Patent Publication Number: US-11041351-B2

Title: Downhole flow diversion device with oscillation damper

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/553,100 entitled “DOWNHOLE FLOW DIVERSION DEVICE WITH OSCILLATION DAMPER” and having a 371(c) date of Aug. 23, 2017, which is a national entry of PCT Application No. PCT/CA2015/051244 filed Nov. 30, 2015, which application claims benefit under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/119,712, filed Feb. 23, 2015, each of which applications is assigned to the current assignee hereof and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Disclosure 
     The present invention relates in general to drill strings and, in particular, to a system, method and apparatus for regulating fluid flow through a drill string. 
     Description of the Related Art 
     A conventional means of drilling for oil or gas includes pumping drilling fluid through a pipe or drill string to a drill bit that is cutting the hole in the rock. The fluid is circulated back up though wellbore in the annular or outer section of the hole. Drilling fluid is beneficial to the drilling process since it clears away pieces of rock that have been cut from the bottom of the wellbore. Without this cleaning action the cut pieces of rock would accumulate near the drill bit and it would become impossible to drill the hole. In general, the higher the level of fluid flow that a drilling operation can achieve, the better that cut pieces of rock or “cuttings” are cleared from the bottom of the wellbore. 
     However, there are several factors that limit the amount of fluid flow, such as the amount of pressure that it takes to pump a large amount of fluid. As the drill string becomes longer or narrower pumping a given amount of fluid encounters more and more resistance and therefore a high amount of pressure is required. With any fluid pump set up there is always a limit to the amount of pressure that can be overcome in order to make the fluid flow, so the size or type of pump that is available on drilling rate can limit the available flow rate. Another limiting factor is the capability of the downhole mud motor. Mud motors are used to make the rock cutting drill bit spin faster than the drill pipe that it is connected to. This is important in cases where a drilling operator may want to drill while holding the drill string still or may want to spin the drill bit faster in general to achieve a higher rate of penetration into the rock. The mud motor works in a fashion similar to a turbine in that mud that flows through the motor turns a rotor which is connected to the drill bit. Energy from the flow in the form of pressure is converted into rotation all work that the drill bit does. These mud motors are designed such that there is a maximum amount of flow that the motors are designed to handle. Forcing more fluid through the motor then it is designed for will result in damage to the motor and will inhibit the drilling process. 
     The desire to flow a high volume of drilling fluid through the well and the need to limit the volume flow rate due to the constraints of the motor are often conflicting. It would be desirable to slow as much fluid as is desired while ensuring that the motor did not experience a rate of flow higher than its design criteria. Thus, improvements in controlling drill string fluid flow continue to be of interest. 
     SUMMARY 
     Embodiments of a system, method and apparatus for controlling fluid flow through a drill string are disclosed. For example, an apparatus may include a housing having an axis, a radial wall with a bore extending axially therethrough, and an aperture formed in the radial wall. The aperture is in fluid communication with the bore. A piston may be located inside the housing and have an orifice configured to permit axial fluid flow through the housing. A spring may be located in the housing and be configured to axially bias the piston to a closed position. A viscous damper can dampen axial motion of the piston. 
     In other embodiments, a method of controlling fluid flow through a drill string may include operating the drill string to drill a hole in an earthen formation; pumping fluid through the drill string to a mud motor such that substantially all of the fluid is flows axially to the mud motor and substantially none of the fluid is radially diverted out of the drill string; and then increasing a flow rate of the fluid such that some of the fluid is radially diverted out of the drill string by a valve before reaching the mud motor, and a remainder of the fluid is flows axially to the mud motor. A viscous damper can dampen motion of the valve in both axial directions. 
     The foregoing and other objects and advantages of these embodiments will be apparent to those of ordinary skill in the art in view of the following detailed description, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the embodiments are attained and can be understood in more detail, a more particular description may be had by reference to the embodiments thereof that are illustrated in the appended drawings. However, the drawings illustrate only some embodiments and therefore are not to be considered limiting in scope as there may be other equally effective embodiments. 
         FIG. 1  is a sectional side view of an embodiment of drill string assembly. 
         FIGS. 2-4  are sectional side views of an embodiment of a system, method and apparatus for limiting fluid flow through a drill string, illustrating a closed position, a partially open position, and a fully open position, respectively. 
         FIGS. 5 and 6  are isometric and side views, respectively, of an embodiment of a sleeve. 
         FIG. 7  is an exploded isometric view of an embodiment of a tool assembly. 
         FIG. 8  is a sectional side view of an alternate embodiment of a tool assembly with an axial damper in a closed position. 
         FIG. 9  is a sectional side view of the embodiment of  FIG. 8  with the axial damper in an open position. 
