American Oil and Gas Reporter - August 2016 - 62

SpecialReport: Hydraulic Fracturing Technology

Variable-Rate Frac Boosts Production
By Jordan Ciezobka,
Debotyam Maity
and Iraj Salehi

The variable-rate design rapidly changed
the fluid pump rate from a predetermined
maximum to a significantly lower rate,
and then rapidly increased back to the
original maximum rate, while pumping
each stage.
Surface fracturing pressure data show
that rapid pump rate changes open additional perforations without physical flow
diverters such as ball sealers or frac balls,
while production log data indicate higher
production from stages with variable
rates. Specifically, the rapid rate changes
produce a "pressure pulse" that travels
up and down the wellbore to open previously unopened perforations and increase
fracture complexity through fluid diversion.
Increased microseismicity was observed while fracturing the stages with
frequent pump rate changes. Regardless
of their type and nature, seismic signals
indicate fragmentation of the stimulated
rock. This could be from shear slipping
or dilatational opening. One also can assume that high signal density is a good
measure of fracturing efficiency.
Production log results showed an average 18 percent increase in production
for the stages pumped with variable rates
versus the stages pumped at a constant
rate. A lower treating pressure was often
encountered after the rapid rate changes,

DES PLAINES, IL-Hydraulic fracturing designs in shale plays typically
utilize a predetermined fluid pump rate,
which once achieved, is held constant
throughout the treatment except for situations where surface pressure limitations
or other conditions prevent it.
But what happens when pump rates
are intentionally varied during treatment
instead of kept constant? Could implementing variable rate hydraulic fracturing
increase stimulation efficiency without
adding additional cost?
Previous hydraulic fracturing and microseismic data collected in the Marcellus
Shale indicated that substantial rapid
changes in fluid pump rates during hydraulic
fracturing can in fact result in increased
levels of microseismic activity emanating
from the stimulated reservoir, which might
be interpreted as creating more fractures,
leading to additional fracture complexity.
To investigate these observations, a
study was conducted in a Marcellus horizontal well. During frac treatment, variable
pump rates were implemented in all the
odd stages while rates were kept constant
in all the even stages to account for
changes in the reservoir along the lateral.

FIGURE 1
Typical Marcellus Frac Stage (Top) and Microseismic Event Density
During Treatment (Bottom)
Treating Pressure
Pump Rate
Proppant
Concentration

4,000.00
3,500.00

2,500.00
2,000.00
1,500.00
1,000.00

Event Density
(events/hr)

3,000.00

Times during
frac stage
corresponding
to pump rate
changes
Microseismic
Event Density
(events.hr)

500.00
0.00

62 THE AMERICAN OIL & GAS REPORTER

leading to the conclusion that unopened
perforations were opened with the induced
rate changes, and resulting pressure pulses.
Moreover, the total production decline
was much slower for the test well treated
with variable pump rates than an offset
horizontal well that did not include the
variable pump rate frac design.
In addition, water hammer frequency
decay analysis shows a predictable trend
in the well with variable pump rate stages.
Throughout the variable pump rate stages,
no proppant transport issues were encountered and the frac stages were completed without any major problems.
Project Motivation
The top of Figure 1 shows a typical
hydraulic fracturing treatment in the Marcellus with treating pressures (red) and
pump rates (blue). The bottom of the
figure shows bore hole microseismic
event density aligned to the treatment
plot, with the shaded columns corresponding to rapid fluid pump rate fluctuations.
In this case, however, the exact sequence
of rate changes was not planned. The
changes were related to pumping equipment or wellbore conditions.
Microseismic event density clearly significantly increases during rapid pump fluctuations, both positive and negative. Similar
pump rate fluctuations and microseismic
event density correlations have been attained
with a dataset consisting of almost 100
fracture stages in the Marcellus.
Furthermore, production log data collected in the horizontal lateral and compared with the microseismic survey results
show the microseismic event count and
event proximity (proximity of each microseismic event to the next closest recorded event) for each fracture stage correlate
with production from each fracture stage.
The log data indicate increased production
coming from fracture stages where the
microseismic event count is high, while
the production contribution also is reduced
in stages where the event count is low.
These findings were the motivation
for the Marcellus field study of the variable-rate frac method. The project sought
to test a hydraulic fracture treatment designed to rapidly fluctuate the pump rate
in a controlled and systematic sequence,
so that rate fluctuations were intended,
rather than a result of unanticipated equipment or wellbore conditions.

American Oil and Gas Reporter - August 2016

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