GB2146127A - Protective sheath in induction logging tool - Google Patents

Abstract

A non-conductive composite sheath protects an inductive logging device contained therein and is designed to be attached to the bottom end of a drill string to allow the logging device to be run to the bottom in highly deviated or partially obstructed wellbores therefore allowing logging of desired wellbore intervals. The sheath comprises a composite, electrically non-conductive wall (10) which houses the logging device. Circumferential electrodes (16, 18, 20) extend around the wall and are electrically connected to the logging device, for example by means of screws (24), for measuring the shallow resistivity of the surrounding earth formation. Metallic joints (12, 14) are fixed at the ends of the sheath for connection to the drill string. <IMAGE>

Description

SPECIFICATION Protective sheath in induction logging tool The invention relates to method and apparatus suitable for logging both vertical and deviated boreholes and evaluating the subterranean formations found therein. The apparatus can take the form of a composite material insulative tool sheath, an induction logging tool including such a sheath, or a tool suite using the sheathed logging tool as attached to the end of a drill string.

The intentional drilling of directional boreholes began in South Africa nearly eighty years ago. A description of this process, by John Hoffman, is found in Bulletin 91 of the American Institute of Mining Engineers (April 11, 1912). The method entailed sequentially drilling two straight but intersecting boreholes.

The technique of intentionally deviating the borehole of an oil well was first widely used in the 1 920's or 1 930's in the Huntington Beach field. Prior to invention of that technique, offshore wells were drilled from piers built out a distance from the shoreline. The number of piers began to impede ship traffic in some Pacific harbours and eventually led to the outlawing of such piers along some portions of the North American West Coast.

One driller's solution to the problem of outlawed piers entailed placing a rig on the shore and deviating the borehole to reach a producing formation offshore. Although the technique worked very well, the driller's failure first to gain permission of either the state or the harbor authorities caused the technique to be disfavored.

However, the legitimate use of controlled directional drilling in shoreline drilling is not the only instance in which the process is useful.

Controlled directional drilling can be used to drill a borehole anywhere a surface obstruction prevents placement of the well site over the point where a well is to be bottomed out.

The obstruction could be a hill, marsh, swamp, river, or freeway.

A deviated well may also be used to control another well which is burning or blowing formation fluids out of control. The deviated well is drilled to intersect near the borehole of the offending well. High pressure mud is pumped through the deviated borehole into the other borehole to control the formation fluids being lost to the fire or blowout.

Controlled deviated drilling techniques may be used in the optimization of reservoir pressure. For instance, if an initial well bottoms out in the upper end or gas cap of a producing formation, it may be wise to plug a lower portion of the well and deviate the borehole from the plugged point into a lower portion of the formation to recover liquid petroleum. Gas is often the major driving force behind the petroleum liquids produced from the same formation. Production of the gas would lower its driving force on the petroleum and ultimately lower the ov rall recovery from the formation.

Probably the most common instances of the use of controlled directional drilling is found in the ubiquitous offshore platform. It is common to drill dozens of wells from a single platform. The expense of building an offshore platform for each well should be apparent to even the casual observer. In any event, the borehole for each well drilled from a platform typically follows a near vertical path to a specified depth into the sea bed and then quickly veers away from its neighboring wells.

Controlled directional drilling is not the only reason for the existence of deviated wellbores.

The process of drilling obliquely from a soft geological layer into a relatively harder subterranean strata will cause the drill bit to swerve from a vertical path. Similarly, insufficient drill collar weight on the lower end of the drill string will cause the bit to wander from the vertical during drilling. Neither situation is a desirable one and much care is taken by the driller to avoid their occurrence.

But whatever the reason for the existence of a deviated wellbore, the step of logging the wellbore once it has been drilled presents special challenges.

Logging a well is done to obtain a wide diversity of information using equally diverse types of instrumentation. In the normal course of events, the well is logged after drilling.

