NL8100250A - ACOUSTIC LOG SYSTEM WITH SWING ENERGY SOURCE. - Google Patents

Description

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Acoustic log system with swing energy source.

The invention relates to methods and systems for measuring acoustic wave runtimes in earth formations in the vicinity of the well borehole. The invention particularly relates to techniques for measuring multiple acoustic wave component (or wave propagation) travel times in earth formations in the vicinity of a well borehole.

The measurement methods utilize sweep frequency transmission techniques and cross-correlation comparison techniques between the transmitted signal and the received signal.

10 Sonorous or acoustic well logging has become an important method for determining the physical properties of earth formations in the vicinity of a well borehole. Measurement of the acoustic compression wave velocity or transit time between a transmitter and a receiver in a well borehole can determine the physical properties of the earth formations indicative of the ability of these formations to deliver oil or gas. For example, a measurement of the compression wave travel time or velocity provides a direct indication of the porosity of the formation in the vicinity of the well borehole. Such acoustic velocity or transit time measurements are therefore practically standard for all new wells being drilled.

In the prior art, acoustic pulse or pulsed sonic logging techniques have been used to measure the travel time or velocity of acoustic waves in the earth formations in the vicinity of a borehole. Such prior art methods typically use pulse-driven acoustic transmitters. An acoustic transmitter is energized or pulsed by pulses and the time length required for the acoustic wave pulse generated by the transmitter to propagate from the transmitter through the earth formations in the vicinity of the borehole and back to an acoustic receiver, located at a spaced distance from the transmitter. By suitably combining the measurements of acoustic wave runtime 8100250, - 2 - with different acoustic receivers spaced at different distances from either a single (or multiple) acoustic transmitter, the acoustic wave runtime or sonic compression wave velocity of propagation of the earth formation.

Highly detailed schemes and geometrical considerations for eliminating the effect on borehole flow measurement and borehole fluids have also been developed.

In later years, it has become desirable to measure acoustic wave mode travel times other than just compression-10 wave velocity. For example, U.S. Pat. No. 4,131,875 describes techniques for measuring the so-called "late arrival" waves or Stonely waves. Other known techniques as disclosed in U.S. Pat. No. 3,354,983 describe the measurement of transverse acoustic wave velocities. In all these techniques, an acoustic impulse is generated by the transmitter and the waveform of the acoustic signal is analyzed at one or more receivers to determine the velocity of the pressure, transverse or stonely waves in the vicinity of the borehole.

Pulsed acoustic techniques depend on the amplitude detection of the arrival of acoustic waves at a receiver. Such techniques are subject to errors, caused by random noise, which occurs when a well log tool is moved through the borehole. Acoustic noise can be caused by the instrument body or centralizers on the instrument body, which scratch along the sides of the borehole as the tool is moved therethrough.

Accordingly, pulsed acoustic techniques with pulsed acoustic transmitters for measuring transverse waves or Stonely waves depend on an extensive interpretation of the waveform of the incoming wave at the receiver. Such interpretations are generally based on theoretical calculations made with simplified mathematical models of the earth formations in the vicinity of the borehole. If the simplified mathematical model turns out to be wrong, the interpretation of the arriving waveform at the receiver may be wrong and its connection to more complicated actual geometric shapes and conditions 8 1 0 0 25 0 * 4 - 3 - which then will be considered taken in the model can lead to erroneous interpretations of the waveform of the arriving acoustic signal.

It would be highly desirable to provide a method for measuring the transit time of different components of acoustic energy (compression or primary wave, cross wave,

Rayleigh or pseudo Rayleigh, direct (fluid) wave, expansion wave and Stonely wave) in earth formations in the vicinity of a well borehole, which was not dependent on a theoretical interpretation of an impending acoustic pulse waveform in terms of a model. The system according to the invention provides for a direct measurement of the transit time of various components of acoustic energy from a transmitter to a receiver in earth formations in the vicinity of a well borehole.

