GB2121189A

GB2121189A - Method and apparatus for induction logging of boreholes

Info

Publication number: GB2121189A
Application number: GB08313578A
Authority: GB
Inventors: James Roger Samworth; Hemant Kumar Patel; John Stanley Fisher
Original assignee: BPB Industries PLC
Current assignee: BPB Ltd
Priority date: 1982-05-25
Filing date: 1983-05-17
Publication date: 1983-12-14
Also published as: GB8313578D0

Abstract

In a method and an apparatus for induction logging to investigate earth formations traversed by a borehole, at least two transmitter/receiver basic elements (T, R1-R4) for induction logging, contained in induction logging means (1, 2) are passed through a borehole, to obtain respective separate electrical signals. A cable (3) supports the induction logging means in the borehole and provides for transmitting the separate electrical signals to the surface. The separate electrical signals are stored (23) at the surface, and may be combined in any desired way (22) to produce one or more displays representative of electrical characteristics of earth formations adjacent the borehole as a function of depth. <IMAGE>

Description

SPECIFICATION Method and apparatus for induction logging of boreholes This invention relates to a method and apparatus for induction logging of boreholes.

Induction logging is a known technique. An instrument, hereinafter referred to as a "sonde", is suspended on a cable and lowered down a borehole in order to obtain information on the nature of the earth in the region of the borehole, and in particular to investigate the possible presence of minerals and oil. The sonde contains a coil (hereinafter "transmitter coil") wound on an insulating mandrel, the coil being activated by an alternating current to generate an electromagnetic field in the KiloHertz range of frequencies. The influence of the surrounding earth formations on the electromagnetic field is investigated by observing the voltage induced in a second coil (hereinafter "receiver coil") which is also wound on the insulating mandrel, so that it is coaxial with and longitudinally spaced apart from the transmitter coil.This arrangement is hereinafter referred to as a "transmitter/receiver basic element".

The signal from the receiver coil in the system described above gives a general measure of the resistivity of the earth formations in the region of the borehole. A number of systems have been proposed for "focusing" the measurements made so that the signal received is representative of the resistivity of relatively narrow sectors of the earth formations. For example, the signal may be representative of the resistivity of an annulus of earth at a certain lateral distance from the borehole (radial or lateral focusing) and/or may be representative of the resistivity of a relatively narrow horizontal band of earth centred at a point somewhere between the transmitter and receiver coils (vertical or longitudinal focusing). Such focusing has been achieved by multiple coil systems employing more than one transmitter coil and/or more than one receiver coil.The signals from the various transmitter/receiver combinations are combined in the sonde and transmitted as a composite signal via the cable to the surface. The signal outputs of various transmitter/receiver combinations may be summed algebraically, optionally with weightings being applied, to obtain measurements characteristic of the earth formations in specific sectors. It is also known to process a signal from a borehole by storing it and then making a predetermined algebraic combination of selected fractions of the signal obtained at different times (and corresponding to different depths in the borehole).

In all cases where multiple coil systems are used, the prior art combines output signals in the sonde itself and transmits one or more such composite signals to the surface. Any further treatment which can be carried out at the surface on such composite signals is limited. Thus, on a single traverse of a borehole by a single sonde, the information obtained is limited and if further information is needed then a further traverse of the borehole by a sonde having different transmitter/receiver combinations is required. This inflexibility of the prior art systems has been overcome by the present invention, which is characterised in that the basic signal data from each transmitter/receiver basic element is transmitted separately to the surface and is stored separately, so that the purity of each element's contribution is preserved.This basic signal data may then be processed and recombined in an unlimited number of ways.

The invention thus provides a method of induction logging to investigate earth formations traversed by a borehole, which comprises disposing induction logging means at a plurality of depths in the borehole, the induction logging means having at least two transmitter/receiver basic elements for obtaining respective separate electrical signals; transmitting the separate electrical signals to the surface; storing the separate electrical signals so that the signal derived from each of the basic elements is preserved in pure form; and combining the separate electrical signals in any desired way to produce one or more displays representative of electrical characteristics of earth formations adjacent the borehole as a function of depth.

The invention also provides apparatus for induction logging to investigate earth formations traversed by a borehole, comprising at least two transmitter/receiver basic elements for induction logging, contained in induction logging means adapted to be passed through a borehole, to obtain respective separate electrical signals; a cable for supporting the induction logging means in the borehole and for transmitting the separate electrical signals to the surface; means for storing the separate electrical signals at the surface; and means for combining the separate electrical signals in any desired way to produce one or more displays representative of electrical characteristics of earth formations adjacent the borehole as a function of depth.