Many wells require a number of logging runs to evaluate various well bore intervals. Some locales, e.g., Norway, require by law that the entire length of the wellbore be logged. Drilling rigs are often rented on a daily basis and consequently anything subtracting from time available to drill is to be avoided. It occasionally may be necessary to log the well before the planned total depth ("TD") is attained to make sure that, e.g., a desired formation is penetrated. The surrounding wellbore forma-' tions are scanned to provide information concerning porosity, density, lithology, and characteristics of the formation fluids. Physical parameters of the borehole, such as its diameter, are measured so that subsequent casing and cementing steps may be efficiently completed.

There are but two methods typically used for the physical step of placing a logging tool in the wellbore and then withdrawing it. The first method is practiced with the drill string out of the hole. A downhole tool or sonde, often weighing several hundred pounds, is lowered in the open borehole upon a logging cable hanging from a pulley on the surface. If the borehole is vertical or nearly so, then gravity may take the logging sonde to the bottom of the hole. However, if the borehole has a dogleg or is otherwise deviated, then relying on gravity to carry the sonde to bottom is a questionable proposition. Even if the borehole is deviated, tsse overall logging costs may be minimized by first attempting this method of inserting the sonde and, if unsuccessful, proceeding to another method.

An improvement to the basic gravity impelled sonde is found in U.S. Patent 4,031,750, Toyoumans et al. This logging instrument utilizes a linear electric motor attached to a set of vanes extending out from the instrument body. The sonde is dropped in the open borehole until it reaches a point where it no longer moves down. The electric motor is then actuated and the vanes reciprocate on the outside of the body and "rows" the device down the borehole. This apparatus apparently is not in wide use.

One logging method used after failure of the free-fall sonde method uses a drill string completely made up of drill pipe, i.e., having no drill collars or drill bit at its lower end. The open-ended drill string is generally run into the hole to a point below the region to be logged. A special logging sonde, having a very narrow diameter, is then attached to a logging cable and pumped down the drill string and out into the open hole below the drill string's lower end. The logging cable is often connected to a recording instrument at the surface. The drill string is raised about 90 feet. The well is then logged by pulling the sonde up and recorçling the data it gathers.

Once the sonde reaches the lower end of the drill pipe, data is no longer recorded and the sonde is pulled all the way to the surface through the drill string. A stand (approximately 90') of pipe is removed from the string. The sonde is again inserted into the drill string and pumped out its lower end. This places the sonde at the print at which logging was terminated in the prior pass. The drill string is again pulled up about the length of a stand of pipe and the sonde subsequently follows it up logging the then-vacated ninety feet. The sonde, once again, is pulled to the surface and another stand of drill pipe removed from the string. This process is repeated ninety feet at a time until a sufficient amount of the wellbore is logged. Obviously this process is slow and laborious. Only ninety feel of the well is logged with each pass.

A variation of this process, used when the welbore has a nonvertical section which prevents insertion of a sonde to a desired interval but has a lower vertical leg, entails running the drill pipe down through the wellbore deviation and into the vertical region above the interval to be logged. The sonde is pumped out through the drill pipe and allowed to fall by gravity through the interval to be logged.

Logging can then be carried out using the operation described above.

One suggested method for increasing the length of wellbore logged with each pass of the sonde is found in U.S. Patent No.

4,062,551, to Base. This process uses a short sub placed in drill string which allows the logging cable to pass through the drill string wall at some midpoint within the well.

The well may be logged for a distance equal to the length of pipe between the pass through sub and the surface before the sonde is pulled from the drill string. Some portion of the remaining drill string must then be pulled to re-install the pass through sub in the drill string.

Similar suggestions for pass-through subs are found in U.S. Patent No. 4,200,197 to Tricon, issued April 29, 1980, and U.K. Patent Application GB 2,094,865A, to Institut Francais de Petrole ("IFP"), published September 22, 1982. Both suggest using a drill string as the impetus for getting a sonde to the bottom of a borehole for logging purposes. Tricon, however, uses the drill stem to drive a drilling head; the drill stem does not rotate. The included downhole tool does not have an insulative sheath. IFP, on the other hand, uses a drill string as an upper part of a suite of logging tools to place the logging tools in a highly deviated portion of a borehole. IFP does not suggest use of an integral, insulating, composite sleeve on any of the tools present in the disclosed suite of tools.

