-
SPE 167803
Influence Factors of Fracability in Nonmarine Shale Changliang
Fang, Mohammed Amro, Technical University Bergakademie Freiberg,
Institute of Drilling and Fluid Mining, Germany
Copyright 2014, Society of Petroleum Engineers This paper was
prepared for presentation at the SPE/EAGE European Unconventional
Conference and Exhibition held in Vienna, Austria, 2527 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 than
300 words; illustrations may not be copied. The abstract must
contain conspicuous acknowledgment of SPE copyright.
Abstract Fracability, the capability of shale that can be
fractured effectively, is the most critical evaluation parameters
in shale gas
production. At present, it is generally recognized that using
mineral composition and rock mechanics parameters to represent
shale fracability is difficult to fully reflect the
comprehensive properties of shale in hydraulic fracturing. However,
so far all
fracturing research about shale gas is almost considering marine
shale and the understanding of shale fracability is confined
and trapped within marine shale. Since shallow deposit of
sedimentary strata in China is non-marine (lacustrine) facies
sediments, China pay increasing attention to non-marine shale,
which is opposite to the situation in Europe, and with the
deepening of shale gas exploration, Europe will focus more on
the non-marine shale in following decades. Due to the fact that
lacustrine shale deposits differ from marine deposits in
sedimentary environment, the former are always with frequent
sand
and mud interbed, interlayers development. Therefore, the
influence factors of fracability in non-marine shale seemed
more
complexes and the research of shale fracability becomes more
significant.
After general contrastive study of sedimentary environment, gas
generation, mineralogy and physical properties of non-marine
and marine shale based on literature data, this paper separately
analysed the fracability influence factors (including
sedimentary environment, mineral composition, diagenesis,
brittleness, nature fracture, etc.) of non-marine shale focused
on
the Mesozoic Triassic Yanchang Formation shale in Ordos Basin of
China. Moreover, comprehensive consideration of all
above influence factors and statement of some fracability
evaluation methods have been implemented. Unified and
quantitative evaluation of fracability influence factors in
non-marine and marine shale has been discussed afterwards. The
summary of main influence factors of fracability in non-marine
shale not only improved the shale fracability research, but
also
could guide the hydraulic fracturing practice.
Introduction
Fracability, the capability of shale reservoir that can be
fractured effectively, is one of most critical evaluation
parameters in
shale gas production. Chong et al. (2010) have summarized
successful approaches towards shale-play stimulation in the last
20
years, and pointed out that fracability, producibility and
sustainability are key factors of shale well-completion.
However, at present, fracability is considered equivalent to
brittleness, low fracability is identified with ductile.
Scholars
generally use brittle mineral composition and rock mechanics
parameters to represent shale fracability, which only reflects
single factors, mineral composition or rock mechanics
characteristic, and is difficult to fully reflect the
comprehensive
properties of shale in hydraulic fracturing.
Moreover, along with the study of non-marine shale going deeper,
existing understanding of shale fracability shows the new
limits that confined and trapped within marine shale. So far all
fracturing research about shale gas is almost considering
marine shale. Since shallow deposit of sedimentary strata in
China is non-marine (lacustrine) facies sediments, China pay
increasing attention to non-marine shale, which is opposite to
the situation in Europe, and with the deepening of shale gas
exploration, Europe will focus more on the non-marine shale in
following decades.
Due to the fact that lacustrine shale deposits differ from
marine deposits in sedimentary environment, the former are
always
with frequent sand and mud interbed, interlayers development.
Therefore, the influence factors of fracability in non-marine
-
2 SPE 167803
shale seemed more complexes and the research of shale
fracability becomes more significant.
This paper separately analysis the fracability influence factors
(including sedimentary environment, mineral composition,
diagenesis, brittleness, nature fracture, etc.) of non-marine
shale focused on the Mesozoic Triassic Yanchang Formation shale
in Ordos Basin of China, after a comparison of sedimentary
environment, gas generation, mineralogy and physical properties
of non-marine and marine shale based on literature data.
Afterwards, evaluation methods of fracability influence factors
in
non-marine and marine shale has been discussed.
