The invention concerns improvements in or relating to induction logging tools.

The logging of geological formations is, as is well known, economically an extremely important activity.

Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.

It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the conditions prevailing in a region from which extraction is to take place.

This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.

The acquisition of such data typically makes use of techniques of logging. Logging techniques are employed throughout the mining industry; in the location of commodities such as (subterranean) ground water; the assessment of rock and ground stability in construction planning and also in particular in the oil and gas industries. The invention is of benefit in all logging activities employing induction logging techniques; and especially in the logging of reserves of oil and gas.

In the logging of oil and gas fields specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids perhaps only one component of which is of commercial value.

This leads to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool (or“sonde’) that is lowered into a borehole formed in the formation by drilling. Many forms of logging tool send energy into the formation and detect the energy returned to the tool that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. electrical signal, database, graphical or tabular representations containing much data about the formation in question).

One particular kind of logging technique, that is known as induction logging, makes use of an induction logging tool. The invention improves the performance of such a logging tool.

During induction logging an induction tool typically is passed into and subsequently removed from a borehole while connected to a surface location using wireline, the nature and purpose of which are well known in the logging art. Like most logging tools the induction tool is an elongate cylinder having at spaced intervals along its length various components whose function is to transmit energy (that in the case of the induction tool is electrical energy) through a geological formation and receive (by induction in the case of the induction tool) energy that is indicative of attributes of the formation. The logging tool converts such energy into electrical signals that may be transmitted via the wireline and/or recorded for later use, processing, transmission, printing or display.

The diameter of an induction logging tool must be less than that of the borehole, which may be e.g. approximately 8” (about 200 mm) in diameter. One known induction logging tool has a diameter of about 3¾” (about 90 mm). In more recent years so-called“slim" or “compact" induction logging tools have also become known. These typically have a diameter of about 2¼” (about 57 mm).

Logging tools may be manufactured as discrete tools or as tool sections that are intended to be deployed fixed to or otherwise in combination with other logging tool (or other tool) sections.

In addition to the wireline deployment outlined above, in which electrical power to operate the logging tool is provided via wireline, it is also possible to configure induction logging tools as self-powered devices. These include on-board power sources and memory modules that obviate the need to connect the logging tool to a surface location using wireline during logging operations. Such logging tools may be deployed in a variety of ways, as will be known to the person of skill in the art, including so-called“pump down" and drill pipe-supported deployment techniques. In simple terms an induction tool includes a transmitter that induces current, according to a per se known technique, in the formation surrounding the tool at the depth to which the tool has been lowered or otherwise conveyed. The induction tool also includes at least one, and in practical versions several, receivers of induced current energy.

Figure 1 illustrates the operation of a simple form of induction tool 10a.

As is apparent from Figure 1 , a transmitter T shown schematically as a coil 1 1 when activated induces currents I in the formation F. These travel through the formation, that includes e.g. a hydrocarbon-bearing fluid (or another quantity) under investigation, to be detected by a receiver R also in the form of a coil 12. The currents I in the formation induce one or more voltage in the receiver coil R that is spaced from the transmitter coil T by a distance selected to make the signal at the receiver R preferentially responsive to the currents circulating in a certain range of distances into the geological formation F around the borehole.

The distance in the formation from which half the signal at the receiver originates is commonly assigned as the“depth of penetration" of that receiver. This contrasts with the term “logging depth ", which refers to the distance along the borehole at which log information is acquired. Such terms are familiar to those of skill in the art and do not require detailed explanation.

An approach adopted in the prior art is to employ in the logging tool intermediate the transmitter T and receiver R at least one secondary coil S (shown in the tool 10b of Figure 2) whose design and location are such as to cancel undesirable, induced voltages that couple directly from the transmitter to the receiver and therefore do not imply information about the formation.

One arrangement adopted in practice in the prior art involves the inclusion of multiple (e.g. four) receiver coils and corresponding secondary coils in the induction logging tool at different spacings from the transmitter coil T. The outputs of the plural receiver and secondary coils can then be combined according to a subtle algorithm that assigns weighting and sign values to the outputs of the coils so as to compensate for various prevalent effects. A tool including multiple coils of this nature is sometimes referred to as an“array tool” or a“multiple array tool”. Such tools were first proposed in the 1980’s. An array tool 10b is visible in Figure 2. In the tool 10b of Figure 2 there are four secondary coils S1 , S2, S3, S4 and four receiver coils R1 , R2, R3, R4. The receiver coils R and the secondary coils S are designed and positioned so as to maximize the desired direct signal cancelling effect.

