Rotary expansible chamber devices – Unlike helical surfaces on relatively wobbling rotating...
Reexamination Certificate
1998-09-18
2001-06-05
Denion, Thomas (Department: 3748)
Rotary expansible chamber devices
Unlike helical surfaces on relatively wobbling rotating...
C418S179000, C175S107000, C029S888023
Reexamination Certificate
active
06241494
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to mud driven motors used in the drilling of oil wells. More particularly, the invention relates to a downhole drilling motor which has a wholly non-elastomeric stator and rotor.
2. Description of the Related Art
The idea of downhole motors for driving an oil well drill bit is more than one hundred years old. Modern downhole motors are powered by circulating drilling fluid (mud) which also acts as a lubricant and coolant for the drill bit. Prior art
FIG. 1
shows a state-of-the-art downhole motor assembly. The assembly
10
generally includes a rotatable drill bit
12
, a bearing/stabilizer section
14
, a transmission section
16
which may include an adjustable bent housing (for directional drilling), a motor power section
18
, and a motor dump valve
20
. The bent housing
16
and the dump valve
20
are not essential parts of the downhole motor. As mentioned above, the bent housing is only used in directional drilling. The dump valve is used to allow drilling fluid to enter the motor as it is lowered into the borehole and to allow drilling fluid to exit the motor when it is pulled out of the borehole. The dump valve also shuts the motor off when the drilling fluid flow rate drops below a threshold. During operation, drilling fluid pumped through the drill string (not shown) from the drilling rig at the earth's surface enters through the dump valve
20
, passes through the motor power section
18
and exits the assembly
10
through the drill bit
12
.
Prior art
FIGS. 2 and 3
show details of the power section
18
of the downhole motor. The power section
18
generally includes a housing
22
which houses a motor stator
24
within which a motor rotor
26
is rotationally mounted. The power section
18
converts hydraulic energy into rotational energy by reverse application of the Moineau pump principle. The stator
24
has a plurality of helical lobes,
24
a
-
24
e
, which define a corresponding number of helical cavities,
24
a
′-
24
e
′. The rotor
26
has a plurality of lobes,
26
a
-
26
d
, which number one fewer than the stator lobes and which define a corresponding plurality of helical cavities
26
a
′-
26
d
′. Generally, the greater the number of lobes on the rotor and stator, the greater the torque generated by the motor. Fewer lobes will generate less torque but will permit the rotor to rotate at a higher speed. The torque output by the motor is also dependent on the number of “stages” of the motor, a “stage” being one complete spiral of the stator helix.
In state-of-the-art motors, the stator
24
is made of an elastomeric lining which is molded into the bore of the housing
22
. The rotor and stator are usually dimensioned to form a positive interference fit under expected operating conditions, as shown at
25
in prior art FIG.
4
. The rotor
26
and stator
24
thereby form continuous seals along their matching contact points which define a number of progressive helical cavities. When drilling fluid (mud) is forced through these cavities, it causes the rotor
26
to rotate relative to the stator
24
. The interference fit
25
is defined by the difference between the mean diameter of the rotor
26
and the minor diameter of the stator
24
(diameter of a circle inscribed by the stator lobe peaks). Motors which have a positive interference fit of more than about 0.559 millimeters (0.022 inches) are very strong (capable of producing large pressure drops) under downhole conditions. However, a large positive interference fit will provoke an early motor failure. This failure mode is referred to as “chunking”.
Interference fit is believed to be critical to the performance and overall life of the motor. In practice, the magnitude of the interference fit (at the time of assembly) is dictated by the expected temperature of the drilling fluid and downhole pressure. High temperatures will cause the elastomeric stator of a motor with negative or zero interference fit to expand and form a positive interference fit. For use at lower temperatures, it is necessary to assemble the motor with a positive interference fit. As mentioned above, a motor with excessive interference fit will fail early. On the other hand, a motor with insufficient interference fit will be a weak motor which stalls at relatively low differential pressure. A motor stalls when the torque required to turn the drill bit is greater than the torque produced by the motor. When this happens, mud is pumped across the seal faces between the rotor and the stator. The lobe profile of the stator must then deform for the fluid to pass across the seal faces. This results in very high fluid velocity across the deformed stator lobes.
In addition to temperature, certain types of drilling fluids may have an adverse effect on the operation of the drilling motor. For example, certain types of oil-based drilling fluid and drilling fluid additives can cause elastomeric stators to swell and become weak. Therefore, the composition of the drilling fluid must also be considered when choosing a motor with the appropriate amount of interference fit.
Those skilled in the art will appreciate that the elastomeric stator of drilling motors is a vulnerable component and is responsible for many motor failures. However, it is generally accepted that either or both the rotor and stator must be made compliant in order to form a hydraulic seal. As mentioned above, it is generally believed that without sufficient positive interference (hydraulic seal) between the rotor and stator, the motor will be weak (generate low torque) and will easily stall.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a drilling motor power section which has no elastomeric parts.
It is also an object of the invention to provide a drilling motor which is virtually immune to chunking.
It is another object of the invention to provide a drilling motor which is operable throughout a wide range of temperatures without adversely affecting the integrity of the stator.
In accord with these objects which will be discussed in detail below, the drilling motor of the present invention includes a non-elastomeric stator and a rotor which are dimensioned for negative interference. The rotor and stator are preferably metallic and made of a thermally and chemically stable metal such as stainless steel. It will be recognized that other non-elastomeric materials, such as ceramics and composites, may also be employed. When a non-elastomeric stator is used, the difference between the outer diameter of the housing (or the stator if no housing is used) and the maximum diameter of the stator profile can be decreased significantly without reducing the stiffness of the stator. A smaller difference in these diameters allows the motor to produce higher torque. According to a preferred embodiment, the amount of negative interference between the rotor and the stator is determined by the largest solid particle expected to pass through the motor. The gap between the rotor and the stator is preferably at least two times the greatest particle size. According to the invention, metallic stators are made by machining or casting. Due to the size limitations imposed by both fabrication techniques, stators according to the invention are fabricated in sections having lengths of 20 to 40 centimeters (8 to 16 inches). The sections are indexed so that each section may be properly aligned with another. The sections are aligned and welded together to form a motor stator of conventional length.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
REFERENCES:
patent: 2527673 (1950-10-01), Byram
patent: 3840080 (1974-10-01), Berryman
patent: 3975121 (1976-08-01), Tschirky
patent: 4764094 (1988-08-01), Baldenko et al.
patent: 4909337 (1990-03-01), Kochnev et al.
patent: 5171138 (1992-12-01), Forrest
patent: 5221197 (1993-06-01), Kochnev e
Koval Vernon E.
Pafitis Demosthenis G.
Christian Steven L.
Denion Thomas
Schlumberger Technology Company
Trieu Thai Ba
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