SPE 168589 Fracability Evaluation in Shale Reservoirs - An Integrated Petrophysics and Geomechanics Geomechanics Approach Xiaochun Jin, Subhash N. Shah, Jean-Claude Roegiers, Bo Zhang, the University of Oklahoma
Copyright 2014, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Hydraulic Fracturing Technology Conference held in The Woodlands, Texas, USA, 4–6 February 2014. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more t han 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Ab st rac t The identification of fracture barrier is important for optimizing horizontal well drilling, hydraulic fracturing, and protecting fresh aquifer from contamination. The word “brittleness” has been a prevalent descriptor in unconventional shale reservoir characterization, but there is no universal agreement regarding its definition. Here a new definition of mineralogical brittleness is proposed and verified with t wo independent methods of defining brittle ness. Formation with higher brittle ness is considered as good fracturing candidate. However, this viewpoint is not reasonable because brittleness does not indicate rock strength. For instance, fracture barrier between upper and lower Barnett can be dolomitic limestone with higher brittleness. A
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assumption is not always true because formation with
measurements due to the limitation of turnaround time
higher brittleness can be fracture barrier. For instance,
and expense. Therefore, B1- B16 are not as practical as B17 -
dolomitic limestone is of high brittleness, but it is a
B21 for the evaluation of brittleness in unconventional
fracture barrier in shale reservoirs because fracture
shale reservoirs.
gradient in shale formation is lower than that in dolomitic limestone formation, and the same fracturing pressure cannot fracture it [ Bruner and Smosna, 2011]. Therefore, brittleness alone is not enough to characterize the fracability of unconventional shale reservoirs. Brittleness is also applied to evaluate rock cutting efficiency, and it is found that the adoption of a single brittleness concept is
From a physical viewpoint, mineralogical brittleness is considered more reliable [ Jarvie et al., 2007; Slatt and Abousleiman, 2011].
In order to obtain confident
interpretation results by B17 , B18 and B19, it is important to compare them with mineralogical brittleness; however, presently,
parallel
comparison
of
these
various
brittlenesses is not available in the literat ure.
not sufficient for the evaluation, specific energy should also be taken into account [Göktan, 1991]. Similarly, in hydraulic fracturing, other parameters similar to specific energy should also be included for improving the formation fracability evaluation [ Altindag, 2010].
The understanding of mineralogical brittleness has deepened in recent years. Originally, the mineralogical brittleness accounted only for the weight fraction of quartz [ Jarvie et al., 2007]. Afterwards, it was observed that the presence of dolomite tends to increase the
In view of the above facts, we attempted to solve the following problems: (1) examine various available definitions
of
brittleness,
redefine
mineralogical
brittleness, and benchmark the definition of brittleness; (2) develop a fracability evaluation model by integrating brittleness
and other parameters,
such as fracture
brittleness of shale, so both the fractions of quartz and dolomite were included [Wang and Gale, 2009]. It is also observed that silicate minerals such as feldspar and mica 1 are more brittle than clay in shale reservoirs. Besides the dolomite, other carbonate minerals, such as calcite in limestone, are also more brittle than clay. Therefore, a
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carbonate,
dolomite,
and
calcite
are
indicated
as
“WQFM ”, “ WCAR”, “WDOL”, and “WCLC ”.
independent to each other, and their trends are similar, all of them can be employed to characterize brittleness of
B18 , B19 and B22 are selected for brittleness evaluation
because they can be derived from well logging data and
unconventional shale, among which B22 is the best brittleness index, followed by B18 , and B19.
are easy to apply. Tracks of B17 and B18 are calculated 3
with internal friction angle , but only B18 is selected for the evaluation here because it is in the range of 0 to 1. The literature formula for B19 is not clear [ Rickman et al., 2008], and after in-house verification, it is redefined as: B19
=
E n
where,
+
vn
2
(2)
E n and v n
are normalized Young’s modulus and
Poisson’s ratio, and are defined below: E n
vn
=
=
E − E min E max vmax vmax
− E min
−v
− v min
(3)
(4)
where, E min and E max are the minimum and maximum dynamic Young’s modulus for the investigated formation, vmin and vmax are dynamic minimum and maximum
B18 0.3
0.6
B19 0.9 0
0.35
B22 0.7 0.3
0.65
1
4
be
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measured
with
various
methods.