         FIG. 10  is an isometric view of a lower portion of the embodiment of  FIG. 8 . 
         FIG. 11  is an exploded isometric view of the lower portion of  FIG. 10 . 
         FIG. 12  is an enlarged isometric view of a portion of an alternate embodiment of a tool assembly. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     Drill string fluid flow may be better controlled with radial ports in the drill string above the mud motor. By choosing the size of the ports carefully, the amount of flow that exits through the ports and the amount of flow that continues on through the drill string into the mud motor can be controlled. The amount of fluid that exits through the ports will vary depending on the back pressure from the mud motor. The back pressure from the mud motor is a factor of how much torque it delivers, so the more torque that is needed or generated by the motor, the higher the back pressure from the motor, and more fluid is radially diverted out of the drill string. The more fluid that is radially diverted out of the drill string, the less fluid that is delivered to the motor. When less fluid goes through the motor the amount of torque that the motor can generate falls, and so a situation can develop where the motor stalls and needs more torque to overcome its boundary condition, which results in less available flow to generate the necessary power. Conversely, an off-bottom situation where there is relatively low amounts of back pressure generated by the motor because there is low or no drilling torque results in a high amount of fluid passing through the motor and a low amount of fluid exiting the drill string. This is problematic because a low torque situation causes the motor to spin faster at a given flow rate and increased amounts of flow will only exacerbate this situation. 
     Some motor manufacturers attempt to solve this problem by drilling a hole through the rotor of the motor so that fluids may still pass through the tool without generating torque or causing damage to the motor. Unfortunately, such rotor holes are static and cannot change shape to adjust for differing flow or pressure conditions. Thus, that solution is subject to the same limitations as the prior art. 
     One possible means to solve this problem is with a tool that varies the size of the outlet orifice depending on flow and back pressure that is used to drill the well, as will be described herein. A variable outlet valve (described below) compensates for abrupt changes in the drilling conditions that can cause the valve piston to move quickly and its mass and momentum may cause it to overshoot the location where it needs to settle upon. In these situations the valve may begin to oscillate and flow may need to be stopped in order to bring the valve to a stable state. This type of oscillatory behavior is rare but can occur if the valve is being used in a position or flow rate where it is just marginally opened. In those cases a small axial movement of the valve piston can represent a relatively large difference in outlet nozzle flow area (i.e., while transitioning from fully closed to somewhat open in a short span of time). Because of this potential for the piston to overshoot and become unstable, it is desirable to include some means of limiting the movement of the piston so that it can more consistently rest in a stable position. 
     Embodiments of a system, method and apparatus for enhanced control of fluid flow through a drill string are disclosed. For example,  FIG. 1  depicts an embodiment of a downhole tool assembly  11  for drilling a well bore  10 . The downhole tool assembly  11  may comprise a variety of configurations. In one embodiment, the downhole tool assembly  11  may include an axis  12 , a plurality of drill pipes  13 , measurement while drilling (MWD) equipment  15 , a fluid flow control tool  17 , a mud motor  19  and a drill bit  21 . The order or sequence of these components may be varied depending on the application. For example, the MWD equipment  15  may be located above or uphole from the drill bit  21 . In some embodiments, the MWD equipment  15  may be axially relatively close (e.g., within about 100 meters) to the drill bit  21 . Likewise, the MWD equipment  15  may be located above but axially relatively close to fluid flow control tool  17 , such that fluid flow control tool  17  is relatively close to the drill bit  21  as well. 
       FIGS. 2-4  are enlarged views of fluid flow control tool  17 . Each drawing depicts a piston  23  in a closed position ( FIG. 2 ), a partially open position ( FIG. 3 ) and a fully open position ( FIG. 4 ). The fluid flow control tool  17  includes a housing  25  having an aperture  27  extending through a radial wall thereof. The aperture  27  may comprise one or more holes, slots, etc. 
     In the illustrated embodiment, a sleeve  29  that is stationary is mounted to the inner bore  31  of the housing  25 . Sleeve  29  has a sleeve aperture  33  that corresponds with aperture  27  in housing  25 . In some embodiments, the sleeve aperture  33  is smaller than and generally complementary in shape to the aperture  27 . In some versions, the sleeve  29  and sleeve aperture  33  are configured to take the brunt of fluid erosion damage away from the housing  25  and aperture  27 , such that aperture  27  may be beveled to reduce erosion. Sleeve  29  may be more readily replaced in fluid flow control tool  17  than housing  25 . Sleeve  29  may be affixed to housing  25  such that it can be considered to be part of the housing  25 , and to ensure alignment between apertures  27  and  33 . 
     Embodiments of the piston  23  also comprise an inner axial orifice  35 . As fluid  37  flows through the orifice  35  it may create a pressure drop and thus a downward force on piston  23 . As long as the flow rate of fluid  37  is low enough, the resultant downward force by the fluid on piston  23  does not exceed the upward force of a spring  41 . Under such conditions ( FIG. 2 ), a shoulder  42  on the piston  23  will remain against an upper stop  43  located on an inner surface of sleeve  29 . In addition or alternatively, the upward axial travel of piston  23  may be limited by landing a lower shoulder  53  of piston  23  on an upper shoulder  51  of sleeve  29 . 
