Abstract:
In a method for logging the location of a fluid depth in a wellbore, an acoustic event is allowed to take place in a specific manner at the ground surface. This acoustic event generates pressure waves. The pressure waves travel in the wellbore downward. The pressure waves traveling in the wellbore are reflected at least also at the fluid depth. At the ground surface, the pressure waves traveling there out of the wellbore are picked up and the time of travel since the acoustic event is measured. The picked-up and measured pressure waves are analyzed and, together with the associated time of travel, the location of the fluid depth is deduced. The acoustic event generates a signal pattern having a predetermined, time-variable frequency spectrum. The signal pattern is emitted as vibration event into the wellbore, travels downward, and is reflected. At the ground surface, the picked-up signals originating from the wellbore are analyzed. During the analysis, vibration events that do not correlate with the emitted signal pattern are filtered out. From the vibration events that are among the picked-up signals and correlate with the emitted signal pattern and from the time of travel since the radiation of the signal pattern, the location of the fluid depth is deduced.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for logging the location of a fluid depth in a wellbore during deep drilling, in which an acoustic event is allowed to take place in a specific manner at the ground surface, said acoustic event generating pressure waves, in which the pressure waves generated by the event travel downward in the wellbore, in which the pressure waves traveling into the wellbore are reflected at least also at the fluid depth, in which pressure waves traveling out of the wellbore to the ground surface are picked up there and the time of travel since the acoustic event is measured, in which the picked-up and measured pressure waves are analyzed and used, together with the associated time of travel, to determine the location of the fluid depth, and a device for determining the fluid depth in wellbores by using such a method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wellbores serve, in particular, for discovering crude oil or natural gas and extracting it from subterranean deposits. The latter extend in horizontal and vertical direction, often over an extensive area, so that a plurality of wellbores are used for each individual deposit in order to recover the crude oil or the natural gas from the deposits. 
         [0003]    In this process, each wellbore has tubing made up of a plurality of individual pipes, vertically adjoining one another, which, when the wellbore is introduced, are inserted one after the other and joined to one another. These pipes of the tubing extend down to and into the oil or gas deposit. 
         [0004]    This means that the desired medium, that is, in particular, crude oil or natural gas, is located in the deepest region of the wellbore and can be extracted. For this purpose, there are pumps or other extracting devices in this region, which bring about a continuous or discontinuous upward extraction within the wellbore. Further quantities of crude oil or natural gas flow from the sides and from below to replace the withdrawn medium. In addition, water and/or other fluids accumulate in this lowest region of the wellbore. The surface of the fluids in relation to the ground surface is referred to as the fluid depth. 
         [0005]    The adjoining pipes of the tubing of the wellbore are joined to one another in such a manner that a relatively tight, mechanically stable joint is formed and, in particular, no sand and no fluids can penetrate through the wellbore tubing into the interior of the wellbore. In this case, the pipes have pipe joints to one another, that is, coupling sites that are formed by the collars and coupling rings. The individual pipes are essentially roughly equal in length. 
         [0006]    Often provided for technical reasons are two tubings that are put in place concentrically in each other. The crude oil or natural gas to be extracted flows upward in an inner pipe and, in an annulus concentrically surrounding the inner pipe from the outside, there takes place, among other things, a pressure equalization. 
         [0007]    The precise location of the fluid depth in a wellbore has already long been of appreciable interest for improving the extraction operation. For example, it is undesirable for the oil pumping operation to withdraw more crude oil from the wellbore than can flow back into this wellbore from the deposit. In end effect, the wellbore would then run dry. Naturally, an effort is also made to withdraw as much crude oil or natural gas from the wellbore as possible. Because the rate at which the crude oil or natural gas flows back cannot be measured and, over the operating time of a wellbore, varies in an unforeseeable manner, the exact location of the fluid depth is a value of interest. If the fluid depth drops over the course of time during each of a number of observations, the fluid level also drops, so that it appears that more crude oil and natural gas is being withdrawn from the wellbore than can flow into the wellbore from the deposit. This would pose a possible danger as extraction continues. On the other hand, if the fluid depth rises, there exists the possibility of increasing the rate of extraction. 
         [0008]    The location of the fluid depth in the wellbore can naturally be several hundred or even greater than 1000 m deep and is accordingly hard to log and determine from the ground surface. Optical measurements are ruled out in this case. 
         [0009]    For the measurement of the water level in a wellbore, which serves as a well for extracting water, an acoustic measurement is proposed in U.S. Pat. No. 5,027,655. A signal having a known frequency is transmitted downward into the wellbore and a returning signal, reflected from the surface of the fluid, is utilized as a response signal. Taking into consideration the speed of sound, it is possible to determine the fluid level. In this case, the wellbore may not have any major discontinuities that act as strong reflectors for the emitted signal and therefore lead to interference with the result. U.S. Pat. No. 5,027,655 proposes, as a further modification, a variation of the frequency in order to determine a resonance frequency or an antiresonance frequency and to deduce from it the location of the water level. 
         [0010]    Undertaken in practice are acoustic measurements such as described, for instance, in U.S. Pat. No. 4,793,178 and U.S. Pat. No. 4,934,186 as well as, in further developments, also in U.S. Pat. No. 5,117,399 and U.S. Pat. No. 5,200,894. A gas canon is fired on the ground surface in the region of the wellbore and, in this way, generates at acoustic impulse, which has a very steep and high front flank. 
