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RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/842,702 filed Sep. 6, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to oilfield drilling, and, more particularly, to detecting existing well casing. 
       BACKGROUND 
       [0003]    In the context of oil and gas fields, infill development involves the redevelopment of an existing oil and gas field. Infill drilling is drilling that occurs within the boundaries of an existing developed gas or oil field. During infill development, there is a possibility of accidentally drilling into existing well casing. In addition, new wells may be connected into or designed around existing oilfield infrastructure, e.g., sidetracking operations. In these two cases, it is desirable to be able to locate existing well casing. 
         [0004]    Therefore, it is a desire to provide a system or method for avoiding accidental drilling into an existing well casing or to facilitate detecting existing well casing for sidetracking operations. 
       SUMMARY OF THE INVENTION 
       [0005]    In view of the foregoing and other considerations, the present invention relates to detecting well casing in a downhole environment. 
         [0006]    In one example, a method for detecting drill casing in a downhole environment is provided. The method includes the steps of generating a plurality of plots of a magnetic field vector (MFV) at a series of depths; and monitoring the plots to detect proximity to the well casing. 
         [0007]    In another example, a system for detecting a drilling casing is provided. The system includes a tool to detect a magnetic field, wherein the tool may be rotated about a longitudinal tool axis to generate a series of magnetic field measurements. The system also includes a processor to generate a series of plots of a magnetic field vector (MFV) based on the magnetic field measurements, wherein each plot comprises a shape that is based on spatial proximity of the tool to a magnetic source. 
         [0008]    The foregoing has outlined the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein: 
           [0010]      FIG. 1  is an example of the system for detecting well casing. 
           [0011]      FIG. 2  is a partial cross section of an example of a sensor package used in the system of  FIG. 1 . 
           [0012]      FIGS. 3A and 3B  are views of the 3-dimensional polar plot of magnetic field vector (MFV). 
           [0013]      FIG. 4  is a diagram illustrating the progressive change in the plot of MFV as the sensor package approaches a well casing. 
           [0014]      FIG. 5  is a comparison of two plots of MFV from  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0016]    As used herein, the terms “up” and “down”; “upper” and “lower”; “uphole” and “downhole” and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, these terms relate to a reference point as the surface from which drilling operations are initiated as being the top point and the total depth of the well being the lowest point. 
         [0017]      FIG. 1  shows a partial cross-section of an example of the system to locate existing well casing, shown generally by 2. Drilling rig  4  suspends or positions drillstring  6  within borehole  8  in rock formation  10 . Drillstring  6  includes drill bit  12  and tool  14 . Tool  14  may be a measurement-while-drilling (MWD) device, logging-while-drilling (LWD) device, or similar tool for conducting measurements of the downhole environment. Tool  14  may be a angle/azimuth tool or a fully steerable tool, for example. Tool  14  includes instrument package or steering unit  16  housed within drill collar  18  of tool  14 . Surface device  20  may transmit or receive data from tool  14  via wired (e.g., via drillstring  6 ), wireless devices (e.g., transceivers or similar devices) or other methods of telemetry (e.g., mud pulses). Surface device  20  may include processor  21  to store and process data. Rock formation  10  is the site of infill development and includes existing well casing  22 . Tool  14  is shown positioned downhole at a depth D from the surface and at a distance P from well casing  22 . System  2  permits the detection of well casing  22  so that casing  22  may be avoided or intersected, depending on the desired operation. 
         [0018]    Existing well casing  22  is typically made from steel or similar ferrous material and represents a relatively low impedance path to magnetic fields. Accordingly, there may be direction and magnitude of magnetic fields near casing  22 . The total magnetic field (TMF), indicated generally at  40 , may vary based on proximity to casing  22 . 
         [0019]    The value for TMF may be expressed as shown below in Equation 1: 
         [0000]      TMF=( M   x   2   +M   y   2   +M   z   2 ) 1/2   (1)
 