       FIG. 3  illustrates the same tool with the fluid flow rate increased such that the downward force that the fluid exerts on piston  23  is equivalent to or exceeds the upward force of spring  41 . Under these conditions, the piston  23  moves axially downward to the “partially open” position shown in  FIG. 3 . The shoulder  42  on piston  23  is located axially below upper stop  43  on sleeve  29 . As the top  45  of piston  23  moves below the top of the sleeve aperture  33  in sleeve  29  (and, thus, the top of aperture  27  in housing  25 ), a flow path begins to open such that some of the fluid  47  escapes out the radial side of the tool  17 . Fluid  47  escapes to the wellbore annulus  49  ( FIG. 1 ) located between the outer surface of downhole tool assembly  11  and the wellbore  10 . The piston  23  finds an axial equilibrium between the downward pressure from fluid  37  through the orifice  35  and the upward force from spring  41 . In some versions, the spring rate of the spring  41  may be selected such that the balancing force is substantially constant throughout the axial range of travel of the piston  23 . 
       FIG. 4  shows the piston  23  in a “fully open” position when it is subjected to an even larger fluid flow rate than that of  FIG. 3 . The fluid flow is divided between fluid  47  through the apertures  33 ,  27  in the side of the tool  17 , and the fluid  37  flowing through the center of the tool  17 . In the fully open position, the fluid flow completely overcomes the spring force of spring  41  and pushes piston  23  completely open. In this condition, fluid flow through apertures  33 ,  27  may be completely unobstructed by piston  23 . In addition or alternatively, the downward axial travel of piston  23  may be limited by landing a lower shoulder  55  ( FIG. 7 ) of piston  23  on an upper shoulder  57  of a sub  13 . 
     In some embodiments, the apparatus or tool  17  may comprise a housing  25  having an axis  12 , a radial wall with a bore  31  extending axially therethrough, and an aperture  27  formed in the radial wall. In some versions, the housing  25  may have has an axial length of about 3 feet to about 12 feet, and an outer diameter of about 3.5 inches to about 8 inches. 
     The aperture  27  may be in fluid communication with the bore  31 . The aperture  27  in the housing  25  may comprise a plurality of apertures  27  formed circumferentially about the housing  25 . In such embodiments, apertures  33  in sleeve  29  may be matched to and register with respective ones of the apertures  27 . 
     In some examples, the aperture  27  may comprise various shapes, such as an elongated slot, or as the teardrop shape shown in  FIGS. 5 and 6 . The aperture  27  may include an upper leading edge  28 . In one example, the upper leading edge  28  is not greater than about 0.030 inches wide in a circumferential direction with respect to the axis  12 . The aperture  27  may narrow or taper in width, such as toward a trailing edge thereof. The angle of taper may comprise, for example, not greater than about 15° from a narrowest end thereof, with respect to the axis  12 . In addition, the aperture  27  may be skewed with respect to the axis  12 , as shown. 
     In the embodiment of  FIG. 12 , each aperture  33  in sleeve  29  may comprise a plurality of holes. The plurality of holes used to form the aperture  33  may be configured in a selected appearance, such as a teardrop shape. For example, spaced-apart circular, and linearly aligned holes  28   a - 28   f  may be provided, rather than a single hole as previously described. Aperture  27  in housing  25  may still comprise a single elongated slot for such embodiments. Each aperture  27  in housing  25  may register with a set of the holes  28   a - 28   f . Sets of a plurality of holes  28 , rather than singular apertures  33  in the sleeve  29 , may make the sleeve  29  more robust and less prone to cracking, thereby increasing its longevity. 
     In still other embodiments, each aperture  33  comprising a plurality of holes may be axially offset with respect to corresponding sizes of holes in other apertures  33 . For example, the largest hole  28   a  in one aperture  33 , may be axially offset by a select distance from another hole  28   a  in a different aperture  33 , which itself may be axially offset by a further slight distance from a third hole  28   a  in a third aperture  33 , etc. Such axial offsets may be repeated for other similarly sized holes in different apertures  33 . 
     A piston  23  may be located inside the housing  25  and have an orifice  35  configured to permit axial fluid flow through the housing  25 . A spring  41  may be located in the housing  25 . The spring  41  may be configured to axially bias the piston  23  to a closed position ( FIG. 2 ). 
     The piston  23  may be movable from the closed position wherein the piston  23  is configured to close the aperture  27  in the housing  25  to substantially block radial fluid flow therethrough when axial fluid flow  37  through the orifice  35  is insufficient to overcome a spring force of the spring  41 . In an open position (which may include any position other than the closed position), the piston  23  may be configured to permit radial fluid flow  47  through the aperture  27  when axial fluid flow  37  through the orifice  35  is sufficient to overcome the spring force of the spring  41  and axially move the piston  23 . In the open position, the piston  23  may be configured to permit substantially unobstructed radial fluid flow through the aperture  27 . 
     Embodiments of the piston  23  may further comprise a partially open position, located between the closed position and the open position, wherein the piston  23  may be configured to reach a force equilibrium between the axial fluid flow  37  and the spring force such that the aperture  27  is only partially obstructed to radial fluid flow  47  by the piston  23 . 