Figure 2 omits numerous features of an induction logging tool that would in practice be present. For purposes of the present disclosure it is important to realise that the components visible in Figure 2 in a completed logging tool would all lie within a cylindrical, parallel-sided e.g. fibreglass sleeve that is filled with an oil and pressure balanced. Such aspects are familiar to the person of skill in the art.

The invention is useable with, but is not limited to, all types of induction logging tool as described herein; and the disclosure hereof of features of the invention includes disclosures in combination with all such logging tool types.

When the downhole conditions (the terms“downhole" and“uphole” being familiar to those of skill in the logging art) permit it, for various reasons induction logging is often the preferred logging approach. A problem arises however when the fluid in the borehole is highly saline, or is for any other reason highly electrically conductive.

In this regard it is commonplace for the borehole in which a logging tool is deployed to be fluid-filled. Such fluid may be naturally occurring or it may be intentionally added after the borehole has been formed. Mixtures of downhole fluids are commonplace.

An induction tool is highly effective when the conductivity of the fluid surrounding the logging tool is low or zero. When a highly conductive fluid is present however, as happens when the salinity of fluid that flows from the surrounding formation into the borehole is high, the performance of an induction logging tool may be compromised.

This is because the tool induces currents in any conductive medium between the transmitter and receivers. This in turn induces current in the receivers. The signals from conductive fluids in the borehole can dominate the signal deriving from the rock, that represents the typically small proportion of conductive fluid within a resistive rock.

The problem of conductive borehole fluids is exacerbated when the conductivity of the rock surrounding the borehole is low. It is obviously worsened even further when the tool is of a compact or slim design, because there then is a relatively large annulus of fluid surrounding the logging tool.

For many years it has been known to encase parts of logging tools with encircling structures largely to protect them or to present them correctly to the borehole and its contents. Thus US 2401371 discloses a generally cylindrical logging tool of the wireline resistivity type defined in part by an outer casing having apertures through which electrodes protrude for the purpose of injecting current into a rock formation.

US 2779915 discloses an insulating cover for the body of a resistivity logging tool, the cover preventing conduction between electrodes forming part of the logging tool and conductive drill pipe on which it is supported during deployment.

US 2964698 discloses an induction logging tool in which a series of coils are provided inside a cylindrical, parallel-sided housing presenting a smooth outer surface of the logging tool.

US 4286217 discloses a logging tool including a number of surface electrodes that are isolated from one another by way of an insulating sheath.

WO83/01336 discloses an insulating sleeve disposed around the end of a drill string.

GB 2146127 discloses a protective sheath for an induction logging tool. Circumferential electrodes are presented on the exterior of the sheath and are electrically connected to internal measuring components by conducting screws.

US 4618828 teaches an insulating segment for a drill string electrode structure used to measure formation resistivity during borehole drilling operations.

US 4929915 teaches a collar-like widener that is secured on the exterior of a small- diameter logging tool in order to stabilise it in large diameter wells. The widener includes electrodes on its outer surface that connect to electrodes formed in the surface of the logging tool, whereby the measurement points of the tool effectively are extended radially outwardly to positions removed from the logging tool surface.

US 2016/0017679 discloses a slip cover for downhole logging tools that prevents the tools from becoming stuck downhole. US 2015/013687 A1 discloses an elongate sleeve for surrounding the exterior of an induction logging tool. The sleeve in US 2015/013687 A1 is discontinuous in certain radial directions extending outwardly from the tool and longitudinally along the sleeve, by reason of including longitudinally extending fluid passageways.

According to embodiments disclosed herein there is provided an induction logging tool or logging tool section including at least one elongate, cylindrical logging tool body having an outer surface of essentially constant outer diameter, the logging tool or logging tool section supporting one or more transmitter and/or receiver coils and the outer surface being surrounded by an electrically non-conducting sleeve that is fixed relative thereto and of greater external dimensions than the outer surface, the sleeve extending outwardly of the outer surface in a manner in use of the logging tool or logging tool section in a borehole excluding borehole fluid from the space occupied by the sleeve, such excluding being uninterrupted in all radial directions extending outwardly from the logging tool through the sleeve. Preferably such excluding also is uninterrupted along the length of the sleeve.