The separate electrical signals can be stored at the surface indefinitely, for example on magnetic tape or discs. The production of a display involving a certain combination of these signals does not affect the storage of the basic data, which is still preserved in pure form. The invention therefore provides the flexibility of being able to produce a variety of displays from the same basic data. It is also possible to produce a display some time after collection of the basic data using software which was not even available at the time the data was collected.

The current fed to the transmitter coil is sinusoidal and the electromagnetic field generated is usually in the frequency range of 1 kHz to 100 kHz, preferably 10 kHz to 50 kHz. The voltage induced in the receiver coil, as a result of the electromagnetic field generated in the surrounding geological formations, and which forms the desired signal component, differs in phase by 1 800 with respect to the current in the transmitter coil. However, a relatively large and undesirable voltage is also induced directly. This latter undesirable component differs in phase by 900 with respect to the current in the transmitter coil. The 900 component can be rejected by suitable electronic means.However, it is preferred to reject the 900 component by providing a small, auxiliary coil (hereinafter referred to as "dummy" coil), which may for example be inserted between the transmitter and receiver coils. Such a dummy coil cancels out most of the 900 component from the signal induced in the receiver coil. The dummy coil does not necessarily have to be in between the transmitter and the particular receiver. One dummy coil can be used to cancel 900 components in several elements. Any residual 900 component is rejected by phase detector means as described below.

Reference is now made to the accompanying drawings, showing a preferred embodiment of the invention, in which: Figure 1 is a diagrammatical representation of a sonde in a borehole showing the arrangement of coils, and associated block diagrams 1 A and 1 B respectively showing the downhole electronics in the sonde and the surface electronics; and Figure 2 is a diagram on a larger scale showing the winding of the coils.

Referring now to Figure 1, a sonde comprises an insulated coil section 1 and a downhole electronics section 2 and is supported from the surface by a cable 3. The various transmitter, receiver and dummy coils are wound on an insulating mandrel in the insulated coil section 1. As shown in Fig.

2, each coil 4 is wound on an insulating fibre glass mandrel 5, and eiectrostatically screened on the inner and outer coil surfaces respectively by screens 6 and 7. The coil array is surrounded by insulating oil within an insulating fibre glass outer tube 8 to form the coil section 1. It has been found suitable to employ a single transmitter coil T and four receiver coils R1, R2, R3 and R4. There are four dummy coils D1, D2, D3 and D4 in between the transmitter coil and the respective receiver coils. In fact, as shown in Figure 1, Dl, D2 and D3 are between T and R1, and D4 is between R1 and R2.The spacing of the transmitter and receiver coils is designed so that suitable combinations of the seprate signals received at the surface can give desired information about the resistivity of various sectors in the region of the borehole.

The spacing of the transmitter, receiver and dummy coils is given in the following Table together with the number of turns and the diameter of each coil.

Table Distance from Diameter Coil Tcoil in mm No. turns (mm) T - 236 71 D1 220 4 71 D2 313 4 71 D3 389 4 71 R1 470 114 71 D4 669 9 71 R2 704 114 71 R3 937 114 71 R4 1350 114 71 The transmitter 9 supplies alternating current to the transmitter coil T. This sets up a 20 kHz electromagnetic field around the T coil. A receiver amplifier 10 takes the microvolt AC emf (VR) induced in the receiver coil R1 and dummy coil D1, and amplifies it into the millivolt range prior to phase detection. A major proportion of the 900 component has been cancelled by the dummy coil Do. A phase detector 11 has a dual role.It receives a current phase reference 12 from the transmitter 9, and rejects any residual 900 component (Vx) present in the signal. In addition, it converts the amplifier output from an AC signal to a DC level. A low pass filter 13 removes the 20 kHz component from the output of the phase detector 11 and supplies the resulting signal to an analogue multiplexer 14. Each receiver and dummy coil has its respective components 1 0, 11 and 1 3 to feed the signal to the multiplexer 14. The analogue multiplexer takes each of the four filtered DC levels from the respective roceiver and dummy coils (and any other analogue transducer measurement) serially in time and passes them to an analogue to digital converter 1 5 which is under the control of a data capture microprocessor 1 6.The outputs from component 1 6 are fed to a cable communications means 1 7 which handles communication between the sonde and the surface, receiving commands from and transmitting data back to the surface. A power supply 1 8 removes power from the cable 3 and produces regulated DC supply rails for general use within the sonde.

