Abstract:
A wireline acoustic probe and associated methods provide enhanced calibration and communication capabilities in a downhole acoustic communication system. In a described embodiment, an acoustic probe is conveyed on a wireline to a position proximate a downhole acoustic transmitter. A command is transmitted acoustically from the probe to the transmitter, causing the transmitter to generate acoustic frequency sweeps. The sweeps are received by the probe proximate a downhole acoustic receiver, permitting an optimum acoustic transmission frequency to be selected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to downhole acoustic communication systems and, in an embodiment described herein, more particularly provides an acoustic probe conveyed on a wireline to facilitate calibration and communication in an acoustic communication system. 
     It is well known to communicate acoustically downhole by transmitting acoustic signals through a tubular string positioned in a wellbore of a well. Components of an acoustic communication system are typically interconnected in the tubular string, so that the components communicate with each other by transmitting acoustic signals through the tubular string extending between the components. The tubular string between the components may be made up of various tubular elements having, for example, differing wall thickness, length, etc. In addition, for different wells with different spacings between the components, the number of connections in the tubular string between the components varies. 
     Unfortunately, the variations in the tubular string material, length, number of connections, etc., between the components makes it difficult to determine beforehand an optimum frequency for acoustic communication between the components. One aspect of the difficulty is that, prior to interconnecting the acoustic communication system in the tubular string, the variations in the tubular string may not be known. Furthermore, after the tubular string has been installed in the well, with the acoustic communication system interconnected therein, it may not be possible to make adjustments to the acoustic frequency used for transmission in the system, and so a preselected frequency must be used, even though it is less than optimum. 
     SUMMARY OF THE INVENTION 
     In carrying out the principles of the present invention, in accordance with an embodiment thereof, an acoustic communication system is provided which utilizes an acoustic wireline probe. Associated methods are also provided. 
     In one aspect of the present invention, the acoustic probe is capable of acoustically communicating with components of a downhole acoustic communication system interconnected in a tubular string positioned in a well. For this purpose, the probe may include a piezoelectric device for transmitting acoustic commands to the system components. In addition, the probe may include an accelerometer for receiving acoustic signals transmitted from the system components. 
     In another aspect of the present invention, the probe may be utilized in a method of calibrating the communication system. For example, the probe may be positioned proximate a transmitter of the system and the probe may acoustically command the transmitter to generate acoustic frequency sweeps. The probe may then be positioned proximate a receiver of the system, where the probe receives the frequency sweeps. The received frequency sweeps may be analyzed to determine an optimum frequency for communication between the transmitter and the receiver. The probe may then be positioned proximate the transmitter, where the probe commands the transmitter to use the optimum frequency for subsequent communication with the receiver. In this manner, the system may be calibrated so that it uses the optimum frequency for acoustic communication, after the system has been installed in the well. 
     In another aspect of the present invention, the probe has the capability of transmitting acoustic signals received by it in either analog or digital form. For example, it may be desirable, for diagnostic purposes, for an operator at the surface to be able to hear the “raw” acoustic signals received by the probe downhole. This is accomplished by the probe transmitting the received acoustic signals via a wireline in analog electrical form. Where a digital form of received acoustic signals is desired, such as for data analysis, the probe may include an onboard digital signal processor, so that the received acoustic signals are transmitted via the wireline in digital electrical form. 
     In still another aspect of the present invention, the acoustic communication system is configured in a unique manner which facilitates its use with the probe. The transmitter is interconnected in the tubular string above a packer and a valve in the string. In this manner, even though the valve may be closed, preventing fluid flow through the tubular string, the probe may still be positioned proximate the transmitter for communication therewith, and communication with other components in the string is enhanced. The transmitter may include a sensor sensing a property of a fluid disposed below the packer. 
    
    
     These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic partially cross-sectional view of a method embodying principles of the present invention; 
     FIG. 2 is a schematic partially cross-sectional view of the method of FIG. 1, wherein further steps in the method have been performed; and 
     FIG. 3 is a schematic block diagram of a wireline acoustic probe embodying principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Representatively illustrated in FIG. 1 is a method  10  which embodies principles of the present invention. In the following description of the method  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein maybe utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
     In the method  10 , a downhole acoustic communication system  12  has been installed in a well. Components of the communication system  12  are interconnected in a tubular string  14  disposed in the well. These components include a transceiver  16 , a repeater  18  and a transmitter  20 , thusly named according to the functions which the respective components normally perform in the communication system  12 . However, it is to be clearly understood that each of the components may have both acoustic transmission and reception capabilities. 
