Abstract:
An apparatus that is usable with a subterranean well includes a magnetometer and a circuit. The magnetometer indicates a strength of a magnetic field that at least partially extends through a portion of a downhole pipe. The circuit is coupled to the magnetometer to indicate a feature present in the pipe based on the indication from the magnetometer.

Description:
BACKGROUND  
         [0001]    The invention generally relates to a downhole magnetic-field based feature detector for detecting features of a downhole pipe.  
           [0002]    Certain downhole oilfield applications, such as perforating applications, require the ability to be able to position a tool at a particular and known spot in the well. For example, a wireline (armored electric cable) service uses a tool assembly (e.g., instrument) that is lowered downhole via a wireline. A depth counter may be used at surface to track the length of the dispensed cable to approximate the depth of the tool assembly. However, because the depth counter does not precisely indicate the depth (primarily because of stretch in the cable), other techniques may be used.  
           [0003]    For example, a more precise technique may use a depth control or depth correlation log (e.g., casing collar locator log), a log that is run while ascending &amp; descending in the well indicates the depths of various casing collar joints of the well. In this manner, the well includes casing collar joints, joints at which casing segments are coupled together to form the well casing. Each casing collar joint includes a casing collar to couple two adjacent casing segments of the well casing together. An air gap may exist between the ends of adjacent casing segments.  
           [0004]    To obtain the depth control log, a wireline tool assembly may be run downhole and include a detection device, called a casing collar locator, to detect the casing collar joints. When the casing collar locator indicates detection of a casing collar joint, the coarse depth that is provided by the depth counter may be used to locate the corresponding casing collar joint on the depth control log. Because the depth control log precisely shows the depth of the detected casing collar joint, the precise depth of the tool assembly may be determined. From this determination, an error compensation factor may be derived. Then, when a perforating gun is positioned downhole, the error compensation factor is used to compensate the reading of the depth counter to precisely position the gun.  
           [0005]    A conventional casing collar locator is a passive device that uses the principle of magnetic inductance to detect casing collar joints. In this manner, the casing collar locator typically includes an electrical coil, or winding, through which a magnetic flux field that is created by one or more permanent magnets passes. When a change occurs in the effective magnetic permeability in the surrounding, such as in the presence of a casing collar joint, a voltage is induced on the coil winding due to the corresponding change in the magnetic flux field (disturbance). Therefore, as the casing collar locator passes the casing collar joint, the change in permeability (caused by such things as the presence of the air gap between adjacent well casing segments and the casing collar) causes a change in the magnetic flux field to induce a signal across the winding. This generated signal may be communicated uphole and observed at the surface of the well. Thus, with this technique of detecting casing collar joints, the casing collar locator must be in motion to produce the signal.  
           [0006]    The quality of the signal may be highly dependent on the degree to which the magnetic permeability changes, or is disturbed. In this manner, the higher the rate of change in the permeability that is experienced by the magnetic flux field, the higher the induced signal (to a finite degree). The degree to which the field is disturbed depends on such factors as the distance, or gap (also called the “stand-off”), between the casing collar locator and the casing; the magnetic properties (i.e., the permeability) of the surrounding well casing; and the degree of change in geometry or bulk-mass of the casing, i.e., the change must be drastic enough and abrupt enough to cause a rapid enough disturbance in the flux field.  
           [0007]    If the field is not sufficiently disturbed, the resulting signal may be too small to be detected at the surface. The signal-to-noise ratio of the signal produced downhole typically places a limit on the degree to which the signal can be boosted, or amplified. Therefore, for these reasons, it may be very difficult to detect joints of casing that is made from a material having a low magnetic permeability, such as Hastalloy, for example. Likewise, collar joints that have no casing collars are difficult to detect, particularly if the joints are “flush” (i.e., each joint has no or almost no air gap).  
