Abstract:
A method and apparatus for changing the speed of a drill bit down hole in a fluid-actuated motor, including a positive displacement motor and a hydraulic motor, is disclosed. The apparatus comprises a bypass valve installed in the motor for controlling flow through and around the power section of the motor. When closed, the bypass valve forces all fluid to flow through the power section of the motor, imparting maximum speed to the drill bit. When opened, a portion of the fluid flow is allowed to flow around the power section of the motor, thereby reducing the speed of the drill bit. The bypass valve may be opened or closed mechanically, electrically, hydraulically, pneumatically, or by any other means, including a removable plug.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
   This application claims priority from U.S. Provisional Patent Application No. 60/676,342, filed Apr. 30, 2005, by inventors Kosay El-Rayes, Nazeeh Melhem, and Peter Shwets, entitled “Method for Shifting Speeds in a Fluid-Actuated Motor,” which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to fluid-actuated motors, including positive displacement motors, known as Moineau pump-type drilling motors, and hydraulic motors, and specifically to a fluid-actuated motor having a variable rotor bypass valve installed therein to alter the rotational speed of the drill bit without the need for the motor to be removed from the well. 
   BACKGROUND OF THE INVENTION 
   In the oil drilling industry, there are two traditional methods of drilling an oil well. One is to attach a drill bit at the end of a drill string, apply downward pressure, and rotate the drill string from the surface so that the drill bit cuts into a formation. The problem with this method is that as the hole becomes deeper and the drill string becomes longer, the frictional forces due to the rotation of the drill string down hole increase, especially in deviated and horizontal wells. 
   The second method is to place a motor down hole near the drill bit. This method requires a special type of motor (or pump) called a positive displacement motor, or PDM. The PDM is also referred to in the oil drilling industry as a Moineau pump or mud motor. It has a long spiral rod inside of it, called a rotor, which spins inside of a stator as fluid is continually pumped down the drill string through the motor. The speed at which a mud motor rotates depends upon the internal geometry of the motor, the flow rate of the fluid that is pumped down the drill string to turn the motor, and the resistance of the formation against the drill bit. Although the pumping of the fluid down the drill string is one factor that determines the speed at which the drill bit rotates, the circulation of the drilling fluid serves other purposes as well. For example, it circulates the cuttings out of the hole and cools the drill bit as it cuts into harder formations. 
   When drilling a hole, an operator frequently encounters the need to change the rotational speed of the drill bit. When drilling through harder, more difficult formations, slower bit speeds are required. When encountering softer formations, an operator may select a faster drill speed to drill quickly through the formation. If an operator cannot change the flow rate of the fluid pumped down the drill string because, for example, the operator needs to maintain some minimum flow rate to circulate the cuttings out of the hole, then the only other option to change drill speeds is to change the internal geometry of the motor. 
   Prior art motors do not have the ability to change their internal geometries down hole without bypassing a portion of the fluid flow outside the drill string. This has at least two deleterious effects. First, not all of the fluid pumped down a drill string will pass through the drill bit to cool it, and, second, not all of the fluid flow pumped down the drill string will be used to circulate the cuttings out of the hole. 
   One way to overcome these problems is to remove the drill string from the hole and replace the motor with one having a different internal geometry or to modify the internal geometry of the motor used. The removal of the drill string to replace a motor is time consuming and expensive. Consequently, there is a need in the art for a method and/or apparatus that allows an operator to change the internal geometry of mud motors down hole without passing a portion of the fluid flow outside the drill string. 
   SUMMARY OF THE INVENTION 
   The present invention allows an operator to change the rotational speed of the drill bit by causing a portion of the fluid that is pumped through the drill string to bypass that part of the power section of a motor that imparts rotational motion on the drill bit without passing any of the fluid outside of the drill string. This is accomplished by means of a bypass valve installed inside, above, or below the power section of the motor. 
   The bypass valve separates the fluid flow through the power section into two paths. One path is directed through that part of the power section that causes the drill bit to rotate while the other path is directed around it. When the bypass valve acts to cause all of the fluid to flow through the power section of a motor, the drill bit will rotate at maximum speed. When the bypass valve acts to bypass a portion of the fluid through a port in the power section, the drill bit will rotate at a slower speed. The actual internal geometry of the fluid flow through the power section in conjunction with the fluid flow pressure maintained at the mud pump determines the actual speed of rotation. After the bypass valve separates the fluid into two flow paths, the flow is recombined inside the motor before it is channeled to the drill bit. This allows all of the fluid that flows down the drill string to cool the drill bit and to circulate the cuttings back up to the surface without any detrimental impact on system performance. 
