WO2001006278A1

WO2001006278A1 - Logging-while-drilling using a directional sonde

Info

Publication number: WO2001006278A1
Application number: PCT/GB2000/002510
Authority: WO
Inventors: Victor Allan; Paul L. Sinclair
Original assignee: Geolink (Uk) Ltd.
Priority date: 1999-07-15
Filing date: 2000-06-30
Publication date: 2001-01-25
Also published as: GB9916576D0

Abstract

A directional instrument for measuring the electrical conductivity of earth formations which may be employed during the drilling of a borehole includes an array of coils set into a pocket in a support element (for example a drill collar) and electronics for processing the measurement into a form suitable for data storage and/or for transmission to surface by known means. The measurement is of the type known as an induction measurement, employing relatively low frequency (10kHz to 200kHz) alternating magnetic fields. The radial response of the measurement can provide information about the radial distribution of formation conductivity, thus facilitating geosteering.

Description

LOGGING-WHILE-DRILLING USING A DIRECTIONAL SONDE

The present invention relates to a directional instrument for insertion into a borehole and for making measurements of electrical properties of rock formations near the borehole whilst drilling. It also relates to a method which is suitable for making these measurements during drilling.

Such measurements are of importance in determining the nature of fluids in the rock formation and have been used since the 1920s in so-called wireline logging, whereby an instrument is inserted into the borehole on a cable after a section of the borehole has been drilled.

From the early 1960s onwards techniques have been developed for making many different kinds of measurement in a borehole contemporaneously with the drilling of the borehole. This technology is known generically as Measurement-While-Drilling, or MWD, by which latter term it will be referred to hereafter.

The earliest commercially successful MWD measurements were of borehole inclination and azimuth, but practical MWD instruments for the measurement of many different rock formation properties were subsequently developed and are now an important component of borehole drilling technology both in exploration and production activities.

Historically the methods used for determining the electrical conductivity of the rock formation in the neighbourhood of a borehole, using instruments lowered into the borehole on a cable have been based principally on one of two distinct methods: either by using electrodes to send electric current into the formation or by iⁿducing such currents using an arrangement of transmitter and receiver coils. It is with the second of these methods, tr.e so called induction-tool technique - that the present invention is concerned. In these instruments a primary alternating magnetic field generated by the transmitter coil induces circulating currents in the formation around the borehole. These currents in turn give rise to a secondary magnetic field which can be detected by the receiver coils . The magnitude and phase of the voltages induced in the receiver coils by the secondary magnetic field are related to the electrical properties, and specifically the electrical conductivity, of the rock formation

Induction tools for measuring formation conductivity have proved to be highly successful for wireline logging: they can provide measurements which extend relatively deeply into the formation in a radial direction and can provide good resolution of boundaries between zones of differing conductivity in the along-hole direction. Such instruments in the wireline logging context are typically operated at alternating frequencies in the range 10kHz-200kHz and are generally constructed in such a way that they provide a symmetrical response m the radial direction. Conventionally the coils employed in the wireline versions of such tools are wound coaxially on a mandrel made from an insulating material and the response of the instrument is thus isolated from the interfering effect of nearby masses of highly conductive metal components . In contrast to instruments used in wireline logging it is a feature of all MWD instrumentation that it has to be carried m the drillstπng, the shaft which carries the drilling b_t into the borehole. The ~echanιcal loads on the drillstnng are extremely high. A typical borehole may be between 4 and 15 inches in diameter and 10,000 feet or more in length: thus it is necessary in most cases to employ high-strength steels or other alloys as the materials for the drillstnng. MWD instrumentation perforce has to operate in very close proximity to large masses of metal.

The current solution to this problem is the use of so-called electromagnetic wave or propagation wave instruments operating at a very much higher frequency, typically 0.4MHz to 2 MHz. At such frequencies the skin-depth in the metal of the drillstnng is very small, allowing electromagnetic isolation from the metallic mass to be achieved relatively easily. Magnetic dipole antennae having only a small number of turns of wire can be used to launch an electromagnetic wave into the formation and to receive the return signals. For a given receiver coil design the gain of the antenna increases with frequency, so receiving antennae having only a small number of turns can provide adequate sensitivity. The depth of investigation of these high frequency tools is however inherently less than that of the lower frequency instruments (other things being equal) because of the smaller skin depth in the rock formations.

