Abstract:
A method and system for monitoring operation of a motor may include initially determining a no load torque versus temperature characteristic of the motor over a range of operating temperatures. After a period of operation, a no load torque value and temperature of the motor may be determined. Motor temperature may be measured by a local thermistor or the like. Motor torque may be determined from measured motor current. The motor torque and temperature may then be compared to the initial torque versus temperature characteristic to determine a change in load of said motor due to break down of motor oil, worn bearings, or similar condition. In some embodiment, the method and system may be used with a downhole tool for drilling a well, such as a rotary steerable system.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to monitoring of electric motors, and in particular to monitoring of electric motors in downhole tools used for drilling, completing, servicing, and evaluating wellbores in the earth. 
       BACKGROUND 
       [0002]    Motors are used in downhole tools for a variety of reasons. Electric motors may be placed in a compensated oil bath environment. Sometimes, the motors have an inbuilt thermistor or temperature sensor other to track the motor temperature and thus its performance. Other times a thermistor can be placed close to the motor to monitor the motor temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Embodiments are described in detail hereinafter with reference to the accompanying figures, in which: 
           [0004]      FIG. 1  is an elevation view in partial cross section of a well with a rotary steerable drilling system according to an embodiment; 
           [0005]      FIG. 2A  is an elevation view of a steering assembly of the rotary steerable drilling system of  FIG. 1  according to an embodiment, showing a control mechanism, a drive mechanism, a steering mechanism, and a bit shaft; 
           [0006]      FIG. 2B  is an axial cross section of the control mechanism of the steering assembly of  FIG. 2A , showing a flow control valve that controls drilling fluid flow rate to the drive element of the drive mechanism of  FIG. 2C ; 
           [0007]      FIG. 2C  is an axial cross section of the drive mechanism of the steering assembly of  FIG. 2A , showing a fluid-powered drive element; 
           [0008]      FIG. 2D  is an axial cross section of the steering mechanism of the steering assembly of  FIG. 2A , showing an offset mandrel counter-rotated by the drive element of  FIG. 2C  for maintaining an angular orientation of the bit shaft of  FIG. 2A  as the steering assembly is rotated; 
           [0009]      FIG. 3A  is a perspective view of the flow control valve of  FIG. 2B , shown with a portion of the housing cut away to reveal rotor and stator flow control plates; 
           [0010]      FIG. 3B  is a transverse cross section taken along line  3 B- 3 B of  FIG. 2B , showing the stator flow control plate of  FIG. 3A  with inner annulus and bypass annulus flow ports; 
           [0011]      FIG. 3C  is a transverse cross section taken along line  3 C- 3 C of  FIG. 2B , showing the rotor flow control plate of  FIG. 3A  for creating a variable flow fluid. pathway; 
           [0012]      FIG. 4  is a block level schematic of a control system of the steering assembly of  FIG. 2A ; 
           [0013]      FIG. 5  is an exemplary plot of oil viscosity versus temperature; and 
           [0014]      FIG. 6  is a flow chart of a method for monitoring the condition of an electric motor according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” “uphole,” “downhole,” “upstream,” “downstream,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation depicted in the figures. 
         [0016]      FIG. 1  is a diagram illustrating an exemplary drilling system  100 , according to aspects of the present disclosure. Drilling system  100  includes rig  102  mounted at surface  101  and positioned above borehole  104  within a subterranean formation  103 . In the embodiment shown, a drilling assembly  105  may be positioned within borehole  104  and. 
         [0017]    may be coupled to rig  102 . Drilling assembly  105  may include drill string  106  and bottom hole assembly (BHA)  107 . Drill string  106  may include a plurality of segments threadably connected. BHA  107  may include a drill bit  109 , a measurement-while-drilling (MWD) apparatus  108  and a steering assembly  200 . Steering assembly  200  may control the direction in which borehole  104  is being drilled. Borehole  104  may be drilled in the direction perpendicular to tool face  110  of drill bit  109 , which corresponds to longitudinal axis  219  of drill bit  109 . Accordingly, controlling the direction of borehole  104  may include controlling the angle between longitudinal axis  219  of drill bit  109  and longitudinal axis  220  of steering assembly  200 , and controlling the angular orientation of drill bit  109  relative to formation  103 . 
