American Oil and Gas Reporter - April 2015 - 77

SpecialReport: Refrac Facts
reservoirs remains a challenge. Often,
limited or even single parameters are utilized (such as brittleness only) to determine
sweet spots for well site selection. Additionally, the fractures generally are placed
equidistantly. This practice may lead to
nonoptimal or highly complex fracture
patterns that are problematic during hydraulic fracturing operations, and may
not lead to optimal production.
The reward of shale plays comes from
applying the appropriate technology (e.g.,
horizontal wells and multistage fracturing,
both in the right place and at the right
time) to reach the sweet spots of unconventional reservoirs based on properly
integrating both geochemical and geomechanical analyses. Employing such
technology facilitates selecting the best
methods for future shale play development,
minimizing bad wells (wells that cannot
produce at economic rates), implementing
better completion strategies, enabling a
more effective fracturing campaign, and
resulting in optimized production and
reservoir management.

that there is a direct relationship between
fundamental properties of shale plays
and their expected ultimate recoveries.
These relationships can be used as a tool
for evaluating shale play feasibility.
For a successful shale play, the effectiveness of the individual wells cannot
be measured by the lengths of wellbores
and the number of fractures, but by the
number of effective wells and fractures
that were placed in areas with brittle and
high kerogen-content rock.
Table 1 shows the main characteristics
of the 12 North American shale plays in
the database. Shale characteristics vary
within the same basin, but the seven necessary elements for a shale oil or gas
play are a laterally extensive shale; more
than 100 feet of shale thickness; greater
than 3 percent total organic carbon content;
thermal maturity with a vitrine reflectance
(Ro) greater than 1.0 in dry gas, 0.5 to
1.0 in wet gas, and less than 0.5 in oil
windows; good gas content (in excess of
100 cubic feet/ton); moderate clay content
(less than 40 percent by volume); and
brittle composition (rich in quartz and
feldspar).
Completion conditions are similar to
shale play characteristics in terms of variations from one play to another. Completion designs for the 12 plays in the
database range from barefoot open-hole,
nonisolated uncemented, preperforated
liner, frac ports/ball-activated sleeves,
and cased and cemented plug-and-perf.
Lateral length and orientation, number
and spacing of stages, proppant type

Assessing Potential Reserves
Geological, petrophysical and geomechanical mapping are the best tools for
screening the potential of shale plays.
This screening strategy includes reservoir
characterization, simulation and completion strategies, and careful examination
of the reasons for success in developing
other shale plays. Each shale formation
has its own criteria and optimal method
of development. It is important to observe
TABLE 2

Mineral Composition of North American Shale Plays
Shale Play

Quartz
(%)

Feldspar
(%)

Clay
(%)

Pyrite
(%)

Carbonate
(%)

Kerogen
(%)

Antrim

40-60

N/A

0-5%

N/A

Bakken

40-90

15-25

2-18

5-40

8-16

Barnett

35-50

6-7

10-50

5-9

0-30

4.0

Eagle Ford

11-29

N/A

46-78

4-11

Fayetteville

45-50

Haynesville

23-35

Horn River

9-60

Lewis

N/A

5-25

N/A

5-10

N/A

20-39

N/A

20-53

4-8

0-3

28-78

4-10

0-9

N/A

Marcellus

10-60

0-4

10-35

5-13

3-50

5.1

New Albany

28-47

2.1-5.1

11-23

3-9

0.5-2.5

N/A

Ohio

N/A

15-57

N/A

7-80

N/A

Woodford

48-74

3-10

7-25

0-10

0-5

7-16

(sand, resin-coated, ceramic, etc.), proppant
mesh size and concentration, and fluid
type (slickwater, cross-linked gel, hybrid,
etc.), pump schedules/rates, and total volumes pumped per stage vary with operating procedures for different shale plays
and even within plays. These variables
can serve as the criteria by which to
measure similarity and differences.
Mineralogical characteristics are the
main sweet spot proxy, especially in planning horizontal wells. Table 2 shows the
mineral compositions of all 12 shale
plays. The higher the quartz, the higher
the organic content. The brittle components
are quartz, feldspar and pyrite. On the
other hand, the rest of the composition
consists in the ductile components. The
brittle proxy is preferred to obtain optimal
fracture geometry.
One factor that makes developing
shale plays challenging is the wide variations in geomechanical properties. Even
a cursory review of the major shale plays
will illustrate clearly that the geomechanical properties of shale rocks vary
significantly among reservoirs as well as
within the same reservoir.
For each shale, the sweet spot segments
of the resource are expected to contribute
to successful treatment and development
that may lead to higher recovery. These
sweet spots control both the production
rate and volume of hydrocarbons that
can be recovered. Average sweet spot
criteria include brittleness, Young's modulus, TOC, Poisson's ratio, vitrine reflectance, kerogen type, mineralogy, the
differential horizontal stress ratio, and
the "fracturability index" (a correlation
tool developed by Texas Tech researchers
as a sweet spot identifier).
Shale Play Expert System

The software (expert system) developed
in this project is based on "if-then" rules
that check input parameters against the
valid ranges of input data from the different
shale plays stored in the database. It then
offers recommendations as to which shale
play characteristics conform to the specific
input parameters. The idea behind the
expert system is to determine which shale
plays can be used as guides according to
desired inputs.
The system consists of two layers: a
back-end layer (database containing the
most common shale plays and the ranges
of different inputs from the plays) and
an interface layer (a suggested method
for using the database, including entering
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American Oil and Gas Reporter - April 2015

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