According to the invention from one aspect there is provided a protective sheath suitable for use as part of an induction logging tool comprising: a generally cylindrical central portion having a substantially electrically non-conductive wall and at least one electrode extending, in the circumferential direction, at least partially around said central portion and in electrical communication with the interior of said wall, and metallic joints fixedly attached to the ends of said central portion and adapted to be connected in a drill pipe.

Each electrode need only extend, in the circumferential direction, partly around the cylindrical central portion but preferably and conveniently, it will extend completely around the cylindrical central portion, i.e. the electrode is annular in shape.

According to the invention from another aspect there is provided a method of logging at least a desired portion of a borehole comprising the steps of: attaching to a lower section of drill pipe at least a protective sheath comprising a generally cylindrical central portion having a substantially electrically non-conductive wall and at least one electrode extending, in the circumferential direction, at least partially around said central portion, a fixedly held inductive logging sonde within said central portion and electrically connected with the circumferentially extending electrodes through the substan tially electrically non-conductive wall of said cylindrical central portion of the sheath, and metallic joints fixedly attached to said central portion and adapted to be connected in said drill pipe, connecting said lower section of drill pipe to additional sections of drill pipe while running said resulting partially assembled drill string into said borehole.

installing a crossover sub on said partially assembled drill string, passing a logging cable through said cros sover sub into the interior of the partially assembled drill string and attaching said logg ing cable to the fixedly held logging sonde, attaching additional sections of drill pipe to the partially assembled drill string until the fixedly held inductive logging sonde passes the desired portion of the borehole, and activating at least the fixedly held logging sonde while removing the drill string from the borehole.

One special application is in logging deviated boreholes or boreholes having bridges or other problems effectively precluding the use of traditional wireline logging apparatus. The invention also may be used in vertical boreholes. In one way of putting the invention into effect an insulative dielectric sheath made of a composite material forms an integral outer housing for an induction logging tool. The tool is of such strength that it may be used as a portion of a logging tool suite which may be installed on the end of a drill string and run to the bottom of highly deviated wellbores.

The sheath is generally cylindrical or tubular with metallic "boxes" (female threaded "receptacles") at each end which allow the sheath with its integral electronics to be included in a tool suite located on a drill string assembly. The body of the sheath desirably is made-up of a resin (preferably epoxy-based) impregnated fiberglass and has a number of discrete conductive bands located on the outside of the sheath to measure the local resistivity of the surrounding earth formations. The fiberglass or other filamentary support material may be either wound from single or multiple sources or may be made up of matte or woven material.

The sheath has high tensile, compressive, and bending strenghts. It has a passageway through its center facilitating the placement of certain logging tools and allowing drilling mud circulation; if such is desired.

The included induction logging device is fairly conventional and preferably of a type that would be used in logging a borehole on a wireline.

The sheathed logging tool is suitable for inclusion with other logging tools, e.g., spontaneous potential (SP) log, density log, neutron log, gamma ray log, sonic log, etc., all situated in a tool suite at the bottom end of a drill string. These drill strings typically have neither drill collars nor drill bits.

Use of the tool suite containing the sheathed tool typically would require a logging cable extending from the tool suite, up the interior of the drill string to a region within a cased portion of the well, out of the drill string through a pass through sub and from there to the surface.

The apparatus is assembled prior to use by installing the tool string at the bottom end of a drill string. The tool string is run into the wellbore to a depth directly above the interval of the open hole to be logged. A side entry or pass-through sub is added to the drill string at this point. This assures that the sub will not enter the open hole even when the deepest logging depth is attained. Once the side entry sub is in place, the wireline cable is pumped down inside the drill string and latched to the tool string electrical connector. The tool string is run in to a point below the interval of the wellbore to be logged.

After the drill string is run to the bottom of the hole, the logging tools are activated. The drill string is then raised the length of a stand, or triplet, of drill pipe (usually about 90') while the hole is being logged. The stand of pipe is removed and placed in the rack.