The invention provides a borehole log instrument with an acoustic transmitter and at least one acoustic receiver spaced a longitudinal distance from the transmitter. Multiple transmitters and receivers can be used as desired. The output signal from the acoustic transmitter of the invention is repeatedly swept over a predetermined frequency range. The transmitter's frequency sweep output is propagated in all different modes of acoustic energy propagation through the earth formations and the borehole and is detected at the spaced receiver. A synchronization signal is also generated at the beginning of each repetitive sweep of the transmitter over its predetermined frequency range. The synchronization signal and the received signal from the receiver are transferred to the surface of the earth through conductors of the well log cable. At the surface, the received signal is converted from analog to digital form and stored in a memory. The station sweep signal is stored in a surface sweep signal memory in digital form.

After completing a sweep of the transmitter and after receiving, digitizing and storing the received signal for a predetermined length of time, the sweep signal from the transmitter cross-correlates with the received signal. Because of the 8100250 φ * - 4 - characteristic sweep frequency pattern applied to the transmit signal, indications are derived from the cross correlation of the arrival of the different modes of acoustic energy propagation at the receiver. The time differences between the synchronization pulse and the arrival of the different modes of acoustic propagation at the receiver can then be explained in terms of transit time of the different modes of acoustic propagation at the receiver. These signals can then be recorded as a function of borehole depth as the well-logging tool is moved through the borehole. The entire swing, transmit, and receive process is performed repeatedly during such borehole instrument movement.

The invention will be explained in more detail below with reference to the drawing.

Figure 1 shows a complete block diagram of a well log system according to the invention.

Figure 2 shows a graph of an acoustic waveform received at a spaced receiver from a pulsed acoustic transmitter as used in the prior art.

Figure 3 graphically depicts a typical sweep frequency waveform applied to the transmit acoustic transducer of the invention.

Figure 4 shows a graph with a sweep frequency signal applied to an acoustic transmitter according to the invention, a composite or mixed mode arrival signal arriving at the acoustic receiver according to the invention, and the output of a cross-correlation between the sweep and the composite arrival signal according to the invention.

Figure 5 schematically shows well logging as a function of the depth of the acoustic compression wave velocity and the granulator output and shows compression and transverse wave arrivals according to the invention.

Figure 1 shows a preferred embodiment of a system for generating and receiving acoustic signals and for logging a well borehole according to the invention.

A well borehole 10 penetrates earth formations 15 and is filled with a borehole fluid 12. A well borehole log probe 11 is suspended via a well log cable 13, which runs over a pulley wheel 14, into the borehole 10. The pulley wheel 14 is electrically or mechanically coupled to a well log recorder 28 of conventional design 5 as indicated by the dotted line 16, so that measurements made by the well bore probe 11 can be recorded as a function of the borehole depth.

The borehole probe 11 includes a fluid-tight, hollow body member of dimensions and adapted to pass through a well borehole. An acoustic transmitter 32 and an acoustic receiver 33 are arranged within the fluid-tight probe 11. The circuit for driving the acoustic transmitter 32 includes a sweep signal storage memory 29, which may include a read-only memory or the like, a digital-analog converter 30, and a filter 31.

The receive acoustic transducer 33 is drawn longitudinally spaced from the transmit transducer 32. Typical spacing distances of 90 to 300 cm may be used as desired. It will be appreciated that the transmit acoustic transducer 32 and receive transducer 33 are acoustically coupled to the borehole through acoustically impedance matching material such as oil or oil filled bellows or the like (not shown) in the art. The transmit and receive transducers can include piezoelectric transducers. The transmit and receive transducers are sized and designed to have a linear or flat response characteristic across the sweep frequency range used in the art of the invention.

While only an acoustic transmitter and an acoustic receiver are drawn in the system of Figure 1, it will be apparent to one skilled in the art that the number of acoustic receivers can be varied, as well as the number of acoustic transmitters, if desired. In such a case, different sweep patterns can be used for each acoustic transmitter to characterize its outgoing acoustic energy from that of any other acoustic transmitter used in the 81 logging instrument.

The sweep signal storage memory 29 contains digital numbers representative of the amplitude of the sweep pattern to be applied to the transmit transducer 32 as a function of time at a preselected sampling interval time or rate. For example, a typical sweep frequency pattern can be drawn by the equation (1). t = T0 2 _. „." - ω.

f (t) = sin / ü) j + (-2l-O) 10 t = Tj

In equation (1), a sine wave whose frequency changes in a linear fashion from at Tj to at is described. Such sweep function amplitudes can be generated by a computer as a function of time and the results 15 then stored in a read-only memory or ROM device for subsequent use in the sub-surface tool and surface equipment as desired. .

Digital signals from the sweep signal storage ROM 29 are sequentially read and converted to analog signals 20 by the digital-analog converter 30. The output of the digital-analog converter 30 is filtered through a low-pass filter 31 to remove the small sample to sample step introduced by the digital-analog converter (ie for removing high-frequency components) and the output voltage signals from the filter 31 drive the transmit transducer.

A typical swing pattern as described by equation (1) is indicated in Figure 3. A synchronization pulse is generated at the start of a swing cycle and is indicated in Figure 4 by synchronization pulse. An acoustic sweep frequency signal with a linearly increasing frequency and starting at a time of about 0.1 milliseconds. after the synchronization pulse, is drawn. The frequency of the transmitter drive signal increases to a time approximately 5 milliseconds following the synchronization pulse, generating an acoustic sweep frequency signal of substantially constant amplitude and linearly varying frequency of, for example, 2 to 12 kHz and duration 8100250 9 - 7 - of about 4 millisec. It will be appreciated that other time durations and sweep frequency ranges can be used if desired.

The acoustic signals detected by the receive transducer 33 are filtered through a bandpass filter 34 to remove interference signals which are far from the passband of the original sweep frequency signal. After filtering, the signals are amplified by an amplifier 35 and applied to a veremetering system 36, which transfers the received acoustic signal waveform to the surface via conductors of the well log cable 13.

Timing of the transmitter sweep situation and the synchronization pulse are controlled by the verge measurement system 36, which contains an accurate frequency clock, such as a crystal controlled oscillator. The synchronization signal drawn in Figure 4 is transferred to the surface so that the surface electronics can be accurately synchronized for each time of starting the transmitter sweep cycle. For a 4 millisec. swing speed and an approximately 10 millisec. receiver record-20 time, as shown in Figure 4, the entire cycle of transmitter sweep and receiver reception transmitted to the surface can be repeated at a repetition rate of 10 to 20 cycles per second. It will be clear to a person skilled in the art that the duration of reception by the receiver and the transmission of received signals is a function of the distance between the transmitter and the receiver. For typical distance on the order of 120 to 180 cm, the 10 millisec. receive signal transmission cycle drawn in Figure 4, suitable.

At the surface, a synchronization detector and timing circuit 18 detects the synchronization signal and outputs an analog-to-digital converter 21, a signal memory 22, a grain memory 24 and a sweep signal memory 19. The receiver signal from the borehole verifier is amplified in an amplifier 20 and converted into digital form by an analog-35 digital converter 21, the time of which is determined by the signal from the synchronization detector and time chain 18. The digital 8100250 9 * - - 8 form of the received signal is then stored in a signal memory 22. At an appropriate time, when the entire receiver signal waveform has been digitized and stored in the signal memory 22, the synchronization detector and timing circuit 18 supplies a peak or output pulse to the swing signal storage 19 and to the signal memory 22, whereby these two signals are supplied as input in digital form in a granulator 23.

The granulator 23 performs a cross-correlation function 10 on the two input signals, as determined by the equation (2).

K = N

φχγ (τ) = Σ (X) (Y, + τ) (2) Κ = -Ν 15 In equation (2), Χ ^ and are discrete functions of time. Thus, the cross-correlation function φχγ is also a discrete function of time. If X ^ and Y ^ each contain N points and the offset value τ is equal to the sampling interval of X ^. and Y ^, the total number of points supplied by the cross-grain correlator 23,20 will be 2N-1. The number of products formed by the cross. .. 2 correlations for example of N points is N.

The digital output of the granulator 23 is supplied to a granulator memory 24, which is also supplied with time pulses from the synchronization detector and time chain 18 as indicated above. The digital output from the granulator memory, upon receipt of a suitable time pulse from the circuit 18, is fed to a digital-to-analog converter 25, where it is converted back into analog form for display as shown in Figure 5. The output from the digital Analog converter 25 is then filtered through a band-pass filter 27 and fed to a recorder 28 for recording as a variable density display as shown in the right half of the well log display as a function of depth shown in Figure 5.

The output of the granulator memory 24 is also dedicated to a transit time computer 26 which calculates the transit time from the transmitter to the receiver for selected arrivals at the receiver such as the compression wave transit time and the cross wave transit time. The compression wave travel time or cross wave travel time is then supplied to the recorder 28 for recording as a function of depth as shown in the left half 5 of the well log display of Figure 5.

In Figure 4, the sweep signal, the composite receiver signal and the cross-correlation of the sweep signal and the composite receiver signals are indicated as a function of time. It will be noted that the cross-correlation-10 output exhibits peaks, which can be explained in terms of compression wave arrival, cross wave arrival, direct wave arrival, and Stonely wave arrival. Transit times for these different acoustic modes can thus be calculated by the transit time computer 26 by comparing these arrivals with the synchronization pulse and deriving the time difference therewith to these arrivals.

One skilled in the art will appreciate that power for the operation of the borehole electronics as well as the surface electronics can be supplied from a surface power supply 17 through conductors of the borehole log cable 13. Suitable borehole power converters (not shown) may be mounted in the borehole probe 11 to provide operating voltages for the electronic borehole systems in a manner known in the art.

Figure 2 shows an acoustic waveform from a pulsed transducer as used in the prior art. The typical acoustic waveform can be interpreted according to propagation rates of different modes of downhole acoustic energy propagation. Thus, the initial arrival is generally explained as that of the compression wave, which is usually propagated faster through the earth formations in the vicinity of a well borehole. Later in the arrival waveform, energy peaks appear, which can be explained as the cross wave, the fluid wave and the Stonely wave-35 parts of the acoustic waveform. Depending on the distance between transmitter and receiver and the amount of reflection occurring within the borehole, interference may occur between the different modes of propagation in known pulsed acoustic transit measurements for the different modes of acoustic propagation. The present invention, using a unique or characteristic variable sweep frequency signal and correlation of this signal with the entire acoustic waveform arriving at the receive transducer, can provide more quickly identifiable output pulses at the cross-correlator output as shown in Figure 4 to separate the different arrivals of acoustic mode propagation in a manner superior to that known in the art. Thus, better acoustic runtime measurements of compression, transverse, Stonely and other modes of acoustic propagation by the invention are provided, which are subject to ambiguous interpretation in the prior art.

It will be apparent to one skilled in the art that the acoustic transmit transducer and acoustic receive transducer of the invention can be mounted on support arms (not shown) and pressed against the borehole wall as desired, rather than being housed in the body of the probe as shown in Figure 1. Accordingly, a back arm (not shown) could be used as desired to press the body of the probe of Figure 1 against a borehole wall. Due to the statistical nature of the cross-correlation when detecting the arriving signals at the receive transducers 25 of the invention, so-called "road noise" or noise generated by movement of the log tool through the borehole is minimized.

It will be clear that changes are possible within the scope of the invention.

