Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Progressive cavity hydraulic machine
This invention relates to machine-building, more specifically, to the
design and manufacturing of positive displacement hydraulic rotary
machine; various embodiments of which are used for oil-field (wells)
applications.
Positive displacement motor (PDM) is commonly used for
directional drilling operations throughout the world and progressive cavity
pump (PCP) having a similar design is widely used in artificial lift. These
machines are often referred to MOYNO system, in reference of to the main
initial business application of such machines.
For such system, reliable and steady operation is essential over long
operating time. Typical life of a PDM in normal drilling conditions is
usually accounted as half a month and may reduce to several hours in case
of severe conditions while aggressive drilling. Deterioration rate of PCP
pumps becomes serious in presence of high sand content or proppant
flowback. Practice demonstrated that most failures of PDM and PCP are
generated by the stator's elastomer. Elastomeric stators usually fail due to
high mechanical loading, wearing due to erosion and abrasion, fluid
incompatibility, high temperature. When operating at high differential
pressure or with progressive erosion, internal leakage increases and the
system performance is progressively reduced.
This invention is aimed to reduction of PDM/PCP performance
degradation by introducing of a fixed positive clearance between the rotor
and the stator and additional calibrated channels providing tolerable
leakage of fluid between hydraulic chambers, allowing elastomeric
coatings of the conventional system to be replaced by more resistant
materials.
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SLB patented [US6241494, Demosthenis G. Pafitis, 1998] and tested
some hydraulic motors with non-elastomeric stator. This type of motor
operates with a clearance estimated as 0.3- 0.6 mm. With such
construction, system plugging with large particles (such as LCM or Fluid
Loss Materials) in some specific conditions (such as motor stall) must be
solved for proper applications.
Prior Art
Helical hydraulic machines are widely used in oil and gas industries:
the terminology MOYNO is often associated to some machine design..
One application of these devices is Positive Displacement Motors
(PDM) when drilling mud is pumped to the drill-string for converting of
flow energy into mechanical for driving the drilling bit. The performance
of PDM depends on motor design, properties of drilling mud (density,
viscosity), downhole environment condition (pressure, temperature,
chemical composition of the fluid) and drilling regimes (required torque
and weight on bit (WOB).
The second application of the same hydraulic operation principle is
the Progressive Cavity Pump (PCP) for lifting of production fluids from the
producing formation to the surface via the production tubing. For such
application, the rotor can be driven by submersible electric motor downhole
(for deep pump) or by a surface unit rotating a rod connected to the PCP
rotor (the latter variant is suitable for moderate depth). A Progressive
Cavity Pump has several advantages in comparison with other types of
pumps: it consists of two main units (rotor and stator), so it is reliable in
= operation, provides a steady flow rate of pumped fluid, it has easy-
controllable flow rate. It is widely used for pumping of heavy oil and other
high-viscosity fluids even with a high percentage of sand in the fluid.
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Thanks to their high performance and reliability, PCP is commonly applied
for heavy oil production.
Standard design of such hydraulic system (PDM and PCP) is a
combination of a metallic rotor with a helical shape and a stator with inner
surface covered with elastomer material providing a tight but flexible rotor-
stator contact along the contact curve throughout the motor. Fig. 1 shows
the details of the power section 18 of PDM/PCP. The power section 18
generally includes the housing 22 which houses the stator 24 within which
a motor rotor 26 is rotationally mounted. The stator 24 has a plurality of
helical lobes 24a-24e, which define the corresponding number of helical
cavities 24a'-24e'. The rotor 26 has a plurality of helical lobes 26a-26d.
This figure refers to a 4-lobe rotor. Such systems (PDM/ PCP) are
considered as positive displacement hydraulic machine: In the ideal case,
the flow is proportional to rotation speed (RPM) while the torque is
proportional to the differential pressure across the system.