US 4 651 101 (Barber) also discloses a partial solution to this problem. This document discloses a low frequency induction MWD instrument employing coils wound in recesses around the perimeter of a drill string or other support element. The drill string/support element defines a long axis which is substantially parallel to the axis of a borehole when the instrument is inserted m the borehole. As a low frequency (10KHz to 200KHZ) is used the coils would usually pick up a considerable signal from the drill string and other metallic elements proximate to the coils. It is disclosed that this problem can be overcome by arranging the instrument to be axially symmetric about its long axis . This results in an instrument having a symmetric response which does not vary with the azimuthal angle around the long axis.

There is now a demand for so called directional measurement while drilling instruments. These are instruments having a response which varies with azimuthal angle about the long axis. Such directional instruments are particularly valuable in so-called horizontal or high-angle drilling. In this type of operation the well is drilled so that it enters the producing formations in a direction which is nearly parallel to the formation bed-boundaries; in most cases this is close to horizontal. This method of drilling has the potential to enhance greatly the producing capability of a single well: for example the along-bed length of the production tubing can be 1000 feet or more, as compared with the conventional method whereby a single well merely crosses from top to bottom of the producing formation which may be only a few feet in vertical extent .

In such horizontal wells it is necessary to steer the wellbore during drilling so that it remains within the producing zone. The directional steering techniques, using MWD, are well-known, but the steering decisions now have to be based on a knowledge of the distance of the wellbore from the top and bottom of the tnm producing zone. Such so-called geosteeπr.σ lnformation car only be obtained from instruments (for exarple conductivity measurements or natural gamma radiation detectors) which can detect the approach of the wellbore to the adjacent (non-producing) rock formations. Non directional instruments may not be useful for determining how to correct the wellbore tra ectory as it will not be clear if the instrument is approaching the top or the bottom of the bed. Hence the need for a directional instrument.

In a first aspect the present invention provides a directional instrument for insertion into a borehole and for making measurements of electrical properties of rock formations near the borehole whilst drilling, the instrument comprising

a support element defining a long axis, the support element being arranged such that the long axis is substantially parallel to the borehole axis on insertion of the instrument into the borehole;

the support element comprising a side pocket formed therein;

a transmitter coil disposed within the side pocket and coupled to a signal generator;

a receiver coil disposed within the side pocket and spaced from the transmitter coil, the receiver coil being coupled to a signal processing circuit; and, an electrically conductive reflectively shaped element formed within the side pocket and spaced from the transmitter coil.

Preferably the transmitter coil and the electrically conductive reflectively shaped element are arranged such that on excitation of the transmitter coil by the signal generator the coil generates a magnetic field which is asymmetric about the long axis of the instrument.

The instrument according to the invention is suitable for use at low frequencies (10 KHz to 200KHz) so allowing a relatively deep investigation into the surrounding formation. As it is a directional instrument, even at low frequencies, it can provide geosteering information.

The instrument according to the invention may be installed in a slot in a drill collar. This has the advantage that the independent parts can be constructed as a separate unit and then installed in the drill collar.

Preferably, the conductive reflectively shaped element extends generally parallel to the axis of the instrument. The support element may have a relative permeability close to unity. The conductive reflectively shaped element may be made of a metal which is highly conductive but non magnetic.

The support element can be electrically conductive and at least a portion of the side pocket can comprise the conductive reflectively shaped element. This removes the need for a separate conductive reflectively shaped element. Certain high strength alloys such as beryllium copper have an electrical conductivity comparable to that of copper and so are suitable for this purpose.

The receiver coil can comprise a main receiver and a bucking receiver coil connected m series opposition to each other. The instrument can further comprise magnetically permeable core material disposed within at least one of the transmitter and receiver coils. This has the advantage of increasing the effective aperture of the coils. It also enables the characteristics of the coils to be varied by for example changing the static magnetic environment of the coils or by altering a direct current in one or more of the coils.

The electrically conductive reflectively shaped element can be V shaped m cross section. This serves to assist in reflecting electromagnetic energy generated by the transmission coil m a direction generally defined by the arms of the V.