     The piston  23  may be configured to generate a pressure differential as fluid  37  flows through the orifice  35  so that the piston  23  pushes against the spring  41 . The orifice  35  may be replaceable within a body of the piston  23 , such that the body is configured to be reusable after the orifice  35  is replaced within the body. In some versions, the orifice  35  may have an inner diameter in a range of about 0.75 inches to about 1.5 inches. In addition, the piston  23  may be formed from a single material, or formed from at least two materials, one of which is harder (e.g., tungsten carbide) than the other (e.g., steel). 
     Embodiments of the apparatus  17  may further comprising a sleeve  29  located between the bore  31  of the housing  25  and the piston  23 . The sleeve  29  may be stationary with respect to the housing  25 . The piston  23  may be movable with respect to the sleeve  29  and housing  25 . In some versions, both axial ends of the sleeve  29  may be sealed with respect to the bore  31  of housing  25 . 
     The sleeve  29  may be consumable. The sleeve  29  may comprise a material that is harder than a material of the housing  25 . For example, the housing may be some form of steel, and the material of sleeve  29  may comprise at least one of tungsten carbide, a ceramic, stabilized zirconia, alumina, and silica. Like the sleeve  29 , the orifice  35  may be consumable and comprise a material that is harder than a material of the housing, and the orifice material comprises at least one of those same materials. 
     The piston  23  and the sleeve  29  may include shoulders  42 ,  43 , respectively that abut each other in the closed position ( FIG. 2 ). The shoulders  42 ,  43  may be axially spaced apart in the open position ( FIG. 3 or 4 ). The shoulders  42 ,  43  may comprise at least one of upper shoulders and lower shoulders. In some versions, the piston  23  may have a range of axial travel in a range of about 1 inch to about 6 inches. 
     In addition, embodiments of the sleeve  29  may comprise a sleeve aperture  33  that registers with the aperture  27  in the housing  25 . The sleeve aperture  33  may be smaller than the aperture  27  in the housing  25 . 
     In some versions, at least some fluid leakage through the aperture  27  is permitted when the piston  23  is in the closed position. In other words, the aperture  27  is not necessarily sealed to stop fluid leaks when the piston is in the closed position. For example, up to about 5% of the fluid entering the apparatus  17  may be permitted to leak through the aperture  27  when the piston  23  is in the closed position. 
     The apparatus  17  may further comprise a labyrinth seal  65  ( FIG. 7 ) between the housing  25  (or sleeve  29 , if present) and the piston  23 . The labyrinth seal  65  may be formed on an exterior of the piston  23 , or could be on the inner surface of housing  25  or sleeve  29 , if present. 
     Embodiments of the spring  41  may have a spring rate and may be configured to apply a force that is substantially constant over a range of axial movement of the piston. For example, the spring  41  may have a spring rate in a range of about 10 lb/in to about 70 lb/in. Examples of the spring  41  may comprise t least one of a coil spring, a Belleville spring stack and a polymer spring. In some embodiments, there is a frictional force between the housing  25  (or sleeve  29 , if present) and the piston  23 . The spring  41  may have a compression preload, such that the frictional force is less than about 5% of the compression preload. 
     The apparatus may further comprise a wash pipe  61  mounted to the piston  23 . The spring  41  may be located between the bore  31  of the housing  25  and the wash pipe  61 . Embodiments of the wash pipe  61  may be sealed to the piston  23  at one axial end and to the housing  25  (e.g., a sub  13  in  FIG. 7 ) at the other axial end. The wash pipe  61  may comprise at least one hole  63  for communicating fluid to and from the spring  41 . Pressure generated by fluid flow through the hole  63  is configured to act as a damper for the axial motion of the piston  23 . 
     In some embodiments, the spring rate may be sufficiently low and the spring  41  is preloaded such that the force provided by the spring  41  is substantially constant over its operating range. In addition, the spring force may be sufficiently high such that at least about 95% of the resistance to downhole movement of the piston  23  may be provided by the spring  41  and not by unpredictable forces like friction. 
     In other embodiments of the tool  17 , the amount of fluid flow through the center (i.e., orifice  35 ) of the tool  17  is substantially constant regardless of the fluid pressure, flow rate, fluid density, etc. The spring rate may be selected such that it is between about 10% and about 15% of the compression preload on the spring  41 . Such a spring  41  may have a relaxed length that is about 2.5 times its compressed length. For example, a spring  41  having a spring rate of 25 lb/in may be compressed to provide a spring force or pre-load of 250 lbs in the compressed state (i.e., when the tool  17  is in the closed position). In order to move the piston  23  a distance of 1.5 inches, the spring force increases by 1.5 times the spring rate. In this example, 250 lbs+(1.5 in×25 lb/in)=282 lbs. Since the fluid pressure difference through the orifice  35  increases with the square of the flow rate, the axial fluid flow rate through the orifice  35  of the tool  17  can be considered to be substantially constant. The actual amount of increase in flow rate at the point where the piston moves to the point where the apertures are fully open can be calculated as increasing by a factor of the square root of the ratio of spring force on the piston in the open position to the spring force on the piston in the closed position, or:
 
