Abstract from 22.4 to 25. For theAbstract from 22.4 to 25. For the

Abstract

 

This paper presents an investigation into mechanical
properties of shale rock from Qassim Province, Saudi Arabia. Uniaxial
Compression test, Schmidt hammer test and porosity estimation was carried out.
For the compression test it was found that the strength ranged for 1.98 MPa to
8 MPa and the strain ranged for 0.53% to 2.5%. For the Schmidt Hammer test, it
was found that the rebound values ranged from 22.4 to 25. For the volumetric
porosity, the measurements indicated that the porosity in the shale rock ranged
between 19.12% and 24.31%. All the values determined in this project match well
with the internationally published values for shale rock.

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Keywords: Shale Rock, Mechanical Properties, Qassim
Province, Saudi Arabia

 

 

 

 

 

 

 

 

 

1.0 Introduction

Shale oil and gas production from organic rich
shale formations is a growing area of technical interest in oil and gas
exploration today. Long horizontal wells with hydraulic fracturing are required
to bring economic production from shale gas reservoirs. Since crack propagation
in hydraulic fracturing occurs under high strain rates, it is important to
understand the fracture behavior of shale rock and its mechanical properties. The
properties of Shale rock are needed in order to be able to design the hydraulic
fracturing systems include: Compressive Strength, Hardness, Porosity, Modulus
of Elasticity, Poisson’s ratio, Fracture Toughness and Permeability.

 

Rocks in general can be classified as : 1.
Igneous rocks 2. Sedimentary rocks , and                    3. Metamorphic rocks.  Sedimentary rocks are formed when products of
weathering are subjected to transportation by water, winds or deposition and
subsequently are compacted or consolidated. Some examples are sandstone, shale,
conglomerate, breccias, limestone, coal and evaporates. Minerals forming the
sedimentary rocks are kaolinite, illite, smectite, hematite, rutile, corundum
and so on. Shale is a fine-grained, clastic sedimentary rock composed of mud
that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles)
of other minerals, especially quartz and calcite  (wikipedia,
2016).

Numerous laboratory and theoretical
investigations have demonstrated how mechanical properties in sedimentary rocks
are affected by porosity (Wyllie,1957); clay content (Han et al. 1986); overburden
stress and pore fluid (King, 1983). Further understanding needs to be developed
on how these parameters control rock strength.

The aim of this paper is to test the mechanical
properties of Saudi Shale Rock from Qassim Province, under  static
loading. Rocks samples were taken from shale formations in Qassim region, Saudi
Arabia. The following tests were carried out as indicate in Table 1. Uniaxial
Compression test, Schmidt hammer test and porosity estimation was carried out.

1.1 Literature Review for Uniaxial
Compression Test of Shale Rock

International
Society for Rock Mechanics (ISRM) describes rock with an UCS in the range of
0.25 to 25 MPa (about 35 to 3,600 psi) as “extremely weak” to “weak” (Ulusay,
2015). A more appropriate upper bound strength limit for weak rocks may be 20
MPa (about 3,000 psi) because there appears to be a difference in the way rock
weaker than this limit behaves when sheared. Strength test data for sandstones
indicate rocks with a UCS below about 20 MPa generally contract when sheared
whereas stronger rocks tend to dilate (Dobereiner and de Freitas, 1986).
Materials that dilate when sheared tend to resist the strains imposed on them
and therefore, are less deformable than materials that tend to contract when
sheared. Therefore, for the purposes of this paper, weak rock is considered to
be rock with an UCS in range of 0.25 to 20 MPa (about 35 to 3,000 psi).

Another
important factor influencing the strength of weak rocks is the porosity, or the
amount of void space in the rock. In general, high porosity correlates with low
strength. Low porosity and high strength is a result of a dense arrangement of
grains and/or cementing agents filling the void space between grains. Table 1
summarizes

some
available strength data for mudstone and sandstone, generally indicating that
mudstone and sandstone with a porosity higher than about 10 and 20 percent,
respectively, will most likely be considered weak rock, if 20 MPa is taken as
the upper strength limit.

