Patent Publication Number: US-2016245078-A1

Title: Modulation scheme for high speed mud pulse telemetry with reduced power requirements

Description:
BACKGROUND 
     Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled. 
     Various types of tools referred to as downhole tools may be conveyed through the boreholes to perform various types of measurements to provide the analysts with the needed information. In order to make efficient use of drilling time, some downhole tools may be disposed on a drill string drilling a borehole so that measurements can be performed while the borehole is being drilled. These types of measurements may be referred to a logging-while-drilling or measurement-while-drilling. 
     Once the measurements are obtained, they can be transmitted by telemetry to a receiver at the surface of the earth so that they can be made quickly available to the analysts without having to remove the drill string from the borehole. One type of telemetry for while-drilling applications is mud-pulse telemetry. In mud-pulse telemetry, downhole data is encoded into a digital format and transmitted by acoustic pulses in drilling mud filling the borehole or interior of the drill string. In that energy is expended to transmit the acoustic pulses, it would be appreciated in the drilling industry if method and apparatus were developed to reduce the energy requirements for transmitting data using mud-pulse telemetry. 
     BRIEF SUMMARY 
     Disclosed is a method for transmitting data from a downhole location to a location at the surface of the earth. The method includes: receiving data using a modulator disposed at the downhole location on a drill tubular in a borehole penetrating the earth; modulating, using the modulator, the data using offset quadrature phase shift keying (OQPSK) and smooth transitions between phase shifts to produce a series of two-bit symbols having smooth transitions in transition intervals between phase shifts of both in-phase and quadrature-phase components of the OQPSK modulated data; transmitting the series of two-bit symbols as an acoustic signal in drilling fluid disposed in the borehole using a mud-pulser; receiving the acoustic signal using a receiver disposed uphole from the downhole location; and demodulating the acoustic signal using a demodulator coupled to the receiver to provide demodulated data. 
     Also disclosed is an apparatus for transmitting data from a downhole location on a drill tubular to a location at the surface of the earth. The apparatus includes: a drill tubular disposed in a borehole penetrating the earth and configured to convey drilling fluid; a modulator configured to receive data from the downhole location and to modulate the data using offset quadrature phase shift keying (OQPSK) and smooth transitions between phase shifts to produce a series of two-bit symbols having a smooth transitions in a transition intervals between phase shifts of both in-phase and quadrature-phase components of the OQPSK modulated data; a mud-pulser configured to transmit the series of two-bit symbols as an acoustic signal in drilling fluid disposed in the borehole; a receiver disposed uphole from the downhole location and configured to receive the acoustic signal; and a demodulator coupled to the receiver and configured to demodulate the acoustic signal received by the receiver to provide demodulated data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  illustrates a cross-sectional view of an embodiment of a downhole while-drilling tool disposed in a borehole penetrating the earth; 
         FIG. 2  depicts aspects of a mud-pulser having a plunger; 
         FIGS. 3A-3C , collectively referred to as  FIG. 3 , depict aspects of a mud-puller having a rotating or oscillating disc; 
         FIG. 4  depicts aspects of offset quadrature phase shift keying (OQPSK); 
         FIGS. 5A-5B , collectively referred to as  FIG. 5 , depicts aspects of smooth transitions in transition intervals between symbols in OQPSK; 
         FIG. 6  depicts further aspects of smooth transitions in transition intervals between symbols in OQPSK; 
         FIG. 7  is a functional block diagram of one embodiment of the mud-pulse telemetry system  100 ; and 
         FIG. 8  is a flow chart for a method for transmitting data from a downhole location on a drill string to a location at the surface of the earth. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method presented herein by way of exemplification and not limitation with reference to the figures. 
     Disclosed are method and apparatus for transmitting data from a downhole tool disposed on a drill string to a receiver at the surface of the earth using mud-pulse telemetry. The method and apparatus use less energy to transmit the same amount of data as prior art techniques. 
