Rotary expansible chamber devices – Unlike helical surfaces on relatively wobbling rotating...
Reexamination Certificate
2002-03-14
2003-08-12
Vrablik, John J. (Department: 3748)
Rotary expansible chamber devices
Unlike helical surfaces on relatively wobbling rotating...
C418S153000, C418S178000
Reexamination Certificate
active
06604922
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates generally to stators used with positive displacement drilling motors. More specifically, the invention relates to a fiber reinforced liner adapted for use with formed stators.
2. Background Art
Positive Displacement Motors (PDMs) are known in the art and are commonly used to drill wells in earth formations. PDMs operate according to a reverse mechanical application of the Moineau principle wherein pressurized fluid is forced though a series of channels formed on a rotor and a stator. The channels are generally helical in shape and may extend the entire length of the rotor and stator. The passage of the pressurized fluid generally causes the rotor to rotate within the stator. For example, a substantially continuous seal may be formed between the rotor and the stator, and the pressurized fluid may act against the rotor proximate the sealing surfaces so as to impart rotational motion on the rotor as the pressurized fluid passes through the helical channels.
Referring to
FIG. 1
, a typical rotor
10
includes at least one lobe
12
(wherein, for example, channels
14
are formed between lobes
12
), a major diameter
8
, and a minor diameter
6
. The rotor
10
may be formed of metal or any other suitable material. The rotor
10
may also be coated to withstand harsh drilling environments experienced downhole. Referring to
FIG. 2
, a typical stator
20
comprises at least two lobes
22
, a major diameter
7
, and a minor diameter
5
. Note that if the rotor (
10
in
FIG. 1
) includes “n” lobes, the corresponding stator
20
used in combination with the rotor
10
generally includes either “n+1” or “n−1” lobes. Referring to
FIG. 3
, the stator
20
generally includes a cylindrical external tube
24
and a liner
26
. The liner
26
may be formed from an elastomer, plastic, or other synthetic or natural material known in the art. The liner
26
is typically injected into the cylindrical external tube
24
around a mold (not shown) that has been placed therein. The liner
26
is then cured for a selected time at a selected temperature (or temperatures) before the mold (not shown) is removed. A thickness
28
of the liner
26
is generally controlled by changing the dimensions of the mold (not shown).
A lower end of the rotor may be coupled either directly or indirectly to, for example, a drill bit. In this manner, the PDM provides a drive mechanism for a drill bit independent of any rotational motion of a drillstring generated proximate the surface of the well by, for example, rotation of a rotary table on a drilling rig. Accordingly, PDMs are especially useful in drilling directional wells where a drill bit is connected to a lower end of a bottom hole assembly (BHA). The BHA may include, for example, a PDM, a transmission assembly, a bent housing assembly, a bearing section, and the drill bit. The rotor may transmit torque to the drill bit via a drive shaft or a series of drive shafts that are operatively coupled to the rotor and to the drill bit. Therefore, when directionally drilling a wellbore, the drilling action is typically referred to as “sliding” because the drill string slides through the wellbore rather than rotating through the wellbore (as would be the case if the drill string were rotated using a rotary table) because rotary motion of the drill bit is produced by the PDM. However, directional drilling may also be performed by rotating the drill string and using the PDM, thereby increasing the available torque and drill bit rpm.
A rotational frequency and, for example, an amount of torque generated by the rotation of the rotor within the stator may be selected by determining a number of lobes on the rotor and stator, a major and minor diameter of the rotor and stator, and the like. An assembled view of a rotor and a stator is shown in FIG.
3
. Rotation of the rotor
10
within the stator
20
causes the rotor
10
to nutate within the stator
20
. Typically, a single nutation may be defined as when the rotor
10
moves one lobe width within the stator
20
. The motion of the rotor
10
within the stator
20
may be defined by a circle
0
which defines a trajectory of a point A disposed on a rotor axis as point A moves around a stator axis B during a series of nutations. Note that an “eccentricity” e of the assembly may be defined as a distance between the rotor axis A and the stator axis B when the rotor
10
and stator
20
are assembled to form a PDM.
Typical stators known in the art are formed in a manner similar to that shown in FIG.
2
. Specifically, an inner surface
29
of the external tube
24
is generally cylindrical in shape and the stator lobes
22
are formed by molding an elastomer in the external tube
24
. Problems may be encountered with the stator
20
when, for example, rotation of the rotor
10
within the stator
20
shears off portions of the stator lobes
22
. This process, which may be referred to as “chunking,” deteriorates the seal formed between the rotor
10
and stator
20
and may cause failure of the PDM. Chunking may be increased by swelling of the liner
26
or thermal fatigue. Swelling and thermal fatigue may be caused by elevated temperatures and exposure to certain drilling fluids and formation fluids, among other factors. Moreover, flexibility of the liner
26
may lead to incomplete sealing between the rotor
10
and stator
20
such that available torque may be lost when the rotor compresses the stator lobe material, thereby reducing the power output of the PDM. Accordingly, there is a need for a stator design that provides increased power output and increased longevity in harsh downhole environments.
Prior attempts have been made to increase stator durability and heat conduction properties. U.S. Pat. No. 6,201,681, issued to Turner, describes fibers disposed in an elastomer material that forms a stator for a helicoidal pump or motor. The fibers are generally arranged to form a two or three dimensional structure within the elastomer material. The fibers are either coated with the elastomer material as they are being woven to form a fabric layer or are formed into the desired arrangement to form a fiber skeleton. After the fiber skeleton is formed, elastomer is then injected into the stator under heat and pressure to complete the process.
However, fiber reinforcement has presented manufacturing difficulties because it is difficult to achieve a desired fiber arrangement using injection molding techniques. Fiber reinforcement via injection molding requires additional manufacturing steps, and the manufacturing processes generally produce either a different concentration of fibers per unit volume of elastomer between the thick portions of the lobes and the thin portions (which reduces the mechanical strength of the liner) or, when fibers are disposed manually, a different number of layers must be applied in the thick portions of the lobes as compared to the thin portions.
Accordingly, there is a need for a liner material that is more durable and is able to withstand prolonged sealing engagement between a rotor and a stator in harsh operating conditions. Moreover, there is a need for a new liner material that is adapted for use with stators that include contoured inner surfaces formed on the stator tube. The liner material should be durable and should be less susceptible to wear and, for example, thermal fatigue. The liner material should also be easy to install so as to achieve a desired fiber concentration proximate selected regions of the rotor or stator.
SUMMARY OF INVENTION
In one aspect, the invention comprises a method of forming a stator for a positive displacement motor. The method comprises forming a liner including at least two resilient layers and at least one fiber layer. The at least two resilient layers are positioned so as to enclose the at least one fiber layer. The liner is positioned in a stator tube, and the stator tube comprises a shaped inner surface including at least two radially inwardly projecting lobes extending helically
Jeffery Brigitte L.
Ryberg John J.
Salazar Jennie (JL)
Schlumberger Technology Corporation
Vrablik John J.
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