Abstract:
An oscillating valve driven by the pressure in a mud pulser develops a pressure variation in the mud pulse signal that is at a frequency that is different than the frequencies of the mud system. The oscillating valve is coupled to or integral with the pulse drive cylinder, whether the drive means for operation of the pulser is upstream or downstream from the orifice, or whether or not the drive means and the poppet are on the same side or opposite sides of the orifice. The oscillating valve is a bistable valve which preferably forms one wall of the drive piston of the pulser. Pressure from the drive cylinder behind the drive piston is directed in such a way as to unseat the bistable valve. When the bistable valve unseats, pressure is bled from within the drive cylinder, which also bleeds the pressure that unseated the bistable valve in the first place. When pressure drops, the bistable valve reseats, and the cycle repeats as long as pressure is being ported to the cylinder, thereby creating a tone in the drilling mud.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the field of measuring while drilling (MWD) systems, and, more particularly, to a system which provides repeated, cyclic pressure oscillations in the transmission of data from borehole sensors to receivers at the surface.  
         BACKGROUND OF THE INVENTION  
         [0002]    Remotely operated sensor packages have been used during the drilling of wells for a number of years. These packages are commonly found in drilling applications where information such as the inclination, azimuth, and various logging sensor measurements of the well are of interest.  
           [0003]    During well drilling operations, drilling fluid, known in the art as drilling mud, is typically pumped down through the drill pipe and then through the drill bit to clean, lubricate, and cool the bit. The drilling fluid then returns to the surface by way of the annulus between the drill pipe and the bore hole or casing, where the drilling mud is cleaned of cuttings so that the drilling fluid can be re-used.  
           [0004]    As early as 1942, it was established that the flowing drilling fluid could be used as a transmission medium for data developed down hole during drilling operations, thus the origin of the term “measuring while drilling”. To transmit information, a device was created that varied the pressure of the drilling fluid in the drill pipe by placing an orifice in the drill string and inserting a poppet into the orifice to form a “pulser”. By repeated insertion and removal of the poppet, a series of pressure increases was created in the drilling fluid that could be detected at the surface and used to convey information. Unfortunately, these pressure increases were of relatively low frequency, generally resulting in a pressure pulse with a rise time of 20-200 milliseconds, a duration of 0.25 to 3 seconds, and a fall time of 20-200 milliseconds. The resulting spectral content of the pulses created down hole was concentrated at frequencies below 20 Hz with the centroid of spectral energy below 3 Hz, and a peak energy centered in the range of 0.1 to 1.5 Hz.  
           [0005]    In addition to severely limiting the data transmission rate, these low frequencies coincide with the noise frequencies generated during drilling. One common technique for improving the signal to noise ratio is to filter the noise. Unfortunately, conventional filtering, which is used to eliminate drilling noise, also removes much of the remaining energy from the transmitted pulse.  
           [0006]    To overcome this shortcoming, the amplitude of the induced pressure pulses was increased. However, erosion of the poppet and orifice by the pressure pulses is a function of the imposed pressure drop. Thus, increasing the pressure drop decreased pulser life. Another problem with simply increasing the amplitude of the induced pressure pulses was the power required to create such pulses. The large power demand meant a large and more powerful prime mover to operate the poppet, and this meant greater weight and cost for the MWD system.  
           [0007]    Therefore, it is an object and feature of this invention to provide a method of modifying the design of positive fluid pulsers that will shift the frequency of the signal away from the region of substantial drilling noise thereby reducing the requirement for the high pressure pulses. It is a further object of this invention to teach a method of generating oscillating pressure signals in the drilling fluid thereby facilitating higher data transmission rates. It is a further object and feature of this invention to provide a general method of valving that allows the oscillation amplitude to be set largely independent of the fluid flow rate.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention addresses these and other drawbacks in the art by creating an oscillating pressure using a control valve that is driven by the pressures in a mud pulser. The present invention is adaptable to the mud pulser whether the drive means for operation of the pulser is upstream or downstream from the orifice, or whether or not the drive means and the poppet are on the same side or opposite sides of the orifice. The invention comprises a toggling or bistable valve and an optional pilot valve bypass port which is inserted in the flow path of the drive fluid of such a pulser and preferably forms one wall of the drive piston of the pulser. In this disclosure, the terms “toggling” and “bistable” are used interchangeably to refer to the valve described herein that moves quickly between open and shut and between shut and open, to effect the function herein described. Pressure from the drive cylinder behind the drive piston is directed in such a way as to unseat the bistable valve.  
           [0009]    When the bistable valve becomes upset it changes state, in the case of unseating the valve, pressure is bled from within the drive cylinder, this also bleeds the pressure that upset the bistable valve in the first place. When pressure drops, then bistable valve returns to the pre-upset condition, and the cycle repeats as long as pressure is being ported to the cylinder, thereby creating a pressure oscillation, a tone, in the drilling fluid.  
