Abstract:
An improved energy efficient intelligent pulser driver used for generating a mud pulse in a MWD (measurement while drilling) application. In the pulser driver, a direct current (DC) powered control circuit activates a three-phase DC brushless motor that operates a servo-valve. Opening of the servo-valve equalizes pressure in a plenum causing the operation of a main valve reducing flow area and causing a pressure spike in the mud column. Closing of the servo-valve creates a reduction in mud pressure that operates the main valve and increases the flow area causing an end to the pressure spike. The servo-valve is powered both in opening and closing operations by the motor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 11/101,033, filed on Apr. 6, 2005, which claims priority from U.S. Provisional Patent Application No. 60/560,047, filed Apr. 6, 2004, and Canadian Application No. 2,463,354, filed Apr. 6, 2004, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a telemetry system, and in particular to a measurement while drilling (MWD) system. More particularly, the present invention relates to a servo-actuator for a downhole mud pulser for sending information from downhole to surface. 
     BACKGROUND OF THE INVENTION 
     The desirability and effectiveness of well logging systems where information is sensed in the well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Mud pulse telemetry systems provide the driller at the surface with means for quickly determining various kinds of downhole information, most particularly information about the location, orientation and direction of the drill string at the bottom of the well in a directional drilling operation. During normal drilling operations, a continuous column of mud is circulating within the drill string from the surface of the well to the drilling bit at the bottom of the well and then back to the surface. Mud pulse telemetry repeatedly restricts the flow of mud to propagate signals through the mud upward to the surface, thereby providing a very fast communication link between the drill bit and the surface. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second. 
     A telemetry system may be lowered on a wireline located within the drill string, but is usually formed as an integral part of a special drill collar inserted into the drill string near the drilling bit. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through a downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well. The information sensed by instrumentation in the vicinity of the drilling bit is encoded into a digital formatted signal and is transmitted by instructions to pulse the mud by intermittently actuating the telemetry valve, which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface where the pulses are detected and transformed into electrical signals which can be decoded and processed to reveal transmitted information. 
     Representative examples of previous mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354; 3,958,217; 4,216,536; 4,401,134; and 4,515,225. 
     Representative samples of mud pulse generators may be found in U.S. Pat. Nos. 4,386,422; 4,699,352; 5,103,420; and 5,787,052. 
     A telemetry system capable of performing the desired function with minimal control energy is desirable, since the systems are typically powered by finite-storage batteries. One such example is found in U.S. Pat. No. 5,333,686, which describes a mud pulser having a main valve biased against a narrowed portion of the mud flowpath to restrict the flow of mud, with periodic actuation of the main valve to allow mud to temporarily flow freely within the flowpath. The main valve is actuated by a pilot valve that can be moved with minimal force. The pilot valve additionally provides for pressure equalization, thereby increasing the life of downhole batteries. 
     Another example of an energy efficient mud pulser is described in U.S. Pat. No. 6,016,288, the mud pulser having a DC motor electrically powered to drive a planetary gear which in turn powers a threaded drive shaft, mounted in a bearing assembly to rotate a ball nut lead screw. The rotating threaded shaft lifts the lead screw, which is attached to the pilot valve. 
     Solenoid-type pulser actuators have also been used to actuate the main pulser valve, however, there are many problems with such a system. The use of a spring to bias the solenoid requires the actuator (servo) valve to overcome the force of the spring (about 6 pounds) and of the mud prior to actuating the main valve. A typical solenoid driven actuator valve is capable of exerting only 11 pounds of pressure, leaving only 5 pounds of pressure to actuate the pulser assembly. Under drilling conditions requiring higher than normal mud flow, the limited pressures exerted by the solenoid may be unable to overcome both the pressure of the return spring and the increased pressure of the flowing mud, resulting in a failure to open the servo-valve, resulting in the main valve remaining in a position in which mud flow is not restricted, and therefore failing to communicate useful information to the surface. 
     A further problem with the use of a solenoid to actuate the pulser assembly is the limited speed of response and recovery that is typical of solenoid systems. Following application of a current to a solenoid, there is a recovery period during which the magnetic field decays to a point at which it can be overcome by the force of the solenoid&#39;s own return spring to close the servo-valve. This delay results in a maximum data rate (pulse width) of approximately 0.8 seconds/pulse, limiting the application of the technology. 
