Abstract:
A measuring while drilling apparatus for use in the drilling of subterranean wells comprises a plurality of interconnected tubular housings which are insertable into and removable from a drill string by a wire line. A hollow plunger on the bottom of the apparatus is vertically shiftable with respect to a fixed diameter orifice disposed in the path of the drilling fluid flow. Fluid pressure forces derived from the drilling fluid effect the movement of the plunger forward and away from the orifice to create pressure pulses which are transmittable through the drilling fluid to the surface. Sensors provided in the housings generate signals, which are converted by a downhole controller, to effect the sequential generation of the positive pressure pulses to be indicated at the surface of the well by a digital readout of the outputs of the various sensors carried in the tubular housings.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to a measuring while drilling (MWD) system, in particular a telemetry system using a pulser that communicates the downhole environmental conditions and borehole directional information to the surface of an operating drilling wellhole. In greater particularity the invention relates to such a system that accomplishes positive pulse signaling by a pulser that momentarily restricts the drilling fluid flow in the drill string. 
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of telemetry systems for transmitting information from the bottom of a well hole to the surface. In particular, this invention relates to the field of mud pulse telemetry where information detected down the well is transmitted to the surface by pressure pulses created in the circulating drilling fluid stream or mud stream in the drill string. 
     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. Systems of this type, i.e., mud pulse telemetry system, provide the driller at the surface with means for quickly determining various kinds of information down the well, most particularly information about the location and direction of the drill string at the bottom of the well. 
     Because of the tremendous investment already made in drill pipe and drill collars, it is highly desirable that the borehole telemetry system be compatible with existing drilling equipment and require minimum or no modification to the drill pipe and drill collars. Mud pulse telemetry is known to offer an effective solution since it does not rely upon electric wires extending to the surface, or other mechanisms which may necessitate modification to existing hardware. Mud pulse telemetry propagates signals through the mud flow in the drill string to the surface at the speed of sound, thereby providing a very fast communication link between the drill bit and the surface. Mud pulse telemetry is usually in the form of a valve located in the vicinity of the drilling bit, which intermittently restricts the flow of mud within the drill string. The 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. 
     A continuous column of mud is circulating within the drill string from the surface of the well to the drill bit at the bottom of the wall during normal drilling operations. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through the downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well at the speed of sound through the drilling mud. The information sensed in the vicinity of the drilling bit, which is to be transmitted to the surface, is encoded into a digital format and that digital formatted signal is used to intermittently actuate the telemetry valve which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface. The pulses are detected at the surface and transformed into electrical or other signals which can be decoded and processed to reveal transmitted information. In a typical oil or gas well drilling mud is circulated through the interior of the drill pipe at flow rates of about 100 to 1200 gallons per minute. The mud pulse telemetry system must operate to partially restrict this flow, therefore the system must control large amounts of energy. The telemetry valve must operate quickly to create a pressure pulse in this high pressure environment to intermittently restrict the flow of mud. This restriction must be sufficient to create a pressure rise in the flow stream that will be detectable at the surface of the well. At the typically high flow rates of mud, considerable force and energy are required to actuate the telemetry valve in the manner necessary to create the desired pressure pulses. 
     A telemetry system which is capable of performing the desired function with a small amount of control energy is extremely desirable. Such a system should lend itself to size reduction and/or miniaturization that can be easily packaged within the confines of conventional drill pipe segment or drill collar. Furthermore, if input power requirements are low enough, downhole power sources such as high temperature batteries can be used to power the telemetry system. 
     SUMMARY OF THE INVENTION 
     The present invention is a positive pulse measuring while drilling system, with the downhole telemetry valve at the downhole end of a pulser, instead of pointing upward facing the pressurized drilling fluid or mud flow. The present invention is powered and controlled by a downhole high temperature battery pack and a microprocessor based controller. The battery pack, controller and all well logging instrumentation may be contained within the drill collar in the vicinity of the drilling bit, or in the bottom segment of the drill string. The controller provides a means to command and control all aspects of well-logging telemetry including, but not limited to, drill bit directional sensing and measurement of environmental conditions of the borehole. Also, the controller will provide a means for encoding those directional sensing and measurements into a digital format for transmission to the surface. 
