Abstract:
The present invention relates to a measurement-while-drilling (MWD) tool having a pressure compensation system and reservoir segregated from fluid integration with actuator mechanism lubricants. The MWD tool can also be serviced on the rig floor without the need to drain, reseal and charge the compensation system.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a measurement-while-drilling tool that is compartmentalized to facilitate rapid field servicing and component replacement without the need to remove the tool from the drilling site for specialized service. In particular, the pressure compensation system and reservoir are segregated from fluid integration with the actuator mechanism&#39;s lubricants. The present invention permits disconnection of tool sections adjacent the pressure compensating reservoir without drawing a vacuum for oil filling of those sections. This also provides for the manufacture of a shorter and less expensive tool. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the exploration of oil, gas, and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. In many locations, it has been found to be advantageous to be able to track the position and direction of the subterranean drill bit during the drilling process. Measurement-while-drilling (MWD) tools have been developed for this purpose. 
         [0003]    MWD tools typically have electromechanical accelerometers mounted with their sensitive axis aligned orthogonally to the spin axis and to each other. Micro-electrical-mechanical systems (MEMS) based accelerometers are also available. Tools may also include other sensors for determining properties such as wellbore temperature and azimuthal direction of inclination of the wellbore. For example, gyroscopes or magnetometers may also be included. The sensors are operated on power provided by a rechargeable battery pack located in the tool. Data recorded from the sensors is sent to onboard memory. The data is coded into pulses similar in theory to Morse code. Each MWD system has its own proprietary code. 
         [0004]    A pulser located in the MWD tool is provided to generate hydraulic pulses at the MWD tool. The pulses create waves in the fluid mud column that reach the surface, where pressure sensors record them. The recorded pulses are filtered to remove normally occurring extraneous signal noise unrelated to the pulses sent by the MWD tool. The filtered pulses are then decoded to reveal the data recorded by the MWD sensors. The data is then displayed at the surface in a manner useful to the drilling personnel. 
         [0005]    Pulses may be generated in a variety of manner, including positive pulse, negative pulse, and combinations thereof. Most conventional MWD units have a pilot valve that actuates a primary pulser. In this manner, a stronger pulse can be sent into the mud column which permits detection and identification apart from system noise and decoding by the surface equipment. 
         [0006]    MWD tools are available in retrievable or non-retrievable designs. Retrievable MWD tools can be removed from the drill string at any time. Non-retrievable MWD tools remain in the drill string until the drill string is removed from the wellbore. Upon retrieval, MWD tools frequently require mechanical and electrical service. 
         [0007]    Conventional pulser units include solenoid and motor driven varieties. Conventional motor driven pulsers utilize a magnetic coupling to transmit power between the motor and a ball screw assembly. The ball screw assembly actuates a pilot valve which in turn operates the pulser. 
         [0008]    Conventional MWD designs have a combined lubrication and pressure compensation system. The system is intended to equalize the internal pressure of the lubrication and compensating reservoir with the external wellbore pressure, while using the compensating fluid for lubrication of the actuator mechanism. A pressure balance rod on which the pilot valve is attached is exposed to both internal and external pressures, and operates better when the internal and external pressures are balanced. Without a balanced system, the motor and actuator must overcome the pressure differential, with energy supplied by the limited batteries source. The reservoir also serves as the lubrication system for the actuator assembly, substantially enclosing the magnetic coupler and ball screw assemblies. 
         [0009]    Due to the harsh environment in which survey tools operate, they are carefully sealed to protect the internal components. When motors are used, special seals are required to prevent the oil from leaking past the magnetic couplers into the motor. The lubrication and compensation reservoir is typically vacuum-filled and sealed on a shop bench. 
         [0010]    A principal disadvantage of known MWD tools is the susceptibility of pilot valves clogging due to lost circulation material (LCM) becoming trapped between the pilot valve and the seat of the valve. In particular, MWD tools utilizing motors and magnetic couplings are limited in torque to the power of the magnet. Magnetic couplings are known to slip in high torque conditions, including interference caused by LCM, and thus rendering false pulse patterns. Magnetic coupling systems are also relatively long, commonly being up to three feet in length. 
         [0011]    A principal disadvantage of the combined lubrication and pressure compensation system of known MWD tools is reliability and serviceability. When direct drive motors are used instead of magnetic couplings, the pressure compensating and lubrication oil must be sealed from the rotating shaft of the motor. The seal against the rotating shaft will have a limited life. The brushes and field coils of the motor will eventually pick-up the metal-iron fines and impurities in the oil, causing the motor to fail. When the motor fails, servicing the motor requires disassembly and drainage of the lubrication and compensation reservoir and, thus, return of the tool to the shop bench for sealing, reassembly and reservoir refilling under vacuum. Conventional MWD tools with lengthy components and integrated lubrication and compensation reservoirs are not, thus, serviceable in the field. 