       FIG. 1  illustrates a cross-sectional view of an embodiment of a downhole tool  10  disposed in a borehole  2  penetrating the earth  3 , which includes an earth formation  4 . The downhole tool  10  is conveyed through the borehole  2  by a drill tubular  5  such as jointed drill pipe or coiled tubing for example. A drill bit  6  is disposed at the distal end of the drill tubular  5 . A drill rig  7  is configured to conduct drilling operations such as rotating the drill tubular  5  and thus the drill bit  6  in order to drill the borehole  2 . In addition, the drill rig  8  is configured to pump drilling fluid  13 , also referred to as drilling mud, through the drill tubular  5  in order to lubricate the drill bit  6  and flush cuttings from the borehole  2 . The downhole tool  10  may include one or more various tools for performing various downhole functions. The tools may include a sensor  8  or formation tester  9  as non-limiting examples. The sensor  8  may be configured to sense various downhole properties such a borehole property, a formation property or a tool property. The formation tester  9  includes an extendable probe  11  configured to seal to a wall of the borehole  2  and extract a sample of formation fluid for analysis downhole or in a surface laboratory. Various sensors in the formation tester may be configured to perform different types of measurements on the sample. Non-limiting examples of the measurements performed by the formation tester  9  or by the sensor  8  include pressure, temperature, density, viscosity, compressibility, radiation, and spectroscopy using optical transmissivity or reflectivity for example. 
     Data sensed or collected downhole (i.e., in the borehole) is transmitted to the surface of the earth  3  by a mud-pulser  12  that is configured to transmit an acoustic signal in the drilling fluid  13 . At the surface, the acoustic signal is received by a receiver  17 . The mud-pulser  12  includes a modulator  14  and downhole electronics  15 . The modulator  14  is configured to receive a bit stream that is composed of data received from the various tools in the downhole tool  10  and to modulate the bit stream into a digital signal. The downhole electronics  15  are configured to operate the mud-pulser  12  to transmit the digital signal as an acoustic pressure signal in the drilling fluid  13 . The downhole electronics  15  may also be configured to process data downhole in order to minimize the amount of data needed to be transmitted via the modulator  14 . Alternatively or in addition, data processing functions may be performed by a surface computer processing system  16 . The downhole tool  10  may also include memory (not shown) for storing measurements that cannot be immediately transmitted to the computer processing system  16  because of limited telemetry bandwidth. A power supply  18  such as a battery or mud turbine powered generator for example supplies power for operation of the mud-pulser  12 . 
     The receiver  17  is configured to receive the acoustic pressure signal using a transducer  19 . The transducer  19  is configured to convert the received acoustic signal into an electrical signal that can be processed. The receiver  17  further includes a demodulator  29  configured to demodulate the acoustic pressure signal into a bit stream that includes the downhole data. The bit stream after further processing is in a format for displaying, storing, or further processing such as by the computer processing system  16 . In one or more embodiments, the computer processing system  16  may be configured to perform the demodulating function. 
       FIG. 2  depicts further aspects of the mud-pulser  12  in a simplified embodiment. In the embodiment of  FIG. 2 , the mud-pulser  12  is a plunger-type mud-pulser that includes a plunger  21  that is configured to move back and forth in a reciprocating motion with respect to a seat  22 . The reciprocating motion with respect to the seat  22  reduces the flow cross-sectional area so as to increase the pressure drop across the mud-pulser  12  and thereby cause acoustic pulses to be emitted into the drilling fluid  13 . An actuator  20  is coupled to the plunger  21  and is configured to cause the reciprocating motion of the piston in accordance with a control signal received from the downhole electronics  15 . The desired acoustic pressure signal may be a phase modulated sinusoidal signal. Instantaneous frequency can be changed in order to provide phase. The actuator  20  receives power from the power supply  18 . It can be appreciated that the actuator  20  can be implemented in various configurations, such as using a shear valve, in order to emit acoustic pulses into the drilling fluid  13  that travel to the surface of the earth. 
