Patent Publication Number: US-7212132-B2

Title: Downhole signal source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a divisional application of U.S. patent application Ser. No. 10/856,439, filed May 28, 2004, entitled “Downhole Signal Source” which is incorporated herein by reference in its entirety. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   GENERAL FIELD OF THE INVENTION 
   The present invention relates generally to methods and apparatus for signaling from one location to another using low frequency magnetic fields. The invention can be used to send a signal from a location near a drill bit in a well drilling operation to a receiver at the earth&#39;s surface, or to a receiver at a different location in the drillstring in the same well, or to a receiver in another well. These and other features of the invention are described in detail below. 
   BACKGROUND OF THE INVENTION 
   In common practice, when it is desired to produce hydrocarbons from a subsurface formation, a well is drilled from the surface until it intersects the desired formation. As shown in  FIG. 1 , a typical drilling operation entails a surface operating system  50 , a work string  100  that may comprise coiled tubing or assembled lengths of conventional drill pipe, and a bottom hole assembly (BHA)  200 . Surface system  50  typically includes a drilling rig  10  at the surface  12  of a well, supporting drill string  100 . BHA  200  is attached to the lowermost end of work string  100 . Operating system  50  is positioned at the surface adjacent to well  12  and generally includes a well head disposed atop of a well bore  18  that extends downwardly into the earthen formation  20 . Borehole  18  extends from surface  16  to borehole bottom  30  and may include casing  22  in its upper zones. 
   The productivity of formations can vary greatly, both vertically and horizontally. For example, in  FIG. 1 , formation  21  may be a producing formation (stratum), while formation  20  above it may be a non-producing formation. The target formation(s) have typically been mapped using various techniques prior to commencement of drilling operations and an objective of the drilling operation is to guide the drill bit so that it remains in the target formation. Thus, in many wells, the lower portion of the borehole deviates from the vertical and may even attain a substantially horizontal direction. In these circumstances, it is desirable to drill the well such that borehole  18  stays within the producing formation  21 . 
   Similarly, it is sometimes desired to guide the drilling of a well such that it parallels another well. This is the case in steam-assisted gravity drainage (SAGD) drilling, in which steam injected through one of a pair of parallel wells warms the formation in the vicinity of the wells, lowering the viscosity of the formation fluids and allowing them to drain into the second well. The second well thus functions as a production well and typically is drilled such that it lies below the injection well. 
   As a result of this deviated, directional, or horizontal drilling, the drill bit may traverse a sizable lateral distance between the wellhead and the borehole bottom. For this reason, and because the degree of curvature of the borehole is often not known precisely, it also becomes difficult to know the true vertical depth of the borehole bottom. Hence, it is preferred to track the position of the bit as precisely as possible in order to increase the likelihood of successfully penetrating the target formation. 
   It is particularly desirable to accurately locate the position of the bottom hole assembly (BHA) during drilling so that corrections can be made while drilling is ongoing. Determining the precise location of the drill bit as it progresses through the formation and communication of that information from the downhole location to the surface are two significant problems that have not heretofore been adequately addressed. Both objectives are made more difficult by the drilling operation itself, which involves at least rapid fluid flow, moving parts, and vibrations. 
   Various methods are traditionally combined to achieve these goals. Gyroscopes and various types of sensors have been used to track bit movement and/or bit position. Electromagnetic (EM) telemetry is one technique used for transmitting information, either to the surface or to another location uphole. Other transmission techniques involve mud pulses or acoustic signaling using the drillstring as the signal carrier. Current techniques are not very accurate or rapid, however, and can result in erroneous calculations of the position of the BHA. Hence, it is desirable to provide a technique for determining the position of a bit in a subterranean formation that eliminates or at least substantially reduce the problems, limitations and disadvantages commonly associated with the known bit-tracking techniques. 
   SUMMARY OF PREFERRED EMBODIMENTS OF THE INVENTION 
   The present invention provides methods and apparatus for signaling from one location to another using low frequency magnetic fields. The invention has many applications and can be used, for example, to locate the position of the bottom hole assembly during drilling. The invention can be used to send a signal from a location near a drill bit in a well drilling operation to a receiver at the earth&#39;s surface, or to a receiver at a different location in the drillstring in the same well, or to a receiver in another well. The invention can also be used for generating a signal at the earth&#39;s surface that can be detected at a downhole location, or as a telemetry transmitter for low frequency communications. 