         [0012]    Another principal disadvantage of known MWD tools is the pressure loss associated with the location of the tool within the confines of the internal diameter of drill collars. Pressure is lost due to high flow rates between the exterior of the MWD tool and the drill collar I.D. This pressure is then unavailable to the drill bit. Pressure (or flow rate) to the drill bit is critical to the rate of penetration and life of the drill bit, and is a significant factor on the calculation of the cost per foot of the drilling operation. The larger the diameter of the MWD tool, the greater the system pressure loss will be. Similarly, the longer the MWD tool, the greater the system pressure loss will be. 
         [0013]    Another principal disadvantage of known MWD tools is material cost. Due to the high velocity of the fluid between the exterior of the MWD tool and the inside diameter of the drill collars, expensive alloyed materials are required. Typically, the housing of conventional MWD tools is made from beryllium copper or a similarly wear resistant material. As a result, larger MWD tool diameters and longer length tools substantially increase the material cost of the tool. 
         [0014]    A principal disadvantage of known MWD tools utilizing motors is that commercially available motors have a significantly larger form factor (profile) than do solenoid systems, thus requiring larger diameter housings and increased material cost. Also, higher-powered commercially available motors capable of extended service are larger in diameter. Additionally, motors are provided with mounting brackets that are external to the circumference of the motor housing. Therefore, increasing the motor power requires increasing the diameter of the tool. Of similar disadvantage, magnetic couplings are relatively lengthy assemblies. 
         [0015]    Therefore, there is a further need to develop an MWD tool which can be serviced at the rig floor without draining the lubrication and compensation system, and without requiring shop delivery to reassemble and refill. There is a further need to develop an improved MWD tool having a shorter length to save cost and drilling efficiency. There is also a need to develop an MWD tool having greater reliability obtained from a more powerful motor driven actuator, without the increasing manufacturing and drilling costs associated with increased tool diameter. Lastly, there is a further need to develop a more compact and effective pressure compensation system. 
         [0016]    There is also a need to accomplish these goals at a reasonable cost. The harsh drilling environment has prevented efforts to accomplish these goals in the past. 
       SUMMARY OF THE INVENTION 
       [0017]    As referenced herein throughout, the terms “downward,” “lower,” “bottom,” and “below” refer to the direction or portion of a part or assembly located or oriented towards the bottom of the wellbore when being used in a drilling assembly. The terms “upward,” “upper,” “top,” and “above” refer to the direction or portion of a part or assembly located or oriented towards the top of the wellbore when being used in a drilling assembly. 
         [0018]    The present invention provides a substantially improved MWD tool. More specifically, the present invention is directed to the pressure balance and actuator assembly portion of the tool which includes a pressure compensation reservoir isolated from the actuator chassis, motor and ball screw assembly. 
         [0019]    The MWD tool of the present invention has a lower pulser assembly that is receivable in a landing sub. The lower pulser assembly has an orifice located at its upper end. A screen housing is connected to the upper end of the lower pulser assembly. The screen housing has a plurality of passages through which drilling fluid enters the MWD tool. 
         [0020]    A pressure balance chassis is removably connected to the upper end of the screen housing. An actuator chassis is removably connected to the upper end of the pressure balance chassis. A motor-electronics chassis is removably connected to the upper end of the actuator chassis. A hollow pressure barrel is slidably positioned over the motor-electronics chassis, actuator chassis, and pressure balance chassis. The pressure barrel is removably connected to the upper end of the screen housing. 
         [0021]    A fluid-filled pressure-compensating reservoir is located in axially sealed relationship within the pressure balance chassis. A pliable bladder circumferentially surrounds the pressure-compensating reservoir. A pressure balance rod extends through the pressure balance chassis, in slideable relationship with the pressure balance chassis. 
         [0022]    A piston is formed on the pressure balance rod, and is located within a cylinder bore portion of the pressure balance chassis. The compensating reservoir is in fluid communication with the portion of the cylinder bore that is above the piston. A pilot valve tip is located on the lower end of the pressure balance rod, and positioned for flow restricting alignment with the orifice of the lower pulser. A plurality of venting perforations is located on the pressure barrel in alignment with the bladder, exposing the bladder to the pressure of the drilling fluid outside of the MWD tool. Drilling fluid pressure acting on the pilot valve tip is thus compensated by fluid pressure in the compensating reservoir acting against the piston. 