           [0010]    The bistable valve includes a pair of adjustable biasing means, preferably springs. Adjusting the steady state tension on the biasing means alters the amplitude of the pressure developed by the bistable a valve. Adjusting the volumetric flow rate through the valve alters the tone of the oscillation. This way, higher or lower drilling fluid flow rates can be accommodated, and a pressure oscillation, or tone, can be developed that can be detected on the surface, regardless of drilling fluid flow rate.  
           [0011]    The present invention reduces the poppet force and stroke required of the poppet, thereby reducing the size, cost, and weight or the MWD tool. Reducing the force and stroke demanded of the poppet also substantially increases its useful life by reducing wear.  
           [0012]    These and other features of the present invention will be immediately apparent to those skilled in the art from a review of the following description along with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a sectional view of a typical drilling system in which the present invention finds application.  
         [0014]    [0014]FIGS. 2 a ,  2   b , and  2   c  are elevational views in partial section of known pulsers with the poppet and orifice in various known configurations.  
         [0015]    [0015]FIG. 3 is a sectional view of a bistable valve component of the present invention adapted to couple to or be formed as an integral part of the pulser of FIGS. 2 a ,  2   b , or  2   c.    
         [0016]    [0016]FIG. 4 is a sectional view of another preferred embodiment of a tone unit of the present invention adapted to couple to or be formed as an integral part of the pulser of FIG. 2 a ,  2   b , or  2   c.    
         [0017]    [0017]FIG. 5 is a graph of the force pressure relationship for various flow rates as a function of valve position for the downstream positive pulsers, shown in FIG. 1.  
         [0018]    [0018]FIG. 6 is a plot of an upstream pressure waveform produced by the bistable valve component of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 illustrates a basic drilling system  100  in a bore hole  102  formed by a typical drill bit  104 . The drill bit  104  is driven by a drill pipe  106  which joins to a bottom hole assembly  108  with a coupling  110 . Drilling mud flows down through the drill pipe  106 , as shown by an arrow  112 , through the bottom hole assembly  108 , through the drill bit  104  and back to the surface by way of the annulus between the drill pipe and the bore hole.  
         [0020]    The bottom hole assembly  108 , between the coupling  110  and the drill bit  104 , is where the present invention finds application. The bottom hole assembly  108  includes one or more sensors  114  adapted to measure parameters of interest. The sensor(s)  114  provide a sensor signal to a transmitter  116  which includes a pulser  118 . The transmitter  116  and pulser  118  vary the pressure in the drilling fluid, which variation is detected at the surface and interpreted to provide the measured data at the surface. These sensors provide an output to a control unit (not illustrated) which drives the mud transmitter containing components of pulser  188  and a bistable valve. The known pulser of FIG. 1 is illustrated for background purposes to illustrate the environment in which the present invention finds application.  
         [0021]    [0021]FIG. 2 a  depicts a hydraulic schematic of a known bottom driven positive pulse MWD pulser  10 . In FIG. 2 a , flow of the drilling fluid, indicated by an arrow  12 , enters the pulser at an inlet  14 , from the direction indicated. In the type of pulser illustrated, an orifice  16  is located upstream of a poppet  18 , although the poppet may preferably be placed upstream of the orifice, as shown in FIG. 2 b . A pressure differential is created across the orifice  16  from an upstream region  20  to a region  22  downstream of the poppet, even when the poppet  18  is in a retracted position as shown in FIG. 2 a . The poppet and orifice are enclosed within a housing  15 , which is preferably a cylindrical or tubular housing. This housing may also be a structural component of the drill string.  
         [0022]    To insure this pressure differential is sufficient for regenerative operation, a spring  24  has one end either attached to a cylinder  26  and the other end to a drive piston  28  or a spring  24  compressed in such a way as to apply some force countering the flow forces on the poppet and forcing the poppet toward the orifice  16 . A portion of higher pressure fluid in the region  20  can be permitted to enter the drive cylinder  26 , behind the drive piston  28 , which is coupled through a drive rod  30  to the poppet  18 . The region  20  is therefore hydraulically coupled to the cylinder  26  through a gallery  32 . This flow maybe interrupted, however, by a pilot valve  34 . By opening the pilot valve  34 , a chamber  36  behind the drive piston  28  is allowed to approach the pressure of region  20 . It will be understood by those skilled in the art that the poppet, drive cylinder, and piston arrangement depicted in FIG. 2 could as well be positioned upstream of the orifice  16 , as will be described below.  