     The pulse signal is generated in the mud flow in the drill string, not the return stream of mud flow around the drill string, or annulus. The system uses a pilot valve to actuate the main valve, and uses the energy of the mud flow to activate the pulser. The movements of the pilot valve are in response to encoded digital signals produced by environmental and directional sensors. The main valve momentarily restricts the mud flow in the drill string, thereby pulsating the mud in the drill string and up to the surface. This configuration of the present invention allows for measuring while drilling and transporting measurement signals uphole to awaiting interpretive surface equipment. 
     The mud signal propagation is accomplished by the main valve&#39;s downhole end moving downwardly into an orifice formed in the downhole end of a sleeve defining a passage for the downward flow of mud. Once that orifice area is reduced by the valve downhole end, momentarily restricting the mud flow, a pressure increase in the mud flow results. When the main valve retracts upwardly, the mud flow area increases and the pressure in the mud flow is returned to normal. 
     The orifice sleeve may be welded to a mule shoe sleeve. The mule shoe sleeve is utilized to orient and seat the pulser in correct angular alignment with the drill collar. The pulser is contained within a series of interconnected tubular housings. The directional sensors are fixedly mounted in the upper portion of the interconnected tubular housings. The lowermost housing defines a shaped slot open at its lower end which cooperates with a key on the mule show sleeve to angularly orient the directional sensors with the drill axis. 
     A plurality of vertically spaced centralizing stabilizers are mounted in a drill collar, or a tubular drill string segment connected to the drill collar, between the bore of the drill collar or tubular drill string segment and the periphery of the plurality of serially connected tubular housings. A hollow central plug projects downwardly out of the downhole end of the lowermost tubular housing and is axially shiftable relative to the downhole end of the lowermost housing between a first position axially spaced from the orifice sleeve and a second position adjacent to the orifice sleeve. A tubular shaft is secured at one end to the plug and extends upwardly through the lowermost tubular housing. An annular piston is formed around the top end of the tubular shaft, the piston having a downhole facing surface and an uphole facing surface. 
     A radial port is provided in the lower tubular housing for supplying pressurized drilling mud to the piston&#39;s downhole facing surface. A compression spring engages the uphole facing end surface of the annular piston to bias the piston and central hollow plug downwardly. A screened mud intake port in the tubular housing is provided for supplying pressurized drill mud to the uphole facing end surface of the annular piston and combines with the spring bias to move the annular piston and central hollow plug downwardly to the second position. 
     A pilot valve is mounted for movement between an opened and closed position relative to the mud flow from the mud intake port for controlling the flow of pressurized drilling mud to the upwardly facing end surface of the annular piston. The pilot valve is operated by a pair of solenoids that provide the required movement of the pilot valve between it&#39;s opened and closed position. The solenoids are in turn operated by a battery powered controller located in the upper portions of the interconnected tubular housings. A plurality of sensors in the upper portion of the interconnected tubular housings provide signals to the controller representing environmental, directional and core information conditions, hereinafter collectively referred to as environmental drilling conditions, which are to be transmitted to the surface. 
     A differential pressure detection diaphragm flow switch is positioned relative to the piston head and the pilot valve in a mud flow chamber formed above the uphole facing surface of the piston. This switch will sense both when mud is flowing and when mud flow has ceased in the mud flow chamber by measuring the differential pressure across the diaphragm. This differential pressure provides movement of the contacts of the flow switch, indicating to the controller that the diaphragm is sensing a pressure differential. This indication represents mud flowing through the screened mud intake port to the mud flow chamber surrounding the uphole facing surface of the main piston. When mud flow ceases in the mud chamber, the differential pressure diaphragm will indicate to the controller by movement of the contacts of the flow switch that the mud flow has ceased. 