       FIG. 3  is a schematic view of an oscillating or rotating shear valve-type pulser  30  for mud pulse telemetry. The pulser assembly  30  is located in the inner bore of a tool housing  101 . The housing  101  may be a bored drill collar in the downhole tool (or bottom hole assembly)  10 , or, alternatively, a separate housing adapted to fit into a drill collar bore. The drilling fluid  13  flows through a stator  102  and a rotor  103  and passes through the annulus between pulser housing  108  and the inner diameter of the tool housing  101 . The stator  102  (see  FIGS. 3A and 3B ) is fixed with respect to the tool housing  101  and to the pulser housing  108  and has multiple lengthwise flow passages  120 . The rotor  103  (see  FIGS. 3A and 3C ) is disk shaped with notched blades  130  creating flow passages  125  similar in size and shape to the flow passages  120  in the stator  102 . Alternatively, the flow passages  120  and  125  may be holes through the stator  102  and the rotor  103 , respectively. The rotor passages  125  are configured such that they can be aligned, at one angular position with the stator passages  120  to create a straight through flow path. The rotor  103  is positioned in close proximity to the stator  102  and is adapted to rotationally oscillate or rotate. An angular displacement of the rotor  103  with respect to the stator  102  changes the effective flow area creating pressure fluctuations in the circulated mud column. 
     As disclosed herein, the modulator  14  implements a digital modulation method referred to as Offset Quadrature Phase Shift Keying (OQPSK).  FIG. 4  depicts aspects of OQPSK as known in the art of digital radio communications. Due to fast acting electronics and radio waves not having mass, phase shifts such as at T are near-instantaneous and the radio waves have jumps where the first derivative of electromagnetic wave amplitude is not continuous over the phase change. For teaching and comparison purposes, aspects of quadrature phase shift keying (QPSK) from which OQPSK is derived are also depicted and discussed further below. Both OQPSK and QPSK transmit a series of two-bit symbols corresponding to four different signal phases (00, 01, 10, 11). 
     QPSK requires changing the phase of the signal via in-phase (I) and the quadrature-phase (Q) for which the possible phase shifts are 0°, ±90°, and 180°. Increasing the number of phase shifts to what is required for QPSK raises the number of needed transitions. As QPSK has four different symbols, then 16 different phase shifts are required, which correspondingly require different transition frequencies; the transition either from ‘01’ to ‘10’ or vice versa requires a high transition frequency. Increasing the carrier frequency is needed because changing from ‘01’ to ‘10’ or vice versa requires changing the ‘I’, and ‘Q’ phases at the same time. Use of OQPSK decreases the power requirements over QPSK by not requiring the high transition frequency while increasing the data transmission rate over other prior art digital modulation techniques such as Binary Phase Shift Keying (BPSK). 
     OQPSK depends on introducing a delay (i.e., offset) for one of the two signal phases either the I-phase or the Q-phase. Adding the delay or offset to one phase guarantees that ‘I’ and ‘Q’ phases will never change together at the same time, accordingly phase transitions will be limited to 0° and ±90° only and does not have a phase shift of 180° as in QPSK. This delay is observed as a delay in the Q-phase as illustrated in  FIG. 3 . The added delay for the Q-phase will ensure that only one bit can change sign at a given time, which will not allow changing from ‘01’ to ‘10’ or vice versa. Accordingly, the high transition frequency will not be needed any more which enhances the signal spectrum and reduces the power requirements, thus, providing more operational time for a power source having a fixed amount of energy. Based on  FIG. 4 , OQPSK has more frequent phase shifts than QPSK, but with less degree values. In addition, OQPSK has the same data rate and bit-error-rate (BER) capabilities as QPSK, assuming the same signal power and bandwidth. 
     Because the mud-pulser  12  has a moveable element such as the plunger  21 , which has mass, the plunger  21  cannot change position or velocity instantaneously or near-instantaneously as electronics for modulating an electromagnetic radio wave. Accordingly, no sudden changes are possible to the acoustic signal or to its first derivative (velocity). The moving element changes its velocity by accelerating for a longer time period, so it can only create a continuous phase moving in the transient between adjacent symbols. Hence, the design of the trajectory of the transient to be followed by the actuator has to be specified. As disclosed herein, OQPSK is implemented for acoustic communication by inserting a transition phase in a transition interval between adjacent symbols as illustrated in  FIG. 5 .  FIG. 5A  illustrates an ideal OPSK modulated signal while  FIG. 5B  illustrates an ideal OQPSK modulated signal and an OQPSK modulated signal with smooth transitions. In the transition intervals illustrated in  FIG. 5B , the instantaneous frequency (i.e., frequency representing a two-bit symbol) is changed to perform a phase shift. The solid line in the transition intervals represents an ideal phase shift, while the dotted line in the transition interval represents the phase shift via an increase in instantaneous frequency. Hence, the first time interval has one distinct frequency while the second time interval has another distinct frequency that is lower than the frequency in first time interval in this example performing the phase shift, then in the third time interval the frequency changes back to the frequency from the first time interval with different phase. This results in smooth transitions that have a continuous first derivative of the signal amplitude with respect to time throughout symbol changes. In one or more embodiments, the width or time of the transition interval is fixed so that the demodulator  29  can regain the information modulated in the acoustic signal at specific points in time.  FIG. 6  depicts aspects of an OQPSK modulated signal having three phase shifts with smooth transitions. In the embodiment of  FIG. 6 , the frequency of the signal in the second and fourth time intervals is half the frequency of the signal in the first and third time intervals. 