Examples
of weak rocks include sedimentary rocks (sandstone, siltstone, shale, claystone
or mudstone, clay-shale, marl, and chalk), some volcanic rocks (tuff,
agglomerate, and breccia), and weathered and altered (hydrothermal or chemical)
rocks of all types. In addition, weak rock conditions can also be produced by
close jointing, shear zones, or faults in the rock mass (Jaeger et. Al. 2007)
and (Klein, 2001).

 

Table 1.  Uniaxial compressive
strength of sandstone and shale as related to porosity

 

Reference

Location

Porosity %

Compressive Strength (MPa)
 

Dobereiner and de Freitas, 1986

Kidderminster (UK)

~31

2 to 3

Ezzat Williams, 2005

Bringelly Shale, Australia

7% to 14%

2.4 to 49

1.2 Literature review of  Schmidt Hammer Tests conducted
for shale rock

A number of studies have indicated the
usefulness of the Schmidt Hammer test on different rocks and have established
its strong correlation with UCS through numerous empirical equations. Schmidt
Hammer Rebound (R) values were directly used in the analysis and were not
converted to UCS, since there is no standard conversion designated for shale.
Table 2 shows the range of R values for Sevier and Rome Shale (Nandi et al.,
2009).

Table 2. Estimated compressive strength of shale rock (Nandi et al., 2009)

Type of Shale
 

Schmidt Hammer Rebound
(R)

Sevier Shale

30

Rome Shale

38

 

 

 

 

1.3 Literature Review of
Porosity in Shale Rock

Porosity is defined as the fraction of a rock
that is occupied by pores. It is a static property, and can be measured in the
absence of flow and determining effective porosity requires fluid flow to
determine if pores are interconnected.

Table 3 presents some data on average porosity
(Manger, 1963). Nearly all the measurements were made at room temperature and 1
atmosphere for different countries and for different cities (Manger, 1963).

Table 3. Average porosity for Shale rock from different
resources (Manger, 1963)

 

Title

 Average Porosity (

)

Shale (Near Ponca City, Oklahoma)

42.5%

Shale (Eastern Venezuela)

33.5%

Shales (Los Manueles field, Venezuela)

20%

Shale (Ponca City and Garber areas, Oklahoma)

17%

Weston Shale (Bonner Springs, Kansas)

15.8%

Chanute Shale (Independence, Kansas)

14.9%

 

 

2.0 Materials and Methods

Shale rock samples were collected from a local
cement quarry in Qassim Province, Saudi Arabia. A total of 40 samples were
obtained for initial testing.

2.1  Specimens
Preparation

Cutting of 40 samples was completed in the
Civil Engineering lab at Qassim University, using their sample preparation
machine. It took five days to complete preparing the specimens to obtain only
20 proper samples out of 40, the rest got damaged during the cutting operation.
Figure 1 shows the shale samples before they
were cut into cubical shapes. Figure 2 shows the sample being cut on a rock
slicing machine.

Figure 1.  Identifying all samples

 

        Figure 2.   Cutting operation

 

The specimens were cut as 50 mm x 50 mm x 50 mm
cubes as per ASTM C 109/C 109M.  There may be some error in the dimensions due
to the difficulty in precision cutting of the soft rocks. Also, there is an
expected dimension error of  ±2 mm, at the maximum.

3.0
Results and Discussion

3.1 Uniaxial Compression Test

 

The laboratory uniaxial compressive strength is
the standard strength parameter of intact rock material. Compressive strength
is the capacity of a material or structure to withstand loads tending to reduce
size (Wikipedia, 2016).

 

Figure 3 shows an MTS universal tensile testing machine used with
a capacity of 25 kN (MTS Website, 2016).

 

           Figure 3.   Fixing specimen

 

Figure 4 shows the first sample of compression
test before the test. While Figure 5 represents the sample after the test, we
can clearly see the failure which started in the horizontal direction.

    

                     

      Figure 4.   Sample  1 before the test               Figure 5.  
Sample  1 after the test

Table 4 shows the results of the compression
test.  The average strength of the  four samples is 2.5 MPa. Table 4 also shows
the strain values of the four samples, the maximum is 2.5 % for sample 3 and
the minimum is 0.53 % for sample  5,
while the average is 1.27 %.

Table 4. Summary of Tensile
Test

 

Sample No.

