GB2050770A - Transmitting signals from well to earth surface - Google Patents

Abstract

A method and apparatus for transmitting signals relating to borehole parameters from a well to the earth surface through an electrical cable. Before transmission of these signals to the earth surface through the cable takes place, signals having a relative high frequency are "stretched" over their time-axis by means of a time base expander (pulse stretcher) so as to reduce their effective frequency and overcome problems associated with increased attenuation at high frequencies.

Description

SPECIFICATION A method and apparatus for transmitting signals from a well to the earth surface through an electrical cable The invention relates to a method and apparatus for transmitting signals from a well to the earth surface through an electrical cable. Said signals are further processed at the earth surface to derive certain data of interest concerning the well.

It is desired to obtain information from wells, on-shore as well as off-shore, concerning a plurality of borehole parameters of interest in the well such, for example, as electrical conductivity, the velocity of sound and nuclear radiation of the earth formation and through interpretation of such measurements, as a result of long experience, it is possible to acquire data of interest for example concerning the presence of a valuable hydrocarbons producing stratum.

So it has long been the practice to log wells, that is to sense and measure various downhole conditions within a well and transmit the acquired signals to the surface of the earth.

Well logging operations as performed today utilize often cabletype logging procedures. In these logging procedures several possibilities may exist; for example in order to conduct the measurement of borehole parameters a measuring capsule (logging tool) is inserted into the well, which capsule senses and measures various parameters of interest and transmits the measured signals up to the earth surface through a cable.

An example of application of such a logging procedure is during drilling of wells. In such logging operations during drilling several logging procedures may exist; for example drilling is stopped and the drill pipe is first removed from the well. Subsequently the logging tool is inserted into the well. Another possibility is the application of continuous logging-while-drilling methods, using a telemetering system. Various telemetering methods and systems have been suggested for use in logging-while-drilling methods.

For example it has been proposed to transmit the acquired signals to the surface of the earth electrically just as in case of the above-mentioned logging procedure, in which the drill pipe is removed.

When using a cable in logging-while-drilling procedures, the drill pipe has been provided with a special insulated conductor and means to form appropriate connections for the conductor at the drill pipe joints.

Other techniques proposed for use in loggingwhile-drilling operations involve the transmission of acoustical signals through the drill pipe.

However, these methods require special equipment such as an acoustic generator and additional equipment and are therefore more complicated than a standard electrical cable.

In view of simplicity the use of a standard electrical cable is preferred in cable type logging procedures. However, in case of using a standard electrical cable the high frequency signal transmission of the cable has a low quality because such a cable usually is not specially designed for high frequency transmission. A special high frequency design would require high cost. The poor high frequency transmission of the cable is due to the fact that the cable normally used in logging operations produces attenuation of the signals, which are transmifled through the cable. It appears that the attenuation of high frequency signals is larger than the attenuation of low frequency signals.Therefore high-frequency signals are often of extremely low amplitude when received at the earth surface terminal of the cable and hence the information concerning the borehole parameters of interest which may be derived from these signals is not an optimum information.

It is therefore an object of the invention to provide a method and apparatus for transmitting signals from a well to the earth surface through an electrical cable without significant attenuation of the signals.

The invention therefore relates to a method for transmitting signals from a well to the earth surface through an electrical cable, said signals relating to a plurality of parameters of interest in the well, and said signals covering a range of frequencies, wherein, before transmission of these signals through the cable takes place, signals having a relative high frequency are "stretched" over their time-axis by means of a time base expander (pulse stretcher) and wherein the "stretched" signals thus obtained are subsequently transmitted through the cable to the earth surface. The stretching of the signals over their tirne-axis (time base expansion) may take place for example in a downhole logging tool, which sensed and measures downhole conditions.

Said logging tool may be located within a drill string.

It will be clear, that a certain ratio between the relative high frequency of the "original" signal relating to the sensed and measured downhole conditions and the frequency of the "stretched" signal, which is subsequently transmitted uphole exists. Said ratio may for example be about 20 in an advantageous embodiment of the invention, but may also be about 30 or more, in an alternative embodiment According to the method of the invention for example a 100 kHz signal, which is stretched over its time-axis to for example 5 kHz may be transmitted to the earth surface through a standard cable without significant attenuation.

Further the method of the invention may be used in an advantageous manner during a loggingwhile-drilling procedure.

The invention is based upon time-base expansion of a signal.

The principle of time-base expansion of signals is known and is simple: the "original" signal to be stretched over its time-axis is read by means of a certain clock-frequency originating from a clock, stored and subsequently read out by means of a clock-frequency, which is lower than the abovementioned first clock-frequency. In practice, a "piece of signal" is supplied to the input of a device, which stretches the "original" signal over its time-basis (time-base expander) and at its output said "piece of signal" appears in a delayed and stretched way. As an example of a time-base expander might be considered a taperecorder.