The signal processing circuit can be adapted to measure the m phase and quadrature components of the signal received by the receiving coil. This greatly facilitates the separation of the signal due to the surrounding formation from the signal due to the conducting components of the instrument, drill collar and adjacent drill string.

Preferably the signal generator provides an oscillating voltage, preferably a sinusoidal oscillating voltage, at a frequency in the range 1 KHz to 200 KHz. Use of such low frequencies greatly increases the depth of penetration of the magnetic field m the surrounding formation whilst maintaining the directional nature of the instrument.

The support element can be a drill collar.

According to a further aspect of the invention there is provided a method of making an axially non symmetric measurement of electrical properties of rock formations near a borehole comprising the steps of

providing an instrument as claimed in any one of claims 1 to 9;

energising the transmitter coil with a periodic signal;

detecting the return signal using the receiver coil;

processing the return signal to obtain the result of the measurement .

Preferably, the step of processing the return signal comprises the steps of

separating the return signal into a component in phase with the periodic signal and a component in quadrature with the periodic signal; and,

processing the component in quadrature to obtain the result of the measurement. Preferably the periodic signal is a sinusoidal signal having a frequency m the range 1 KHz to 200KHz.

The instrument may be e^mployed on any rigid assembly used tc enter a wellbore, not necessarily a drillstnng. Such assemblies are used in the industry for diverse purposes, for example to obtain well logs where conventional wireline cannot be used or to conduct maintenance operations in the wellbore. Although in some cases such assemblies cannot be rotated from the surface, nevertheless information can be obtained about their orientation in the wellbore, so that the asymmetric response pattern of the instrument may still provide useful information.

The invention will now be described by way of example only and not in any limitative sense with reference to the drawings in which :

figure 1 shows a cross section of an instrument according to the invention positioned within a borehole. The plane of the cross section is perpendicular to the axis of the borehole;

figure 2 shows the instrument of figure 1 in cross section. The plane of the cross section is parallel to the axis of the borehole;

figure 3 shows the instrument of figures 1 and 2 in cross section through axis B of figure 2;

figure 4 shows a schematic view of the magnetic field created by an excitation of the transmission coil; figure 5 shows a schematic view of the current induced in the surrounding formation by excitation of the transmission coil;

figures 6 and 7 show block diagrams of signal processing circuits for processing the signal received by one or more of the receiving coils of the instrument according to the invention.

Figure 1 shows a cross-section of a borehole 1 in an earth formation 2. The axis of the borehole extends perpendicularly from the plane of the page. Positioned within the borehole is the directional instrument comprising a drill collar 3. The drill collar 3 is a part of a drillstnng which runs between the drilling machinery at the earth's surface and a drilling bit. The drill collar 3 defines a long axis of the instrument which extends substantially parallel to the axis of the borehole, and forms a support element which houses the electrical components of the instrument. The drill collar 3 incorporates one or more passages 4 (which may be offset from the axis) to conduct the drilling fluid from surface to the drill bit. There is a generally V-shaped longitudinal slot (or pocket) 5 in a longitudinal section of the periphery of drill collar 3 and this slot contains an electrically conductive reflectively shaped element 6 and a coil array 7. The coil array 7 is housed in an insulating tube 8 which may be made from a fibre-resm composite and which is embedded in an abrasion resisting material 9. A suitable abrasion-resisting material is an aluminium oxide and epoxy resin composite made from Duralco 4460 as manufactured by Cotronics Corporation of Brooklyn, New York and containing at least 50% by volume of aluminium oxide powder as supplied by Norton Industries (a division of Saint-Gobain) , of Worcester, Massachussets , USA. Down the centre of the coil array 7 there pass shielded cables 10 in the form of twisted pairs contained m a cylindrical shield having an outer layer of copper and an inner layer of a highly magnetically permeable material sucn as mu-metal. These special shields are necessay to minimise cross-talk and the induction of spurious voltages directly into the cables themselves. These cables carry the electrical supply to the transmitter coil and the signals from the receiver coils.