         [0020]    In Equation 2, M x , M y  and M z  are the orthogonal magnetic field values sensed by the magnetometers  24 . 
         [0021]    Detection of casing  22  based on the distortion of static fields using static measurements, however, may prove difficult. System  2  measures TMF  40  to detect or locate casing  22  by taking advantage of the drilling process itself. 
         [0022]      FIG. 2  shows a partial cross-section side view of steering unit  16 . During rotary drilling, steering unit  16  detects the rotation of drillstring  6 , indicated as direction  32 . When pipe rotation ceases, steering unit  16  may be used to perform a survey of the borehole  8  within rock formation  10 . Steering unit  16  includes three orthogonal magnetometers  24  that are used to conduct the survey. Each sensor  24  is substantially normal to tool axis  28  and has an axis  30  that is substantially aligned along the x-axis, y-axis or z-axis. Steering unit  16  or tool  14  may include processor  26  to store and process data. During drilling, the x-axis and y-axis magnetometers  24   a  and  24   b  are presented with a varying magnetic field vector for magnetic field  34 . As each axis  30  of the magnetometers  24  alternatively aligns with magnetic north, the field strength that the magnetometer  24  receives is at a relative maximum. Similarly, as a given axis  30  becomes oriented 90° to magnetic north, the measured field strength is at a relative minimum. 
         [0023]    Because magnetometers  24   a  and  24   b , which are positioned normal to drill collar  18  rotating about tool axis  28  as it rotates in direction  32 , pass through a maximum and minimum magnetic field strength as described, the field strength may be mapped at other angular orientations as well.  FIG. 3A  shows an example of a plot  36  of the polar magnetic vectors of earth field strength in formation  10  superimposed on a top-down view of magnetometers  24 , e.g., showing the x-axis and y-axis.  FIG. 3B  shows a sketch of plot  36  showing the z-axis contribution measured from magnetometer  24   c  as zero. It will be understood by those of ordinary skill in the art that this contribution is not necessarily always zero and may be distorted based on nearby ferrous objects and by the presence of magnetic dip which may give a natural, non-zero, z-axis component. Accordingly, polar plot  36  is a 3-dimensional plot representing an accumulation of vectors  38  with angle θ i  and field strength M i . Plot  36  may be determined from an average of multiple samples for vectors  38  over time. Plot  36  may be calculated by surface processor  21  or tool processor  26 . 
         [0024]    Polar plot  36  may be expressed as a magnetic field vector (MFV), which describes polar plot  36  in matrix form as shown below in Equation 2: 
         [0000]    
       
         
           
             
               
                 
                   
                     MFV 
                     _ 
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             Θ 
                             1 
                           
                         
                         
                           
                             M 
                             1 
                           
                         
                       
                       
                         
                           
                             Θ 
                             2 
                           
                         
                         
                           
                             M 
                             2 
                           
                         
                       
                       
                         
                           ⋯ 
                         
                         
                           ⋯ 
                         
                       
                       
                         
                           
                             Θ 
                             i 
                           
                         
                         
                           
                             M 
                             i 
                           
                         
                       
                       
                         
                           ⋯ 
                         
                         
                           ⋯ 
                         
                       
                       
                         
                           
                             Θ 
                             f 
                           
                         
                         
                           
                             M 
                             f 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0025]    In Equation 2, Θ i  is the apparent field direction; Θ f  is the maximum angle (e.g., 360°) of angular displacement during rotation; and M f  is the average field at the maximum angle. 
         [0026]    The value |MFV| is related to TMF, and may be expressed as shown below in Equation 3: 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     MFV 
                      
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           i 
                           f 
                         
                          
                         
                             
                         
                          
                         
