DE2758770C2 - Device and method for transmitting acoustic signals - Google Patents

Description

The invention relates to a method and a device for transmitting acoustic signals via a drill pipe string located in a borehole according to the preamble of claims 1 and 12 respectively.

The need for devices to transmit well data to the surface during drilling has existed in the oil industry since the advent of modern drilling techniques. With recent drilling to ever deeper depths and with technological advances enabling the determination of well parameters that can be usefully used outside the well during drilling, the need for such a telemetry system has become ever greater. As a result, efforts in the oil industry to develop such systems have increased accordingly. For example, such a need arises when the driller requires a means of communication between the drilling site and the surface, for example for information regarding the formation being drilled. Since the optimum combination of speed and weight on a drill bit varies considerably depending on the type of formation being drilled, such as sand, shale, limestone, chert, etc., the driller is unable to select the optimum drilling speed without this information.

Attempts have been made to develop systems that record while drilling, and one such device is described in U.S. Patent No. 2,755,431. However, such a system has not yet found widespread use in the art for a variety of reasons. Some systems use cables to transmit information from the borehole to the surface. This requires completely pulling out the cable or making cable connections at the surface each time a section of pipe is added. This is a laborious and time-consuming operation and has not been widely adopted. Attempts have also been made to create electrically conductive paths in a pipe string using pipe couplings that have electrical conductors. However, such systems have not been developed to the point where they would be suitable for large-scale use. Although the technical feasibility of such a system has been demonstrated, it would require special drill string, which is very expensive.

The deviation of the borehole from the vertical or the direction of this deviation is another essential parameter for drilling. Such direction monitoring information is essential in drilling, where a deviation is intended to drill into reservoirs which are inaccessible or expensive to reach by vertical drilling. An early example of such drilling is the California drilling fields at Huntington Beach and Ventura. These drilling fields are located on the Pacific coast. Most of the reservoirs are under the sea. When these fields were developed in the 1930s and 1940s, it was necessary to develop methods and tools for controlling directional drilling so that oil could be drilled into the submarine using onshore drilling rigs. Directional drilling was, as it is now, very complicated and expensive because of the lack of facilities for surveying and transmitting data from the drilling site to the surface. The data is obtained by photographic or chemical means and transferred to equipment which is brought to the surface either by pulling the pipe or by placing the equipment at the end of a wire or cable which is pulled out of the well, interrupting the drilling process. This is costly and time consuming and is particularly disadvantageous in modern drilling techniques because of the sometimes extreme depths of drilling, which necessarily means that it takes a long time to obtain the data by wireline. The high cost of drilling rigs, particularly in difficult environments such as offshore where drilling time is extremely expensive, is also a major factor when interrupting drilling is necessary to obtain the desired data.

In the 1940s a number of companies recognized the economic potential of a telemetry system and began research into it. This work was carried out independently of one another, but after examining many possible transmission methods, they came to the same conclusion that the most promising method would be sound transmission through the metal of the drill pipe. Electromagnetic transmission or radio transmission was considered a poor secondary solution because of the strong attenuation of such signals in the earth's formations. Knowing that sound attenuation in steel is very low, it was logical to assume that sound signal transmission through the metal wall of the drill pipe would be relatively easy. However, this has been shown not to be the case. In 1948 a system was built to investigate the possibilities of acoustic drill pipe telemetry. The system consisted of a pulse sound source located in the borehole and a surface assembly for receiving the transmitted sound signal and measuring its amplitude in each of three frequency bands. The sound source contained a battery-powered motor that wound a spring. When the spring was fully wound, it was released and actuated a mass to deliver a loud hammer blow to the end of the drill pipe. The receiving device consisted of an accelerometer attached to the drill pipe, the output of which was connected to an amplifier, which in turn was connected to three bandpass filters to split the energy spectrum into low, medium and high frequency bands. The results of this field study were disappointing. The attenuation varied somewhat between the three bands, but was so high even in the best range that no further attempts were made in this direction. It was concluded that acoustic telemetry was not possible with the technology available in 1948. The telemetry research project was abandoned and was not resumed until 1969 when it was realized that the use of amplifiers would be beneficial in overcoming the high attenuation.

At this time, research was being conducted into the field of casing perforation. Casing perforation is an essential step in the completion of oil and gas wells. It involves drilling and casing the well through the oil or gas producing sand, as opposed to the earlier and less satisfactory practice of placing the casing just above the gas or oil producing sand and drilling into it to complete an open hole. There was interest in radioactive (gamma) recording as a means of recording cased wells, both to control the casing perforators more accurately and to identify other potential producing zones behind the casing. In 1948, a well recording research laboratory was established and one of the major projects of this laboratory was well site telemetry. The research program began similarly to that described above. After studying the alternatives, acoustic drill pipe telemetry was again chosen as the most promising solution for determining drill pipe acoustic attenuation. The final tests of this program showed that it was not possible to carry out acoustic drill pipe telemetry in this way. This test was carried out as follows: The borehole sound source consisted of a set of containers arranged so that each time they were activated they would drop a section of pipe sleeve approximately 1 m. At the surface a geophone was used as a detector which fed the signal into a system comprising a seismic amplifier and a recorder.

The attenuation thus determined was so high that it was felt that transmission of sound through the drill pipe was impractical. Mud pulse transmission was therefore adopted, and the greatly reduced data transmission possible with the mud pulse system was accepted. Work on the mud pulse telemetry system continued, and attempts were made to market the system as a means of recording while drilling. The conclusion that mud pulse telemetry was the only possible approach evidently influenced subsequent telemetry research to the extent that most of the research now being carried out in the area of drill string telemetry focuses on the technique known as mud pulse telemetry. The mud pulse system requires more complex components and has a slower data rate compared to the potentially cheaper and faster acoustic drill pipe system.