The
popular
measurement methods are listed in the following: •
Chevron
Notched
Short
Rod
(CNSR)
strength, uniaxial compressive strength, and velocity of Method
Chevron
Notched
Semicircular
Bend
(CNSCB)
Singh, 1992]. In addition, fracture toughness, tensile
strength, and acoustic velocity were measured on samples
Method [Chong and Kuruppu, 1984] •
primary acoustic wave were derived from experimental data of different types of rocks [ Barry, Whittaker and
[ Bubsey et al., 1982] •
Young’s modulus, Poisson’s ratio, hardness, tensile
Chevron Notched Brazil Disk (CNBD) Method [ Zhao and Roegiers, 1993]
of Woodford shale [Sierra et al., 2010]. The laboratory data of Woodford shale in the literature were taken to verify the accuracy of the existing correlations in
Fracture toughness measurement of rock is more difficult
predicting fracture toughness of shale. The comparison
and complex than other rock mechanics tests. To save
results are included in Table 1.
time and expense, correlations of fracture toughness with Table 1 Error Analysis of Correlations for Fracture Toughness Equation
Correlation of Fracture
Coefficient of Determination
Number
Toughness
R
2
Error between Predicted and Measured K IC
1
K IC
=
0.271 + 0.107 × σ t
0.86
12.47%
2
K IC
=
0.313 + 0.027 × E
0.62
23.82%
3
K IC
= −1.68 +
0.65 × V p
0.90
491.78%
4
K IC
0.708 + 0.006 × σ c
0.72
No measured data
=
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toughness anisotropy on hydraulic fracturing [Chandler,
increases with increasing E . Considering the uncertainty
Meredith and Crawford , 2013].
in the coefficients of fracture toughness correlation, 11.59 is not the exact boundary value, and it may be slightly higher or lower than 11.59. Based on the analysis, it is concluded that the fracture energy does not always increase with increasing Young’s modulus. However, because Young’s modulus of most sedimentary rocks is greater than 11 GPa, it can be stated that higher the Young’s modulus, more difficult is to fracture the
Figure 2 Demonstration of anisotropy of rock mechanics property 3.2 Strain Energy Release Rate
Strain energy release rate is the energy dissipation per
formation. Since fracture toughness is linearly correlated with Young’s modulus, higher the fracture toughness, more energy will be dissipated during the creation of new fracture surface.
unit surface area during the process of new fracture creation [ Barry, Whittaker and Singh, 1992]. According
4.
Fracture Barrier
to failure criterion, when strain energy release rate
One of the key steps to successful hydraulic fracturing is
reaches its critical value GC , fracture starts propagating
the identification of fracture barrier before fracturing
from the preexisting fracture. In hydraulic fracturing,
operation [ Economides and Nolte, 2000]. Hydraulic
given a certain amount of energy, lower the GC , more
fractures should be contained within the pay zone,
fracture surface area will be created. The critical value GC
unintentional invasion into fresh aquifer or fault zone will
is the fracture energy that is independent of the applied
adversely affect future hydrocarbon production rate or
load and the geometry of the body [ Irwin, 1957]. In
lead to environmental pollution [ Bruner and Smosna,
present study, GC is assumed not to vary with fracture
2011]. For instance, Barnett shale is underlain by the
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horizontal in-situ stress corresponds to higher closure FI 1
stress; •
Poisson’s ratio: If it is not in active tectonic stress
=
Bn
where,
+
2 Bn
area, higher Poisson’s ratio corresponds to higher minimum horizontal in-situ stress, which is a fracture barrier; •
Fracture
toughness:
higher
fracture
toughness
Bn
=
•
(7)
and GC _ n are normalized brittleness and strain
B − Bmin Bmax
GC _ n
=
higher fracture toughness; •
energy release rate, and are defined as:
corresponds to higher breakdown pressure, the same fracture gradient cannot fracture formation with
GC _ n
− Bmin
GC _ max GC _ max
− GC
− GC _ min
(8)
(9)
Brittleness: relatively less brittle formation exhibits
where, Bmin and Bmax are the minimum and maximum
plasticity, fracturing in plastic formation consumes
brittleness and GC_min and GC_max are the minimum and
more energy;
maximum critical strain energy release rate for the
Bonding strength at the interface: slippage at the
investigated formation.
interface may stop fracture penetration into the
According to the analysis in Section 3.2, for most
adjacent formation.
sedimentary rocks, GC increases monotonically with increasing Young’s modulus and fracture toughness.
