Abstract:
A method and apparatus for measuring properties of fluid flowing through a bore, using a signal transmitted substantially perpendicularly to the direction of flow. A preferred embodiment uses a pair of acoustic transducers mounted in the annular body of the fluid conduit. The tool is constructed so as to be able to operate for a period of years in a downhole environment without maintenance or replacement. An transducer is also described that is adapted to fit within the dimensions of an annular body without suffering a reduction in the length of the transducer backing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates generally to apparatus and methods for acoustically determining various properties of a fluid flowing through a conduit. More particularly, the present invention relates to an acoustic transducer system for monitoring production fluids in completed wells. Still more particularly, the present invention comprises a tool having one or more opposed transducer pairs that are acoustically isolated from the tool body and transmit acoustic signals across the fluid stream. Applications of the present invention also include an angled transducer backing that allows transducers to be used in any annular device, including tubing and rotating logging heads. Transducers configured in accordance with the present invention can be used in either pitch-catch or pulse-echo mode. 
     BACKGROUND OF THE INVENTION 
     Description of the Related Art 
     Production of hydrocarbons from geologic formations in the earth is typically accomplished by drilling a well to the desired formation. Once the desired drilling objectives have been met and prior to the start of production, the well undergoes a completion process. The completion process entails cementing the annulus between the casing and the borehole wall and isolating the production zone(s) from the rest of the strata. The casing is perforated at the production zone(s) so as to allow the formation fluids to drain into the casing. To isolate the production zones from other fluids in the casing, production tubing is installed inside the casing. The formation fluids, comprising mainly gaseous or liquid hydrocarbons, flow upward through the production tubing to the surface, where they are captured for processing. 
     Once a well has been completed in this manner, it may produce steadily and require no further attention for many years. Nevertheless, for a variety of reasons, it may be desirable to monitor the flow of fluids through the production tubing, using sensors installed in sensor modules that are deployed as part of the production tubing string. For example, the controlling equipment that receives the fluids from the formation may need to be modified or optimized, depending on the nature of the fluids it receives. Similarly, it may be desirable to monitor each producing zone in order to detect the encroachment of water, gas, or other undesired fluid into the well. It is often necessary to maintain data on the rate at which each of the flowstream constituents is produced for one or more wells in a reservoir in order to monitor the effectiveness of a reservoir production scheme, detect faults in the production equipment for an individual well, and/or in determine royalty payments for produced hydrocarbons. Hence, the pressure, temperature, flow rate, density, chemical composition and water content are all properties of the fluid stream whose measurement would be advantageous. 
     Because many wells are remote and/or offshore, it is often desirable to install equipment that allows unmanned or automatic mode monitoring of the producing well. For example, it is desirable to provide a production monitoring system that can measure the desired parameters and transmit the resulting data to a remote data-receiving site. 
     Similarly, because the monitoring system is downhole, maintenance or replacement of the monitoring system requires a trip into the hole and a corresponding interruption in production, Hence, it is desirable to maximize the period for which a production system can operate without maintenance or replacement. An ideal production monitoring system would be able to operate for at least twenty and up to thirty or more years without interruption, so as to reduce the amount of downtime and maximize efficiency. The corrosive environment that exists downhole reduces the types of equipment that are suitable for this application. 
     In addition, it is further desirable to minimize the amount of flow disruption caused by the monitoring sensors or tool. This typically means that the configuration of the sensors must correspond to the inside diameter of the production tubing. Hence, it is further desired to provide an effective production monitoring tool that can fit within the dimensions of the production tubing itself. 
     Finally, it is important that the monitoring system be capable of assessing a wide range of fluids. Production from most oil wells takes the form of a multicomponent fluid stream. For a typical oil well this stream may include crude oil, brine, hydrocarbon gases, various inorganic gases, and minor amounts of particulate matter. The proportion of each constituent of the production flowstream varies from well to well, and in a single well can vary significantly over time. 
     Various systems currently exist that purport to meet these requirements. Early methods for determining the fractional representation of the various fluids, i.e. liquids, gases, and combinations thereof, within the flowstream of an oil well relied on manual sampling and analytical procedures. A representative sample of the flowstream was collected and through physical separation and chemical analysis the fractional representation of each constituent was determined. Manual analysis is still used today in many instances, particularly where accuracy is particularly important. However, manual testing is relatively expensive, especially in remote oil fields or where frequent updating is necessary. Further, collecting small volume samples that accurately represent the flowstream is difficult, especially in high pressure, high temperature production systems. 