   In some embodiments, the apparatus of the present invention is particularly useful as a tool for sending a signal from the bit location that can be detected at the surface and used to determine the location of the bit. The present invention avoids the deficiencies of prior devices and offers an alternative way to determine the position of the BHA. In preferred embodiments, the invention includes placing a signaling apparatus at the bit and tracking its position during the entire drilling process. For this method to work, the signal source must be strong and stable enough even for deep and extended-reach wells. 
   In certain embodiments, a synchronization signal and using said synchronization signal is provided and used to control modulation of the magnetic field created by the magnet. Controlling the modulation of the magnetic field may include doubling the frequency of, taking the absolute value of, or squaring the synchronization signal. The modulated magnetic field can be sensed by receivers that may detect a phase shift between said synchronization signal and said modulated magnetic field and or amplitude variations in said modulated magnetic field. There may be a plurality of receivers spaced apart from said bottomhole assembly, and the receivers may be located at or below the earth&#39;s surface. 
   In alternative embodiments, the invention can also be used to generate a signal at the earth&#39;s surface that can be detected at a downhole location. 
   In some embodiments of the present invention, the signal source may be a rare earth permanent magnet used in conjunction with a shield made of high permeability soft magnetic alloy. By precisely controlling the motion of the shield, the permanent magnet can be made to function as a precise oscillating signal source that can be tracked by magnetometers at the surface for accurate position monitoring of the BHA. In alternative embodiments, the frequency and/or phase etc. of the motion of the shield can be modulated in response to data acquired by downhole instruments using well-known digital encoding schemes, transforming the signal source into a transmitter that can communicate LWD data to surface receivers. 
   In certain embodiments, the present invention comprises a magnet and a shield moveable relative to said magnet between a first position in which said magnet is relatively exposed and a second position in which said magnet is relatively shielded. The magnet can be an electromagnet. The present system may further comprise means for providing a synchronization signal and means for controlling movement of the shield in response to the synchronization signal so as to modulate the magnetic field created by the magnet. The means for controlling the shield movement may include means for doubling the frequency of, taking the absolute value of, and/or squaring the synchronization signal. The apparatus may further include a downhole sensor generating a signal and means for modulating the magnetic field in response to the signal from the downhole sensor. 
   Thus, the embodiments of the invention summarized above comprise a combination of features and advantages that enable them to overcome various problems of prior devices systems and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
   It should be appreciated that the present invention is described in the context of a well environment for explanatory purposes, and that the present invention is not limited to the particular borehole thus described, it being appreciated that the present invention may be used in a variety of well bores. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the preferred embodiments of the present invention, reference will now be made to the accompanying drawings, wherein: 
       FIG. 1  is a schematic elevation view, partly in cross section, of a drillstring including a bottom hole assembly (BHA) in a subterranean well; 
       FIG. 2  is a simplified perspective view of a signal source in accordance with a preferred embodiment of the invention; 
       FIG. 3  is a cross sectional view of the signal source of  FIG. 2  incorporated into a downhole tool; 
       FIGS. 4 and 5  are end views of a signal source in accordance with a first alternative embodiment, in closed and open positions, respectively. 
       FIG. 6  is a simplified view of a slotted sleeve that can be used in certain embodiments of the present invention; 
       FIG. 7  is a plot illustrating the dependence of magnetization on temperature, where Ms is the saturation magnetization; 
       FIG. 8  is a schematic diagram illustrating an embodiment of a system incorporating a signal source in accordance with the present invention; and 
       FIGS. 9A–D  are plots illustrating a transmitted signal (A), the same signal after squaring (B), the squared signal after filtering (C), and a comparison of all three modes through one cycle of the original signal (D). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, reference to “up” or “down” are made for purposes of ease of description with “up” meaning towards the surface of the wellbore and “down” meaning towards the bottom of the wellbore. In addition, in the discussion and claims that follow, it is sometimes stated that certain components or elements are “electrically connected.” By this it is meant that the components are directly or indirectly connected such that an electrical current or signal could be communicated between them. 
   According to the present invention, the strong magnetic moment of the rare earth permanent magnet is used together with the shield made of high permeability soft magnetic alloys. By precisely controlling the motion of the shield, the permanent magnet is transformed into a precise oscillating signal source that can be tracked by magnetometers at the surface for accurate position monitoring of the BHA. Alternatively, the speed/phase etc. of the motion of the shield can be modulated with data acquired by downhole instruments through well-known digital encoding scheme, thus transform the signal source into a transmitter that can communicate LWD data to surface receivers. 