         [0018]    Steering assembly  200  may include an offset mandrel (not shown) that causes longitudinal axis  219  of drill bit  109  to deviate from longitudinal axis  220  of steering assembly  200 . The offset mandrel may be counter-rotated relative to the rotation of drill string  106  to maintain an angular orientation of drill bit  109  relative to formation  103 . Steering assembly  200  may receive control signals from a control unit  113 . Control unit  113  may include an information handling system with a processor and a memory device, and may communicate with steering assembly  200  via a telemetry system  111 . In certain embodiments, control unit  113  may transmit control signals to steering assembly  200  to alter longitudinal axis  220  of drill bit  109  as well as to control counter-rotation of portions of the offset mandrel to maintain the angular orientation of drill bit  109  relative to formation  103 . In certain embodiments, a processor and memory device may be located within steering assembly  200  to perform some or all of the control functions. Moreover, other BHA  107  components, including MWD apparatus  108 , may communicate with and receive instructions from control unit  113 . 
         [0019]      FIGS. 2A-D  are diagrams illustrating an exemplar steering assembly  200 , according to aspects of the present disclosure, that may be used, in part, to maintain a drill bit in a geo-stationary position during drilling operations.  FIGS. 2B-D  depict illustrative portions of steering assembly  200 . Steering assembly  200  may include a housing  201  that may be coupled directly to a drill string or indirectly to a drill string, such as through a MWD apparatus. Housing  201  may include separate segments  201   a - c , or may include a single unitary housing. In certain embodiments, each of the segments may correspond to a separate instrument portion of steering assembly  200 . For example, section  201   a  may house the control mechanisms, and may communicate with a control unit at the surface and/or receive control signals from the surface and control mechanisms within the steering assembly. In certain embodiments, the control mechanisms may include a processor and a memory device, and may receive measurements from position sensors within the steering assembly, such as gravity tool face sensors that may indicate a drilling direction. The control mechanism may also receive measurements form temperature sensors and current sensors. Section  201   b  may include drive elements, including a variable flow pathway and a flow-controlled drive mechanism. Section  201   c  may include steering elements that control the drilling angle and axial orientation of a drill bit coupled to bit shaft  202  of steering assembly  200 . 
         [0020]    Referring to  FIGS. 2B and 2C , in certain embodiments, steering assembly  200  may be coupled, directly or indirectly, to drill string  106  ( FIG. 1 ), through which drilling fluid may be pumped during drilling operations. The drilling fluid may flow into an annulus  205  around a flow control module  206 . The drilling fluid may then either flow to an inner annulus  208 , in fluid communication with a fluid-powered drive element  209 , or may be diverted to a bypass annulus  207 . The rotational speed of fluid-powered drive element  209  may be controlled by the amount and rate of drilling fluid that flows into inner annulus  208 . Accordingly, a flow control valve  210  may be included within flow control module  206  and may selectively control the proportion of drilling fluid that enters inner annulus  208  to drive fluid-powered drive element  209  to the portion of flow that is bypassed. 
         [0021]    According to aspects of the present disclosure, in certain embodiments, flow control valve  210 , therefore, may be used to control the rotational speed of fluid-powered drive element  209  by varying the amount or rate of drilling fluid that flows into inner annulus  208 . However, other variable flow fluid pathways may be provided using a variety of valve configurations that may meter the flow of drilling fluid across a fluid-powered drive mechanism. 
         [0022]      FIG. 2C  illustrates a fluid-powered drive element  209  in fluid communication with inner annulus  208 . In the embodiment shown, fluid-powered drive element  209  may include a turbine, but other fluid-powered drive mechanisms may be possible, including but not limited to a mud motor. Turbine  209  may include a plurality of rotors and stators that generate rotational movement in response to fluid flow within inner annulus  208 . Turbine  209  may generate rotation at an output shaft  211 , which may be coupled, directly or indirectly, to an offset mandrel  212  ( FIG. 2D ). In the embodiment shown, a speed. reducer  213  may be placed between turbine  209  and output shaft  211  to reduce the rate of rotation generated by turbine  209 . 
         [0023]    In certain embodiments, a generator  214  may be coupled to fluid-powered drive element  209 . Generator  214  may be magnetically coupled to a rotor  209   a  of turbine  209 . Generator  214  may include a wired stator  214   a . Wired stator  214   a  may be magnetically coupled to a rotor  209   a  of rotor  209  via magnets  215  coupled to rotor  209   a , As turbine  209  rotates, so does rotor  209   a , which may cause magnets  215  to rotate around wired stator  214   a  thereby generating an electrical potential within which may be used to power a variety of control mechanisms and sensors located within steering assembly  200 , including control mechanisms within segment  201   a.    