Figure 2 shows a longitudinal section of the drill collar 3 m a plane indicated by the line AA in Figure 1. Coil 20 is the transmitter coil and coils 21 and 22 are two receiver coils. Above the coil array 7 is a chamber containing electronic section 23 and a power supply 24, memory storage system 25 and a system for encoding data for transmission to surface. The systems (24, 25 and the data encoding system) are well-known in the art, are included for completeness only and are not to be taken as representing any specific configuration of these elements. Additionally there may be a further system (not shown) for transmitting data from the instrument to surface and again such systems are well-known. Figure 2 also shows schematically a compensating piston 27 and hydraulic oil fill 28. The purpose of the piston 27 and the oil fill 28 is to balance the internal pressure in the coil array with the external pressure due to the fluid in the wellbore thus eliminating pressure-generated stresses on the coil array itself. It is to be understood that the piston 27 has sufficient stroke length to permit pressure balancing throughout the range of temperatures and pressures at which the instrument is to be used. Such pressure-balancing systems are well known in downhole equipment and no detailed description is necessary here.

Figure 3 shows a more detailed transverse cross-section taken across the drill collar 3 in the plane indicated by the line BB m Figure 2 and indicating the construction of coil 21. In this embodiment the construction of the coils 20, 21 and 22 are similar, although they differ in length and number of turns, and the construction of coil 21 is shown by way of example. The coil is wound using the type of multi-conductor wire known as Litz wire on an insulating tubular former 30 made of a fibre-resm composite which is in turn accommodated in the coil array housing 8. Within the tubular former 30 there is placed a magnetic core 31 , which may conveniently be constructed by stacking a suitable number of toroidally shaped cores. An improvement in the properties of such a stacked core may be obtained by lapping the individual elements so that their meeting surfaces are in excellent contact with each other. In alternative embodiments the cores may be constructed from cores of other shapes, for example rods or tubes. Suitable toroidal cores are type MPP powder-permalloy cores manufactured by Arnold Engineering of Merengo, Illinois, USA. Also seen m Figure 3 is the cross section of the shielded conductors 32. As is well known, it is important to minimise capacitative coupling between the individual coils of the coil array and for that purpose electrostatic shields are fitted over the windings of each coil. Such shields may conveniently be made of a material of high electrical conductivity such as copper, are slotted in the direction of the axis of the coil in order to minimise the flow of tangential induced currents in the shields themselves, and are electrically grounded.

In use the transmitter coil 20 is energised by passing through it an alternating current at a fixed frequency, which in this described embodiment is around 20kHz, but which may m alternative embodiments be anywhere m the range 10kHz to 100kHz. Alternatively the instrument may be operated at more than one selected frequency or swept frequency range in order further to enhance the resolution and processing of the measurement or the automatic maintenance of the instrument calibration. Typically the current is of the order of a few amperes and is supplied from an oscillator and amplifier of conventional design but using parts, materials and constructional techniques which are suitable for use in the downhole environment.

The alternating current n the transmitter coil gives rise to a primary alternating magnetic field. The primary alternating magnetic field interacts with the receiver coils 21,22, with the electrically conductive reflectively shaped element 6, with the drill collar 3, and with the fluid m the borehole 1.

At the frequency employed, much of the incident energy is absorbed in the conductive material of the reflectively shaped element and drill collar. The magnetic field can only penetrate the conductive material to a shallow depth, customarily defined as the skin depth the depth in the material at which the field reaches 1 /e (where e is the base of natural logarithms) of its incident value. Both in the reflectively shaped element and m the conductive material of the drill collar the penetration of the field is relatively shallow: the skin depth at 20kHz is about 0.7 mm in aluminium and 2.4 mm m stainless steel.

Because the reflection is imperfect and because the magnetic field cannot readily penetrate the materials of the reflectively shaped element and the drill collar, the primary magnetic field due to the energised transmitter coil is effectively steered around the drill collar as shown in the schematic field pattern of Figure 4. But because of the partial reflection from the V-shaped element 6, the field is also to some extent concentrated and developed further in a radially outwards direction away from the open end of the V-shaped element 6.