     Automated flowstream analysis systems have been developed to avoid much of the expense associated with manual testing. Early automated systems relied on gravity driven physical separation of the constituents of the flowstream. Such systems are not accurate for applications where the flowstream includes an oil-water emulsion, which cannot be gravity separated. Such systems are also of limited use for heavy oil reservoirs, where the density difference between the produced oil and brine is too small to provide significant driving force for gravity separation. Gravity driven automated analysis systems also tend to be bulky, expensive, and require careful maintenance. 
     Many additional, alternative techniques have been proposed to measure flow characteristics of fluids contained in tubular conduits. For example, some known techniques are based on sensing and correlating local pressure fluctuations or sensing the pressure field set up by a venturi or vortex element. Other techniques use measurements of the effect of the fluid stream on various types of radiation, such as gamma rays, to determine fluid properties. These techniques have obvious disadvantages, including safety and the continuous decay of the radiation source. 
     Acoustic monitoring systems use ultrasonic transducers that, through known acoustic signaling and signal processing techniques, are able to derive information about the fluid stream. Specifically, the speed of sound for a fluid can be calculated using the known distance between a transmitting transducer and a receiving transducer and the measured time required for a signal to traverse that distance. The transducers are typically annular rings that are spaced apart along the axis of the tool. Acoustic signals are transmitted axially, parallel to the direction of the fluid flow. An example of such a device is described in U.S. Pat. No. 4,003,252 to Dewath. The apparatus disclosed in the Dewath patent uses transducers recessed in the acoustically damping liner of a tube. 
     None of the prior art devices have been entirely satisfactory, however. Many of the conventional multiphase flow metering systems do not provide an accurate indication of the flow velocity of when the fluid flow is multiphase. Other conventional systems are prohibitively difficult to install or incorporate intrusive metering arrangements requiring interruption or alteration of the flow. Many conventional systems are compatible with only a limited range of pipeline designs and, accordingly, have limited utility. Finally, many systems are not capable of extended operation in the corrosive downhole environment. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an acoustic fluid stream monitoring system that can be positioned within an annular shell. Another object of the present invention is to provide a means for measuring the flow of multicomponent streams, including streams comprising both gas and liquid, through a pipe. Still another object of the invention is to provide a tool that is capable of withstanding the downhole environment for at least twenty years. 
     The present invention comprises a novel acoustic transducer configuration that can be mounted in an annular shell having the same dimensions as the tubing that transmits the fluid stream. In a preferred embodiment, at least two transducers are positioned on opposite sides of the annular shell, with their operative faces directed into the center bore of the shell. Preferably, the transducers face each other across the bore. In another preferred embodiment, transducers are positioned with their operative faces directed out toward the formation. 
     The present invention further comprises a novel transducer configuration that allows a transducer having a relatively long backing to fit into the reduced space available in an annular volume without severely compromising the signal amplitude available from that transducer. In the novel transducer configuration, the transducer face has an orientation that is normal or substantially normal to a radius of the tubing, but the backing that extends behind the transducer face is angled relative to both the transducer face and the head radius, so that its length is not limited by the available annular thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the present invention, reference will now be made to the accompanying Figures, wherein: 
     FIG. 1 is a schematic representation of a fluid monitoring system constructed in accordance with the principles of the present invention and in place in a competed well; 
     FIG. 2 is a cross-sectional end view of a sensor sub constructed in accordance with the principles of the present invention; 
     FIG. 3 is a partially cut away side view taken along lines  3 — 3  of FIG. 2; and 
     FIG. 4 is a longitudinal cross-section of the sensor sub shown in FIG. 2; 
     FIGS. 5A-B are longitudinal and normal cross-sectional views of a first alternative embodiment of the present sensor sub; 
     FIG. 6 is a cross-sectional end view of a second alternative embodiment of the present sensor sub; 
     FIG. 7 is a schematic representation of a preferred embodiment of a novel transducer configuration according to the present invention; 
     FIG. 7A is a schematic representation of an alternative embodiment of the present transducer; 
     FIG. 7B is a cross-sectional view taken along the lines B—B of FIG. 7A; 
     FIG. 7C is a schematic representation of a second alternative embodiment of the present transducer; 
     FIG. 8 is a schematic representations of a first alternative embodiment of a novel transducer configuration; 
     FIG. 9 is a schematic cross section showing four pairs of transducers configured so as to yield a plurality of signals that can be used in a tomographical analysis; 
     FIG. 9A is a schematic diagram showing a pair of multi-transducer rings axially spaced apart so as to permit correlation flow measurements for inhomogeneous fluids; 
     FIG. 10 is a cross-section showing a pair of transducers mounted in an annular shell so as to ring outward; 
     FIG. 11 is a cross-section showing a pair of transducers mounted in an annular shell so as to ring inward; and 
     FIG. 12 is a plot of a signal output from a sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, a completed well  10  in a formation  11  typically includes a casing  12  and production tubing  14 . Cement  28  fills the annulus between casing  12  and formation  11 . One or more packers  16  are used to isolate zones along the well axis from one another. At one or more producing zones  17 , the casing is perforated as at  18  so as to allow the ingress of formation fluids. A passage  20 , preferably valved, allows the fluid to enter production  14  tubing from the annulus  19  between casing  12  and production tubing  14 . Passage  20  can pass through sensor sub  22  as shown, or can be remote (not shown). If remote, fluid entering passage  20  must be monitored by the controlling sub sensors. The fluids then flow up to the surface through the bore  15  of production tubing  14 . 