Despite the realization that the attenuation values are high, the applicant resumed research in the field of acoustic drill pipe telemetry in 1968 in the hope of being able to overcome the difficulties by using a number of amplifier stations. Based on the attenuation measurements in 1948, which were about 12 dB per 300 m, it seemed possible to use a system of amplifiers spaced along the drill pipe, each receiving data from the station downstream at one frequency and transmitting it to the next station upstream at a different frequency. To do this, a transmitter-amplifier system was built. To achieve maximum discrimination against noise, transmission was digital, using either a single crystal-controlled frequency switched on for 1 and off for 0, or, in some cases, just a pair of closely spaced frequencies, one frequency being 1 and the other 0. The new system differed from the 1948 attempt only in that discrete frequencies were used instead of a broadband source such as that provided by the weight and spring. To use the multiple amplifier system, three transmission frequencies were required for the on-off logic or six transmission frequencies for the dual frequency logic. An arbitrary choice was therefore made. For the dual frequency logic system, the following frequency pairs were chosen: 860-880 Hz, 1060-1080 Hz and 1260-1280 Hz. All of these frequencies were within a band for which the 1948 test showed an attenuation of 10 to 12 dB for a range of 300 m. The first field test was carried out using the 860-880 Hz band. The test confirmed the assumed attenuation value of 10 to 12 dB per 300 m and showed that the amplifier system could be operated as planned.

However, when the band was moved to the 1060 to 1080 Hz band, it was found that the attenuation was so great that no satisfactory data could be obtained to determine the exact rate of attenuation. Over a period of just over a year from these initial tests, a number of other frequencies were tested, but none were found to be equal to the 860 Hz band. It should be noted that there was no basis for selecting one frequency over another; the selection was entirely arbitrary. It was also found that the attenuation at the 860 Hz frequency varied considerably from one test to another. This seemed to depend on the condition of the drill pipe, but it was not understood how. In new drill pipe or in very good condition, the attenuation at 860 Hz was 10 to 12 dB per 300 m, while in badly worn drill pipe the attenuation was often 30 dB or more per 300 m. In the search for an explanation of these results, the reference "Passbands For Acoustic Transmission In An Idealized Drill String", Barnes and Kirkwood, Journal of Acoustical Society of America, Vol. 51, No. 5 (1972), pp. 1606-1608, was found. This reference gives a theoretical analysis of a drill string as an acoustic filter and shows that there should be a number of relatively narrow bandpasses separated by wider stopbands in which no sound transmission can occur. This publication seemed to provide some explanation for the strange results of the applicant's experiments. However, it turned out that the applicant's most successful test frequency, i.e. 860 Hz, fell exactly within one of the stopbands of the reference. Other frequencies that the applicant had also investigated, for example 760 Hz, should be bandpasses with good transmission according to the reference, which was contrary to the experimental data. Interest in the theoretical analysis of the literature was therefore lost and attempts were made again to find three transmission bands based on a random selection. However, this random selection procedure proved to be very laborious, disappointing in its results and very time-consuming to carry out.

The above statements show that the experiments carried out so far in the field of acoustic telemetry in a drill pipe have encountered great difficulties.

The preamble of claims 1 and 12 is based on a system previously developed by the applicant for transmitting acoustic signals via a drill pipe string, as described in DE-OS 24 40 538. In this system, a transmitting and receiving device are coupled to the two ends of the pipe string, between which the transmission of acoustic signals serving as data carriers takes place. A frequency-modulated signal is used for this data transmission, with the signal modulation usually alternating between two fixed frequencies separated by approximately 50 Hz on a 1 kHz carrier. Several relay stations are arranged along the length of the drill pipe string, which amplify the signals and implement their transmission function in order to avoid interference caused by the superposition of the incoming and outgoing signals. The acoustic signals emitted by the transmitter and the relay stations have carrier frequencies of 800 Hz, 1000 Hz and 1200 Hz respectively. However, these transmission frequency ranges of the acoustic signals are not adapted to the frequency-dependent passbands of the drill pipe string.

US patents 39 30 220 and 38 89 228 also describe acoustic borehole telemetry devices, which can be traced back to the applicant and which essentially have the same structure as that known from DE-OS 24 40 538. These systems also work with transmission frequencies of 800 Hz, 1000 Hz and 1200 Hz.

In addition, various methods for remote measurement of borehole data during drilling are described in the ERDÖL-ERDGAS magazine from August 1976 on pages 262-263. Acoustic methods are also mentioned, although neither the structure of such a telemetry device nor specific frequency ranges for the transmission of the acoustic signals are specified.

In view of this state of the art, the object of the present invention is to provide a device and a method for transmitting acoustic signals via a drill pipe string, wherein the transmission takes place with minimal signal attenuation and low interference sensitivity.

This object is achieved by a device or a method having the features of the characterizing parts of claims 1 and 12. According to this, the transmission frequency ranges of the acoustic Signals are adapted to the passbands of the drill rod pipe string, whereby the lowest frequency of each transmission frequency range is selected approximately so that the length of a pipe string section at this frequency corresponds to half the wavelength of the acoustic signal. The acoustic signal transmission with the system according to the invention thus takes place in frequency ranges that correspond to the natural passbands of the drill rod pipe string used, ie are matched to the pipe length and the pipe condition, so that an optimal adaptation to the drill rod used and to the conditions of the drilling operation can be achieved.

The transmitted acoustic signal can be encoded or modulated to transmit information from one location to another along the pipe string, for example from the bottom of the borehole to the surface.

Preferred embodiments and developments of the invention are specified in the subclaims. They relate in particular to the specific frequency ranges for lengths of pipe string sections used in practice, the arrangement and construction of the transmitting and receiving devices, and amplifier devices arranged along the drill pipe string, which simultaneously carry out frequency conversion.

