1. Field of the Disclosure
The present disclosure is related to the field of electric wireline well logging tools. More specifically, the present disclosure is related to systems for two way communication of signals between logging tools disposed in wellbores and a recording and control system located at the earth's surface
2. Background of the Art
Electric wireline well logging tools are used to make measurements of certain properties of earth formations penetrated by wellbores. The measurements can assist the wellbore operator in determining the presence, and quantity if present, of oil and gas within subterranean reservoirs located within the earth formations.
Well logging tools known in the art are typically extended into the wellbore at one end of an armored electrical cable. The cable includes at least one, and commonly includes as many as seven, insulated electrical conductors surrounded by steel armor wires. The armor wires are included to provide abrasion resistance and tensile strength to the cable also provide the mechanical strength to suspend logging instruments in the borehole. The cable supplies electrical power to the logging tools and provides a communication channel for signals sent between the logging tools and a recording system usually located near the wellbore at the earth's surface.
Logging tools known in the art can provide many different types of measurements of the earth formation properties, including measurements of electrical resistivity, natural gamma-ray radiation intensity, bulk density, hydrogen nucleus concentration and acoustic travel time, among others. Still other logging tools, generally called “imaging” tools, provide finely detailed measurements, meaning successive measurements can be made at axial and radial spacings of as little as several hundredths of an inch, of resistivity and acoustic pulse-echo travel time in order to generate a graphic representation of the visual appearance of the wall of the wellbore.
It is generally beneficial to the wellbore operator to be able to combine as many different types of logging tools as is practical into one continuous instrument package (generally called a “tool string” by those skilled in the art). The benefit to the operator is to reduce the number of times logging tools must be extended into the wellbore, which can save a considerable amount of operating time. Combining a large number of measurements generally requires that large amounts of signal data be sent to the recording system at the earth's surface.
A particular problem in combining large numbers of measurements in the tool string is that the large amount of signal data which must be transmitted can cause the required signal data transmission rates to exceed the signal carrying capacity of the cable. This problem is particularly acute when the imaging tools are included in the tool string because of the very fine measurement spacing, and consequently the large increase in the amount of signal data, of imaging tools relative to other types of tools.
Conventional cables are used for two-way transmission of signals in addition to supplying power to the downhole logging tool assembly. The upward communication comprises the data recorded by the individual logging tools while the downward communication comprises control commands to the logging tools. The control commands may include instructions for setting the parameters used in the tools. Some prior art devices rely on the half-duplex mode wherein at any one time, only one way communication is possible. This is inefficient and leads to a decrease in the total throughput of the communications links.
Orthogonal Frequency Division Multiplexing (OFDM) has been widely used in the telecommunication industry for Asymmetric Digital Subscriber Lines. The present disclosure addresses the problems with implementing OFDM for the problems of communication through a cable into a borehole.
One embodiment of the disclosure is a method of communicating information through a downhole cable. The method includes: selecting a plurality of channels for communicating the information; simultaneously encoding and modulating the information at a first end of the cable; receiving signals at a second end of the cable responsive to the encoded and modulated information; and demodulating and decoding the received signals to provide an estimate of the information.
Another embodiment of the disclosure is a system for communicating information through a downhole cable. The system includes: a first processor configured to simultaneously encode and modulate the information at a first end of the cable in a plurality of channels; a receiver at a second end of the cable configured to receive signals responsive to the encoded and modulated information in the cable; and a second processor configured at the second end of the cable configured to demodulate and decode the received signals to provide an estimate of the information.
The present disclosure is best understood with reference to the accompanying figures in which like numerals refer to like elements and in which:
a-1f (Prior Art) show cross-sections of commonly used wirelines for logging applications;
a shows an exemplary constellation for TCM in the present disclosure;
b shows a block diagram of the TCM encoding process showing the convolutional coding and set partitioning;
a-1f show cross-sections of commonly used logging wirelines, the most common being the 7 conductor cable of
Communications sent from the surface to the downhole location primarily comprise instructions for control of the logging tool 205: its depth, rate of movement, and the control of the FE sensors on the logging tool. Communications from the downhole location to the surface comprise the measurements made by the formation evaluation sensors along with information regarding the location and orientation of the logging tool 205 corresponding to the measurements made by the FE sensors.
The basic approach taken here is to use Orthogonal Frequency Domain Multiplexing (OFDM). In OFDM a large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. This is illustrated in
OFDM has a number of benefits. The multiplexing with minimal overlap means that the available spectral bandwidth is efficiently used. The orthogonality results in low interference between the sub-carriers. The narrow bandwidth of the individual sub-carriers makes the system robust with respect to narrow band noise: only a few sub-carriers would be affected by the noise. No equalization of the signal levels in the different sub-carriers is necessary. The method is also characterized by a low symbol rate (SR), which results in low inter-symbol interference (ISI). Another advantage of OFDM is that the core calculations can be implemented using a Fast Fourier Transform (FFT) or an Inverse Fast Fourier Transform (IFFT): operations that can be implemented using the limited capabilities of a downhole processor. Specific problems associated with implementation of OFDM for communication between a surface location and a downhole location through a wireline cable are discussed next.