By“logging tool section" is meant a section or length of logging tool componentry that can be assembled with other sections into a composite tool. Such a composite tool may have as its primary function a logging operation; or alternatively logging may be only one of plural functions carried out.

Equally,“logging tool" as used herein is not limited to a tool that carries out logging to the exclusion of other types of operation. The terms“logging tool" and“logging tool section" are used somewhat synonymously herein.

A logging tool or logging tool section according to embodiments of the invention disclosed herein advantageously permits an induction logging tool to be used even when a highly conductive fluid surrounds the logging tool in the borehole. This is achieved by reason of the non-conducting sleeve displacing the conductive fluid such that significantly less of it remains in the borehole between the exterior of the non-conducting sleeve and the interior wall of the borehole. As a result the detrimental effect of the conductive fluid is reduced or eliminated compared with the prior art, with the beneficial consequence that corrections for it are rendered small or unnecessary.

Such benefits arise even if the conductivity of the rock surrounding the logging tool is low and/or the logging tool is of a typical slim or compact diameter. The non-conducting sleeve additionally permits a relatively narrow tool to be stabilised in a relatively large borehole, but this is a secondary benefit compared with that of displacement of the conductive fluid.

Such a logging tool is likely to be highly attractive to operators of oil and gas fields. As implied above, induction logging is very popular yet high salinity (or other causes of high conductivity) of borehole fluid may cause perhaps 20% of boreholes in a field to be unsuitable for this type of logging. In such cases it hitherto has been necessary to employ other logging methods. The data generated by such techniques may not be available in the same form as the induction logs. The need to present log data according to differing protocols and/or to convert log types can reduce confidence in the log results. The invention permits a significantly greater percentage of the boreholes in a field (and in some cases all of them) to be logged using one and the same induction technique.

A logging tool or logging tool section according to embodiments as disclosed herein further provides a number of unexpected advantages over the sleeve arrangement of US 2015/0136387 A1.

The elongate fluid passageways of US 2015/0136387 A1 are stated to permit the flow of fluid longitudinally relative to the sleeve, but conductive fluid in the passageways appears to induce noise voltages in the receiver coils. US 2015/0136387 A1 proposes the inclusion of multiple electrodes in the passageways in an attempt to ground any currents and try to to avoid the indicated problem. Such additional features are not needed in the embodiments disclosed herein.

Furthermore the design of the sleeve in US 2015/0136387 A1 appears to require that the sleeve occupies the entire cross-section of a borehole in which it is inserted, but as a practical matter this is almost certain to be impossible owing to variations in the cross- section of a borehole along its length.

Yet a further problem arising in use of the sleeve of US 2015/0136387 A1 is that the longitudinal passageways are likely to become plugged if the fluid in the vicinity of the sleeve is drilling mud, that usually is intended to block and fill any cavities encountered downhole.

Aspects of embodiments disclosed herein obviate such disadvantages of the prior art. In further embodiments disclosed herein there is provided a non-conducting sleeve for fitting on the outer surface of an elongate, cylindrical induction logging tool or logging tool section, the sleeve defining an elongate, hollow member including an inner, circular cross- section bore the diameter of which nominally is a sliding fit relative to an induction logging tool and a sleeve wall that when the sleeve is immersed in surrounding fluid uninterruptedly excludes surrounding fluid from the volume defined by the boundaries of the sleeve wall. The diameter of the inner bore is selected to suit the diameter of the logging tool with which the sleeve is intended to be used.

Various optional features of embodiments are described in the dependent claims forming part of this disclosure. Such optional features include the possibility, as explained herein, of the sleeve (whether considered as part of a logging tool or logging tool section, or independently) defining a sleeve wall having inner and outer longitudinal walls that are spaced from one another along the length of the sleeve whereby the sleeve wall includes a hollow sleeve wall interior.

The hollow sleeve wall interior optionally may contain a non-conducting filling fluid; and at least the outer longitudinal wall is flexible thereby permitting the dimensions of the sleeve to increase on filling of the hollow sleeve wall interior with fluid.

Thus the sleeve may be made to be selectively inflatable. This provides a benefit in that when the sleeve is secured to or otherwise used in conjunction with a slim logging tool such as those described above, during deployment the sleeve may be in a deflated state.

As a consequence the external dimensions of the logging tool including the sleeve are to all practical purposes no greater than those of the slim logging tool. In turn this means that the advantages of a slim logging tool in terms of its ability to undergo deployment in e.g. deviated, squeezed or caved-in boreholes (these concepts being well known in the borehole art) are retained during deployment.