     The pilot valve is positioned in the uphole end of the mud chamber, in line with a mud flow passage, thereby controlling the flow of mud to the upwardly facing end of the piston. This valve has two positions: open, pressurized mud flows past the valve into the mud flow chamber; and closed, the mud is prevented from flowing into the mud flow chamber. The main compression spring alone will not move the piston and plug downwardly. The pressurized drilling mud applied to the uphole facing end surface of the piston will combine with the spring bias to move the piston and plug downwardly to its second position relative to the orifice. 
     Once pressure decreases on the uphole side of the hollow plug, due to the cycling of the pilot valve, the pressurized mud on the downhole face of the piston begins to force the hollow plug upwardly. As mentioned, the top portion of one of the lower sections of the lower tubular housing has a radial port for supplying pressurized drilling mud to the piston&#39;s downhole facing surface. This force pressing upwardly on the piston&#39;s downhole facing surface will add upward pressure to the piston head, sufficient to urge the annular piston and the hollow central plug upwardly to the first position axially spaced from the orifice. 
     To minimize the force required to shift the pilot valve, the pilot valve, its operating shaft, and the two solenoids which are successively connected to the operating shaft, are all disposed within a separate chamber which is filled with oil. The pressure of the oil around these elements is maintained equal to the surrounding pressure of the mud flow through the incorporation of flexible diaphragms or membranes in the wall of the oil chamber. Thus, the movement of the pilot valve to its open position can be accomplished with a minimum of electrical energy supplied to the solenoids and can be returned to its closed position by a weak spring. The pressure equalization of the pilot valve assures a long life for the downhole batteries. 
     The present invention utilizes a telemetry program stored within the memory portion of the controller that provides the instructions for the command and control function for the pulser. The best mode of operation of the pulser utilizing the stored program can best be understood by presetting initial conditions for positive pulse transmission while drilling. In the initial condition, there is no mud flow and the pressure in the mud flow chamber is the same as in the flow sensor chamber. The pilot valve is in its closed position. The main spring has forced the main valve end into the downmost position in the main valve orifice. The flow switch is in the no-flow state, indicating no mud flow. 
     Pumping begins with mud flow being forced thru the main valve orifice with the main valve end extended into the orifice. This creates a lower pressure in the mud flow below the orifice. The hollow shaft opening provides a conduit for reducing the pressure in the mud flow chamber. This reduced pressure results in a condition where the pressure in the flow sensor chamber is greater than the pressure in the mud flow chamber. This pressure differential forces the plunger shaft to move downward and therefore changes the state of the flow switch. The same pressure differential causes the main valve end to retract upward and out of the orifice until it reaches an equilibrium position. The controller having sensed mud flow from the flow switch and knowing the preconditions of the main valve signals the first solenoid to energize thereby pulling back the pilot valve&#39;s forward end from its orifice. Approximately 80 milliseconds later the controller energizes the second solenoid to hold the retracted pilot valve&#39;s end away from the pilot valve orifice. At this time, the first solenoid is de-energized and the second solenoid holds the pilot valve&#39;s shaft in a retracted position with reduced electrical energy. The pressurized mud begins to flow through the pilot valve&#39;s orifice into the mud chamber, engaging the piston head of the main valve and producing a downward pressure. A portion of this mud flow traverses the center core of the main valve and exits the lower end into the mud stream of the drill collar. The added pressure upon the piston head combined with the force of the main spring urges the piston downwardly, pushing the main valve downwardly until it&#39;s forward end is adjacent the orifice, thereby reducing the orifice area and creating a pressure increase in the drilling mud, thus producing a positive pulse. 
     The controller will continue alternating the pilot valve between open and closed positions causing the main valve to create positive mud pulses in the mud flow until all of the desired information gathered from the plurality of sensors is transmitted to the surface by way of pulses in the mud flow pressure. 