Referring now to
Referring now to
Reed-Solomon codes are block codes. This means that a fixed block of input data is processed into a fixed block of output data. In the case of the most commonly used R-S code (255, 223)—223 Reed-Solomon input symbols (each eight bits long) are encoded into 255 output symbols. Corresponding the R-S coding 403 at the transmitter, there is an R-S decoding in the receiver 451, the output of which is the recovered data stream.
An inherent limitation of digital processing systems is that data can be read (or transmitted) faster than it can be processed. For this reason, interleaving is provided in the system 405. What this means is that the blocks of data coming from the R-S coding are not sequentially arranged; instead, they are interleaved. This ensures that during the time a block of data is being processed, a number of blocks are temporarily bypassed and the next block that is accessed by the processor is the block that sequentially follows the block being processed. As a counterpart to the interleaving 405, there is a de-interleaving 453 in the receiver.
Still referring to
Fundamental to TCM is the concept of a constellation. For the purposes of the present disclosure, we adopt the definition given in Wikipedia. A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. It displays the signal as a two-dimensional scatter diagram in the complex plane at symbol sampling instants. In a more abstract sense, it represents the possible symbols that may be selected by a given modulation scheme as points in the complex plane. Measured constellation diagrams can be used to recognize the type of interference and distortion in a signal. The term “constellation” is defined as the entire ensemble of signals possible with a particular modulation method. For the present disclosure, the modulation may include Phase-Shift Keying and Amplitude Shift keying.
As noted in Ungerboeck (1982), Signal waveforms representing information sequences are most impervious to noise-induced detection errors if they are very different from each other. Mathematically, this translates into the requirement that signal sequences should have large distance in Euclidean signal space. The essential new concept of TCM that led to the aforementioned gains was to use signal-set expansion to provide redundancy for coding, and to design coding and signal-mapping functions jointly so as to maximize directly the “free distance” (minimum Euclidean distance) between coded signal sequences. This allowed the construction of modulation codes whose free distance significantly exceeded the minimum distance between uncoded modulation signals, at the same information rate, bandwidth, and signal power. The term “trellis” is used because these schemes can be described by a state-transition (trellis) diagram similar to the trellis diagrams of binary convolutional codes. The difference is that in TCM schemes, the trellis branches are labeled with redundant nonbinary modulation signals rather than with binary code symbols.
a shows an exemplary constellation used with the present disclosure. The number of points in the constellation is a function of the signal-to-noise ratio. This means that the lower numbered channels (corresponding to lower frequencies) will have more elements in the constellation than the higher numbered channels. The design of TCM can be interpreted in terms of the convolutional coding with the set partitioning of signal constellations. This is illustrated in
Returning to
A pre-equalization of the signals is done 407. The pre-equalization is discussed with reference to
With pre-equalization, the spectral correction is done at the transmitter, so that the OFDM has a spectrum shown by 811. A resulting change in a time-domain signal is indicated by 813 and the received OFDM spectrum is 807′. The noise spectrum is now 811, i.e., flat.
Returning to
Next, a prefix is added to the signals. This is necessary due to the dispersive nature of the communication channel and the resulting frequency-dependent time delay. This is best understood with reference to
b shows that the generated symbol length has been increased by adding a prefix p at the beginning and end of each symbol, so that the symbol k 911 is longer than the symbol 901, and the symbol 913 is longer than the symbol 903. After passing through the communication channel with the delay, the result is shown in 915 and 917. Also shown are intervals corresponding to the original symbol length. It can be seen that at all three of the frequencies, the received symbol is not corrupted by signal from other frequencies.
The reverse operation in the receiver is denoted in
An interpolation of the data is done 417. In one embodiment of the disclosure, this involves an upsampling by a factor of 4. A first order Lagrange interpolation may be used. This upsampling reduces problems with the Digital to Analog converter (DAC) 421 caused by transitions between successive symbols. The output of the DAC 421 is sent to the communications driver 423 and on to the wireline 475. The output of a driver 471 in the receiver is fed to the Analog to Digital Converter (ADC) 469. The output of the ADC is oversampled to avoid problems caused by transitions before being sent to the symbol synchronization and de-prefixing 465. The decimating 463 brings the data back to 128 samples in the time domain.
The channel compensation 459 removes the channel gain on the symbol signals in the frequency domain.
It should be noted that the communication between the surface location and the downhole location could be in a simplex mode, a half duplex mode and/or a full duplex mode as the terms are understood in the art.
The operation of the transmitter and receivers may be controlled by the downhole processor and/or the surface processor. The modulation/encoding and demodulation/decoding are done by the downhole processor and the surface processor respectively Implicit in the control and processing of the data is the use of a computer program on a suitable machine readable medium that enables the processor to perform the control and processing. The machine readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and Optical disks. The results of the processing include telemetry signal estimates relating to measurements made by downhole formation evaluation sensors. Such results are commonly stored on a suitable medium and may be used for further actions in reservoir development such as the completion of wells and the drilling of additional wells.
This application claims priority from U.S. Provisional Patent Application Ser. No. 61/179,988 filed on May 20, 2009.
Number | Date | Country | |
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61179988 | May 2009 | US |