Once the logging tool including the sleeve has reached the borehole depth at which logging is to commence the sleeve can be inflated using a non-conducting filling fluid in order to provide the benefits of the invention in terms of displacement of the conducting, surrounding borehole fluid and consequent improvements in the logs generated by the logging tool. Optionally the inner longitudinal wall is integral with or secured to the exterior of the logging tool body.

In cases of the sleeve being secured to the logging tool body this may be achieved, within the scope of the invention, by way of one or more breakable fixings. As a result if the logging tool bearing the sleeve becomes stuck in a downhole location the logging tool can be separated from the sleeve, withdrawn from its hollow interior and recovered.

The foregoing option however does not rule out the alternative possibility of the sleeve not being fixed, or being fixed only by friction between the sleeve and the logging tool, should this prove desirable.

The invention also is considered to reside in use of a sleeve according to the invention as defined herein to encircle an induction logging tool or logging tool section in a borehole in a manner excluding borehole fluid from the space occupied by the sleeve.

The advantages of the logging tool or logging tool section of embodiments described herein over the arrangement of US 2015/0136387 A1 are also relevant mutatis mutandis to the sleeve disclosed herein.

There now follows a description of preferred embodiments of the invention, by way of non- limiting example, with reference being made to the accompanying drawings in which:

Figure 1 is a schematic representation showing the basic operational principles of an induction logging tool;

Figure 2 shows one arrangement of transmitter, secondary and receiver coils of an array induction logging tool;

Figure 3 is a transverse cross-sectional view of a logging tool including secured thereon a sleeve in accordance with embodiments of the invention;

Figure 4 is a transverse cross-sectional view of one embodiment of a sleeve according to the invention; and

Figure 5 is a transverse cross-sectional view of a logging tool having secured thereon another embodiment of sleeve according to the invention.

Referring to Figure 3 a logging tool or logging tool section 10 is represented schematically as including elongate cylindrical logging tool body 10’. Logging tool body 10’ may contain and/or support the components of an induction logging tool or logging tool section that one of skill in the art would expect to be present. Such components, which include e.g. transmitter, receiver and secondary coil features non-limitingly exemplified by Figures 1 and 2, are omitted from the drawings for ease of viewing.

The outer surface 14 of the logging tool body is of constant outer diameter along the length of logging tool body 10’, and to almost all practical purposes is the same as the exterior of a conventional induction logging tool or logging tool section. In the illustrated embodiment the logging tool body 10’ exhibits slim or compact dimensions, especially its diameter; but in alternative embodiments larger diameter logging tool bodies 10’ are possible.

An electrically non-conducting, rigid, elongate, hollow, essentially cylindrical sleeve 16 encircles and is secured on the outer surface 14 so as to encircle the logging tool body 10’ over a major part of its length. The sleeve 16 is a cylinder of constant cross-sectional dimensions over the major part of its own length between two tapered end sections 17, 18 as illustrated.

The sleeve 16 is formed with a constant diameter, circular internal bore 19 extending along its length and terminating in openings 21 , 22 at the ends of the sleeve 16. The bore is a sliding fit on the outer surface 14 whereby on assembly of the logging tool or section the sleeve may be slid onto the outer surface 14 and moved along it until it is in a correct position for fixing.

In view of the foregoing features and the end tapers 17, 18 the sleeve 16 defines a sleeve wall 23. In the embodiment of the invention illustrated in Figure 3 the thickness of the sleeve wall 23 is constant between the tapered ends 17, 18. This provides for a plain cylindrical outer surface of the sleeve 16 in the Figure 3 embodiment.

In other embodiments the sleeve 16 may have a non-constant wall thickness between the tapered ends. This may provide for particular effects such as variations in the shape of the sleeve along its length, as may be desired in order to achieve specific performance effects.

Such effects may arise in respect of the displacement of fluid as described herein when the logging tool supporting the sleeve is in a borehole; the ability of the logging tool 10 and sleeve 16 to pass along a fluid-filled borehole; or features that facilitate the handling of the sleeve when it is not in a borehole. The sleeve 16 is keyed to the logging tool body 10’ in the exemplary embodiment by way of one or more transverse bores 24 extending radially through the sleeve wall 23 at the tapered ends. The transverse bores on assembly of the logging tool 10 and sleeve 16 may be caused to align with blind bores 26, of which one example is visible in Figure 3, formed in the outer surface 14 of the logging tool body 10’.