     There are several advantages to using QPSK or OQPSK over other digital modulation techniques for digital encoding for acoustic signal transmission. While BPSK transmits one bit per symbol, QPSK and OQPSK transmit two bits per symbol corresponding to four different phases (00, 01, 10, 11). Accordingly, the data rate can be doubled at the same symbol rate. QPSK and OQPSK can give the same BER as for BPSK in case of doubling the energy used per symbol, which means the same energy per bit is maintained. Accordingly, increasing the data rate, with the same BER of BPSK comes at the cost of doubled transmitted power. 
     However, QPSK has a drawback. As noted above, QPSK requires changing the phase of the signal via in-phase (I) and the quadrature phase (Q) for which the possible phase shifts are 0°, ±90°, and 180°. Increasing the number of phase shifts to what is required for QPSK raises the number of needed transitions. As QPSK has four different symbols (00, 01, 10, 11), then 16 different phase shifts exist, which require correspondingly different transition frequencies; the transition either from ‘01’ to ‘10’ or vice versa requires a high transition frequency. Increasing the transition frequency is needed, because changing from ‘01’ to ‘10’ or vice versa requires changing the ‘I’, and ‘Q’ phases at the same time. OQPSK overcomes this drawback (by not transitioning from ‘01’ to ‘10’ or ‘10’ to ‘01’) in addition to having the same benefits of QPSK such as—a higher possible data transmission rate and similar BER as for BPSK assuming doubling the energy per symbol. Thus, OQPSK decreases the values of the transition frequencies needed for QPSK and consequently decreases the power requirements for OQPSK. 
     Accordingly, the power saved by using OQPSK may also be used for increasing the energy used per transmitted bit and thus the signal-to-noise ratio. Higher energy per bit can be achieved by reducing the data transmission rate to be the same as for BPSK, and using the saved power from the reduced transmission rate for increasing the signal power. 
     A Forward Error Correction method can be combined with the proposed OQPSK method for reducing bit error rate. Error coding methods add error detection and correction capabilities to the disclosed telemetry system in order to increase the reliability of the system. Different error coding methods (i.e. block codes, cyclic codes, convolutional codes, and Reed-Solomon) may be combined with OQPSK. As one example, Turbo codes, which are based on convolutional coding, are considered for use with OQPSK. With respect to Turbo/Convolutional code, Convolutional coding can be used to protect the data on the bit level, which performs encoding bit by bit. Accordingly the decoder should not buffer an entire block before generating the associated code-word. Convolutional coding is relevant to the present telemetry system which has bits transmitted and received serially rather than in large block. Convolutional code is specified by three parameters (n, k, K), where k/n is the code rate and determines the number of data bits per coded bit. K is called the constraint length of the encoder where the encoder has K-1 memory elements. The convolutional encoder can be represented in the form of a state diagram that provides outputs S 1  and S 2  where the encoder manipulates the incoming data to provide the S 1  and S 2  outputs. Convolutional codes also can be done in a recursive systematic manner (RSC) in which S 1  is not affected by the encoder, but is the input data stream. Generally, Recursive encoders provide better weight distribution for the code. The difference between them is in the mapping of information bits to code-words. The idea of recursive systematic encoders is used for the turbo coders, where two component RSC encoders in parallel are used separated by an interleaver. The two RSC component encoders are usually identical. The interleaver is used to de-correlate the encoding process of the two encoders. As the number of iteration grows, the decoding performance improves. Alternatively, the two RSC encoders can be replaced also in a series. 