However the current design of industrial PDM and PCP (or screw
pumps) put limits on their application. Current design of industrial
hydraulic machines for oil industry is based on a solid rotor (metal or
composite) and a stator comprising of a metallic housing lined with
elastomer material. The interference between the lobes of rotor and
elastomeric stator at contact lines defines cavities for trapping fluid which
is transported along the system axis from the intake to the outlet thanks to
the machine rotation. For a positive displacement motor, hydraulic pressure
difference between cavities defined by the helical stator and helical rotor
drives the rotor in the proper direction. In case of PCP, the rotor rotation
pushed the fluid across the pump, creating the delta pressure between
successive chambers. It is commonly believed that the efficiency of such
design depends on the tightness of contact (interference between 24d and
26c, see Fig. 1) between the stator and rotor along the helical-type contact
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curve [1]. This generates complex requirements onto the properties of
elastomer. It must sustain differential pressure across the cavities to
prevent
leakages, while being flexible enough for tight contact, but strong enough
to withstand wearing from the sliding of the rotor and erosion; It must also
withstand thermal impact due to friction, chemical aging due to oil or
hydrocarbon elements over wide temperature range in the well. As a
consequence to these requirements, these elastomeric materials are
expensive; they require special methods of depositing on the liner to make
a stator for high geometrical precision after molding, including
consideration of post-reaction settling and swelling.
The prior art for design of positive displacement hydraulic machine
(especially drilling motors) assumes a strong positive seal called a positive
interference. When higher downhole temperatures are anticipated, the
positive interference is reduced during assembly to allow thermal
expansion of the elastomer. The mud weight and vertical depth also must
be considered as they influence the hydrostatic pressure applied to the
stator's elastomer and cause it to shrink. Drilling motors (such as SLB
PowerpackTM ) are commonly available with different stator elastomers.
The choice of elastomer depends on the downhole conditions. However the
conditions can vary during motor operation, so it is desirable to have a
stator-rotor pair which is universal for standard and challenging conditions
of temperature and pressure. The choice of interference can be predicted by
various models: for example, the Schlumberger software called PowerFit is
used to calculate the desired interference fit for a PowerPackTM Steerable
Motor.
However by looking into typical performance curve of PDM (such as
PowerPackTM Steerable Motor), it may be observed that considerable
leakages exist in operational range of differential pressure. This means that
part of the sealing lines become open during the operation allowing part of
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the flow to leak from cavity to cavity in place of moving with cavity
rotation and translation..
An alternative method of using the stator-rotor pair with negative
interference (or positive clearance) was, described in patent US6241494
(Demosthenis Pafitis et al., 2001). This patent teaches that a small
clearance between the rotor and stator is quite tolerable for good
performance of a hydraulic motor. Fig. 2 shows the concept of this design.
There is a positive clearance between rotor's lobes 126b, 126c, 126d and
the stator, but there is also at least one contact line between the rotor and
the stator as it is shown between 126a and 124a'. During the rotation of the
rotor it rolls over the stator and the contact line moves as well. This design
was tested and showed acceptable performance. Positive clearance helps
avoiding some problems: Fine particles pass trough the system without any
problem. This applies extremely well to particles smaller than the gap.
However in some applications, large particles are present: in drilling, LCM
particles usually have a larger size (1 mm of particles or several millimeters
of square flakes), while in other applications sand particles may be present.
These large particles can create plugging of the cavities between rotor and
stator, especially if flow is forced into the machine while the machine is
stalled. In this latest condition, the hydraulic machine acts as a particles
filter.
A minimal contact between two parts of helical hydraulic machine
reduces friction in the rotor-stator pair, the abrasive and erosive
deterioration of the surfaces, thus extending the service life of a motor or
pump: this minimum gap may be imposed by a rotor guidance system, as
explained in later section. Publication on heavy oil production methods
[SPE 0059276] also emphasizes that a PCP can achieve high efficiency
without "interference fit" between the rotor and stator. For pumping of
fluids with a high viscosity (like heavy oil), the design based on "sloppy
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fit" helps to the preserve the chromed or boronized surface of a rotor. This
means that such a screw pump comprising a hard rotor and hard stator
exhibits a low mechanical abrasion, low erosion, while keeping the head of
pump at desired level. The gap of several tens microns is typically
recommended in publication for high-viscosity fluids.
The use of a metal-metal pair (or composite-composite pair) removes
the chemical-related problems of elastomer: aging by drilling
fluid/production fluid, reaction with gases dissolved (H2S, CO2), sudden
decompression of the elastomer material saturated with gases after lifting of
device to the surface.
The use of regular design of PCP/PDM often faces the problem of
plugging of cavities by sand or loss-circulation materials (LCM) suspended
in the fluid. This problem becomes acute with a metal-metal pair (or
composite-composite pair) in the hydraulic system. There exists a method
to limit and even suppress the plugging effect with a special valve
(US6371206, Preventing of Sand Plugging of Oil Well Pumps, 2002).
