GB2076039A

GB2076039A - Apparatus for, and a Method of, Signalling Within a Borehole While Drilling

Info

Publication number: GB2076039A
Application number: GB8016759A
Authority: GB
Original assignee: RUSSELL ATTITUDE SYST Ltd
Current assignee: RUSSELL ATTITUDE SYST Ltd
Priority date: 1980-05-21
Filing date: 1980-05-21
Publication date: 1981-11-25
Also published as: FR2483006B1; CA1205376A; NO159615C; NO159615B; NL8102401A; NO811446L; GB2076039B; FR2483006A1

Abstract

A telemetry sub 1 for a drill string 2, 3 comprises upper and lower portions 6 and 7 electrically insulated from one another by an insulator 8. The sub 1 incorporates a transformer 12 having its secondary connected between the upper portion 6 and the lower portion 7. In operation a measurement data signal in the form of pulses is supplied to the primary of the transformer 12 by suitable electronic circuitry 11 and a pulsating current signal is thereby induced in the secondary. This pulsating signal is passed along the drill string towards the surface, a return path for the signal being provided by the earth formations through which the borehole 10 is drilled. The signal is passed to a receiving station on the surface either directly or by way of one or more repeater subs of the same general construction as the sub 1. Such a system is capable of transmitting signals at a high rate and with an acceptable signal-to-noise ratio. Furthermore the subs are relatively inexpensive to produce so that a large number of repeater subs may be distributed along the drill string if desired. <IMAGE>

Description

SPECIFICATION Apparatus for, and a Method of, Signalling within a Borehole while Drilling This invention relates to apparatus for, and a method of, signalling within a borehole while drilling. The invention also concerns a telemetry sub for a drill string.

Various measurements-while-drilling (MWD) systems have been proposed for taking measurements within a borehole while drilling is in progress and for transmitting the measurement data to the surface. Since time is not lost in stopping the drill and inserting a measurement probe into the borehole in such systems, MWD systems lead to a substantial saving in drilling time and hence to a considerable reduction in the overall cost of the drilling operation. Those systems which have so far been suggested are of four basic types, namely mud pressure pulse systems, electromagnetic systems, hard-wire systems and acoustic systems. A wide variety of electromagnetic (EM) systems have been proposed, all of which rely on the transmission of electromagnetic data signals through the earth and/or drill string.However, none of the EM systems so far proposed has been shown to give a reasonable signal-to-noise ratio for the signals detected at the surface, and accordingly the data transmission rate is severely limited.

An EM system is proposed in U.S. Specification No. 2,354,887 (D. Silverman et al) in which the measurement data signals are supplied in the form of pulses to a toroidal coil surrounding the drill string in the vicinity of the drill bit. A varying magnetic field is associated with the pulsating current in the coil and this field induces signals in the form of pulsating current in the drill string.

The return circuits for the currents which flow in the drill string as a result of this action are through the earth itself. The transmitted signals are detected at the surface by means of receiving station connected between the drill string and a grounded electrode at a distance from the drill string. However, due to attenuation of the signals in the earth and the core losses in the coil, the signal-to-noise ratio of the detected signals is poor, and the system is therefore unsatisfactory in practice.

It is an object of the invention to provide signalling apparatus constituting an EM system which is inexpensive and is capable of transmitting signals at a high rate and with an acceptable signal-to-noise ratio.

According to the invention the signalling apparatus incorporates a transformer having one winding electrically connected between a first portion of the drill string for drilling the borehole and a second portion of the drill string which is electrically insulated from the first portion, and signal transmitting and/or receiving means electrically connected to the other winding of the transformer, the first portion providing a current path for signals travelling between the transformer and a signal receiving and/or transmitting station at a remote location along the drill string, and a return path being provided between the transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which the borehole is being drilled.

Preferably said one winding is the secondary winding of the transformer and said other winding is the primary winding of the transformer and transmitting means are electrically connected to the primary winding for transmitting signals to a remote receiving station.