The overall effect of the interaction of the primary alternating magnetic field with the reflectively shaped element 6 and the drill collar 3 is that the primary alternating magnetic field develops in an asymmetric pattern around the drill collar in such a way that flux density at a given radial distance away from the drill collar will, in an otherwise symmetrical and homogenous environment, be greater in the direction of the open end of the V-shaped reflectively shaped element 6 than m any other direction. It will be at a minimum in the direction 180° away from the V-shaped element. The primary alternating magnetic field is asymmetric about the long axis of the instrument. The magnetic field pattern varies with azimuthal angle about the long axis of the instrument. This field pattern is shown schematically in figure 4. This primary field is similar in nature to that developed in conventional radially symmetric wireline induction logging tools but is in this case created around a metal drill collar in the borehole and has the asymmetric properties described in the previous paragraph.

In the receiver coils 21,22 the primary field generates an induced voltage which is in quadrature phase relative to the transmitter current. Coils 21 and 22 are connected in series opposition and the resultant signal is taken as the input to electronic circuitry which is capable of phase discrimination of the received signal with respect to the transmitter current. Coils 21 and 22 may be designed so that the magnitudes of the signals developed in them due to induction from the primary field are equal: for a particular spacing between the coils this can be done, for example, by increasing the number of magnetic cores in the further spaced coil 22 or by changing the position of the cores in either coil or by increasing the number of turns on coil 22 relative to the number on coil 21. In this case and considering for the moment only the response to the directly linked field from the transmitter 20, the resultant quadrature signal from the series-opposed coils 21 and 22 will be zero. This technique, of adjusting a so-called bucking coil connected in series opposition with another receiver coil so as to cancel, in the resultant signal, the voltages induced in the said coils by the alternating magnetic flux generated directly from the transmitter coil is well-known in induction logging tools.

The primary alternating magnetic field also interacts with the material of the reflectively shaped element 6 and the drill collar 3. Eddy currents flowing in these items induce a secondary alternating field which also links the receiver coils 20 and 21 and generates voltages m these coils. Because of the inductive nature of the circulating current loops m the highly conductive materials these voltages are also in quadrature (or nearly so) with the transmitter current and can also be nulled in the resultant series-opposition signal by adjustment of the magnetic cores, or adjustment of the numbers of turns in the coils or adjustment of the spacing of the coils one from another.

The primary alternating magnetic field also interacts with the material of the earth formation surrounding the instrument and causes secondary currents to flow in the formation at right angles to the plane of the field lines in the vicinity. Such currents, known as Foucault currents, have a magnitude which is generally proportional to the electrical conductivity of the material through which they pass. These currents are shown schematically m figure 5. As before, these currents give rise to a secondary field which links to the receiver coils . When (as is usually the case) the formation is of relatively low electrical conductivity, the voltage induced in the receiver coils is substantially in phase with the transmitter current.

The coil array can be adjusted physically (or electronically as will be described later) to null the large quadrature signals developed from the primary field and also from the neighbouring conductive masses of the drill collar and reflectively shaped element. The effect of any of these adjustment methods is to achieve effectively zero mutual induction between the coils 21 and 22. Achievement of this condition is important because it avoids the high-gain electronic amplifiers being saturated by large directly induced signals. It also optimises conditions for minimising the sensitivity of the instrument to conductive fluid the borehole. Once the balance condition has been achieved, the residual error in the wanted m-phase signal is small and can be handled by subtraction without introducing excessive error.

The in phase signal in the individual receiver coils is due to secondary fields arising in a relatively large vertical section of the earth formation around the instrument and cannot respond abruptly when the instrument first enters a section of contrasting conductivity. However the subtracted signal from the two relatively closely spaced receiver coils represents only the secondary field in the general region of the formation lying between the two receivers and is therefore capable of providing better resolution of abrupt changes in the formation conductivity. This general technique, of subtracting the signals from two relatively closely spaced receiver coils, and other more complex methods, are well-known in the art and are mentioned here only for completeness.

Figure 6 shows a block diagram of the electronic processing circuits for use with the receiving coil array configuration described above. The electronic processing circuits provide means to measure both the m-phase and quadrature components of the received signal, for the reasons detailed in prior text. The m-phase (I signal) and quadrature phase (Q signal) measured components are referenced to the sinusoidal transmitter 20 circulating current. In this embodiment, the clock circuit 72 generates a stable high frequency square wave, used to synchronise the micro-controller 75, data acquisition and sine wave generation. The clock _s fed to the phase control circuits 73, which generates the reference I and Q signals, at 90 degrees apart. The phase control circuits 73 also allow the fine adjustment of the phase relationship between I and Q, and the transmitter excitation current. The digital sine wave synthesizer 74 generates a pure sine wave, preferably in the range of 1 OKHz to 200KHz, and in this embodiment 1/32 of the square wave clock input. The power amplifier (PA) 63 proceeds to drive the transmitter 20 with an amplified sinusoidal current, the magnitude of which, is a function of the primary magnetic flux.

