Natural fractures are very common in shale gas plays. It is often presumed that because the formations are so tight, gas can be produced economically only when extensive networks of natural fractures exist. The creation of large fracture surface area in contact with the reservoir is considered essential to commercial success. This is facilitated by multistage hydraulic fracturing of long horizontal wells using large volumes of low- viscosity (low-cost) fracturing fluid. However, the efficiency of this process in terms of water usage is now coming under close scrutiny. The success of these operations is beyond doubt, but what can be inferred about the accuracy of this conceptual picture in light of many years’ accumulated production data? What does production data tell us about the role of natural fractures? This presentation addresses these issues by using a semianalytic shale gas production model to analyze and interpret production data from many shale gas wells across several different plays. Ian Walton is a senior research scientist at the Energy & Geoscience Institute of the University of Utah and an adjunct professor in the department of chemical engineering. He holds a PhD in applied mathematics from the University of Manchester. Walton has more than 25 years of petroleum industry experience, most recently as a scientific advisor for Schlumberger, and more than 15 years of university teaching experience.

1. Primary funding is provided by
The SPE Foundation through member donations
and a contribution from Offshore Europe
The Society is grateful to those companies that allow their
professionals to serve as lecturers
Additional support provided by AIME
Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl

2. Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl
2
Ian C. Walton
The Role of Natural Fractures in Shale Gas
Production: What Does Production Data Tell Us?
iwalton@egi.utah.edu

3. Presentation Outline
• Introduction: Importance of shale gas
• Natural fractures
• Production analysis
• Which fractures contribute?
• Some inferences
• Summary
3

4. US Dry Gas Production
4

5. Shale Gas: a Worldwide Resource
5

6. US Shale Gas Production
6

7. Shale Gas Production:
Perceptions
7
• Large volume of gas in place
• Matrix permeability is extremely small ~ 200 nD
• Gas can be produced economically through:
• multiple hydraulic fracture treatment of long horizontal wells
• use of slickwater as frac fluid
• natural fractures; complexity matters

8. Natural Fractures
8

9. Creation of Fracture Complexity
9
• Natural fractures are often
sealed or mineralized.
• Calcite fill in the fractures
has a very weak
attachment to shale.
• Natural fractures open at
about 60% of the stress
level needed to fracture
the formation.
• Weak planes in the shale
will break into natural
fractures on application of
stress.

10. Are natural fractures necessary for
economic production?
10
• Water-filled cleats are
essential for Coal Bed
Methane production.
• Narrow, calcite-sealed,
natural fractures reactivated
during hydraulic fracturing
are regarded as essential to
produce gas from shale.

11. Impact of Natural Fractures on
Production
11
Carlson and Mercer (1991) summarized the
consensus view of the production process as
“ because the formations are so tight gas can be
produced only when extensive networks of
natural fractures exist.”

12. Natural Fractures: some
industry opinions
“Presence and ability to open and
maintain flow in primary and
secondary natural fracture
systems are keys to shale gas
production.”
“Development of the flow paths (natural fractures)
adjacent to the main hydraulic fracture will act as
leakoff that limits outward fracture extension growth”
12
OR

13. Some Unresolved Questions
13
• What is the role of the natural fracture network?
Essential or detrimental?
• How much productive fracture surface area do we
need, how much do we create and how can we
increase it?
• What does production data tell us?

14. How much fracture surface area
do we create?
14
Mass balance
• Frac fluid: Frac surface area
~ 100 MMsqft
• Proppant: Propped frac
surface area ~ 2-3 MMsqft
Example
15 transverse hydraulic fractures each
200 ft high and 500 ft across
Frac surface area = 2*15*200*500 sqft
= 3 MMsqft

15. Created Fracture Surface Area
• Hydraulic fracturing creates a
propped fracture surface area
~ 2–3 MMsqft.
• Connecting the hydraulic fractures
with natural fractures may open
up an enormous fracture surface
area ~ 100 MMsqft.
But how much of this surface area
contributes to productivity?
15

16. Some Shale Gas Production Data
Baihly et al. (2010) SPE 13555
16
Gasrate(Mcf/d
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 1 2 3 4 5 6
Gasrate(bcf/year)
time (years)
Production Data from Sample Wells in the
Barnett Shale

17. 0
500
1000
1500
2000
0 50 100 150 200 250 300
dailyproduction
time(days)
data
"b=1.6"
"b=2"
0
0.5
1
1.5
2
2.5
0
500
1000
1500
2000
0 1095 2190 3285 4380 5475 6570 7665 8760
cumulativeproduction,bscf
dailyproduction,Mscf/day
time (days)
data
"b=1.6"
"b=2"
"b=1.6"
"b=2"
10
)1(
1


 bfor
tbD
q
q
b
i
i
Conventional Decline Analysis—Arps
Conventional Production Data Analysis
Techniques: Decline Curve Analysis
Advantages
•Quick, easy to use, familiar
Disadvantages
•Limited basis in the physics
•Not appropriate for transient flow
•No insights into production drivers
•Extrapolation is dangerous
•Exponent b varies in time

