Elastomer swell can
occur either when the elastomer is affected by production fluids or treatment
chemical that are incompatible with it (chemical swell) or due to an increase
in temperature (thermal swell).
Chemical swell is
generally caused by the elastomer coming in contact with high API gravity crude
oils or incompatible treating chemicals; both of which have a fairly high
percentage of aromatics. Chemical swell generally permanent and nonreversible
Source of the swelling
has been removed.
Thermal swell on the
other hand is purely due to an elevation in temperature resulting in the
elastomer expanding due to this increase in temperature. This expansion also
results in an increase in the interference fit between the rotor and stator
thereby in an increase in operating torque as well. Thermal swelling can be
predicted and is not permanent. Once the temperature decrease the thermal
expression of the elastomer also decreases.
A bond failure occurs
when the bonding agent that keeps the elastomer in the stator tube fails.
This can occur at two
interface is between the bonding agent and the elastomer. In this instance,
there is no elastomer test on the stator tube but there is bonding agent. A
view of the backside of the elastomer will show it to be smooth. The stator
tube will also be smooth with a gray or black color.
is also known as Gas permeation occurs when gas; under pressure; enters the
elastomer matrix and expands due to a pressure drop. When the gas expands, it
often results in blister or bubbles forming within the elastomer. This pressure
drop can be the results of the event such as the fluid level equalizing in the
wellbore after a shut down or from the pulling of the pump.
The expanding gas within the elastomer matrix
can sometimes expand to the point that the elastomer ruptures. This is called
explosive decompression. The rapidly
decompressing of the gas entrained within the elastomer matrix tears and chunks
the elastomer when it expands.
The word delamination
contains the word lamination which means “layer”. In elastomeric terms, this
means that the elastomers had not knit through its cross-section resulting in
layer-like striation within the elastomer. This is often the result of
temperature during the injection process. These lamination or layers create
weak areas within the elastomers and a prime location for gas that has
permeated into the elastomers to gather. When a pressure drop occurs, the gas
has entered these laminated areas expands thereby tearing these weak areas.
Visually looking at the laminated areas, the internal surface of the laminated
are smooth; unlike a torn surface where the surface are rough and jagged.
Improper Rotor Spacing:
Improper rotor spacing
is not a material or application related failure but a failure due to improper
installation procedure. For a given set of well condition, the sucker rods will
stretch a given amount. This stretch is dependent upon the rod size, total
dynamic head and the effective cross-sectional areas of the pump. If the proper
amount of stretch is not accounted for, the rotor placement within the stator
could either be too high or too low. In either case, excess stresses are placed
on the rotor and sucker rods and may result in a torsional fatigue of the
either the rotor or the sucker rods.
In case of not enough space out:
Rotor runs on the tag
bar, thus putting the rotor in compression and potentially causing premature
failure of the rotor and tag bar.
If a torque anchor is
not used the additional torque of the rotor running on the tag bar could result
in the stator/tubing unscrewing.
In case of too much space out:
Reduced contact with
the pump stages & over pressuring of the pump or excessive unidirectional
bending of the rotor causing it to break.
A pump that has run dry
its elastomer hard, brittle and extensively cracked. In an extreme run dry
condition, the contour will be completely gone.
Abrasive wear occurs
when the elastomer is worn from the presence of abrasives in the produced
fluids. As the percentage of abrasives increases, the chances of prematurely
wearing the seal lines that are formed by the interference fit between the
rotor and stator also increase. The hardness and angularity of the pumped
abrasive can also affect the wear rate.
The total differential
pressure seen across the pump as well as the pump’s rotational speed play a key
role in how abrasive affect the elastomer. As a result of the elastomer
wearing, the slip within the pump increases and as a result, the production
will drop off.
The data and the related case studies are
mentioned below at page no.6 & 7.