American Oil and Gas Reporter - July 2015 - 50

SpecialReport: Horizontal Well Operations
Additional multiwell studies are needed for proof of concept, Abou-Sayed
notes. "The potential for refracturing is
tremendous, and can be quite rewarding for
companies that understand both the forces
affecting performance and the relative
economic benefit of each well," he says.
As a case in point, Abou-Sayed offers
the Haynesville Shale, where he says he
studied 300 wells. "For an independent operator in North Louisiana, we initially selected more than 90 candidate wells,
which we slimmed to 60," he recalls. This
was further slimmed, based on economics, safety and potential interference."
In mid-2014, Abou-Sayed reports, the
operator refractured six wells. The first well
added 2 billion cubic feet of proved reserves. The operator is evaluating the well
performance for further work assessment.

uneconomic refracs also have been performed in almost every reservoir type.
Vincent says his research demonstrates that the reasons to pursue refracs
are different in each formation.
"The key to a successful refrac program
is to first identify the suspected failure
mechanisms and then design a field test
structured to confirm and address the
problems," he advises. "Blindly pumping
a refrac without trying to learn what was
done incorrectly the first time is engineering malpractice."
Vincent has assessed the results from
hundreds of refracs, including more than
300 performed in the Bakken Shale. He reports the Bakken wells have had a wide
range of outcomes.
"Some Bakken refracs doubled initial
well productivities, while some failed attempts so thoroughly damaged the wells
that they subsequently were plugged
and abandoned," Vincent reports. "But
there are specific categories of Bakken
candidates that have shown more than a
90 percent economic success rate, with
every refrac attempt adding in excess of
80,000 barrels of reserves, easily exceeding a 300 percent rate of return on the refrac investment."
Although calculating incremental reserves shortly after a refrac is challenging,
Vincent observes that oil reservoirs provide
a unique "tracer" that helps confirm whether
new areas of the reservoir are contacted.

Challenging Conventional Design

Refracs have been performed successfully in every formation type, and in
both gas and oil reservoirs, suggests Mike
Vincent, an independent consultant who
specializes in refrac training.
In his refrac schools, Vincent says he
shares the results from refracs in 32 oilbearing and 28 gas-bearing formations-including sandstones, shales, limestones, diatomites, conglomerates, coals and other
rock types-demonstrating that refrac opportunities are not limited to specific formation conditions. However, he cautions,

FIGURE 2
Oil Production Response after Refrac and Tri-Frac (Bakken Well)
6,000

What is the mechanism?
Salt dissolution?
Rearrangement of proppant pack?
Gel removal?

5,000

D1 Initial

4,000

D1 Refrac
BOPM

D1 Trifrac
3,000

2,000

1,000

0
0

50
Months

50 THE AMERICAN OIL & GAS REPORTER

100

"Often, after restimulation, we will see
the gas-to-oil ratio drop. This return to the
near-original GOR plus pressure data
confirms that the refractures reached new
areas of the reservoir, and help substantiate the operator's estimate of incremental reserves" he explains.
Vincent says he also reviewed horizontal refracturing results in the Bakken to
identify where economic success was
most likely. He says he found that refracturing wells initially fractured with sand
had a higher economic success rate than
similar wells initially fractured with ceramic proppant. He reasons that durable proppants either have delayed or avoided the
need to refrac some wells.
Successful refracs have been performed in a wide range of Bakken well
types, including single laterals, multilaterals, cemented, barefoot and uncemented liners. With the enormous range of well
configurations in the Bakken, Vincent says
he considers it a "refrac playground"
with interesting opportunities to learn on
each well.
Identify Optimal Treatment

According to Vincent, suspected failure mechanisms should be ranked for candidate wells before designing the refrac
treatments. "For example, in some fields
there is strong evidence that fractures are
'salting up,' or filling with scale or fines,"
he reports. "In others, the dominant damage mechanism is the fracture closing near
the well. The initial treatment overflushed
the sand, and the near-wellbore void has
collapsed."
By understanding the failure mechanism, Vincent says, operators can identify the optimal treatment. "Some wells may
be rejuvenated with a tiny refrac because
they have a near-wellbore problem," he attests. "Pumping millions of pounds of
proppant would be far more expensive
than necessary."
Vincent illustrates how mechanisms
and treatments vary using a series of
real-world examples. For one Bakken
well completed with millions of pounds of
proppant, a refrac containing zero proppant
was performed. In this case, the sandless
refrac was pumped about 15 months after
the well had commenced production. As
shown in Figure 2, monthly oil production

American Oil and Gas Reporter - July 2015

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