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.