Flow(open)=Flow(closed)×sqrt(282/250)
 
Flow(open)=Flow(closed)×1.06.
 
     So, even though the spring force increases by 13% (282/250) as the piston moves into an open position, the flow that is allowed to pass axially through the tool only increases by 6%. 
     Should the tool be configured such that the rate was 15% of the preload, the preceding calculation would be done as follows:
 
Flow(open)=Flow(closed)×sqrt(306.25/250)
 
Flow(open)=Flow(closed)×1.10.
 
     Therefore, in the case where the spring rate is configured to be 15% of the preload value, with a 1.5″ axial movement of the piston the axial flow through the tool increases by 10%. 
     In other embodiments, a method of controlling fluid flow through a drill string may comprise operating the drill string to drill a hole in an earthen formation; pumping fluid through the drill string to a mud motor such that substantially all of the fluid is flows axially to the mud motor and substantially none of the fluid is radially diverted out of the drill string; and then increasing a flow rate of the fluid such that some of the fluid is radially diverted out of the drill string before reaching the mud motor, and a remainder of the fluid is flows axially to the mud motor. The valve opening may be proportional to the fluid flow rate. Pumping may comprise insufficient fluid pressure to overcome a mechanical force biasing a valve to a closed position. In some versions, increasing the flow rate may comprise opening a valve with fluid pressure that overcomes a mechanical force biasing the valve to a closed position. In other versions, increasing the flow rate may comprise variably controlling an amount of fluid that is radially diverted and the remainder of the fluid flowing axially to the mud motor. 
     Embodiments of a method of controlling fluid flow through a drill string may comprise operating a drill string to drill a hole in an earthen formation; pumping fluid through the drill string; closing a piston in the drill string to direct substantially all of the fluid to a mud motor; and then changing a parameter of the drill string such that the piston moves to an open position allowing at least a portion of the fluid to be diverted away from the mud motor. 
     When operating the tool, the impact of tool  17  that will be noticed at the surface of the well is that once the flow rate is increased to the point that the tool opens, the stand pipe pressure (or surface operating pressure) will increase more slowly with any further flow rate increases. Thus, once the piston in the tool begins to open (i.e., from one of the partially open positions to the fully open position), the fluid pressure does not substantially increase even with an increase in fluid flow rate. This is due to the fact that pressure of the fluid at the surface is a function of the drilling fluid flow rate through the surface piping, the drill pipe, and the bottom hole assembly (BHA, or MWD, mud motor, drill bit, etc.). As fluid flow opens the tool, an increasing amount of fluid bypasses the BHA through the radial aperture. Thus, even though the fluid flow rate may increase, the fluid pressure through the BHA is substantially constant. Increases in fluid pressure can originate from more fluid flow through the surface piping and the drill string. 
     For example, the tool  17  may be configured with the following constants. The ID of most of the tool components is about 2 inches, which will be the number used in flow calculations for Bernoulli&#39;s equation. The piston/orifice combination may be considered a single part for these purposes. Further, for the purposes of calculation it can be thought of as a toroid (donut) shape with a cross-sectional area that is a function of its ID and OD and will, in conjunction with the orifice pressure drop (delta P), determine the downward force that the piston applies to the spring. The OD of the piston may be 3 inches. The ID of the orifice may be determined based on flow rate. 
     In this example, the spring has a spring rate of 25 lb/in and is compressed (preloaded) in the closed state such that it applies a force of 200 lb on the piston. The spring may be compressed 8 inches for this example. Incidentally, and not considered in this calculation, the force on the piston increases slightly as it moves downwards. If the piston moves down by one inch the force will increase by 25 lbs to 225 lbs. 
     In one example, the tool may be set up so that only 250 gpm of fluid will go axially through the tool and that any increase in flow rate will be allowed to exit through the radial apertures. A flow rate of 250 gallons per minute is equivalent to 962.5 cubic inches per second. In this example, the density of the fluid flowing through the tool can be about 10 ppg (pounds per gallon), or 6.9 slugs/cubic ft. 
     This may comprise an iterative calculation (where the orifice diameter determines the pressure drop at a given flow rate, but it also can determine the cross sectional area over which the pressure is applied. Thus, the calculation could be performed many times. However, the ID does not drastically affect the area as much as it affects pressure drop. Accordingly, a good starting estimate for orifice size is sufficient to bring the calculation to a satisfactory conclusion. 
     For example, if the orifice ID may be estimated at 1.2 inches. If the piston has an OD of 3.00 inches, then the cross sectional area is: 
     A=pi*((Piston OD/2)squared−(Orifice ID/2)squared)=5.93 sqin. This is the area that the delta P acts on to push against the spring. 
     With this area, the pressure drop (delta P) that will start to move the spring is: 
     deltaP=preload force/cross sectional area. 
     So, delta P=200 lb/5.93 sqin=33.7 psi. Or, 4853 lbs/square foot. 
     The velocity of the fluid may be determined as it goes through the 2″ ID section of the tool. If the design goal is 250 gpm, velocity may be calculated as V=Q/A where Q is the volume flow rate. For consistent units, the calculation in feet per second is: for flow rate 962.5 cubic inches per second, and area is 3.14 sq in, the inlet velocity is 306.4 in/second or 25.5 ft/second. 
     Bernoulli&#39;s equation for pressure drop across an orifice is:
 