The signal from the cable 3 is received on the surface by cable communication means 1 9 which are the counterpart of the downhole unit 17, the two units exchanging information in the uphole and downhole directions. A sonde interface 20 recognises the signal received from the sonde and converts the digital data format used in communication into nominal units of conductivity in a format recognised by the central processing unit 21. A plotter 22 enables the computed results of combinations of the four separate receiver outputs to be plotted or logged against a depth axis. A tape drive 23 is the source of the system software as well as a means of data storage. A control console 24 comprises a video display unit and keyboard by which the operator commands the sonde and the system to operate.A power supply 25 provides the source of power to transmit down the cable to the sonde.

It is possible for the invention to be carried out with coil arrangements different from that shown in Figure 1. For example, the dummy coil in a basic element can be on the lower side of the transmitter coil relative to the receiver coil. A basic element can also have the receiver coil below the transmitter coil, and the dummy coil above the transmitter. Similarly, the basic elements as shown in Figure 1 can be inverted such that the transmitter coil is towards the top of the sonde. Furthermore, it is possible to have more than one transmitter coil, although it is preferred only to have one transmitter coil. A coil array may theretore combine a permutation of all of the above basic element forms.

The system described above thus provides at the surface the raw signal data from each transmitter/receiver basic element preserved in pure form. This data is stored on tape and can be recombined and processed in any desired way. Thus, all the data is produced by a single pass through the borehole with a single instrument. The predominant response of a single transmitter/receiver basic element depends mainly on the transmitter/receiver spacing. The four basic signals can accordingly be combined at the surface with appopriate weightings to achieve radial focusing. Vertical focusing is achieved by introducing a time factor (the time being dependent on sonde depth), so that the signals are combined at the surface with appropriate depth offsets. By suitable programming, a variety of induction logs can be produced immediately in the field from the basic data.Furthermore, as the basic data has been stored, further information can be obtained simply by a change of program, with no need for a further pass through the borehole. It is also possible, by suitable adaptation of computer software, to employ the same sonde in a variety of different borehole and geological conditions. This flexibility obtained by the present invention is not possible with the systems previously used.

Where radial focusing is achieved by combining signals in the sonde itself, as in the prior art, it has generally been necessary to employ multiple transmitter coils as well as multiple receiver coils to achieve focusing at one or more radial distances from the borehole. This same result can be achieved in the present invention with a single transmitter-multiple receiver system. The latter therefore requires a shorter coil section, so that the sonde is both shorter and lighter than previous instruments.

Claims

1. A method of induction logging to investigate earth formations traversed by a borehole, which comprises disposing induction logging means at a plurality of depths in the borehole, the induction logging means having at least two transmitter/receiver basic elements for obtaining respective separate electrical signals; transmitting the separate electrical signals to the surface; storing the separate electrical signals so that the signal derived from each of the basic elements is preserved in pure form; and combining the separate electrical signals in any desired way to produce one or more displays representative of electrical characteristics of earth formations adjacent the borehole as a function of depth.

2. A method as claimed in claim 1, wherein the induction logging means comprises at least two basic elements sharing the same single transmitter coil.

3. A method as claimed in claim 2, wherein the receiver coils of the respective basic elements are all arranged on the same side of the transmitter coil.

4. A method as claimed in claim 1, wherein each basic element includes an auxiliary coil between the transmitter and receiver coils to cancel out the voltage induced in the receiver coil directly from the transmitter coil.

5. A method as claimed in claim 1, wherein the induction logging means includes multiplexing means for processing the signals from the respective receiver coils serially in time, whereby the processed signals are transmitted separately to the surface.

6. Apparatus for induction logging to investigate earth formations traversed by a borehole, comprising at least two transmitter/receiver basic elements for induction logging, contained in induction logging means adapted to be passed through a borehole, to obtain respective separate electrical signals; a cable for supporting the induction logging means in the borehole and for transmitting the separate electrical signals to the surface; means for storing the separate electrical signals at the surface; and means for combining the separate electrical signals in any desired way to produce one or more displays representative of electrical characteristics of earth formations adjacent the borehole as a function of depth.