     A complete appreciation for the invention and many of the advantages thereof will be readily perceived by reference to the following detailed description, taken in conjunction with the following drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIGS. 1A-1G collectively constitute a vertical sectional view of a measuring while drilling tool embodying this invention with FIG. 1A being the lower portion of such tool and FIG. 1G being the uppermost portion of such tool; FIGS. 1A-1D illustrate the positions of the components of the tool when the main mud flow restricting valve is in its lowermost, operating position. 
     FIGS. 2A-2D are views respectively similar to FIGS. 1A-1D but showing the components of the tool in the positions occupied when the mud flow constricting valve is in its inoperative position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawings schematically illustrate a measuring while drilling (MWD) tool embodying this invention. As is conventional, drill collar 1, formed of a non-magnetic metal, is positioned downhole and has connected to its lower end 1a a conventional drilling tool (not shown). Such drill collar 1 is threadably connected at its upper end to the lower end of a tubular drill string (not shown), which extends from the drill collar to the well surface. Alternatively, a special tubular segment, formed of non-magnetic material, either forms an extension of a conventional drill collar or is incorporated in the lower end of the drill string, preferably immediately above the drill collar. The drill collar 1, or the alternative tubular segment, houses a majority of the elements embodying this invention, and, for reference convenience, either structure will be referred to as a tubular drill string segment 1. 
     An orifice sleeve 4 is clamped in position within the lower end of the tubular drill string segment 1 by a threaded retaining sleeve or jam nut 4a which is threadably engaged with threads provided in the interior of the lower end of the tubular drill string segment 1. Orifice sleeve 4 defines a cylindrical passage 4b through which a pressurized drilling fluid, such as drilling mud, flows to actuate the drilling tool. The pressurized drilling fluid is supplied from the well surface through the drill string segment 1. After exhausting from the drill tool, the drilling fluid flows upwardly around the exterior of the drilling string segment 1 to the well surface, carrying with it the formation particles released by the action of the drilling tool, where the formation particles produced by the drilling tool are conventionally removed from the drilling mud. 
     The apparatus comprising the present invention constitutes a plurality of serially interconnected housings 6a, 6b, 6c, 6d, 6e, 6f, 6g, and 6h as is shown in FIGS. 1A-1G. A plurality of centralizers 5a, 5b, 5c and 5d are conventionally mounted within the tubular drill string segment 1 in vertically spaced relation, to position the interconnected tubular housings 6a-6h concentrically within the tubular drill string segment 1, thus defining an annular passage around the exterior of the interconnected housings for the downward flow of the pressurized drilling fluid. 
     That pressurized drilling fluid not only activates the drilling bit but also acts as a communication conduit for a plurality of sensors S, to be described, that provide environmental information to a controller C for transmission to the surface. The individual types of sensors that provide environmental information are well known in the art. When location sensors are used, they must all be specifically aligned with the axis of the drilling tool. Such alignment is accomplished by utilizing a mule shoe centralizer 5a as the lowermost centralizing unit. Mule shoe centralizer 5a may be formed as an integral part of orifice sleeve 4, or welded thereto. Mule shoe centralizer 5a incorporates a inwardly projecting radial lug or key 5f, which cooperates with a triangular slot 6j formed in the lowermost interconnected tubular housing 6a to effect alignment of the environmental sensors S within the interconnected housings with the axis of the drilling tool. The angular position of key 5f is fixed by a set screw 2 traversing the wall of tubular drill string segment 1 and engaging an axis slot 4c in orifice sleeve 4. Thus, the mule shoe centralizer 5a is aligned by orifice sleeve 4 and the tubular housing 6a is aligned by the orifice sleeve 4 with set screw 2. 