         [0011]    Alternatively known, besides the firing of a gas canon, is the generation of an impulse by implosion from a device offered by Friedrich Leutert GmbH &amp; Co. KG under the name “Acoustic Well Sounder AWS.” This method is suitable when, in the wellbore, there exists an overpressure with respect to the surrounding air. To this end, the uppermost section of the tubing in the wellbore is closed off by a pipe that can be shut at the upper end and at the lower end. In the first measurement step, the upper valve is then opened and the gas contained in the pipe escapes into the surroundings. Then, the upper shutoff valve is closed again. Afterwards, the lower shutoff valve in the wellbore is opened and the gas present under overpressure in the wellbore enters the pipe abruptly. This abrupt expansion of the gas present in the wellbore into the pipe also generates an impulse having a very steep and high front flank. Then, in the next step, the gas that is now contained in the pipe from the wellbore is expelled into the surroundings and the next measurement can take place. 
         [0012]    The pressure wave generated by the firing of the gas canon or else by the described implosion travels down through the wellbore to the fluid depth and is then reflected there. In this process, it is subject to the physical laws of sound waves. 
         [0013]    However, the speed of sound in the wellbore is not known, because a complicated gas mixture, which, moreover, is not constant in time and which consists of, among other things, methane, nitrogen, carbon dioxide, argon, and many other gases, is present in the wellbore between the ground surface and the fluid depth and behaves in no way like an ideal gas and also changes in composition both with time and over the depth of the wellbore in an unforeseeable manner. This means that it is not possible to deduce automatically the distance of travel of a sound wave from its time of travel and it is not yet possible to make a quantitative statement about the distance of the path traveled from the time of travel of the sound wave from the ground surface to the fluid depth and, after reflection, back to the ground surface. Hence, the fluid depth cannot be determined only from the time of travel. 
         [0014]    Also, smaller reflections take place at the individual joints of the pipes of the wellbore tubing, so that portions of the sound or pressure wave are reflected back even before the reflection from the fluid surface and can be recorded above at the ground surface by using an appropriate receiving device. These waves thus have a shorter distance of travel and hence also a shorter time of travel. 
         [0015]    However, because the individual pipes are, as mentioned, of roughly equal length and the discrete reflections at the edges of the joints can be observed, an entire series of reflection lines of sound waves result, before the impingement of the sound wave reflecting from the fluid level, and these sound waves have reached only the individual pipe joints, that is, the coupling sites between the pipes, and been reflected there. If these reflecting sound waves can be identified and adequately clearly detected in the noise, then, by subtracting the reflection lines, it is possible to determine how many pipe joint lines lie between the ground surface at the wellbore and the fluid depth. With a known average pipe length, it is then possible to deduce the approximate location of the fluid depth and, by comparison of several measurements at intervals in time, it is also possible, in a later analysis, to deduce whether the fluid depth is rising or falling. 
         [0016]    A problem is that there are multiple reflections, because each sound wave that reflects at a joint and travels upward is partially reflected anew at all of the above-lying joints and these doubly reflected sound waves travel downward and result in additional triple and multiply reflected sound waves that impinge at the ground surface due to renewed reflection and interfere with and appreciably hamper the identification of the reflection lines that are of actual interest. Hence, when the gas canon is fired, a great deal of care must be applied to achieve an initial sonic event that is as clean as possible, that is, to obtain a flank that is as steep and as high as possible, which enables a later identification of desired and undesired reflection lines. 
         [0017]    Proposed in U.S. Pat. No. 6,085,836, in view of this complicated situation, is to employ three separate signal generators, each of which emits a signal of a certain frequency, with the three frequencies being different from one another. These three signal generators are intended to then emit the sound waves generated by them separately from one another, first of all within a drill string, then outside of a drill string, and, finally, additionally in an annulus. Through reception of the reflected signals in the various wellbore regions and the assignment of the monofrequency sound waves to the initial signals, it is hoped to improve the measurement results and to make the measurements more reliable. The expense for signal generators that operate separately from one another and operate in separate regions of the wellbore cross section is appreciable. In practice, this proposal has not gained acceptance. 
         [0018]    Besides the difficulties already discussed in the analysis of reflecting sound waves, it is desirable to prevent all interfering noise due, for instance, to running extraction pumps and the like, which could also interfere with the measurement result or falsify it. Thus, in such measurements using methods of prior art, it is necessary to interrupt the drilling operation for the time of the measurement in many cases. 
         [0019]    In practical application, such an arrangement of devices involving a gas canon and a receiving device as well as an associated electronic analysis and evaluation device is then employed in succession at the various wellbores of an oil field or other deposit. The analysis then takes place largely manually after all measurements have been concluded. From the resulting measured values, experts can then, insofar as required undertake measures at the individual wellbores to adjust the rate of extraction, for example. 
         [0020]    In spite of the fundamental functional ability of such methods, there exists the desire in expert circles to be able to undertake this operating procedure at lower cost and, insofar as possible, also faster. To be taken into consideration in doing so is the fact that technical personnel are needed over a prolonged period of time for the operation in order to appropriately carry out and analyze measurements from a large number of wellbores. The arrangements of prior art are exacting and have to be used in places where the wellbores are located, which is often the case in impassable terrain that is not favorably located for vehicle access. 
