Abstract:
One aspect of the disclosed subject matter is seen in a directional drilling instrument that comprises a chassis, a first sensor and a second sensor. The chassis has a longitudinal axis, and the first sensor is coupled to the chassis and oriented at a first angle (w1) relative to the longitudinal axis of the chassis. The second sensor is coupled to the chassis and oriented at a second angle (w2) relative to the longitudinal axis of the chassis. The first and second angles are non-identical and non-orthogonal relative to the longitudinal axis.

Description:
BACKGROUND 
       [0001]    The disclosed subject matter relates generally to instruments useful in the drilling of wells and, more particularly, to a directional drilling instrument that may be useful in guiding the direction of the borehole as it is being drilled. 
         [0002]    When drilling vertical, horizontal, or extended reach wells, obtaining accurate measurements of inclination and azimuth is a fundamental requirement. Typically, the drilling equipment includes a directional drilling instrument located within the drill string near the drill bit. The directional drilling instrument is configured to collect data regarding its orientation, and communicate that data to the surface where it may be analyzed to determine its current position, such that the position of the drill bit may be derived. Collecting and analyzing this data over a period of time allows the operator of the drilling process to accurately guide the drill bit to a desired location that will enhance production from the resulting well. 
         [0003]    Typically, as is shown in  FIG. 1 , a conventional directional drilling instrument  10  is comprised of a plurality of sensors  12 ,  13   14 , such as accelerometers and/or magnetometers (e.g., typically three of each), that are positioned orthogonally to one another to form a 3-axis sensor capable of collecting data in a 3-dimensional system (e.g., X, Y, and Z coordinates) that may be used to locate and guide drilling. The magnetometers and accelerometers are oriented, relative to each other, so that their like axes are coincidentally aligned—X accelerometer parallel to X magnetometer, Y accelerometer parallel to Y magnetometer and Z accelerometer parallel to Z accelerometer. This alignment simplifies the mathematics involved in solving for the axes specific outputs, with respect to the orientation of the directional drilling instrument. The sensor  12  is oriented with its sensitive axis perpendicular to the instrument Z axis such that movement of the directional drilling instrument  10  along the Z axis will produce an output signal corresponding to the magnitude of such a movement. As will be appreciated by those skilled in the art, movement of the sensor  12  along the X and Y axes will result in a near-zero or non-significant output signal. The sensor  13  is oriented with its sensitive axis perpendicular to the X axis such that movement of the directional drilling instrument  10  along the X axis will produce an output signal corresponding to the magnitude of such a movement. And finally, the sensor  14  is oriented with its sensitive axis perpendicular to the Y axis such that movement of the directional drilling instrument  10  along the Y axis will produce an output corresponding to the magnitude of such a movement. 
         [0004]    Historically, oil well drilling trajectories were originally most often vertical wells. However, over time the trajectories have varied to low angle slant wells (build and hold), to directional “S” shaped wells (Directional Drilling, with gradual changes in both inclination and direction) then to Extended Reach (3D designer well paths) and then to the current standard, “Long Lateral” horizontal wells. Currently, the typical development drilling well profile consists of a vertical segment drilled deep enough to get close to the production depth, a curved section with a borehole angle build up rate (and direction) calculated for the best fit between the hole size and the completion tubular program as well as the specified reservoir target(s) and the production volume. The horizontal section is designed for enhanced resource recovery. The horizontal section of these wells may commonly be almost as long, or longer, than the vertical section. Wells of this type only produce at desired capacities if they are actually located accurately in the target reservoir. Accordingly, wellbore position uncertainty is critical to well productivity. 