   In underbalanced drilling, the fluid pumped down the drill string is composed of a mixture of fluid and gas. The fluid that is diverted around the power section when the bypass valve is open may then comprise the gas. 
   In one embodiment, the bypass valve is attached to the bottom portion of the rotor of a typical mud motor. As mentioned above, a rotor is a long spiral rod that spins inside of a stator. The fluid that is pumped down the drill string passes through and around the rotor. The portion of the fluid that passes around the rotor causes the rotor to spin. The portion of the fluid that passes through the center of the rotor has no effect on the rotor&#39;s rotational speed. By placing a bypass valve along the fluid path through the center of the rotor, the fluid that passes through the center of the rotor can be manipulated and controlled. In this embodiment, closing the bypass valve blocks the fluid from passing through the center of the rotor and forces all of the fluid flow around the rotor. This configuration imparts maximum rotational speed to the drill bit. Opening the bypass valve allows a portion of the fluid flow to pass through the center of the rotor. By altering the flow paths inside the motor, the rotational speed of the drill bit can be manipulated and set. 
   The bypass valve attaches inside of a motor and consists of a rotor adapter and a housing. The rotor adapter attaches to the end of the rotor and has an inner diameter, or cavity, that allows fluids to pass from the center of the rotor into the housing. A cam inside the housing is configured to rotate axially along the flow path each time the mud pump controlling the fluid flow down the drill string is cycled on and off. When the mud pump is turned on, fluid flow forces the cam into contact with one or more stationary splines on the inner diameter of the housing. As the cam continues to move forward, an outer axial surface on the cam contacts an angled surface on the spline and forces the cam to rotate axially along the flow path. Each time the cam is rotated, a different set of slots along the outer diameter of the cam slide in between splines on the housing. The length of each slot changes with each rotation. When the flow pump is initially turned on, the slot that initially slides along the splines is short, resulting in the cam traversing only a part of the path downwards towards the lower end of the housing. When the flow pump is turned off, a biasing spring at the bottom of the housing pushes the cam upwards to its original position. The next time the flow pump is turned on, the cam is rotated again and a longer slot is selected, allowing the cam to traverse the full length of the path inside the housing as it is pushed downwards by the fluid pressure against the biasing spring at the bottom of the housing. When the cam is allowed to traverse the full length of the housing, a radial exit hole in the cam aligns with a radial exit hole in the housing to provide a flow path from the center of the rotor to the inside diameter of the motor containing the bypass valve. This allows a portion of the fluid in the drill string to flow through the center of the rotor. When a shorter slot is selected, the radial holes in the cam do not align with the radial holes in the lower housing. Consequently, the flow of fluid through the center of the rotor is blocked and all fluid passes around the rotor, allowing the rotor to turn at its maximum designed speed. 
   Each time the cam is rotated, a longer or shorter slot is alternatively selected, and the bypass valve is alternatively opened or closed. In another embodiment, three different slot lengths may be used and alternatively selected, one slot fully closing the bypass valve, another slot partially opening the bypass valve, and the last slot fully opening the bypass valve. In such an embodiment, the operator may select one of three speeds for the motor. 
   In other embodiments, the bypass valve may be opened and closed by an electrical motor installed in the tool. A wireline running tool having electric cables is inserted: into the bore and connected to the electric motor. The wireline running tool applies electric power and signals to the motor to open and close the bypass valve. 
   The valve may also be configured to open and close mechanically. A wireline running tool is inserted into the bore and physically connected to a valve that opens by mechanical pull. An upward force applied to the wireline tool physically opens the valve. Alternatively, the valve may be configured to open when heavy force is applied to the top of the bypass valve. The force may be a heavy bar dropped on top of the valve while the valve is inside the drill string causing the valve to shift to an open or closed position. 
   The bypass valve may also be configured to open by hydraulic, pneumatic, or other means. Electrical, mechanical, hydraulic, and pneumatic means of opening and closing valves in a drill string are well known in the art. 
   In even another embodiment, the amount of fluid that flows through the bypass valve when open is controllably selected by the size of a replaceable nozzle that installs inside the cam. The replaceable nozzle is configured to restrict a certain amount of flow through the cam and the housing when the bypass valve is open, thereby allowing a drilling operator to pre-set the speed of the drill bit. 