         [0021]    The problem of the invention is therefore to propose a class-specific method and a class-specific device by means of which an improved or simplified determination of the fluid depth in wellbores is possible. 
       SUMMARY OF THE INVENTION 
       [0022]    This problem is solved in accordance with the invention for a class-specific method in that the acoustic event generates a signal pattern having a predetermined, time-variable frequency spectrum, in that the signal pattern is emitted as a vibration event into the wellbore, travels downward, and is reflected, in that the signals originating from the wellbore and picked up at the ground surface are analyzed, in that vibration events that correlate with the emitted signal pattern are filtered out of the picked-up signals during the analysis, and in that, from the vibration events that are among the picked-up signals and correlate with the emitted signal pattern and from the time of travel since the emission of the signal pattern, the location of the fluid depth is deduced. 
         [0023]    In a device, this problem is solved in accordance with the invention in that, at the ground surface, a vibration-emitting device is provided, which generates signal patterns with a predetermined, time-variable frequency spectrum and emits them into the wellbore, in that a measuring device for receiving signals originating from the wellbore is arranged and set up, in that an electronic analysis unit is provided, which analyzes signals picked up by the measuring device and is set up so that the vibration events correlating with the emitted signal patterns are deduced and, from these correlated vibration events and the time of travel, the location of the fluid depth is deduced. 
         [0024]    By using a method and/or a device according to the invention, it is possible to measure and display continuously and automatically the fluid depth at oil and gas extraction sites or at other deep drillings. The use of such a method and/or such a device affords a continuous knowledge about the behavior of the fluid depth at individual extraction sites of a crude oil or natural gas field. This continuous knowledge can make a decisive contribution to the optimization of the yield. 
         [0025]    The method according to the invention as well as the device according to the invention may be employed not only in the extraction of crude oil, natural gas, or other natural resources, in particular fluid or gaseous natural resources, for the determination of the fluid depth, but also in geothermal applications. There, too, it is of interest to determine the location of the fluid depth in deep drillings in the wellbores created there, because an observation of the water level can also be important for geothermal applications. 
         [0026]    The method and device according to the invention can also be employed in other applied cases in which, during deep drillings, the precise location of a boundary layer between a liquid and a gas may be of interest in wellbores. 
         [0027]    Whereas the previously applied method for determining the location of the fluid depth is undertaken manually in each case by personnel in most cases and leads to temporary knowledge, which affords only unsatisfactory results, this is changed fundamentally by the invention. 
         [0028]    In contrast to the methods of prior art, the invention does not work with an individual, as steep as possible flank of an acoustic event, which one attempts then to identify, following various reflections, from the mixture of the totality of returning signal patterns. Instead of this, work is done with a frequency spectrum, which, moreover, is also time-variable. The signal pattern, which is predetermined both in terms of time course and frequency, is emitted into the wellbore and—just as in prior art—reflected at the various pipe joints, even multiply if need be. 
         [0029]    The measuring device in the device according to the invention at the ground surface thus picks up again a very complex and, at first glance, also very “confusing,” complicated packet of signal patterns. On the basis of a recorded log, a user would never be capable manually of interpreting, in turn, the picked-up log showing the time-variable frequency spectrum. 
         [0030]    However, the electronic analysis device is capable of determining, from the known composition of the emitted signal pattern in the picked-up composition, which reflections have actually taken place, because the individual signal patterns have indeed experienced, in their totality, the same reflection event, and the mathematical correction of a possible frequency-dependent, different reflection property of the various pipe joints is possible. 
         [0031]    Due to the fact that a signal pattern passes through each individual reflection event, a corresponding determination can take place with appreciably greater precision than in prior art. The hitherto interfering multiple reflections now are no longer interfering, because they, too, are each passed through by an entire signal pattern and, on account of the time change of the individual frequencies of the emitted signal, can even be identified unequivocally. 
         [0032]    As a result of these highly precise possibilities, vibration events that do not correlate with the emitted signal pattern because, namely, they are not based on a reflection of these signal patterns, but rather on wellbore-specific vibration events, can also be cleanly identified. Thus, they no longer interfere with the analysis and can be identified as wellbore-specific working events, even in the case when the signal amplitude caused by the working event is greater than that of the signal pattern generated in the device for the measurement. 
         [0033]    In practice, this results in the appreciable advantage that the measurements by means of the method according to the invention and the device according to the invention can be carried out without having to interrupt the extraction operation. The extraction of, for example, crude oil from the wellbore can thus be continued with a running pump. The pump noises, the noises of moving parts and joints, and also the noise caused by the flowing fluid itself, which are obviously created in this extraction operation, do not interfere with the determination of the location of the fluid depth, because the noises in question are all noises that create vibration events that do not at all correlate with the signal patterns that are transmitted downward from the ground surface into the wellbore for purposes of the measurement. They can thus be excluded without question during the determination of the location of the fluid depth. 
         [0034]    These working events specific to the wellbore can thus be filtered out during the determination of the exact location of the fluid depth, so that this determination can focus on the vibration events of actual interest. 
         [0035]    In the process, it is especially of advantage when the ensuing analysis in the electronic analysis unit can then compare these picked-up and filtered-out additional vibration events also against, for instance, the other values provided in memory storage—for example, with typical vibration events that are emitted by an extraction device in the wellbore and were picked up, for instance, during an earlier measurement. These typical vibration events, which are to be recognized as being known, are then identified without any problem and in a proper manner and can be ignored. 