     Communication of information to the well surface is accomplished by encoded signals producing pressure surges in the downward flow of the pressurized drilling fluid. Such pressure surge is accomplished by the axial movement of a hollow poppet or flow plug 8 moving downwardly into orifice 4b defined by orifice sleeve 4 and restricting the flow of pressurized mud, thereby increasing the pressure of the mud. Flow plug 8 threadably secured by threads 8a to the interior of an upwardly extending hollow shaft or plunger 9. The top end of plunger 9 (FIG. 1B) has an annular piston 10 threadably secured thereto by threads 10a. The cylindrical periphery of annular piston 10 has a seal 12a and a wiper ring 12b providing a sealing engagement with the bore of a cylindrical sleeve 12, which is mounted in the second interconnected tubular housing 6b. Piston 10 is shown in FIG. 1B in its lowermost operative position, wherein flow plug 8 is disposed within the orifice 4b. A bore diameter adjustment sleeve 10d is secured within the bore of annular piston 10 by the upwardly facing end 10c of annular piston 10. 
     That lowermost position of annular piston 10, and hence that of flow plug 8, is determined by the engagement of the downwardly facing annular surface 10b of annular piston 10 with the top edge of the lowermost tubular housing 6a. Annular piston 10 is biased to this lowermost position partially by a compression spring 14 and partially by the pressurized mud flow as will be explain, forthwith. Compression spring 14 is positioned between an internal shoulder 6k provided in the second interconnected tubular housing 6b and the upwardly facing annular surface 10c of annular piston 10. Upward movement of the annular piston 10 is limited by the compacting of the compression spring 14. 
     It should be noted that annular piston 10 is therefore movable within a mud flow chamber 13 defined within second interconnected tubular housing 6b and extending into the third interconnected tubular housing 6c. Immediately below the lowermost position of the downwardly facing annular surface 10b of the annular piston 10, one or more radial ports 11 are provided in the tubular housing 6b. These ports permit a portion of the pressurized drilling fluid to operate on the downwardly facing annular surface 10b of annular piston 10 and impact an upward bias to such piston. It should also be noted that the downwardly facing end surface 8b of the flow plug 8 is subjected to an upward fluid pressure force by the pressurized drilling mud flowing around it. The pressurized mud is also free to move upwardly through the hollow bore of the shaft or plunger 9 and to enter mud flow chamber 13 when no pressurized drilling mud is entering mud flow chamber 13 from the upwardly adjacent pilot valve mud flow chamber 15, to be described. 
     Pilot valve mud flow chamber 15, which is in the lower portion of the third inter-connected tubular housing 6c, is in communication with mud flow chamber 13 that extends upwardly from tubular housing 6c into tubular housing 6c. Housing 6c has a large slot 16 cut in the side wall of mud flow chamber 15, which is covered by a screen 18 secured by screws 18a to the periphery of tubular housing 6c. Pressurized drilling fluid can then pass through the screen 18 and into the chamber 15. A small diameter fluid passage 20 is defined by an internally projecting shoulder 22 formed in the lower portion of the third interconnected tubular housing 6c. A pilot valve sleeve 21 is mounted in small diameter passageway 20 and secured in place by the radial screw 18b. 
     The passageway 20 defined by the pilot valve sleeve 21 is normally closed by a plunger element 24 which, as shown in FIG. 1C, is in close contact with the bore of the pilot valve sleeve 21. Plunger element 24 has a wear resistant sleeve 24a formed on its lower end which closely engages the bore of pilot valve sleeve 21. When plunger 24 is in close contact with passageway 20, pressurized mud flow into chamber 13 is reduced. When there is limited pressurized mud flow into chamber 13 there is not sufficient downwardly pressure exerted by compression spring 14 onto annular piston head 10 to retain plunger 9 and hence flow plug 8 in their most downward position. This decrease in pressure causes a differential pressure across flow plug 8, i.e. low pressure on the uphole facing end of flow plug 8 and high pressure on the downhole facing surface of flow plug 8. This action provides flow plug 8 with the necessary pressure condition to begin retracting from orifice 4b in orifice sleeve 4 until an equilibrium pressure position measured across flow plug 8 is achieved. As stated earlier, pressurized mud flow onto piston surface 10a via intake ports 11 continuously urges piston 10 upwardly compressing spring 14 in chamber 13. When plunger 24 is in the open position pressurized mud flows into chamber 13 onto piston surface 10c adding the needed partial pressure bias to the force of spring 14 to urge piston 10 downwardly, thereby moving plug 8 downward into orifice 4b. 