     Moreover, the linear alignment of the solenoid must be exactly tuned (i.e. the magnetic shaft must be precisely positioned within the coil) in order to keep the actuator&#39;s power characteristics within a reliable operating range. Therefore, inclusion of a solenoid within the tool adds complexity to the process of assembling and repairing the pulser actuator, and impairs the overall operability and reliability of the system. 
     Existing tools are also prone to jamming due to accumulation of debris, reducing the range of motion of the pilot valve. Particularly when combined with conditions of high mud flow, the power of the solenoid is unable to clear the jam, and the tool is rendered non-functional. The tool must then be brought to the surface for service. 
     Stepper motors have been used in mud pulsing systems, specifically, in negative pulse systems (see for example U.S. Pat. No. 5,115,415). The use of a stepper motor to directly control the main pulse valve, however, requires a large amount of electrical power, possibly requiring a turbine generator to supply adequate power to operate the system for any length of time downhole. 
     Repair of previous pursers has been an as yet unresolved difficulty. Typically, the entire tool has been contained within one housing, making access and replacement of small parts difficult and time-consuming. Furthermore, a bellows seal within the servo-poppet has typically been the only barrier between the mud flowing past the pilot valve&#39;s poppet and the pressurized oil contained within the servo-valve actuating tool, which is required to equalize the hydrostatic pressure of the downhole mud with the tool&#39;s internal spaces. Therefore, in order to dissemble the tool for repair, the bellows seal had to be removed, causing the integrity of the pressurized oil chamber to be lost at each repair. 
     Furthermore, a key area of failure of MWD pulser drivers has been the failure of the bellows seal around the servo-valve activating shaft, which separates the drilling mud from the internal oil. In existing systems, the addition of a second seal is not feasible, particularly in servo-drivers in which the servo-valve is closed by a spring due to the limited force which may be exerted by the spring, which is in turn limited by the available force of the solenoid, and cannot overcome the friction or drag of an additional static/dynamic linear seal. 
     It remains desirable within the art to provide a pulse generator that has an energy efficiency sufficient to operate reliably and to adapt to a variety of hostile downhole conditions, has reduced susceptibility to jamming by debris, and is simpler to repair than previous systems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate or mitigate at least one disadvantage of previous mud pulsers and pulse generators. 
     In a first aspect, the present invention provides a downhole measurement-while-drilling pulser actuator comprising a servo valve movable between an open position which permits mud flow through a servo-orifice and a restricted position which restricts mud flow through the servo-orifice, the servo-valve powered to the open position and powered to the closed position by a reversible electric motor. 
     In one embodiment, the servo valve includes a servo-poppet powered by the motor in reciprocating linear movement towards and away from the servo-orifice. 
     In a further embodiment, the actuator may include a rotary to linear conversion system for converting rotary motion of the reversible electric motor into linear reciprocating movement of the poppet. The rotary to linear conversion system may include a threaded lead screw held stationary and driven in rotation by a rotary motor. In this embodiment, the lead screw may be threadably attached to a ball nut from which the poppet depends, whereby the rotary motion of the motor causes rotation of the screw to result in driven linear movement of the ball nut and the poppet in either direction. 
     In a further embodiment, there is provided a servo-controller for controlling the powering of the servo-valve by the electric motor. The servo-controller may further be capable of sensing the position of the poppet with respect to the servo-orifice, such that the poppet position is sensed when mud flow through the servo-orifice is restricted or unrestricted, and wherein the amount and direction of rotation of the motor from the sensed poppet position is counted and stored by the controller. 
     In another embodiment, the sensed position of the orifice restriction is calibrated as the fully closed position of the poppet. The poppet&#39;s travel is thereby monitored and controlled during operation to avoid unneeded collision or frictional wear between the poppet and the servo-orifice. The servo controller may sense the position of the poppet by sensing whether movement of the poppet is impeded, and the servo-controller counts the number of rotations of the motor until the poppet is impeded and compares the number of rotations to an expected number of rotations to determine the position of the poppet with respect to the servo-orifice. The expected number of rotations can be preset to allow a predetermined rate of mud flow past the servo-orifice when the poppet is moved away from the servo-orifice by the preset expected number of rotations. 
     In a still further embodiment, the servo-controller may include a debris clearing command that is initiated when the number of rotations counted is not equal to the expected number of rotations. The debris clearing command may cause the motor to rapidly reciprocate the poppet to dislodge any debris present between the poppet and the servo-orifice. 
     In another embodiment, the attachment between the poppet and the motor comprises a dynamic seal to isolate the motor, rotary to linear conversion system and related drive components from the drilling mud in which the poppet and orifice are immersed when in operation. 