         [0036]    In an especially preferred embodiment, moreover, it is possible to undertake simultaneously a monitoring of fixed and mobile process equipment components of the extraction site. This preferred embodiment provides, besides the hitherto conventional registration of electrical or mechanical malfunctions that have already occurred at extraction devices, a knowledge of deviations from the normal state that have crept in. As a result, the possibility of a more targeted preventive maintenance and service is created. 
         [0037]    To be taken into consideration is indeed the fact that, at wellbores over the course of the operating time, failure and damage can also occur to the extraction devices, for instance. An early detection of such failure, damage, and other defects is of great value. Thus, failure due to, for instance, drill string breakage, leads to a very expensive retrieval and repair operation, which means a prolonged downtime for the entire wellbore. However, if a corresponding failure is detected early on, a markedly less costly replacement or repair of the drill string can be undertaken before damage occurs. 
         [0038]    Such failure is often indicated already by vibration events, which, as such, are not significant and are also not noticed by persons who happen to be present at the wellbore, but which, through changes in the vibrational relationships and a monitoring of these changes, are indicated by the occurrence of additional vibrations that were not observed in the previous measurements, for instance. 
         [0039]    The preferred embodiment makes use then of the possibility of considering more closely vibration events that are specific to the above-mentioned wellbore and do not trace back to the signal pattern that was emitted and reflected for the determination of the location of the fluid depth. 
         [0040]    As mentioned above, the vibration events that do not correlate with the emitted signal pattern can be compared in the analysis unit with other values provided in memory storage—for example, with typical vibration events that can be assigned to normal working events in the wellbore. 
         [0041]    However, if there occur vibration events that cannot be assigned or perhaps can be assigned to a vibration event deposited in memory storage that indicates a specific flaw, the user can be appropriately informed and can pursue this unknown vibration event, which was picked up in addition to the actual measurement and may possibly point to a malfunction of the extraction device, the drill string, or other elements. 
         [0042]    In doing so, the user can be afforded right away an indication as to whether a certain flaw and what flaw is to be presumed and urgently inspected or whether a vibration event that is totally unknown and cannot be assessed by the analysis unit is being observed. 
         [0043]    The method and/or the device allow, besides the automatic determination of the location of the fluid depth, also simultaneously a monitoring of moving process components for unspecific vibration events. Thus, for example, string anomalies as well as the running characteristics of downhole rotary pumps or motors or gears of the above-ground drive units can be monitored. Even for simple embodiments, 16 or more different malfunction scenarios can be evaluated individually or mathematically linked to one another and tracked in each cycle of measurement. 
         [0044]    For further expanded process equipment monitorings, it is also possible to attach an additional sensor or sensor group to the analysis unit. 
         [0045]    In accordance with the invention, then, it is possible in the preferred embodiment to measure the fluid depth by using only one sensor and analysis system as well as simultaneously to undertake the monitoring of extraction sites for this failure and damage. 
         [0046]    The simultaneous observation possibilities of, on the one hand, possible flaws and, on the other hand, independent of each flaw consideration, the location of the fluid depth affords the operator of the wellbore appreciable advantages. Thus, he can undertake much more frequently a checking of the fluid depth at markedly lower cost, because, together with the determination of the location of the fluid depth, parts of the maintenance and flaw monitoring tasks can also be fulfilled at the same time, without incurring additional cost for the use of technical personnel or devices in the process. 
         [0047]    Also possible is an inspection for an eventual correlation between a perhaps observed flaw and the location of the fluid depth during the observation of the flaw. This affords the possibility of long-term observations at intervals in time and the recording of the vibration events observed in each case, so that, in retrospect, it can additionally be established whether a possibly occurred flaw was already indicated during the preceding measurement. 
         [0048]    It is of special advantage that, unlike the case for conventional equipment, particularly a stationary use at an extraction site becomes feasible due to the clearly simplified construction design. The equipment is then arranged in a stationary manner and monitors the wellbore regularly or even permanently, preferably continuously. 
         [0049]    However, there are also embodiments that continue to enable a mobile use. 
         [0050]    The method works in such a manner that a signal generator generates low-frequency electrical signals. These signals are converted to a signal pattern by means of a vibration converter in a measuring head. These signal patterns can propagate as pressure waves via the gaseous media in the wellbore, which are present between two pipes that run one inside the other. 
         [0051]    Returning waves are also created at each of the pipe joints of the piping, as is the case for measurements of prior art. Here, too, the emitted wave forms are reflected at the boundary layer between gaseous and solid or fluid media, which may be present between the two pipes, hence also at the fluid depth. 
         [0052]    The returning waves and the reflections of the boundary layer between the gaseous and the fluid media are received in the measuring head by an electronic sensor and then transmitted further to the analysis unit. 
         [0053]    The analysis unit, in turn, calculates the number of pipe joints and the distance between the signal generator and the boundary layer between the fluid and gaseous media from the initial signal and the received return signals by correlation in the frequency and time range. 
         [0054]    Unlike the case up to now, however, no explosion or implosion having a large, steep amplitude is produced by using a gas canon in order to obtain the measuring signal. 
         [0055]    Instead of this, in accordance with the invention, signal amplitudes that are as small as possible and have a known, continuous-time course are used. 