           [0008]    Another difficulty associated with a conventional casing collar locator is its mass and size. In this manner, the conventional casing collar locator may include two or more permanent magnets, one or more coils, and one or more coil cores, or bobbins. The combination of all of these components imparts a large mass to the casing collar locator. This large mass, in turn, may cause a significant force to be exerted on the casing collar locator during perforating operations due to the high acceleration and shock that is placed on this large mass. This force may damage the casing collar locator if extensive measures are not undertaken to properly pack the casing collar locator in the string.  
           [0009]    Besides having a large mass, the casing collar locator typically is quite bulky, as the locator may extend from six to eighteen inches and beyond, not including the pressure housing and connections. The tool string that houses the casing collar locator is therefore long and cumbersome. Tool length is very important particularly when the tool string is conveyed on a wireline and when working with high well pressure. Having a long tool string can present major operational and safety problems with pressure control equipment, such as the lubricator and riser pipe. Therefore, it is typically important to conserve every inch of a tool string, particularly in perforating applications.  
           [0010]    Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.  
         SUMMARY  
         [0011]    In an embodiment of the invention, an apparatus that is usable with a subterranean well includes a magnetometer and a circuit. The magnetometer indicates a strength of a magnetic field that at least partially extends through a portion of a downhole pipe. The circuit is coupled to the magnetometer to indicate a feature present in the pipe based on the indication from the magnetometer.  
           [0012]    In another embodiment of the invention, an apparatus that is usable with a subterranean well includes a magnet and a winding. The magnet establishes a flux field near the apparatus, and the flux field at least partially extends through a portion of a downhole pipe. The winding generates a signal produced by a change in a strength of the flux field to indicate a feature of the pipe. The longitudinal dimension of the apparatus does not exceed approximately two inches.  
           [0013]    In yet another embodiment of the invention, an apparatus that is usable in a subterranean well includes a first winding, a second winding, a powered interface and an unpowered interface. The first winding generates a first signal in response to a change in a magnetic field that at least partially extends through the first winding and at least partially extends through a portion of a downhole pipe to indicate a feature of the pipe. The second winding generates a second signal in response to a change in the magnetic field to indicate detection of the feature of the pipe. The magnetic field at least partially extends through the second winding. The first interface is coupled to the first winding to communicate the first signal to the surface of the well when the apparatus is in a powered mode, and the second interface is coupled to the second winding to communicate the second signal to the surface of the well when the apparatus is in an unpowered mode.  
           [0014]    Additional advantages and other features of the invention will become from the following description, drawing and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]    FIGS.  1 , 3 , 4  and  6  are schematic diagrams of a magnetic field-based detector according to different embodiments of the invention.  
         [0016]    [0016]FIG. 2 is a schematic diagram of circuitry of the feature detector of FIG. 1 according to an embodiment of the invention.  
         [0017]    [0017]FIG. 5 is a schematic diagram of circuitry of the feature detector of FIG. 4 according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to FIG. 1, an embodiment  10  of a downhole magnetic-field based feature detector in accordance with the invention may be used to locate magnetically distinguishable features of a tubular member, or pipe, that surrounds the feature detector. In this context, the term “pipe” may include a well casing, a valve, a casing collar joint or other tubular structure that has ferromagnetic properties and has a passageway for receiving the feature detector  10 . As an example, in some embodiments of the invention, the feature detector  10  may pass through a central passageway of a well casing  20  along a longitudinal axis  11  of the casing  20  for purposes of detecting features of the casing  20 , such as a casing collar joint  22 . Unlike conventional casing collar detectors, the feature detector  10  does not need to move to generate a signal to indicate a potential feature (a collar joint, for example) of the portion of the pipe near the detector  10 . Instead, the feature detector  10  includes one or more magnetometers  44  (an upper magnetometer  44   a  and a lower magnetometer  44   b  depicted in FIG. 1, as examples) to sense the strength of a magnetic flux field that is created by a magnet  40  of the detector  10  and extends through a portion of the pipe near the detector  10 . Thus, while stationary, the feature detector  10  may be used to detect features of the surrounding pipe.  