       FIG. 7  is a functional block diagram of one embodiment of a mud pulse telemetry system  100 . As shown therein, data from downhole sensors (DATA IN) are input to the mud pulse telemetry system  100 . The mud pulse telemetry system  100  contains circuits and a processor for processing and transmitting the data to the surface. In the downhole system, the data is compressed. The compression scheme  40  may encompass data scaling and/or any data compression technique known in the art of digital information transmission. Optionally, the compressed data may be error protection encoded ( 41 ) by an encoder  41  before it is modulated ( 42 ) and converted to an acoustic signal by the transmitter ( 43 ) such as a mud-pulser valve. The acoustic signal then propagates to surface through the mud channel  50  where it is received and digitized by the receiver  44  such as a transducer. Optionally, the digitized signal (i.e., electrical digital signal) may then be pre-processed to remove noise (block  45 ) and reduce signal distortions (block  46 ) caused by the mud channel  50 . Subsequently, the signal is de-modulated ( 47 ), error protection decoded ( 48 ) by a decoder  48  and decompressed ( 49 ) to provide output data (DATA OUT). 
       FIG. 8  is a flow chart for a method  80  for transmitting data from a downhole location to a location at the surface of the earth. Block  81  calls for receiving data using a modulator disposed at the downhole location on a drill tubular in a borehole penetrating the earth. Block  82  calls for modulating, using the modulator, the data using offset quadrature phase shift keying (OQPSK) and smooth transitions between phase shifts to produce a series of two-bit symbols having smooth transitions in transition intervals between phase shifts of both in-phase and quadrature-phase components of the OQPSK modulated data. The transition intervals may be of the same time duration or two or more transition intervals may have different time durations. A smooth transition may be characterized by a change of instantaneous frequency of the acoustic signal and one or more of the smooth transitions may be characterized by a continuous first derivative of amplitude with respect to time. Block  83  calls for transmitting the series of two-bit symbols as an acoustic signal in drilling fluid disposed in the borehole using a mud-pulser. Block  84  calls receiving the acoustic signal using a receiver disposed uphole from the downhole location. The term “uphole” relates to the receiver being closer to the surface of the earth via the borehole. Block  85  calls for demodulating the acoustic signal using a demodulator coupled to the receiver to provide demodulated data. The method  80  may also include transmitting the demodulated data to a signal receiving device such as a display or printer for displaying the data to a user or a storage medium or memory for storing the data. 
     The method  80  may also include (1) applying forward error correction encoding before modulating the data using an encoder such as at  41  in  FIG. 7  and (2) applying forward error correction decoding to the demodulated data uphole of the downhole location using a decoder such as at  48  in  FIG. 7 . In one or more embodiments, the forward error correction encoding is turbo coding. The method  80  may also include sensing a downhole property using a downhole sensor and transmitting sensed data to the encoder. The method  80  may further include extracting a sample of formation fluid using an extendable probe, sensing a property of the formation fluid using the downhole sensor to provide sensed formation fluid data, and transmitting the sensed formation fluid data to the modulator. The method  80  may also include generating a two-dimensional image representation of the sensed data before transmitting the sensed data to the modulator. 
     In support of the teachings herein, various analysis components may be used, including a digital and/or an analog system. For example, the mud-pulse telemetry system  100 , the downhole tool  10 , the downhole sensor  8 , the formation tester  9 , the mud-pulser  12 , the modulator  14 , the downhole electronics  15 , the receiver  17 , the transducer  19 , the demodulator  29 , the encoder  41 , the decoder  48 , and/or the computer processing system  16  may include digital and/or analog systems. The system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, optical or other), user interfaces (e.g., a display or printer), software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a non-transitory computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure. 
     Further, various other components may be included and called upon for providing for aspects of the teachings herein. For example, a power supply (e.g., at least one of a generator, a remote supply and a battery), cooling component, heating component, magnet, electromagnet, sensor, electrode, transmitter, receiver, transceiver, antenna, controller, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure. 
     Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” and the like are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list of at least two terms is intended to mean any term or combination of terms. The term “configured” relates one or more structural limitations of a device that are required for the device to perform the function or operation for which the device is configured. The terms “first,” “second,” and the like do not denote a particular order, but are used to distinguish different elements. 
     The flow diagram depicted herein is just an example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. 
     It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed. 
     While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.