Contrastive study of non-marine and marine shale
Research on non-marine stratum is a very important field of oil
and gas exploration in China, because distribution area of non-
marine sedimentation is large and more than half of oil and gas
are found in continental strata in China (Wang, 2012).
Therefore, all characteristics of non-marine shale in this paper
are concluded from Chinese lacustrine formations, while
characteristics of marine shale mainly come from literature of
American shale strata.
Shale, extensively developed in non-marine stratum, has a
distinct characteristics compared with the marine strata.
Different
sedimentary histories make different tectonic and sedimentary
characteristics of shale that differ from other shale. In area,
the
scale of non-marine shale relatively smaller compared with
marine shale. In tectonic, non-marine shale have weaker late
reformation, better preservation condition, obvious inheritance,
and stronger basement relief than marine shale. In
sedimentation, non-marine shale presents frequent sand and mud
interbed, developed interlayers, thick gross thickness and
changing greatly in single layer pay thickness.
In mineralogy, non-marine shale has higher mud content, lower
quartz content, and higher feldspar content; brittle minerals
in
non-marine shale mainly include quartz, feldspar and carbonate
while quartz and carbonate mainly in marine shale (Rickman,
2008). Relatively higher content of Illite, a certain amount of
chlorite, and very low content of kaolinite are indication of
continental sedimentary environment (Thomas, 1984).
Organic matter is sensitive to water depth and climate change in
deposit, so deep lacustrine facies, shallow lake facies, and
limnetic facies develop different types of kerogen. Overall
thermal evolution of organic matter in non-marine shale is
lower
than that in marine shale, mainly in oil-generating window. In
the aspect of gas generation, non-marine shale gas usually is
pyrolysis gas, and associated with oil. The phenomenon of oil
and gas co-existence often appears in shale.
Though shale has low porosity and extremely low permeability
compared with conventional reservoirs, many survey of
literature with mercury intrusion test, Scanning Electron
Microscope, et al. indicates that lacustrine shale generally has
lower
porosity and permeability than marine shale. Moreover,
lacustrine shale seldom appears nature fractures with length
across
core sample of Barnett shale, and micro fractures developed in
non-marine shale is obvious poorer than marine shale.
Table 1 shows the comparison of marine and non-marine shale in
sedimentary, organic matter, mineralogy and physical
properties. The different between marine and non-marine shale
determines that influence factors of fracability in non-marine
shale cannot be completely copied of that in marine shale.
Table 1 - Comparison of marine and non-marine shale in
sedimentary, organic matter, mineralogy and physical properties
Shale Marine Non-marine
Tectonic
area large small, limit late reformation strong weak
preservation condition good good inheritance obvious basement
relief weak strong
Sedimentation
sand and mud interbed seldom frequent gross thickness thick
interlayers developed single layer pay thickness changing
greatly
Mineralogy
mud content lower higher quartz content higher lower feldspar
content lower higher key brittle minerals quartz & carbonate
quartz, feldspar, carbonate illite content higher chlorite content
certain amount kaolinite content very low
Organic Matter thermal evolution higher lower types sapropel -
mixture mixture - humics gas generation pyrolysis
Physical Characteristics
porosity low lower permeability low lower nature fracture
developed poor developed
-
SPE 167803 3
Influence factors of fracability
Fracability is capability of the reservoir to be fracture
stimulated effectively which is comprehensive reflection of
shale
geological and reservoir characteristics. Influence factors of
fracability in shale mainly include shale brittleness, brittle
mineral
content, nature fracture, diagenesis, and sedimentary
environment. The fracability influence factors of non-marine
shale
focused on the Mesozoic Triassic Yanchang Formation shale in
Ordos Basin of China are separately analyzed in this part.
Brittleness
Shale brittleness is the most important influence factor of
fracability. Higher brittleness can make more induced fractures
when
reservoir takes hydraulic fracturing. The more mud shale has,
the heavier plasticity shale is, and plastic deformation will
be
produced in fracturing, so fracture network is forming simply.
When content of brittle mineral such as quartz is relatively
higher, shale become more brittle that fracture network will be
more complex. Therefore, the brittleness is higher, the
fracture
network is more complex, and fracability is higher.