                           M 
                           i 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0027]    TMF may be used as a factor to indicate close proximity to existing well casing  22 . Similarly, the value for MFV may be affected by the presence of ferrous materials such as those found in casing  22  or the magnetic anomalies in casing  22  caused, for example, by previous pipe inspections. Through the use of metrics related to the shape of the MFV at any given drilling depth D, system  2  allows a user to identify, in a progressive manner, relative proximity P to a magnetic anomaly such as casing  22 . 
         [0028]    Referring  FIGS. 3A and 3B , plot  36  corresponds to an MFV that is not subject to any distortion, such as that caused by nearby ferrous objects such as casing  22 . Accordingly, plot  36  may correspond to the MFV at a depth D and distance P relatively distant from casing  22 , e.g., at spudding. 
         [0029]      FIGS. 4 and 5  show an example of how the shape of the polar plot of MFV changes as tool  14  approaches casing  22 . With further reference to  FIG. 1 , as distance P decreases, the plot for MFV will manifest changes that a user may monitor to detect and locate casing  22 . Referring to  FIG. 4 , plot  42  corresponds to the polar plot of MFV at depth D 1  and distance P 1  from casing  22 . Plot  44  corresponds to the polar plot of MFV at depth D 2  and distance P 2  from casing  22 , where depth D 2  is greater than D 1 , and distance P 2  is less than P 1 . Similarly, plots  46 ,  48  and  50  correspond to polar plots of MFV as tool  14  is positioned downhole at greater depths D and closer proximity P to casing  22 , respectively. 
         [0030]    Initially, when tool  14  is at depth D 1  and distance P 1  from casing  22 , casing  22  does not exert any magnetic field distortion upon the MFV generated by tool  14  (e.g., plot  42  is similar to plot  36  of  FIG. 3A ). As depth D 1  increases to D 5 , tool  14  approaches casing  22 , and therefore, the distance P 1  decreases to P 5 . As P 1  decreases to P 5 , the magnetic field distortion caused by casing  22  increases. This distortion results in a change of shape of the plot of MFV. 
         [0031]      FIG. 5  shows the plot  50  superimposed upon plot  42  to illustrate the difference in shape between plot  42  at depth D 1  and plot  50  at depth D 5 . In contrast to plot  42 , plot  50  includes additional quadrant area  54  that distends outwards from quadrant area  58  (which corresponds to the initial area for that quadrant in plot  42 ). The shape of additional quadrant area  54  is substantially defined by inflection points  52 . The shape of additional quadrant area  54  indicates the relative positioning and proximity of casing  22  because quadrant area  54  is caused by magnetic field distortion generated by casing  22 . For example, additional quadrant area  54  is most distorted or extended at point  56  at an angle θc, e.g., distance E is greatest at point  56 . Note that this distance E corresponds to the largest vector  38  (shown in  FIG. 3 ) and therefore corresponds to direction θc in which the magnetic distortion caused by casing  22  has substantially the greatest effect on the plot of MFV. Accordingly, the shape and size of additional quadrant area  54  indicates the probable location of casing  22 . In this manner, a user may monitor the transition of plot  42  to  50  to observe the emergence of additional quadrant area  54  to determine the existence and location of casing  22 . Guidelines may be developed for selected plot shapes that correspond to casing proximity. A user, tool  14  or surface device  20  may compare the real-time plots of MFV to these guidelines to determine whether a well casing is nearby. Tool  14  may transmit an alert or signal to a user or surface device  20  upon determining a correspondence with a stored guideline that indicates the likelihood of a nearby well casing  22 . 
         [0032]    For the purposes of clarity, the contribution along the z-axis for the MFV plots are not shown in the examples discussed above in connection with  FIGS. 4 and 5 . Given that the plots for MFV may exhibit reflection symmetry, a user may need to consult the contribution along the z-axis, in addition to that along the x-axis and y-axis to determine the correct direction in which casing  22  is positioned relative to tool  14 . For example, where drilling is not truly vertical, the z-axis contribution may change over time and may be non-zero. As another example, magnetic dip may need to be considered due to magnetic vector directions that are not parallel to the Earth&#39;s surface. The relationship between the x-axis and y-axis contributions are further changed as the sensor package  16  approaches a long narrow object such as casing  22 . In addition, the TMF or vector sum of the x-, y- and z-axis may be altered, increasing or decreasing depending upon the polar relationships among tool  14 , well casing  22  and magnetic north. The shape of the MFV plot will distort and change in direct relation to the above factors. 
         [0033]    Typically, time is a premium during drilling operations. As a result, a slow, incremental approach to acquiring MFV data is generally not feasible. In survey-only operation, a mode often used in operating directional tools in a substantially vertical well, the magnetic sensors are normally not queried at all during drilling. In contrast, system  2  collects data from magnetic sensors  24  during times when magnetic sensors are typically dormant and takes advantage of the rotation that occurs from the drilling process. For example during the time required to penetrate the depth of one drill pipe joint. In addition, MFV data may be collected over a shorter depth interval at any time by stopping drilling and pulling bit  12  off the bottom. As a result, the disclosed system and method provide an economical process for locating casing, whether for avoidance or planned sidetracking. 
         [0034]    The speed with which magnetic field measurements may be taken may be based on the rotation rate of drillstring  6 , and the sample rate of magnetometers  24 , among other factors. For example, if magnetometer  24  is rotated at about 1 RPS, then a rate of 1 sample per 1/120 second for each axis will be required to map the fields to an angular resolution of about 3°. The sample rate would be 120 samples/second for each sensor  24   a  and  24   b  (x-axis and y-axis) and the z-axis measurement would simply give the inclination of the x and y field plane. The rate of 120 samples/second corresponds to 1 “averaged” sample from each sensor  24   a  and  24   b  about once every 8.33 milliseconds. The components of tool  14  may limit the number of raw samples that can be obtained during this time, e.g., the base raw data rate of the analog-to-digital converters (ADCs) of magnetometer  24 . The repetitive nature of drilling rotation allows the averaging of several MFV data sets to provide an MFV vector of relatively high accuracy for a period of several minutes. 
         [0035]    In a further example, a magnetometer sensor package may have a sample rate shown in Equation 4 below: 
         [0000]      3 sensors×512 samples/0.403 seconds=3811 samples/second  (4)
 
         [0036]    This sample rate, divided primarily between magnetometer  24   a  (y-axis) and magnetometer  24   b  (x-axis), would provide a rate of 1906 samples/second/sensor or 1906 samples/360 rotations. This would provide a capability of 1 sample per 0.2°, which is greater than the selected resolution discussed above. Thus, a 16 sample average would provide an angular resolution of about 3° at a nominal RPM of 60. 
         [0037]    From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a system for casing detection that is novel has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow.

Summary:
A system and method for detecting casing is provided. In one example, a method for detecting drill casing in a downhole environment includes the steps of generating a plurality of plots of a magnetic field vector (MIN) at a series of depths; and monitoring the plots to detect proximity to the well casing.