Preferred embodiments of the invention are explained in more detail with reference to the drawings. In the drawings,

Fig. 1 shows schematically an arrangement for carrying out an acoustic pipe string telemetry test,

Fig. 2 is a diagram of the test data measured with the arrangement according to Fig. 1,

Fig. 3 and 4 Diagrams of acoustic passbands derived from a comparison of the measurement results with theoretical data,

Fig. 5, 6 and 7 diagrams illustrating the influence of the flexibility and stiffness of the pipe string connectors on the acoustic passbands,

Fig. 8 is a schematic view of an acoustic drill pipe telemetry system, with the electronic devices of the system at the shaft bottom and at the surface shown in block diagrams,

Fig. 9 is a block diagram of an amplifier device of the telemetry system according to Fig. 8 and

Fig. 10 is a schematic representation of the telemetry system according to Figs. 8 and 9 when using a plurality of amplifier devices and when performing frequency mixing.

The above-mentioned Barnes and Kirkwood reference gives theoretical data and a theoretical analysis of a drill string as an acoustic filter. The reference shows that the drill string has a number of relatively narrow pass bands separated by wider stop bands in which no sound transmission can occur. In the above discussion of the circumstances leading to the invention, it has been shown that the theoretical data of this reference do not correlate with the data obtained from experiments. Additional experiments were therefore carried out to grasp the solution to the problem of acoustic transmission in a drill string.

It was considered that if the drill pipe is to act as a tuned transmission line capable of passing certain frequencies and blocking others, this property can be determined in a transient test analysis as is done for electrical transmission lines. A pulse test was designed by introducing a strong acoustic pulse of short duration into one end of a drill pipe string suspended vertically in a borehole. The test arrangement is shown schematically in Fig. 1. The upper end of the pipe string 11 carries the spigot end 13 of a threaded connection having a plate 15 welded to the upper end which is an acoustic coupler acting into the pipe string. The lower end of the pipe string is similarly received in the end 17 of a threaded connection having a plate 19 at its lower end. Attached to the plate 19 is a chamber formed by a section of pipe 21 . At the lower end of the chamber is mounted a threaded cap 23 having an O-ring seal 25. A conventional crystal accelerometer 27 , attached directly to the plate 19 , extends downward into the chamber 21 and is housed therein. Connected to the output of the crystal 27 is a preamplifier 29 which adapts the relatively low level output of the crystal to the relatively low impedance input of a tape cassette recorder 31 also located in the chamber 21. The recorder uses a cassette tape having a playing time of 60 minutes for one side. The recorder is turned on at the surface and run down the pipe into the borehole so that the total test time from that point is 60 minutes. After reaching a pipe depth of 95.4 m (313 ft) in the borehole, the first sound transmission test is carried out. A series of sound pulses is produced by striking a ball peen hammer 33 sharply against plate 15 at the top of the test string in the following manner. First one pulse is produced. Then several seconds are allowed to elapse before a series of 10 pulses are produced on the plate, spaced 1 second apart. If the ball peen hammer is struck sharply and allowed to spring back, it produces a sharp pulse of less than 1 millisecond and of a relatively high energy level. After the first series of pulses, additional pipe sections are added to the string so that the recorder can be located at a depth of 160.6 m. After an initial code of two pulses identifying the second test, the 10-pulse count is repeated. These measures are repeated at depths of 180.1 m (1919 ft), 381.9 m (1253 ft) and 477.3 m (1566 ft). Then enough time has passed that the tape has run out and no additional values can be obtained. The impulse test produces an impulse with energy up to a maximum frequency, which is determined by the sharpness and duration of the impulse. For example, if the hammer impulse has a duration of 1 millisecond, the impulse contains energy that includes all frequencies from 0 to 1000 Hz. The ball hammer method of the tests described also produces frequencies above 1000 Hz.

After recording the pulse data, the recording device is switched off again by pulling the tube available. After recording this raw data on tape, the problem of data analysis begins. The sound signal recorded on the cassette tape is in a "time domain", that is, the signal recorded on the tape is a continuous recording of amplitude over time.

In order to analyze the frequency spectrum of the recording, it is necessary to convert the recording into the frequency domain through the mathematical process of a Fourier transform. This process is too complicated to be done by hand and requires the use of high-speed digital computers. The "time domain" data must therefore be converted to digital form for input to the computer.

Seismic data processing equipment frequently uses the Fourier transform technique. Thus, many geophysical data processing centers have at their disposal the equipment for digital conversion and analysis of seismic recordings. One problem with using these facilities to analyze acoustic data is that seismic recordings typically contain frequencies only in the range 0 to 100 Hz, with little or no useful data above 100 Hz. When digitally converting any data, it is required that the time increment between two points of digitization be short enough to produce at least two points per complete cycle at the highest frequency of the recording, otherwise errors would be introduced which could not be later corrected. Geophysical data are usually digitized every 2 milliseconds. If any frequency has a cycle that is completed in less than 2 digitizing intervals, fewer than two points on a frequency cycle will be obtained, so the waveform is not adequately described. This yes-no-yes frequency level is called the Nyquist frequency and is 500 Hz in geophysical data processing equipment. In order to reduce the number of digital values to the minimum required for recording and to exclude any possibility of exceeding the Nyquist frequency, all seismic digitizers send the input data through a very sharp low-pass filter designed to exclude all frequencies above about 250 Hz before digitization. Since the invention aims to examine passbands of up to 2500 Hz, this limitation must be eliminated by frequency filtering. There was no other digitization option available. A special digitizing device for the purpose of the invention could not be built for cost reasons. It was found that the recordings made from the sound pulses in the situation described can be scaled up to the seismic frequency range by recording the pulses at a tape speed of 19.05 cm/s, then playing the tape at 4.76 cm/s and making another recording on the cassette tape. This procedure reduces all frequencies on the first tape by a factor of 4. However, this is still not enough to bring the 2500 Hz band below the seismic digitization limit of 250 Hz. Therefore, the second cassette recording is re-recorded at a speed of 19.05 cm/s and played back on the cassette tape at both 9.53 cm/s and 4.76 cm/s, thus obtaining two sets of recordings with total frequency reductions of 8:1 and 16:1 respectively. Digitization and processing at both tape speeds is necessary because at the 16:1 frequency division the lower frequencies of interest, below 500 Hz, are below the low frequency response of the tape recorders, which is about 30 Hz. On the other hand, the 8:1 reduction is not sufficient to bring the 2500 Hz range into the passband of the seismic digitizer.

