American Oil and Gas Reporter - February 2015 - 83

SpecialReport: Improved Oil Recovery

Frac Technique Ups Stimulated Volume
By Tao Wan,
Xingbang Meng
and James J. Sheng
LUBBOCK, TX.-A modified zipper
fracture technique developed in a manner
different from a zipper frac in which the
fractures are initiated in a staggered
pattern has been demonstrated to improve
the contact area with the reservoir and
increase the effective stimulated volume.
Studies show that enhancing fracture
complexities in shale gas formations is
critical to improving stimulation treatment
and production performance. A miscible
carbon dioxide flood can reduce oil viscosity significantly. Reduced oil viscosity
combined with the increased area contacted
by hydraulic fractures can be the dominant
recovery mechanism.
Problems associated with gas injection
in conventional well patterns, such as
early breakthrough and channeling through
high permeability zones, likely will not
happen in nano-permeable shale oil or
gas reservoirs.
This article examines gas injection to
enhance natural gas condensate recovery
in a horizontal well pair: a gas injection
well and a producing well that is stimulated
in a modified zipper-frac pattern. The
approach integrates the advantages of hydraulic zipper fracturing, horizontal wells
and miscible gas flooding.
Miscible gas flooding has shown the
potential to improve recovery in shale
gas and oil reservoirs in simulations and
experimental work. Texas Tech University
developed a compositional model to simulate complex interactions between the
injected gas and reservoir fluids that were
not modeled precisely by black-oil simulation.

Our simulation of the Eagle Ford Shale
indicates that recovery increased to 86
percent for 4,000 days of secondary production, compared with 39 percent during
five years of primary recovery in a 100foot fracture spacing, staggered zipperfrac pattern. The investigation of CO2 injection in a modified zipper-fractured
horizontal well pair provides an insight
into EOR performance in nano-shale
reservoirs.
This article also discusses the modified
zipper-frac technique used to stimulate
adjacent horizontal wells to maximize
the exposure of new reservoir rock. Although the laterals expose more of the
nanodarcy-permeability shale rock, the
desired end result in hydraulic fracture
stimulation is to maximize coverage
around each lateral.
The benefit of the modified zipperfrac technique from a geomechanical
consideration is to increase stimulated
reservoir volume and complexity in successive fracturing stages, because the net
pressure created by the stimulation stage
on the first well helps divert the fracture
direction.
Experiment Design
Numerous numerical simulations show
that well productivity in shale oil and
gas reservoirs depends primarily on the
size of the fracture network and the stimulated reservoir volume (SRV), which
provides highly conductive conduits to
communicate the matrix with the wellbore.
The natural fracture complexity is critical
to the well's production performance,
and it also provides an avenue for injected
fluids to displace the oils.
Two horizontal wells are drilled through
the reservoir, and casing is set and ce-

TABLE 1

Initial
Comp.

Pc (atm)

Tc (k)

Acentric
Fac.

TABLE 2
Reservoir Properties
for the Model Input
Initial Reservoir
Pressure

Shift

6,425 psia

Reservoir Temperature

Peng-Robinson EOS Fluid Description
of Eagle Ford Condensate Lumping
Components

mented. The two horizontal wells are
spaced closely, one above the other, forming a gas injector and producer pair like
a steam-assisted gravity drainage system.
The perforation design plays a critical
role in the stimulation treatment. The effectiveness of the perforating process in
this horizontal well pair, cased-hole completion system depends on the perforation
location. Before selecting components
for a perforating job, the first task is to
understand how to prevent the hydraulic
fractures in the injection well from communicating directly with the producer.
This requires perforating a staggered pattern of effective entrance holes through
the pipe and cement.
Once the perforation and staging design
are completed for one lateral, the perforation scheme for successive laterals is
based on a staggered design in adjacent
wells. There are no perforations in the
injector in the same locations as the perforations in the producer. The primary
objective is to prevent the two wings created in the injector from communicating
with the producer.
In this case, there is no connection
between the hydraulic fractures propagated
in the injector and the horizontal production well. The solvent injected will not
break through to the producer through
the hydraulic fractures created in the injector. The CO2 injected into the injector's
hydraulic fractures effectively pushes the
oil toward the hydraulic fractures of the
production well. Components in the oil
transfer to the vapor phase, and components in the injection gas dissolve in the

335o F

Saturation Pressure

4,456 psia

Rock Compressibility

5.0 E-06

CO2

0.1618

72.8

304.2

0.225

44.01

0.094

0.7098

45.4

190.6

0.008

16.043

0.099

Porosity

6 percent

C2-3

0.079

45.7

330.5

0.119

36.0

0.16964

Permeability of Shale

100 nanodarcies

C4-6

0.026

34.9

453.8

0.226

72.0

0.2986

Water Density

62.4 lb/cu ft

0.02

26.0

606.0

0.359

128.0

0.4904

C22+

0.0034

14.9

869.7

0.788

310.0

1.0589

Hydraulic Fracture
Conductivity

83.3 md-ft

FEBRUARY 2015 83

American Oil and Gas Reporter - February 2015

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