         [0024]    Referring to  FIGS. 2C and 2D , output shaft  211  may be coupled, directly or indirectly, to an offset mandrel  212 . Output shaft  211  may impart rotation from turbine  209  to offset mandrel  212 , so that offset mandrel  212  may be rotated independently from housing  201 . Offset mandrel  212  may be coupled to output shaft  211  at a first end and may include an eccentric receptacle  217  at a second end. Bit shaft  202  may be at least partially disposed within eccentric receptacle  217 . Eccentric receptacle  217  may be used to alter or maintain a longitudinal axis  219  of bit shaft  202  and a drill bit (not shown) coupled to bit shaft  202 . Bit shaft  202  may be pivotally coupled to housing  201  at pivot point  218 . Bit shaft  202  may pivot about pivot point  218  to alter a longitudinal axis  219  of bit shaft  202 . In certain embodiments, eccentric receptacle  217  may cause bit shaft  202  to pivot about pivot point  218 , which may offset longitudinal axis  219  of bit shaft  202  relative to longitudinal axis  220  of steering assembly  200 . In addition to allowing bit shaft  202  to pivot relative to housing  201 , pivot point  218  may also be used to impart torque from housing  201  to bit shaft  202 . The torque may be imparted to a drill bit  109  ( FIG. 1 ) that is coupled to bit shaft  202  and that may share longitudinal axis  219  of bit shaft  202 . Longitudinal axis  219  of bit shaft  202  may therefore correspond to a drilling angle of steering assembly  200 . 
         [0025]    During drilling operations, a drill string coupled to housing  201  may be rotated, causing housing  201  to rotate around longitudinal axis  220 . The rotation of housing  201  may be imparted to bit shaft  202  as torque through pivot point  218  using balls  290 . The torque may cause bit shaft  202  to rotate about its longitudinal axis  219  as well as longitudinal axis  220  of steering assembly  200 . When longitudinal axis  219  of bit shaft  202  is offset relative to longitudinal axis  220  of steering assembly  200 , this may cause the end of bit shaft  202  to rotate with respect to longitudinal axis  220 , changing the angular direction of bit shaft  202  and corresponding bit with respect to the surrounding formation, 
         [0026]    In certain embodiments, offset mandrel  212  may be counter-rotated relative to housing  201  to maintain the angular orientation of bit shaft  202 . For example, a drill string may be rotated in a first direction at a first speed, causing steering assembly  200  to rotate at the first direction and the first speed. To maintain the angular orientation of bit shaft  202  with respect to the surrounding formation, flow control valve  210  ( FIG. 2B ) may be actuated to provide a desired flow rate of drilling fluid across fluid-powered drive element  209  so that offset mandrel  212  is rotated in a second direction, opposite the first direction, at a second speed, the same as the first speed. Notably, with offset mandrel  212  rotating opposite housing  201  at the same speed, eccentric end  217  of offset mandrel  212  may remain stationary with respect to the surrounding formation (geo-stationary), maintaining the angular orientation of hit shaft  202  relative to the formation while still allowing bit shaft  202  to rotate about its longitudinal axis  219 . Likewise, the angular orientation of bit shaft  202  may be altered relative to the surrounding formation by rotating offset mandrel  212  at any other speed than the rotational speed of housing  201 . 
         [0027]    Referring to  FIG. 2B, and 3A-3C , housing section  201   a  may house the control mechanisms, and may communicate with a control unit at the surface and/or receive control signals from the surface and control mechanisms within the steering assembly. The drilling fluid may flow from drill string  106  ( FIG. 2 ) into annulus  205  defined around flow control module  206 . The drilling fluid may then either flow to an inner annulus  208 , in fluid communication with a fluid-powered drive element  209 , or may be diverted to a bypass annulus  207 . Flow control module  206  may include a flow control valve  210  having a rotor flow plate  232  that may be rotated and selectively positioned with respect a stator flow plate  230 . 
         [0028]    Stator flow plate  230  includes a bypass port  231  that is in fluid commination with bypass annulus  207  and an operating port  233  that is in fluid communication with internal annulus  208 . Rotor flow plate  232  rotates with respect to stator flow plate  230 . Rotor flow plate has a singular port formed therethrough that can be selectively positioned to provide total flow to bypass annulus  107 , total flow to internal annulus  208 , or to split in varying proportions between the two flow paths. Rotor flow plate may include a ramp surface to facilitate flow. 