Shale brittleness is usually represented by Poisson ratio and
Young modulus these two rock mechanical parameters. Poisson's
ratio reflects the ability of shale failure under pressure and
Young's modulus reflects the ability of keeping crack after the
fracturing. The higher Young's modulus and lower Poissons ratio
is, the higher brittleness is. In general, Young's modulus of shale
is 10 to 80 GPa, Poissons ratio is 0.20 to 0.40 (Tang, 2012).
Rickman (2008) use brittleness index calculated by following
formulas to determine brittleness quantitatively:
where is brittleness index, dimensionless; is static Young's
modulus, 10 GPa; is static Poisson's ratio, dimensionless; is
normalized Young's modulus, dimensionless; is normalized Poisson's
ratio, dimensionless.
Sondergeld (2010) have concluded a Brittleness Index formula
from the proportion of quartz-carbonate-clays which leads to
the observation that the most brittle section of Barnett shales
have abundant quartz, the least brittle have abundant clays,
and
those with abundant carbonate are moderate. He has compared two
Brittle Indexes, one from the mineralogy and the other one
from the Poissons Ratio and Static Youngs Modulus, and
summarized that both indexes are similar. It defined as:
Where BI is Brittleness Index, %;
is content of quartz, %;
is content of clay, %; is content of carbonate, %.
It is obvious that Brittle Index from the mineralogy could not
be used in non-marine shale, since not only quartz content is
high in shale, but also feldspar content plays a key role in
brittle mineral content.
Brittle Mineral Content
Brittle mineral content is a key influence factor of pores and
micro fractures development in shale matrix, gas content,
fracturing methods and so on. The higher brittle mineral content
is, the stronger shale brittleness is. Quartz is the main
brittle
minerals in shale reservoir, that some reports replace brittle
mineral content with quartz content. Though scholars realized
that
besides quartz, feldspar and dolomite are brittle components in
shale reservoir as well, low content of feldspar and dolomite
are always ignored when reservoir is evaluated, especially in
marine shale reservoir.
Within the perspective of rock failure mechanism, main
ingredient of quartz is silicon dioxide, which has high
brittleness, and
easily broken forming fractures under external force. In
general, the higher quartz content of shale reservoir, the more
natural
-
4 SPE 167803
fractures developed. So that in hydraulic fracturing operations,
more induce fractures are produced, and fracturing efficiency
increased. It is generally recognized that minimum quartz
content is 25%, optimum value is 35%.
Brittle minerals in non-marine shale mainly include quartz,
feldspar and carbonate while quartz and carbonate mainly in
marine shale. Mesozoic Triassic Yanchang Formation shale in
Ordos Basin of China is typical lacustrine shale, which have
16.0% ~ 44.0% quartz content, 12.0% ~ 32% feldspar content, and
23.0% ~ 64.0% clay content (Guo, 2012). This data shows
that feldspar is a main brittle mineral which should not be
ignored in non-marine shale.
Nature Fracture
The existing of nature fractures is the performance of
geo-stress inhomogeneity. The development zone of nature fractures
is
usually the zone with weak geo-stress. Nature fracture reduces
the tensile strength of shale, and changes the geo-stress near
wellbore. The change of geo-stress will influence induced
fractures creating and extending. Therefore, developed nature
fractures could increase fracability.
Natural fracture is the weak links on rock mechanics. It can
enhance the effect of hydraulic fracturing, and fracture
pressure
can be as low as 50% of fracturing in the shale reservoir
without nature fractures, research shows. Moreover, induced
fractures
and natural fractures influence each other, and fracturing
direction is controlled by natural and induced fractures at the
same
time (Tang, 2012).
Natural fractures seem to be ubiquitous in shale gas plays. It
is often said that their presence is one of the most critical
factors
in defining an economic or prospective shale gas play. However,
natural fractures in shale should be represented into natural
micro fractures (scale at micron or nanometer) and common bigger
scale natural fractures. Natural fractures developed well in marine
shale, but not in non-marine shale. In lacustrine shale, only
micron and nanometer scale micro fractures exist, no
bigger scale fractures developed.