When a signal, being recorded with a certain tape velocity is played back with a lower velocity, timebase expansion takes place.

Time-base expansion may in practice be obtained with electronic means.

The invention also relates to an apparatus for sensing and measuring downhole conditions in a well and provided with one or more transducers, which provide signals relating to a plurality of parameters of interest in the well, said signals covering a range of frequencies, wherein said apparatus comprises a time-base expander (pulse stretcher) to "stretch" relative high frequency signals over their time-axis before they are transmitted to the earth surface through an electrical cable.

In an advantageous embodiment of said apparatus for sensing and measuring downhole conditions in a well the time base expander "stretches" for example a 100 kHz signal to for example a 5 kHz signal, said 5 kHz signal being transmitted to the earth-surface through a standard cable without significant attenuation.

A certain ratio between the frequency of the signal to be "stretched" at the input of the timebase expander and the frequency of the "stretched" signal at the output of the time-base expander exists, said ratio for example being about 20. However, said ratio may also be about 30 or more.

Preferably said apparatus for sensing and measuring downhole conditions in a well may be a downhole logging tool, which is provided with one or more transducers.

Time-base expanders (pulse stretchers) are known as such for well-defined applications, for example in a television set, in which they are applicated as a line delay circuit.

The present invention will now be described by way of example in more detail with reference to the drawings, in which Figure 1 shows curves, which represent the principle of time-base expansion of a signal; Figure 2 shows a block scheme of a time-base expander; Figure 3 shows a logging tool, which logging tool is used in a well logging procedure and a well drilling system, and especially a logging-whiledrilling system, in which the drill pipe is not removed.

Referring now to Figure 1 there is illustrated the principle of time-base expansion of a signal.

In Figure 1 curve a shows a possible form of a signal in this case a measured downhole signal.

The time-base of this signal should be expanded, which means that said signal is to be stretched over its time-axis, which axis is represented as the horizontal axis in figure 1.

For this purpose the signal is sampled in a suitable device in a sampling process by means of a clock signal originating from a clock. The form of said clock signal is represented by signal b, in Figure 1. After sampling the "original" downhole' signal a sampled signal c is obtained and is stored in a suitable device for example a shift register or a digital memory. To derive the desired time-base' expanded signal the stored sampled signal c is read out by means of a clock-signal d having a frequency which is lower than the frequency of the clock-signal b.

The read-out signal is represented in Figure 1 as a signal e and this signal can be considered as a sampling of a signal as represented by curve f. It will be clear that the curve f corresponds with the "original" curve a. However, the curve f appears later in time and is stretched over its time-axis.

The ratio between the two clock frequencies b and d determines the "stretch" (time expansion) of the original signal. These clock frequencies are chosen in such a way, that the "stretched" downhole signal can be transmitted through the cable to the earth surface without significant attenuation.

A block scheme of a time-base expander which for example may be applied in a logging tool is shown in Figure 2. A measured down-hole signal having a relative high frequency, the time-base of which should be expanded, is supplied through the input 1 of the time-base expander and an inputamplifier 2 to a suitable device 3, in which device the signal is sampled by means of a clock signal vt with a certain clock frequency originating from a clock (not shown) and stored, for example in a shift register or a digital memory. The upper limit of the relative high frequency is determined by the sample frequency. Storing the sampled signal may be done in one analog shift register; however, in an advantageous embodiment of the invention two parallel shift registers may be used, in which case a higher maximum sample frequency may be reached.

However, the use of a shift register is not essential; after sampling the signal may be converted in a digital number by means of an analog/digital convertor. In that case a digital memory may be used for storing the signal.

To derive the desired time-base expanded signal the sampled signal, which is stored in the device 3, subsequently is read out by means of a clock signal 2 originating from a clock (not shown) and having a clock frequency which is lower than the frequency of the clock signal (Pi The read out time-base expanded signal is supplied through an output amplifier 4 to the output 5 of the time-base expander and is subsequently transmitted through a cable to the earth surface.

If a digital memory has been used as a storing device, the time-base expanded signal may be converted into an analog voltage before being transmitted uphole.

Additional equipment which may be used in the embodiment of Figure 2 such as a device to sense and measure the effect of temperature on a shift register or a device for automatic recording and reproducing is known to those skilled in the art and will not be described.

Referring now to Figure 3 there is illustrated a well 1, which in this example is being extended through the earth's crust by means of, for example a rotary drilling technique. In this example an onshore drilling technique is shown. However, the present invention may also be used to transmit relative high frequency signals, which are acquired during an off-shore operation.

Drilling operations are carried out utilizing a drill bit 2 attached to the lower end of a drill string 3.

