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spotlight: shale
FLUID THINKING: Proppant
suspended in cross-linked
gel — different frac fluid
formations are evaluated
to optimise compatibility of
proppants with frac fluid.
Photos: Santrol
Making
frac
proppants
go farther
UPSTREAM TECHNOLOGY Q3 2013
39
spotlight: shale
When it comes to increasing
production of a shale well,
the frac job is the prime goto solution. The success
of a hydraulic fracturing
operation rests on much, from
understanding the reservoir to
calling on the right proppant
type to keep the fractures open.
Jennifer Pallanich hears from
sector specialist Santrol how
proppant technology is evolving
to meet the shale play challenges.
O
ne of the main
hurdles in hydraulic
fracturing is sand
settling out of the
frac fluid before it reaches
the farthest point within
the fractures. As Van Smith,
executive vice president at
Santrol, explains: “The big gap
in what shale operators have
needed for years is something
that has a lower effective
density, which allows them to
propagate the proppant farther
into the fracture.”
Proppant selection has often
come down to compromise
- coarser sands offer better
permeability, but the coarser,
heavier sands settle faster. To get
the distance desired, operators
may have to pump a finer sand
than they prefer.
Now, with Santrol’s recent
purchase from Soane Energy
of self-suspending proppant
technology, hailed as an industry
game changer, operators “will be
able to pump what they
really want to pump”, Smith
says. The new technology,
he adds, is expected to
“substantially” increase the
surface area of propped fractures
to the reservoir, thereby driving
up production rates.
According to Vinay Mehta, vice
president of technical excellence
and innovation at parent
group Fairmount Minerals,
the coarser the proppant, the
better the value proposition of
the self-suspending proppant
(SSP). Heavier proppants tend to
settle more than their lighter
counterparts, so the SSP will
be able to keep those afloat
longer.
The benefit of proppants
remaining afloat for longer
periods of time — up to seven
days under laboratory-static
conditions — is that they can
travel farther, which ultimately
can lead to better production
rates.
Only a few microns in size,
the SSP coating is described by
Mehta as a “fast-acting water
absorbent material”. Initial
versions of the material were
sensitive to water composition,
and it behaved differently in
different waters. Santrol has a
solution to help the new coating
retain its effectiveness even
in hard water, Mehta says.
Further, it “has the potential
to eliminate guar gum and its
derivatives”.
Guar is a bean, which is
ground into guar gum for use
as a viscosifier in frac fluids to
suspend sands. Because it is
an agricultural product — 85%
of the world’s crop is grown in
India — prices can be volatile
due to fluctuating supply. As
of mid-2012, this was “pretty
expensive stuff”, Mehta says,
but with the adoption of the new
SSP technology the vagaries of
40 UPSTREAM TECHNOLOGY Q3 2013
RESIN RESEARCH:
With an eye to
developing nextgeneration proppants,
a Santrol Technology
Center research
associate in Sugar
Land works with
a gel permeation
chromatograph
to determine the
polymer molecular
weight and
distribution of the
resin used to coat
proppant.
“The STC
happens to be
one of the only
facilities in
the proppant
industry that
can evaluate
the flowback
resistance in
the lab.”
Vinay Mehta,
Fairmount Minerals
the guar market are lessened as
factors.
“Self-suspending proppant is
like a coating that allows it to
suspend in frac fluid and helps
carry down the wellbore,” he
explains. “But once it is carried
down the wellbore, that SSP
coating is not needed. Therefore,
we induce breakdown of that
fluid.”
The coating envelopes the
traditional proppant, for example
northern white sand, in a polymer
and suspends it in the hydraulic
fracturing fluid. Once placed
in the fracture, the polymer is
removed by traditional breakers,
leaving the proppant in place and
suspended in the fracturing fluid
throughout the delivery process.
This reduces the need for fluid
viscosifiers, such as guar and its
derivatives, cross-linkers, friction
reducers, biocides, gel stabilisers,
or high pH buffers.
As Santrol director of
marketing Nick Johnson points
out, the proppant suspends
itself longer than cross-linked
guar fluids do, enabling it to
travel farther. With field trials
scheduled for this autumn
using the SSP, initially with
regular sand — the technology
is also applicable to resin-coated
sand and ceramics — Johnson
says Santrol is not yet ready
to commercialise the new
technology.
The recently opened Santrol
Technology Center (STC) in
Sugar Land, Texas and its sister
innovation centre in Ottawa,
Illinois carried out the laboratory
evaluation work leading up
to May’s purchase of the SSP
technology from Cambridge,
Massachusetts-based Soane.
