American Oil and Gas Reporter - April 2015 - 67

SpecialReport: Refrac Facts

Refrac Timing

The timing of a refrac depends on the condition that created
the need for a refrac and whether that condition can be reversed
by refracturing. The timing for some conditions may be "never."
Insufficient remaining reserves or geologic factors such as regional fracture systems that allow the first wells in a field to produce
from a large area, reducing reserves and pressure across the field,
are examples of the "never" classification.
Timing may be influenced by completion- or reservoir-dominated problems. Completion-dominated reasons for a refrac include overflushing of proppant away from the wellbore and production of fluids (and proppant) before the fracture closes. Flow
capacity in the intersection area of a transverse fracture with the

FIGURE 1
Inverse Variation of GOR and Oil Rate over Time
Oil Production
180

Gas-to-Oil Ratio

1,200

160
1,000

Oil (bbl/d)

140
120

800

100
600

80
60

GOR (Mcf/d/bbl)

in a single well.
Early production rates are proving the technology and some
flattening of decline curves has been reported. One example is
a refrac pilot program implemented by BP America Production
Co. in the Woodford Shale (see SPE 168607, presented at the 2014
Society of Petroleum Engineers Hydraulic Fracturing Technology Conference).
There are many ways to pump a refrac. A single large refrac
with diverting is one of the best methods for cost control and effective treatment. This type of refrac may be a departure from
the large initial designs that had 10 to 30 or more stages, and the
focus is on reopening and stabilizing initial fracture paths as well
as taking advantage of stress realignments provided by production to increase contact areas in the reservoir.
Established fractures and new areas are both targets and can
be combined in a single refrac, if diverting options are fitted to
the design. Adding perforations is a possibility as long as the number of open perforations is low enough to allow fracture extension and new breakdowns.
There are several common approaches to focusing energy into
effective refracturing, including:
* Cementing off existing perforations where tracers or production logs have indicated no flow, and then adding new perforations at different points along the lateral;
* Running a liner to seal off old perforations and then reperforating;
* Refracturing the existing perforations with or without adding
a diverting agent;
* Adding perforations along the wellbore and fracturing new
and old perforations (with and without diverting agents);
* Restoring proppant at the intersection of the fracture and
the wellbore (this can be highly effective if proppant was overflushed or produced by early flow); and
* Refracturing specific intervals with a straddle packer on tubing or coiled tubing.
The type of refrac depends on well conditions and local economics.

400

40
200
20
Refrac
0

0
0

10

20
30
Months on Production

40

50

Source: Modified from Vincent, Insight Consulting 2010

horizontal well is a vulnerable component in producing a multifractured horizontal well.
Flow velocity through this highly restricted area may be in excess of 100 feet per second, easily enough to remove untrapped
proppant either during the last injection steps or early production
flow before fracture closure. In low-permeability formations such
as shales, fracture closure may take several days before proppant
flowback will no longer be a possibility.
Examining declines in gas and oil production is instructive
to both demonstrate the importance of natural fractures on liquid flow through the shales and to express the timing of a refrac
by showing natural fracture closure. Drawing on Mike Vincent's
extensive refracturing work (see, for example, CSUG/SPE
136757, presented the 2010 Canadian Unconventional Resources
& International Petroleum Conference, and SPE 134330, presented at the 2010 Annual Technical Conference & Exhibition),
a schematic of gas-to-oil ratio and liquids rate behavior in a
Bakken producer was overlaid with very rough trend lines for
GOR and oil rates that showed near mirror-image reactions (Figure 1).
This reworked figure highlights the obvious closure of the enhanced, but unpropped liquid flow routes in this well's drainage
area. Gas can move more easily through narrower fractures than
more viscous liquids. Refracturing opened, but did not stabilize,
the drainage paths, as seen by the relatively quick decreasing oil
rate and the increasing GOR. Refracturing for naturally fractured
formations must focus on creating and stabilizing complex fracture development.
Refrac Economics

Refrac economics depend on well cost versus cost and returns
on an effective refrac design. An analysis of 16 refractured Eagle Ford oil wells showed an average EUR increase of 57 percent, with the increase ranging from 10 to 184 percent (see SPE
173340, presented at the 2015 Hydraulic Fracturing TechnoloAPRIL 2015 67



American Oil and Gas Reporter - April 2015

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