When the well pump is idle and there exists a risk of sedimentation of
suspended particulate atop the idle pump, a piece of production tubing is
closed by the valve. When this valve is reopened, a sudden pressure
variation is created to that remove the accumulated particulates from atop.
But this method does not address the problem of particle accumulation
inside the pump or aggregation of loss-circulation material inside PDM
cavities. It was observed in experiments (such as made in Schlumberger by
Demos Pafitis) and others that concentration of LCM can become so high
(especially during PDM stalling) that it was impossible to restart the motor
after a single motor stalling.
Summary of the Invention
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The object of this invention is to improve the design of positive
displacement motors and progressive cavity pumps.
Said object is achieved by using a positive displacement motor
comprising a rotor and a stator of helical shape without elastomeric coating
or liner installed with a clearance wherein said stator is hard with an
elastic
modulus of at least 10 times the elastic modulus of elastomers used in
stators, further wherein said clearance is 0.05 ¨ 0.5 mm. Preferably, said
rotor and/or stator are additionally covered with a wear-resistant coating.
Said motor may comprise multiple sections comprising said rotor and said
stator.
= Also, said object can be achieved by using a positive displacement
motor comprising a rotor and a stator without elastomeric coating or liner
installed with a clearance wherein the lobes of said rotor have through
channels hydraulically connecting the,chambers formed by said lobes. The
channels in adjacent lobes are preferably arranged not in line. Preferably,
the axis of at least part of said channels is curved: Typically, the diameter
of said channels is 2 ¨ 10 mm. The rotor surface may additionally have
grooves of 5 ¨ 10 mm width and depth of 0.5 to ¨ 10 mm depending on
operation conditions. Typically, one cavity has at least 2 grooves and one
lobe pitch has at least two channels. Said motor may also comprise multiple
sections comprising said rotor and said stator.
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Some embodiments disclosed herein relate to a progressive cavity hydraulic
machine comprising a multi-lobed rotor and a stator of helical shape without
elastomeric
coating installed with a clearance wherein said stator is a hard metal, alloy,
ceramic, or
composite material, wherein said clearance is 0.05 - 0.5 mm, and wherein a
surface of the
rotor or stator comprises a groove, wherein dimensions of the groove comprise
a width of
5mm to lOmm and a depth of 0.05 mm to 1 Omm.
By design analogy, similar holes and channels could be installed in the stator
in
hydraulically equivalent position.
Also, the previously described local channels (either in the rotor and stator)
could be replaced by a spiral grooves (either in rotor or stator). The groove
angular
orientation can be either in the same or opposite direction to the component
in which one is
formed. However, its pitch and the number of grooves should be such that at
least one
opening is present in the sealing line of each cavity between the stator and
rotor. With such a
system, the
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sealing line of each cavity has an opening for any angular position of the
rotor: the opening is "apparently" moving axially during the rotor rotation,
allowing the cleaning over the whole sealing length after one rotation.
The invention relates to the oil and gas industry, in particularly, to
the field of design of helical hydraulic machines. The design of hydraulic
machine is offered with a small positive clearance between a solid rotor and
solid stator. The size of clearance depends on the properties of fluid
transported through the machine. Minimal clearance is also chosen in
accordance with process of manufacturing and assembling of the rotor in
the stator. Also the minimal clearance is enough for passing of most of
small particulates expected in the fluid thus reducing sand plugging,
abrasion and erosion caused by particle flow at high velocities. To deal
with particles bigger then the clearance a number of special channels
through the lobes of either rotor or stator are evenly placed hydraulically
connecting adjoining cavities located at both sides of the lobes. The flow
goes through the clearance but also passes through those channels flushing
the cavities while operation of the motor. Another alternative is to form
spiral grooves in the surface of the rotor or stator. The lobe hole allows the
flow through the pump of "cubical or spherical" large particles while the
surface channels or grooves allows the flow through the pump of "flat"
large particles.
In normal operation, most of the flow is passing trough the pump via
the rotation of the progressive cavities: only the particles related to the
leak
rate needs to pass trough the clearance, the lobe holes, channels or spiral
grooves.
In stalling mode of a motor operation, the whole flow is becoming a
leak trough the system: larger amount of particles needs to be handled via
plugging and locking the motor. With limited concentration of large
particles, the size of lobes holes, channels or spiral grooves can allow the
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flow even during stall condition. For large concentration, other means may
be required such as a pressure bleed-off valve (installed on a hollow rotor)
which allow to by-pass part of the flow outside of the hydraulic motor.