     In a further aspect, the present invention provides a method for causing the generation of a mud pulse by a controlled pulser&#39;s main pulse valve comprising the steps of: powering a pulser servo-valve in a first direction using a rotary motor such that mud is permitted to flow past a servo-orifice to activate a main mud pulse valve; and powering the servo-valve in a second direction using the rotary motor such that mud flow past the servo-orifice is restricted to deactivate the main mud pulse valve. 
     In one embodiment, the method further comprises the step of cutting power to the motor to hold the servo-valve in a particular position within its range of motion to tailor the actuator&#39;s effect on the main pulse valve and thereby tailor the pressure and duration characteristics of a mud pulse. 
     In another aspect, the invention provides a servo-controller for use with a downhole measurement-while-drilling pulser actuator, the servo-controller comprising a sensor, memory, control circuitry, and an operator interface. 
     In one embodiment, the sensor is a mudflow sensor, pressure sensor, temperature sensor, rotation-step counter, position sensor, velocity sensor, current level sensor, battery voltage sensor, timer, or an error monitor. 
     In another embodiment, the memory stores time-stamped or counted sensed events together with an event-type indication. The servo-controller may be programmable to cause an action within the actuator responsive to a sensed event, a time, an elapsed time, a series of sensed events, or any combination thereof. 
     In a further embodiment, the user interface provides information from memory to the operator, and may allow an operator to alter the programming of the control circuitry. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIGS. 1A  and B are a longitudinal cross sectional view of the upper and lower portions of an embodiment of the mud pulser during mud flow through the servo orifice; and 
         FIGS. 2A and 2B  are a longitudinal cross sectional view of the upper and lower portions of an embodiment of the mud pulser during mud flow restriction by the poppet. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an apparatus and method for actuating a mud pulser telemetry system used during well-drilling operations. The present apparatus allows a servo-valve to be powered both in opening and closing to activate a main mud pulser valve, and does not rely on a solenoid system. The powered opening and closing of the servo-valve results in various functional and economic advantages, including the ability to clear debris from the restricted portion of the mud flowpath, and faster data rates due to elimination of inherent operating delays in the solenoid systems of previous tools, with the end result of providing a pulser driver which consumes a minimal amount of DC power while providing more force with which to drive the servo-valve&#39;s poppet in each direction. Therefore, the actuator remains functional at a comprehensive range of downhole drilling conditions. 
     Furthermore, in the embodiment shown in the Figures, the present device is designed to have several independent, interconnected housings, and employs a double seal between the oil compartment and the drilling mud, which simplifies assembly and repair of the tool. The assembly/disassembly is simplified to reduce repair turnaround time by using modular components. 
     Additionally, the use of a stepper motor, electric load sensors, and control circuitry in a powered-both-directions servo-valve system will allow for self-calibration of the tool and self-diagnosis and error correction unavailable in other systems. In an embodiment of the invention, as shown in  FIGS. 1A and 1B , a three-phase stepper rotary motor  1  is monitored and controlled by a servo-controller  10 , the rotary movement of the motor  1  being converted into linear movement of a poppet  21 , thereby opening and closing a servo-valve  20  to actuate a mud pulser main valve (not shown). Communication of information to the well surface is accomplished by encoded signals, which are translated to produce pressure surges in the downward flow of the pressurized mud. It is recognized that although the drilling fluid is generally referred to as mud, other drilling fluids are also suitable for use with the present invention, as is well known in the art. 
     With reference to the Figures, the mud pulser actuator is lowered downhole and, in the embodiment shown, generally includes a plurality of serially interconnected housings  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8 , an electrical connector  9 , a servo-controller  10  for controlling the operation of a rotary motor  1 , and a servo-valve assembly  20  that is driven in linear motion by the rotary motor  1 . The servo-valve assembly includes a poppet  21  capable of linear reciprocating movement to and from a seal surface  22  of a servo orifice  23 , thereby opening and closing the servo orifice  23  to allow or prevent the passage of pressurized mud and thereby actuate a pulser (not shown, connected to the lower end  2   a  of the lowermost housing  2 ) to generate a pressure pulse for telemetric purposes. 