These steps continue until the pass-through sub reaches a point near the surface. Logging is interrupted-permanently if an appropriate portion of the hole has been logged or temporarily if an additional length is to be logged.

If additional borehole is to be logged, a length of drill pipe equal to the length of the casing is removed. The pass through sub is reinstalled, the logging cable is inserted and the drill string run back in the hole to the point where logging had been temporarily interrupted. Logging may then be continued.

Logging may alternatively take place in the opposite direction with appropriate modifications to the assembly process.

It is possible for significant distances within the borehole to be logged while foregoing the necessity of pulling the logging tools each time a stand of pipe is removed.

The invention will be better understood from the following description, given by way of example and with reference to the accompanying drawings, in which: Figure 1 schematicaly depicts, in axial cross-section, one form of composite insulative tool sheath in accordance with the present invention.

Figure 2 shows an assembled tool suite using the tool sheath. The tool string could mount on the bottom end of a drill string.

Figure 3 schematically depicts the step of logging a deviated borehole using the drilling and logging apparatus.

Fig. 1 shows, in a schematic partial cutaway, the construction of the insulative dielectric tool sheath. The tool sheath is simple in its makeup. It is made up of the composite housing 10, an upper box 12, a lower box 14, and a number of detector electrodes (electrodes 16, 18 and 20 are shown) which extend circumferentially around the tool sheath (ire. they are annular in shape). Upper box 1 2 and lower box 14 may be held in Rlace by pins 22. The electrodes have electrical pass-through devices of some type such as screws 24. Screws 24 allow electrical continuity between the electrodes and the shallow resistive measuring portion of the inductive logging device or sonde (not shown) placed within composite housing 10. The screws may abe used to help secure and centralise the sonde within the housing.The inductive logging sonde may have exterior electrodes, each electrically connected, by way of the screws 24, to a corresponding circumferential electrode 16, 18, 20.

Composite housing 10 may be made in a number of ways. The preferred method entails laying up a fiberglass matte or woven cloth on a mandrel having the approximate shape of the inside of composite housing 10, impregnating that matter with an epoxy based resin, and allowing the matte or weave to cure into a hardened solid. Precoating the mandrel with a mold release material is usually necessary.

Alternatively, the mandrel may be inserted in a lathe and single or multiple fibres spun onto the form. Longitudinal strength may be given by giving the fibres 15 or more of pitch during winding or by using interspersed longitudinal fibres. Once the rough composite housing is separated from the mandrel then both the outside surface and inside bore of the housing may be machined if necessary Circumferential slots suitable for accepting deo tectors 16, 1 8 and 20 can then be cut.

Electrodes 1 6 and 20 are typically about 1 6 inches in length. Electrode 18 may be as small as 1.5 inches in length. The gaps between electrode 18 and both of electrodes 16 and 20 may be 0.5 inches. The ends of housing 10 are then machined and threaded to take the male ends of boxes 1 2 and 14.

Boxes 12 and 14 are then desirably bonded and pinned in place using pins 22. Electrodes 16, 1 8 and 20 are then set in place and attached to the appropriate screws 24. Care must be taken to assure accessibility to screws 24 for later assembly.

Composite housing 10 may be an epoxy fiberglass laminate such as RANDOLITE made by the Randolpfr Company of Houston, Texas, or KEMLOX "G" made by the Keystone Engineering Company also of Houston. Another fiber which may be suitable for this service is KEVLAR (RTM). The resulting composite should be resistant to heat in excess of 400 F and possess sufficient tensile, compressive, and flexural strength, during the disclosed usage, to maintain its structural integrity. The dielectric constant preferably should be in the neighborhood of 3-5.5 at all frequencies between 50 hertz and 1 megahertz.

The upper and lower boxes 10 and 1 2 are preferably made of known materials suitable for use in drill strings. Various steels are wellknown and aluminum alloys are gaining wide acceptance.

The details of the induction logging sonde included within the sheath have been omitted from the drawing and this disclosure for the purpose of keeping the disclosure simple.