18. Semi-analytic Production Model
Based on dual-porosity/dual-permeability model applied to
multiply-fractured shale reservoirs.
18

19. Theory suggests that for a substantial period of
time cumulative production and production
rate can be approximated by
where Cp depends on
• Pressures (bhfp, pore or reservoir pressure)
• Reservoir quality/ GIP (permeability, porosity)
• Gas properties (viscosity, compressibility,
equation of state)
• Productive fracture surface area
 

 mm
s
wr
p
kc
p
pp
AC
22


Production Model
 
m
m
m
k
Lc
T
2
2



20. Example of Application to
Production Data
0
200
400
600
800
1000
1200
0 0.5 1 1.5 2 2.5
cumulativeproduction(MMscf)
sqrt(time) years^0.5
The slope of the straight line is equal to the production
coefficient . .
Its value depends on reservoir properties and productive
fracture surface area. 20
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 1 2 3 4 5 6
Gasrate(bcf/year)
time (years)
Production Data from Sample Wells in the
Barnett Shale

21. More Examples
0
500
1000
1500
2000
2500
3000
3500
0 1 2 3 4 5
cumulativeproduction
(mmscf)
sqrt(time) (months^0.5)
3 stages
8 stages
10 stages
12 stages
24 stages
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 0.5 1 1.5 2
cumulativeproduction(bscf)
sqrt(time) (years^0.5)
Montney Shale Gas Production
21

22. Barnett Production: sweet spots?
22
Good
wells
Average
wells
Poor
wells
Tarrant
DentonWise

23. 0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
0 0.25 0.5 0.75 1 1.25
CumulativeProbability(%)
Cp (bcf/year^0.5))
core horizontal-2974 wells
core vertical-3886 wells
tier 1 horizontal- 3865 wells
tier 1 vertical-251 wells
tier 2 horizontal-687 wells
tier 2 vertical-401 wells
Barnett Production Analysis

24. Estimate of Productive Fracture
Surface Area Using Production Data
• Measure the production coefficient,
• Estimate relevant reservoir and fluid properties
• Infer productive fracture surface area
0
1
2
3
4
5
6
7
8
9
10
0 100 200 300 400 500
Surfacearea(MMsqft)
matrix permeability (nD)
poor producer
average producer
good producer
Productive fracture surface
area in the Barnett:
1-5 MMsqft
24
 

 mm
s
wr
p
kc
p
pp
AC
22



25. How much fracture surface area
do we create?
25
Mass balance
• Frac fluid: Frac surface area
~ 100 MMsqft
• Proppant: Propped frac
surface area ~ 2-3 MMsqft
Example
15 transverse hydraulic fractures each
200 ft high and 500 ft across
Frac surface area = 2*15*200*500 sqft
= 3 MMsqft

26. End of linear flow-
fracture to fracture interference
Usually there is no
indication of
interference for at
least 5 years
26
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
0 2 4 6 8
CumulativeOil(bbl)
sqrt (time) (Months After Initial Production)^0.5
Cumulative Oil

27. The impact of fracture spacing on
the time to fracture interference
0
2
4
6
8
10
12
14
16
18
20
0 50 100 150 200 250 300
matrixdrainagetime(years)
fracture spacing (ft)
permeability=10nd
permeability=100nd
permeability=200nd
Usually there is no
indication of
interference for at
least 5 years
27
 
m
m
m
k
Lc
T
2
2



28. Inferences from Production
Data Analysis
• Productive fracture surface area ~1–5 MMsqft.
• Productive fracture volume scales approximately
with volume of proppant placed.
• No indication of fracture-fracture interference even
after several years of production.
Productive fracture spacing is at least 100 ft.
Re-opened natural fractures may not be productive!!
28

29. Natural fracture conductivity
In principle, even unpropped fracs
should have sufficient conductivity for
efficient production from shales.
But ….
• conductivity is limited by two-phase
characteristics of flow in narrow
channels.
•Fracs may continue to close due to
creep

30. Summary
• A fast and robust physics-based production
analysis method has been developed,
specifically for very low permeability reservoirs.
• A new metric for well productivity has been
proposed:
the production coefficient, .
30

31. Summary, continued
• Analysis of shale gas production data has provided
new insights into role of the natural fracture
network: only a small fraction of the total fracture
surface area is productive.
• Natural fractures may not be productive unless we
can clean them up;
they may even be detrimental.
31

32. Thank you for attending!
Questions?
32

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34. Where does the frac fluid reside?
• 90% is used up in opening the
natural fracture network (induced
unpropped fractures).
• As these fractures close, the fluid
is imbibed into the formation.
• After closure, 20-30% is returned
to the surface.
• The balance remains in the
induced unpropped fractures and
in the matrix surrounding them.

35. Are Natural Fractures Detrimental?
If production comes
through the main
propped fractures,
then we need to
maximize the length
of the propped
fractures.
35
The natural fractures generate large leakoff which
reduces the length of the propped fractures.