         [0023]    A secondary opening defining a bleed bore  38  is installed in the chamber  36  to serve as a controlled leak or an operating valve that allows an equilibrium to be established between the force behind drive piston  28  and the force of the drilling mud impinging on the poppet  18  as a result of fluid movement and the difference in pressure between the regions  20  and  22 . The secondary opening  38  also allows pressure within the chamber  36  to return to downstream pressure at region  22  when the pilot valve  34  is closed. This reduction in pressure allows the drive piston  28 , the connecting rod  30 , and the poppet  18 , to return to an off pulse position. A pressure relief valve  40  is employed to effectively maintain the pressure in the drive cylinder so that the pulses are of constant amplitude regardless of flow rate.  
         [0024]    [0024]FIG. 2 b  and  2   c  depict other configurations of a known pulser, and like structural components are provided with like element numbers. In FIG. 2 b , the poppet  18  is positioned upstream of the orifice  16 . One drawback of the configuration of FIG. 2 b  is that fluid flow as shown exerts a closing force on the poppet against the orifice, a force which must be overcome in retuning the poppet to the retracted position. This drawback is overcome by the configuration of FIG. 2 c  by placing the actuator upstream of the orifice while placing the poppet downstream of the orifice. However, the configuration of FIG. 2 c  includes the drawback of the rod  30  going through the orifice, and thereby taking up some of the cross sectional area for fluid flow through the tool. It is to understood by those skilled in the art that the present invention may be used effectively without further adaptation with any of the configurations of FIGS. 2 a ,  2   b , and  2   c.    
         [0025]    Assuming that the pilot valve  34  allows upstream pressure into drive cylinder  26 , the force on the piston  28  within the drive cylinder  36  such as is illustrated in FIG. 2, is given by the equation:  
         Force=Piston Area*( P   20 - P   22 ),  
         [0026]    where P 20  is the pressure at region  20 - and P 22  is the pressure at region  22 .  
         [0027]    The pulser thus far described provides one poppet position (i.e. one level of back pressure) for a logical “1”, and another poppet position (i.e. another level of back pressure) for a logical “0”. As previously described, the frequencies produced by this arrangement can be masked by the background noise of the mud system, such as for example by the mud pump providing the drilling fluid flow, and other background noise. The present invention, however, is directed to providing a frequency variation at either or both of the logical “0” and/or “1” to move the frequency of the data carrying system away from the frequencies of the natural background noise, so that the logic transmitted by the system is more easily detected.  
         [0028]    [0028]FIG. 5 illustrates typical relations between the force on the poppet of a pulser, the displacement of the poppet from the orifice and the resulting pressure drop across this poppet orifice pair as a function of the same displacement of FIG. 2. FIG. 5 shows the poppet force required to develop a particular pulse pressure is a parametric function of flow rate. The exact shape of these curves is controlled by the rate of momentum change in the fluid traversing the orifice which is controlled by the shape of the poppet and orifice. These illustrated curves are a subset of an infinite number of such curves for a variety of fixed flow rates as indicated by the designations FR 1  and FR 2  in FIG. 2. FIG. 5 also indicates that the required stroke length and the displacement of the poppet from the orifice necessary to achieve this given pressure excursion is also a parametric function of flow rate. This is relevant because wells are typically drilled with positive displacement pumps so that flow rate does not vary with a variation of circuit pressure around the fluid circuit. As can be readily discerned, for a variety of volumetric flow rates, approximately the same poppet force is require to attain a desired pulse pressure however this force is obtained at different displacements from the orifice. Therefore, the actual positions of the poppet relative to the orifice for both the onpulse and offpulse conditions will vary with flow rate. If the poppet force is set by the structure of the pulser then the pulse amplitude will be nearly constant over a range of flow rates. In the absence of this force the poppet will be driven away from the orifice. Therefore, by adjusting the force of  1 i insertion of the poppet into the orifice a given pressure drop can be obtained somewhat independent of the flow rate. A similar situation occurs when the poppet is located upstream of the orifice except the sense of force is reversed as the absence of this force will result in seating the poppet onto the orifice. This can be accomplished by matching either matching the bleed rate of the orifice  38  to the desired flow rate or by supplying a pressure relief valve  40 . In the first case the pulse amplitude must be set for the minimum flow rate and any greater flow rate results in a higher pressure pulse and more rapid wear of the poppet and orifice. In the second case the pressure relief valve  40 , wears at a high rate and during the course of a drilling well the valve characteristic may change sufficiently to inhibit pulser operation. In either case, the pulser will tend to wear out quickly. The present invention, in contrast, substantially increases the lifetime of pulser operation. This is accomplished by including an oscillation in the mud column that is above the frequency of the drilling noise reducing the required pressure excursions while still providing a detectable signal at the surface.  