However, the electronics necessary for a multicoil induction logging sonde such as disclosed here, are well-known. For instance, an article in December, 1 962 issue of GEOPHY SICS entitled "Basic Theory of Induction Logging and Application to Study of Two Coil Sondes" by Moran and Kunz, discusses the theory and operation in some detail. Similarly, Volume 1 Principals of "Log Interpretation" by Schlumberger also discusses the theory of operation. Several suitable induction logging sondes are available from Gearhart or Schlumberger.

Fig. 2 shows a tool suite or tool string suitable for installing on the bottom of a drill string. The inductive logging device is shown at 26.

Beginning at the top of the tool string is a female drill pipe connection 28. Just below that may be placed a gamma ray sleeve 30.

A gamma ray log or sleeve measures the natural radio-activity of a formation. Carbonates which are shale free and sandstones are low in radioactive material and, consequently, give low gamma ray readings. The gamma ray response increases as the amount of shale in the formation increases due to the concentration of radioactive material therein. Clean sandstones can, however, give high gamma ray readings whenever micas or potassium feldspars are present. Shortly below the gamma ray sleeve is a set of circulation holes 32. Many of the prior art devices used for enclosing pump down sondes were not capable of circulating drilling mud during the time the drill stem is being inserted into the wellbore.

A stabilizer is found below the circulation holes in Fig. 2. This stabilizer as well as the other shown in the drawing are there for the express purpose of keeping the bodies of the various logging devices from rubbing against the borehole wall and afford some degree of centralization. Fluted stabilizers are shown but well-known spring stabilizers and so-called "rubbers" are also suitable in this service.

Next down the string is the opening for a compensated neutron pad 36. This tool continuously emits high energy neutrons from a radioactive source. These neutrons collide with the nuclei of the formation material. Each collision causes the neutron to lose some of its energy and the amount of energy lost per collision depends on the relative mass of nuclei the neutron hits. Maximum energy loss occurs when the neutron hits a hydrogen nucleus. The greatest amount of energy loss of the neutron is, therefore, due to the hydrogen concentration in the formation. Since hydrogen is a major portion of both petroleum and water, the neutron log measures the liquid filled porosity.

Below the next stabilizer 34 is a compensated density pad opening 38. This device consists of medium energy gamma ray source which emits gamma rays into the formation.

These gamma rays collide with the electrons in the formation and, at each collision, a gamma ray particle loses some of its energy.

This interaction is referred to as Compton Scattering. The scattered gamma rays which reach the detector, located at a fixed distance, are counted as an indicator of formation density. The number of Compton Scattering collisions is related directly to the number of electrons in the formation, and the electron density is in turn related to the true bulk density of the formation. Because the bulk density of the formation depends upon matrix density, porosity, and fluid density filling of the pores, both fluid density and matrix density must be known in order to calculate porosity. Consequently, the density log must be run with a gamma ray log.

Below the next stabilizer 34 lies the inventive induction device 26. In induction logging, a high frequency alternating current of constant intensity is sent through a transmitting coil. The alternating magnetic field which is created induces secondary currents in the formation. These currents flow as ground loop currents, creating magnetic fields that induce signals in the receiver coils located within the tool. The sheath of sheathed inductive logging device 26 is adapted to accept and hold such an inductive logging sonde. Preferably, the inductive logging sonde held within the sheath will be selected from those currently available induction logging sondes which measure the resistivity of the formation at shallow, medium, and deep distances from the sonde itself.In such a preferred embodiment, the external electrodes 16, 18 and 20 are electrically connected to the shallow resistivity measuring portion of the logging sonde via electrical pass-through devices such as screws 24 (shown in Fig. 1). Since the mud used in drilling the well may have been infused into the formation near the hole, comparison of the three resistances can give some indication concerning the makeup and content of specific subsurface strata which is beyond the drilling fluid invasion. For instance, small differences between deep and shallow resistivity may mean a hard and nonporous rock.

Fresh water and hydrocarbons both have high resistivities; salt water has a low resistivity.