     Plunger 24 is threadably secured to the end of an elongated shaft 25 which extends upwardly through the third tubular housing 6c. Shaft 25 is threadably secured to a spring mounting shaft 26 which extends upwardly through tubular housing 6d, through nipple 3 into tubular housing 6e where it is connected to the actuating core 48a of a solenoid 48. A pair of light easily compressible springs 28a and 28b which surround shaft 26, are separated by a sleeve 30 which also slidable surrounds the shaft 26. Upper spring 28b abuts a spacer sleeve 28c which is secured in the hollow nipple 3. 
     As mentioned, the top end of the spring supporting shaft 26 is threadably secured to the bottom end of a solenoid shaft 46a. Solenoid shaft 46a is in turn threadably connected to the actuating core 48a of an actuating solenoid 48. When it is desired to move the plunger 24 upwardly out of closing engagement with the bore of valve element 21, the actuating solenoid 48 is energized by current supplied from a downhole battery pack BP (FIG. 1F) conventionally mounted in one of the additional interconnected tubular housings secured to the top end of the housing 6g. These same additional tubular housings may also mount the various sensors S (FIG. 16) for making the desired measurements. 
     The fourth interconnected tubular housing 6d is provided along the major portion of its length with a plurality of axially spaced ports 31 through which the drilling fluid may readily flow. The pressurized drilling mud flowing through the ports 31 acts upon a flexible tubular diaphragm or membrane 32 which is sealed to axially spaced shoulders 34a on a tubular diaphragm support sleeve 34. The opposite ends of support sleeve 34 are respectively sealably mounted within a cylindrical wall surface 35a of tubular housing 6d and a cylindrical recess 3a in the bottom of nipple 3. 
     While the pressurized drilling mud cannot enter the interior of the flexible diaphragm sleeve 32, the entire interior of the diaphragm sleeve 32, as well as the plunger shaft 25 and the spring mounting shaft 26 may be subjected to a pressure equal to the drilling mud pressure. This feature is sealed by appropriate seals which define a chamber 39 for oil which is fillable through an oil fill port 33 and closed by a plug 33a. The oil chamber 39 extends from the plunger 24 upwardly to the top end of the fifth tubular interconnected housing 6e which is sealed by a conventional pressure bulkhead 40a sealably mounted in a second nipple 40 connected in the tubular housing string which passes electrical leads required for electrical elements in the oil filled chamber 39 upwardly into an air or nitrogen filled chamber 41 defined by interconnected housings 6f, 6g, 6h and 6i. 
     To ensure that the plunger 24 is subjected to the same internal oil pressure as the mud pressure entering through the slot 15, a secondary flexible diaphragm or bellows 38 is secured between the top end of the plunger 24 and the bottom end of a guide block 60. The pressurized oil is distributed around and through holes 26b in the spring mounting shaft 26 and along the periphery of the plunger shaft 26. Such distribution of the pressurized oil is assisted by radial ports 34b formed in the membrane support sleeve 34 and by a plurality of radial ports 44a formed in an oil distributing sleeve 44 surrounding the spring 28a, the sleeve 28c and the upper spring 28b. The various holes and ports eliminate trapped pockets of air. As a result, only a light spring force is required at any time to effect the movement of the plunger 24 into its closed position with respect to the passage way through the valve sleeve 21, regardless of the fluid pressure of the drilling mud flow. To accommodate electrical wires coming from a differential pressure switch 54 to be later described, an axial slot 44b is provided in sleeve 44. 