   Referring now to  FIGS. 2 and 3 , one preferred embodiment of a signal source  10  in accordance with the present invention includes a permanent magnet  12 , a magnetic shield  14 , and a drive mechanism  16  for shifting shield  14  relative to magnet  12 . Magnetic shield  14  is slidable axially into and out of surrounding engagement with magnet  12 , as indicated by arrow  26 . Drive mechanism  16  engages one end of shield  14  and provides the motive force needed to advance and retract the shield. Referring now particularly to  FIG. 3 , signal source  10  is preferably mounted inside a cylindrical non-magnetic drill collar  20 , along with a drive means  30 . The assembly formed in this manner preferably has a central bore  22  therethrough such that the drill collar can be included in a drill string. 
   In the embodiment shown in  FIGS. 2 and 3 , magnet  12  is generally cylindrical and shield  14  likewise comprises a cylindrical shell. Shield  14  preferably includes an end cap  17  and a cylindrical inner surface  15  having a diameter only slightly larger than the outside diameter of magnet  12 . Shield  14  is preferably moveable between first and second positions in which magnet  12  is, respectively, exposed and shielded. 
   In  FIG. 3 , shield  14  is shown in an intermediate position, with magnet  12  partially exposed and partially shielded. The length of arrow  26  illustrates an approximate range of movement for shield  14 . As shield  14  moves along the length of magnet  12 , the fraction of magnet  12  that is exposed changes. Correspondingly, the magnetic field emanating from magnet  12  changes as shield  14  attenuates it. When magnet  12  is wholly within shield  15 , the magnetic field emanating from the tool  100  will at its minimum. In certain embodiments, the movement of shield  14  relative to magnet  12  can be controlled so as to produce a sinusoidal modulation of the magnetic field that extends beyond the tool. Likewise, the movement of shield  14  can be controlled such that the magnetic field cycles in a sawtooth manner, or according to any preferred function or modulation. 
   In an alternative embodiment of the invention, depicted in  FIGS. 4 and 5 , the shield consists of two or more partial circumferential sections  40 ,  42 . Sections  40 ,  42  are preferably configured such that together they can be closed to form a shield that encloses the circumference of magnet  13 . 
   In still another embodiment, shown in  FIG. 6 , the shield can comprise two or more concentric cylindrical shells, each generally having the configuration shown at  50  and each having a plurality of longitudinal slots  52  therethrough. The magnet is disposed within the innermost shell. When the concentric shells are positioned such that the slots in each shell are aligned with the slots in the other shell(s), the magnet is exposed. Similarly, when the shells are positioned such that the slots do not align, the magnet is shielded. 
   It will be understood that the configurations shown herein are merely illustrative of the manner in which the magnetic material and the shield could be configured. Various other arrangements of the components of the tool will be understood by those skilled in the art. 
   Magnet 
   In order to have the highest available magnetic energy, rare-earth based permanent magnets such as Nd/Fe/B and Sm/Co are preferred. With a magnetic energy (BxH) max  in excess of 200 KJ/m 3 , Nd/Fe/B magnets are the strongest permanent magnets available today. Sm/Co magnets typically have a lower magnetic energy, at about 150 KJ/m 3 . 
   As is known, permanent magnets are made of ferromagnetic materials. One of the characteristics of ferromagnetic materials is the existence of a critical temperature (T c ) called Curie temperature. Above this temperature, ferromagnetic materials lose their magnetization and become paramagnetic. The transition is gradual within a temperature range; even before the temperature of the magnet reaches its Curie temperature, the magnet starts to lose its magnetization. This behavior can be described by the molecular field theory that gives the temperature dependence depicted in  FIG. 7 . Hence, if a permanent magnet is to maintain 80% of its magnetization in the downhole environment, it must operate in temperatures no higher than 0.7×T c , where T c  is the Curie temperature. For Sm 2 Co 17 , T c  is 700–800° C., while it is 300–350° C. for Nd 2 Fe 14 B. Therefore, for deep wells where the bottom hole temperature is high, Sm 2 Co 17  magnets are preferred. 
   Shield 
   In order to modulate the strength of the permanent magnet, shield  14  is preferably made of a magnetically soft alloy such as Mumetal® (Ni/Fe/Cu/Mo) or Supermalloy, with high magnetic permeability. Various suitable magnetically soft metals are known in the art, including CO-NETIC AA®, which has a high magnetic permeability and provides high attenuation, and NETIC S3–6®, which has a high saturation induction rating that makes it particularly useful for applications involving strong magnetic fields. NETIC S3–6 and CO-NETIC AA are trademarks of Magnetic Shield Corp., 740 N. Thomas Drive, Bensenville, Ill. 60106. In embodiments where it is desired to achieve very high attenuation ratios in a very strong field, it may be preferred to use both alloys. In these instances, the NETIC S3–6 alloy is preferably positioned closest to the source of the field so as to protect the CO-NETIC AA alloy from saturation. Alternative metals that are suitable for use in shield  14  include but are not limited to Amumetal® and Amunickel® from Amuneal Manufacturing Corp., 4737 Darrah Street, Philadelphia, Pa. 19124, USA. 