     Mechanical System 
     A rotary-to-linear coupling system  30   a ,  30   b  (hereinafter referred to as coupling system  30 ) is used to translate the torque from the rotary motor  1  into linear movement of the servo-valve shaft  24 , which is preferably a series of connected shafts for transferring linear movement from the coupling system  30  to the servo poppet  21 . Preferably, the servo shaft includes a spline shaft  24   a , which passes through a spline coupling  24   b  that can be used to prevent rotation of the shaft  24   a  when necessary. The coupling system  30  also includes seals which serve to isolate the rotating mechanism from the downhole mud. 
     In the embodiment pictured in  FIGS. 1A and 1B , the rotary motor  1 , is electrically powered through an electrical connection  9 , by a power source (not shown). When activated, the motor  1  rotates a lead screw  31  that is mounted within a bearing support  32 , causing a ball nut  33  to move threadably along the lead screw  31 . Linear movement of the ball nut  33  results in dependent linear movement of the servo shaft  24 , and servo poppet  21 . When driven in the forward direction, the rotary motor  1  will cause linear movement of the poppet  21  away from the servo-valve seat  22 , to allow passage of pressurized mud through the servo-orifice  23  to activate the main mud pulser valve to close. When the motor  1  drives the lead screw  31  in the reverse direction, poppet  21  is urged towards the seal surface  22  to cover the servo orifice  23 , as shown in  FIG. 2B , and mud is therefore prevented from passing through the servo orifice  23  to actuate the mud pulser main valve to open. 
     The spline shaft  24  is surrounded by lubricating fluid, which must be pressurized against the downhole hydrostatic pressure. As shown, a pressure compensator in the form of a membrane or bellows  42  allows reservoir fluid to substantially equalize the pressure via a part  43 . The pressure compensator be a membrane, bellows, piston type or other type known in the industry. In addition to a bellows seal  40 , an additional seal  41  may be added to hold oil inside the chamber of the tool, with the bellows seal  40  preventing mud from reaching the additional seal  41 . The dual seal  40 ,  41  maintains the integrity of the lubrication chamber during operation and during replacement of the bellows seal  40  during maintenance. The addition of this seal  41  does not negatively impact performance of the actuator due to the improved power characteristics of the system, as will be discussed below. 
     In a preferred embodiment, the construction of the device allows most downhole clogs, where debris in the mud may stop the poppet  21  from sealing with the seal surface  22 , to be easily cleared as will be described below, and the serially interconnected housing design allows simple and rapid repair of the tool when necessary. 
     The valve assembly  20  is preferably composed of a wear resistant material such as tungsten carbide or ceramic to maximize the efficiency of the tool and to minimize maintenance of the tool, and is preferably replaceable. 
     Operation 
     When restriction of mud flow by the main valve is desired, the rotary motor  1  will be activated by the servo-controller  10  in the forward direction. As shown in  FIG. 1B , forward powering of the rotary motor  1  will cause the lead screw  31  to turn in the forward (for example, clockwise) direction, thereby raising the ball nut  33  and lifting the servo poppet  21  from the servo-valve seat  22 . This will allow mud flow to pass unrestricted through the servo-orifice  23  to actuate the main mud pulse valve, restricting mud flow to generate a pulse that is transmitted to the surface. The current-consuming portion of the circuit is then shut down until a further signal is received from the servo-controller  10 . The lack of current to the motor  1  results in the motor  1  being immovable and therefore acting as a brake to prevent further movement of the poppet  21  until further activation of the motor  1 . 
     Subsequently, when the servo-controller  10  initiates reverse motion by the motor  1 , the lead screw  31  is rotated in the reverse direction (in the example, counterclockwise) by the motor  1 , causing the ball nut  33  and servo shaft  24  to move towards the servo-valve seat  22  as shown in  FIG. 2B . Closure of the servo-valve  20  causes opening of the main mud pulser valve to allow mud to flow unrestricted to the surface. The current-consuming portion of the circuit is then shut down until a further signal is received from the servo-controller  10 . The motor again acts as a brake until further power is applied (by shorting its coils together). 
     The lead screw  31  and ball nut  33  may be replaced by an alternate system of rotary to linear conversion, however a lead screw  31  and ball nut  33  are advantageous as they are relatively small in size and may be provided with bearings to provide a low-friction mechanism with high load capacity, durability, and low backlash tolerance. The lead screw  31  may be held in contact with the motor  1  by a bearing support  32  or any other suitable means. 