         [0023]    Also, upon disconnection of the motor-electronics chassis from the actuator chassis for service of the tool, the fluid in the compensating reservoir remains axially sealed within the pressure balance chassis. 
         [0024]    In another preferred embodiment, the cross-sectional area of the cylinder bore portion of the pressure balance chassis is substantially equal to the sum of the cross-sectional area of the pilot valve tip plus the cross-sectional area of the pressure balance rod. 
         [0025]    In another preferred embodiment, a case-hardened gear train housing is provided that has a circular external diameter that is approximately equal to the external diameter of the motor housing. Electrical connections are rerouted through the rear plate of the motor. The actuator chassis has a portal for receiving the gear train and longitudinal slits at the portal. Fasteners located in holes through the slits permit slight compression of the chassis to secure the gear train and motor in place. 
         [0026]    A principal advantage of the present invention is that it provides a more compact and effective pressure compensation system. Another advantage of the present invention is that it provides segregation between actuator lubricants and pressure compensating fluids, permitting optimized selection of each. 
         [0027]    Another advantage of the present invention is that it provides enhanced serviceability on the rig floor, by permitting disassembly without draining the lubrication system or the pressure compensating system, and thus does not require return of the tool to the shop bench for reservoir refilling and sealing. 
         [0028]    A principal advantage of the present invention is that it provides an MWD tool with increased actuator power, which is less susceptible to clogging due to lost circulation material (LCM) becoming trapped between the pilot valve and the seat of the valve. Another advantage of the present invention is that it provides an MWD tool having a shortened coupling system. 
         [0029]    Another advantage of the present invention is that it provides an MWD tool with increased actuator power within a smaller diameter and shorter length, thus reducing the pressure loss to the drill bit. Another advantage of the present invention is that it provides an MWD tool with a lower material cost. 
         [0030]    As referred to hereinabove and throughout, the “present invention” refers to one or more embodiments of the present invention, which may or may not be claimed, and such references are not intended to limit the language of the claims, or to be used to construe the claims in a limiting manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The objects and features of the invention will become more readily understood from the following detailed description and appended claims when read in conjunction with the accompanying drawings in which like numerals represent like elements. 
           [0032]    The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. 
           [0033]      FIG. 1  is a side sectional view of a landing sub of the type that may be used in conjunction with the preferred embodiments of the present invention. 
           [0034]      FIG. 2  is a partially-sectioned side view of MWD Tool  1 , illustrated in accordance with a preferred embodiment of the present invention. 
           [0035]      FIG. 3  is a side sectional view of the lower pulser assembly of the MWD Tool  1 , illustrated in accordance with a preferred embodiment of the present invention. 
           [0036]      FIG. 4  is a cross-section of a portion of the MWD Tool  1 , illustrating a screen housing connected to a pressure balance assembly, which is connected to an actuator assembly, which is connected to a motor-electronics assembly in accordance with a preferred embodiment of the present invention. 
           [0037]      FIG. 5  is a cross-section of a portion of the MWD Tool  1 , illustrating a pressure balance assembly connected between a screen housing and an actuator assembly in accordance with a preferred embodiment of the present invention. 
           [0038]      FIG. 6  is a side view of the actuator assembly connected between a pressure balance assembly and a motor-electronics assembly, illustrated in accordance with a preferred embodiment of the present invention. 
           [0039]      FIG. 7  is a side view of the actuator chassis of  FIG. 6 , illustrated rotated 90°. 
           [0040]      FIG. 8  is an isometric view comparing the prior art motor and gear train housing with the motor and gear train housing of a preferred embodiment of the present invention. 
           [0041]      FIG. 9  is an isometric view comparing the prior art motor, rear plate and electric connections with the motor, rear plate and electric connections of a preferred embodiment of the present invention. 
           [0042]      FIG. 10  is a side sectional view of a motor and electronics assembly illustrated in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0044]      FIG. 1  is a side sectional view of a landing sub  10 . A landing sub  10  is provided for location in the collar section of a drill string. Landing sub  10  is positioned in the drill string above the drill bit and mud motor, if one is used. A muleshoe  12  having a lower helix  14  is located internal to landing sub  10  for receiving MWD Tool  1 . Muleshoe  12  is secured in position within landing sub  10  by a plurality of set screws  16 . A locator key  18  projects radially inward for lateral angular positioning of MWD Tool  1  in muleshoe  12 . A wear cuff  20  is located at the top of muleshoe  12  for limiting the insertion of MWD Tool  1  in landing sub  10  and for absorbing vibration and impact during operation of MWD Tool  1 . An orifice sleeve  22  is located at the bottom of muleshoe  12 . A carbide orifice  24  is located inside orifice sleeve  22 . 
