Moineau pump type drilling motors or downhole positive displacement drilling motors are extensively used for drilling boreholes from the surface to a desired location within a selected underground hydrocarbon producing formation. To operate the drilling motor, a pressurized fluid is pumped into and circulated through a progressing axial fluid cavity or chamber within the power unit of the motor formed between a helical-lobed rotor and a compatible helical-lobed stator comprising tile power unit. The force of the pressurized circulating fluid being pumped into the axial cavity between the rotor and stator causes the rotor to rotate within the stator. The rotation of the rotor is transferred to the drill bit through a driveshaft.
Various circulating fluids may be used to actuate the downhole motor, such as mud, water, air or other gases. Thus, the hydraulic or pneumatic energy of the pressurized circulating fluid is converted into the mechanical energy of the rotating driveshaft and the attached drill bit. Further, the bit rotation speed or rotations per minute ("RPM") is directly proportional to the circulating fluid flow rate between the rotor and stator. If for any reason the motor is operated above a maximum desirable RPM for the particular motor, there is a tendency for damage and increased or accelerated wear to the motor.
Excessively high or damaging RPMs of the driveshaft have been found to particularly occur in positive displacement motors operated or actuated by a compressible fluid such as air or other gases. Specifically, excessive RPMs have been found to occur whenever the motor is pulled up off of the bottom of the drilled borehole or the weight on bit is otherwise removed from the drill bit or significantly decreased such as when the weight on bit is drilled off.
The decreased weight on bit results in a runaway condition caused by the sudden lowering of the pressure and consequent expansion of the compressed fluid, such as the compressed gas or air, inside the drill string and motor normally present during the drilling mode or performance of the drilling operation. As indicated, the pressure drop across the motor's power unit, including the rotor and stator, normally provides the energy for the creation of the rotary motion of the driveshaft and bit when torque is generated at the bit in the drilling mode. Thus, an excessive or sudden reduction in pressure within the motor has a tendency to create excessive RPMs of the driveshaft. In other words, the decreased weight on bit reduces the torsional resistance to the rotor of the motor, which reduces the pressure resistance and thus the pressure within the motor. The reduction in pressure within the motor permits the expansion of the compressed fluid resulting in excessive motor speed and rotation of the driveshaft.
This runaway condition is particularly prevalent when the motor is actuated by compressed air or gas as compared with the same motor driven by a flow of drilling mud. In fact, it has been found that runaway RPMs when utilizing compressed air or gas can be as high as 5 to 8 times the rated maximum RPM for the motor. Consequently, serious damage and accelerated wear results to both the rotating and stationary parts comprising the motor.
Several devices and systems exist for controlling the flow of drilling fluid through the power unit which are dependent upon and reactive to the pressure of the drive fluid within the motor.
For instance, U.S. Pat. No. 4,339,007 issued Jul. 13, 1982 to Clark describes a control system for a progressing cavity hydraulic downhole drilling mud motor for controlling the pressure drop of the fluid through the motor so that it does not become excessive (such as may be caused by increased torsional resistance of the rotor). The control system includes a valve sub attached to an upper end of a power unit including a rotor and stator, which valve sub is located above the rotor and the stator. The valve sub comprises a valve housing secured to the stator and a flow valve linked with the rotor and positioned within the valve housing to control the flow of fluid through the valve housing. The flow valve is movable between an open and closed position in response to the fluid pressure within the motor, however, the valve is normally biased towards the open position.
Further, U.S. Pat. No. 5,351,766 issued Oct. 4, 1994 to Wenzel also describes a flow restrictor for controlling the rate of mud flow through the bearing assembly of a mud lubricated drilling motor. In particular, a first seal, coupled to an outer housing, is biased by springs towards a second seal, coupled to an inner member, to bring it into sealing engagement therewith to form a mechanical seal having a first inner side and a second outer side. A first fluid flow passage extends from the interior of the inner member to the first side of the mechanical seal, while a second fluid flow passage extends from the second side of the mechanical seal to the exterior of the outer housing. A number of grooves extend from the first to the second side of the mechanical seal, which turns the mechanical seal into a flow restrictor.
In operation, drilling mud passes through the first fluid flow passage to the first side of the mechanical seal and then through the grooves from the first side to the second side of the mechanical seal. The mud is then vented to the exterior of the outer housing through the second fluid flow passage. The pressure with which the first seal and the second seal are engaged is determined by the biasing force of the springs applied to the seals. Therefore, the springs are selected based upon the desired flow rate through the mud motor.
U.S. Pat. No. 4,768,598 issued Sep. 6, 1988 to Reinhardt describes a valving apparatus for protecting a downhole fluid pressure motor from excessive fluid pressures within the motor, which apparatus is mounted directly above the motor. The apparatus includes a flow plug and a piston for shifting the position of the flow plug. Upon the occurrence of a predetermined fluid pressure across the motor, the fluid pressure moves the piston upwardly, which concurrently causes an upward movement of the flow plug to produce a flow constriction in the fluid flow path of the pressurized fluid. The upward motion of the piston also opens a bypass flow path around the motor to reduce the fluid pressure being applied to the motor.
If the operator responds to the excess pressure by raising the drill string at the surface, the fluid pressure will be reduced within the motor and the piston will move downwardly to its initial position. Downward movement of the piston results in downward movement of the flow plug and permits the fluid flow path through the motor to be re-established. Thus, the device is actuated by and reactive to the pressure within the motor.