         [0029]    [0029]FIGS. 3 and 4 depict alternative structures for developing that frequency variation in the manner of a tone. Referring first to FIG. 3, the cylinder  26  receives the drive piston  28  which is coupled to the rod  30 . A variable volume and pressure chamber  36  is enclosed by the cylinder and piston. In the present invention, a bistable valve  50  forms the other wall to enclose the chamber  36 . The bistable valve  50  provides an oscillator which develops a time-varying pressure superimposed on the pressure pulse signal from the pulser.  
         [0030]    The bistable valve  50  comprises a body  52 , preferably cylindrical in cross-section, of the same diameter as the cylinder  26 . The secondary opening  38  (see FIG. 2) comprises a bore  54  which feeds into a valve chamber  56 . The valve chamber extends from the bore  54  the length of the body to an opening  58  and vents into the casing downstream of the cylinder  26  through a vent opening  59 . Enclosed within the chamber  56  is an axially movable valve disk member  60  which is integrally formed with a valve stem  62 . The valve disk member  60  seals against the bore  38  when in the position shown in FIG. 3. The stem  62  slidably receives a sleeve  64  which abuts against a shoulder  66  formed by the intersection of the disk member  60  and the stem  62 .  
         [0031]    Attached to or integrally formed with the sleeve  64  is a pivot  68 . Attached to the pivot  68  is an elongate arm  70  which receives one end of a first spring  72 . The other end of the first spring  72  is attached to a tension adjusting screw  74 . In operation, as the disk member  60  moves axially back and forth, the arm  70  rotates about the pivot  68 , under the control of spring action from the first spring, as further explained below.  
         [0032]    The sleeve  64  rides against a second spring  76 . The disk member  60 , the sleeve  64  and the second spring all ride within an alignment sleeve  78 . During operation, the alignment sleeve  78  remains stationary in relation to the body  52 . The compression of the second spring  76  is controlled by the position of a set screw  80 , which also receives the tail end of the stem  62 . A vent hole  81  is provided to prevent hydraulic lock of the tail end of the stem  62 . The adjustment of the set screw  80  determines the pressure within the chamber  36  at which the disk member  60  unseats from the bore  54 . Thus, together, the first and seconds springs determine the frequency and amplitude of the pressure variations created by the axial movement of the disk member  60 . Simple actuation of the bistable valve  50  allows the poppet  18  to oscillate between the pressure settings of the bistable valve.  
         [0033]    A typical wave form of the pressure signal created by the bistable valve is shown in FIG. 6. The ordinate of FIG. 6 is shown in terms of pressure differential, the absolute pressure will depend on the volumetric flow rate of the drilling fluid, and tension on the springs contained within the valve. As pressure in the chamber  36  exceeds the combined forces holding the bistable valve closed, the disk member unseats and toggles into the open position, the pressure in the chamber  36  drops rapidly, depicts as a rapid fall in the ΔP waveform. As pressure in the drive cylinder decreases the flow rate through the bistable valve decreases reducing the force on the face of the disk member. Spring pressure from the second spring then activates the toggle and shuts the bistable valve. The closed valve causes pressure in the drive cylinder to increase as flow rate into the drive cylinder is greater than the flow rate out of the drive cylinder and the drive cylinder inlet is at upstream pressure. This results in are generative pressure increase in the drive cylinder and a corresponding rapid increase in the waveform. This rise and fall in drive cylinder pressure creates a rapid oscillation of the poppet, which is coupled to the piston  28 , alternately restricting and releasing the restriction of flow in the drilling fluid stream thus creating an oscillating pressure tone in the drilling fluid.  
         [0034]    The frequency of these oscillations can be controlled by placing additional valves that control the volumetric rate of fluid flow in the circuit between the bore  54  through valve  50  and the drive cylinder  26 . In the alternative, the frequency of theses oscillations can be controlled by replacement of the bistable valve with a valve controlling the rate of fluid flow through the fluid path into the drive cylinder. Such a valve can be a plug valve, a needle valve, another valve type with multiple fixed orifices, or a combination of valve types.  
         [0035]    [0035]FIG. 4 depicts another preferred embodiment of the bistable valve, in this case a valve  82 . The valve  82  accomplishes the same function as the valve of FIG. 3, and includes the same body  52  which forms a wall of the volume  36 . It also includes the same second spring,  76 , set screw  80 , bore  54 , chamber  56 , and vent  59 , which are numbered with the same reference numbers as in FIG. 3. A disk member  84  seals off the bore  54 , as before, but has received a different reference number because the disk member includes a detent  86  adapted to receive a first spring  88 . In this embodiment, the first spring comprises a leaf spring. The second spring  76  opposes movement of the disk member  84 , as before, but the second spring  76  is retained by a sleeve  90  and a shoulder  92  of the disk member  84 . The setting of the first spring is adjusted by an alien screw  94 , while the set screw adjusts the tension of the second spring  76 .  
         [0036]    The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.