In the Figure 3 embodiment the transverse bores 24 extend radially at equiangularly spaced intervals about the transverse cross-section of the taper. As an example, six such bores, or any other practical number, may be provided. A single transverse bore 24 is visible in Figure 3 for ease of viewing.

A fastener 27 is received in each transverse bore 24 and extends into the adjacent, aligned blind bore 26 to secure the sleeve 16 and logging tool body 10’ against longitudinal relative movement.

In preferred embodiments each fastener 27 is a shear pin that shears on its transverse bending loading exceeding a threshold. The fasteners 27 may thus secure the logging tool body 10’ and the sleeve 16 one to the other unless the sleeve 16 becomes stuck in a borehole during movement of the logging tool 10 therein.

At such a time further attempts at moving the logging tool (e.g. by withdrawing wireline attached to its in-use uphole end) cause shearing of the shear pins with the result that the logging tool body 10’ and the sleeve 16 are no longer secured together. As a result the logging tool 10, which is part of a logging string that may contain radioactive sources, may be slid out of the sleeve 16 and recovered. The sleeve 16, which is relatively cheap to manufacture, may be left in situ in the borehole; or if desired equipment may be conveyed to it to remove it so that it does not impede borehole access.

Other forms of fastener for securing the sleeve 16 and logging tool body 10’ together are possible within the scope of the invention.

A respective o-ring 28 or other seal is provided at either end of the sleeve 16 in a respective seal recess 29 formed extending radially outwardly in the inner wall of the bore 19. The o-ring seals 28 prevent the ingress of fluid or other matter between the sleeve 16 and the outer surface 14 of logging tool body 10’. Figure 4 shows the sleeve 16 in the absence of the logging tool 10. The invention resides in the sleeve 16 when considered on its own, in addition to when it is in combination with a logging tool or similar cylindrical component.

The sleeve of Figures 3 and 4 is in one embodiment made from a non-electrically conducting solid, rigid, polymer-resin composite such as fibre-reinforced plastic. One non- limiting example of such a material is Kemlox HT-2 produced by Keystone Engineering Company of Houston, TX, USA. More generally, glassfibre is a preferred material for solid wall versions of the sleeve 16.

In use the logging tool 10 and sleeve 16 are assembled essentially as shown in Figure 3, although in practice it is likely that more than one fastener 27 would be employed. The assembly then would be conveyed to a chosen depth in a fluid-filled borehole in a condition permitting operation of the logging tool 10 to acquire induction log data.

The outer diameter of the sleeve 16 is less than the diameter of the borehole. As noted one known borehole diameter is approximately 8” (about 200 mm), although in practice boreholes are almost never of uniform dimensions from place to place along their lengths, and indeed considerable variations in the sizes and shapes of nominal 8” boreholes arise.

One preferred, non-limiting outer diameter of the sleeve 16 is 90 mm. This is sufficient to exclude borehole fluids from the vicinity of the logging tool 10 in accordance with the aim explained herein while permitting the logging tool 10, supporting sleeve 16, to move along the borehole in uphole and downhole directions as desired.

Other diameters of sleeve 16 are possible within the scope of the invention, the upper limit of the diameter essentially relating to the dimensions of the borehole in which the apparatus is to be deployed.

The logging tool 10 then would be activated to log the borehole, being moved typically in an uphole direction as this occurs.

During such logging activity the space taken up by the sleeve 16 encircling the logging tool body 10’, that otherwise would be occupied by borehole fluid, is free of such fluid. If the fluid and/or the conductivity of the rock surrounding the borehole are such that conduction in the fluid would in the absence of the sleeve 16 adversely dominate the signals generated at the receiver coils of the logging tool, the detrimental effect of the fluid is minimised or eliminated. As a result any correction of the receiver coil outputs applied to compensate for the surrounding borehole fluid effects described herein is much more likely to be effective than in prior art arrangements, or in some situations would become very small with respect to the formation log signal.

As explained should the logging tool 10 having the sleeve 16 secured thereto become stuck in the borehole e.g. pulling on wireline connected to the logging tool body 10’ would cause shearing of the fastener(s) 27. As a result recovery of the logging tool 10 would remain possible.

In an alternative embodiment, shown in Figure 5, the sleeve 16 may be formed with a hollow sleeve wall 23’. This is defined principally by inner and outer longitudinal walls 31 , 32 extending along the length of the sleeve to define an elongate annular shape of the hollow sleeve wall 23’ as illustrated.