         [0019]    In this manner, in some embodiments of the invention, the magnetic axis of the magnet  40  is generally parallel to the longitudinal axis  11  of the casing  20  so that flux lines  42  of the magnetic flux field extend between the poles of the magnet  40  in a dipole pattern. The upper magnetometer  44   a  may be located above the north pole of the magnet  40 , and the lower magnetometer  44   b  may be located below the south pole of the magnet  40 , as an example.  
         [0020]    As depicted in FIG. 1, some of the flux lines pass through a portion of a wall of the well casing  20 , and as a result, the strength of the magnetic field may be controlled by features of the casing  20 , as the magnetic field is a function of the effective permeability of the path through which the flux lines  42  pass. Thus, the permeability is affected by the different features of the well casing  20 , and as a result, the strength of the magnetic field that is detected by the magnetometers  44  is affected by the different features of the well casing  20 . By detecting the strength of the magnetic field, the feature detector  10  may determine when a particular feature (the casing collar joint  22 , for example) is in proximity to the detector  10 .  
         [0021]    For example, the casing collar joint  22  that is depicted in FIG. 1 is formed from the union of two well casing segments  20   a  and  20   b  that are coupled together by a casing collar  24 . In this manner, a lower tapered end  28  of the upper casing segment  20   a  extends an upper portion of the collar  24 , and an upper tapered end  30  of the lower casing segment  20   b  extends into the lower portion of the collar  24 . The two ends  28  and  30  do not meet inside the collar  24 , but rather, an air gap  26  exists between the ends  28  and  30 . Thus, the combination of the air gap  26  and the casing collar  24  creates a significantly different permeability for the flux lines  42  of the feature detector  10  when the detector  10  is near the collar joint  22  than the permeability that is present when the feature detector  10  is near a portion of the well casing  20  away from the collar joint  22 .  
         [0022]    The one or more magnetometers  44  provide an indication of the strength of the magnetic field, and because this strength is different when the feature detector  10  is near the collar joint  22  than when the feature detector  10  is away from the collar joint  22  near a straight section (for example) of the well casing  20 , the presence of the collar joint  22  may be detected by comparing the different magnetic field strengths.  
         [0023]    The feature detector  10  is to be compared to a conventional casing collar locator that relies on a change in the sensed magnetic field to induce a signal on a winding for purposes of indicating detection of a casing collar joint. Thus, the conventional casing collar locator does not generate a signal if the locator is not moving. In contrast, the feature detector  10  measures a magnetic field strength, regardless of whether the feature detector  10  is moving or not. The difference in field strengths may be used to determine if a casing joint or other feature has been detected.  
         [0024]    In general, the changes or disturbances to the established magnetic flux field are caused by such changes as the geometry of the pipe; gaps in the pipe (such as the air gaps present in collar joints); anomalies in the pipe, such as heavy pitting, cracks, or holes such as perforations; sudden changes in distance or stand-offs between the feature detector  10  and the pipe; other changes in the magnetic properties (e.g., permeability) of the pipe; and changes in the bulk-mass of the pipe.  
         [0025]    Among the other features of the feature detector  10 , in some embodiments of the invention, the detector  10  may include a tubular non-magnetic housing  39  that has a longitudinal axis that is generally aligned with the longitudinal axis  11  of the well casing  20  when the detector  10  is located inside the casing  20 . As its name implies, the housing  39  protects and provides sealed containment of the magnetometers  44 , the magnet  40  and circuitry  46  of the feature detector  10 . As an example, the housing  39  may be connected to a wireline cable  64  (see FIG. 2) that extends to a surface of the well to position the feature detector  10 , to communicate signals from the feature detector  10  to the surface and to possibly provide power to the detector  10 .  