     Sensor Sub 
     The sensors of the present monitoring tool are preferably located in a sensor sub  22  that forms part of the production tubing  14  at or above the entry point for the fluids that are to be monitored. Referring now to FIGS. 2 and 3, a first, preferred embodiment of an acoustic monitoring system  30  comprises an outer shell  32  and an inner shell  40 . Outer shell  32  is preferably adapted to support the mechanical load applied to the production string and is therefore substantially solid. Inner shell  40  has a central bore  43  through which production fluid passes on its way to the surface. Inner shell  40  is preferably adapted to support one or more pairs of transducer assemblies  42   a,    42   b,  described in detail below. One of transducer assemblies  42   a,b  is a transmitter adapted to provide an acoustic signal to the fluid in bore  43  and the other is adapted to receive the acoustic signal from the fluid. Because an acoustic signal is transmitted more quickly and efficiently by solid metal than by a fluid, however, the portion of the acoustic signal transmitted circumferentially from transducer assembly  42   a  to transducer assembly  42   b  along a path through the metal body of inner shell  40  will overshadow and obscure the portion of the signal that is transmitted through the fluid in bore  43  unless measures are taken to attenuate the former. According to one embodiment, these measures include the use of signal-damping bushings  44  that support transducer assemblies  42  in inner shell  40  and the placement of multiple longitudinal slots  46  in the body of inner shell  40 . Bushings  44  are preferably made of epoxy potting or Teflon®. Slots  46  preferably alternate between slots  48  that intersect the end of inner shell  40  and slots  49  that do not. When configured this way, slots  46  break up the circumferential path that would otherwise allow the acoustic signal to be transmitted efficiently from transducer assembly  42   a  to transducer assembly  42   b  through the body of inner shell  40 . By configuring slots  46  as shown, the length of the path through the shell body is greatly increased, with a corresponding attenuation in the signal as it is transmitted along that path. In other instances, it may be acceptable to provide fewer slots, or slots that are differently configured, but in any event it is desirable to lengthen the path through the tool body sufficiently to prevent it from interfering with the signal travelling along the desired path. 
     It is preferred but not required to fill slots  46  with a signal damping material, such as epoxy potting or Teflon®. The length and number of slots  46  can be adjusted to ensure that the portion of the signal transmitted through inner shell  40  is sufficiently attenuated. Outer shell  30  does not include signal attenuating slots because this would tend to weaken outer shell  30 , whose purpose is to provide mechanical support. Nevertheless, the portion of the acoustic signal that is transmitted circumferentially through outer shell  30  is insignificant, because transducers  42  are acoustically isolated from outer shell  30 , preferably by signal-damping bushings made of epoxy potting or Teflon®. 
     While transducer assemblies  42  are shown extending radially outward through outer shell  30 , this is not necessarily the case. Transducer assemblies  42  can be any desired length, so long as they fit within the dimensions of the sensor sub and so long as their backings are long enough to adequately attenuate the backing reverberations that are generated when each transducer is fired. 
     Referring now briefly to FIG. 4, the sensor sub  22  may have an outside diameter that is greater than the outside diameter of production tubing  14 . For example, outer shell  30  and inner shell  40  can be connected to production tubing  14  by a pair of end pieces  50  that provide a transition from the outside diameter of production tubing  14  to the outside diameter of outer shell  30 . End pieces  50  are preferably welded to the ends of outer shell  30  and inner shell  40  at welds  31 ,  41 , respectively. End pieces  50  should be at sufficient distance from the transducers so that acoustic arrivals from the end pieces cannot interfere with the fluid measurements. An example of a suitable distance is ten times the fluid path length. 