         [0056]    In this case, resort is preferably had to a concept that is described mathematically in U.S. Pat. No. 6,272,441 B1, whereby, in contrast to that publication, low-frequency and not high-frequency signals are used. It was found in a manner surprising to the expert that this method, which was conceived for quite different areas of application in radar technology in the high-frequency range of about 100 MHz to 10 GHz, is especially suitable in accordance with the invention for the application purposes in a wellbore when, also in contrast to what is provided for in that publication, not high-frequency signals but rather low-frequency signals are used, such as lie in the frequency range of sound waves, for instance. 
         [0057]    Thus, it is particularly preferred when the method is carried out with frequencies of between 5 Hz and 1000 Hz, preferably of between 5 Hz and 100 Hz. 
         [0058]    In tests, these frequency ranges have proven to be particularly suitable. 
         [0059]    Furthermore, the method has the great advantage that, during the measurement, no exchange of the medium in the measuring device takes place with the surrounding air, because the measuring head is a closed system. 
         [0060]    This is an appreciable advantage in comparison to the known method, for instance, in which, by means of an explosion, overpressure in the atmosphere in the wellbore is exploited, which, however, results in portions of the gas in the wellbore escaping into the surrounding air, which may encounter objections for reasons of technical safety and also for ecological reasons. 
         [0061]    It becomes possible by means of the invention to arrive at a continuous-time and automatically operating monitoring mode both during the measurement of the location of the fluid depth and during the monitoring of process equipment. Created is an innovative, fully electronic combined system that is suitable for stationary and mobile use at crude oil and natural gas extraction sites and can be fabricated as equipment in series as well. The method and the device make possible a very economical use even in the case of unfavorable external conditions. 
         [0062]    The rate of recovery of the measuring operations for determining the location of the fluid depth may be selected practically at will and, depending on the external requirements, a time range of between a few seconds and intervals of months may be selected at will. 
         [0063]    It is provided for in a particularly preferred manner that the analysis unit generates an electrical signal corresponding to each measured fluid depth. The value of this electrical signal is compared constantly to the target value of the fluid depth noted in the analysis unit. If the value for the measured fluid depth rises or falls, the power of the extraction device, that is, in particular, the pump at the wellbore, is changed automatically until the measured signal for the fluid depth has attained once again a predetermined target value. 
         [0064]    Thus, by controlling the pump power, a control loop is created. For stationary equipment, it is thus possible, in the absence of technical personnel, to undertake an automatic control of the wellbore operation. 
         [0065]    Such a control loop is not possible as such by using the methods and devices of prior art. The latter are able to determine a value for the location of the fluid depth at the wellbore practically only manually, which can then be utilized by users who are present, after checking and manual comparison with other values, for corresponding correction of the control of extraction devices. In accordance with the invention, however, the determination of the location of the fluid depth is very precise on the one hand and also safe and thus reliable and, on the other hand, it is fully automatic and does not require any user who checks and uses the measured values determined. The resulting control loop is thus not only very reliable and precise, but, furthermore, also markedly uncomplicated and hence cost-advantageous. 
         [0066]    A preferred embodiment of the device is characterized in that a remote transmission device is provided, which undertakes a remote transmission of picked-up and/or analyzed results. 
         [0067]    In such an embodiment, the continuously registered data of the location determination of the fluid depth can be employed for comparison against a deposit model of an entire crude oil and natural gas field and hence serve to optimize the control of all equipment in the region of the deposit. 
         [0068]    In this embodiment, the information of the individual monitorings of the respective process components can be transmitted to a control room. It is also possible to link the individual analysis units, memory storage units, and display units on site into a network controlled by the control room. It is then possible at any time to make changes in parameters via remote access, to visualize the current measured values of the connected analysis units on site, and to display the behavior of the fluid depth of the individual extraction regions in three dimension, for example. 
         [0069]    It is especially preferred when the vibration-emitting device has a permanent magnet and an excitation coil at a vibrating membrane. 
         [0070]    This represents a reversal of the construction of a classical vibration generator, such as is known, for example, from conventional loudspeakers. By means of such a combination of permanent magnet and diaphragm as vibrating unit, it is possible to create a robust vibrator that is resistant to acids and bases. The construction makes possible operation both as an analog oscillator (that is, one that follows exactly the curve course of the electrical signal) and as a digital oscillator, which vibrates at an excitation frequency between two end points without intermediate positions. 
         [0071]    Tests have resulted in an especially elegant possibility for setting up the device according to the invention. It is characterized in that the vibration-emitting device and the measuring device resort at least in part to identical elements. 
         [0072]    Namely, it is possible to construct the vibration-emitting device, which, in end effect, is similar to a kind of loudspeaker, in common with the measuring device as well. This measuring device requires similar elements in order to be able to register pressure variations and thus operates, in end effect, fundamentally like a kind of microphone or pressure sensor. 
         [0073]    Through the use of identical elements (in particular, the identical membrane), it is possible to dispense with a separate and very costly sensor, because, indeed, only certain characteristics of this pressure sensor are needed. 
         [0074]    Surprisingly found to be usable have been devices that are used in other connections, for instance for siren tone generators. Such generators for producing a siren tone have emitting powers and characteristics such as those that diaphragm speakers also have. In particular, they also have a corresponding membrane. 
         [0075]    The corresponding elements of the siren tone generators are used and, in the region of the membrane, holes are used to achieve an internal pressure equilibration in the installed state so as to take into consideration possible overpressures in the wellbore, which are not of interest for registering the pressure variations and, instead, would only encumber the membrane. 