By the simple measures described, it is possible to shift the impulse test data into the seismic frequency domain and digitize them so that they can be transformed into the frequency domain and analyzed using conventional seismic data processing techniques, provided that a suitable frequency multiplier is used for the processed data to compensate for the slowing down process. This lengthy process was applied to a number of impulse test recordings made during the tests described above.

As can be seen in Fig. 2, the computer output of this analytical process was plotted as a spectral power density versus frequency curve for each of the five depths. The results of this analytical process are very interesting. Even at the shallow depth of 95.4 m (313 ft), preferred frequency passbands are evident as peaks on the curves of Fig. 2. As more pipe is added to the string, up to the maximum of 477.3 m (1566 ft), these passbands become sharper, with transmissions outside these bands dropping to almost zero.

The authors Barnes and Kirkwood of the reference described at the beginning were qualitatively correct in their prediction that a drill pipe behaves like a mechanical filter that passes certain frequency bands and blocks other frequency bands. Fig. 3 shows a comparison of the data measured by the inventors with the theoretical data of Barnes and Kirkwood for a drill pipe with a length of 9.45 m. If one compares the theoretical bandpass frequencies with the measured data of the impulse test, as shown in Fig. 3, one can see that the band points according to the Barnes and Kirkwood reference are almost completely out of phase with the measured data. This is especially true for the frequency range from about 600 Hz to 1500 Hz, which is the preferred range for acoustic telemetry, ie in this range there is almost no agreement between the prediction according to Barnes and Kirkwood and the measured data. It is found that in the range 480 Hz to 1740 Hz all the blocking bands in the measured data lie entirely within the passbands predicted by Barnes and Kirkwood. Similarly, the blocking bands according to Barnes and Kirkwood lie almost entirely within the passbands measured in the actual drill pipe tests. Since the passbands are in each case wider than the adjacent blocking bands, there is necessarily an overlap between the measured and theoretical passbands. This is clearly coincident in view of the complete lack of agreement between the measured and calculated blocking bands.

As might be expected, the boundaries between the pass and block bands are not as sharply defined as in the theoretical paper by Barnes and Kirkwood. This is clearly evident from the considerable attenuation in the edges of each pass band. Although only five pass bands were clearly identifiable in the measured data, there is a pattern in the arrangement which indicates that other pass bands also exist. The lowest frequency f _A of each pass band can be approximately determined by the following formula: °=c:30&udf54;H&udf53;vu10&udf54;&udf53;vz2&udf54;&udf53;vu10&udf54;where n is an integer and the speed of sound v _S in the drill pipe is 5318.8 m/sec. The fundamental frequency f _A is thus determined such that one length of the drill pipe is half the wavelength at this frequency. The average connection length of the drill pipe used is 9.39 m (30.8 ft) excluding the thread. This results in a fundamental frequency of
f _A = @W:5318.8¤m/s:2ó9.39¤m&udf54; = 283 Hz.
It can be seen that the lower frequency ends of the five passbands determined in the experiments fall very close to 1, 2, 3, 4 and 5 times this frequency.

In view of this recurring pattern, it is clear that a lower transmission band must also exist with an output frequency of 0x283 Hz. This band extends to 0 Hz since it is clear that the drill pipe transmits "DC" displacements without attenuation. This "fundamental" passband is lost in the analysis because of the division of the frequency by 8 or 16 as explained above. Even division by 8 would place the appropriate center frequency of this lowest passband at 17 Hz, which is well below the lower frequency response of the cassette tape recorder used for this purpose. It is also expected that transmission bands will occur at higher multiples than 5 times the fundamental frequency. These transmission bands are weaker since the natural attenuation increases with increasing frequency.

In a separate test, satisfactory transmission was observed down to a depth of 110 m (700 ft) using a frequency of 2304 Hz, which is in the passband with the lowest frequency of 2264 Hz, corresponding to 8 times 283 Hz.

The width of the passbands is somewhat imprecise because there is a gradual roll-off rather than a sharp boundary as is present in the definition of passbands. In any case, the preferred operating range is in a 150 Hz band starting 20 Hz above the output frequency f _A of each passband calculated by the formula above. The 20 Hz spacing of the band eliminates the slopes that occur at the edges of the passbands. Of course, telemetry can be carried out in this slope range, but there is a little attenuation in the 150 Hz band above this distance. Due to the less attenuation at the lower frequencies, the lower frequency passbands are somewhat wider. Therefore, a certain transmission can be expected plus or minus 100 Hz from the center frequency, while passbands above 2000 Hz can be narrower.

