American Oil and Gas Reporter - July 2015 - 55

SpecialReport: Horizontal Well Operations
* A frenetic drilling pace, reducing
frac engineers' role to scheduling without
attention to frac durability.
Vincent says these may be the most difficult to avoid in large companies because
they are part of "the way things are
done." Investigating and addressing these
issues typically requires management involvement, since remedies require compromises among drilling, geology, operations
and completions personnel.
Designing The Refrac

"Refracs can improve production and
reserves by several mechanisms," observes Vincent.
He says fracture geometry may be enlarged to increase reservoir contact. Fracture conductivity may be improved by addressing underlying problems such as
embedment, cyclic stress, proppant degradation, scaling, salting, and so on.
New fluids and proppants may be
used to improve frac-fluid compatibility
and re-energize natural fissures, Vincent
continues. Stress field alterations may
lead to superior containment and reorientation.
In any case, Vincent says, significant
engineering effort is required to design the
optimal refrac. The approach is not as routine as a daily drilling and completion strategy. Once the potential mechanisms are
identified, tests can be performed to narrow the list, he says.

For example, Vincent offers, if a nearwellbore problem is suspected, it can be
tested by pumping a very small refrac with
a durable, conductive proppant. Success
would support the idea that it was a nearwellbore problem. "Otherwise, the results
support a diagnosis of problems with the
fractures away from the well," he remarks.
If the problem appears to be a salt
mechanism, a hot water wash with freshwater and no new proppant is a good test,
Vincent mentions. "Monitoring the salinity of the flowback water will reveal
whether salt is, indeed, the problem," he
says, adding that a test process can be devised for every potential damage mechanism.
Although more than 200,000 refracs
have been performed over the past 60
years, much remains to be learned about
refracturing in unconventional reservoirs,
according to Vincent. "The opportunity to
increase production and reserves through
refracturing is enormous," he holds.
Figure 4 illustrates how a refrac and
subsequent tri-frac incrementally generated higher oil recoveries without adding
new stages or perforations. "Although the
well produced 400 barrels of oil a day and
was highly profitable as initially stimulated, the initial frac job failed to reveal the
true potential of the well," Vincent reveals.
"Only after refracturing did the operator
recognize that the formation was capable
of producing at much higher rates."

A more conductive refrac design succeeded in doubling daily well productivity to 1,000 barrels of oil. "Most operators
would have stopped there and designated
this refrac procedure as the new 'recipe'
and began applying this cookbook to all
other wells in the field," Vincent says.
"Luckily, this operator continued experimenting and found an improved design
that could again more than double production with a tri-frac, clearly increasing
production by more than 1 million barrels
of incremental oil. These improved techniques were performed in more than 200
other refracs, tri-, and quad-fracs in the
field."
There also is an opportunity to improve
original frac designs by studying refracs,
Vincent adds. "In every field we have examined, we know the fractures were not
optimized," he states.
Vincent says he believes the key to improving original fractures is to consider the
potential failure mechanisms. "There is
something to be learned on each refrac job,
and these lessons can be carried back to improve initial frac design," he concludes.
"One key benefit of disciplined refracturing is that it helps us do a better frac job
in the first place. Refracs are great, and I
love to help clients restimulate old wells.
But an even more lucrative outcome is
when they discover how to treat the well
correctly the first time and avoid the expense of refracturing."
❒

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JULY 2015 55

American Oil and Gas Reporter - July 2015

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