These devices and systems are designed to control the pressure drop or the fluid flow through the motor or to control excessive pressure within the motor. They do not specifically address the runaway condition described above nor are they reactive to or actuated by the weight on bit. However, various attempts have been made to specifically address the runaway condition and to avoid the damage and wear caused by the resulting excessive RPMs. These attempts have not been completely satisfactory.
Several attempts to provide a solution to the runaway condition include a clutch mechanism or clutch arrangement to prevent rotation of the driveshaft when the weight on bit is reduced. For instance, Canadian Patent Application No. 2,071,612 published Dec. 19, 1993 by Wenzel describes a clutch mechanism for preventing an uncontrolled increase in the speed of a drilling motor during air drilling. The clutch mechanism is located within a lubricant filled bearing chamber defined between an outer housing and an inner mandrel. The bearing chamber is sealed to prevent drilling fluids from communicating with the chamber. The clutch mechanism includes a first clutch means secured to the interior of the housing and a second clutch means secured to the exterior of the inner mandrel.
When placed in compression during drilling, the first and second clutch means are spaced apart within the bearing chamber to permit the relative rotation of the housing and inner mandrel. When placed in tension, the first clutch means lockingly engages the second clutch means to prevent the relative rotation of the housing and inner mandrel. The clutch means are preferably comprised of mating teeth or splines to ensure relative rotation does not occur.
Further, U.S. Pat. No. 3,964,558 issued Jun. 22, 1976 to Fogle describes a downhole drilling device including a fluid turbine to produce torque and a positive displacement fluid motor to regulate the speed of an output shaft connected to both the turbine and the motor. Further, Fogle describes an over-running clutch to aid in start-up of the turbine and to prevent overspeed of the turbine. The clutch may be located anywhere in the drive train between the turbine and the motor and is generally described as a one-way overrunning clutch arrangement. No further description of the specific structure of the clutch arrangement is described.
Other solutions to the runaway condition described above have resulted in motors which have a relatively complex or complicated structure and mechanism of operation. For instance, U.S. Pat. No. 5,174,392 issued Dec. 29, 1992 to Reinhardt discloses an apparatus for controlling the power supplied to a drill bit by a downhole fluid powered motor to prevent the motor from rotating the bit at high speeds when there is little or no weight on bit. Further, the apparatus is specifically designed to prevent the high speed rotation of the drill bit while permitting full circulation through the bit. Specifically, when weight is removed from the bit, a bypass is opened and the fluid is directed past the motor and through the drill bit.
When fluid is circulated through the motor, the fluid is directed into the motor and is split into two flow paths. A first path is defined between the rotor and stator of the motor, while a second path is defined through a flexible member contained within the bore of the stator. A bypass seal or valve member is provided within the flexible member for selectively sealing the second flow path. The fluid paths again commingle below the location of the bypass seal or valve member via crossover ports extending between the first and second flow paths. The commingled fluid is then directed through the driveshaft to the drill bit.
The bypass seal is actuated by a centre rod extension which extends through the driveshaft from the bypass seal to an end adjacent the drill bit. The application of weight on bit acts upon the adjacent end of the centre rod extension and thereby moves or actuates the bypass seal.
When little to no weight is applied to the bit, the bypass seal is moved to a position within the bore of the driveshaft such that fluid is permitted to flow through the flexible member. As a result, due to the pressure resistance necessary to pass through the first flow path by rotating the rotor within the stator, the fluid tends to flow through the path of least resistance, being the second flow path. As a result, zero to slight rotation of the rotor only is experienced, while full circulation is maintained through the drill bit.
When weight is applied to the bit, the bypass seal is moved upward by the centre rod extension out of the bore of the driveshaft and into the flexible member for sealing engagement therewith. Drilling fluid cannot therefore pass through the second flow path through the flexible member and is forced into the first flow path, causing rotation of the rotor within the stator. When the motor is picked up off bottom or the weight on bit is drilled off, the bypass seal is again moved out of the flexible member to permit fluid flow and so that the fluid again bypasses the rotor and stator. Alternately, rather than closing the flexible member to prevent flow through the first fluid path, the bypass seal may only act to restrict the flow through the flexible member.
In an alternate embodiment of Reinhardt, as shown in FIG. 10, the bypass seal or valve member is located within the bore of the driveshaft below the level of the cross-over ports such fluid flowing through a fluid path defined between the rotor and stator is directed through the cross-over ports into the bore of the driveshaft. Thus, the bypass valve controls the passage or flow of the drilling fluid through the bore of the driveshaft.
In the alternate embodiment, when weight is removed from the bit, the bypass seal is moved downward to a position within a constricted portion of the bore of the driveshaft to seal therewith and prevent all fluid flow therethrough. Thus, the column of drilling fluid is held in the string. Alternately, the bypass seal may act only to restrict the fluid flow through the bore of the driveshaft. Conversely, when weight is applied to the bit, the weight pushes the bypass seal upwards out of engagement with the constricted portion of the bore of the driveshaft such that fluid may flow past the seal. Thus, fluid flow between the rotor and stator and through the driveshaft is permitted.
Thus, there remains a need in the industry for a device for controlling the runaway condition associated with downhole fluid driven drilling motors when weight on bit is removed from the drill bit. More particularly, there is a need for such a device for use with downhole fluid driven drilling motors, wherein the circulating drive fluid is comprised of compressed gas or air. Further, there is a need in the industry for a drive fluid flow restrictor device or valve for use in a downhole drilling assembly of the type comprising a fluid driven motor.