The location of the initial frequency of each passband is not fixed but is a function of the length of the individual joints of the drill pipe. The initial frequency locations given above are accurate for the typical drill pipe lengths used in the petroleum industry, i.e., 9.6 m (31.5 ft) including threaded joints. Some offshore drilling rigs use 13.7 m (45 ft) drill pipe. In such rigs, the transmission frequency shifts because there is no one set of frequencies that is optimal for both 9.6 m and 13.7 m pipes. For 13.7 m pipes, the fundamental frequency is 196 Hz. The frequencies for the passbands are multiples of this frequency. Assuming that the preferred passbands fall in the range of 500 Hz to 1500 Hz, the corresponding 3, 4, 5, 6, 7, and 8 times of 196 Hz form the lower end of the passbands at frequencies of 588, 784, 980, 1176, 1372, and 1568 Hz, respectively.

Analyzing the discrepancy between the theoretical analysis of the drill pipe transmission passbands according to Barnes and Kirkwood and the measured data from the impulse tests, a discrepancy between the theoretical predictions and the measured data is found by comparing the interval between the center frequencies of adjacent passbands. In the measured test data this interval is 270 Hz for a pipe of 9.6 m, while the corresponding interval calculated from the theoretical analysis is 310 Hz. In seeking an explanation for this difference, it is found that Barnes and Kirkwood used a factor of 6000 m/s for the speed of sound in the drill pipe. This is the speed usually assumed for structural steel in a form in which all dimensions are substantially the same. However, it is known that the speed of sound in long thin bars is considerably lower and is about 5200 m/s. Inserting this value for the speed of sound into the Barnes and Kirkwood equations for the pressure waves, the results shown in Fig. 4 are obtained. A comparison of these two curves shows that the interval between the central frequencies of the transmission bands is almost the same. However, the measured and theoretical data still do not agree in that there is a large horizontal shift with respect to the position of the central frequency of the passbands. This shift is sufficient for the stop bands of the theoretical data to cover almost half the width of each of the measured passbands. No way has been found to correct the parameters according to the Barnes and Kirkwood model in such a way as to eliminate this error. This This fact, together with the measurements of the test program, led to the conclusion that the model of drill pipe behavior underlying the theoretical data contains a fundamental error.

The Barnes and Kirkwood model of drill pipe consists of a length of drill pipe of uniform cross-section made using welded joints of considerably larger cross-section. In this model, these joints are much stiffer than the pipe. This periodically repeated discontinuity in stiffness produces the pattern of transmission and barrier bands predicted by the theoretical data. Although the most obvious difference between the welded joints and the pipe is the increase in size and mass, another difference is that the joint includes a threaded joint. The acoustic properties of the threaded joint are very difficult to analyze. It appears that the threaded joint makes the joint more compliant than the drill pipe rather than stiffer. One reason for believing that the thread, rather than the additional metal, is the influencing factor comes from test results on heavily worn drill pipe. Before determining the correct position of the passbands according to the method described above, many tests were carried out at 960 Hz, a frequency corresponding to the lower end of a measured passband. When the pipe was in good condition, the tests showed that satisfactory transmission could often be achieved at this frequency. When the pipe showed severe wear, the results were consistently negative. The two most obvious effects of wear on pipe joints are a noticeable reduction in the outside diameter of the joint and an increase in the play of the threaded joint. The outside diameter of the joints, which is considerably larger than that of the drill pipe itself, is reduced by the rotation of the pipe in contact with the walls of the well during drilling. If the extra metal of the pipe joint were the determining factor in blocking certain frequencies, one would expect that selective removal of metal from the pipe joints would reduce this effect and give nearly constant transmission at all frequencies. On the other hand, if the greater compliance in the thread were the determining factor, thread wear would be expected to increase the compliance, making the passband boundaries sharper and the cutoff at other frequencies greater. Experimental data clearly show that the latter explanation is closer to the true conditions than the former. Based on these results and to confirm the theory developed from these experiments, a computer program was written to analyze the properties of drill pipe string in which the joints are more compliant than the pipe body. Since no way is known to calculate the relative compliance of the pipe joint and the pipe, this quantity was introduced as one of the variables in the program. Fig. 5 shows a comparison of the measured values with the computer predictions at two different compliance ratios and at two different reciprocal stiffness ratios. At the compliance ratio of 2:1, the size and location of the transmission bands agree fairly well with the experimental data. At a compliance ratio of 10:1, the transmission bands become much narrower. In fact, they are too narrow for practical telemetry with multiple amplifiers. This confirms in theory the earlier observations that severely worn threads prevent transmission at frequencies near the edge of the transmission band. A pipe with threads 10 times more compliant than the pipe body cannot mechanically support itself during drilling, so this extreme situation is unlikely to occur in practice.

It has been found that by using a more realistic sound velocity in the drill pipe, say 5200 m/s, and taking into account the fact that the threaded joint of the pipe joint is more compliant and not stiffer than the drill pipe, and by using these differences in the mathematical equation of Barnes and Kirkwood, data can be obtained which give a better agreement between the theoretical and experimental data. This comparison is shown in Fig. 6. Although it is known that the threads of the pipe joints in a pipe string are more compliant than the drill pipe, it is not possible to calculate how much more compliant they are. Therefore, in order to obtain a model for the computer, a number of ratios were used. By trial and error it was found that a compliance ratio of 7:1 gives a range which agrees fairly well with the experimental data. From Fig. 2, the difficulty of extracting an accurate bandwidth from the test data can be appreciated, since the amplitude gradually falls off at both ends of each band. There is therefore a considerable margin of error in the compliance ratio of 7:1. This ratio undoubtedly changes with the age of the pipe. The threads of the pipe become more compliant as a result of wear, while the stiffness of the pipe body does not change significantly. In the calculations made, the speed of sound was shifted upward by an amount of 122 m/s (400 ft/s). This measure was taken to adjust the calculated passbands to best agree with the measured data. This change is only 2% from the handbook value for the speed of sound in long thin rods. The change is in the direction of the speed of sound in a solid steel body. It is not known whether this difference reflects a real difference in the speed of sound in the pipe compared with thin rods or whether it indicates an error in the data. An error in the data of 2% is quite possible in view of the multiple acquisition processes required to adapt the measurement data to the seismic data processing equipment used to analyze the frequency array.