Also in normal operating mode, the rotor has some sliding while
rolling/rotating effect. Small particles would be compressed and dragged
in the area of cavity sealing. As this area is continuously moving and
covering the whole periphery after one rotation, wear would be appearing
over the whole surface of the rotor and stator. This effect can be prevented
or limited by the use of guidance mechanism on the extremity of the
hydraulic system to insure no direct contact of the rotor onto the stator.
Detailed Description of the Invention Including Examples and
Drawings
The current invention presents an improved design of a device
described in the Schlumberger patent US6241494 [Demos Pafitis, 20011
claiming a principle of positive clearance for hydraulic motor with non-
elastomeric stator. The current invention is based on this Schlumberger's
invention as a basic concept for further improvement in design and
operation.
Similarly to the basic concept, the stator of the helical machine is
made of non-elastomeric material that helps to avoid the problems inherent
to conventional elastomeric stators (low strength, high deformation under
operational loads, aging, chemical and thermal sensitivity, gas-induced
swelling, temperature expansion).
The material for the stator manufacturing is metal, alloys, ceramic, or
composite suitable for downhole conditions. The material for the rotor is
the same or a hard material with similar temperature expansion coefficient
in the operation range.
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Now the stator is rigid so that its elastic module is at least 10-100
times higher than in the typical elastomers used for the conventional
stators. Special thin coating may be used on the stator (or/and rotor) to
enhance their resistance to erosion and wearing.
Since the device according to invention has no elastomeric elements,
it can be assembled for operation at high temperatures (>140 degC).
The rotor rotates in the stator with a prescribed and constant
clearance rotor-stator. The said clearance is determined to be wider than the
2...3 size of particles corresponding to the top of particle size distribution
in the operational fluid. The preferable clearance interval is from 0.05 mm
¨0.5 mm.
Brief description of the drawings:
Fig. 1 is a cutaway view of motor/pump showing the rotor and the
stator (left side).
A cross-section taken along A-A line is shown in the right side. The
top diagram illustrates a positive interference between a solid rotor and
elastomer-coated stator (prior art design).
Fig.2 shows another modification of prior art device with a positive
clearance. The view is similar to Fig. 1.
Fig. 3 shows the longitudinal sectional view of a stator-rotor pair
according to the invention (left side) and cross-section along the A-A line
(embodiment 1).
Fig. 4 is the cross-sectional view of a rotor according to invention
(embodiment 2) illustrating the allocation of drilled holes in the rotor body.
Fig. 5 is a 3D projection of the rotor according to the invention.
Fig. 6 is a typical operational curves for a prior art motor
(elastomeric stator with positive interference depicted in Fig. 1) as well as
d
for solid rotor ¨ solid stator pair with a fixed gap.
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Fig. 7 is a typical mechanical power curve as a function of pressure
drop across the power section for prior art motor (elastomeric stator with
positive interference) and a motor according to the invention (solid rotor ¨
solid stator pair with a fixed gap).
Embodiment 1
Fig. 3 shows that at the both ends of the hydraulic machine there are
two additional sections 128 and 130. Two support sections at the ends and
the hydraulic machine constitute the hydraulic section of the PDM (or
PCP). This section can be the motor (or the pump) itself, but alternatively
the downhole unit may comprise multiple sections connected together. This
allows one to increase the power while, if said sections are short enough,
reducing the cost of each individual section and lowering the negative
effect of well curvature by providing flexible connections between said
sections. These sections incorporate a special guiding mechanism which
ensure consistent rotation and nutation of the rotor 126 inside the stator 124
and provide the support for the rotor 126 so that the rotor does not see
contact with the stator within the power section between the sections 128
and 130.
The cross-section in Fig. 3 shows the expected position of the rotor
126 inside the stator 124 with positive clearance along its entire perimeter.
This will eliminate the friction and abrasive wearing within the poser
section. Also this will decrease the filtering of the small particles since
they
gap for them will exist on the full round basis.
The guiding mechanism drives the rotations of the rotor so that its
rotational speed about its own axis and nutational speed about the axis of
the stator are insured to be in the following relation [W. Tiraspolsky,
Hydraulic Downhole Drilling Motors, Editions Technip, Paris, 1985, p.
246]: =
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nõõ, =Z2 *n ,.õ,
where Z2 is the number of lobes of the rotor.
This may be achieved by using a special gear mechanism or simply
repeating the geometry of power section but with tight clearance closed to
zero.