         [0029]    Referring back to  FIG. 2B , the position of rotor flow plate  232  with respect to stator flow plate  230  may be determined by flow control module  206 . In an embodiment, flow control module  206  includes an electric motor  250  located within a pressure compensated oil bath. Motor  250  may include a rotor  252 , a stator  254  and output shaft  256 , bearing assemblies  258 , and a pressure compensating piston  260 . A thermistor, thermocouple, or other temperature sensor  259  may be located within motor  250 , within the oil bath, or on a surface of electric motor  250 . Output shaft  256  is connected to rotor flow plate  232  of flow control valve  210 . 
         [0030]    In the embodiment illustrated, electric motor  250  operates as servo motor, stepper motor, or the like, adjusting and maintaining a desired rotational position of rotor flow plate  232  within a limited rotational window. However, other types of motors and arrangements may be used as appropriate. 
         [0031]      FIG. 4  is a block level schematic of an overall control system  299  of steering assembly  200 .  FIG. 5  illustrates a method for using a thermistor and electric motor  250  to monitor the condition of the oil and other components of steering assembly  200 . Although described in the context of a downhole steering assembly, the system and method of the present disclosure may be used to monitor conditions of any suitable electric motor. 
         [0032]    Referring to  FIGS. 2B and 4 , electric motor  250  may be used to selectively actuate and maintain position of flow control valve  210 . Electric motor  250  may be disposed in a compensated oil bath environment along with bearings  258  that carry the load imposed by shear valve  350 . In particular, a selective portion of total drilling fluid flow may be ported to fluid-powered drive  209  by flow control valve  210  to control the speed thereof, and the speed of the cam.  217  which controls the tool face. Cam speed and position may be measured at sensor  300  and is provided as negative feedback to a summer unit  302  along with the target tool face value. The output of summer  302  is input to a proportional-integral-derivative controller  306 , which in turns actuates or varies electric motor  350  to reposition flow control valve  210  appropriately. 
         [0033]      FIG. 5  is a flow chart that outlines a method  400 , for controlling and assessing the operating condition of motor according to one or more embodiments. Although described with respect to the presently disclosed embodiment, this method is not limited to such. Indeed, in any circumstance where is may be desirable to determine the health of a motor that is remotely located, such as a downhole motor, the following method may be appropriate. 
         [0034]    At step  404 , a torque versus temperature characteristic of the motor over a range of operating temperatures during a first period of time is determined. This torque versus temperature characteristic may be used as a benchmark for future assessment of motor condition. Accordingly, it may be preferable to establish the torque versus temperature characteristic under pristine conditions, with fresh clean oil and new bearings, for example. 
         [0035]    Torque versus temperature characteristic may be determined in a lab environment, for example, by measuring torques and the motor is heater through a range of operating temperature. 
         [0036]    For an electric motor, torque may be determined by measuring motor current draw. One way of doing this is to actuate the motor and measure the drag torque by measuring the current. Motor torque is given by: 
         [0000]      T=kti   Eq. 1
 
         [0000]    where T is torque, kt is the torque constant of the motor, and i is the current. Because the oil bath viscosity varies with temperature, it is necessary know the temperature at which the torque is measured. The temperature may be measured using a thermistor or any other temperature measuring device. 
         [0037]    At step  406 , the motor may be located at a point downhole and been used for a period of time. Motor torque and motor temperature are determined, again under no load conditions. As above, motor torque may be determined from motor current using Equation 1. In the absence of any drilling fluid flow, when the motor is actuated, then the motor has to overcome only the viscous friction of the oil and the nominal bearing friction, because there is no operational load. 
         [0038]    At step  408 , by comparing the motor torque and temperature value to the pristine torque versus temperature characteristic, the condition of the motor setup, especially changes in viscosity of the oil or bearing condition, may be determined. Thus by knowing the temperature and drag torque at the start of operation and after a given number of operating hours, the operator can make an informed decision about the condition of the oil and bearings and the motor setup in general. 
         [0039]      FIG. 6  is an exemplary oil viscosity verses temperature plot that may allow an operator to assess a nominal viscosity value at a temperature and thereby able to changes in oil viscosity based on motor operating characteristics that affect no load torque values. 