Strength (MPa)

Strain (%)

1

2.13

0.91

2

2.38

1.26

3

3.53

2.5

4

1.98

1.16

 

Comparison with Published Results  

It is important to compare the results with
previous ones, to observe and discuss the differences between them. Table 6
represents the uniaxial compressive strength of shale rock as related to
porosity, from literature, in comparison with Qassim Shale results as
investigated in this paper. The average porosity of Qassim Shale as calculated
was 22.54 %. The average UCS of Qassim Shale was 2.5 MPa which is well within
the range of internationally published values as shown in Table 5.

Table 5. Comparison of UCS between Qassim Shale and the
previous results

 

Reference

Location

Porosity %

Compressive
Strength (MPa)
 

Dobereiner and de Freitas, 1986

Kidderminster (UK)

~31

2 to 3

Ezzat Williams, 2005

Bringelly Shale, Australia

7% to 14%

2.4 to 49

Current study

Qassim, Saudi Arabia

19% to 24%

1.98 to 3.53

 

 

3.2 Schmidt Hammer Test

 

A total
of 3 samples were tested by Schmidt hammer. The dimensions of samples are 50mm
x 50mm x 50mm. N-type Schmidt Hammer device was used to measure rock hardness.
The N type Schmidt Hammer has an impact energy of 2.2 N.m. Figure 6 shows the
Schmidt hammer used to conduct the test.

 

 

 

 

 

 

                                                      

Figure 6.   Schmidt hammer

 

Table 6 shows  pictures of the three samples before and after
the test. It also shows the rebound and strength values obtained. The maximum
rebound value was in sample 2 by the value 25, where sample 3 has the minimum
value of 22.4. Please note that the empirical relation used to estimate the
compressive strength by Shalabi et al. (2007) might not be true for the specimens
used in this test.

 

 

 

 

 

 

Table 6. Schmidt hammer test results

 
Name

 
Picture before test

 
Picture after test

Rebound value
(R)

 
 
 
 
SH-2016-01

 
 
 
   
     23.5

 
 
 
 
SH-2016-02

 
 
 
   
      25
 
 
 

 
 
 
 
 
SH-2016-03

 
 
 
 
      22.4

 

Comparison with Published Results 

Tables 7 and 8 show the results of this study in
comparison with previously published results (Nandi et al. 2009).

 

 

 

 

Table 7. Comparing results with Sevier Shale (Nandi et al. 2009)

 

 
Sample Number of current study

Absolute
Difference of Schmidt
Hammer Rebound Value
(R)

Percentage
Difference of Schmidt
Hammer Rebound Value
 (%)

SH-2016-01

6.5

21

SH-2016-02

5

16

SH-2016-03

7.6

25

 

 

Table 8. Comparing results with Rome Shale (Nandi et al. 2009)

 

 
Sample Number of current study

Absolute
Difference of Schmidt
Hammer Rebound Value
(R)

Percentage
Difference of Schmidt
Hammer Rebound Value
 (%)

SH-2016-01

14.5

38

SH-2016-02

13

34

SH-2016-03

15.6

41

 

The depth from where the Sevier and Rome samples
taken is not mentioned.

 

3.3 Determination of Porosity

Porosity is the fraction of a rock that is
occupied by pores. It is a static property and it can be  measured in the absence of flow. Determining
effective porosity requires fluid flow to determine if pores are interconnected
(Texas A report, 2016).

     (1)

Where:

?: Porosity (%).

Vb: Bulk Volume (cm3).

Vp: Pore Volume
(cm3).

Vm: Matrix
Volume (grain volume) (cm3).

 

The Bulk volume (Vb) of the sample
was measured by volumetric displacement method, by immersing it in the beaker
with water. The Matrix volume (Vm) was measured by crushing the
sample to grain size as shown in Figure 7 and immersing it in a container
filled with water.

 

Figure 7.   Crushing the sample with the hammer to grain
size

 

 

 

 

 

 

 

Figure 8.   Sample 1 before test

 

 

A
typical calculation for sample 1 is shown below.

 

Vb= 136 ml

Vm= 110 ml

Pore Volume (Vp) :
Vp = Vb-Vm = 136 – 110 = 26 ml

Porosity (?):

Substituting
the results of the pore, bulk and matrix volume in equation (6.1):

 

 = 19.12%

 

Table 9. Summary of results

Sample
No.

