American Oil and Gas Reporter - July 2015 - 52

SpecialReport: Horizontal Well Operations
FIGURE 3
Results of Refracturing with Small Proppant Volumes
(Two Bakken Wells)
These refracs utilized less than
10% the mass of proppant
placed in the original
treatments

8,000

7,000

When a tiny refrac (<100,000 lbs) makes a
well "better than new," it is clear we are
addressing near-wellbore issues.

6,000
C1 Initial

BOPM

5,000

C1 Refrac
C2 Initial

4,000

C2 Refrac
3,000

Late term: deal with scale,
acidize, pull pumps, etc.

2,000

1,000

0
0

40
Months

after refracturing slightly exceeded initial
production rates. Few participants in his
refrac courses predict that outcome, according to Vincent.
Interestingly, he points out that the production decline rate was virtually the
same for both the initial completion and
the refrac. Consequently, about 15 months
after pumping the refrac (30 months after
commencing initial production), the same
Bakken well was again refractured using
no proppant. The "tri-frac" again improved production, but this time to about
60 percent of IP.
As noted in Figure 2, unusual treatments such as these unpropped jobs can
help identify failure mechanisms. Careful
evaluation of flowback fluids can determine whether the freshwater injection removed salt or gel residue from the proppant pack, providing key insights that can
improve future treatment designs by including salt inhibition or higher concentrations of gel breakers, Vincent says.
Figure 3 shows results after refracturing two Bakken completions that had nearwellbore problems. In both cases, the refracs were performed within the first
year of production.
Vincent reports that the refrac treatments pumped less than 10 percent of the
proppant mass placed in the original treatments, bull-heading fewer than 100,000
pounds of ceramic proppant into wells orig52 THE AMERICAN OIL & GAS REPORTER

inally fractured in 7-10 stages with more
than 1 million pounds of sand each.
"These tiny refracs made the wells 'better than new,' repairing a near-wellbore issue unrelated to fracture half-length, diversion, or other mechanisms," he describes.
In a third example, wells initially
stimulated with ceramic proppant were
restimulated with small sand refracs.
Vincent says one refrac was economic,
but the other two collapsed within two
months of showing encouraging initial
production. Even a tri-frac was unable to
restore initial productivity, suggesting that
bull-heading small-diameter sand into the
existing fractures caused permanent damage, he reasons.
"Many curious things have been documented in refracs," Vincent concludes.
"Success depends on careful analysis and
testing."
Failure Mechanisms

In the Bakken, Marcellus, Eagle Ford,
Niobrara and other plays where propped
fractures have intersected adjacent wells,
Vincent reports that pressure communication testing has shown that the created fractures provide insufficient initial conductivity, and progressively collapse or "heal"
over time, providing essentially zero hydraulic continuity after six to 24 months.
"The ramifications of fracture collapse are that companies are drilling very

closely spaced wells, placing perforation
clusters much closer than otherwise needed, and increasingly conducting refracs,"
he states.
Vincent says there are myriad conditions that can cause failure, each with its
own best resolution. "Determining the failure mechanism is critically important in
defining a refracture strategy and ensuring that refracturing is economic," he says.
He advocates careful analysis and
testing prior to beginning a refrac program
to determine the best method for each well.
"There is no 'one size fits all' approach
that has yielded the best technical and economic result," he insists.
"The list of failure mechanisms is
long, and cannot be analyzed properly and
addressed by a single discipline," Vincent
adds.
Instead, he says he believes multidisciplinary asset teams should analyze candidate wells to hypothesize the key mechanisms, then use field trials to confirm the
best approach instead of simply repeating
the first successful refrac design.
Although he is adamant that all problems are multidisciplinary, Vincent reluctantly groups fracture collapse mechanisms
into three categories: engineering, geology and "unclaimed." He says each well
may suffer from one or more failures in
one or more categories. Understanding the
failure mechanisms involved in loss of
continuity and production decline aids in
identifying the best refracturing method.
It also can help in predicting the initial and
long-term effects of refracturing.
The first category of problems, according to Vincent, arises from the original
completion design. It includes proppants,
fluids and perforations. These problems are
commonly "owned" by the engineering
discipline. Proppant-related failures include:
* Insufficient strength and durability,
which leads to degradation and fines
plugging;
* Thermal degradation of sand-based
proppants;
* Overflushing of proppant from the
near-wellbore area in transverse fractures;
* Insufficient placement of proppant
throughout the network, both lateral and

American Oil and Gas Reporter - July 2015

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