     The presently described system of using a stepper motor  1  to drive a servo-valve has several advantages. The powering of the servo-valve  20  in both directions allows greater direct control of the servo-valve  20 , avoids the previous necessity of using a return spring in the servo assembly, and therefore the energy required is similar to that of the force of the downhole mud flow. This results in an energy efficient system, and results to date indicate that the presently described system can supply a force of 100 pounds of pressure for less energy than previous systems, particularly than those which employ a solenoid activator. Thus, the present system can overcome higher pressures on the poppet valve  21 , allowing the system to clear itself of debris, and permitting use in a wide range of downhole conditions, including conditions of higher pressure and higher volume mud flow, and in conditions when the mud is contaminated or is very dense. 
     Use of a rotary motor powering the servo-valve in both directions also allows the system to be more responsive than solenoid systems, resulting in a faster data rate with more accurate or precise pulse-edge timing. Experimental results indicate that data rates of 0.25 seconds/pulse are possible with this system, as compared to 0.8 to 1.5 seconds/pulse in solenoid systems. 
     Flow Detection &amp; Diagnostic Software 
     The servo controller detects the position of the poppet  21  against the servo-valve seal  22  by counting the number of rotations made by the motor until further movement of the poppet is impeded. For example, if the poppet  21  is generally programmed to attain an unseated position that is three forward motor rotations away from the seated position, upon seating activation by the servo-controller  10 , the motor will turn three reverse rotations, at which point further rotation will be impeded due to seating of the poppet  21  on the seal  23 . On unseating activation by the servo controller  10 , the motor will turn three complete forward rotations to return the poppet to its pre-programmed unseated position. Seating can be sensed by an increase in current drawn by the motor, from which a large opposing force (like stopped motion due to valve seating) is inferred. The control circuitry also senses rotation of the motors and can count rotations and direction of rotation. 
     Debris may enter the device with the mud, potentially causing jamming of the poppet. The servo controller  10  can be programmed to detect and clear jams from the servo-valve  20 . For example, debris may become lodged at the servo-valve seal  22 , preventing the poppet from fully sealing against the valve seal  22 . In such a situation, the motor would be prevented from completing its three reverse rotations. This is sensed by the servo-controller  10 , which will then attempt to dislodge the debris. The dislodging sequence may include rapid reciprocation of the poppet  21  towards and away from the seal  22 , or may include further reverse rotations on the subsequent reverse rotation. For example, if the motor was able to turn only two reverse rotations, the servo-controller  10  will recognize that the valve did not properly close, and will adjust one or more subsequent forward and/or reverse rotations to ensure that the poppet  21  is able to seat against the valve seal  22 . Similarly, debris may cause the poppet to not fully open, resulting in appropriate corrective action by the servo-controller on the next motor  1  activation. In either case, a processor provides a report of measurements recorded and controls the following cycle of the brushless motor&#39;s rotation accordingly. 
     The ability to detect and clear most jams within the tool allows a more robust design of the tool in other respects. For example, as the tool can easily clear particulate matter from the servo-valve assembly, the tool can be provided with larger and fewer mud ports, and may include reduced amounts of screening. Screening is susceptible to clogging, and so reducing screening leads to longer mean time between operation failure of the device in-hole; and will reduce the velocity of any mud flow through the tool, reducing wear on the bladder and other parts. Further, the removal of several previously necessary components (such as the return spring, transformer, and solenoid and related electronics) contributes to a tool of smaller size (in both length and diameter) that is more versatile in a variety of situations. For example, embodiments with outside diameter less than 1⅜″ (approaching 1″) or length less than four feet have been achieved, although these dimensions are not by way of limitation, but by example only. 
     Custom software also has the ability to track downhole conditions, and also uses a sensor to detect mudflow. When mudflow is detected, a signal is sent to the Directional Module Unit (not shown), to activate the overall system. The system also has the ability to time stamp events such as start or end of mudflow, incomplete cycles or system errors, low voltages, current, and the like, as well as accumulated run-time, number of pulses, number of errors, running totals of rotations or motor pulses. Wires or conductors may also be easily passed by the pulser section to service additional near-bit sensors or other devices. The software that detects the mudflow can be configured for different time delays to enable it to operate under a larger variety of downhole drilling conditions than its predecessors. The mudflow detection capability can also be used to calibrate or confirm the closed position of the poppet. 
     In addition, a user may monitor such data as well as any downhole sensors using a user interface attachable to the tool. Such sensors may include pressure or temperature sensors, rotation step-counters, travel or depth sensors, current levels, battery voltage, or timers. The user could monitor each component of the actuator to determine when the tool must be removed from downhole for repair. A user may, in turn, program an activity to cause an action or correction in response to a sensed event. 
     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.