With this system the size of the transformer and the number of turns of the primary and secondary windings of the transformer may be chosen so as to reduce core loss. Moreover the transformer, which is advantageously a toroidal transformer, may be made very small so that it is inexpensive to produce and may be accommodated within a small chamber in the wall of the drill string. Since the core loss is small, the power requirement of the signal transmitting and/or receiving means is small, or the order of 1W, for example, and may be supplied by a battery.

In a preferred embodiment of the invention, the first and second portions are the upper and lower portions respectively of a special telemetry sub, the portions being separated by an electrical insulator, and the transformer is disposed within a chamber in the wall of the sub together with signal transmitting means comprising a battery and signal processing means.

Since the degree of attenuation of the transmitted signals is dependent on the distance over which the siganls are transmitted and the electrical conductivity of the earth formations through which the borehole is being drilled, it may be advantageous to provide one or more repeater subs between the sub containing the measuring instrument and the surface. Each repeater sub may contain identical signal transmitting, signal receiving and amplifying means, with the signal transmitting means being the same as that contained in the sub containing the measuring instrument. Since the subs are relatively inexpensive to produce, a large number of repeater subs may be provided as necessary along the drill string.

The invention also provides a method of signalling within a borehole while drilling, utilising a drill string having first and second portions electrically insulated from one another, which method comprises passing signals between signal transmitting and/or receiving means and a signal receiving and/or transmitting station spaced apart along the drill string by way of a transformer having one winding electrically connected between the first and second portions and the other winding electrically connected to the signal transmitting and/or receiving means, the first portion providing a current path for signals travelling between the transformer and the signal receiving and/or transmitting station, and a return path being provided between the transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which the borehole is being drilled.

The invention further provides a telemetry sub for a drill string, comprising first and second portions electrically insulated from one another, a transformer having one winding electrically connected between the first and second portions, and signal transmitting and/or receiving means electrically connected to the other winding of the transformer, the first portion providing a current path for signals travelling between the transformer and a signal receiving and/or transmitting station at a remote location along the drill string when the telemetry sub is disposed in a drill string, and a return path being provided during drilling between the transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which a borehole is being drilled.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagram of part of a drill string incorporating a telemetry sub in accordance with the invention; and Figure 2 is a longitudinal section through a telemetry sub in accordance with the invention.

Referring to Figure 1, which is intended only to illustrate the principle of the invention and not to show the detailed construction of the telemetry sub, the telemetry sub 1 is shown disposed between two drill string sections 2 and 3 within a borehole 1 0. The sub 1 is coupled to the section 2 by means of a male screwthreaded coupling 4 on the sub 1, and to the section 3 by means of a female screwthreaded coupling 5 on the sub 1. The section 2 is coupled to a rotary table (not shown) at the surface by way of other sections and the section 3 is coupled to a drill bit (not shown) optionally by way of other sections.

The sub 1 comprises an upper portion 6 electrically connected to the section 2 and a lower portion 7 electrically connected to the section 3. The upper and lower portions 6 and 7 are electrically insulated from one another by an insulator 8. Within the side wall of the sub is a chamber 9 containing electronic circuitry 11 and a toroidal output transformer 12 having its secondary connected between the upper portion 6 and the lower portion 7. In operation a measurement data signal in the form of pulses is supplied to the primary of the transformer 12 by the electronic circuitry 1 the measurement data being, for example, the data outputted by an instrument for measuring the orientation of a borehole such as that described in U.K. Patent Specification No. 1,509,293.A pulsating current signal is thereby induced in the secondary of the transformer 12, and this signal is passed to the surface by way of the section 2 and the further sections of drill string between the section 2 and the surface. A return path for the signal is provided through the earth formations through which the borehole 10 is drilled.

The telemetry sub shown in Figure 1 may be either a transmit-only sub for transmitting measurement data supplied to it by an adjacent measuring instrument or a repeater sub which receives a signal supplied to it along the drill string, processes this signal and transmits the processed signal. In the former case the electronic circuitry 11 will comprise a battery and a signal processing circuit for modulating the current supplied by the battery in accordance with the measurement data.