   In still another embodiment, the bypass valve may also be configured to open and close automatically based upon the type of formation encountered during drilling. When the drill bit encounters a harder formation, more weight is needed to press through it. The increased weight increases the friction on the bit and the pressure experienced by the motor. The bypass valve can be configured to respond to the increased pressure by, for example, opening one or more spring-loaded outlet valves. When the increased pressure experienced by the motor overcomes the closing forces of the spring-loaded outlet valves, the outlet valves open, diverting a portion of the fluid flow around the power section of the rotor and slowing the speed of the drill bit. The spring-loaded outlet valves may be configured to adjust to the amount of pressure experienced by the motor, allowing the amount of fluid to flow around the power section of the motor to be a function of the pressure experienced by the motor. 
   In addition to the above embodiments, a removable plug may be dropped down the drill string to plug the bypass valve, preventing the bypass valve from diverting fluid around the power section of the motor or, alternatively, closing off all fluid flow through the motor. The removable plug may be pre-installed and removed by a wireline running tool by applying an upward force that shears the plug from its pre-installed position. Both the installation and removal of plugs from downhole tools are well known in the art and are applicable to a downhole tool having a bypass valve described herein. 
   A method of shifting speeds of a motor consistent with the description above is as follows: installing on a drill string a motor capable of changing rotational speeds of a drill bit; drilling into a first formation; opening a bypass valve to change the rotational speed of the drill bit; and continue drilling into the first formation or into a second formation. An alternate method consistent with automatic selection of drill speeds is as follows: installing on a drill string a motor capable of changing speeds; drilling into a formation; sensing a change in the formation resulting from increased or decreased frictional forces on the drill bit; and opening or closing a valve to change the rotational speed of the drill bit. 
   The invention described herein is not limited to mud motors or to applications for drilling through down hole formations, but applies to any motor that uses fluidic means for turning a drive shaft where control of the rotational speed of the motor is accomplished by manipulating the flow of fluid through the power section of the motor, such as a turbine motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the open position attached above the power section of the motor. 
       FIG. 2  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the closed position attached above the power section of the motor. 
       FIG. 3  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the opened position attached below the power section of the motor. 
       FIG. 4  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the closed position attached below the power section of the motor. 
       FIG. 5  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the opened position attached inside the power section of the motor. 
       FIG. 6  is a view of an exemplary embodiment of a positive displacement motor having a bypass valve in the closed position attached inside the power section of the motor. 
       FIG. 7  is an exploded view of an exemplary embodiment of a bypass valve. 
       FIG. 8  is a view of the exemplary embodiment of the bypass valve of  FIG. 7  with the components interconnected. 
       FIG. 9  illustrates the movement of the index ring relative to the housing and flow piston when fluid flow pressure is initially applied. 
       FIG. 10  illustrates the positioning of the index ring, flow piston, and housing relative to one another after the fluid flow pressure has been initially applied. 
       FIG. 11  illustrates the alignment of a slot milled on the outer radial surface of the index ring with a spline in the inner diameter of the housing when fluid flow pressure is applied a second time. 
       FIG. 12  is a two-dimensional layout of the slotted outer surface of the index ring consistent with the exemplary embodiment of  FIGS. 7-11 . The figure shows the pattern of alternating between a deep slot, item  280 , and a shallow slot, item  250 . 
       FIG. 13A  is a view of an exemplary embodiment of a removable flow plug inserted into an exemplary embodiment of a positive displacement motor. 
       FIG. 13B  is an enlarged view of a portion of the exemplary embodiment of the removable flow plug of  FIG. 13A . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagram of an exemplary embodiment of a typical positive displacement motor  10  (“PDM”), or mud motor. The top side  15  of the motor connects to a drill string (not shown). The bottom side  20  connects to a drill bit  185 . The power section  40  comprises a rotor  42  and stator  45 . When a mud pump is turned on, fluid  70  enters the drill string, flows through the power section  40  and exits the bottom side  20  of the motor. 
     FIG. 2  is a diagram of an exemplary embodiment of a typical positive displacement motor  10  having a bypass valve  150  attached above the power section  40  of the motor  10 ;  FIGS. 3 and 4  show the bypass valve  150  attached below the power section  40  of the motor  10 ; and  FIGS. 5 and 6  show the bypass valve  150  attached inside the power section  40  of the motor. Because operation of the bypass valve is similar regardless of whether it attaches above, below, or inside the power section of a motor, only the operation of the bypass valve of  FIGS. 1 and 2  need be explained. 