     Referring now to FIGS. 5A and 5B, an alternative embodiment for sensor sub  22  is shown in cross-section. According to this embodiment, the housing is a single annular shell  60  having a central bore  61  and a plurality of generally longitudinal bores  62  extending therethrough. Shell  60  preferably has an outer diameter that is significantly larger than the outer diameter of the adjacent pipe. Bores  62  are preferably slightly inclined with respect to the tool axis, as shown. Each bore  62  defines a cavity within which is preferably positioned a signal damping slug  63 . Each slug  63  preferably includes at least one end portion  65  having a diameter substantially equal to the inside diameter of the bore  62  in which it is received. In this manner slug  63  provides support for the bore  62  and helps reduce the risk that bore  62  will be crushed. Each slug  63  preferably also includes a neck portion  67  having a diameter that is smaller than the inside diameter of the bore. This creates an annular space around neck  67 , which helps prevent the acoustical transmission. In some embodiments, an acoustical attenuation fluid may be provided in the gap. Suitable acoustical signal attenuators are also fully described in U.S. Pat. No. 5,510,582, which is incorporated herein in its entirety. Alternatively, bores  62  are preferably filled with a signal-damping material such as tungsten loaded rubber. Bores  62  are preferably provided in sufficient number and in such spacing as to attenuate the portion of the acoustic signal that is transmitted circumferentially through shell  60  enough to prevent it from obscuring the portion of the signal transmitted through the fluid in central bore  61 . In this embodiment, shell  60  supports both transducers  42  and the mechanical load applied to the production string. 
     As shown in FIG. 6, another alternative embodiment for sensor sub  22  comprises a sub body  70  having a central bore  71  therethrough. In this embodiment, the path of the circumferential tool body signal is lengthened by removing alternating segments of the tool body, as at  72 ,  74 . The location and shape of the removed material must be designed to maintain sufficient mechanical strength to withstand tubing stresses that occur during operations and packer setting. 
     Transducers 
     Referring now to FIG. 7, a preferred transducer  100  for use in the present invention includes a piezoelectric element  102  mounted on a backing member  110 . Piezoelectric element  102  and backing member  110  are enclosed in a housing  120  that comprises an acoustical window  122 , a flexible corrugated sleeve  124  and a rigid sealing base  126 . The outer surface  104  of piezoelectric element  102  preferably contacts the inside of window  122 . In one preferred embodiment, corrugated sleeve  124  acts as a spring that biases piezoelectric element  102  toward window  122 . Housing  120  is preferably sized such that there is a small annular space  103  surrounding piezoelectric element  102  mounted and backing member  110 . This volume is preferably filled with a liquid having good acoustical properties, such as silicone oil or Fluorinert®. The sealed configuration in which the backed piezoelectric transducers are supported in the tool preferably includes means for balancing hydrostatic pressure on the inside of the transducer housing with the hydrostatic pressure outside the transducer housing. Because the hydrostatic pressure of the fluid in the production tubing (i.e. outside the transducer housing) fluctuates with depth and in response to variations in pressure at the surface and variations in fluid weight, the volume of the fluid inside housing  120  needs to fluctuate so as to maintain a pressure balance. The pressure balance is needed if any component of the transducer, such as the backing, permanently deforms under pressure. In the present device, variations in fluid volume are achieved by flexing of corrugated sleeve  124 . Fluctuations in hydrostatic pressure are matched by expansions or contractions of the fluid surrounding transducer  102  and corresponding movement of sleeve  124 , thereby allowing hydrostatic pressure to remain equal inside and outside the housing. FIGS. 7A and 7C depict alternative embodiments of transducer housings that include pressure balancing systems. In particular, in FIG. 7A, transducer  102  is housed in a can  134 , which includes a fluted wall  135  (FIG. 7B) that allows the volume within the can to fluctuate. As in FIG. 7, piezoelectric element  102  and backing member  110  are surrounded by a fluid  113 , such as Fluorinert®. In FIG. 7C, the volume of can  136  is fixed and the volume of the fluid  113  surrounding piezoelectric element  102  and backing member  110  is allowed to vary through the flexing of a diaphragm  138 . While one side of diaphragm  138  contains the fluid inside can  136 , the other side of diaphragm  138  is in fluid communication with the borehole fluid. Still another alternative configuration for transducer  100  is shown and described in U.S. Pat. No. 5,644,186, which is incorporated herein in its entirety. 