When using the computer model to determine the optimum compliance ratio, interesting and surprising findings emerge. As expected, increasing the compliance ratio narrows the passbands, but it is surprising that the change is entirely in the high end of each passband. The low end does not change at all. This can be seen by comparing the dashed lines in Fig. 6 for a compliance ratio of 20:1 with the solid lines. lines for a ratio of 7:1. It is found that the lowest frequency limit of each passband falls exactly on a multiple of a frequency for which the length of a connected drill pipe corresponds to half a wavelength. This frequency can be calculated as follows:
Fundamental frequency = @W:5318.8¤m/s:2ó9.39¤m&udf54; = 283 Hz,
where 5318.8 m/s is the speed of sound and 9.39 m is the length of the pipe excluding the threads. It is noted that the successive passbands begin at frequencies that are 0, 1, 2, 3, 4, 5, 6, etc. times this frequency. As shown in Fig. 6, this low frequency end point does not change with compliance ratio. Only the high frequency end shifts as the compliance ratio changes.

Fig. 7 shows the influence of the drill pipe length on the position and width of the calculated passbands. The lowest first curve of Fig. 7 is for a drill pipe with a length of 9.5 m (31.3 ft). The second curve for 9.1 m (30.0 ft) was assumed to be the probable lower length limit for standard drill pipe. The third curve is for a drill pipe with a length of 13.7 m (45.0 ft) as is sometimes used on drilling rigs in near-shore waters. It is interesting to note the position of 860 Hz on the first and second curves in view of the erroneous attenuation values that have resulted for this frequency. For a drill pipe with a length of 9.5 m, the frequency of 860 Hz is safely within the passband. For a drill pipe with a length of 9.1 m, the lower limit of the band has shifted to 890 Hz. It may be that a so-called bad tube from the earlier tests, which showed a strong attenuation at 860 Hz, was actually just a "short" tube.

Fig. 8 shows a schematic of a telemetry system for the purposes of the invention. A drill pipe string 35 is suspended in a borehole and has a plurality of pipe sections (not shown) which are connected by threaded pipe connections in a conventional manner.

A series of amplifiers 37 , shown schematically, are installed at regular intervals in the tubing string. The function of each amplifier is generally to pick up or receive an acoustic signal from the drill pipe string, amplify it and transmit it further along the pipe as an acoustic signal.

A wellbore parameter sensor 39 outputs an analog signal which is converted to a digitally encoded signal by an analog-to-digital converter 41. An example of such a sensor is a device for determining the orientation of a wellbore using an air gap control tool. The signal may also be generated in the form of pulse width data which is also converted to digital data for transmission in the system. The signal generated by the sensor is always fed to an analog-to-digital converter which converts the analog voltages to an encoded digital signal using "1"s and "0"s for all information transmissions. The output of the A/D converter 41 is fed to a shift register 43 which receives the digital signal and outputs the information to be transmitted in a timed sequence in conjunction with a timing control mechanism 45 . The output of the shift register is connected to a switch 47 which is operated by an oscillator 49 which in turn is operated at the desired transmission frequency which is within the passbands described above. The output of the A/D converter 41 and the shift register 43 is either "on" or "off" according to the digital encoding "1" or "0". When "on" or "1" comes from the shift register, the switch is operated so that the output signal of the oscillator is fed to a power amplifier 51 which in turn amplifies the power of the oscillator signal. The amplified signal is fed to a sound source 53. The sound source is an electromechanical device which converts the electrical energy into acoustic energy which is then applied to the drill pipe. Such a sound source can be a fixed frequency device or a crystal controlled device. One type of sound source uses a coil which, when energized by electrical energy at, say, 920 Hz, causes a rod to oscillate lengthwise at a frequency of 920 Hz in the coil. This motion is directed into the tube to produce a pressure wave at a frequency of 920 Hz. The analog data received by the detector is thus converted into a binary code, which in turn is converted into an acoustic tone which is transmitted only when "1" or "on" appears in the data. This transmission of the tone is for a fixed period of time and within a clocked sequence to decode a decoding at the surface by means of an adapted clocked decoding mechanism.

A timing control system suitable for the purposes of the invention may operate as follows: The time allowed for each data bit is 200 milliseconds. When "1" is transmitted, the signal is on for 100 ms, the remaining 100 ms are used for decay in the pipe. If the next digit is also "1", the signal is again transmitted for 100 ms, while 100 ms is off. If the next signal is "0" or "off", no signal is passed through, so it is quiet for 200 ms. To have a reference time, a synchronous signal is used. Such a scheme allows 8 bits for a word, so that the 200 ms intervals are repeated eight times, the ninth digit then being in the form of a parity bit. The logic circuit is designed so that when the "1"s in the 8-bit data stream add up to an even number, the ninth bit is given a "1" or "on". If the "1"s in the 8-bit data stream add up to an odd number, the ninth bit or parity bit is "0", meaning no signal passes through. Thus, each word in the scheme consists of 8 bits plus a parity bit. The parity bit provides a means of checking for errors. In fact, if the odd-even scheme fails in the presence or absence of the parity bit, it is seen that signals have been lost in the transmission. When 9 words (8 bits plus parity bit) have passed through, a discrete synchronous signal, for example in the form of a passing time frame or a series of "1's", etc. This system requires a minimum of power, since the sound source is only activated when a data item in the form of a "1" or a parity bit passes through. Power in the system is continuously used only for driving the clock control mechanism or for other devices which have a low power consumption. Thus, the system which uses only a battery as a power source can be kept in operation for a considerably longer time than, for example, a system which constantly transmits a passband frequency and has devices for modulating the signal depending on the data information being measured.

