American Oil and Gas Reporter - April 2015 - 72

SpecialReport: Refrac Facts
Whereas conventional wells are completed to accommodate future remediation,
this has not been true for unconventional
wells. To speed production and maximize
economics, operators relied on methods
that accessed the formation as efficiently
and inexpensively as practical, starting
with plug-and-perf techniques, which
were later joined by open-hole completions
with packers and ball-drop sleeves. Future
refracs simply were not part of the completion plan.
To be fair, there have been virtually
no restimulation technologies available
to be considered. That could be changing,
however, as new completion technologies
enter the market.
In theory, refracs make a lot of sense.
There are hundreds of thousands of existing wellbores in unconventional formations. Some percentage of those wells
should be potential refrac candidates, and
recompletions should cost less than new
drills. It is known that there are a lot of
reserves in place-more than 90 percent
of the original reservoir content in tight
formations-bypassed by the original completions. In fact, a successful refrac can
restore production to levels on par with
the IP rate. Finally, restimulation is a
proven technology that works well in
conventional vertical wells.
Properly executed, when refracturing
in unconventional wells:
* New fractures along the lateral
should contact virgin reservoir rock. Many
wells completed in the early days of the
shale boom suffered from poorly designed
or incorrectly placed fracs. As a result,
pay zones were missed or not stimulated
to their full potential.
* Existing fractures should extend to
contact untapped formation. It is believed
that stress reversal effects can reorient
the fracture extensions to propagate perpendicular to the original fractures, adding
significant reservoir contact.
* It should restore the conductivity
of the original fractures and clean up
near-wellbore damage, which can improve
well performance dramatically. Studies
have shown that, over time, proppants
break down under formation stresses,
and asphaltenes and fines settle into
critical conductivity pathways, resulting
in premature production declines.
Unfortunately, all these require effective zonal isolation and a lot of hydraulic
energy, and wellbore issues from the
original completion make those very difficult to achieve in practice.
72 THE AMERICAN OIL & GAS REPORTER

Refrac Wellbore Issues
The original stimulations could be accomplished with sheer brute force because
there were few, if any, wellbore issues.
With new pipe and no restrictions or
leaks, rates and pressures could always
be matched to the job, and stages were
easily isolated for stimulation. That is
not the case with refracs. Existing wellbores are full of perforations, and in the
case of open-hole completions, often are
full of ball-seat/sleeve restrictions. Each
of these completion methods presents its
own obstacles that must be overcome for
a refrac to be successful.
By nature, wells completed with plugand-perf methods present significant wellbore-integrity challenges. The casing or
liner is riddled with holes, making pressure
control, zone isolation, and fluid circulation
economically impractical, if not impossible.
Open-hole completions with ball-drop
frac sleeves have no cement or other
barrier behind the casing string; the
original stimulation was allowed to flow
behind a long section of casing and enter
the reservoir through the path(s) of least
resistance.
This type of completion presents three
problems for restimulation. First, because
there is so much open formation outside
the frac sleeve, the secondary treatment
will not be effective at the lower rates
that can be pumped through a straddle
system; the available force cannot be focused. Second, interzone communication
past isolation packers is not uncommon,
and there is a high chance of full communication to the zone above the refrac
straddle assembly. This could introduce
enough debris on top of a refrac bottomhole assembly to lead to extensive fishing
operations. Finally, with open communication behind casing, there is a heightened
well control risk with a straddle assembly
in the well.
Although most ball seats can be milled
out, any remaining restriction or milling
damage can prove problematic for refrac
straddle tools. Seals on straddle systems
are prone to getting stuck in restrictions,
and the jagged and unpolished inside diameter of a milled sleeve can ruin the
seals.
The completion method most conducive for refracs appears to be a relatively
new technology that represents only a
tiny fraction of unconventional completions to date: single-point, cemented completions. This method combines cemented

casing and frac sleeves with a coiled tubing-deployed completion tool that isolates
each stage for single-point stimulation.
The advantages for refrac are the fulldrift wellbore, easily located and isolated
stages, and a cemented annulus that prevents communication between stages.
For existing wells, refracs must be
performed using a straddle assembly, but
the frac rates typically are sufficient,
thanks to the single-point injection. Also,
new stages can be added using sand-jet
perforating. However, it is still important
to secure the wellbore above the target
zone in case the refrac treatment manages
to communicate through the formation
to the stage above.
Limited Options
Unfortunately, even for promising candidate wells, currently available refrac
options have some challenging limitations.
Complications can quickly lead to cost
overruns, project abandonment because
of mechanical problems, and even loss of
a well. However, as refrac demand grows,
the industry surely will respond with new
technology and techniques, just as it did
at the outset of the shale revolution.
But for now, there are only a handful
of ways to address the wellbore problems:
squeezing off all perforations with cement,
installing expandable steel liners, pumping
permeability modifiers and temporary
blocking agents, and using a coiled tubing
straddle assembly. The method used depends on the nature of the formation and
the condition of a particular well.
To restore pressure integrity, perforations could be sealed off with cement
squeezes. The cost can be prohibitive
except, perhaps, for older completions
with short laterals and few stages. It is
also unlikely that the squeeze would withstand a high-pressure, high-rate frac.
Expandable steel liners have been installed to create a fresh wellbore through
the lateral section. The drawbacks are
high cost, reduced collapse rate and the
reduced inside diameter of the liner,
which limits recompletion options. However, expandable scab liners can accommodate higher pumping rates for a plugand-perf recompletion.
Biodegradable balls, gels and particulates have been used with some success
to seal all but the target zone, allowing a
treatment to be pumped. This method assumes that the bridging material will follow the path of least resistance, which
would be perforations that received the



American Oil and Gas Reporter - April 2015

Table of Contents for the Digital Edition of American Oil and Gas Reporter - April 2015

Contents
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