Finally below stabiliser 34 and at the bottom end of the tool string is a bull plug 40 with an off-centre circulating port 42. Drilling mud may pass through this suite of tools all the way from upper box 28 through circulating hole 42. Bull plug 40 may be eccentric, that is to say that the- point need not be in the centre of the shaft so that, during insertion, rotating the string may allow it to bypass bridges and other obstacles in the borehole.

The tool string shown is strong, slightly more flexible than 4 1/2 inch drill pipe, and other than the pads, has no moving parts or movable sheaths. Consequently, it is fairly simple to operate and aprovides a significant amount of quality data concerning the subsurface strata through which it passes.

Fig. 3 shows the inventive tool as installed in the tool string 44 which was discussed above with regard to Fig. 2. Tool string 44 is installed at the bottom end of a string of drill pipe sections 46; these drill pipe sections are typically about 30 feet in length. The drill string is shown in a borehole 48 which is deviated nearly 70 from vertical. Since the inventive technique would normally be used to log a hole during the time drilling is proceeding or shortly after the well has been drilled to TD, a drilling rig 50 is shown at the surface 52. Borehole 48 has a string of casing 54 installed down to casing shoe 56. The remainder of the borehole below casing shoe 56 is open hole. A pass-through sub 58 is shown in the drill string located at the bottom of casing 54 near casing shoe 56.The passthrough or side entry sub 58 allows a logging cable to extend from tool string 44 by connector 60 up through the interior of various drill pipe sections 46 out side entry sub 58 into the annular area between casing 54 and the drill pipe sections 46. Logging cable 62 then continues up out of the borehole and over a pair of sheaves 64 and 66 into a winch 68.

For the purposes of clarity, the drill string handling equipment required for movement of the drill string and the surface safety equipment required both by prudence and law have not been depicted in drilling derrick 50. However, these devices are so well-known that no additional disclosure is considered necessary.

Pass-through or side entry sub 58 is shown at its lowest point. Although, for the purposes of the disclosed method, a number of different crossover subs would be acceptable, the preferable sub is shown in U.S. Serial No.

468,532, filed February 22, 1983, by A. P.

Davis, O. M. Knight, and J. W. Stoltz. In order to protect the wire from the rigors of the open borehole, logging cable 62 normally would not be allowed to venture into the open hole below casing shoe 56. Consequently, the drill string shown in Fig. 3 is ready to log a portion of the hole upward from its depicted position to a point up the hole which is equal in distance as is the pass-through sub 58 from surface 52. The drill string is merely tripped out of the hole, logging is paused every 90 feet (a triplet of drill pipe 46), the triplet is removed from the string, logging is recommenced and continues until time comes for removal of another triplet of drill pipe. This process continues until side entry sub 58 reaches the surface. At that time, side entry sub 58 is removed from the string and the logging cable withdrawn.If additional wellbore remains to be logged, an amount of drill pipe equal in length to the distance between casing shoe 56 and ground surface 52 is removed from the drill string and the side entry sub reinstalled in the string. The logging cable 62 is then pumped down the interior of the drill string through the side entry sub and latched with tool string connector 60. The tool string 44 is then run in to a position to log previously unlogged portion of the borehole 48. This procedure is repeated until the entire zone of interest is logged.

This technique has a number of significant advantages. The tool string is made up of logging tools that are potentially of a very high resolution in that they need not be miniaturized to be pumped down within the interior of a drill string. The fact that a drill string is used to insert the tool string to the bottom allows very accurate depth correlation.

The process should save drilling rig time in that the tools are positively placed by pushing, if necessary, rather than being passively inserted as is the case with wireline logging apparatus. The borehole need not be conditioned prior to running a sonde as is often the case with wireline logging apparatus. Wireline devices are susceptible to a number of problems in open boreholes, particularly those which are high angle. These problems can be summarized as washouts in which the tools fall and become lodged, doglegs, bridges, and ledges into which the tools nose during their downward path and lose momentum, mud balls, cutting buildups, or heavy muds, all of which impede the downward motion of the logging tool towards the bottom, and key seats found in the upper edge of the boreholes causing the logging tool to hang up upon removal.

