American Oil and Gas Reporter - August 2019 - 91

SpecialReport: Hydraulic Fracturing Technology
turn limits the total number of perforations
to which fluid and proppant are delivered.
If one attempts to pump into too many
perforations, some may not be stimulated
effectively.
Various studies where wellbore fracture
diagnostics were deployed by, among
others, ConocoPhillips, Devon Energy,
Liberty Resources, Schlumberger and
Shell, show that perforation cluster effectiveness (i.e., the number of clusters
being stimulated during a frac job) was
often not much better than 50%-70%.
This has driven operators to find ways
to improve perf cluster effectiveness in
liquids-rich shale basins while also changing their strategies in order to maximize
fracture density using more clusters. The
graphs in Figure 4 show how these attempts have changed limited-entry strategy
parameters in one such liquids-rich basin.
Of primary interest are the rate per
perforation, the number of perforations
and the number of perforation clusters.
However, as most of the data is not available in the public domain, we must settle
for rate per foot per stage, a proxy for
the hydraulic fracture density created
along the lateral.
As the graphs show, rate per foot per
stage is up significantly, requiring service
companies to bring ever-more horsepower
to location. After a long period of reducing
stage intervals in lateral feet, stage intensity
appears to be stagnating in these basins
at about 200 feet/stage. Some operators
now are increasing their feet per stage
while using a more aggressive limitedentry strategy as a way to reduce cost by
pumping fewer stages while keeping proppant and fluid intensity per lateral foot
unchanged. Operators also are exploring
more efficient ways to treat these intervals
by increasing cluster count per stage and
rate while keeping the number of perforations about the same.
Limited-entry perforating, when done
properly, can save in stage counts and
lower well cost while creating a denser
fracture network. While service companies
have provided volume discounts on larger
proppant volumes, they also have reduced
their charges per hydraulic horsepowerhour, making up for this reduction by
pumping more hours every day.
Limited-entry continues to lower operators' cost per boe and continues to
spread the wealth in the form of more
prolific oil and gas production. This ultimately results in spreading the wealth to
energy consumers around the world, who

FIGURE 4
Evolution of Limited-Entry Perforation Strategy in Liquids-Rich Basin

Stabilizing ft/stage

Increasing clusters/stage

Slightly increasing perforations/stage

Increasing rate ft/stage

have saved more than a collective $1 trillion a year through shale revolution efficiencies since 2014.
Unruly Parent Wells
Many aspects of modern frac designs
focus on creating a balanced distribution
of fractures and placing an equal distribution
of proppant within these fractures. Extreme
limited-entry methodology is creating a
more equal distribution of fractures along
wellbores, creating higher-producing modern child wells than ever before.
One thing stands in the way of these
productive children: bad behavior from
their unruly parents. Their parents, back
in 2008-12, often were completed with
poor frac designs. That was not entirely
their fault. Technology still had to come
a long way to, for example, make plugand-perf in cemented wellbores an effective technique in horizontal wells with
measured depths of 15,000+ feet.
Among the worst behaviors were the
"Hail Mary" frac (a single large fracture
in a horizontal wellbore where it was
hoped the pump-and-dump frac would
stimulate a large portion of the lateral).
In the extreme, a single fracture could
have been created in a well, resulting in
frac fluid and proppant intended for
dozens of clusters all going in one place,
possibly creating a fracture with a halflength in excess of 1,000 feet.
This bad parent behavior likely is responsible for the fact that operators sometimes see, for example, near-immediate
frac hits during small-volume diagnostic
fracture injection tests in child wells,

which are observed almost instantly in
wells hundreds of feet away. This is not
caused by the child-well fracture, but by
the extensive conductive flow path created
in the parent.
In a 2015 infill-well-spacing pilot
project in the Williston Basin, Liberty
Resources documented some of these
observations. In this case, the instantaneous shut-in pressures in a child well
showed production depletion from fractures in a few parent wells, one of which
was treated with a single Hail Marystyle frac and another with a six-stage,
open-hole sliding sleeve completion (for
information on the company's Bakken
completion strategy in the Williston, see
"'Extreme' Perforating Strategy Key to
Liberty Resources' Bakken Completion
Design," AOGR, September 2018).
As shown in Figure 5, the Liberty Resources project observed three common
offset-well bottom-hole pressure behaviors
in parent wells from multistage fracture
treatments in a child well:
· »1,000 psi frac hits during every
stage (orange);
· »100 psi frac hits that stopped after
about 10 stages as proppant from the
parent well was pushed back into the
liner and limited further communication
with that well (yellow); and
· Minimal interference in an offset
well one location over from the yellow
well (blue).
The cost of cleaning up the yellow
well can be on the order $400,000.
Therefore, operators have started opting
to have pump-down services available
AUGUST 2019 91

American Oil and Gas Reporter - August 2019

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