   Motor 
   Motive force for moving shield  14  relative to magnet  12  is preferably provided by drive means  30 , which is housed inside drill collar  20 . Drive means  30  is preferably an electric motor, but can be any other suitable mechanical drive device. It will be understood that, depending on the type of power source selected, it may be necessary to provide gearing and the like in order to allow drive means  30  to cause the desired movement of shield  14 , whether that be rotational, translational, or other. 
   Use of the Downhole Transmitter 
   As mentioned above, one preferred use for a transmitter of the type disclosed herein is as a field source for a downhole absolute positioning system. The purpose of such a system is to allow a precise determination of the position of the bottomhole assembly. This can be done by using the present signal source to generate an ultralow frequency signal (0.1 Hz to 0.01 Hz, depending on depth, with greater depths requiring lower frequencies) that is extremely stable and precisely synchronized with a surface clock. The transmitter itself can be a transmitter of the type herein disclosed or a large electromagnet. A highly stable synchronization signal makes it possible to operate in a very narrow bandwidth, which in turn makes it possible to receive the signal with a minimum of noise and improves the quality of the resulting telemetry. 
   When the present invention is used to assist in location of a bottomhole assembly, for example, it is preferably positioned in the drillstring adjacent to the BHA. The present signaling devices may not be in physical contact with the BHA, but the greater the distance between the BHA and the signaling apparatus, the less precise will be the information relating to location of the BHA. Because precise location of the signal source is achieved by a combination of phase shift and amplitude measurements, timing is particularly important in this embodiment. 
   In other embodiments, the downhole signal source need not be synchronized to an synchronization signal. This type of system can be used when it is desired to generate a signal at the earth&#39;s surface that can be detected at a downhole location, or when the system is used as a telemetry transmitter for low frequency communications. In still other embodiments, an array of three or more surface sensors can be used locate the signal source using triangulation techniques, with or without a synchronization source. 
   In spite of the frequency stability requirement, it is not necessary to carry a precise clock (good to about 1 millisecond over 200 hours) downhole. Nonetheless, in some embodiments, a downhole clock may be preferred. In one embodiment, illustrated in  FIG. 8 , a precise clock  100  is located at the earth&#39;s surface. Clock  100  is used to synchronize a system that includes a downhole signal source in accordance with the present invention. In the embodiment shown in  FIG. 8 , clock  100  is electrically connected to a surface sine wave transmitter  112 , which in turn is electrically connected to a surface antenna  114 . Clock  100  can be an atomic clock, a clock obtained from the GPS system, an over controlled system of oscillators, or any other suitable precise clock. 
   Still referring to  FIG. 8 , a signal  118  from surface antenna  114  is transmitted through the earth and are received at a downhole receiver  120 . The received signal from the downhole receiver  120  is preferably passed through a preamplifier  122  into a digital-to-analog converter and then through signal processing means that use the received signal to synchronize the downhole system. In a preferred embodiment, the signal processing means comprise a CPU  124  that applies a squaring algorithm and a low pass filter to the received signal. CPU  124  also implements control logic that drives a downhole system clock. The output of the low pass filter is preferably sent to a digital-to-analog (D/A) converter  126 . The output of D/A converter  126  is preferably amplified by an amplifier and then used to control drive means  30 . In embodiments where an electromagnet is used, the output of the D/A converter can be used to operate to the electromagnet, preferably with amplification. 
   Regardless of the source of the drive signal, the signal source  10  ultimately generates a signal  130  that comprises a variable magnetic field. Signal  130  is detected by a sensing device  140 , which preferably comprises an array of at least two receivers  142 ,  144 ,  146 ,  148 . Sensing device  140  may or may not be located near antenna  114 . If a surface synchronization source is used, the phase and/or amplitude of the received signal  130  can be used to locate the signal source. Timing-induced errors can be mitigated by using a digital phase lock loop circuit or other suitable means. In alternative embodiments, the frequency and/or the phase of signals  130  can be modulated so as to transmit signals from the borehole bottom to the surface, such as, for example, signals indicative of measurements made by downhole sensors and/or MWD equipment. 
   Clock  100  is preferably used to generate a sine wave at one-half the frequency of the signal that is to be transmitted by the downhole transmitter ( FIG. 9A ). In an alternative embodiment, the clock signal can be induced directly into the drillstring and sensed as an electric field across an insulating gap in the bottomhole assembly or by any other current-sensing means. It is well known that if a sinusoidal signal is squared, that the resulting signal contains only even harmonics of the fundamental signal. In particular, the Fourier series representation of a rectified sine wave is given by Equation (1) and is illustrated in  FIG. 9B . 
   