     To prevent angular movements of the plunger 24, an axial slot 44c is provided in oil distributing sleeve 44. A dowel pin 44d traverses slot 44c and engages sleeve 25. This prevents twisting and tearing of bellows 38. 
     The successive interconnected tubular elements 6f, 6g, 16h and 6i provided above pressure bulkhead 44a do not contain oil. The oil is blocked by the seals shown in the drawings. A downhole driver circuit C (FIG. 1F) for the solenoids may be mounted in housings 6f. A battery pack BP may be conventionally mounted in housings 6f and 6g and a plurality of sensors S for measuring downhole environmental conditions and appropriate microprocessor based controller MP may be mounted in tubular housing 6h. 
     At the top end of tubular housing 6i, a retrieving plug 6k is sealably mounted in the top of the housing 6i and permits the entire string of interconnected tubular housings to be removed from the drill string by wireline when repair or adjustments are necessary. 
     Returning now to the solenoids, the actuating solenoid 48 is of conventional construction, including a ferrous core 48a mounted in a ferrous sleeve 48b around which a plurality of turns of wire 48c are mounted to provide, when energized, a magnetic force to pull the plunger 24 upwardly to a limiting position defined by a ferrous barrier element 48d. Since it is highly desirable to minimize the draining of electrical energy from the downhole batteries in order to actuate the solenoids, the driver or control circuit C includes a plurality of capacitors within which energy is stored and, when triggered by a signal received from the controller, effects the discharge of the stored energy into the solenoid coil 48c. 
     In a similar manner, the holding solenoid 46 includes the previously mentioned ferrous core 46a which is slidably mounted within a ferrous sleeve 46b around which an actuating coil 46c is wound. To maintain the pilot valve actuating shaft 26 in its upper position without expending substantial amounts of battery energy, a ferrous disc or clapper 46d is secured to the lower end of the core 46a and is brought into closely spaced relationship with a radial flange 46e of ferrous material formed on the bottom end of the ferrous sleeve 46b. A thin disc 46f of non-magnetic material is mounted adjacent to the ferrous flange 46e. Thus, the energization of holding solenoid 46 after the actuating solenoid 48 has been actuated, will bring the clapper 46d into closely spaced relationship with the ferrous flange 46e and will hold the clapper and the connected extension shaft 26a in the uppermost position with a minimal amount of energy drain from the downhole batteries. The purpose of the non-magnetic disc 46f is to make sure that the permanent magnetic attraction of the clapper 46b by the ferrous sleeve 46b and flange 46e will not continue to hold the actuating shaft 24 and the extension shaft 26 in their upper positions when the holding solenoid 46 is deenergized. The actuating solenoid 48 is, of course, deenergized immediately upon the energization of the holding solenoid 46. 
     The medial portion of the tubular housing 6c is additionally provided with an axially extending bore 25c. A plunger shaft 50 is mounted in the bore 25c and at its lower end is engagable by elastomeric diaphragm, held by a pressure plate 52, which is responsive to the differential pressure between that existing in the chamber 13 and the oil pressure existing within a flow switch chamber 25a defined between slide block 60 and the inner wall of housing 6c. Downward movement of the diaphragm in response to flow of drilling fluid causing reduced pressure in the chamber 13 will cause a downward movement of the plunger shaft 50, thereby actuating a flow sensor switch 54 in chamber 25a. Actuation of flow sensor switch 54 is transmitted by wires (not shown) to inform the controller C that mud flow has been established through the drill string. The controller C then takes over the control of the pilot valve 24 to move it upwardly from its closed position to permit pressurized drilling positive fluid to flow from chamber 15 into chamber 13 and thus effect downward displacement of the piston 10 with the resultant creation of fluid pressure pulses due to the partial closing of the orifice 4b defined by the orifice sleeve 4 as shown in FIG. 1a. 
     Modifications of this invention will be readily apparent to those skilled in the art and it is intended that all such modifications be encompassed within the scope of the appended claims.