         [0076]    Therefore, this hole makes possible, for the first time, the use of high-sensitivity pressure sensors. 
         [0077]    As tests have shown, the simultaneous use of the membrane as an element of the signal emitter or the vibration-emitting device and the measuring device makes it possible to attain an appreciably greater sensitivity. The form and number of the detectable echoes markedly increase. 
         [0078]    In order to be able to exploit these advantages, an electronic changeover switch is inserted into the circuit. The changeover switch has to separate the operation between signal generation and signal reception and also control the corresponding further transmission of the signal. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0079]    An exemplary embodiment of the invention will be described in greater detail below on the basis of the drawing: Shown therein are: 
           [0080]      FIG. 1  is a schematic illustration of a wellbore having a device according to the invention; 
           [0081]      FIG. 2  is an enlarged illustration of a measuring head of the device according to the invention; 
           [0082]      FIG. 3  is a schematic illustration of the further transmission of the signal in a first embodiment; and 
           [0083]      FIG. 4  is a schematic illustration of the further transmission of the signal in a second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0084]    Illustrated in  FIG. 1  is a wellbore  10 . The wellbore  10  extends from the ground surface  11  downward to a deposit for crude petroleum or crude oil. The crude oil  12  forms, together with additional fluids, a mixture that is separated by a boundary layer  13  from the various gases and gaseous media  14  forming above it. These gases are, among others, nitrogen, argon, and other components of the atmosphere and, in addition, methane and other gases that form above the crude oil. The composition varies over the course of time and also with the depth of the wellbore  10 . 
         [0085]    The precise location of the boundary layer  13  between the fluid substances containing the crude oil  12  and the gaseous media  14  constitutes a fluid depth. The precise location of this fluid depth  13  or of the boundary layer varies over the course of time, depending on the rate at which the crude oil  12  and the other fluids enter the wellbore  10  from the side and from below. 
         [0086]    In order to extract the crude oil  12  from the wellbore  10  to the ground surface  11 , a piping is provided. This piping  20 , which extends vertically in the wellbore  10 , comprises an inner pipe  21  and an outer pipe  22 , which concentrically surrounds the inner pipe and simultaneously forms the pipe casing. The crude oil  12  is extracted upward through the inner pipe  21 , while the pressure is equilibrated in the annulus between the inner pipe  21  and the outer pipe  22 . 
         [0087]    Both the inner pipe  21  and the outer pipe  22  of the piping  20  comprise a plurality of individual pipe sections, which are lowered in succession from the ground surface  11  into the wellbore  10  and, in the process, successively joined to one another via pipe joints  23 . These pipe joints  23  are not seamless, but rather form collars, which are illustrated in highlighted manner in the schematic illustration in  FIG. 1 . 
         [0088]    Illustrated above the ground surface  11  in a purely schematic manner is, in addition, a pump  30 , which undertakes the extraction. Parts of the extracting device are naturally located as well (not illustrated) in deeper regions of the wellbore  10 . 
         [0089]    The fluid or crude oil  12  that is extracted to the ground surface  11  by means of the pump  30  is conveyed away via a pipeline  31  for further use. 
         [0090]    A device  40  in the region of the ground surface  11  at the head of the wellbore  10  has, among other things, a vibration-emitting device  41 . This emits a vibrational signal, which is to be described further below, in the direction  42  downward into the wellbore  10 . The vibrational signal or the signal pattern  42 , in the form of pressure waves, is reflected upward at the boundary layer  13  at the level of the fluid depth and additionally at all pipe joints  23 , so that, here, a plurality of returning signals or signal patterns  43  result. 
         [0091]    These returning signal patterns  43  are picked up in a measuring device  44  in the device  40 . This measuring device  44  is, for example, a pressure sensor. 
         [0092]    In the right area of the illustration in  FIG. 1 , it is indicated that the generation of the signal patterns  42  is accomplished in the vibration-emitting device  43  by way of an electrical signal generator  51 . 
         [0093]    The returning signal patterns  43  picked up in the measuring device  44  are analyzed in an electronic analysis unit  52 . The corresponding results of the electronic analysis unit  52  are stored in a memory storage unit  53  and, in the embodiment illustrated, visualized in a display unit  54 . The results can also, as illustrated, be transmitted further via a remote link  55 , such as, for instance, a radio link or by means of a local area network or a wide area network, in order to enable the simultaneous analysis of several such devices  40  at several wellbores  10 . The remote link  55  can also include a remote control, which enables tapping or else access to control. 
         [0094]    Further indicated is an external signal sensor  56 , which can pick up additional signals as well and feed them to the electronic analysis unit  52 . The data from the electronic analysis unit  52  that originate from the external signal sensor  56  and, in particular, from the measuring device  44 , can be used for controlling the pump  30  and, in this way, the rate of extraction of the crude oil  12  from the wellbore  10  can be optimized. 
         [0095]    The electrical signal generator  51  generates low-frequency electrical signals. These signals are fed to the vibration-emitting device  41  in the device  40  at the upper end of the wellbore  10 . There, they are converted into signal patterns  42  by means of a vibration converter (not illustrated). These signal patterns  42  are emitted as pressure waves and can propagate in the wellbore  10  via the gaseous media  14 , which are located in the annulus between the inner pipe  21  and the outer pipe  22 . 