A repeater sub is shown in longitudinal section in Figure 2. Like parts are given the same reference numerals in this figure as in Figure 1. It will be noted that the annular insulator 8 extends between the chamber 9 and the inside wall of the sub in the vicinity of the chamber 9, and that the insulator is generally S-shaped in section so as to strengthen the joint between the upper and lower portions 6 and 7. The chamber 9 is closed off from the outside by a cover 13 which is sealed at its edges by means of a seal 14, and is electrically insulated in such a manner that it does not provide a current path between the upper and lower portions 6 and 7. The chamber contains, in addition to the output transformer 12, an input transformer 15, an input amplifier 16, a signal processing circuit 18, and an output amplifier 1 7.

The input and output transformers 1 5 and 1 2 could be replaced by a single transformer whose function is both to receive and transmit signals.

The repeater is arranged to receive and transmit signals in a multiplexed manner. Thus, in a first period of, for example, one second, an input signal transmitted by a transmit-only sub or by another repeater sub further down the drill string is detected by the input transformer 1 5 whose primary is coupled between the lower sections of the drill string and the earth return, and is amplified by the input amplifier 16. In a second period of, for example, 100 milliseconds, the signal is digitally processed by the circuit 18 so as to remove errors, and, in a third period of, for example, one second, the processed signal is amplified by the output amplifier 1 7 and transmitted by the output transformer 12 either to a receiver at the surface or to another repeater further up the drill string.

Since the telemetry subs are relatively inexpensive to produce and the bandwidth, and hence the data transmission rate capability, ofsignal drops off as a function of the distance ovbr which it is transmitted, repeater subs may be provided with a spacing as low as 1000 feet along the drill string if necessary. This then 'enables signals to be transmitted at a data rate of about 100 bits per second with a power input of about 1W per sub and a bandwidth of about 100 Hz.

If, in the system proposed in U.S. Specification No. 2,354,887, the drill string and conductive return through the earth are considered as a single turn secondary winding of a toroidal transformer having the toroidal coil surrounding the drill string as its primary, the core losses in the system disclosed in that specification and the .system illustrated in the accompanying drawing may be compared.

If the area of cross-section of the toroidal core is Ac and the maximum saturation flux density for the core material is BRAT, then clearly Xmax < BSATAC (i) If the impedance of the secondary is Z5 then the power injected into the secondary can be written as Ps=Vs2/2Zs (ii) The maximum value of the emf induced in the secondary can be written as Vs=Ns.5bmax.es (iii) where N5 is the number of turns on the secondary, fmaX is the maximum value of the magnetic flux through the toroidal core cross-section, and =2.7ref where f is the frequency of the primary drive.

From equations (i), (ii) and (iii) it follows that (2.Ps.Z5)112 Ac > (iv) Ns.2..f.BsAT Typical values for Silicon Iron toroidal transformer core material may be taken as: Bmax=1 .2 Tesla Core Loss at 200 Hz=5.5 watts.kg-' Core Density=7 gm.cm~3 From previous calculations at 200 Hz, the impedance Z5 of the drill pipe and earth can be taken as 0.17 ohm.

Thus, for a toroid of average diameter 14 cm surrounding the drill string having a single turn secondary winding, these typical values indicate a minimum value for the cross-sectional area of the core of 3.9 cm2 and resulting minimum core volume of 1 56 cm3, and the minimum mass of the core is calculated at 1.1 kg when the case for Pus=1 watt is considered. Thus, in order to deliver 1 watt to the secondary circuit at 200 Hz, the resulting core loss is about 6 watts. Furthermore such a core will be expensive to manufacture.

By contrast, if the toroidal transformer of the system illustrated in the drawing has 100 turns on its secondary, the minimum value for the cross sectional area of the core is about 4 mm2 and the core may therefore be extremely small so that the core loss is negligible.