         [0045]      FIG. 2  is a partially sectioned side view of MWD Tool  1 , illustrating MWD Tool  1  engaged in landing sub  10 . MWD Tool  1  has an upper end and a lower end, the lower end being disposed closest to the drill bit. A lower pulser assembly  100  comprises the lower end of MWD Tool  1 . A screen housing  200  is attached above lower pulser assembly  100 . A pressure balance assembly  300  and actuator assembly  400  are attached above lower pulser assembly  100 . A motor and electronics assembly  500  is attached above actuator assembly  400 . A lower centralizer  2  is typically attached above motor and electronics assembly  500 . A battery barrel  600  is attached above motor and electronics assembly  500 . An upper centralizer  4  is typically attached above battery barrel  600 . A Navigation Master Assembly  700  is attached above upper centralizer  4 . A lifting connection  6 , such as a rope socket, is typically attached above Navigation Master Assembly  700 . 
         [0046]      FIG. 3  is a side sectional view of lower pulser assembly  100 . Lower pulser assembly  100  is comprised of a helix  102  connected to a hollow hydro chassis  104 , with various internal components communicating between them. Helix  102  fits in mechanical engagement with lower helix  14  on muleshoe  12  in landing sub  10 . Helix  102  has a hollow center. A longitudinal slot  106  is provided on the exterior surface of helix  102  for vertical engagement with locator key  18 . The upper end of helix  102  is thread connected to hollow hydro chassis  104 . 
         [0047]    A wear ring  110  is located on the external surface of helix  102  at the connection of helix  102  to hydro chassis  104 . Wear ring  110  engages wear cuff  20  when seating MWD Tool  1  inside landing sub  10 . A hollow push rod  112  slidably extends through the center of helix  102  and beyond the top and bottom ends of helix  102 . A poppet  114  is attached to the lower end of push rod  112 . Poppet  114  is normally made of a hard, wear resistant material, such as carbide. A hollow orifice  116  is formed through the center of poppet  114  to allow drilling fluid to pass through. 
         [0048]    A plurality of longitudinal surface flats  118  are located on the lower end of push rod  112 , above poppet  114 , to provide reduced friction passage of mud when push rod  112  slides through helix  102 . The opposite, upper end of push rod  112  extends into a cylinder chamber  120  inside hydro chassis  104 . A piston  122  is located in cylinder chamber  120  and thread connected to the upper end of push rod  112 . A piston orifice  124  is secured inside piston  122 . Piston orifice  124  allows drilling fluid a continuous flow path through to the interior of push rod  112  and piston  122 . In a preferred embodiment, a plurality of small holes  128  is circumferentially located in hydro chassis  104  above the position of piston  122 . 
         [0049]    A compression spring  130  is located inside chamber  120  of hydro chassis  104 . Spring  130  is partially compressed between the end of piston  122  and a circular ledge  132  at the upper end of chamber  120 . In this manner, spring  130  urges piston  122  downward, which in turn urges poppet  114  to close in relation to orifice  24 , restricting flow around poppet  114 . 
         [0050]    A passage  140  is provided between chamber  120  and the upper end of hydro chassis  104 . A valve seat  142  is located in the end of the passage. The upper end of hydro chassis  104  is threaded for connection to pressure balance assembly  300  and actuator assembly  400 . Valve seat  142  receives a valve tip  332  of pressure balance assembly  300  and actuator assembly  400 . 
         [0051]      FIG. 4  is a cross-section of a portion of the MWD Tool  1 , illustrating screen housing  200  connected to pressure balance assembly  300 , which is connected to actuator assembly  400 , which is connected to motor-electronics assembly  500 . Screen housing  200  is thread connected to the upper end of hydro chassis  104 . When connected, valve seat  142  of hydro chassis  104  is located inside the hollow center of screen housing  200 . A perforated mud screen  202  is attached to screen housing  200  to permit the flow of drilling fluid into screen housing  200 . 
         [0052]    Pressure barrel  8  is thread connected to the upper end of screen housing  200 . In the preferred embodiment illustrated, three separable chassis are connected in series to support the working components inside pressure barrel  8 . A pressure balance chassis  310  ( FIG. 5 ) is located at the lower end of pressure barrel  8 . An actuator chassis  410  ( FIG. 6 ) is located in the middle portion of pressure barrel  8 . A motor-electronics chassis  510  ( FIG. 10 ) is located at the upper end of pressure barrel  8 . 
         [0053]    Removal of pressure barrel  8  provides access to pressure balance chassis  310 , actuator chassis  410 , and motor-electronics chassis  510 . Pressure barrel  8  can be removed without loss of oil from a pressure compensation reservoir  328 . 