The bucking receiver 21 and mam receiver 22 are connected series opposition, cancelling the direct mutual coupling from receivers to transmitter primary field i.e. the induced bucking receiver 21 and mam receiver voltages 22 attributed to the transmitter primary field, are equal and opposite in polarity.

Connected in series with the transmitter 20, a transformer 61 terminated with an appropriate resistance element (Rs) 62, provides voltage components m direct proportion to the transmitter circulating current. The transformer signal, receiver signal and a short circuit element are the inputs for the double pole, three way, multiplexer 64. Under command from the micro-controller 75, the multiplexer sequentially routes the signals to the high ga , low noise amplifier (LNA) 65. The low noise amplifier provides voltage gam in the region of 80dB, for signals centred around the transmitter frequency, preferably between the ranges of 1 OKHz to 200KHz. The voltage amplified signals output from the LNA provide the mputs of the I phase sensitive detector (IPSD) 69 and the Q phase sensitive detector (QPSD) 66. The phase sensitive detectors extract the I and Q phase components of interest from the input signal, supplying the resultant signal (s) to filters 67, 70. The filters remove undesirable harmonic content before conversion to digital format by the analogue to digital converters (1 and 2) 68, 71, under control from the micro-controller. 24-bit Data words are communicated to the micro-controller for further calculation. Calculated resistivity data is stored and passed to suitable borehole data telemetry means by the data telemetry and memory circuits 76.

The asymmetric arrangement of coils, materials, borehole fluid and surrounding rock does not lend itself to exact mathematical analysis even when some simplifications are introduced. However the electrical and geometrical properties of the instrument can be predicted by using 3-dιmensιonal finite element analysis. Computer programs to perform such analyses are commercially available and their use in this connection is well-known the art. It can be seen however that the asymmetric arrangement of at least one of the reflectively shaped element and the transmitter coil around the long axis of the instrument results in an asymmetric secondary field in the surrounding formation. This in turn results m an instrument with a response which varies azimuthal angle around the long axis of the instrument. It has been noted above that the operation of zeroing the quadrature output from the series-opposed receiver coils may be carried out by making physical changes in the coils such as adding or subtracting turns of wire or varying the number or position of the magnetic cores inside the coils. In embodiments such as this preferred embodiment which magnetic cores are employed in at least one of the receiver coils there is an alternate method of making this adjustment which is particularly favourable if the instrument is to be installed in one of many drill collars or other elements having differing properties such as diameter and constructional material. It is known that the effective magnetic permeability of ferromagnetic materials is a non-linear function of the static magnetic flux within the material .

In an alternative embodiment there is applied to at least one of the receiver coils or to a separate winding on the same coil former, a d.c. bias current which maintains a static or slowly varying flux m the magnetic core. This bias current is controlled using negative feedback and an integrator in the configuration commonly known as proportional-integral control. The error signal for the controller is the previously described quadrature signal from the receiver. Such a system allows the magnetic permeability of a core made of appropriate material to be dynamically adjusted m such a way that the condition of mutual balance may be maintained even when the instrument is installed in an environment, such as that of a borehole, in which temperature changes, pressure changes and mechanical stresses might otherwise cause drift in the mutual balance condition. Figure 7 shows the detail of a method for realising this automatic adjustment. The quadrature phase signal (Q phase) is routed to a simple two way multiplexer 77, enabled during the receiver acquisition phase only. The feedback signal provides the input to the integrator error amplifier 78, which produces an amplified voltage output 79 as a function of the quadrature signal. The integrator amplifier also has the additional functions of limiting the frequency components of the output coil drive signal 79 , as the mutual balance drift is slow m time, and restricting the noise introduced to the receivers conductors and hence the high gam LNA inputs.