The guiding mechanism in the special sections 128 and 130 does not
contribute to the motor performance and it is specially designed to have
enhanced wearing resistance to keep the rotor in right position to the stator
within the power section. This may be achieved by protecting the guiding
mechanism from the main fluid and its abrasive particles passing through
the motor or by use of special material or coating on the wearing surfaces
of the guiding mechanism. Tungsten carbide can be used for the guidance
system.
If multiple short motor sections are being used, It can be
recommended to a guidance systems at the extremity of each short sections.
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Embodiment 2
In another embodiment the problem of plugging of the hydraulic
machines is addressed by placing additional preferably round holes through
the lobes of either the rotor or stator: (see Fig. 4, 5 for holes in rotor).
The
holes through the adjoining rotor lobes should not be on a straight line to
ensure flushing effect (see Fig. 5). The axis of those holes may not be
parallel to the axis of the rotor or may be curved allowing good
machinability.
Diameter of the holes is large enough to allow the passage of LCM
particles (loss circulation material particles) or any other particles bigger
then the clearance. The preferred size of transport holes is from 2 to 10
mm. In presence of flake type particles (potential type of LCM), in addition
to the holes a set of groves may be made on the surface of rotor (or stator)
with typical width of 5...10 mm and depth 0.5...2 mm. As the holes the
grooves may be evenly distributed along the rotor (or stator) having at least
2 grooves along the length of one helical lobe pitch of the system.
As alternative to these local grooves, spiral grooves can be machines
either on the rotor or stator. In practical application, the sealing area is
moving on the periphery of the rotor/stator during the rotation of the rotor:
each point of the rotor and stator will be covered by the sealing area during
one rotation. With the proper pitch of the spiral, a limited number of
openings is present in the sealing area for any position of the rotor. The
spiral can be forwards of backwards, but its pitch must accordingly
adapted. With such a construction, opening in the sealing area is moving
axially during the rotor rotation, allowing cleaning of the clearance.
The holes are evenly placed along each lobe of rotor (or rotor) so that
there are at least 2 holes over the lobes pitch. By such the way with running
motor there will always be at least 1 channel connecting adjoining
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chambers. In the presence of the filtering problem (when the big particles
are filtered out by the clearance), it is preferable to pass most of leakage
flow through those holes, and a smaller part of the contaminated fluid or
drilling mud should be squeezed through the peripheral clearance. To
achieve that goal, the holes diameter is determined (based on the large
particle size) first and then the number of holes in one rotor lobe per one
pitch is determined so that to achieve the summed area of all holes per each
cavity to be bigger then the area of the peripheral clearance. Upper limit for
the number of holes is determined by overall leaks area and expected motor
performance (more area for the leakage means less performance for the
motor).
An illustration of this concept is shown in the Fig. 4. The hole 100c
is drilled through lobe 126c and the hole 100d is drilled through lobe 126d.
Relative position (angles a 1 and a2) of these holes is the same for each
lobe so that the in-hole and out-hole are not on the straight line along the
rotor axis. The holes 100d and 100d' are neighboring along the cavity and
are located on the same surface of the lobe 126d with helical pitch between
them a3 as half of stator pitch. Fig. 5 shows the same holes in 3D picture
with expected flow pass flushing the cavity between the lobe 126a and
126d.
This allocation of holes in lobes provides a flushing flow from one
cavity to another. Additional holes induce a minor loss in the performance
of a hydraulic motor, but allow sustain no-stall (no-plugging) operation in
challenging conditions.
Combinations of lobes holes, channels or spiral grooves can insure
optimum operation when the fluid has a wide range of particles.
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Embodiment 3
The extreme case of flow leakage is for the motor stalled condition:
the entire flow has to be forced through the clearance and lobe holes. There
is a risk that the clearance is reduced by the large particles as not sweeping
action is achieved as there is no rotation. Then flow normally passing
trough the clearance is redirected into the lobe holes: this increases the
pressure drop across the motor under stalling for a certain time. This may
also induce erosion in the lobe holes. An alternative method to limit these
effects is to install a pressure limiting valve in the motor: when the
pressure
across the motor increases above a pre-defined threshold, the valve opens
and by-passes part of the flow outside the clearance between rotor and
stator. In practical application, this valve can be installed inside the
central
axial bore of hollow rotor. This installation can be made in a similar
fashion as the conventional rotor nozzle used occasionally when motors
have to be operated at extremely high flow rate. The valve can be a
"conventional" pressure limiting valve such as a ball closed by a spring
against the pressure. When such a pressure limiting valve is installed in the
rotor by-pass central hole, it is directly submitted to the differential
pressure across the motor.