The ends of the sleeve 16 of Figure 5 are formed as respective, tapered, rigid end caps 33, 34 that are secured to the otherwise open, respective ends of the hollow sleeve wall 23’. As a result the hollow interior of the sleeve wall 23’ is enclosed.

A non-conducting filling fluid 36, such as an oil, is filled inside the hollow sleeve wall 23’ and provides in essence a similar function to the material of the wall 23 of the rigid version of the sleeve 16 of Figures 3 and 4. However at least the radially outer longitudinal wall 32, and in practice both the longitudinal walls 31 , 32, are flexible being made from a flexible, tough material such as certain grades of polymer. In some embodiments the walls 31 , 32 or at least one of them additionally is elastic.

End cap 34 includes formed in its outer surface a normally closed, openable filling port 37 communicating via a fluid passage 38 with the hollow interior of the sleeve wall 23’. This permits filling of non-conducting filling fluid into the interior of the sleeve wall 23’. Such filling may take place under pressure in order to overcome any elasticity of the material of at least outer longitudinal wall 32.

Numerous variants on the filling arrangement illustrated are possible and are within the scope of the invention.

The embodiment of Figure 5 functions similarly to that of Figures 3 and 4, but offers some additional advantages. Among these is an ability to adjust the cross-section of the sleeve 16 in proportion to the amount of filling fluid 36 filled into the hollow interior of sleeve wall 23’. This may be desirable when particular borehole dimensions are encountered.

Also it is possible to select the filling fluid 36 in accordance with specific performance requirements as may arise in certain boreholes.

As explained herein the Figure 5 embodiment offers a particular deployment advantage when used in conjunction with a slim logging tool 10. One of the significant benefits of such a tool is that it can negotiate deviated, squeezed or caved-in boreholes better than a larger diameter tool. By employing an inflatable sleeve 16 it is possible to retain this advantage during deployment, since it is possible to effect deployment with the sleeve 16 in a deflated condition. At such a time the sleeve adds little additional diameter to the exterior of the logging tool, such that the ability of the tool 10 to access difficult-to-reach borehole locations is not compromised.

Once the logging tool 10 including the sleeve 16 reaches a location at which logging is to commence the sleeve 16 can be inflated so that the non-conducting, increased diameter benefits of the sleeve become available.

Filling of the sleeve 16 may be effected by e.g. a downhole oil pumping technique of a kind that is familiar to those of skill in downhole tool conveyance subjects.

It also is possible at the end of a logging operation to deflate the sleeve 16 by allowing the oil to drain from the hollow wall (or by pumping the filling oil out of the hollow wall) thereby returning the logging tool to its slim diameter and aiding its recovery in an uphole direction via any sections of the borehole that are hard to negotiate. Several arrangements for causing draining of the oil are possible. As a non-limiting example one may consider the use of a remotely activatable shear component that on activation opens a drain port.

A further benefit of the ability to inflate and deflate the sleeve 16 is that its diameter can be adjusted as desired to suit particular conditions encountered in downhole environments.

It is desirable that each of the end caps 33, 34 is secured to the outer surface 14 by way of a transverse aperture 24, blind aperture 26 and (shear) fastener 27 combination in order to prevent sliding of one or other of the end caps that may result from the non-rigid interconnection between them defined by the sleeve 23’. In the event of shearing of one or both of the shear fasteners 27 occurring following sticking of the sleeve inside a borehole the filling fluid is likely to leak from inside the sleeve wall 23’ with the result that the sleeve parts remaining in the borehole following recovery of the logging tool 10 collapse to a relatively small form. This is unlikely to impede borehole access and operation; and any parts of the sleeve left in situ as a result may readily be cleared from the borehole.

The end caps 33, 34 are tapered at their longitudinal ends but also include constant diameter cylindrical portions that sealingly mate with the otherwise open ends of the sleeve wall 23’. The constant diameter portion of end cap 34 accommodates the filling port 37 and fluid passage 38. The end caps may be made from a rigid material and desirably are machined from a non-conductive material such as polyether ether ketone (PEEK).

The invention provides several advantages over the prior art, chief among these being the elimination of a need for extreme compensation (which often is impossible to provide) for highly conductive borehole fluid when induction logging is undertaken. The benefit of the invention is particularly noticeable when a slim or compact logging tool body is employed, and/or the conductivity of the rock to be logged is low.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention.