Patent Application
20200064505
This invention pertains generally to well-logging technology for acoustically determining the cement bond between casing and formation in a cased wellbore. More specifically, the invention pertains to cement-bond logging technology that enables in-situ data acquisition at a first acoustic frequency by a logging tool disposed in a wellbore and transmission of the acquired data at a different frequency.
As is well-known in the art, holes may be drilled into the ground to access fluid deposits (e.g., water, oil, and natural gas) in subterranean formations. Often, the boreholes are lined with a tubular casing and cement is injected into the annulus between the outer surface of the casing and the borehole wall. The cement serves a variety of purposes, such as structurally supporting the casing and isolating zones (e.g., to prevent unwanted migration of fluids into an aquifer).
Cement bond logging provides in situ measurement of characteristics of the cement in the annulus between casing and borehole wall. A typical cement-bond-logging tool utilizes acoustic transducers to measure travel or reflection characteristics of acoustic signals in the borehole environment. The basic approach is well-known in the art. See, e.g., U.S. Pat. Nos. 3,729,705, 4,685,092, 4,740,928, and 5,377,160; Wang, et al., Understanding Acoustic Methods for Cement Bond Logging, 139 Journal of Acoustic Society of America 2407-2416 (May 2016), available at https://doi.org/10.1121/1.4947511.
In operation, a cement-bond-logging tool may be disposed in the borehole at the end of a wireline. The wireline is attached at one end to the tool disposed in the borehole and at the other end to a surface system. The surface system is electrically connected to the borehole-disposed tool through the wireline and typically includes a power supply for powering the tool, a transceiver for communicating with the tool, and a computer for controlling the tool and collecting information from the tool. Between the tool and the surface system, the wireline is partly spooled on a winch. The winch is used to position the tool along the longitudinal axis of the borehole by spooling out line to allow the tool to go “deeper” or winding up line to move the tool “shallower.” (“Deeper” and “shallower” here refer to the length along the borehole's longitudinal axis from the surface. The borehole is not necessarily vertical.)
In typical operation, a transducer in a cement-bond-logging tool disposed within a borehole is electrically operated to produce an acoustic signal. The acoustic signal travels from the tool to the casing (and through the casing) and then back to the tool (along various paths). The tool receives the return acoustic signal through a transducer, converts the acoustic signal to an electric signal, performs some level of signal processing on the electric signal, and transmits the processed signal along the wireline to the surface system for recording and display. It is possible to infer information about the cement in the annulus from the return acoustic signal. For example, the quickest return path for the acoustic signal is typically through the casing (as opposed to, for example, the formation, the borehole fluid, or the tool housing). The better the bond between the cement and the casing, the less acoustic energy returned along the casing. Thus, one can infer information about the casing-cement bond from the amplitude of the beginning of the return acoustic signal: lower amplitude indicates a better bond. (If the formation path is faster than the casing path, then a higher amplitude at the beginning of the return signal indicates a better bond.)
Two connected issues that affect the quality of a cement-bond-logging tool are: (1) the signal-to-noise ratio at the tool and (2) the signal-to-noise ratio at the surface. The first issue is related to the operating frequency of the transducers. Receiving too far off the receiver's resonance (at other than the transducer's resonant frequency) can significantly lower the signal-to-noise ratio at the tool. A transducer's resonant frequency is generally inversely related to its size—the smaller the transducer the higher the resonant frequency. Thus, smaller transducers require higher operating frequencies. The second issue is also related to the operating frequency of the transducers. Generally, the greater the frequency of the signal transmitted along the wireline, the greater the transmission losses. The wireline losses for the received signal increase with transducer operating frequency.
These two issues can be in tension—a higher transducer operating frequency may improve the signal-to-noise ratio at the tool (because it is closer to the resonant frequency of the transducer) but will increase transmission losses. Conversely, a lower operating frequency may improve transmission losses, but at the cost of degraded signal at the tool. In effect, the wireline losses either impose a size constraint on the transducers or require operating transducers well above the resonant frequency. Often, the borehole operating environment imposes a transducer size constraint in tension with the wireline-loss size constraint: the borehole requires small transducers (to reduce the diameter of the tool) with resonant frequencies well above what is acceptable for wireline losses. This size constraint is exacerbated for multi-transmitter applications. The typical cement-bond-logging tool operates at around 20 kHz and surface systems often have limited ability to process cement-bond-logging signals of frequencies greater than about 25 kHz.
Accordingly, there is a need for technology to enable a higher transducer operating frequency to improve the signal-to-noise ratio at the tool without increasing the transmission losses along the wireline and while maintaining compatibility with existing surface systems. This would, for example, enable use of smaller, higher-frequency, transducer configurations than is the practice in the art of cement-bond logging. For example, instead of the transducers conventionally used in a cement-bond-tool, the frequency-shifting technology disclosed herein can allow the use of several small stacked piezo transducers azimuthally dispersed in transmitter-receiver pairs to provide high-quality azimuthal cement-bond information. (A stacked piezo transducer is comprised of several stacked layers of piezoelectric material.)
This invention includes technology for shifting the frequency of an electric signal generated in response to a received acoustic signal before driving the shifted-frequency signal on a wireline. This technology enables a novel cement-bond-logging tool that utilizes stacked piezoelectric transducers that operate at a much higher frequency than can be transmitted on the wireline.
In one aspect of the invention, a cement-bond-logging tool includes an acoustic transmitter and receiver, an analog-to-digital converter (ADC), a memory, a digital-to-analog converter (DAC), a digitizing clock, and a converting clock. The ADC converts the analog signal of the acoustic receiver to a digital signal which is stored in memory (at least in part). The digitizing clock sets the sampling rate for the ADC. The DAC converts the digital signal stored in memory to an analog signal. The converting clock sets the sampling rate for the DAC. The sampling rate for the ADC is different from the sampling rate for the DAC. Thus, in operation, an acoustic signal received at the receiver is frequency shifted through the ADC/DAC conversions. The receiver or transmitter may be a piezoelectric stacked transducer
In another aspect of the invention, a cement-bond-logging tool includes a transmitting acoustic transducer configured to generate an acoustic pulse, a receiving acoustic transducer configured to generate an electric signal in response to receipt of an acoustic signal, and a circuit for shifting the frequency the electric signal generated by the receiving transducer. In operation, the transmitting transducer produces an acoustic pulse having a first frequency, the acoustic pulse travels through a borehole environment, the receiving transducer generates an electric signal having the first frequency in response to the receipt of the transmitted acoustic pulse, and the circuit shifts the first frequency to a second frequency. Typically, the second frequency is lower than the first frequency to enable smaller or higher-frequency transducers (e.g., piezoelectric stack transducers) and lower transmission losses when transmitting the electric signal to, e.g., a system for analysis, display, or storage.
In another aspect of the invention, a method for determining the quality of a bond between cement and casing includes placing an acoustic transmitter and receiver in a borehole, operating the transmitter to generate an acoustic pulse, operating the receiver to generate a signal in response to receipt of the acoustic pulse as transmitted through a borehole environment, digitizing the receiver signal at a first rate, converting the digitized signal to an analog signal at a second rate (thereby shifting the frequency of the signal), and driving the analog signal on a wireline. Typically, the second rate is lower first rate to enable smaller or higher-frequency transducers (e.g., piezoelectric stack transducers) and lower transmission losses when transmitting the electric signal to, e.g., a system for analysis, display, or storage. In a further aspect of the invention, the analog signal may be extracted from the wireline and its frequency may be shifted before analysis, display, or storage of the signal. In a further aspect of the invention, a portion of the receiver signal may be discarded before digitization or before converting the digitized signal to the analog signal. The discarded portion may be replaced before analysis, display, or storage of the signal.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings where:
In the summary above, and in the description below, reference is made to particular features of the invention in the context of exemplary embodiments of the invention. The features are described in the context of the exemplary embodiments to facilitate understanding. But the invention is not limited to the exemplary embodiments. And the features are not limited to the embodiments by which they are described. The invention provides a number of inventive features which can be combined in many ways, and the invention can be embodied in a wide variety of contexts. Unless expressly set forth as an essential feature of the invention, a feature of a particular embodiment should not be read into the claims unless expressly recited in a claim.
Except as explicitly defined otherwise, the words and phrases used herein, including terms used in the claims, carry the same meaning they carry to one of ordinary skill in the art as ordinarily used in the art.
Because one of ordinary skill in the art may best understand the structure of the invention by the function of various structural features of the invention, certain structural features may be explained or claimed with reference to the function of a feature. Unless used in the context of describing or claiming a particular inventive function (e.g., a process), reference to the function of a structural feature refers to the capability of the structural feature, not to an instance of use of the invention.
Except for claims that include language introducing a function with “means for” or “step for,” the claims are not recited in so-called means-plus-function or step-plus-function format governed by 35 U.S.C. § 112(f). Claims that include the “means for [function]” language but also recite the structure for performing the function are not means-plus-function claims governed by § 112(f). Claims that include the “step for [function]” language but also recite an act for performing the function are not step-plus-function claims governed by § 112(f).
Except as otherwise stated herein or as is otherwise clear from context, the inventive methods comprising or consisting of more than one step may be carried out without concern for the order of the steps.
The terms “comprising,” “comprises,” “including,” “includes,” “having,” “haves,” and their grammatical equivalents are used herein to mean that other components or steps are optionally present. For example, an article comprising A, B, and C includes an article having only A, B, and C as well as articles having A, B, C, and other components. And a method comprising the steps A, B, and C includes methods having only the steps A, B, and C as well as methods having the steps A, B, C, and other steps.
Terms of degree, such as “substantially,” “about,” and “roughly” are used herein to denote features that satisfy their technological purpose equivalently to a feature that is “exact.” For example, a component A is “substantially” perpendicular to a second component B if A and B are at an angle such as to equivalently satisfy the technological purpose of A being perpendicular to B.
Except as otherwise stated herein, or as is otherwise clear from context, the term “or” is used herein in its inclusive sense. For example, “A or B” means “A or B, or both A and B.”
An exemplary cement-bond-logging tool 100 as disposed in a borehole and connected to a surface system 110 via a wireline 102 is illustrated in
The cement-bond-logging tool 100 includes an electronics section 100a, an acoustic transmitter 100b, and an acoustic receiver 100c. The acoustic transmitter 100b generates an acoustic signal that travels through the borehole environment to return to the tool 100 where it is detected at the acoustic receiver 100c. The acoustic receiver 100c receives the return signal. Typically, the transmitter 100b and receiver 100c are piezoelectric transducers that convert electrical energy into mechanical vibration (the transmitter) and mechanical vibration into electrical energy (the receiver). The transmitter 100b is coupled to the borehole fluid 112 such that application of an electrical signal to the transmitter, which causes the transmitter 100b to vibrate, will generate a wave in the borehole fluid 112 (the acoustic signal). The receiver 100c is coupled to the borehole fluid 112 such that it will vibrate in response to a wave in the borehole fluid 112. The electronics section 100a includes: (1) circuitry for operating the transmitter 100b (e.g., applying the electrical signal to generate the acoustic signal), (2) circuitry for operating the receiver 100c (e.g., receiving the return acoustic signal to generate a return electrical signal), (3) circuitry for processing the return electrical signal, and (4) circuitry for communicating with the surface system 110 (e.g., to enable the surface system 110 to control operation of the transmitter 100b and receiver 100c and to collect the return electrical signal or a representation thereof).
The paths in
The cement-bond-logging tool 100 depicted in
In the typical operation of a cement-bond-logging tool, an electrical signal causes the transmitter to generate an acoustic pulse having a predetermined frequency (e.g., 20 kHz). The acoustic pulse will travel from the tool into the borehole environment causing various components to vibrate at the frequency of the pulse. This acoustic energy will travel through the borehole environment (e.g., borehole fluid, casing, cement, formation) and return to the receiver. The receiver vibrates at the frequency of the pulse and thereby generates and electrical signal at that frequency. The duration of the received signal depends on the extent to which the borehole components ring. Typically, the duration of the received signal is about 5-25 milliseconds. The transmit-receive cycle is repeated as the tool moves through the hole (e.g., as the winch winds the wireline in and raises the tool to the surface).
Three idealized curves representing the acoustic signals received at the tool (and converted to electric signals through the receiving transducer) are presented in
The amplitude 502a of the received casing-path signal 502 is a function of the bond between the casing and the cement in the annulus between the casing and the borehole wall. The better the bond, the more acoustic energy that is transmitted through the casing and the lower the amplitude 502a. The worse the bond, the more acoustic energy that remains in the casing path and the higher the amplitude 502a. The amplitude 502a, as registered by the receiver, is also a function of the amplitude of the signal produced by the transmitter and the receiver's ability to convert the acoustic energy to electrical energy. These, in turn, are functions of the frequency of the signal. Generally, smaller transducers perform better at higher frequencies.
The amplitude of the received formation-path signal 504 also provides information indicative of cement-bond quality. For example, the better the bond between the cement and the formation the more acoustic energy travels through the formation and the greater the amplitude of the received formation-path signal 504.
Ideally, the cement-bond-logging tool will capture the return signal through at least the borehole-fluid arrival time 506b. And it will send this signal to the surface system 110 via the wireline. In practice, it is common to capture about 2 milliseconds of the return signal (measured from the firing pulse of the transmitter).
Because of wireline losses, it is difficult to send a return signal to the surface system 110 when the frequency of the signal is much above 20 kHz. As a result, for higher frequency operation, e.g., at 100-120 kHz, it is typically the envelope of the signal that is captured and returned as opposed to the signal itself. This does not provide the same information as the full signal.
A processor 616 controls firing circuit 618 to supply a high voltage pulse (e.g., 1000 VDC) to a transmitter transducer 100c. The transmitter transducer 100c vibrates in response to the pulse and produces acoustic energy with a frequency of about 120 kHz. The 120 kHz acoustic energy travels through the borehole environment to be received at a receiver transducer 100b that converts the acoustic signal to an electrical signal. The processor 616 controls an analog-to-digital converter (ADC) 610 to sample the electrical signal from the receiver 100b at a sampling period defined by an ADC clock 614 provided by the processor (e.g., 1 million samples per second). Typically, the ADC clock starts a substantially the same time as the firing pulse (within about 5-10 nanoseconds). The processor 616 stores the sampled signal 612 from the ADC 610 in memory 620.
The processor 616 controls a digital-to-analog converter (DAC) 604 to convert the sampled receiver signal stored in memory 620 to an analog signal 606 provided to the line driver 602 at a sampling period defined by a DAC clock 608. The DAC 604 provides the analog signal 606 to a line driver 602 that provides the signal to the surface system 110 through the wireline 102. The DAC clock 608 is slower than the ADC clock 614 and is chosen so that wireline losses are acceptable, for example, the DAC clock 608 may operate at 168 thousand samples per second to simulate a signal of about 22 kHz.
The various electronic components of the system illustrated in
As illustrated in
At the surface system 110, the signal is corrected for the difference in ADC clock and DAC clock and for any discarded samples from the ADC signal. The resultant signal provides travel time and amplitude information for an operating acoustic frequency higher than what would be achievable if the signal was not frequency shifted (because of wireline losses). And it provides more information than is provided when only the envelope of the signal is provided by the cement-bond-logging tool.
Exemplary cement-bond-log presentations are shown in
While the foregoing description is directed to the preferred embodiments of the invention, other and further embodiments of the invention will be apparent to those skilled in the art and may be made without departing from the basic scope of the invention. And features described with reference to one embodiment may be combined with other embodiments, even if not explicitly stated above, without departing from the scope of the invention. The scope of the invention is defined by the claims which follow.