         [0026]    Referring also to FIG. 2, in some embodiments of the invention, the circuitry  46  may include a filter  54  that receives a voltage (called V1) from the upper magnetometer  44   a  and a filter  56  that receives a voltage (called V2) from the lower magnetometer  44   b . In this manner, the V1 and V2 voltages indicates the magnetic field strengths sensed by the upper  44   a  and lower  44   b  magnetometers, respectively. The filters  54  and  56  may be peak detectors, for example, to detect the peaks of the V1 and V2 voltages for purposes of filtering lower magnitude voltages (i.e., noise) from the V1 and V2 voltages. Other filters (low pass and/or bandpass filters, as examples) may be used.  
         [0027]    The output terminals of the filters  46  and  56  provide signals to a controller  60  that determines when a casing joint or other feature of the well casing  20  has been encountered based on the indications from the magnetometers  44   a  and  44   b , in some embodiments of the invention. When a particular feature has been detected, the controller  60  may, in some embodiments of the invention, communicate an indication of the feature to the surface via a telemetry interface  62 . As an example, the telemetry interface  62  may establish communication with the wireline cable  64  that extends to the surface of the well. Depending on the particular embodiment of the invention, the controller  60  may communicate to the surface a direct indication of the strength of the magnetic field or alternatively may communicate an indication of an actual feature detected.  
         [0028]    The controller  60  may also control the on/off operation of the magnet  40  for embodiments where the magnet  40  is formed from an electrical coil, or winding  51 . In this manner, the controller  60  may couple the winding  51  to a signal source  53  (an AC or DC source) via a switch  61  to create the magnetic field. In other embodiments of the invention, the magnet  40  may be a permanent magnet. The size and the position of the magnet  40  relative to the casing  20  may be adjusted to achieve different results, such as sensitivity, radius of investigation, etc.  
         [0029]    Each magnetometer  44  includes a magnet sensor  43  that generates a signal indicative of the strength of the sensed magnetic field. As just a few examples, the magnet sensor  43  may be a Hall-effect sensor, a silicon-based sensor (e.g., an anisotropic magnetoresistive (AMR) sensor or a giant magnetoresistive (GMR) sensor), a superconducting quantum interference device (SQUID), a Search-Coil, a magnetic flux gate, or a magnetoinductive device.  
         [0030]    As depicted in FIG. 1, in some embodiments of the invention, two or more magnetometers  44  may be used to achieve different modes of detection, such as integration of differential detection (for directional or focused detection, such as movement in only single axis).  
         [0031]    In some embodiments of the invention, the feature detector  10  may not have an explicit magnet source. For these embodiments, the magnetic flux field comes from the “natural magnetism” of the pipe. For example, FIG. 6 depicts an embodiment  200  of a magnetic field-based feature detector that does not include the magnet  40 , although the feature detector  200  may otherwise have a design that is similar to the feature detector  10 .  
         [0032]    The feature detectors (such as the feature detectors  10  and  200  and other feature detectors that are described below) that are described herein may offer one or more of the following advantages over conventional casing collar locators. The feature detector may not (in some embodiments) use an inductance to detect casing collars and thus, may be immune to problems related to detecting a change in inductance. The feature detector may be able to detect changes in pipes that have low magnetic permeability. The feature detector may be able to detect changes from large stand-offs (large air gaps between detector and the surrounding well casing), thus eliminating the need for “outrigger” arms. The feature detector may be sensitive only to changes in the longitudinal axis, thereby eliminating noise that is generated in a conventional casing collar locator when the locator is moved in transverse directions due to vibrations and/or cable movement. The mass and overall size of the feature detector may be greatly reduced, as compared to conventional casing collar locators. Regardless of whether the feature detector is moving, the detector indicates whether the detector is near a casing collar joint. The feature detector may provide precise downhole depth control and measurement that may be used for such applications as double shooting with oriented perforating where the shot spacing is critical; precise setting of orienting packer or whipstocks; and positioning a special cutter or splitter over a critical area of pipe, such as splitting a pipe joint for pipe recovery. Other and different advantages are possible.  