30 8100250

Claims (17)

1. System for measuring acoustic energy propagation characteristics of earth structures in which a well borehole is provided, characterized in that a fluid-tight hollow body member of dimensions and adapted for passage through a well borehole, and a housing, means for repeatedly generating acoustic energy outputs with a variable range of frequencies from a lowest frequency to a highest frequency in a characteristic sweep frequency pattern, longitudinally spaced members from the generating means for detecting acoustic energy propagated from the borehole generators and earth formations in the vicinity of the borehole and to generate signals representative thereof, means for granulating the characteristic sweep frequency pattern and the signals representative of the propagated acoustic energy, for deriving correlation output signals as an indication for or the arrival at the detectors of different modes of propagation of acoustic energy in the borehole and the earth formations in the vicinity of the borehole, and means responsive to the correlation output signals for deriving measurements of the speed of propagation of the different modes of propagation of acoustic energy. System according to claim I, characterized in that means are provided for recording those measurements of the propagation speed of the different modes of acoustic energy propagation as a function of the borehole depth. System according to claim 2, characterized in that means are provided for recording the correlation output signals as a function of the borehole depth. System according to claim 1, characterized in that the means for correlating the characteristic sweep frequency pattern and the signals representative of the propagated acoustic energy are provided with means for digitizing the characteristic sweep frequency pattern and the signals 81 00 25 0 - 12 - representative of the propagated acoustic energy and means for digitally cross-correlating the digitized signals. System according to claim 4, characterized in that memory means are provided for storing the digitized characteristic sweep frequency pattern and the digital representative signals before inputting these signals to the digital cross correlation means of the digitized signals. System according to claim 1, characterized in that the generating means and the detecting means are spaced in the longitudinal direction over a distance of a few tens of centimeters from each other. System according to claim 6, characterized in that the generating means and the detecting means are spaced longitudinally from each other over a distance of 120 to 360 cm. System according to claim 1, characterized in that the characteristic sweep frequency pattern has an approximately linearly varying frequency pattern as a function of time. System according to claim 8, characterized in that the characteristic sweep frequency pattern comprises an approximately linearly varying frequency pattern as a function of time and varying from approximately 2 to 12 kHz. System according to claim 9, characterized in that the duration of the characteristic sweep frequency pattern is about four milliseconds. is. 11. Method for measuring acoustic energy propagation characteristics of earth formations in which a well borehole is arranged, characterized in that in a well borehole acoustic energy output signals are generated with a varying range of frequencies from a lowest frequency to a highest frequency in a characteristic swing frequency pattern, detecting a longitudinal distance from the position where the characteristic swing frequency pattern is generated, in a well borehole, of acoustic energy propagated through the borehole and the 8100250-13 earth formations in the vicinity of the borehole and generating of signals representative therefor, correlation of the representative signals and the characteristic sweep frequency pattern for deriving correlation output signals as an indication of the arrival with the detection modes of acoustic energy propagation in the earth formations and the borehole, and derivation due to these correlation output signals, indications of the velocity of propagation of the different modes of acoustic energy in the borehole and the earth formation in its vicinity. 12. A method according to claim 11, characterized in that said steps are repeated at different depths in a well borehole and registration of these indications of propagation speed as a function of the borehole depth is made. Method according to claim 12, characterized in that the correlation output signals are recorded as a function of the borehole depth. 14. The method of claim 11, characterized in that the graining of the representative signals and the characteristic sweep frequency pattern is performed digitally by digitizing the signals and digitally cross-correlating these signals to provide digital correlation output signals. Method according to claim 11, characterized in that the characteristic sweep frequency pattern is an approximately linearly varying frequency pattern as a function of time. A method according to claim 15, characterized in that the linearly varying frequency pattern ranges from about 30 to 12 kHz. 17. Device and method substantially as described in the description and / or shown in the drawing. 8 1 00 25 0