Claims

1. Apparatus for signalling within a borehole while drilling, incorporating a transformer having one winding electrically connected between a first portion of the drill string for drilling the borehole and a second portion of the drill string which is electrically insulated from the first portion, and signal transmitting and/or receiving means electrically connected to the other winding of the transformer, the first portion providing a current path for signals travelling between the transformer and a signal receiving and/or transmitting station at a remote location along the drill string, and a return path being provided between the transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which the borehole is being drilled.

2. Apparatus according to claim 1, wherein said one winding is the secondary winding of the transformer and said other winding is the primary winding of the transformer, and signal transmitting means is electrically connected to the primary winding for transmitting signals to a remote receiving station.

3. Apparatus according to claim 2, wherein the signal transmitting means includes signal processing means for modulating a carrier signal in accordance with the measurement data received from a measuring instrument.

4. Apparatus according to claim 1, 2 or 3, incorporating a plurality of signalling units distributed along the drill string, each comprising a transformer having one winding electrically connected between respective first and second portions of the drill string and signal transmitting and/or receiving means electrically connected to the other winding, one of said units being a transmit-only unit for transmitting measurement data supplied to it by a measuring instrument and the or each other unit being a repeater unit for receiving a signal supplied to it along the drill string, processing the signal and transmitting the processed signal.

5. Apparatus according to any preceding claim, wherein the transformer is a toroidal transformer.

6. Apparatus according to any preceding claim, wherein the transformer and the transmitting and/or receiving means are accommodated within a chamber in the wall of the drill string.

7. Apparatus according to any preceding claim, wherein the signal transmitting and/or receiving means is adapted to be supplied by a battery.

8. Apparatus according to any preceding claim, wherein the first and second portions are the upper and lower portions respectively of a special telemetry sub, the portions being separated by an electrical insulator.

9. A telemetry sub for a drill string, comprising first and second portions electrically insulated from one another, a transformer having one winding electrically connected between the first and second portions, and signal transmitting and/or receiving means electrically connected to the other winding of the transformer, the first portion providing a current path for signals travelling between the transformer and a signal receiving and/or transmitting station at a remote location along the drill string when the telemetry sub is disposed in a drill string, and a return path being provided during drilling between the transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which a borehole is being drilled.

10. A telemetry sub according to claim 9, being a transmit-only sub for transmitting measurement data supplied to it by a measuring instrument.

11. A telemetry sub according to claim 9, being a repeater sub for receiving a signal supplied to it along the drill string, processing the signal and transmitting the processed signal.

12. A telemetry sub according to claim 9, 10 or 11, wherein the transformer and the transmitting and/or receiving means are accommodated within a chamber in the wall of the sub.

13. A telemetry sub according to claim 12, wherein the chamber is closed off from the outside by a cover.

14. A telemetry sub according to any one of claims 9 to 13, wherein the first and second portions are insulated from one another by an annular insulator which is generally S-shaped in radial section.

1 5. A method of signalling within a borehole while drilling, utilising a drill string having first and second portions electrically insulated from one another, which method comprises passing signals between signal transmitting and/or receiving means and a signal receiving and/or transmitting station spaced apart along the drill string by way of a transformer having one winding electrically connected between the first and second portions and the other winding electrically connected to the signal transmitting and/or receiving means, the first portion providing a current path for signals travelling between the transformer and the signal receiving and/or transmitting station, and a return path being provided between the - transformer and the signal receiving and/or transmitting station by way of the second portion and the earth formations through which the borehole is being drilled.

1 6. A method according to claim 15, which method comprises successively passing signals between a plurality of signalling units, each comprising a transformer having one winding electrically connected between respective first and second portions of the drill string and signal transmitting and/or receiving means electrically connected to the other winding, the first of said units being a transmit-only unit which transmits measurement data supplied to it by a measuring instrument and the or each other unit being a repeater unit which receives a signal supplied to it from a previous unit, processes the signal and transmits the processed signal.

1 7. Apparatus for signalling within a borehole while drilling, substantially as hereinbefore described with reference to the accompanying drawings.

1 8. A telemetry sub for a drill string, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

1 9. A method of signalling within a borehole while drilling, substantially as hereinbefore described with reference to the accompanying drawings.