         [0033]    Referring to FIG. 3, in some embodiments of the invention, a feature detector  100  may be used in place of the feature detectors  10  and  70 . Unlike conventional feature detectors, the feature detector  100  is constructed with components to minimize the overall size and weight of the detector  100 . Due to its low mass, the force that is exerted on the feature detector  100  during perforating operations is significantly less than the force exerted on larger, conventional casing collar locators. As an example, a longitudinal dimension (called d 1 ) of the feature detector  100  may be near two inches or even less.  
         [0034]    To achieve its miniature size, the feature detector  100  includes two or more permanent magnets (an upper magnet  103  and a lower magnet  104  depicted as examples) that are each formed from a material (SmCo-30, for example) that has a high magnetic strength. The feature detector  100  also includes a winding  102  that has a high number of turns (40,000 for example) and is located between the permanent magnets  103  and  104  to provide a signal (called V) indicative of the rate at which the flux through the winding  102  changes. The winding  102  may have a high number (approximately 40,000 or more, depending on the particular embodiment) of turns, as compared to the number of turns for a winding used in a conventional casing collar locator. The winding  102  is formed on a bobbin  107  that is formed of a highly permeable magnetic material (Carpenter electrical iron, for example).  
         [0035]    [0035]FIG. 4 depicts another embodiment  120  of a magnetic-field based feature detector. The feature detector  120  may or may not have the miniature design of the feature detector  100 , depending on the particular embodiment of the invention. The feature detector  120  provides dual modes: a first mode in which the feature detector  120  provides a detection signal without requiring power to be provided to the detector  120  and a second mode in which the feature detector  120  provides a detection signal when the detector  120  receives power. It may be desirable to have an unpowered detector for certain perforating applications. However, in other applications, it may be desirable to power the feature detector  70  for purposes of increasing the strength of the detection signal.  
         [0036]    In some embodiments of the invention, the feature detector  120  includes a winding  126  for the powered mode and a winding  128  for the unpowered mode. The windings  126  and  128  may be bi-filar wound on the same bobbin  125  and may be electrically isolated from each other. The feature detector  102  may include an upper permanent magnet  122  and a lower permanent magnet  124  that cooperate to establish a magnetic field, as depicted by the flux lines  130  that extend through along the longitudinal axis of the windings  126  and  128 .  
         [0037]    The two different windings  126  and  128  are needed for the two different modes due to the signal loss that occurs when a single winding is used for both the powered and unpowered modes. In this manner, with a single winding, the output terminals of the winding drives the signal straight onto the cable when no power is received and drives the signal onto the cable via amplifiers when power is received. Due to this dual use, during the powered mode of operation, the wireline cable may attenuate the signal to the point that the amplifier may not have a sufficient signal.  
         [0038]    Thus, to address this problem, the feature detector  120  includes the two windings  126  and  128  for the different modes and includes different interfaces to drive the signals that are produced by these windings  126  and  128  onto the wireline cable  64 . For example, FIG. 5 depicts possible circuitry  150  that may be used. The circuitry  150  includes an instrumentation amplifier  152  that has its input terminals coupled to the terminals  139  of the winding  126  and provides a signal to a powered interface  154  to drive an indication of the signal from the winding  126  onto the cable  64  during the powered mode. The circuitry  150  also includes an unpowered interface  158  (a resistor network, for example) that is coupled to the terminals  140  of the winding  140  to drive an indication of the signal from the winding  128  onto the cable  64  during the unpowered mode.  
         [0039]    While the embodiments above are all invented to detect physical (geometrical) changes and changes in magnetic permeability in pipe when the embodiments are conveyed (moved) axially, they are also sensitive to sudden movement in the transverse axis. As such, they can be used for detecting sudden transverse movement of down-hole tools such as explosive devices, for example perforating guns and cutters fire. In one embodiment, the output of any of the above embodiments, can be used to trigger an output signal onto the cable for real-time detection of the detonation event at surface. It may in another embodiment be recorded downhole for post event confirmation of the event for example the detonation.  
         [0040]    While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.