American Oil and Gas Reporter - August 2017 - 53

SpecialReport: Hydraulic Fracturing Technology

VFRs Illustrating Value As Frac Fluids
HOUSTON-Viscosity-building friction
reducers (VFRs) are demonstrating value-adding benefits in hydraulic fracturing
operations in horizontal resource plays
across the country.
In addition to reducing pipe friction
during fracturing, VFR fluids develop
higher viscosities than traditional slickwater systems, providing operational advantages ranging from reduced water and
chemical requirements to on-the-fly design
change flexibility and high fracture conductivities. Moreover, by incorporating
fit-for-purpose monomer structures, effective breakers and other advanced polymer chemistries, VFR fluids exhibit higher
regain conductivities than guar-based
linear and cross-linked gels.
Fracturing strategies in unconventional
reservoirs generally include a combination
of two objectives: Develop as much fracture complexity and surface area contact
as possible during early treatment stages,
and increase the conductivity of the fracture network in the near-wellbore and
intermediate fractured region to sustain
high production rates. Accordingly, treatment pumping schedules typically incorporate small sand sizes in the early stages,
followed by higher concentrations and/or
larger-sized proppant in later stages. That
can require using two or more fluid types
during a frac job, with operators often
pumping slickwater in early stages and a
higher-viscosity fluid in later stages. In
contrast, VFR fluids allow the entire
pumping schedule to be executed with a
single fluid. Viscosity increases are controlled by simply increasing the concentration of the liquid friction reducer.
Polyacrylamide-based polymer systems
(PAM) have been used for years in
drilling, acid stimulation and enhanced
oil recovery, usually incorporating highmolecular weight systems to attain good
fluid stability under a variety of harsh
conditions. As PAM began to be used to
reduce friction during fracturing treatments, monomer chemistries evolved to
create lower-molecular weight systems
that are cleaner than guar and modified
guar gelling agents.
The advantages of using VFRs as frac

fluid include lower cost, reduced logistical
complexities, enhanced physical properties,
optimized fracture placement given higher
conductivities, and on-the-fly design
change capabilities. By using a single
fluid system that can be adjusted to
provide a range of viscosities, fewer
chemicals are required on location, simplifying logistics and reducing tank requirements. In fact, overall chemical volumes can be reduced by one-third or
more when using a VFR-based treatment.
Because of the higher proppant concentrations that VFR fluids can transport,
total water volumes also can be reduced
by 30 percent to place an equivalent
proppant volume as slickwater.
Another area of cost reduction is the
expense of materials and services themselves. Although difficult to quantify with
changing market conditions, several operators have reported lower overall chemical costs when switching from hybrid or
cross-linked gel to VFR fluids. In the
Eagle Ford Shale, for example, Earthstone
Energy Inc. has documented overall completion cost savings of 35 percent, of
which 17 percent are attributed to pressure
pumping charges.
Additional cost savings arise from
needing less fracturing equipment on location. Gel hydration units and dry gel
handling equipment are not required be-

cause of the rapid hydration rate of VFR
polymer. Also, with the friction reducing
properties of VFR fluids, pumping pressures are lower than for cross-linked gels
pumped at similar rates-equating to lower
onsite hydraulic horsepower requirements.
Friction Reduction
Figure 1 shows the friction reduction
obtained with a VFR fluid system at various friction reducer concentrations. The
test is started by pumping freshwater
through a 3/8-inch laboratory flow loop.
The freshwater exhibits high differential
pressure at the start of the test. As friction
reducer loading is increased to 0.5 gallon
per thousand gallons of water (gpt), a 70
percent reduction in friction occurs. As
the concentration of friction reducer loading is progressively increased from 0.5
to 3.0 gpt, only a slight decrease in
friction reduction occurs (from 70 to ~60
percent).
The PAM-based VFR fluid described
in this article yields elevated viscosities
at relatively low concentrations using a
complex branching structure of the
monomer. The lowered polymer loadings
provide for a cleaner and more cost-effective system. Clean breaks can be
achieved even at low temperatures using
a liquid breaker. While the VFR performs
best in mixing waters with low hardness,

FIGURE 1
VFR Fluid Friction Reduction at Various Concentrations
76
72
68
64
60
Pressure Drop (psi) & Friction Reduction (%)

By Steve Collins,
Mark Van Domelen,
Wayne Cutrer
and Michael Ruegamer

1.0 gpt

0.5 gpt

56
52

2.0 gpt

3.0 gpt

48
44
40

Flow Rate (gpm)
Delta P (psi)
% Friction Reduction

36
32
28
24
20
16
12

8
4
0

0

1

2

3

4
Elapsed Time (min)

5

6

7

8

AUGUST 2017 53



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