     The transmitter  20  normally collects data from sensors, such as sensor  22 , and transmits the data to the repeater  18 . The repeater  18  receives the data and retransmits it to the transceiver  16 . The transceiver  16  receives the data and communicates it to a surface computer or control terminal  24 . 
     The transmitter  20  is interconnected in the tubular string  14  above a valve  26  and a packer  28 . For measurement of a property, such as temperature or pressure, of a fluid below the packer  28 , the sensor  22  is in communication with the fluid via a passage, such as a tube  30 , extending from the sensor to below the packer. Of course, the sensor  22  could be positioned below the packer  28  with wires permitting communication between the sensor and the remainder of the transmitter  20 , without departing from the principles of the present invention. 
     Note that the configuration of the communication system  12  as depicted in FIG. 1 is especially useful in operations such as formation testing, where the valve  26  may be closed to allow buildup of formation pressure, and the valve may be opened to permit drawdown of formation pressure. These pressure changes may be sensed by the sensor  22  and transmitted to the surface (along with other data, such as temperature, etc.) using the communication system  12 . 
     Positioning the transmitter  20  above the valve  26  and the packer  28  provides several advantages. For example, the transmitter  20  does not have to transmit an acoustic signal to the repeater  18  through the valve  26  or the packer  28 , and is instead positioned somewhat closer to the repeater. As another example, such placement of the transmitter  20  permits an acoustic probe  32  to be conveyed on wireline  34  through the tubular string  14  and into the transmitter, even though the valve  26  may be closed. However, it is to be clearly understood that the communication system  12  may be differently configured, without departing from the principles of the present invention. Furthermore, the probe  32  may be otherwise conveyed, for example, it could be conveyed by coiled tubing, etc. 
     Preferably, the transceiver  16 , repeater  18  and transmitter  20  are programmed prior to being installed in the well, so that they may communicate with each other using one or more acoustic frequencies determined by expected characteristics of the tubular string  14 . Since the tubular string  14  between the transceiver  16  and repeater  18  is typically different from the tubular string between the repeater and the transmitter  20 , different frequencies may be used for transmission in these different portions of the tubular string. For example, the transmitter  20  may transmit data to the repeater  18  at one frequency, and the repeater may transmit the data to the transceiver  16  at another frequency. 
     In the method  10 , use of the probe  32  enables the communication system  12  to be calibrated so that it utilizes the optimum frequency for acoustic communication through each respective portion of the tubular string  14 . In this manner, an optimum frequency for communication between the transmitter  20  and repeater  18  may be determined and utilized, and another optimum frequency for communication between the repeater  18  and transceiver  16  may be determined and utilized. 
     The probe  32  is initially positioned proximate the repeater  18 . Preferably, the probe  32  is positioned inside the repeater  18  for maximum acoustic coupling via the metal of a centralizer  33  attached to the probe, but this is not necessary in the method  10 . The centralizer  33  extends between the probe  32  and the repeater  18 , or otherwise provides acoustic coupling between the probe and the tubular string  14 . However, it is to be understood that acoustic coupling between the probe  32  and the repeater  18  may be accomplished by other means, without departing from the principles of the present invention. 
     In one unique aspect of the probe  32 , it is capable of transmitting, rather than merely receiving, acoustic signals. To initiate the calibration process, the probe  32  transmits a command to the repeater  18 , causing the repeater to generate repetitious acoustic frequency sweeps, that is, the repeater repeatedly transmits a broad range of acoustic frequencies through the tubular string  14 . 
     The probe  32  is then repositioned in the tubular string  14 , so that it is proximate the transmitter  20 . FIG. 2 depicts the probe  32  positioned inside the transmitter  20 . Again, it is preferable that the probe  32  be positioned inside the transmitter  20 , but this is not necessary. 
     While positioned proximate the transmitter  20 , the probe  32  receives the frequency sweeps generated by the repeater  18  and transmitted through the tubular string  14 . Acoustic coupling between the probe  32  and the tubular string  14  is provided by the metal of the centralizer  33 , but as discussed above, other means of providing acoustic coupling may be used, without departing from the principles of the present invention. 
     The probe  32  transmits the received frequency sweeps to the surface via the wireline  34  for analysis. For example, frequency spectrum analysis software, such as SpectraPlus™, may be used to select one or more optimum frequencies having high amplitudes at the transmitter  20  as compared to other frequencies. As used herein, the term “optimum” is used to describe a frequency having a comparatively high amplitude as transmitted through a portion of a tubular string. 