         [0054]      FIG. 5  is a cross-section of a portion of MWD Tool  1 , illustrating pressure balance assembly  300  connected between screen housing  200  and actuator assembly  400 . To support the working components, a pressure balance chassis  310  is connected between screen housing  200  and an actuator chassis  410 . In the preferred embodiment illustrated in this view, the lower end of pressure balance chassis  310  is thread connected inside the upper end screen housing  200 , which is attached inside the lower end of pressure barrel  8 . Pressure balance chassis  310  is generally tubular, but otherwise has a relatively complex profile. A smooth interior bore  312  is provided through pressure balance chassis  310  for receiving a pressure balance rod  314  in sliding relation. Pressure balance rod  314  has a carbide tip  332  connected at its lower end which operates in opening and closing relationship with valve seat  142  of hydro chassis  104 . 
         [0055]    Pressure balance chassis  310  has a central relief portion  316  with a reduced external diameter. A pliable bladder  320  surrounds central relief  316 . Bladder  320  is circumferentially secured and sealed to the external surface of pressure balance chassis  310  on both sides of central relief  316  in a fluid-tight manner, by clamps  322 , which may be wrapped buss wire, or another clamping device. Circumferential grooves  324  may be provided on pressure balance chassis  310  for receiving clamps  322 . The volume between central relief  316  and bladder  320  provides a compact and uniquely isolated pressure-compensating reservoir  328 . Venting perforations  330  are provided through pressure barrel  8  above bladder  320  to facilitate equalization of fluid pressure between the exterior of pressure barrel  8  and pressure-compensating reservoir  328 . 
         [0056]    The central portion of pressure balance rod  314  includes a circumferentially raised piston  334 . Piston  334  is located inside an enlarged cylinder portion  336  of interior bore  312  of pressure balance chassis  310 . Reservoir fluid passages  318  provide fluid communication of compensating fluid between pressure compensating reservoir  328  and cylinder  336 . One or more circumferential grooves  338  in piston  334  has a piston seal  340  located therein, forming a seal between piston  334  and cylinder  336 . Piston seal  340  prevents passage of compensating fluid in cylinder  336  beyond piston  334 . 
         [0057]    In a preferred embodiment, pressure balance chassis  310  has a receptacle  350  on its upper end. As best seen in  FIG. 7 , a cylindrical bushing  450  is formed on the lower end of actuator chassis  410 . When actuator chassis  410  is connected to pressure balance chassis  310 , bushing  450  is received in receptacle  350  in close tolerance engagement, forming the upper end of cylinder  336 . 
         [0058]      FIG. 6  is a side view of actuator assembly  400  connected between pressure balance assembly  300  and motor and electronics assembly  500 , illustrated in accordance with a preferred embodiment of the present invention.  FIG. 7  is a side view of actuator chassis  410  of  FIG. 6 , illustrated as rotated  900 . 
         [0059]    As seen in these views, actuator chassis  410  has a central bore  412  for receiving pressure balance rod  314  in sliding relation. A seal groove  414  is formed in central bore  412  within bushing  450  for location of one of more rod seals  342 . In a more preferred embodiment, an external seal groove  416  is provided on the exterior of bushing  450  for location of one or more chassis seals  418  for sealing between bushing  450  of actuator chassis  410  and receptacle  350  of pressure balance chassis  310 . In this embodiment, rod seal  342  and chassis seal  418  provide the upper end seals of the pressure compensating system. 
         [0060]    One or more rod seals  342  surrounds pressure balance rod  314  to prevent passage of compensating fluid above or beyond rod seal  342 . In the preferred embodiment illustrated, one or more circumferential grooves  346  on actuator chassis  410  has a rod seal  342  located therein, forming the seal between the upper end of pressure balance rod  314  and pressure balance chassis  310 . In a more preferred embodiment, a secondary rod seal  344  is provided adjacent to rod seal  342  to enhance sealing of the pressure-compensating fluid. 
         [0061]    In the above described configuration, piston  334  operates to move pressure balance rod  314  in response to pressure changes at the surface of bladder  320  from within the well bore. Unique to the present invention, the exposed cross-sectional area of cylinder portion  336  is designed to be substantially equal to the sum of cross-sectional area of carbide valve tip  332  plus the cross-sectional area of pressure balance rod  314 . The result is a force-balanced movement of pressure balance rod  314  in either direction, in response to pressure changes acting on substantially equal areas. In this manner, an isolated and minimized volume pressure-compensating reservoir  328  can be used and still ensure downhole hydrostatic pressure does not impair or impede the ability of the actuator assembly  410  to operate valve tip  332 . 