When the acoustic signal is applied to the pipe, it creates a pressure wave that travels in both directions in the pipe. The amplifiers 37 in the pipe string are spaced apart and receive the acoustic signal when it is still strong enough to be easily detected. The system of amplifiers operates to detect "1" or "on" and then re-emit a signal at a different frequency when activated by the acoustic signal representing a "1".

In Fig. 8 there is shown schematically the surface equipment for receiving an acoustic signal emitted from a sound source either at the well site at the bottom of the drill string or at an amplifier station 37. In either case the acoustic signal is received in the form of a pressure wave in the pipe at the surface by a signal pickup device or acoustic receiver 71. The receiver 71 may be in the form of a crystal accelerometer which converts the acoustic signal into electrical energy. A preamplifier 73 increases the amplitude of the electrical signal from the receiver on the pipe for further processing at the surface. This electrical signal is further fed via a cable or radio link to a decoding or demodulating section having a narrow band filter which only passes the frequency from the preceding sound source and is tunable to such a frequency as to eliminate as much noise from the signal as possible. The filter 75 passes this so-called pure data to a synchronous detector circuit 77 which recovers the clock associated with the downhole circuitry to put the data into its word-bit scheme as described for downhole transmission. This clock-synchronized data is then passed to an intermediate circuit 79 which separates and sorts the data words to correspond to the analog value of the parameters measured in the downhole, which are then read out in analog or digital form.

The amplifier section shown in Fig. 9 operates as follows: A crystal accelerometer 55 connected to the pipe picks up the signal transmitted in the pipe at a discrete frequency, e.g. 920 Hz. The accelerometer converts the acoustic signal into an electrical signal which contains the frequency transmitted in the drill pipe and some noise. The signal from the accelerometer can be very weak, e.g. 1 mV, or relatively strong, e.g. several volts. In order to accommodate such a wide variation in signal amplitudes, the output of the accelerometer is fed to an amplifier 57 which has a system 58 for automatically controlling the gain. This system 58 controls the signal which is fed to a narrow passband filter 59. The filter "listens" only to the specified frequency, e.g. 920 Hz, and is designed to operate with as narrow a band as possible, taking into account uncontrollable variables. At the frequency of 920 Hz transmitted in the example, the filter passes frequencies from 918 to 922 Hz to ensure that other frequencies used in the system, i.e. 940 and 960 Hz, are reset in the filter. This close discrimination is possible by using a crystal controlled oscillator in the transmit section. The filter operates very effectively when it receives a fixed amplitude signal. The system 58 receives the output of the amplifier 57. If it is too strong, it sends a feedback signal to the amplifier, reducing the output of the amplifier and vice versa. Since the amplifier section also has a transmit section which outputs a 30 V signal, this strong signal would activate the system 58 to reduce the gain too much to effectively amplify the data signals. Therefore, an electronic switch 61 is arranged in the circuit which switches off the amplifier and the gain control when the amplifier sound source 62 is transmitting and remains switched on during the rest of the time for reception of the next data bit. Each data bit received actuates a reset device 65 which resets a clock generator 63 , thereby controlling this switching device so that the input is switched so that it does not receive the retransmitted pulse. This circuit remains activated for a sufficient time to prevent the sound source from ringing and thus interfering with the receiver.

The amplifier filter thus emits a pure 920 Hz signal which is only present when a transmission ("1" or "on") is received. At other times this signal is absent. The output signal from the filter is fed to a delay section 67 which does not activate the amplifier transmitter until the receiver is switched off, thus causing a phase shift of the transmission with respect to the reception. In the example system described, the amplifier transmitter operates at 940 Hz.

The system may use additional amplifier sections, depending on the depth. If the depth of the bore, pipe age, etc., dictate a telemetry system where more than one amplifier section is required, the successive sections may be operated at frequencies of 940 and 960 Hz, using the different frequencies alternately, as shown schematically in Fig. 10. In this example, the spacing between the amplifiers is 610 m. Three frequencies are used. The total spacing between the transmitters operating at the same frequency is 2440 m, which provides sufficient attenuation of the signal. This prevents a stray signal from a station operating at the same frequency from being confused with passing data signals. The spacing between the amplifiers and frequency mixing are determined by the signal loss and receiver signal lock. in the power state. The signal transmitted by each acoustic transmitter or source naturally travels in both directions along the pipe, so that the transmitter transmitting a 920 Hz signal near the surface of Fig. 10 will transmit the signal both downward and upward, the upward direction being the desired direction if the data is to be transmitted from bottom to top. The stepped frequency arrangement with three different frequencies used by three different amplifiers, the amplifiers spaced apart in the drill string, favorably discriminates as to the upward direction of travel of the acoustic signal.

In the foregoing, the telemetry system has been described essentially for the purpose of transmitting data from the borehole to the surface. It is also possible to use the system in such a way that data, control signals or the like are transmitted from the surface into the borehole so that an operation in the borehole can be carried out under control from the surface.