     The transceiver  16  receives the frequency sweeps transmitted from the repeater  18  through the tubular string  14  to the surface. The received frequency sweeps are then analyzed to select an optimum frequency for communication between the repeater  18  and the transceiver  16 . Thus, the method  10  permits an optimum frequency to be determined for communication between the repeater  18  and the transmitter  20 , and permits an optimum frequency to be determined for communication between the repeater and the transceiver  16 , although these optimum frequencies may be different from each other. 
     Once the optimum frequencies have been determined, the probe  32  transmits a command acoustically to the transmitter  20  to cause the transmitter to use the appropriate optimum frequency for communication between the repeater  18  and transmitter  20 . The probe  32  is then positioned proximate the repeater  18 . The probe  32  acoustically transmits a command to the repeater  18  to cause the repeater to use the appropriate optimum frequency for communication between the repeater and transmitter  20 , and to use the appropriate optimum frequency for communication between the repeater and the transceiver  16 . The transceiver  16  may be reprogrammed at the surface, using the computer  24 , to use the appropriate optimum frequency for communication between the repeater  18  and the transceiver. 
     As an alternate calibration method, the computer  24  could be used to cause the transceiver  16  to generate frequency sweeps, which are received proximate the repeater  18  by the probe  32 . The probe  32  could then be positioned proximate the transmitter  20 , which could be commanded by the probe to generate frequency sweeps. These frequency sweeps could be received by the probe  32  positioned proximate the repeater  18 . The frequency sweeps as received by the probe  32  could be analyzed to determine optimum frequencies for transmission in the portions of the tubular string  14  as described above. The probe  32  may then be used to reprogram the repeater  18  and transmitter  20  to use the optimum frequencies for transmission in the respective tubular string portions. Therefore, it may be seen that the particular order and direction of frequency sweep generation and reception may be varied in the method  10 , without departing from the principles of the present invention. 
     The communication system  12  is, thus, calibrated to use optimum frequencies for communication in the method  10 . Additionally, the method  10  permits calibration of the system  12  so that it uses different frequencies for communication in different portions of the tubular string  14 . Furthermore, the method  10  permits the system  12  to be calibrated after it has been interconnected in the tubular string  14  and installed in the well. 
     The probe  32  includes unique features which enhance its usefulness in the method  10  and in the communication system  12 . Due to its ability to transmit acoustic signals, the probe  32  may be used to transmit information or instructions to the repeater  18  or transmitter  20  downhole, before optimum frequencies have been determined for transmission through the tubular string  14 . Therefore, even though it may not initially be possible for the transmitter  20  to communicate with the repeater  18  for exchange of data or instructions, the probe  32  may be used to communicate with either of them. 
     Due to the ability of the probe  32  to transmit received acoustic signals to the surface or another remote location via the wireline  34 , the probe may be used to communicate downhole, even though there may be a problem with the acoustic communication system  12 . For example, the probe  32  may be positioned proximate the transmitter  20  for download of data stored therein. In this situation, the probe  32  commands the transmitter  20  to acoustically transmit data stored therein, such as indications of fluid properties sensed by the sensor  22 . The data received by the probe  32  is transmitted via the wireline  34  directly to the surface. 
     Referring now to FIG. 3, a schematic diagram of the probe  32  is representatively illustrated. The probe  32  includes a stack of piezoelectric crystals  36  and a stack driver  38  for transmitting acoustic signals. The probe  32  also includes an accelerometer  40  for receiving acoustic signals. 
     The accelerometer  40  and stack driver  38  are connected to an interface board  42  for electrical communication with the wireline  34 . Thus, electrical signals on the wireline  34  may be used to cause the stack driver  38  to actuate the piezoelectric stack  36  to transmit an acoustic signal, and acoustic signals received by the accelerometer  40  may be transmitted to the surface via the wireline. 
     The probe  32  includes a digital signal processor  44  for converting the raw acoustic signal received by the accelerometer  40  into digital electrical form before being transmitted to the surface via the wireline  34 . The signal processor  44  is interconnected between the accelerometer  40  and the interface board  42 . However, the accelerometer  40  is also directly connected to the interface board  42  so that, when desired, the raw acoustic signal may be transmitted in analog electrical form via the wireline  34 , for example, for diagnostic purposes. 
     A power supply  46  receives power from the wireline  34  via the interface board  42  and converts it as needed to power the driver  38 , accelerometer  40  and processor  44 . However, a battery or other type of power source, and any additional power source, may be used in the probe  32 , without departing from the principles of the present invention. 
     Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.