Minimum horizontal stress Poisson’s ratio Bonding strength Fracture toughness
Therefore, it is more practical to define fracability index with Young’s modulus and fracture toughness. The mathematical model of fracability index in terms of brittleness and fracture toughness is defined as follows: B
K
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E n
=
7
E max E max
where,
−
E max
−
E
E min
(13)
and E min are the maximum and minimum
Young’s modulus for the investigated formation. FI 1,2,3 is in the range of 0 to 1.0. Formation with FI = 1.0
is the best fracture candidate, and formation with FI = 0 is the worst fracture candidate. From Figs. 4-6, It is concluded that: (1) Fracture toughness and Young’s modulus can be substitutes for critical strain energy in calculating fracability index because they show t he same trend as critical strain energy does; (2) formation with brittleness close to 1.0 might not be
good
for
fracturing,
because
its
Young’s
modulus/fracture toughness might be higher, which might lead to lower fracability index; (3) formation with lower Young’s modulus/fracture toughness might not be a good candidate, because its brittleness might be lower, which prohibits the formation of connected complex fracture network, and leads to lower fracability index.
Figure 5 A cross plot of brittleness and fracture toughness shows increasing fracability index
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6 7 ( AT90) , photoelectric factor (PEF ) , compressional and
can act as barrier if it is thick enough and there are
8 shear sonic slowness ( DTC , DTS ) , and bulk density
not enough natural fractures within it [ Bruner and
9 ( RHOB) ;
Smosna, 2011].
Step 2: Calculate dynamic Young’s modulus ( E ),
(3) FI does not always increase monotonically with
dynamic Poisson’s ratio (ν), fracture toughness (K IC ),
increasing brittleness. For instance, different trends
mineralogical brittleness ( B22), fracability index (FI 1 , FI 2 ,
of brittleness and FI are observed in F 3, F 5, Bar 3,
or FI 3), and plot their tracks;
Bar 4, and Bar 6. The reason for the difference is
Step 3: Locate fracture barrier by comparing FI in
attributed to different trends of fracture toughness.
adjacent formations, and compare the optimized intervals
(4) F 1 through F 5 (the red shaded sections) are selected
of fracture barriers with interpretation results from Step 1;
as fracturing candidates, and Bar 1 through Bar 6 (the
Step 4: Screen hydraulic fracturing candidates of higher
yellow double arrows) are considered as fracture
FI within the pay zones;
barriers by comparing FI of adjacent formations. For
Step 5: Place horizontal well in the middle of each target,
well-A, when FI is greater than 0.7, it is considered
or at the depth that is good for connecting different targets
as fracturing candidate, otherwise, it is not. (5) Bar 6 is thicker than Viola limestone shown in blue
adjacent to thin fracture barriers. As for the field development plan, it will be helpful to have a 3-D geological model of fracability index, but it is not constructed in this paper due to the absence of logging data
covering
a
field.
It
will
assist
the
geomodeler/engineer refine the horizontal well trajectory within the pay zone, and optimize the locations and intervals of perforation clusters.
shaded section, which was interpreted by geologist. Bar 6 is considered as a fracture barrier because its FI
is much lower than that in F 5, which is attributed to lower
brittleness.
Complex
connected
fracture
network is not easy to create in formation with lower brittleness. (6) Whether a formation can act as fracture barrier depends on: differences of FI in adjacent formations
SPE 168589
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is so for upper Barnett and Marble Falls. The
In conclusion, based on the above analysis, fracability
inconsistency is caused by the fact that fracability
index
index
identification
model
accounts
for
both
mineralogical
model
provides of
an
fracture
additional barrier,
tool
for
screening
the good
brittleness and ener gy dissipation, which makes more
candidates for hydraulic fracturing within pay zones,
sense from physical viewpoint.
optimizing the horizontal well trajectory and perforation cluster spacing.
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Figure 7 Screening Hydraulic Fracturing Candidates with Fracability Index. According to logging interpretation (mainly based on NPHI , PEF , AT90, and GR), the pay and non-productive zones are represented by yellow and blue colors, respectively. The red shaded sections are hydraulic fracturing candidates; yellow double arrows represent fracture barriers. The fracability index track is derived with Eq. 10.
SPE 168589
7.
11
higher fracture toughness might be a good fracturing
Discussion and Conclusions
This paper develops an integrated petrophysics and geomechanics approach for characterizing fracability in unconventional
shale
reservoirs.
The
mineralogical
brittleness is redefined as B22, which consists of silicate
candidate if its brittleness is high enough. The fracability index model is successfully applied to optimize the hydraulic fracturing and horizontal well drilling of WellA in Barnett shale.
minerals (quartz, feldspar, and mica), and carbonate
Only opening mode fracture is considered in this paper
minerals
when developing fracability index model. It is not fully
(mainly
dolomite
and
calcite).