   Referring to  FIG. 1 , bypass valve  150  is installed inside motor  10  in fluid flow path  70  in the drill string. When bypass valve  150  is open, a portion of the fluid flow  175  in path  70  passes through bypass channel  170 . In a typical mud motor having a rotor  42  and stator  45 , the flow around the rotor  42  is shown by flow path  180  and the flow through the center of the rotor  42  is shown by bypass path  175 . In other motors, such as turbines, bypass path  175  represents flow through a bypass port in the turbine power section and flow path  180  represents flow through the turbine blades or fins. Because only a portion of the fluid flow from the drill string flows around the rotor  42  when bypass valve  150  is open, the rotor  42  rotates at less than its maximum speed. 
   When bypass valve  150  is closed, as shown in  FIG. 2 , all fluid flow is forced to flow around the rotor  42 . In this configuration, bypass flow  175  through the center of the rotor  170  is blocked. For other motors, such as a turbine, bypass flow  170  represents the flow through a bypass port in the turbine power section, and flow path  180  represents flow across the turbine blades or fins. Thus, when bypass valve  150  is closed, all flow is forced across the turbine blades or fins and the turbine rotates at its maximum speed. 
   When bypass valve  150  is open ( FIG. 1 ), the fluid flow  70  through the drill string is separated into two flow paths, bypass path  175  and flow path  180 . The two paths are recombined at  160  and sent to the drill bit  185 . None of the flow through bypass path  175  is diverted outside the drill string. By recombining the two flow paths, all fluid flow pumped down the drill string from the surface is used to cool the drill bit and circulate cuttings out of the hole. 
   Referring to  FIG. 7 , a mud motor bypass valve  100  of the type consistent with the present invention includes a rotor adapter  110 , a housing  120 , a replaceable nozzle  140 , a nozzle piston  145 , a spring  160 , and a cam  130 . The rotor adapter  110  connects to the bottom of a mud motor rotor (not shown) on a drill string, though in other embodiments, it may connect to the top of the rotor. The bottom of the housing  120  attaches to the top of the motor drive shaft (not shown). The cam  130  includes an index ring  130   a  and a flow piston  130   b , both with milled outer, axial surfaces  133  and  230  for axially rotating the index ring  130   a  relative to the flow piston  130   b . The bypass valve  100  of  FIG. 7  replaces the upper U-Joint of a drive shaft in a typical mud motor. 
   Referring to  FIG. 8 , when the mud pump is turned on at the surface, fluid is pumped down a drill string to entrance cavity  112 . When the fluid enters the entrance cavity  112 , pressure builds up along the top surface  131  of the nozzle piston  145  and forces the index ring downwards in tandem with the flow piston  130   b  and against the upward biasing force of a spring  160 . The fluid flowing around the rotor does not pass through the bypass valve  100  until the radial exit holes  130   c  ( FIG. 10 ) on flow piston  130   b  ( FIG. 10 ) align with radial exit holes  120   a  ( FIG. 10 ) on housing  120 . 
   Referring to  FIGS. 8 and 9 , flow piston  130   b  has a slotted surface  210  ( FIG. 8 ) for sliding along spline  220  ( FIG. 9 ), which is part of housing  120 . Spline  220  prevents flow piston  130   b  from rotating inside housing  120 . As index ring  130   a  moves downward, milled surface  230  engages spline  220  on the housing at slanted surface  240 . Slanted surface  240  corresponds to milled surface  230  for engaging the index ring  130   a  and causing the index ring  130   a  to rotate relative to flow piston  130   b . Rotation continues with continued downward movement of the index ring  130   a  until spline  220  reaches slotted surface  250 , as illustrated in  FIG. 10 . Referring now to  FIG. 10 , at this point, slotted surface  250  impedes any further downward movement of index ring  130   a , and radial exit holes  130   c  on flow piston  130   b  remain above radial exit holes  120   a  on housing  120 , preventing the fluid entering through entrance cavity  112  from escaping through the housing  120 . Housing  120  is configured to block fluid flow through the bypass valve  100  unless the radial exit holes  130   c  on flow piston  130   b  aligns with radial exit holes  120   a  on housing  120 . The index ring  130   a , flow piston  130   b , and housing  120  remain in their relative positions, as shown in  FIG. 10 , for as long as fluid pressure is applied to the drill string from the surface. In this configuration, bypass valve  100  effectively blocks all fluid passing through the center of the rotor resulting in the drill bit turning at its maximum speed. 