         [0062]    A pipe plug  348  in pressure balance chassis  310  provides access for fluid filling of pressure-compensating reservoir  328 . The configuration detailed above isolates the pressure compensation fluid from other moving parts within the actuator assembly  400 . Importantly, this permits utilization of lubricants that may be less suitable in performance as pressure compensating fluids, and vice-versa. In particular, grease has been shown to be a superior lubricant for the actuator ball screw, but is inferior in performance as a compensating fluid. 
         [0063]    The above-described design further permits service to most component portions of MWD Tool  1  without the need to return the tool to the shop to replace the compensating fluid. Simple bench service of the tool can be performed without the loss of the critical compensating fluid. This is increasingly useful as newer tools are beginning to have reconfiguration options related to the well conditions and required information. 
         [0064]    Actuator chassis  410  is located centrally inside pressure balance vent barrel  8  between pressure balance chassis  310  and motor-electronics chassis  510  and secured to each. Actuator chassis  410  has a central bore  412  for receiving pressure balance rod  314  in sliding relation. A shoulder bolt  420  connects the upper end of pressure balance rod  314  to a ball screw nut connector  422  on actuator chassis  410 . Axial movement of nut connector  422  results in movement of pressure balance rod  314  and opening and closing of valve tip  332  in relation to valve seat  142  of hydro chassis  104 . 
         [0065]    As assembled, a gear train  530  of a motor  514  extends into the interior of actuator chassis  410 . A ball screw  434  is operatively connected at one end to gear train  530 . In the preferred embodiment, an Oldham connector  428  is connected to motor gear train  530 . A thrust bearing assembly  430  and thrust bearing shaft  432  are attached to Oldham connector  428  at one end. At its opposite lower end, ball screw  434  is thread connected to a ball screw nut  436  and nut connector  422 . Shoulder bolt  420  limits the longitudinal travel of nut connector  422  within a longitudinal slot  452 . Rotation of nut connector  422  is also prohibited. 
         [0066]    Operation of motor  514  moves nut connector  422  and pressure balance rod  314  longitudinally to open and close valve tip  332  in relation to valve seat  142 . In the normal, non-actuated position, pilot valve  352  is closed, causing main poppet  114  to remain open. In the actuated position, pilot valve  352  is opened, causing main poppet  114  to close. Closing poppet  114  creates a marked restriction in the total flow area causing a pressure spike, or pulse, in the supply side of the fluid system, which is received by pressure monitoring equipment at the surface of the well. 
         [0067]      FIG. 10  is a side sectional view of motor and electronics assembly  500  illustrated in accordance with a preferred embodiment of the present invention. Motor-electronics chassis  510  supports motor  514 , gear train  530 , and a motor electronics circuit board  536 . A compressible rubber snubber  538  internally secures motor-electronics chassis  510 . The engagement of motor-electronics chassis  510  to actuator chassis  410  and the design on gear train  530  are unique to the present invention. 
         [0068]      FIG. 8  is an isometric view comparing the prior art motor and gear train housing with motor  514  and gear train housing  532  of the preferred embodiment of the present invention. As seen in  FIG. 8 , the prior art gear train housings have radially extending flanges A for axial attachment to a structural component of the MWD Tool  1 . 
         [0069]      FIG. 9  is an isometric view comparing the prior art motor, rear plate and electric connections with the motor, rear plate and electric connections of a preferred embodiment of the present invention. As seen in  FIG. 9 , the prior art motors have the electrical wiring extending radially outward from the upper end of the motor. 
         [0070]    The present invention incorporates a higher speed motor  514  than is conventionally used, for the purpose of increasing the reliability of MWD Tool  1 . To accommodate the preferred motor having sufficient speed and power without increasing the diameter of MWD Tool  1 , to match the increased size of motor  514 , it was necessary to modify motor  514  and gear train  530 . Additionally, to provide a modular MWD Tool  1  that can be separated for servicing at motor  514  without draining pressure-compensating reservoir  328 , gear train  530  and motor  514  are modified. 
         [0071]    As seen in  FIG. 8 , a new gear train  530  has been designed without connection ears or other radially extending features. In the preferred embodiment, a housing  532  of gear train  530  has a circular external diameter approximately equally to (= or &lt;0.05 larger than) the external diameter of motor housing  516 . In the preferred embodiment, gear train housing  532  is case hardened. A small radius  534  is provided as shown to remove the stress riser and prevent crack initiation when compressive forces are applied. As seen in  FIG. 9 , motor housing  516  and a rear plate  518  of motor  514  are modified to have electrical connections  520  extending directly rearward through rear plate  518 , rather than radially outward through motor housing  516 . 