Bowker (2008) have showed the image of mineralized natural
fractures in a Barnett shale sample, the natural fractures are
across the shale core, and filled with white mineral (Figure
1).
Figure 1 - Mineralized natural fractures in a Barnett shale
sample (Kent Bowker, HAPL Technical Workshop, 2008)
Figure 2 shows the micro fractures in non-marine shale sample,
fractures with 2-4 wide and 100 long exist in black organic matter.
It is obvious smaller than bigger fracture in Barnett shale
sample.
Diagenesis
The morphology of minerals, clay mineral composition and pore
types are different in distinct diagenetic stages of shale,
which will influence fracability of shale reservoir. The
vitrinite reflectance ( ) is considered as an important index
reflecting the thermo-evolution history of organic materials, and
it is also the most suitable parameters to reflect the diagenesis
in shale.
-
SPE 167803 5
Figure 2 - Micro fractures in Yanchang formation shale.
Table 2 shows in four diagenetic stages, reservoir features have
changed with mineral changing. In low maturity stage, shale
brittleness is mainly under the influence of clay mineral
composition. With the increase of maturity, Shale brittle minerals
and
reservoir porosity increase, and fractures developed, so that
fracability is increasingly higher. The higher mature is, the
faster
fracability increase.
Table 2 - Key features bear on fracability in different
diagenetic stages.
Diagenetic Stages Key Features
< < 3 Mesogenetic stage A period Porosity decrease
3 < < Mesogenetic stage B period Hydrocarbon generation
and expulsion
< < Telogenetic stage Brittleness increase
> Over mature stage Clay minerals stable; higher
fracability
However, in non-marine shale is generally lower than marine
shale. In Yanchang shale, is between 0.8% and 1.2%, average is
1.0%, which is in the mesogenetic stage A period, the stage of oil
and gas coexistence.
Table 3 - and quartz content of marine and non-marine shale (an
adaptation of Chen, 2011).
Facies Shale Quartz Content
Marine Barnett 1.0% ~ 1.9% 38% ~55% Ohio 0.4% ~ 1.3% 35% ~ 47%
Lewis 1.6% ~ 1.88% 22% ~ 52%
Lacustrine Yanchang 0.8% ~ 1.2% 16.0% ~ 44.0%
Biyang 0.57% ~ 1.08% 14% ~ 25%
Sedimentation
Different sedimentary histories make different tectonic and
sedimentary characteristics of shale. In lacustrine
sedimentation,
lake has smaller area for plants and animals deposit, compared
with sea in marine sediment environment. Lake also has less
organic matter than sea, but in deep lake, the amount of organic
matter is considerable.
Sediment environment could influence thickness, interbed and
interlayers of shale, which affect reservoir properties,
ultimately
affect gas generation, storage and migration. Of cause,
reservoir properties will determine fracability. But influence is
so
complex that need more investigation.
Influence factors of shale fracability are not isolated from
each other. Various factors influence each other, and represent
fracability characteristics together.
-
6 SPE 167803
Fracability evaluation
According to the above research, gas shale reservoir fracability
correlated with every complex influence factors. It is so hard
to
evaluate fracability comprehensively. Yuan (2013) put forward a
method that using fracture toughness and brittleness index
identify Fracability index:
where is Fracability index; is brittleness index; is type I
Fracture toughness; is type II Fracture toughness.
Fracture toughness is an important factor of representation
difficulty level of reservoir fracturing. It reflects the ability
of
keeping fracture extending forward after fracture formed in the
hydraulic fracturing.
This evaluation method could establish spatial distribution of
fracability index, according the rock mechanics parameters in
different location of reservoir. Fracability index is accurate
to the certain location. However, only mechanics parameters are
taken into consideration. It is still cannot fully reflect the
problem that a comprehensive problem simplified to a mechanical
problem.
Tang (2012) established a mathematical model of Fracability
Index to evaluate the fracability. Calculation steps of
Fracability
Index are that:
1. Normalize all parameters values with different units or
different dimension;
2. Determine the weights of different factors that affect
fracability;
3. Weight Standardized value and weight coefficient.
Mathematics calculating formula of Fracability Index is as
followed:
where FI is Fracability Index, dimensionless;
is the standardization values of reservoir parameter,
dimensionless; is the weight coefficient of reservoir parameter,
dimensionless;
c is correction coefficient, take experience value according to
the different characteristics.