     Referring now to FIG. 8, an alternative transducer  300  for use in the present invention includes a piezoelectric element  302  mounted on a backing member  310 . Piezoelectric element  302  has an exposed surface and a backing surface and is preferably, but not necessarily, non-circular. Backing  310  preferably comprises a cylindrical base  312  and a tapered extension  314 , which terminates at a face  316 , which corresponds in shape to the shape of the backing surface of piezoelectric element  302 . Piezoelectric element  302  is affixed to face  316  in a conventional manner. Backing  310  is mounted in a recess with piezoelectric element  302  oriented toward the fluid in the central tool bore and its innermost surface  319  preferably in fluid communication with a hydrostatic pressure balancing system. Tapered extension  314  may have any cross-section, but a conical or pyramid-shaped extension may be most advantageous due to manufacturing considerations. Regardless of the shape of extension  314 , it preferably shares a common axis  318  with backing  310 . It is further preferred, but not necessary that the transducer mount include an O-ring  320  that forms as seal between the tool body and backing  310 . This seal prevents the oil or other fluid used in the fluid-pressure balancing system from leaking out around the transducer. It is also preferred that the surfaces of tapered backing extension  314  not touch the material from which the tool body is formed, so as to avoid undesired secondary reverberations. It will be understood by those skilled in the art that individual features of the foregoing system can be altered or substituted without altering the overall operation of the system. 
     Tomography 
     While the transducers in FIGS. 2-6 are shown as a single pair opposing each other along a diameter of the tool, it is contemplated that some instances it will be desirable to construct a tool where this is not the case. For example, it is necessary to provide multiple acoustic paths through the fluid when it is desired to characterize a multiphase, inhomogeneous fluid. Hence, in FIG. 9 a sensor sub  400  includes an array comprising four transmitter/receiver pairs. The first,  410   a-b,  oppose each other along a diameter  410   c.  Second and third pairs  412   a-b  and  413   a-b  oppose each other along chords, respectively  412   c  and  413   c,  that transect the central bore. Chords  412   c  and  413   c  may or may not be parallel to the diameter between  410   a  and  410   b.  Finally, a fourth pair,  414   a-b,  oppose each other along a line  414   c  that is preferably substantially perpendicular to  410   c.  The composite information received from these transducer pairs can be used in a sparse tomographic analysis to yield information about the number and relative proportion of phases present in the fluid stream. The tomographic information may include attenuation and speed of sound along each path. It will be understood that a tomographic array in accordance with the present invention may have more or fewer than four pairs of opposed transducers. 
     In addition, a correlation flow meter can be formed by using two rings of transducers like those shown in FIG.  9 . As shown in FIG. 9A, two or more tomographic arrays  400  such as the one shown in FIG. 9 can be spaced apart a distance d along the longitudinal axis of the tool. For multiphase fluids, the correlation between patterns in the first ring and those in a second ring will have a time delay related to the movement of the pattern from one ring axially to the other. The flow velocity equals the axial distance between the rings divided by the time it takes for the pattern to move axially from one ring to the next. Hence, using cross correlation between arrays, in conjunction with pattern recognition, one can estimate the delay time t d  for a pattern to travel the distance d. The flow velocity of the material contained in the inhomogeneity is equal to v f =d/t d . 
     Regardless of how many transducers are used, it is not necessary that the face of each transducer be perpendicular to a radius of the bore passing through its center point. More specifically, and in contrast to the transducers commonly in use in prior art tools, the longitudinal axis of each transducer (eg.  318  in FIG. 8) does not necessarily lie on a radius of the logging tool, but may be shifted so that it does not intersect the tool axis. It will be understood that this angle can be varied as desired, depending on the desired direction of the transducer radiation pattern. Placing the transducer axis at an angle means that the body of the transducer backing can be made longer than would otherwise be possible. As discussed above, it is desirable to provide as long a backing as possible, so as to maximize attenuation of the backing reverberations that are generated when transducer  302  is fired. For example, in FIG. 10, the face of the left-hand transducer  350  lies at an angle of 74 degrees with respect to a tool radius passing through its center, while the face of the right-hand transducer  352  lies at an angle of 90 degrees with respect to a tool radius passing through its center. Similarly, in FIG. 11, the axis of each transducer  354  lies on an angle with respect to a tool radius through the center of the active (ringing) face. In FIG. 10, transducers  350 ,  352  face outward from the tool body, while in FIG. 11, transducers  354  face in toward the bore. 