The amplified voltage output 79 gives rise to a DC bias current, flowing m the direction indicated by the schematic arrows 83. This DC bias current is selected to be sufficient to introduce an appropriate static magnetic flux m the ferromagnetic material, resulting in an effective decrease in the effective permeability of the magnetic cores 31 to achieve mutual balance. The discrete network 82 positioned between the receiver coils and input multiplexer 64, contains capacitors on each of the receiver connections, providing a low impedance path for the high frequency (20KHz) signals and a blocking element for the circulating DC bias current, isolating the sensitive high gam LNA from the DC bias currents.

In the embodiment shown in figures 1 and 3 the electrically conductive reflectively shaped element is a separate component to the drill collar. In an alternative embodiment the drill collar itself forms the electrically conductive reflectively shaped element . In the embodiment described above the instrument comprises a transmitter coil and a plurality of receiver coils. In an alternative embodiment the instrument comprises only one transmitter coil and one receiver coil. In an alternative embodiment the instrument comprises a plurality of transmitter coils and a single receiver coil. In an alternative embodiment the instrument comprises a plurality of transmitter coils and a plurality of receiver coils. In this embodiment the plurality of transmitter coils and/or the plurality of receiver cols may be arranged in arrays.

Claims

1. A directional instrument (3) for insertion into a borehole (1) and for making measurements of electrical properties of rock formations (2) near the borehole whilst drilling, the instrument comprising:

a support element (3) having a longitudinal axis, the support element being arranged such that the longitudinal axis is substantially parallel to the borehole axis on insertion of the instrument into the borehole;

the support element comprising a side pocket (5) formed therein,-

a transmitter coil (20) disposed within the side pocket

(5) and coupled to a signal generator;

a receiver coil (21, 22) disposed within the side pocket (5) and spaced from the transmitter coil (20), the receiver coil being coupled to a signal processing circuit; and,

an electrically conductive reflectively shaped element

(6) formed within the side pocket (5) and spaced from the transmitter coil (20).

2. A directional instrument as claimed in claim 1 wherein the transmitter coil (20) and the electrically conductive reflectively shaped element (6) are arranged such that on excitation of the transmitter coil by the signal generator the coil generates a magnetic field which is asymmetric about the longitudinal axis of tne instrument .

3. A directional instrument as claimed either of claims 1 or 2 , wherein the conductive reflectively shaped element (6) is generally parallel to the longitudinal

4. A directional instrument as claimed in any one of claims 1 to 3, wherein the support element (3) is electrically conductive and at least a portion of the side pocket (5) forms said conductive reflectively shaped element (6) .

5. A directional instrument as claimed n any one of claims 1 to 4, wherein the receiver coil comprises a main receiver coil (22) and a bucking receiver coil

(21) connected in series opposition to each other.

6. A directional instrument as claimed in claim 5, further comprising magnetically permeable core material (31), disposed within at least one of the transmitter (20) and receiver coils (21, 22) .

7. A directional instrument as claimed m any one of claims 1 to 6, wherein the conductive reflectively shaped element (6) is generally V shaped in cross section.

8. A dιrectιona_ instrument as claimed m any one of claims 1 to ^~ , wherein the signal processing circuit is adapted to rsasure the phase and quadrature components of the signal received by the receiving coil (21, 22).

9. A directional instrument as claimed in any one of claims 1 to 8 , wherein the signal generator provides an oscillating voltage, preferably a sinusoidal oscillating voltage, at a frequency m the range 1 KHz to 200KHz.

10. A directional instrument as claimed in any one of claims 1 to 9, wherein the support element is a drill collar (3).

11. A method of making an axially non symmetric measurement of electrical properties of rock formations (1) near a borehole (1) comprising the steps of

providing an instrument as claimed m any one of claims 1 to 10;

energising the transmitter coil (20) with a periodic signal;

detecting the return signal using the receiver coil (21, 22);

processing the return signal to obtain the result of the measurement .

12. A method as claimed in claim 11 wherein the step of processing the return signal comprises the steps of

processing the component in quadrature to obtain the result of the measurement.

13. A method as claimed in either of claims 11 or 12, wherein the periodic signal is a sinusoidal signal having a frequency in the range 1KHz to 200KHz.