Delta  P =(density×(orifice fluid velocity)squared)/2−density×(inlet fluid velocity)squared)/2
 
     The delta P and inlet velocity are known, and the equation may be configured for orifice velocity.
 
Orifice Velocity=sqrt((2*delta  P /density)+(inlet fluid velocity)squared)
 
     Thus, Orifice velocity=sqrt((2*4853/(6.9))+(25.4)squared) 
     Orifice Velocity=45.3 ft/s 
     Converted to in/s, velocity is 543.6 in/s 
     And back calculating an orifice area, A=Q/V, so A=962.5/543.6=1.77 sqin. 
     And finally, the orifice diameter becomes sqrt(4*Area/pi)=sqrt(4*1.77/3.14159) 
     Diameter=1.50 inches. 
     This calculation provides an orifice diameter of 1.50 inches gives a pressure drop of 33.7 psi at a flow rate of 250 gallons per minute. This calculation is slightly different from the original estimate of 1.20 inches. The area difference that this equates to is 5.3 inches squared as opposed to the original estimate of 5.93 inches, which is a difference of 0.63 square inches or 10%. The formula may be recalculated with this new estimate to yield a more precise value. With a new estimate of a 1.5 inch orifice, recalculating the numbers provides an orifice value of 1.48 inches. A value of 1.48 inches is sufficiently close to the previous iteration value of 1.50 that the calculation can be considered to be complete. 
     Embodiments of the tool described herein solve the problems described above with a piston assembly that moderates the amount of flow that exits the tool. The holes in the sides of the tool can be partially closed to change their size. As the holes are made smaller, a larger portion of the flow is directed downward through the motor. As the holes are enlarged, more of the flow is directed radially outward to bypass the motor and yet still aid in the hole cleaning process. The moderation of hole size can be done very quickly, typically in a fraction of a second. Rapid hole size selection addresses issues such as motor stalls and stick-slip, which can occur and can be resolved very quickly. 
     In some embodiments, the piston assembly comprises a sleeve that slides axially to open or close one or more holes in the tool. The holes may comprise a variety of shapes, such as axially elongated shapes. An orifice is attached to the sleeve to generate a pressure difference across the orifice that depends on the amount of fluid flow. Pushing the sleeve and orifice upwards is a spring with a spring rate that is as low as is reasonable given the other mechanical constraints of the tool. The spring may be preloaded such that a high amount of force is required to make the sleeve initially move from the seated position, but relatively low additional force may be required to push the sleeve down to its fully open position. Thus, the position of the piston may be correlated with the amount of fluid flow that exits through the side of the tool, rather than the amount of flow that is directed down hole to the motor. Accordingly, the spring may have a relatively constant force over its range of travel. The downward force from the fluid is generated by flow through the orifice. Since the downward force balances with the upward spring force, the flow through the orifice may remain relatively constant as well. Fluid flow that is in excess of an amount required to push the sleeve down may be directed out the side of the tool. 
     A motor “stalls” when its rotor stops turning and fluid flow is backstopped such that the fluid stops flowing through the motor. With the embodiments described herein, motor stalls are avoided since pressure drops through the orifice allow the sleeve to move upward to close the radial holes and direct more fluid down through the orifice to the motor where it is needed to correct the stall. 
     An embodiment of a tool described herein solves the previously described problems by including a valve mechanism that moderates the amount of flow that exits the tool. The holes in the side of the tool are designed so that they can be partially covered to change their size. As the holes are made smaller, a larger portion of the flow is directed downward through the tool to the motor. As the holes are enlarged, a more significant portion of the flow is directed out to the annulus to bypass the motor yet aid in the bottom hole cleaning process. The moderation of the size of these holes may be done quickly, typically in much less than one second, to avoid motor stalls and stick-slip. 
     In some versions, the valve mechanism is a sleeve that slides down to open holes in the tool that are axially elongated. Attached to this sleeve is an orifice that generates a pressure difference across it that depends on the amount of flow. Pushing the sleeve and orifice upwards is a spring with a rate that is as low as is reasonable given the other mechanical constraints of the tool. This spring is preloaded such that a high amount of force is required to make the sleeve move from the seated position but relatively little additional force pushes the sleeve all the way down into it fully open position. In this way the position of the valve is correlated with the amount of flow that exits through the side of the tool and not with the amount of flow that is directed down parts. That is, the spring can have a relatively constant force over its range of travel. And since the downwards force that balances the spring is generated by flow through the orifice, that means that the flow through the orifice will stay relatively constant as well. Any amount of flow that is higher than the flow required to push the sleeve down to the point where a slot becomes open is directed out the side of the tool. 
     The spring force that works to push the piston up against the flow and pressure reduce force, and a damping device or “dash pot” that works to provide a force that is opposite of the direction of movement may be used. Including a damping device slows the movement of the main sleeve or piston to prevent the sleeve from building up enough momentum that it significantly moves past its point of equilibrium in the event of a change of drilling conditions (either flow or back pressure). Without a damping device and especially if the slot is not sufficiently narrow the piston may overshoot the equilibrium point at which point the hydraulic and spring forces would be reversed and the piston would move back in the other direction, causing the valve to oscillate open and shut rather than to stay at a stable position, as is desired. 