         [0040]    This method may allow the establishment of a condition monitoring system without the requirement for any additional sensors apart from those that are likely being used in the tool. At the same time, optimal maintenance procedures can be established to change the oil and/or bearings of electric motor  350  based on operational data. 
         [0041]    In summary, a system and method for drilling a wellbore and a method for monitoring operation of a motor have been described. Embodiments of the system for drilling a wellbore may generally have: A steering assembly having a housing and steerable bit shaft, a tool face of the bit shaft controllable by an electric motor disposed within the steering assembly; a drill bit coupled to the bit shaft; a drill string operable to rotate the housing of the steering assembly in a first direction; a temperature sensor coupled to the electric motor; a telemetry system coupled to the temperature sensor; and a surface control unit in communication with the telemetry system, the control unit operable to receive temperature data from the temperature sensor via the telemetry system and to receive motor current data of the electric motor and determine a change in operating condition of the electric motor by comparison with a torque versus temperature characteristic of the motor under pristine no load conditions. Embodiments of the method for drilling a wellbore may generally include: Providing a steering assembly having steerable bit shaft, a tool face of the bit shaft controllable by a motor disposed within the steering assembly; determining a torque versus temperature characteristic of the motor over a range of operating temperatures under a pristine no load condition; coupling a drill bit to the bit shaft; coupling the steering assembly along a drill string; rotating the drill string in a first direction to rotate a housing of the steering assembly and the drill bit to drill the wellbore to a first depth while at least occasionally operating the motor to control the tool face; determining a first torque value of the motor under a no load condition at the first depth; determining a first temperature value of the motor at the first depth; and comparing the first torque and temperature values to the torque versus temperature characteristic to determine a change in load of the motor. Embodiments of the method for drilling a wellbore may generally include: Determining a torque versus temperature characteristic of the motor over a range of operating temperatures during a first period of time; determining a first torque value of the motor at a first point in time after the first period of time; determining a first temperature value of the motor at the first point in time; and comparing the first torque and temperature values to the torque versus temperature characteristic to determine a change in load of the motor. 
         [0042]    Any of the foregoing embodiments may include any one of the following elements or characteristics, alone or in combination with each other: An electric current sensor coupled to the electric motor and to the surface control unit; an electric generator disposed within the steering assembly and electrically coupled to the electric motor for powering the electric motor; the electric current sensor coupled the surface control unit via the telemetry system; a drive mechanism disposed within the steering unit and selectively fluidly coupled to the drill string, the electric generator coupled to the drive mechanism; a flow control valve fluidly coupled between the drill string and the drive mechanism, the electric motor coupled to the flow control valve for positioning the flow control valve; an offset mandrel coupled between the drive mechanism and the bit shaft for controlling a tool face of the drill bit; rotating by a drive mechanism an offset mandrel with respect to the housing in a second direction opposite the first direction to control the tool face; controlling the speed of the drive mechanism by the motor; powering the drive mechanism with a fluid flow; controlling the fluid flow to the drive mechanism by a flow control valve; positioning the flow control valve by the motor; the motor is an electric motor; the method further comprises measuring a motor current to determine motor torque; measuring the first temperature value by a temperature sensor coupled to the electric motor; telemetering the first temperature value to a control unit located at the surface of the wellbore; rotating a generator by the drive mechanism; providing the motor current by the generator; measuring a first motor current value at the first depth; telemetering the first motor current value to the control unit; operating the motor in an oil bath environment; determining a pristine oil viscosity value at the first temperature value; determining a first oil viscosity value at the first temperature for the first depth on the change in load of the motor; determining the torque versus temperature characteristic under a no load operating condition; determining the first torque and temperature values under the no load operating condition; the change in load relates to an efficiency of the motor; determining the torque versus temperature characteristic under a pristine motor condition; determining the first torque and temperature values under a used motor condition; the motor is an electric motor; measuring a motor current to determine motor torque; operating the motor in an oil bath environment; detennining a pristine oil viscosity value at the first temperature value; determining a first oil viscosity value at the first temperature for the first point in time based on the change in load of the motor. 
         [0043]    The abstract of the disclosure is solely for providing the reader a way to determine quickly from a cursory reading the nature and gist of technical disclosure, and it represents solely one or more embodiments. 
         [0044]    While various embodiments have been illustrated in detail, the disclosure is not limited to the embodiments shown. Modifications and adaptations of the above embodiments may occur to those skilled in the art. Such modifications and adaptations are in the spirit and scope of the disclosure.