus if desire coarse allowance may be made for this effect by adjustin the depth of suspension of each casing element relativ to previously placed casings prior to release of that elemen Typically 100 metres of drill string may carry sufficien helices for casing about 60 metres of borehole with substant ially a single layer of casing. The drill pipe can the be reloaded each time it is retracted to inspect or chang the drill bit. In the existing casing techniques, each tubular stee 1R CI-.'Λϊ casing length usually depends right down from the borehole head. In the present invention each expanded helix is gener¬ ally self-supporting against the ' borehole wall. Thus it is now possible to case only selected lengths of a borehole, omitting if so desired lengths where the formation does not require casing. Furthermore the strip material of the helix is thin as compared with the conventional steel casing pipes. It is therefore possible in the present invention to case a borehole length with two or more concentric helices expanded concentrically within one another so as wholly or partially to overlap and mutually reinforce one another. The second and subsequent concentric helices may be installed on a subsequent run or substantially simultaneously with the first helices. They may be wound in the same or opposite sense as the first helices. Where they are wound in the same sense the respective turns may be out of phase to pro¬ vide a measure of sealing against fluid ingress into the borehole over that cased length. Figure 3 shows an ar-range- ment wherein first and second strips 4, 11 are wound to form first and second helices 6, 12 on a common mandrel 5, which may be a drill pipe section. Helices 5 and 12 are wound in the same sense as one another with their respec¬ tive turns out of phase so that each substantially seals the spiral gap 7 around the other. Where an upper length of borehole has been cased according to the invention, it, is desired to continue drill¬ ing at the same diameter as the first length. That is accom¬ plished by use of an under-reamer drill bit as known in the art, or by other suitable means. The clamps 8 have been described above as releasable by means of explosive bolts. Other remotely releasable clamps may be employed. For example the clamps may be pro¬ vided with a weakness in lieu of the explosive bolt. The weakness may then ' be sheared e.g. by application of a high pressure hydraulic pulse down the drilling mud filling the borehole. Alternatively the stressed helix may be releasably secured, e.g. by a shear bolt, to the mandrel itself. Finally the present invention may be employed i cooperation with existing casing techniques where desired particularly where there is a requirement for isolatin water or gas-bearing formations when the characteristic of both methods can advantageously complement each other For example a borehole, or a length of a borehole, may b cased according to the present invention with a number o helices which may or may not overlap one another and thereb substantially protect the borehole against collapse. conventional casing, or production tubing, string of stee pipe sections may then be lowered down within the helicall cased borehole. The helical casing thus protects the conven tional casing string from direct borehole contact. Thi combined technique may be effected progressively down borehole. A first upper length of borehole is drilled cased according to this invention with released stresse helices which thus substantially protect the new borehol from collapse, and a conventional casing tube string i lowered down through said upper length carrying an externa casing packer at its lower end. The packer is actuated e.g. hyraulically, to seal the annulus between a lower en of this tube string and the borehole wall. The borehol is now both protected from collapse, and water, gas or oi zones above the packer are isolated from lower zones. lower length of borehole is then drilled, with an under reamer drill, and cased with released helices accordin to this invention. Those helices can pass down the tub string when they are clamped, but expand to a greater diam eter than the tube string. The packer is then release and the first tube string progressively increased in lengt and lowered down through the second helically cased borehol length. The packer is again actuated to again seal th annulus between the borehole and casing. This procedur may be repeated as often as necessary to provide a borehol cased throughout its length with constant ' diameter conven tional tubing, which would not have been possible in thos borehole conditions in the prior art. Additional external casing packers may be placed at appropriate points in the continuous conventional casing string to provide additional isolation of formations and to provide protection from b.ow- cuts to surface between the borehole and the casing wall.

Claims: CLAIMS : 1. A method of casing a borehole (1) characterise by disposing in the borehole a strip (4) of material i the form of an axially elongate helix (6), and causing th diameter of the disposed helix to increase. 2. A method according to claim 1 characterised by prior to said step of disposing the helix (6), stressin said strip (4) to provide said helix in a condition of tor sional stress and releasably securing said stressed heli (6) around a mandrel (5) with securing means (8), and wherei said step of causing the diameter to increase is effecte by releasing said securing means (8). 3. A method according to claim 2 characterised i that said steps of stressing said strip (4) and releasabl securing said stressed helix (6) are effected on a discret length of said strip material wound around said mandre (5) prior to insertion of he secured stressed helix an said mandrel into the borehole. 4. A method according to claim 2 characterised i that said steps of stressing said strip (4) and releasabl securing said stressed ' helix (6) are effected by windin strip material onto said mandrel (5) at the borehole head, said mandrel being progressively inserted into the borehole as the helix is progressively formed and secured thereon. 5. A method according to any one of claims 2 to 4 characterised in that said mandrel (5) comprises a length of borehole drill pipe. 6. A method as claimed in any one of claims 2 to 4 characterised by disposing in the borehole at least one second said strip (11) of material in the form of an axially elongate helix (12) extending at least partially within OMPI said first-mentioned helix (6), and causing the diameter of the second helix (12) to increase. 7. A method according to claim 6 characterised by providing on said mandrel (5) said second strip (11) of material in the form of an axially elongate stressed second helix (12) extending concentrically within said first- mentioned helix (6), said helices being wound in the same sense as one another with their respective turns out of phase, and said stressed helices being releasably secured by said securing means (8). 8. A method according to any one of claims 2 to 4 characterised by providing a succession of said secured stressed helices (6) on a common said mandrel (5) or on a string of individual said mandrels disposed in said bore- hole, and releasing the securing means (8) associated with each said helix when that helix ' is substantially in a desired position in the borehole. 9. A method according to claim 8 characterised in that the securing means (8) associated - with at least one said secured stressed helix (6) in said succession is released after that helix has been lowered axially through at least one • said helix of expanded diameter whose securing means had previously been released. 10. A method of casing a borehole with steel casing material characterised by winding strip steel material (4) around a mandrel (5) to form an axially elongate resilient helix (6) in a condition of torsional stress, releasably securing said stressed helix around said mandrel with secur¬ ing means (8), disposing said secured stressed helix and mandrel at a desired depth in a borehole, releasing said securing means so that the stored energy of said torsional stress tends to unwind the helix with a consequent reduction in its axial length and an increase in its diameter whereby the helix bears against the wall of the borehole to provide a self-supporting casing therefor, and withdrawing the mandrel. 11. A borehole (1) having a self-supporting casing ' comprising a strip of material (4) in the form of an axially elongate helix (6) bearing against the wall of the borehole under the influence of stored torsional stress energy in the material of the helix. 12. An arrangement for use in casing a borehole char¬ acterised by a strip of material (4) wound around a mandrel (5) to form a helix (6) in a condition of torsional stress, and securing means (8) releasably securing said stressed helix around said mandrel. W ~

Inventors: IBALL E
Applicants: IBALL E
Application year: 1980
Main classification: E21B7
IPC codes: E21B7, E21B17, E21B43