         [0072]    Referring to  FIG. 7  and  FIG. 10 , tabs  454  extend axially upward from the upper end of actuator chassis  410 . Tabs  512  extend downward from the lower end of motor-electronics chassis  510 . Tabs  454  and  512  engage complementary slots for aligned engagement of actuator chassis  410  and motor-electronics chassis  510 , and preventing relative rotation between actuator chassis  410  and motor-electronics chassis  510 . 
         [0073]    As seen in  FIG. 7 , a pair of diametrically opposed slits  456  extends longitudinally downward from the upper end of actuator chassis  410 . A pair of threaded fastener holes  458  intersects each slit in perpendicular relation. Threaded fasteners  460  are located in holes  458 . The upper end of actuator chassis  410  has a portal  462  located internally of fastener holes  458 . As seen in  FIG. 10 , when motor-electronics chassis  510  is attached to actuator chassis  410 , portal  462  receives gear train  530  and motor  514  is received beneath tabs  454  and  512 . 
         [0074]    The case-hardened housing  532  of gear train  530  and the addition of radius  534  provide a gear train housing  532  that can be compressively secured in portal  462  by tightening threaded fasteners  460  on actuator chassis  410 . Compression of longitudinal slits  456  by fasteners  460  secures gear train  530  in portal  462  of actuator chassis  410 . This provides a novel method of removably securing motor  514  in place that removably accommodates the larger motor  514  and provides convenient serviceability. 
         [0075]    A lower centralizer  2  is typically attached above motor and electronics assembly  500  to centralize the lower portion of MWD Tool  1  in the drill string. A battery barrel  600  is attached above motor and electronics assembly  500 . Battery assembly  600  provides the power needed to operate MWD Tool  1 . An upper centralizer  4  is typically attached above battery barrel  600  to centralize the upper portion of MWD Tool  1  in the drill string. A Navigation Master Assembly  700  is attached above upper centralizer  4 , and contains sensors to take the required directional and inclination measurements. A lifting connection  6 , such as a rope socket, is typically attached above Navigation Master Assembly  700  to permit retrieval of MWD Tool  1 . 
       OPERATION OF THE INVENTION 
       [0076]    During operation, the drilling fluid has two primary flow paths controlled by the presence and operation of MWD Tool  1 . A first flow path is through the interior of MWD Tool  1 , and has an interior flow rate Q I . A second flow path is around the exterior of MWD Tool  1 , and has an exterior flow rate Q E . Generally: 
         [0000]    
       
      
       Q 
       I 
       +Q 
       E 
       =Q 
       T  
      
     
         [0000]    where Q T  is the total flow rate of the drilling fluid system. Interior flow rate Q I  and exterior flow rate Q E  vary in accordance with the operation of actuator assembly  400 . Operation of actuator assembly  400  alters the internal and exterior flow paths, which changes the flow rates of each as the drilling fluid seeks the path of least resistance. Designating the sub-symbol ‘0’ to indicate actuator assembly  400  at rest, and designating sub-symbol ‘1’ to indicate actuation of actuator assembly  400 , the flow rate equation above can be expended as follows: 
         [0000]    
       
      
       Q 
       I(0) 
       +Q 
       E(0) 
       =Q 
       T  
      
     
         [0000]    
       
      
       Q 
       I(1) 
       +Q 
       E(1) 
       =Q 
       T  
      
     
         [0077]    Therefore: 
         [0000]    
       
      
       Q 
       I(0) 
       +Q 
       E(0) 
       =Q 
       I(1) 
       +Q 
       E(1)  
      
     
         [0078]    In summary, in the normal, non-actuated position, pilot valve  352  is closed, causing main poppet  114  to remain open. In the actuated position, pilot valve  352  is opened, causing main poppet  114  to close. Closing poppet  114  creates a marked restriction in the total flow area, which causes a pressure spike, or pulse, in the supply side of the fluid system. Pressure monitoring equipment at the surface of the well senses and records the pressure spikes. System software decodes the sequence of pressure spikes to obtain well direction data. 
         [0079]    Referring now to the operation in greater detail, in the normal, non-actuated position, pilot valve  352  is closed and flow through the interior flow path is ‘essentially’ zero. This necessarily maximizes flow through the exterior path. Thus: 
         [0000]        Q   I(O) ≈0 
         [0080]    Therefore: 
         [0000]    
       
      
       Q 
       E(0) 
       =Q 
       I(1) 
       +Q 
       E(1)  
      
     
         [0081]    The entrance of the interior flow path is mud screen  202 . However, since valve tip  332  closing with valve seat  142  blocks flow past valve seat  142 , the drilling fluid simply flows over mud screen  202 , bypassing the entrance to the interior flow path. Thus the entire flow rate Q T  generated by the pumps at the surface is directed to the exterior flow path at flow rate Q E(0) . 