         [0005]    The orthogonal orientation of the sensors in a conventional directional drilling instrument  10  is problematic in these types of wells. For example, the orthogonal orientation of the sensors  12 ,  13 ,  14  causes the output of one or more of the sensors to be either almost zero or full scale when the directional drilling instrument is oriented so that the direction of drilling substantially aligns with one of the X, Y, or Z-axis, such as would occur in the vertical or horizontal portions of the well. For example, when the directional sensor  10  is oriented vertically with the gravitational vector generally aligned with the Z-axis, the X and Y-axis sensors  13 ,  14  will typically experience little variation, and thus, be at or near zero output, whereas the Z-axis sensor  12  will undergo substantial variation and may be at or near maximum output. Similarly, when directional sensor  10  is oriented horizontally with the gravitational vector, as shown in  FIG. 1B , for example, generally aligned with the Y-axis, the Z and X-axis sensors  12 ,  13  will typically experience little variation, and thus, be at or near zero output, whereas the Y-axis sensor  14  will undergo substantial variation and may be at or near maximum output. 
         [0006]    Turning now to  FIG. 1C , a stylistic representation of the sensor  12  is shown in a position where the directional drilling instrument is in a substantially vertical orientation. In this orientation, gravity is acting in a direction that substantially aligns with the sensitive axis of the sensor  12 . Similarly, as is shown in  FIG. 1D , with the directional drilling instrument  10  in a substantially vertical orientation, the sensor  13 , on the other hand, has its sensitive axis at a right angle to the gravity vector. Thus, movement of the drilling instrument in a substantially vertical direction can produce very high output signals from the sensor  12  and very low output signals from the sensor  13 . Those skilled in the art will appreciate that the sensor  14  will have similar characteristics to the sensor  13  with respect to near vertical drilling. 
         [0007]    Both conditions (zero and full scale) are sub-optimum for the sensors in use in directional drilling instruments, and the corresponding accuracy of the resulting calculations suffers a significant degradation. Of course, this degradation in the calculations will undesirably affect the ability of the operator to guide the drilling process and locate the well at its desired location. 
         [0008]    Moreover, those skilled in the art will appreciate that the sensors  12 ,  13 ,  14  are more prone to physical damage when the sensitive axis is perpendicular to the direction that a force is applied to the sensor. For example, when drilling a vertical section of the well such that the sensitive axis and gravity vector of the sensor  12  are aligned, a substantial jarring or shock in the vertical direction can result in damage to and ultimate failure of the sensor  12 , whereas the sensors  13 ,  14  are less likely to be damaged. 
       BRIEF SUMMARY 
       [0009]    The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
         [0010]    One aspect of the disclosed subject matter is seen in a directional drilling instrument that comprises a chassis, a first sensor and a second sensor. The chassis has a longitudinal axis, and the first sensor is coupled to the chassis and oriented at a first angle (w 1 ) relative to the longitudinal axis of the chassis. The second sensor is coupled to the chassis and oriented at a second angle (w 2 ) relative to the longitudinal axis of the chassis. The first and second angles are non-identical and non-orthogonal relative to the longitudinal axis. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    The disclosed subject matter will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0012]      FIG. 1A  is a stylistic representation of one embodiment of a conventional directional drilling instrument deployed in a vertical section of a well bore; 
           [0013]      FIG. 1B  is a stylistic representation of one embodiment of a conventional directional drilling instrument deployed in a horizontal section of a well bore; 
           [0014]      FIG. 1C  is a stylistic representation of one embodiment of a conventional single axis accelerometer that may be employed in the directional drilling instrument of  FIG. 1A  as a Z-axis sensor; 
           [0015]      FIG. 1D  is a stylistic representation of one embodiment of a conventional single axis accelerometer that may be employed in the directional drilling instrument of  FIG. 1A  as an X-axis sensor; 
           [0016]      FIGS. 2A-2D  are stylistic representations of an embodiment of a desired orientation of sensors within a directional drilling instrument; 
           [0017]      FIG. 3A  is a side view of one embodiment of a chassis of a directional drilling instrument configured to receive a plurality of sensors; 
           [0018]      FIG. 3B  is a side view of one embodiment of a sensor in an orientation where its sensitive axis is skewed from the gravity vector; 
           [0019]      FIG. 4A  is a stylistic representation of the directional drilling instrument of  FIGS. 1 and 2  disposed in a vertical segment of a well; 
           [0020]      FIG. 4B  is a stylistic representation of the directional drilling instrument of  FIGS. 1 and 2  disposed in a horizontal segment of a well; and 
           [0021]      FIGS. 5A and 5B  stylistically illustrate the angular measurements W and T that may be used to define the orientation of the sensors set forth in  FIGS. 2A-2D  and other alternatives. 
       