         [0096]    The returning signals or signal patterns  43  are then created, as mentioned, at the pipe joints  23  of the individual pipe sections of the piping  20 . The emitted wave forms are additionally reflected at the boundary layer  13  between the gaseous media  14  and the fluids or solid and fluid media containing crude oil  12  in the lower region of the wellbore  10 . 
         [0097]    The returning signal patterns  43  are received by the measuring device  44  in the device  40  and converted to electrical signals and transmitted further to the electronic analysis unit  52 . This electronic analysis unit  52  calculates, from the initial signal and the received returning signal patterns  43  by correlation in the frequency and time range, the number of pipe joints  23  above the fluid depth with the boundary layer  13 , and from it, in turn, the distance between the vibration-emitting device  41  and the boundary layer  13  between the media  12  and  14 . 
         [0098]    The length of the individual pipe sections  23  is known or else is measured and, in contrast to the composition of the gaseous medium  14 , is constant over time. This length of the pipe sections can be deposited in the memory storage unit  53  and will then be analyzed by the electronic analysis unit  52 . 
         [0099]    The electronic signal generator  51 , in contrast to prior art, generates very small signal amplitudes. These signals have a predetermined continuous-time course. The electronic signal generator  51  can, by contrast, correspond to an arrangement such as that described schematically in U.S. Pat. No. 6,272,441 B1. However, in contrast to this known arrangement, low-frequency signals are used. 
         [0100]    Large or steep amplitudes are not necessary. 
         [0101]    The device  40  with the vibration-emitting device  41  and the measuring device  44  is a closed system, which, in a preferred embodiment, is treated in more detail in  FIG. 2 . Therefore, during and also between various measurements of the fluid depth  13 , no equipment medium is vented to the surrounding air. 
         [0102]    The electronic analysis unit  52  generates, in each case depending on the measured values supplied to it by the measuring device  44 , an electrical signal, which corresponds to the respective fluid depth  13 . In a preferred embodiment, the magnitude of this electrical signal is compared constantly against the adjusted target value of the fluid depth that is known in the electronic analysis unit  52  or the attached memory storage unit  53 . If, then, the value of the measured fluid depth  13  rises or drops, the power of the pump  30  is automatically changed by means of a control signal until the signal corresponding to the fluid depth  13  attains once again the desired target value. 
         [0103]    The current values of the fluid depth  13  calculated in the analysis unit  52  from the measured values of the measuring device  44  are stored in the memory storage unit  53  and may, as needed, be transmitted via the data link  55  to higher-level process monitoring and control systems. 
         [0104]    From the individual measured values of the extraction sites, together with their respective wellbores  10 , that are connected to this higher-level system, it is possible to produce automatically a near-real-time, three-dimensional image of the fluid depth  13  of the respective deposits with their plurality of wellbores  10 . Via the remote access  55  to the individual analysis units  52 , it is possible, for optimization of the rates of extraction of all of the deposits, to adjust at any time the individual stored target values of the fluid depth  13  for the respective wellbores. The target value can thus be changed separately for each wellbore  10  depending on new higher-level knowledge. 
         [0105]    The measuring operation of the fluid depth  13  in a specific wellbore  10  can be carried out in any arbitrary time interval practically without any limitation in the frequency of the measuring operations. 
         [0106]    In addition to the reflected and returning sonic signals  43 , the measuring device  44  also picks up vibration events of the process equipment devices of the extraction site at the wellbore  10 . These vibration events afford information about the technical state of movable parts of the extraction devices, such as, for example, about the pump  30  and its components or else about movable parts of strings of piping  20 . The picked-up vibration events are analyzed continuously in the analysis unit  52  on site and examined for significant changes in the time and frequency range as well as in the phase position. The analytical method can include standard methods of automatic machine monitoring (so-called CMS). 
         [0107]    The analysis unit  52  offers for machine monitoring the connection of the above-mentioned external, additional signal sensor  56  or else a signal sensor group. These external signal sensors  56  also enable a monitoring of those vibration events within the process equipment that are not directly transmitted via the equipment components or the process media to the measuring device  44 . 
         [0108]    Both spontaneous system changes and those that creep in are identified, stored in the memory storage unit  53 , transmitted using the remote transmission device  55 , and/or visualized. 
         [0109]    The simultaneous measurement of the location of the fluid depth  13  and the machine characteristics leads to appreciable advantages, because the two values can also be related to each other and hence make possible appreciable synergistic effects and added knowledge for the analysis. Thus, for example, the location of the fluid depth  13  can be taken into consideration in the interpretation of the vibration events coming from the wellbore  10 . 
         [0110]    A particularly suitable embodiment for simultaneously carrying out the two measuring methods accomplishes this with only one signal emitter, that is only one vibration-emitting device  41 , and only one signal receiver, that is, only one measuring device  44 . The embodiment is illustrated in  FIG. 2  and shows a diaphragm speaker that is suitable for this. 
         [0111]    A bar magnet or an iron core  61  is mounted by way of an elastic diaphragm  62  in a pressure housing  60 . Mounted on the outside of the pressure housing  60  is an impulse magnet  63 , which transmits the electrical signals generated by the electronic signal generator  51  to the bar magnet or iron core  61 . The electrical signals of the electronic signal generator  51  are converted in this way to vibrations of the iron core  61 . Connected to the iron core  61  is a schematically illustrated membrane  64 , which thus vibrates along with the iron core  61 . 