The
new
definition of mineralogical brittleness is proven with two
understood
independent definitions of brittleness: B18 (sinusoidal
contributes to the stimulated reservoir volume. 3-D
function of internal friction angle) and B19 (modified from
fracability index model could be constructed with the
Rickman’s
Young’s
method developed in this paper if well logging data from
modulus and Poisson’s ratio). The parallel comparison of
a field is available. It will help user optimize the
B18 , B19 and B22 benchmarks the definition of brittleness.
trajectory of horizontal well and perforation cluster
brittleness
based
on
dynamic
Fracability index model is developed by integrating fracture energy (critical strain energy release rate) and brittleness. For most sedimentary rocks, critical strain
whether
only
sliding
mode
fracture
spacing in multistage hydraulic fracturing. Expanding data from fracture diagnostic tests will be helpful for further proving the validity of fracture index model.
energy release rate can be substituted with fracture toughness or Young’s modulus in the calculation of fracability index. The objectives of fracability index model are: (1) to search formations with great potential to create both connected complex fracture network and maximum stimulated reservoir volume; (2) to locate the
Acknowledgement: The authors would like to thank Prof. Ahmad Ghassemi, Prof. Roger Slatt, Yawei Li, Ting Wang and Yuqi Zhou of the University of Oklahoma, Dr. Jerome Truax of LINN Energy LLC, Dr. Zhengwen Zeng of BP, Dr. Xu Li of Shell, Dr. Gang Li of FTS
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12
References
Alassi, H., R. Holt, O.-m. Nes, and S. Pradhan (2011), Realistic Geomechanical Modeling of Hydraulic Fracturing in Fractured Reservoir Rock, paper presented at Canadian Unconventional Resources Conference, Alberta, Canada, November 15-17, 2011. Altindag, R. (2003), Correlation of specific energy with rock brittleness concepts on rock cutting, Journal of the South African Institute of Mining and Metallurgy, 103(3), 163-171. Altindag, R. (2010), Assessment of some brittleness indexes in rock-drilling efficiency, Rock Mechanics and Rock Engineering, 43(3), 361-370. Andreev, G. E. (1995), Brittle failure of rock materials: test results and constituti ve models, Taylor & Francis. Barry, N., N. R. Whittaker, and S. G. Singh (1992), Rock fracture mechanics principles design and applications, ELSEVIER, Amsterdam-London-New York-Tokyo. Bishop, A. W. (1967), Progressive Failure-with special reference to the mechanism causing it., paper presented at Geotechnical Conference, Oslo, Norway. Bower, A. F. (2011), Applied mechanics of solids, 1 ed., CRC press. Bruner, K. R., and R. Smosna (2011), A Comparative Study of the Mississippian Barnett Shale, Fort Worth Basin, and Devonian Marcellus Shale, Appalachian Basin. National Energy Technology Laboratory Rep., DOE/NETL-2011/1478. Bubsey, R., D. Munz, W. Pierce, and J. Shannon Jr (1982), Compliance calibration of the short rod chevron-notch specimen for fracture toughness testing of brittle materials, International Journal of Fracture, 18 (2), 125-133. Chandler, M., P. Meredith, and B. Crawford (2013), Experimental Determination of the Fracture Toughness and Brittleness of the Mancos Shale, Utah, paper presented at European Geosciences Union General Assembly, Vienna, Austria, April 7-12, 2013. Chang, C., M. D. Zoback, and A. Khaksar (2006), Empirical relations between rock strength and physical properties in sedimentary rocks, Journal of Petroleum Science and Engineering, 51(3), 223-237. Chong, K., and M. D. Kuruppu (1984), New specimen for fracture toughness determination for rock and other materials, International Journal of Fracture, 26 (2), R59-R62. Chong, K. K., W. Grieser, A. Passman, H. Tamayo, N. Modeland, and B. Burke (2010), A Completions Guide Book to
