American Oil and Gas Reporter - August 2019 - 94

SpecialReport: Hydraulic Fracturing Technology

Expandable Liner Keys Refrac Success
By Amrut Athavale,
Adebayo Onafowokan,
Carl Kasdano,
Sunil Lakshminarayanan
and Lisset Sousa
HOUSTON-The original frac designs
pumped on many of Occidental's Petroleum's older Bone Spring horizontal wells
in the Delaware Basin in Southeast New
Mexico used less sand and wider cluster
spacing than its modern wells. Hydraulic
fracturing design has evolved significantly,
with newer Bone Spring wells showing
increased productivity. More proppant
and fluid, tighter cluster spacing, and
longer lateral lengths are some of the
main drivers enhancing production. This
evolution of frac design has led to the
conclusion that many of the older wells
were understimulated.
To recover the reserves left behind in
understimulated wells, Oxy has conducted
several trials to restimulate older Bone
Spring wells and return them to profitable
production. Some used particulate diverters
to isolate and divert flow, while others involved coiled tubing and packers. The
biggest challenges to the success of these
refracs were isolating existing perforations
and ensuring that the project was economic.
Oxy's New Mexico business unit teams
felt that installing a new liner inside the
original cased lateral was the only way to
have a completely isolated wellbore that
can be fractured as a "new" well.
To qualify different mechanical isolation
technologies, the company has performed
four full-lateral refrac field trials to test
both cemented and expandable liners in
Bone Spring wells with vintage completions. Well-defined refracturing candidate
selection criteria were developed based
on several parameters tied to improved
production and economic thresholds to
identify wells that can yield the highest
returns on the capital invested. Equally
important are wellbore preparation and
lateral cleanout criteria to ensure that the
liner can be run to bottom and expanded
successfully, and a comprehensive study
of the candidate well's history (including
interventions and chemical treatments).
The results show that with effective
mechanical isolation, highly effective
modern frac designs can be applied to
refracs successfully. One cemented liner
refrac showed poor isolation during treat94 THE AMERICAN OIL & GAS REPORTER

ment. Expandable liners showed good
isolation, but some expandable technologies were better than others. Rigorous
planning and engineering were considered
crucial to ensuring trouble-free operational
execution and to delivering an optimum
wellbore for refracturing purposes.
Operational issues were encountered
on each of the refracturing candidate
wells, but all four wells nevertheless were
stimulated successfully using the latest
frac design. The refractured wells all had
significant increases in production. The
full-lateral refrac results unlocked the
unrecovered reserves at a lower cost than
drilling new wells, and all four refracs
outperformed the expected type curve
and yielded positive economics.
Selection Criteria
The primary driver for the refracturing
work was to boost base production and
unlock stranded reserves with low capital
investment. Older wells producing from
the Bone Spring formation in several different areas in New Mexico were analyzed
by the reservoir teams. Field trial wells
were selected and ranked based on the
wells' performance and reservoir characteristics, including:
· Stimulated lateral length, with longer
laterals expected to have better estimated
ultimate recovery since the potential for
contacting more reservoir increases with
lateral length;
· Original completion design, with
older completions having low sand volumes
per foot, long cluster spacing and fewer
stimulated stages over the lateral length;
· Cumulative production (wells with
high cumulative production would represent a risk to stimulating new rock because of reservoir depletion);
· Forecasted EUR based on incremental production generated from the
new stimulated rock volume (SRV);
· Rate of return (project economics
had to meet a pre-established ROR threshold); and
· Partner participation (wells with
100 percent working interest were preferred candidates).
In addition, the teams also investigated
wellbore and operational conditions, including:
· The condition and characteristics
of the installed production casing (size,
weight, grade, connection, etc.):

· The wellhead configuration, considering that wellhead equipment with a
10,000-psi rating would not require a
wellhead isolation tool to pump modern
frac treatments in refrac candidate wells;
· Lateral access and mechanical issues, such as well intervention data showing parted casing or a mechanical obstruction (sleeves or permanent flowthrough plugs) that would reduce effective
lateral length, or extensive clean-outs or
difficult mechanical issues in the laterals
that would increase cost; and
· The ability of existing facilities to
take on additional volumes, and the effect
on project economics of any required capacity upgrades.
The wellbore conditions in the four
Bone Spring wells selected for refracturing
were favorable based on the selection
criteria. The produced volumes in all four
wells were low, and the forecast EUR
values using the original production trends
were not economic. Since the produced
volumes were low, the level of depletion
around the wellbore was presumed to be
low, providing the opportunity to increase
the SRV and contribute to higher recovery
by placing additional frac stages with
higher proppant volumes per foot in the
lined laterals.
Cemented Liner Well
Regardless of the isolation technologies
or techniques used, proper planning, engineering design, and reacting efficiently
to operational challenges during lateral
preparation and refracturing treatment
were key to controlling costs and preserving project economics.
Cemented liners were being used to
perform refracs in wells in Oxy's Texas
Delaware Basin acreage, so the New
Mexico team decided to use the same
technology to refrac Candidate No. 1. A
22-stage modern frac treatment was
planned in the well's 3,400-foot long
lateral in the Bone Spring formation. The
plan for preparing Candidate 1 well was
to seal off existing perforations using
liquid diverter material, install a 41⁄2- by
4-inch tapered casing string inside the
existing 51⁄2-inch production casing, and
cement it in place for interstage isolation.
The artificial lift equipment was pulled.
A 27⁄8-inch workstring was run to the
kickoff point, and two liquid diverter
pills were bullheaded to close off open
American Oil and Gas Reporter - August 2019

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