     
       
         
           
             
               
                 
                    
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         ω 
                         · 
                         t 
                       
                       ) 
                     
                   
                    
                 
                 = 
                 
                   
                     2 
                     π 
                   
                   - 
                   
                     
                       2 
                       π 
                     
                     · 
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                       ⁢ 
                       
                         
                           2 
                           
                             
                               
                                 ( 
                                 
                                   2 
                                   · 
                                   n 
                                 
                                 ) 
                               
                               2 
                             
                             - 
                             1 
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               2 
                               · 
                               n 
                               · 
                               ω 
                               · 
                               t 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   Whether the procedure is carried out using analog electronics or digital electronics, the concept is the same: take the absolute value of the received signal (or square it) and low pass filter it ( FIG. 9C ). The fundamental frequency of the resulting signal will be exactly twice that of the transmitter at the earth&#39;s surface. The signal will contain higher order harmonics which can be filtered out downhole, if desired (the higher the order of the harmonic, the more this signal will be attenuated as it propagates through the earth, back to the earth&#39;s surface).  FIG. 8  illustrates one possible way of carrying a preferred procedure out using mostly digital electronics. It should be appreciated that the digital functions could be replaced with analog functions if desired, but since the frequencies used are so low, the required signal processing is well within the capabilities of present technology. 
     FIGS. 9A–D  illustrate the waveforms, individually and together ( 9 D) that result in a preferred signal processing technique that is suitable for use in the present invention. It will be understood that any other synchronization signal source or other signal processing techniques can be used in the present invention and that the signal(s) need not be sinusoidal. 
   Advantages 
   Compared with active sources using active dipole source energized by alternating current, the new signal source will be stronger, more stable, and more accurate. The present signal source can be used to precisely locate a BHA while drilling. It can also be used to improve depth reference in wireline logging operations by reducing errors related to cable stretching due to thermal expansion, sticking/stuck wireline tools, etc. Coupled with digital coding schemes, the present signal source can also be employed as a transmitter to send downhole tool and or formation data to surface receivers, thus provide an additional communication channel for LWD. 
   While certain preferred embodiments have been disclosed and described, it will be understood that various modifications may be made thereto without departing from the scope of the invention. For example, the type, size and configuration of the magnet and of the shield can be varied. Likewise, the mode of movement of the shield relative to the magnet can be altered or varied. To the extent that the claims include a sequential recitation of steps, it will be understood that those steps need not be completed in order and that it is not necessary to complete one step before commencing another.