     It is desirable to keep any downhole device, and the damping device or system in particular, as simple as possible to extend longevity and reduce the chance of failure. Additionally, friction does make an adequate means of damping because of the difference between static and dynamic friction. As static friction is generally larger than dynamic friction, the largest damping force would occur when the device is not moving, which is when the least amount of damping force is required, and vice versa. A much better means of damping is a viscous damping system where fluid is forced to flow between two defined volumes as a piston moves. That way, the faster the piston moves, the faster fluid is forced to flow to equalize the volumes. The faster that fluid moves through a small orifice separating the two volumes the larger the pressure drop across that orifice. The larger that pressure drop is, the larger the hydraulic force on the piston, which will always be opposite to the direction of movement. 
     Since the damping force need not be precisely defined and that it needs only to be within a given range, there is no need to know exactly the properties of the fluid that move between the two volumes. Because of this it is possible to use whatever drilling fluid is being used to drill the well as the viscous damping fluid. This allows the design to become quite simple in that there is no need for an oil-filled section, nor the further necessity to have it pressure and temperature compensated to allow for downhole use. Substantially any volume filled with a non-compressible fluid and used downhole must be allowed to change volume to allow for compression and thermal expansion. With the necessity for an oil-filled section removed, the housing of the damping unit can be completely open to the drilling fluid. The bypass orifice or means for fluid to move past the piston can be a quasi-seal around the outside of the piston. For example, a metal piston ring is used. The piston ring allows fluid to move past it as long as the fluid is moving very slowly. A piston ring will not allow high rates of fluid to move past it. 
     The dash pot described herein may comprise two main parts. A housing that is generally a hollow cylinder and defines a volume that a piston can move axially within., may be mounted to the main body or housing of the valve sub. A piston which moves within that housing is mounted to the valve sleeve or piston and moves axially as the main piston moves. The volume defined on each side of the damper piston changes in size as the main piston moves, thereby changing volume, necessitating flow into or out of that volume, and providing a damping force for the main piston. 
     Referring now to  FIGS. 8-11 , an alternate embodiment of a tool  100  is depicted with a damper at its lower end. Tool  100  many comprise any combination of the features and elements described herein for the other embodiments. 
       FIG. 8  shows the tool  100  in a “closed” position. The main piston or sleeve  110  is in the “up” position. A damping housing  103  is mounted to a main body  114  through the use of webbing  115  ( FIGS. 10 and 11 ). The damping piston  106  is connected to the main piston sleeve  110  by a wash tube  101 , and a damping piston shaft  108  therebetween. 
       FIG. 9  shows the tool in an “open” position. With the main piston  110  moved down (for reasons described elsewhere herein), the teardrop-shaped holes  111  (also described elsewhere herein) are exposed in the side of the main body  114 . Holes  111  permit some of the fluid flow to exit to the well bore annulus. Accordingly, there is a reduced amount of fluid flow down through the main orifice  113 . In this way, the pressure drop across the orifice  113  can balance the spring force and so that the main piston  110  moves to a position where equilibrium is maintained. 
     In operation, when the tool  100  is in the open position, the volume  112  within damping housing  103  defined between the damping piston  106  and the interior of the damping housing  103  is larger than it is when the tool is closed ( FIG. 8 ). As the main piston  110  opens or closes and volume  112  changes in size, fluid must enter or exit the volume  112  by flowing past the piston ring quasi-seal  107 , which seals against the inner surface of the damper housing  103 . Using a piston ring  106  instead of a more conventional elastomer seal coupled with a small hole in the damping piston permits greater simplicity and improved reliability. 
     In some embodiments of the tool  100 , the wash tube  101  protects the spring  121  and couples the main piston  110  to the damping piston  106 . The damping piston  106  has a shaft seal  102  within the damper housing  103 . A snap ring  104  secures the damper piston  106  to the damper piston shaft  108 . Another seal  105  is located between the damper piston  106  and the damper piston shaft  108 . The damper piston  106  also has a piston ring quasi-seal  107  that engages the inner surface of the damper housing  103 , while permitting limited fluid flow past it. A damper piston shaft connection spider  109  further supports damper piston shaft  108 , while readily allowing fluid flow through wash tube  101 . 
     Still other embodiments may include one or more of the following items: 
     Item 1. An apparatus for drilling, comprising:
         a housing having an axis, an entrance at one axial end and an exit at an opposite axial end, a radial opening in a side of the housing extending from an interior of the housing to an exterior of the housing;   a sleeve slidably mounted in the interior of the housing that closes or partially closes the radial opening in the housing as the sleeve moves axially upward toward the entrance;   a spring mounted in the housing that biases the sleeve toward the entrance, and an axial aperture in the sleeve is configured to generate a pressure differential as fluid flows through the axial aperture to push the sleeve down against the spring; and   a viscous damping device coupled between the sleeve and the housing for damping axial motion of the sleeve in both axial directions.       