     Materials 
     In order to provide an acoustic tool that is capable of surviving downhole for the desired period, the materials from which the various components of the tool are made are selected for long life and compatibility with the other parts of the tool. For instance, fluid components of the tool, such as are used in the pressure balancing system, preferably comprise innert hydrocarbons. Examples of suitable fluids include silicone oil and perfluorinated hydrocarbons, such as that marketed by the 3M corporation under the trademark Fluorinert®. 
     For the transducers, the backing, piezoelectric material and window are each constructed of conventional materials. Preferred backings are tungsten-loaded epoxies, or tungsten-loaded rubbers. Lead zirconate titanates (PZT) are preferred for the piezoelectric elements and titanium is preferred for the acoustic windows. The PZT is particularly suitable, as it is matched in acoustic impedance to titanium. 
     Circuitry 
     A suitable driver circuit for operating the acoustic transmitter of the present invention is any circuitry that provides the piezoelectric transducers with sufficient ringing energy can be substituted. Furthermore, any device capable of generating a detectable acoustic signal in the fluid can be substituted for the transmitting transducers and any device capable of generating a detectable electrical signal in response to the acoustical signal transmitted by the fluid can be substituted for the receiving transducers described below. 
     Referring briefly to FIG. 12, an exemplary received signal is shown with time periods t 1 , t 2 , t 3 , and t 4  marked thereon. In operation, a switch is closed for a time period t defined as t 1 ≦t≦t 2 . Time t 1  corresponds to the end of the transmitter-induced spurious receiver signal and time t 2  corresponds to slightly less than the earliest possible occurrence of a flow-path acoustic signal. Both values are preset, based on calibration of the tool in a controlled environment. During this period, the background noise level is determined by rectification and peak-detection of the filtered acoustic output. This background energy level is stored. The voltage is then multiplied by predetermined multipliers x 1  and x 2  to give a first threshold  THRESHOLD    1  and a second predetermined threshold  THRESHOLD    2 . Time t 4  is defined as the time at which the output from a first comparator indicates that the received signal has exceeded  THRESHOLD    1 . Similarly, time t 5  is defined as the time at which the output from a second comparator indicates that the received signal has exceeded  THRESHOLD    2 . The two times, t 4  and t 5 , are used to extrapolate back to the time at which the output voltage first became non-zero, i.e. the actual time of the first break t b . For example, using a linear extrapolation, t b =t 4 −(t 5 −t 4 )=2t 4 −t 5 . Alternative techniques for determining the first arrival time of the signal transmitted through the fluid are equally acceptable, including setting t b =t 4 . In the latter instance, only one level detector is required. Regardless of how t b  is determined, using t b  and the known distance between the transmitter and receiver, the speed of sound for the formation fluid can be calculated. 
     If it is desired to obtain the real amplitude of the oscillatory signals, the energy envelope of the signal can be determined. The energy envelope is obtained from the equation: 
     
       
           A   0 =[( V   2 +(2 πf )−2( dV/dt ) 2 ] ½   (1) 
       
     
     where V is the received signal and f is the approximate center frequency of the received signals. This amplitude is measured by full-wave rectification and filtering. 
     For situations with only one transmitter and one receiver, the attenuation can be estimated by using narrow band electrical filtering to obtain amplitude at several frequencies. The ratios of amplitudes vs. frequencies can be used to estimate the dB per wavelength of attenuation, provided that the frequency dependence of the transducer is relatively constant with time. Stable electronics must be selected. An example of a suitable technique for processing the signal is disclosed in reference to FIG. 5B of U.S. Pat. No. 5,726,951, which is incorporated herein in its entirety. 
     The optimal frequency range for the present tool depends on the characteristics of the well in which it will be placed and can be determined using known techniques. The preferred tool preferably balances signal strength with accuracy by operating in the range of 50 kHz-200 kHz when the fluids are gaseous and in the range of 100 kHz-2 MHz when the fluids are liquid. 
     The system described herein provides many advantages over prior art tools, including accurate measurement of inhomogeneous fluids over prolonged periods. 
     It will be recognized that various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. For example, the transmitters, sensors and signal processing means described herein can be used to advantage in a range of applications, including those such as are described in U.S. Pat. No. 5,841,734, which is incorporated by reference in its entirety. Thus, while the principal preferred construction and mode of operation of the invention have been explained in what is now considered to represent its best embodiments, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically illustrated and described.