Claims (20)

1. A protective sheath suitable for use as part of an induction logging tool comprising: a generally cylindrical central portion having a substantially electrically non-conductive wall and at least one electrode extending, in the circumferential direction, at least partially around said central portion and in electrical communication with the interior of said wall, and metallic joints fixedly attached to the ends of said central portion and adapted to be connected in a drill pipe.

2. A protective sheath according to claim 1, wherein the interior of said sheath is adapted to accept and fixedly hold an inductive logging sonde therein, said logging sonde being capable of logging as well on a wireline.

3. A protective sheath according to claim 2 wherein said induction log has exterior electrodes, the cylindrical central portion has three electrodes placed circumferentially thereabout and adapted to be used in measuring the formation resistivity near the borehole, and each of the electrodes of said central portion is in electrical communication with a corresponding electrode on said fixedly held logging sonde.

4. A protective sheath according to claim 3 or 4, wherein said fixedly held logging sonde is centralised by screws through the electrodes extending circumferentially around said sheath, said screws being also adapted to provide said electrical communication to the exterior electrodes of the fixedly held logging sonde.

5. A protective sheath according to any preceding claim, wherein said substantially electrically non-conductive wall comprises epoxy and fibreglass.

6. A protective sheath according to claim 5, wherein said fibreglass is in the form of a woven fabric.

7. A protective sheath according to claim 5, wherein said fibreglass is in the form of a matte.

8. A protective sheath according to claim 5, wherein said fibreglass comprises a filament wound circumferentially throughout the wall of said cylindrical central portion.

9. A protective sheath according to any preceding claim, wherein said metallic joints are both female.

10. A protective sheath according to any preceding claim, wherein said metallic joints are joined to said cylindrical central portion by adhesive bonds and pins.

11. A protective sheath according to claim 3 or any one of claims 4 to 10 as appended to claim 3 and connected in a drill string made up of a number of sections of detachable drill pipe to which the sheath is removably attached and a logging cable connected to the inductive logging sonde within at least a portion of said drill string.

1 2. A protective sheath according to claim 11 also comprising a crossover sub located between two sections of said detachable drill pipe and adapted to permit the logging cable to exit the interior of the drill string.

13. A method of logging at least a desired portion of a borehole comprising the steps of: attaching to a lower section of drill pipe at least a protective sheath comprising a generally cylindrical central portion having a substantially electrically non-conductive wall and at least one electrode extending, in the circumferential direction, at least partially around said central portion, a fixedly held inductive logging sonde within said central portion and electrically connected with the circumferenti ally extending electrodes through the substantially electrically non-conductive wall of said cylindrical central portion of the sheath, and metallic joints fixedly attached to said central portion and adapted to be connected in said drill pipe, connecting said lower section of drill pipe to additional sections of drill pipe while running said resulting partially assembled drill string into said borehole.

installing a crossover sub on said partially assembled drill string, passing a logging cable through said crossover sub into the interior of the partially assembled drill string and attaching said logging cable to the fixedly held logging sonde, attaching additional sections of drill pipe to the partially assembled drill string until the fixedly held inductive logging sonde passes the desired portion of the borehole, and activating at least the fixedly held logging sonde while removing the drill.string from the borehole.

14. A method according to claim 13, wherein the borehole is deviated from the vertical.

1 5. A method according to claim 13, wherein the cylindrical central portion of said protective sheath is comprised of fibreglass and epoxy.

16. A method according to claim 13, wherein the fibreglass is a woven fabric.

1 7. A method according to claim 15, wherein the fibreglass is a matte.

1 8. A method according to claim 15, wherein said fibreglass comprises a filament wound circumferentially throughout the wall of said cylindrical central portion.

1 9. A protective sheath suitable for use as part of an induction logging tool substantially as hereinbefore described with reference to the accompanying drawings.

20. A method of logging at least a desired portion of a borehole, substantially as hereinbefore described with reference to the accompanying drawings.