Neftegaz.RU #3-17 ENG

OFS
conveyed fluid corrosive properties.
Therefore, during the downhole
drilling motor operation it is
necessary to pay special attention
to the selection of appropriate drill
mud. The elastomer as technical
material should have a low gas
impermeability and water tightness,
as well as chemical resistance.
However, most elastomers are
able to absorb gas and lightly
corrosive fluids. The following are
typical changes the elastomers are
subjected to: swelling, shrinkage,
solidification, softening [2, 4, 5].
Furthermore, the bottomhole
temperature is a limiting factor for
engine operation. Series-production
domestic engines are designed
for long-term operation at the
bottomhole temperature up to
100°С. At the rise in temperature
in IRP-1226 rubber (used in most
domestic engines) irreversible
changes of mechanical properties
occur leading to faster wear
of the stator elastomer insert,
performance deterioration and
premature failure of the down hole
motor power section.
As a result, it was decided to carry
out the experimental researches
on evaluation of stability of
IRP-1226 rubber specimens at
engine temperature rise under the
influence of different mediums.
During this experiment the
process of drill-string running
was simulated; running speed
was assumed to be 1.5 m/s.
The following parameters were
determined as the design basis:
total vertical depth – 2,670 m,
geothermal gradient – 3°С/100
m, drillpipe stand length – 30
m (deemed time for making
a connection – 4 min). The
following items were calculated
in accordance with the initial
parameters: experiment duration –
384 min, engine end temperature
– 80°С.
Simulation of drill-string running
and consequently the rise of drill
mud temperature were carried out
in a desiccator. Test specimens
were made in the shape of cylinder
with diameter up to 43 mm and
thickness up to 11.5 mm. They
were kept in plastic containers
with total immersion into the
liquid medium. Processing the
experiment results, the changes
of the specimen weight and its
diameter at the temperature rise in
the liquid medium were assessed.
Initial measurement of input
parameters was made at the
temperature of 25°С; further
measurements were made after
every 5°C rise in temperature,
simulating then the drill-string
running for 165 m (23.5 min of the
experiment). Reaching the depth of
1,680 m, the specimen parameters
were measured after every 10°С
rise in temperature, simulating then
the drill-string running for 330 m
(50 min of the experiment).
Processing and analyzing the
obtained data, the following
correlations were exposed. After
completion of the experiment, the
weight reduction was observed for
all specimens. Nevertheless, in the
temperature range of 25 to 50°С
most specimens had no expressed
tendency towards weight variation;
its random variation was observed.
The exceptions were the specimens
immersed in the salt brine;
these specimens demonstrated
the tendency towards the
weight reduction throughout
the experiment. The specimens
immersed in solution based on
diesel fuel, oil and multigrade lowtemperature
hydraulic oil (see Table
1) were subjected to maximum
relative weight variation. The weight
reduction can be attributed to the
washing-out of rubber plasticizer
from IRP-1226 specimens.
For all specimens, it was recorded
the growth in diameter as the
temperature became closer to
80°С. The temperature range of 25
to 40 – 50°С has no clear tendency
towards an increase or reduction
in size of the specimens which is
illustrative of the potential danger
for elastomer. The maximum
relative diameter change was
demonstrated by the specimens
immersed in multigrade lowtemperature
hydraulic oil, salt brine
and oil. The specimens placed in
the alkaline solution (see Table 1)
were subjected to the minimum
relative diameter change.
Thus, the whole temperature
interval studied for the analyzed
dispersing mediums may have
a negative impact on the DDM
stator. Uncontrolled variation of
engine performance can occur
within the range of 25 – 50°С
due to the change of gap width
and consequently the tightness
FIG. 1. Dependence of IRP-1226 specimen diameter change from temperature in the presence
of different dispersing mediums
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