    
    
       [0022]    While the disclosed subject matter is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosed subject matter to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosed subject matter as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0023]    One or more specific embodiments of the disclosed subject matter will be described below. It is specifically intended that the disclosed subject matter not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the disclosed subject matter unless explicitly indicated as being “critical” or “essential.” 
         [0024]    The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the disclosed subject matter with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0025]    As will be discussed in more detail throughout, a directional drilling instrument is described that utilizes one or more of the principals associated with sensor axis orientation to provide for a directional drilling instrument (or any like navigational platform) that reduces individual sensor output degradation arising from the orientation of the instrument body. The principals described herein can be applied to a variety of sensors that may be utilized in directional drilling instruments, including accelerometers and magnetometers. The principals described herein allow the use higher order mathematical algorithms to detect and correct for misalignments in either the directional drilling sensor itself, a corresponding bottomhole assembly, or both. Moreover, the principals described herein may also be used to facilitate the detection and compensation for other environmental conditions, including, but not limited to, drillstring magnetism, adjacent cased hole well bores (magnetic interference), well bore geometric stability, drilling fluids (type and condition), thermal effects and individual sensor failure/s. 
         [0026]    In one embodiment, the sensors within the directional drilling instrument may include a cluster of two, three or four accelerometers and two, three or four magnetometers where one or more of the sensor axes may be skewed relative to a conventional right-handed array. Additionally, one or more of the axes may be skewed relative to each other. This orientation creates a condition where each of the sensors delivers an output signal that falls near the middle of its capable range of output signals. Such a situation is highly desirable in that it enhances the accuracy of the measurement made by each individual sensor, and thus, the accuracy of the directional drilling instrument as a whole. 
         [0027]    Those skilled in the art will appreciate that the principals set forth herein may be utilized to produce a directional drilling instrument that may provide accurate results in situations where only two sensors are employed. Such a two-sensor embodiment may be implemented to provide a lower cost design, or alternatively to allow a three or four-sensor instrument to continue to operate and provide acceptable results in the event that one or more of its sensors are damaged. 
         [0028]    Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIGS. 2A-2D , the disclosed subject matter shall be described in the context of a four-sensor directional drilling instrument that includes four sensors  202 - 205 . In one embodiment, the sensors  202 - 205  take the form of accelerometers manufactured by Honeywell as part number QAT-185 or magnetometers manufactured by Microtesla as part number 220368-PL-01, or both. Those skilled in the art, however, will appreciate that other specific sensors may be utilized without departing from the spirit and scope of the instant invention. 
         [0029]      FIG. 2A  illustrates a conventional right-hand coordinate system oriented with its Z-axis  210  generally corresponding to the longitudinal axis of the directional drilling instrument. The X and Y-axes  211 ,  212  are positioned at 90° relative to the Z-axis and to each other. In conventional directional drilling systems, sensors are generally aligned with these axes. However, in the illustrated embodiment of the directional drilling instrument it may be seen that a first sensor  202  is positioned on a new axis A, which is located by rotating the original X-axis  211  through an angle of about −60° (clockwise in the XY plane) to form an axis X′ and then elevated by an angle of about 30°. As is shown in  FIG. 2B , a second sensor  203  is positioned on a new axis B, which is located by rotating the original X-axis  212  through an angle of about −30° (clockwise in the XY plane) to form an axis X′ and then depressed by an angle of about 30°. As is show in  FIG. 2C , a third sensor  204  is positioned on a new axis C, which is located by rotating the original X-axis through an angle of about 165° to form an axis X′ and then elevated by an angle of about 60°. As can be seen in  FIG. 2D , a fourth sensor  205  is positioned on a new D axis, which is located by rotating the original X-axis through an angle of about −150° (clockwise in the XY plane) to form an axis K and then depressed by an angle of about −45°. Thus, the newly formed coordinate system A, B, C and D has the following relationships:
       A to B=−145°;   B to C=−150°;   C to D=135°; and   D to A=150°       
 
         [0034]    Turning now to  FIG. 3A , one embodiment of a chassis  400  is shown in which 4 sensors  402 - 405 , such as accelerometers and/or magnetometers may be located according to the angular relationships set forth with respect to  FIGS. 2A-2D . Generally, the chassis  400  has a longitudinal axis  410  that aligns with a Z-axis  415  of a conventional 3-dimension coordinate system, which also includes an X-axis  420  and a Y-axis  425  as references for describing the orientation of the sensors  402 - 405 . 
         [0035]    The first sensor  402  is located in a pocket  430  formed in the chassis  400 , wherein the pocket  402  includes a longitudinal axis  435  that is skewed from the 3-dimension coordinate system  415 ,  420 ,  425  according to the relationships described above. Further, the second, third and fourth sensors  403 - 405  are similarly positioned in pockets  431 - 433  with each of these pockets having longitudinal axes  436 - 438 , respectively. The axes  436 - 438  are also skewed from the 3-dimension coordinate system  415 ,  420 ,  425  according to the relationships described above. Those skilled in the art will appreciate that the chassis  400  may be formed from any suitably rigid material, including plastics, metals, etc., and the pockets  430 - 433  may be formed by casting or forming the chassis  400  with the pockets  430 - 433  formed therein. Alternatively, the chassis  400  could be initially formed without pockets and the pockets  430 - 433  could be subsequently formed therein via a machining or similar process. The size and construction of the pockets  430 - 433  may be sufficient to securely retain the sensors  401 - 404  in a desired orientation while limiting movement between the sensors  401 - 404  and the chassis  400 . It is envisioned that any of a variety of conventional retention systems may be employed. For example, in one embodiment it may be useful to retain the sensors  401 - 404  using a conventional snap ring arrangement; however, other methods, including, various mechanical and chemical processes may be employed, including, but not limited to, gluing, soldering, welding, screws, bolts, nuts, etc. 
         [0036]    As can be seen in the stylistic drawing of  FIG. 3B , the sensor  402 , for example, when constructed according to the principals set forth herein has its sensitive axis  480  skewed from the gravity vector  482  when the instrument  400  is vertically oriented. This orientation causes the sensor  402  to detect gravity, when oriented vertically at only a portion of its full-scale capability. 
         [0037]    Turning now to  FIG. 4A , a stylized representation of an instrument  500  having four accelerometers  501 - 504  arranged according to the principals set forth herein is shown, and these accelerometers  501 - 504  have the following angular relationships to the longitudinal axis of the instrument  500 : 45°, 60°, 60°, and 30°, respectively. The mathematical formula for calculating the output of the accelerometer  504  is as follows: 
         [0000]      Output=sin(Angle of sensor+Angle of Instrument relative to the Gravity vector)×Gravity
 