This method can quantitative calculate Fracability Index of
shale reservoir, and obtain distribution features in the plane,
according to the distribution of different parameters on the
plane, to optimize fracability zone. It is obviously more
comprehensive, but a lot of experiences are needed.
Conclusions
In hydraulic fracturing, Fracability is needed to give a
representation of difficult level. Non-marine shales have more
complex
properties. After analyze influence factors of fracability
existing in non- marine shale, two fracability evaluation methods
have
been discussed. We can conclude as follows:
1. Marine shale and non-marine shale are different in many
aspects. When a non-marine shale reservoir plan to be developed,
copy the marine shale reservoir experience is inadvisable;
2. Influence factors of fracability in non-marine shale
reservoir are more complex. Influence factors of shale fracability
are not isolated from each other, and various factors influence
each other. So more factors need to be considered when
hydraulic fracturing is designing;
3. Fracability evaluations are not comprehensive that need more
investigated, especially fracability of non-marine shale
reservoir.
Acknowledgments
We would like to thank Faculty of Engineering, China University
of Geosciences for Chinese literatures, and Institute of
Drilling engineering and Fluid Mining, TU Bergakademie Freiberg
for all help.
-
SPE 167803 7
References
K.K. Chong, W.V. Grieser, et al., 2010, A Completions Guide Book
to Shale-Play Development: A Review of Successful Approaches
Towards Shale-Play Stimulation in the Last Two Decades: CSUG/SPE
Paper No 133874, Presented at the Canadian Unconventional
Resources & International Petroleum Conference, Calgary,
Alberta, Canada, 19-21 October.
X.Z Wang, J.C. Zhang, et al., 2012, A preliminary discussion on
evaluation of continental shale gas resources: A case study of
Chang 7
of Mesozoic Yanchang Formation in Zhiluo-Xiasiwan area of
Yanchang. Earth Science Frontiers, 2012, 19(2):192-197.
Pick Rickman, Mike Mullen, Erik Petre et al., 2008, A practical
use of shale petrophysics for stimulation design optimization: all
shale
plays are not clones of the Barnett shale: SPE Paper No 115258,
Presented at SPE Annual Technical Conference and Exhibition,
Denver, Colorado, USA 21-24 September.
Thomas F, Moslow, 1984, Depositional models of shelf and
shoreline sandstones: American Association of Petroleum Geologists,
1984:
99-102.
Q.N. Zeng, B.S. Yu, et al., 2013, Reservoir characteristics and
controlling factors of Yanchang formation shale in southeast of
Ordos
basin: Special Oil & Gas Reservoirs, 2013(1).
Y. Tang, Y. Xing, et al., 2012, Influence factors and evaluation
methods of the gas shale fracability: Earth Science Frontiers,
2012, 19(5):
356-363.
C.H. Sondergeld, K.E. Newsham, et al., 2010, Petrophysical
considerations in Evaluating and producing shale gas resources: SPE
Paper
No 131768, Presented at the SPE Unconventional Gas Conference,
Pittsburgh, Pennsylvania, USA, 23-25 February.
Q. Guo, F. Shen, et al., 2012, Discussion on stimulation
technology of shale gas reservoir in Yanchang formation, Ordos
basin:
Petroleum Geology and Engineering, 2012, 26(2), 96-98.
X. Chen, et al., 2011, Study and application of fracturing
techniques for continental shale reservoir in Biyang depression of
Nanxiang
Basin: Petroleum Geology and Engineering, 2011, 25(3),
93-96.
J.L. Yuan, et al., 2013, Fracability evaluation of shale-gas
reservoirs: Acta Petrolei Sinica, 2013, 34(3): 523-527.
T. Zhu, et al., 2012, Pooling conditions of non-marine shale gas
in the Sichuan Basin and its exploration and development
prospect:
Natural Gas Industry, 2012, 32(9): 16-21.