Porosity
(

)

1

19.12%

2

24.15%

3

24.31%

 

The Porosity average of all the samples
calculated as:

 

Comparison with Published Results 

Table 10 shows the porosity of Shale rock of previous
published results (Manger, 1963) in compare with Qassim Shale rock.

 

Table 10. Comparison with published values (Manger, 1963)

Title

Published
Average Porosity (

)

Qassim
shale Average Porosity (

)

Difference
Between Average Percentage

Shale (Near
Ponca City, Oklahoma)

42.5%

22.53%

+
20.03%

Shale
(Eastern Venezuela)

33.5%

+
10.03%

Shales
(Los Manueles field, Venezuela)

20%


2.53%

Shale
(Ponca City and Garber areas, Oklahoma.)

17%

 – 5.53%

Weston
Shale (Bonner Springs, Kansas)

15.8%


6.73%

Chanute
Shale (Independence, Kansas)

14.9%


7.63%

 

It can be seen that the measured values of
porosity for Qassim Shale lie well within the range of published values in
literature.

 

 

4.0 Conclusion and Future Work

The uniaxial compressive strength was tested
for four different samples, the strength ranged for 1.98 MPa to 5 MPa and had
an average of 2.5 MPa, while the strain ranged between 0.53% to 2.5% and had an
average of 1.27%. The Schmidt hammer test results found that the rebound values
ranged from 22.4 to 25. The displacement method was used to determine
the porosity of Qassim Shale rock, the results were (?1= 19.12%, ?2=24.15% and
?2=24.31%) and the average porosity of was  ?avg= 22.53%.

The shale
samples for this pilot study were taken from a cement quarry. In future samples
can be derived from the drilling “CORE”, because that would give more meaningful
results for Shale gas exploration. Also, another suggestion is to calculate
modulus of elasticity and Poisson’s ratio for shale rock. Schmidt hammer can be
done with a lighter L-type Schmidt hammer, which has impact energy 0.735 N.m . In addition, impact tests can also be conducted.

Acknowledgements

The authors would like to thank undergraduate students Suhail A
Aswailem, Abdurehman S Alquba and Ahmed M. Alhasan for conducting the
experiments as a part of their senior project.

References

 

Dobereiner, L. & de Freitas, M. H.
Geotechnical properties of weak sandstones. Geotechnique 36, 79-94
(1986).

 

Faisal
I. Shalabi , Edward J. Cording , Omar H. Al-Hattamleh a Estimation of rock
engineering properties using hardness tests, 
Engineering Geology, 90: 3-4, 138-147, 2007.

 

Han, D., Nur, A., and Morgan, D., 1986, Effect
of porosity and clay content on wave velocity in sandstones: Geophysics, 51,
2093-2017.

http://www.ele.com/Product/rock-classification-hammer,
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http://www.mts.com/en/products/producttype/test-systems/load-frames-uniaxial/servohydraulic/elastomer/index.htm,
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https://en.wikipedia.org/wiki/Compressive_strength ,
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Jaeger, J.C., Cook, N.G.W. and Zimmerman, R.W., Fundamentals of Rock
Mechanics, Fourth Edition, Blackwell Publishing, 2007.

 

King, M.S., 1983, Static and dynamic elastic
properties of rocks from the Canadian Shield, Int. J. Rock Mech. Min. Sci.
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Klein,
S., an approach to the classification of weak rock for tunnel projects, Chapter
64, Jacobs Associates, Rapid Excavation and Tunneling Conference (RETC)
Proceedings, 2001.

 

Manger, G.E., “Porosity and Bulk Density of
Sedimentary Rocks,” Geological Survey Bulletin 1144-E, US Atomic Energy
Commission, 1963.

Nandi,
A, Liutkus, CM, & Whitelaw, MJ. Geotechnical characterization of Sevier and
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Reservoir Petro-physics, Laboratory
Determination of Porosity, presentation by Texas A& M university. www.uis.no.labPorosity.ppt,
visited on 27/4/2016 at 10:00 PM.

Ulusay, R. Editor, The ISRM Suggested Methods for Rock
Characterization, Testing and Monitoring: 2007-2014, Springer International
Publications, Switzerland, 2015.

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M. R., Gregory, A. R., and Gardner, G. H. F., An experimental investigation of
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