American Oil and Gas Reporter - September 2019 - 77

explains. "This lets the operator transition
to cross-link-like transportation characteristics without changing the fluid system."
Noles says HVFRs' popularity will
continue to grow, but cautions that HVFRs
are a poor fit for reservoirs with high
frac gradients and extreme temperatures.
"The Haynesville Shale is a great example," he illustrates. "The wells there
tend to be deep and hot. HVFRs are
shear-sensitive, so by the time they reach
the perforation, they have thinned too
much to create fractures with enough
separation. A cross-linked gel may be
harder to pump, but the frictional resistance
it creates is helpful for separating the
rock and allowing fluid and sand to enter
the fractures.
"Every year, HVFRs get better at tolerating variations in fluid chemistry,"
Noles says. "For example, we are coming
up with HVFRs that perform well in the
higher TDS fluids. Our goal is to extend
TDS tolerances from 50,000-150,000
parts per million to 200,000 ppm to
reduce operators' need to dilute produced
water with freshwater when they are
building completion fluids."
CoilChem is investigating how water
quality affects HVFR performance and
experimenting with pretreatment techniques, Noles mentions. "We have quite
a bit of testing left, but the preliminary
data suggests the right pretreatment will
improve HVFR performance and the
well's production," he reports.
Mitigating Damage
FRs are an essential component of
the completion fluid, acknowledges Scott
Bailey, senior scientist with Flex-Chem
Corp, but he also warns of the downsides
associated with a negative charge. "When
they are down hole, this charge causes
them to cross-link with minerals in the
shale and create polymer derivatives that
plug the formation," he explains.
That plugging can greatly reduce the
well's production, Bailey says. "In the
Mid-Continent area, we see wells' production rates decline 85%-90% within the
first year largely because of plugging,"
says Bryce Conway, Flex-Chem's president.
"In some wells, even the initial production
is much lower than it should be given
their location and the offset performance."
Samples from these wells generally
have FR polymer derivatives containing
metals from the formation, Conway says.
"Polyacrylamide-based FRs' tendency
to cross-link with the formation is sufficiently well-known that operators are experimenting with alternative FRs to reduce
the damage. For example, they are trying
anionic, nonionic and cationic polymers,
as well as HVFRs," Conway observes.

Conway says his company's experience
suggests that in many applications, it can
be more economic and effective to prevent
or repair the blockage with breakers that
pull the metals out of the polymers. "This
transforms them from a shoe-rubber-like
material into a low-viscosity, water-like
liquid that flows out of the well," he says.
Flex-Chem has applied such breakers
to 700 Mid-Continent wells, particularly
in the Meramec and Woodford formations.
According to Conway, nine out of 10
wells have seen production improvements
after the treatment. "In 35% of the wells,
we see production above the IP. This has

happened even in wells that are four
years old."
"The breakers should yield similar results in other shale plays, such as the
Marcellus, Utica and Eagle Ford," Conway
predicts. "Like the Meramec and Woodford, these plays have metal-bearing clays
such as illite, smectite, chlorite and pyrite
that cross-link with slickwater completions' friction reducers."
Flex-Chem has been working with operators to identify variables in the completion
fluid's design and the formation's geology
that determine the severity and type of
damage, Senior Scientist Bailey reports.

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American Oil and Gas Reporter - September 2019

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