Claims (18)

1. Device for transmitting acoustic signals via a drill pipe string ( 35 ) located in a borehole and composed of individual sections, with
a device ( 41-53 ) for transmitting acoustic signals at a predetermined transmission frequency via the pipe string ( 35 ) and
a device ( 71-79 ) for receiving the acoustic signals, which is arranged at a distance from the transmitting device ( 53 ) on the pipe string ( 35 ),
characterized in that the devices ( 41-53; 71-79 ) for transmitting and receiving the acoustic signals are designed to transmit or receive a predetermined transmission frequency which lies in a frequency range corresponding to the passbands of the pipe string ( 35 ), the initial frequency f _A of which is determined approximately according to the following equation: °=c:30&udf54;&udf53;vu10&udf54;&udf53;vz2&udf54;&udf53;vu10&udf54;wherein
n : integer,
V _S : speed of sound in the pipe string [m/sec] and
l : length of a pipe string section [m].
2. Device according to claim 1, characterized in that the devices ( 41-53; 71-79 ) are designed to transmit or receive a transmission frequency which lies in a frequency range whose initial frequency f _A is +20 Hz and whose frequency bandwidth is 150 Hz. 3. Device according to claim 1 for transmitting signals through a pipe string, the sections of which have a length of approximately 9.5 m, characterized in that the devices ( 41-53; 71-79 ) are designed to transmit or receive a transmission frequency which lies in a frequency range which has an initial frequency of n · 283 Hz+20 Hz and a frequency bandwidth of 130 Hz. 4. Device according to claim 1 for transmitting signals through a pipe string, the sections of which have a length of approximately 13.7 m, characterized in that the devices ( 41-53; 71-79 ) are designed to transmit or receive a transmission frequency which lies in a frequency range which has an initial frequency of n · 196 Hz + 20 Hz and a frequency bandwidth of 100 Hz. 5. Device according to one of claims 1 to 4, characterized in that the devices ( 53, 71 ) for transmitting and receiving acoustic signals are arranged at the bottom or at the mouth of the borehole. 6. Device according to one of claims 1 to 5, characterized in that the transmitting device ( 53 ) is designed to convert an electrical input signal into a corresponding acoustic signal and the receiving device ( 71 ) is designed to convert the received acoustic signal into a corresponding electrical signal. 7. Device according to claim 6, characterized by a device ( 39 ) located in the borehole for detecting a borehole parameter, with which an electrical signal corresponding to the detected parameter is to be generated and passed on as an input signal to the transmitting device ( 41, 43, 45, 47, 49, 51, 53 ). 8. Device according to claim 7, characterized in that the device ( 39 ) for detecting a borehole parameter is designed to generate an analog signal representing the parameter and the transmitting device ( 41-53 ) is used to convert this analog signal into a digitally coded pulse sequence, whereby an acoustic signal with a pulse sequence describing the detected parameter is to be generated. 9. Device according to claim 8, characterized in that the acoustic pulse sequence is to be received by means of the receiving device ( 7-79 ), converted into a corresponding electrical signal, a clock synchronization of the electrical signal with the pulse sequence generated by the transmitting device ( 41-53 ) is to be established and a signal corresponding to the parameter detected in the borehole is to be generated. 10. Device according to one of claims 1 to 9, characterized by at least one amplifier device ( 37 ) which is arranged on the pipe string ( 35 ) between the transmitting and receiving devices ( 41-53, 71-79 ) and by means of which an acoustic signal with a predetermined transmission frequency is to be received and a signal corresponding to this signal is to be transmitted with a second transmission frequency which is also in a frequency range corresponding to the passbands of the pipe string. 11. Device according to claim 10, characterized by an amplifier device ( 37 ) with which the acoustic signal is to be received at the second transmission frequency and a signal corresponding thereto is to be transmitted at a third transmission frequency which is also in a frequency range corresponding to the passbands of the pipe string. 12. A method for transmitting acoustic signals via a drill pipe string ( 35 ) composed of individual sections located in a borehole, using a transmitting and a receiving device ( 41-53, 71-79 ) which are coupled to the pipe string at a first and a second location arranged at a distance therefrom, respectively, wherein
either at the first or second location an electrical signal with a predetermined frequency is generated which corresponds to the data to be transmitted,
from the transmitting device ( 41-53 ) corresponding to this electrical signal an acoustic signal with a discrete transmission frequency,
this acoustic signal is guided via the pipe rod ( 35 ) to the receiving device ( 71-79 ), and
an electrical signal corresponding to the received acoustic signal is generated by this receiving device ( 71 ),
characterized,
that the transmission frequency of the acoustic signal is generated by the transmitting device ( 41-53 ) in accordance with the passbands of the pipe string ( 35 ) in a frequency range whose initial frequency is approximately predetermined according to the following equation: °=c:30&udf54;°=b:1&udf54;H&udf53;vz2&udf54;H@°=b:1&udf54;HHwherein
n : integer,
v _S : speed of sound in the pipe string [m/sec] and
l : length of the pipe string section [m].
13. Method according to claim 12, characterized in that the transmission frequency is generated in a frequency range whose initial frequency f _A is +20 Hz and whose frequency bandwidth is 150 Hz. 14. A method according to claim 12, in which pipe string sections with a length of approximately 9.5 meters are used, characterized in that the transmission frequency is generated in a frequency range whose initial frequency is n · 283 Hz + 20 Hz and whose frequency bandwidth is 130 Hz. 15. A method according to claim 12, in which pipe string sections with a length of approximately 13.7 meters are used, characterized in that the transmission frequency is generated in a frequency range whose initial frequency is n · 196 Hz + 20 Hz and whose frequency bandwidth is 100 Hz. 16. A method according to any one of claims 12 to 15, characterized in that a borehole parameter is determined at a location on the tubing string, that an electrical signal is generated which represents the determined parameter, and that the transmitting device ( 41-53 ) is actuated in response to the generated electrical signal, whereby the acoustic signal is generated at the discrete frequency. 17. Method according to claim 16, characterized in that an analog signal is used for the signal representing the parameter, which is converted into a digitally coded pulse sequence, and that the transmitting device is actuated in accordance with this digitally coded pulse sequence. 18. Method according to one of claims 12 to 17, characterized in that the acoustic signal with the predetermined transmission frequency is received by an amplifier device ( 37 ) arranged between the transmitting device ( 41-53 ) and the receiving device ( 71-79 ), and from this a signal corresponding to the received signal is transmitted with a different transmission frequency which is predetermined in a frequency range corresponding to the passbands of the pipe string ( 35 ).