         [0082]    Without flow through passage  140 , and with the entire system flow Q T  directed externally, a low-pressure zone is created in chamber  120  of hydro chassis  104 . The low pressure of chamber  120  and high flow rate between poppet  114  and orifice  24  forces poppet  114  and push rod  112  upwards, overcoming the force of spring  130 . Fluid in chamber  120  exits poppet  114  as piston  122  moves upward. 
         [0083]    With poppet  114  open in relation to orifice  24 , the drilling fluid has a direct path over MWD Tool  1  and a large area of passage. Drilling fluid flows along the exterior of MWD Tool  1  and into the interior of the drill string. When this drilling fluid enters landing sub  10 , it flows by muleshoe  12  between poppet  114  and orifice  24 , and continues downward through the interior of the drill collars below. Thus, when the interior flow path is closed, the exterior flow path is largely unrestricted, and pressure loss traveling past MWD Tool  1  is lowest in the non-actuated position. 
         [0084]    Valve tip  332  and valve seat  142  comprise an operable pilot valve  352 . When actuator assembly  400  operates (opens) pilot valve  352 , main poppet  114  closes. When pilot valve  352  is opened, drilling fluid flows through mud screen  202  into screen housing  200 . Inside screen housing  200 , the drilling fluid continues past valve tip  332  and through valve seat  142  and into the hollow interior of hydro chassis  104 . The drilling fluid asserts downward pressure on piston  122 , assisted by the force of spring  130 , forcing piston  122  downward and closing poppet  114 . The drilling fluid continues through the restricted interior space of piston  122 , piston orifice  124  and the interior of push rod  112 . The drilling fluid then exits through the end of poppet  114 , and finally exits MWD Tool  1  and continues downward through the interior of the drill collars below. 
         [0085]    At the same time, drilling fluid flows at a significantly reduced rate along the external fluid path along the exterior of MWD Tool  1 , past poppet  114  and into the unrestricted interior of the remaining drill string. This is possible because poppet  114  does not fully engage orifice  24 , leaving a restricted passage between them. When the drilling fluid enters landing sub  10 , it flows by muleshoe  12  between poppet  114  and orifice  24 , and continues downward through the interior of the drill collars below. Since the external flow rate Q E(1)  is less than the non-actuated external flow rate Q E(0)  by the amount of the internal flow rate Q I(1) , there is now insufficient fluid pressure to force poppet  114  upwards against spring  130  to separate poppet  114  from orifice  24 . 
         [0086]    Referring to  FIG. 5 , the configuration detailed above isolates the pressure compensation fluid from other moving parts within the actuator assembly  400 , including motor  514 , protecting motor  514  from premature failure. Importantly, this permits utilization of lubricants that may be less suitable in performance as pressure compensating fluids, and vice-versa. In particular, grease has been shown to be a superior lubricant for the actuator ball screw, but is inferior in performance as a compensating fluid. Similarly, oils that maintain constant viscosity over the operating temperature range of MWD Tool  1  are superior pressure-compensating fluids, but may be inferior to certain greases, such as a ball screw or thrust bearing lubricant. 
         [0087]    Referring to  FIG. 6 , isolated pressure-compensating reservoir  328  performs in conjunction with motor  514  retention system of the present invention. Upon disconnection of motor-electronics chassis  510  from actuator chassis  410  for service of motor  514  or gear train  530 , the fluid in pressure-compensating reservoir  328  remains axially sealed within pressure balance chassis  310 . 
         [0088]    Still referring to  FIG. 6 , the design of gear train housing  532  and motor  514  permit the use of the largest diameter motor  514  with the confines of the internal diameter of pressure barrel  8 . This is achieved by providing gear train housing  532  with a circular external diameter approximately equally to (= or &lt;0.05 larger than) the external diameter of motor housing  516 . Compression of longitudinal slits  456  by fasteners  460  secures gear train  530  in portal  462  of actuator chassis  410 . This configuration further permits removal of motor  514  and gear train  530  from actuator chassis  410  for servicing, without disconnection of an axially aligned fastener, and without draining the pressure-compensating fluid from the pressure-compensating reservoir  328 . 
         [0089]    One result of the design described is that it is significantly shorter than prior art designs, and is smaller in diameter than conventional designs using a motor having the same performance characteristics. This reduces the material cost of MWD Tool  1 . An additional benefit is that the shorter length and smaller diameter reduce the drilling fluid pressure loss through MWD Tool  1 , providing more hydraulic horsepower to the jet nozzles of the drill bit. 
         [0090]    It will be readily apparent to those skilled in the art that the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. 
         [0091]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.