A combination of the first two said embodiments can be employed.
The guidance systems are installed to reallocate the friction and abrasive
milling from rotor-stator pair to the guidance system and provide passage
of fine particles through a constant-width clearance. The holes in the lobe
(rotor or stator) body ensure flushing for bigger particles while channels
and/or spiral grooves ensure flushing for flat particles.
Finally the third embodiment can be added to the previous
combinations to allow proper behavior during motor stalled condition,
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while the fluid contains a large amount of large and/or flat particles. In
that
case, a certain amount of flow is by-passed ou of the cavities between rotor
and stator.
The new design may be constructed by incorporating the guiding
mechanism or the flushing channels or the by-pass vavle system separately
or as a combination of the three solutions in one design. In any version
there will be the full range of the above mentioned advantages due to the
use of non-elastomeric stator that will increase the reliability of the tool.
Example 1
To estimate efficiency of a motor with these additional channels in
the body of rotor, let us consider the motor to be of the same type and size
as the working prototype described in the patent US 6,241,494. In that case
the stator's OD was 6.75 inches (172 mm) and stator pitch was 27.8 inches
(706 mm). Estimation of leakage flow areas through the clearances of
0.3mm is 212 MM2.
If the size of the big particles is 4-6 mm, the disclosed device may
have 2 holes of 8.5 mm in diameter per each cavity: this gives additional
area of 113 mm2 that is about a half of leakage area in the working original
prototype. Thus putting away secondary hydrodynamic effects, to achieve
approximately the same performance as the prototype was, the new design
must have clearance of 0.15 mm (a half of prototype clearance). The area
of the clearance will constitute 106 mm2 (i.e., less than the holes area) and
most of leaks will go through the holes that will help to avoid the plugging
problem. If the size of the big particles is only 1-2 mm the, new design may
have 16 holes of 3 mm in diameter per each cavity to achieve the same
performance and flushing effects at the same clearance 0.15 mm between
the solid rotor and solid stator.
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Fig. 6 and Fig. 7 illustrate the comparison of theoretically expected
performance of new design versus conventional design of a PDM with
elastomeric stator. Points on the graphs represent typical data for motor
(such as SLB PowerPak A675SP4548 with normal interference fit 0,016
inches). That is a conventional PDM motor with elastomeric stator. The
solid curves 1 are the curves approximating the experimental points of the
conventional motor. The dashed theoretical curves 2 and 3 represent the
estimations for a hydraulic motor with 2 channels of 8 mm in diameter per
each cavity and the clearance 0.1mm and 0.2 mm. All the curves are
plotted for the same and constant flow rate 300 gal/min (0.02 m3/sec).
Although the numbers on the graphs correspond to a particular motor, the
shape of the curves is similar to that for all types of conventional hydraulic
motors [ PowerPack Steerable Motor Handbook, Schlumberger, 2004, page
99-192] .
In Fig. 6 the RPM curve of the conventional motor drops
significantly: this reflects the fact of widening a gap because of
deformation in the elastomeric coating (linear model of gap growth had
been used to best fit the experiments). Since the new design of the device
according to the invention has a fixed clearance (both stator and rotor are
made solid), the leakage occurs by a different law. There exists a point
where the leakages in the constant-gap design become even less than for a
motor with elastomeric stator (prior art) at the same pressure drop per
power stage.
Fig. 7 shows the calculated performance of new design in terms of
useful mechanical power. The effectiveness declines with the increase in
the clearance, but there is an interval of pressure drop (high pressure drop),
where the disclosed device is more efficient (the gap is smaller than for a
motor with elastomer-covered stator). Considering the clearance of 0.1 mm,
the curve says that the expected maximum power will stay as it was in
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conventional motor but stalling torque is expected to be 40% higher, so that
the new motor will be able to operate in more aggressive drilling regime.
With clearance of 0.2 mm there will always be the less power within the
same range of operation but the range and the power may be increased
simply by increasing the flow rate.
Thus Fig. 6 and Fig. 7 demonstrate that the disclosed device is less
effective in operation at low differential pressure but will better fit
aggressive drilling conditions when high torque is required in the same size
of the tool.
The conclusions about lower friction, plugging resistance,
temperature durability, and longer service time are valid not only for a
progressive cavity motor illustrated in the example, but also for a
progressive cavity pump with a non-elastomeric stator.