The drill string terminated at its upper end in a kelly 4, which is polygonal in cross section and extends through a rotary table 5 on the floor of the drilling rig. The kelly is received in the rotary table in a slidable torque-applying relationship by means of a kelly bushing (not shown). The rotary table is powered by a prime mover 6 through a suitable drive mechanism such as a chain drive 7.

The drill string is suspended from the derrick structure of the drilling rig by means of a hook 8 which is attached to a travelling block 9. The travelling block in turn is suspended by a suitable cable arrangement 10 from the crown block (not shown) which is located in the upper portion of the derricle structure. The kelly is connected to the hook through a rotary swivel 11 which permits rotation of the drill string relative to the hook and travelling block. Drilling liquid from a container 12 (commonly called a "mud pit") is circulated by a pump 13 through a conduit 14 into the swivel 11 and thence downwardly through the interior passage of the drill string to the bit 2.The drilling liquid then passes outwardly into the wellbore through appropriate ports in the drill bit and is circulated to the surface of the well through the annulus between the drill string and the wall of the well. At the surface the mud is withdrawn from the annulus through a conduit 1 5 and recirculated to the mud pit 12. The conduit 14 is equipped with a desurger system 1 6 which functions to suppress undesirable "noise" in the drilling liquid introduced by pulsations produced by the pump 13.

Located within the drill string 3 near the drill bit is a downhole logging tool 17, which comprises one or more transducers for sensing and measuring downhole conditions. The measured signals may range over certain frequencies.

The logging tool 1 7 comprises a time-base expander (pulse stretcher) (not shown) in which relative high frequency signals are stretched over their time-axis. The logging tool 1 7 comprising the time-base expander is connected by means of a cable with a suitable receiving equipment 1 8 at the earth surface, in which equipment the signals are further processed to produce the readout functions representative of the measured downhole conditions.

At the earth surface several ways to process the transmitted signals may exist; for example the transmitted signals may be "retransformed" to the original relative high frequency signal, from which signal information concerning downhole parameters may be derived; another possible way is not to "retransform" the transmitted signals and to maintain said signals as such at the earth surface; for, knowing the stretching over its timeaxis this can be taken into account in further processing the signals.

Said receiving equipment 1 8 is known for a man skilled in the art and will not be described.

Although the description of Figure 3 relates to a logging-while-dnlling procedure, the invention is not restricted thereto; also a logging procedure may be applicated, in which the drill string is removed and subsequently a measuring tool (logging tool) comprising a time-base expander, which is connected with the earth surface by means of an electrical cable, is inserted in the bore-hole, that may have been completed.

Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings.

Such modifications are intended to fall within the scope of the appended claims.

Claims (14)

1. A method for transmitting signals from a well to the earth surface through an electrical cable, said signals relating to a plurality of parameters of interest in the well, and said signals covering a range of frequencies, wherein, before transmission of these signals through the cable takes place, signals having a relative high frequency are "stretched" over their time-axis by means of a time-base expander (pulse stretcher) and wherein the "stretched" signals thus obtained are subsequently transmitted through the cable to the earth surface.

2. The method as claimed in claim 1, wherein the time-base expansion of the signals takes place in a downhole logging tool, which senses and measures downhole conditions.

3. The method as claimed in claim 2, wherein said downhole logging tool is located within a drill string.

4. The methods as claimed in any one of the claims 1-3, wherein the relative high frequency signals have a frequency of 100 kHz.

5. The method as claimed in any one of the preceding claims, wherein the signals are stretched over their time-axis to signals having a frequency of 5 kHz.

6. The method as claimed in any one of the preceding claims, wherein a certain ratio between the relative high frequency of the measured downhole "original" signal and the frequency of the "stretched" signal which is subsequently transmitted uphole exists, said ratio being about 20.

7. The method as claimed in any one of the claims 1-6, wherein the method is used during a logging-while-drilling procedure.

8. An apparatus for sensing and measuring downhole conditions in wells and provided with one or more transducers, which provide signals relating to a plurality of parameters of interest in the well, said signals covering a range of frequencies, wherein said apparatus comprises a time-base expander (pulse stretcher) to "stretch" relative high frequency signals over their time-axis before they are transmitted to the earth surface through an electrical cable.

9. The apparatus as claimed in claim 8, wherein the relative high frequency signal has a frequency of 100 kHz.

10. The apparatus as claimed in any one of the claims 8 or 9, wherein the "stretched" signal has a frequency of 5 kHz.

1 The apparatus as claimed in any one of the claims 8-1 0, wherein a certain ratio between the relative high frequency of the measured downhole signal and the frequency of the "stretched" signal exists, said ratio being about 20.

12. The apparatus as claimed in any one of the claims 8-11, wherein said apparatus is a downhole logging tool, provided with one or more transducers.

1 3. Method substantially as described in the specification.

14. Apparatus substantially as described in the specification and illustrated in Figures 1-3 of the accompanying drawings.