   When fluid pressure is released from the drill string, spring  160  ( FIG. 8 ) forces flow piston  130   b  and index ring  130   a  upwards towards its initial position. Index ring  130   a , however, remains partially rotated. As the spring pushes index ring  130   a  upwards, milled surface  260  ( FIG. 10 ) passes above spline  220 . Spline  220  no longer holds index ring  130   a  in place relative to flow piston  130   b . Milled surfaces  230  and  290  cause index ring  130   a  to rotate relative to flow piston  130   b  by sliding along milled surfaces  270  on flow piston  130   b  due to the continually applied force of reset spring  165  ( FIG. 8 ) pushing the index ring  130   a  ( FIG. 10 ) downwards against flow piston  130   b  ( FIG. 10 ), allowing slot  280  ( FIG. 10 ) to position itself above spline  220  to cause additional rotation the next time fluid pressure is applied to the drill string. 
   Referring now to  FIG. 11 , when pressure is reapplied to the drill string, index ring  130   a  is again forced downwards towards spline  220 . This time, however, slanted surface  240  on spline  220  contacts the top of angled surface  290  next to slot  280 , causing index ring  130   a  to rotate until slot  280  is aligned with spline  220 , as shown in  FIG. 11 . Slot  280  is longer than slot  250  ( FIG. 10 ) so that index ring  130   a  will continue to move downwards until spline  220  contacts surface  300 . At this point, radial exit holes  130   c  on flow piston  130   b  will be aligned with radial exit holes  120   a  on the housing  120 . This alignment opens a flow path between entrance cavity  112  and the annulus  310  ( FIG. 1 ) between housing  120  and the motor  10  ( FIG. 1 ). As fluid flows along this path, less fluid flows around the rotor, causing the speed of the rotor to decrease. The fluid flowing through and around the rotor are then recombined in the annulus and sent to the drive shaft and drill bit. 
     FIG. 12  is a two-dimensional rollout diagram of the milled outer surface of the index ring  130   a . The figure shows that in one embodiment, slots  280  alternate with slots  250  along the surface. Referring now to  FIGS. 10-12 , the length of slots  280  are milled such that when the index ring  130   a  moves downwards towards the bottom of the housing  120 , the radial exit holes  130   c  of the flow piston  130   b  will align with the radial exit holes  120   a  of housing  120 . The length of slots  250  are milled such that when fluid pressure is applied to the drill string and index ring  130   a  is pushed downwards towards the bottom of the housing  120 , spline  220  will hold the index ring and flow piston  130   b  in a position where the radial exit holes remain out of alignment. Because the index ring  130   a  rotates only one slot at a time each time power to the mud pump is cycled and because slots  250  and  280  are milled in alternating succession, the bypass valve will alternate between an open position and a closed position each time the mud pump is cycled. In this configuration, the mud pump rotates at two speeds, one speed corresponding to the open position and another speed corresponding to the closed position. 
   In other embodiments, the slots shown in  FIG. 12  may have more than two different lengths and cause more than two different sets of radial exit holes  130   c  in the flow piston to align with radial exit holes  120   c  in the housing. In this configuration, the amount of fluid flow that can be bypassed will vary with each setting resulting in a motor having more than two selectable speeds. 
     FIG. 13  shows a typical positive displacement motor  10  having a bypass valve (not shown) consistent with the invention herein and having a removable flow plug  420  for plugging the bypass valve. In this embodiment, the flow plug  420  is pre-installed at the surface and removed by a wireline tool by shearing the plug  420  from the valve. The plug  420  prevents fluid from entering the bypass channel  170  and thereby changing the speed of the motor when the bypass valve is open. If the bypass valve is of the type that opens and closes by cycling the mud pumps, the removable flow plug  420  prevents fluid flow pressure from entering the bypass channel  170  and activating the cam. The mud pump may be cycled any number of times without opening and closing the bypass valve. Other types of removable plugs for plugging an annulus in a downhole tool are well known in the art and can be used for this type of application. 
   It will be apparent to one of skill in the art that described herein is a novel method and apparatus for adjusting the speed of a mud motor down hole without the need to pull the motor out of the hole. While the invention has been described with references to specific preferred and exemplary embodiments, it is not limited to these embodiments. The invention may be modified or varied in many ways and such modifications and variations as would be obvious to one of skill in the art are within the scope and spirit of the invention and are included within on the scope of the following claims.