         [0000]      Output=sin(30°+0°)×Gravity
 
         [0000]      Output=0.5 G. 
         [0038]    Thus, when the instrument is arranged vertically, the output of the accelerometer  504  is advantageously at about its mid-range. Those skilled in the art will appreciate that similar mathematical relationships exist between accelerometers  501 - 503 , based on the degree to which each accelerometer is skewed from the gravity vector such that each of their output signals is a fraction of full scale. 
         [0039]    Turning now to  FIG. 4B , when the instrument  500  is oriented horizontally, such as in the horizontal portion of a wellbore, the angle of the instrument relative to the gravity vector is about 90°, and thus, the mathematical calculations for determining the output of the accelerometer  504  in the horizontal orientation is as follows: 
         [0000]      Output=sin(Angle of sensor+Angle of Instrument relative to the Gravity vector)×Gravity
 
         [0000]      Output=sin(30°+90°)×Gravity
 
         [0000]      Output=0.866 G. 
         [0040]    Thus, the output of the accelerometer  504 , whether it is vertically or horizontally oriented, is advantageously at a fraction of full scale. Those skilled in the art will appreciate that similar mathematical relationships exist between accelerometers  501 - 503 , based on the degree to which each is skewed from the gravity vector such that each of their output signals is a fraction of full scale in both horizontal and vertical orientations. 
         [0041]    Those skilled in the art, having the benefit of the instant description, will appreciate that the precise angular orientation of each of the accelerometers/magnetometers relative to the longitudinal axis of the instrument may vary substantially without departing from the spirit and scope of the instant invention. For example, it is anticipated that varying the angular orientation of each of the sensors by as much as 5° from their designed orientation in any plane will nevertheless produce results having acceptable accuracy for at least some applications. 
         [0042]    Those skilled in the art will also appreciate that the sensors may be oriented in a variety of positions and still result in acceptable accuracy. For example, three embodiments that describe orientations that may be useful are set forth in Tables I-III below. In these tables, a simpler convention is adopted to define the orientation of the individual sensors, as opposed to the convention used to describe the embodiment set forth in  FIGS. 2A-2D . In the embodiments set forth below, the orientation of the sensors A, B, C, and D are defined by two angular components, W and T. W, as shown in  FIG. 5A , represents the angle of the longitudinal axis of the individual sensor relative to the longitudinal axis of the tool, and thus, would theoretically fall in the range of 0°-180°. T, as shown in  FIG. 5B , represents the angle of the longitudinal axis of the sensor as viewed from the top of the tool, and thus, would theoretically fall in the range of 0°-360°. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Sensor 
                 W 
                 T 
               
               
                   
               
             
             
               
                 A 
                 23° 
                 180° 
               
               
                 B 
                 85° 
                 337° 
               
               
                 C 
                 102°  
                  94° 
               
               
                 D 
                 68° 
                  43° 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Sensor 
                 W 
                 T 
               
               
                   
               
             
             
               
                 A 
                 20° 
                 210° 
               
               
                 B 
                 90° 
                 330° 
               
               
                 C 
                 90° 
                  90° 
               
               
                 D 
                 70° 
                  30° 
               
               
                   
               
             
          
         
       
     
         [0043]    Table III, set forth below, represents the angles W and T for the embodiment set forth in  FIGS. 2A-2D . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                 Sensor 
                 W 
                 T 
               
               
                   
               
             
             
               
                 A 
                 120° 
                  60° 
               
               
                 B 
                  30° 
                 330° 
               
               
                 C 
                 150° 
                 195° 
               
               
                 D 
                  45° 
                 150° 
               
               
                   
               
             
          
         
       
     
         [0044]    The particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.