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Jin, X., S. Shah, J.-C. Roegiers, and B. Hou (2013), Breakdown Pressure Determination-A Fracture Mechanics Approach, paper presented at SPE Annual Technical Conference and Exhibition, New Orleans, Louisi ana, USA, Sep 30 - Oct 2, 2013. Kundert, D., and M. Mullen (2009), Proper evaluation of shale gas reservoirs leads to a more effective hydraulic-fracture stimulation, paper presented at SPE Rocky Mountain Petroleum Technology Conference, Denver, Colorado, April 14-16, 2009. Lawn, B., and D. Marshall (1979), Hardness, toughness, and brittleness: an indentation analysis, Journal of the American ceramic society, 62(7-8), 347-350. Li, Q., M. Chen, Y. Jin, Y. Zhou, F. Wang, and R. Zhang (2013), Rock Mechanical Properties of Shale Gas Reservoir and Their Influences on Hydraulic Fracture, paper presented at 6th International Petroleum Technology Conference, Beijing, China, Mar 26 - 28, 2013. Nasseri, M., and B. Mohanty (2008), Fracture toughness anisotropy in granitic rocks, International Journal of Rock Mechanics and Mining Sciences, 45(2), 167-193. Protodyakonov, M. (1962), Mechanical properties and drillability of rocks, paper presented at Proceedings in the. Quinn, J. B., and G. D. Quinn (1997), Indentation brittleness of ceramics: a fresh approach, Journal of Materials Science, 32(16), 4331-4346. Rice, J. R. (1968), Mathematical analysis in the mechanics of fracture, Fracture: An advanced treatise , 2, 191-311. Rickman, R., M. Mullen, J. Petre, W. Grieser, and D. Kundert (2008), A practical use of shale petrophysics for stimulation design optimization: All shale plays are not clones of the Barnett shale, paper presented at SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, September 21-24, 2008. Sehgal, J., Y. Nakao, H. Takahashi, and S. Ito (1995), Brittleness of glasses by indentation, Journal of materials science letters, 14(3), 167-169. Sierra, R., M. Tran, Y. Abousleiman, and R. Slatt (2010), Woodford Shale mechanical properties and the impacts of lithofacies, paper presented at Proceedings symposium on the 44th US rock mechanics symposium and 5th US–Canada rock mechanics symposium, Salt Lake City, Utah, USA, June 27 - 30, 2010. Simonson, E., A. Abou-Sayed, and R. Clifton (1978), Containment of massive hydraulic fractures, Old SPE Journal, 18 (1), 27-32. Singh, S. (1986), Brittleness and the mechanical winning of coal, Mining Science and Technology, 3(3), 173-180.
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Appendix
14
Table A-1 Selected Expressions of Brittleness
Formula
Variable declaration
Test Method
Reference
B1 = (H m-H)/K
H and H m are macro and micro-hardness, K is bulk modulus
Hardness test
[ Honda and Sanada, 1956]
B2 = qσ c
q is percent of debris (<0.6 mm diameter); σ c is compressive strength
Proto impact test
[Protodyakonov, 1962]
B3= εux×100%
εux is unrecoverable axial strain
[ Andreev, 1995]
B4 = (ε p- εr )/ ε p
ε p is peak of strain, εr is residual strain
[ Hajiabdolmajid and Kaiser , 2003]
B5 = τ p- τ / r τ p
τ p and τ r are peak and residual of shear strengths
B6 = ε / r εt
εr and εt are recoverable and total strains
B7 = W r /W t
W r and W t are recoverable and total strain energies
Stress strain test
[ Hucka and Das, 1974]
B8 = σ c /σ t B9= (σ c - σ t )/( σ t + σ c) B10 = (σ cσ t )/2
σ c and σ t are compressive and tensile strength
[ Bishop, 1967]
Uniaxial compressive strength and Brazilian test [ Altindag, 2003]
B11 = (σ cσ t ) . /2 B12=H/K IC
H is hardness, K IC is fracture toughness
B13 = c/d
c is crack length, d is indent size for Vickers indents at a specified load; empirically related to H/K IC
B14=Pinc /Pdec
Pinc and Pdec are average increment and decrement of forces
B15= F max /P
F max is maximum applied force on specimen, P is the corresponding penetration.
Hardness and fracture toughness test
[ Lawn and Marshall, 1979] [Sehgal et al. , 1995]
Indentation test
[Copur et al., 2003] [Yagiz, 2009]
H is hardness, E is Young’s modulus, K IC is fracture toughness
Hardness, stress strain, and fracture toughness test
[Quinn and Quinn , 1997]
φ is internal friction angle
Mohr circle or logging data
[ Hucka and Das, 1974]
B19 = (E n+vn)/2
E n and vn are normalized dynamic Young’s modulus and dynamic Poisson’s ratio defined in Eqs. 3-4.
Density and sonic logging data
Modified from [ Rickman et al., 2008]
B20= (W qtz)/W Tot
W qtz, is the weight of quartz, W Tot is total mineral weight.
B16 =
H×E/K IC 2
B17 = 45°+ φ/2 B18 = Sinφ
B21= (W qtz+W dol)/W Tot
W qtz and W dol are weights of quartz and dolomite, W Tot is total mineral weight.
B22= (W QFM +W Carb)/W Tot
W QFM is weight of quartz, feldspar, and mica; W Carb is weight of carbonate minerals consisting of dolomite, calcite, and other carbonate components. W Tot is total mineral weight.
[ Jarvie et al., 2007] Mineralogical logging or XRD in the laboratory
[Wang and Gale , 2009] Defined in this paper