“There’s a learning curve,
spotlight: shale
Proppant
evolution
SSP VERSUS SAND: Self-suspending proppant
technology, shown applied to raw sand in the jar on the
right (time elapsed: 1 hour), helps maximise hydrocarbon
production by substantially increasing the surface
area of propped fractures to the reservoir. Santrol sees
the proppant, suspended in the fracturing fluid, as a
step change in the decades-old challenge of evenly
distributing proppant into the full length of a created
hydraulic fracture. The jar on the left shows the sand
settling to the bottom of a cross-linked gel solution.
F
UP CLOSE: Magnified to 30x (left to right) Santrol’s Super LC curable resin-coated proppant, Power Prop
procured resin-coated proppant, and the northern white sand the company mines for use as proppants.
but that’s not stopping us
from doing some innovation at
this point,” Mehta says of the
$5-million STC facility, which
became operational toward
the end of last year and now
employs 10 scientists. Its goal,
Mehta says, is not just to fully
characterise materials but to
understand their performance
and failure modes and develop
new materials and resins.
Sand under scrutiny
Santrol, which made its first
resin-coated sand in 1976, is
today working to optimise
coating resins based on
chemistries of the surfaces they
are intended to coat. All of the
company’s sand is premium
northern white.
Sand “by its very nature is
irregular in shape”. “It creates
significant point loading,” Mehta
says. This point loading, or stress
and pressure, can crush the
sand particles, leading to the
creation of fines. Using a resin
helps reduce the generation of
fines by providing a little bit of
elasticity, or cushion, between
the particles and the stresses.
An additional benefit is that any
fines generated are contained
within the resin coating, instead
of falling out.
According to Mehta: “All
sands have in general certain
surface contaminations and
impurities,” such as calcium
carbonate and iron oxide. “So we
want to make sure we optimise
the resins against those.”
Calcium carbonate, “the chief
culprit”, can dislodge itself under
high stress, which in turn can
prompt the coating to dislodge
itself, reducing porosity and,
ultimately, production. Using
state-of-the-art analytical tools
in its new R&D centre, Santrol
is working to overcome this
problem by optimising the resin
coating to produce the best
adhesion to the silica surface.
Flowback, and the costly
pumping and production
equipment downtime it can
cause, is another industry issue
under close scrutiny at the
centre. “The STC happens to
be one of the only facilities in
the proppant industry that can
evaluate the flowback resistance
in the lab and therefore design
our proppants to minimise the
flowback,” he says.
Proppant design is a balancing
act that must accommodate
properties such as flowback
resistance, conductivity, and
crush resistance.
One way to address flowback
concerns is to use a curable
resin-coated proppant, which
enters the well in granular form.
Downhole, a certain temperature
and pressure trigger the proppant
to bond, creating a proppant pack
that prevents frac sand from
flowing back into the wellbore.
“As frac fluids
became more
advanced,
our resincoated sand
technology
had to keep
pace and stay
customisable.”
Van Smith,
Santrol
orty years ago, there
were two common
choices when it came to
proppants — frac sand
or aluminum bauxite, a fired
ceramic.
“The price difference between
those two is a multiple of 100,”
says Van Smith, executive vice
president at Santrol. There was
often a need for something
higher performing than
basic frac sand, but pumping
aluminum bauxite was
frequently “overkill”, he adds.
Santrol — a combination of
“sand” and “control” — came
into being in 1976 with the
commercialisation of a curable
resin-coated proppant that was
designed to help prevent flowback
after hydraulic fracturing but also
tended to provide a larger postfrac width than sand alone.
The resin coatings work by
containing within the coating
any fines or dust created when
the crush resistance of a particle
or sand grain is exceeded — this
keeps the fines or dust from
entering into the wellbore,
therefore helping with sand
control. Research indicates
just 5% of fines can reduce flow
capacity of a well by 60%.
Proppant technology evolved
further in the early 1990s with
the development of high-viscosity
frac fluids capable of carrying
sand to its intended destination
in deep vertical natural gas wells
with bottom hole temperatures
exceeding 300° Fahrenheit.
The high-temperature borate
cross-linked guars that followed
later made the frac fluids thicker.
However, the resin-coated sands
were affecting viscosity. In
response, Santrol came up with
a solution that encapsulates the
resin-coated sand in an inert
outer shell — characterised as
“similar to a peanut M&M” by
Smith, who was involved in that
project.
“As frac fluids became more
advanced, our resin-coated sand
technology had to keep pace with
that and stay customisable,” he
adds.
UPSTREAM TECHNOLOGY Q3 2013
41