     Item 2. The apparatus of any of these items, wherein the viscous damping device comprises a damper housing and a piston configured to move axially relative to each other in response to axial movement of the sleeve relative to the housing. 
     Item 3. The apparatus of any of these items, wherein the piston is connected to the sleeve and the damper housing is stationary. 
     Item 4. The apparatus of any of these items, wherein the damper housing is connected to the sleeve and the piston is stationary. 
     Item 5. The apparatus of any of these items, further comprising a seal between the piston and the damper housing. 
     Item 6. The apparatus of any of these items, wherein the piston and damper housing are configured to allow fluid communication between an outer diameter of the piston and an inner diameter of the damper housing as the piston moves in both axial directions. 
     Item 7. The apparatus of any of these items, further comprising a quasi-seal between the outer diameter of the piston and the inner diameter of the damper housing to permit fluid communication therethrough. 
     Item 8. The apparatus of any of these items, further comprising a damper piston shaft connecting the piston to the sleeve. 
     Item 9. The apparatus of any of these items, further comprising a wash tube coupled between the sleeve and the piston, the wash tube having a connection spider for coupling to a piston shaft of the piston, and the damper housing comprises a tubular body having damper webbing for coupling the damper to the housing, and both the connection spider and the damper webbing readily permit axial fluid flow through the wash tube and the around the damper housing, respectively. 
     Item 10. The apparatus of any of these items, where a fluid that fills the damping housing is drilling fluid used for drilling operations, such that the viscous damping device operates exclusively with the drilling fluid rather than a separate hydraulic fluid. 
     Item 11. The apparatus of any of these items, wherein the radial opening in the housing, comprises a plurality of elongated slots, the aperture in the sleeve comprises a plurality of apertures in the sleeve, and each aperture comprises a plurality of linearly aligned holes that selectively register with respective ones of the elongated slots. 
     Item 12. The apparatus of any of these items, wherein the plurality of linearly aligned holes are circular, differ in size from each other, and are configured in a teardrop shape. 
     Item 13. The apparatus of any of these items, wherein each aperture is axially offset with respect to other ones of the apertures. 
     Item 14. A method of controlling fluid flow through a drill string, comprising:
         operating the drill string to drill a hole in an earthen formation;   pumping fluid through the drill string to a mud motor, such that substantially all of the fluid flows axially to the mud motor and substantially none of the fluid is radially diverted out of the drill string; then   increasing a flow rate of the fluid such that some of the fluid is radially diverted out of the drill string by a valve before reaching the mud motor, and a remainder of the fluid flows axially to the mud motor; and   viscously damping the valve in both axial directions with an axial damper that is responsive to the axial fluid flow.       

     Item 15. A method of controlling fluid flow through a drill string, comprising:
         operating a drill string to drill a hole in an earthen formation;   pumping fluid through the drill string;       

     closing a piston in the drill string to direct substantially all of the fluid to a mud motor; then 
     changing a parameter of the drill string such that the piston moves axially to an open position allowing at least a portion of the fluid to be diverted away from the mud motor; and 
     viscously damping the piston in both axial directions with an axial damper that is responsive to the axial fluid flow. 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable those of ordinary skill in the art to make and use the invention. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. 
     In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Also, the use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. 
     After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.