         [0112]    The vibrations, as mentioned above, are low-frequency. They lie, in particular, on an order of magnitude of between 5 Hz and 1000 Hz, preferably between 5 Hz and 100 Hz. The vibrations of the membrane  64  are transmitted to the gaseous medium lying below the membrane  64  and said membrane, in turn, is in connection with the gaseous medium  14  in the annulus between the inner pipe  21  and the outer pipe  22  in the piping  20  in the wellbore  10  or else constitutes its uppermost region. 
         [0113]    The electric signals of the signal generator  51  are thus transmitted as sonic signals or pressure waves or signal patterns  42  to the gaseous medium  14  in the wellbore  10  between the inner pipe  21  and the outer pipe  22 . 
         [0114]    The measuring device  44  with the signal receiver is located within the device  40  in a screw-on housing. This is indicated only schematically in  FIG. 2 . The data received by the measuring device  44  are transmitted further to the electronic analysis unit  52 , which, here, in turn, is symbolized by an arrow. 
         [0115]    Finally, a shut-off valve  65  is provided, which allows an adjustment of the pressure in the pressure housing  60  in the region outside of the membrane  44 . 
         [0116]    The entire construction is set up such that a use in tests up to interior pipe pressures of 300 bar was possible. 
         [0117]    The advantage of the illustrated construction, which is indicated schematically in  FIG. 2 , consists in the fact that no cable leadthroughs into the pressure housing  60  are required. Furthermore, the housing can be designed to be relatively small. 
         [0118]    Illustrated in  FIG. 3  is a schematic illustration of the form of the signal transmission, which, for instance, is employed for a use of a diaphragm speaker and a measuring head from  FIG. 2 . 
         [0119]    Seen is a measuring device  44 , which operates as a signal sensor. It transmits its signals further to the electronic analysis unit  52 . The latter has a low-frequency amplifier  71 , which appropriately amplifies the signal of the measuring device  44  and transmits it further to a unit  72  for a noise pattern analysis and to a unit  73  for a fluid depth determination. Afterwards, the data of the units  72  and  73  then go to a memory storage unit  53  and to a display unit  54  as well as any further data processing elements. 
         [0120]    From there or directly from the electronic analysis unit  52 , the data are transmitted further to the electronic signal generator  51 , which, in turn, transmits them further via a second low-frequency amplifier  74  to the signal emitter or the vibration-emitting device  41 . 
         [0121]    Illustrated in  FIG. 4  is an alternative embodiment. Fundamentally, it has the same elements  41 ,  44 ,  51 ,  52 ,  53 ,  54 ,  71 ,  72 ,  73 , and  74 . However, there are additional elements. 
         [0122]    This embodiment may then be employed to advantage when the use of a common device as a vibration-emitting device  41  and a measuring device  44  is accomplished, that is, when they are able to use the same membrane and also additional identical elements. In this case, a corresponding actuation and switching is employed in order to control the use for various application purposes. 
         [0123]    Seen here, therefore, is a device  75  for synchronization or for use as a signal path changeover switch. This device  75  is supplied, on the one hand, with the data of the measuring device  44  and it transmits this data of the electronic signal generator  51  further to the vibration-emitting device  41 . It ensures that the membrane can be used in each case as an emitting and receiving element. 
         [0124]    The device  75  can then act directly on the other elements or receive data and information from them and thereby enables an even more direct and more informational analysis. 
         [0125]    Instead of the two fully separated signal paths in  FIG. 3 , the branch generating the signal and the branch analyzing the signal are now connected together via an electronic changeover switch. This device  75  disposes over its own intelligence. Depending on the parameters set by the signal generator  51 , which are set depending on the construction of the wellbore  10  and a maximally possible terminal depth of the fluid depth  13 , the device  75  has to switch the sending path at the proper moment and activate the receiving branch. 
         [0126]    On account of the different parameter settings, this operation varies in time. Nonetheless, it is possible to operate the phases precisely in relation to the sending signal. 
         [0127]    Such changeover switches exist in the field of radar technology, but may also be modified for the low-frequency region in accordance with the invention. 
         [0128]    A noise pattern analysis can equally take place, even when the fluid depth measurement is activated. Only for the time of emission of the excitation signal is the noise pattern analysis briefly interrupted. 
         [0129]    In the embodiment of  FIG. 4 , a fluid depth measurement is thus possible using a combined sensor. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           10  Wellbore 
           11  Ground surface 
           12  Crude oil 
           13  Boundary layer or fluid depth 
           14  Gaseous medium 
           20  Piping 
           21  Inner pipe 
           22  Outer pipe 
           23  Pipe joints 
           30  Pump 
           31  Pipeline 
           40  Device 
           41  Vibration-emitting device 
           42  Signal patterns or pressure waves 
           43  Returning signal patterns or pressure waves 
           44  Measuring device 
           51  Electronic signal generator 
           52  Electronic analysis unit 
           53  Memory storage unit 
           54  Display unit 
           55  Remote transmission device 
           56  External signal sensor 
           60  Pressure housing 
           61  Iron core 
           62  Diaphragm 
           63  Impulse magnet 
           64  Membrane 
           65  Shut-off valve 
           71  Low-frequency amplifier 
           72  Unit for a noise pattern analysis 
           73  Unit for a fluid depth determination 
           74  Second low-frequency amplifier 
           75  Device