Page 194 - Fundamentals of Gas Shale Reservoirs

174 GEOMECHANICS OF GAS SHALES

of organics in the range of 10–15 GPa. Kumar (2012) con­ considerable time for sample preparation, for example,
ducted nanoindentation measurements on a total number of polishing the sample ends and precise length to diameter
144 organic‐rich shale samples. His measurements were ratio. In addition, the water content and the irregularity of
performed on different types of shales. He concluded that: the ends of the samples can also cause errors in the measure­
ments results (Bieniawski, 1968; Dey and Halleck, 1981;
• Woodford samples show Young’s modulus of 23–80 Farmer, 1992; Hoek and Brown, 1980; Hudson et al., 1972).
GPa. Samples with lower E values (23–30 GPa) showed Furthermore, the UCS test can be only used for testing
either high concentration of TOC or high clay content; homogeneous and intact rock and not for heterogeneous,
whereas samples with higher E values (60–80 GPa) damaged, layered, or fractured rocks. This is due to the
showed relatively low TOC, porosity, and clay content. existence of the weakest plane, that is, joint or a pre‐existing
Hardness of this formation varies from 0.54 to 7.2 GPa crack, in the core sample which determines the failure of the
and shows higher hardness values (6–7 GPa) due to rocks. Point load, indentation, or Schmidt hammer tests are
high quartz content and low TOC and porosity values. alternative tests which are used to obtain an estimation of
• Barnett samples gives Young’s modulus of 39–78 GPa rock strength (Bieniawski, 1974; Broch and Franklin, 1972;
while higher E values were found to be either relatively Chau and Wong, 1996; Rusnak and Mark, 1999; Szwedzicki,
low in TOC and porosity, or high in carbonate content. 1998). In fact, point load and indentation tests are very use­
• Haynesville samples have a Young’s modulus of 31–79 ful as they can be used to assess rock strength with very
GPa where samples with higher E were found to be small size samples, while the Schmidt hammer test even
either high in carbonate or low in TOC. Hardness for allows testing the strength of the outcrops. Yet, all of these
Haynesville samples with average value of 1.1 ± 0.6 indirect methods suffer from many drawbacks. For instance,
GPa is attributed to the high carbonate content of both the point load and the Schmidt tests can be remarkably
samples. affected by the elastic properties, the sample size, and the
• Eagle Ford samples were found to have a Young’s mod­ water content of the samples (Aydin and Basu, 2005; Thuro
et al., 2001; Tsiambaos and Sabatakakis, 2004; Tsur‐Lavie
ulus of 31–57.5 GPa. Samples with higher carbonate and Denekamp, 1982).
content exhibit higher Young’s moduli. Hardness of The nanoscratch test is one of the most recent methods
these samples obtained was between 0.45 and 1.5 GPa. used for determination of UCS. This test requires a small‐
• Ordovician shale with high average carbonate content scale sample for measurement and does not suffer from any
of 73 ± 4 wt% showed Young’s modulus of 49–57 GPa. of the disadvantages mentioned earlier for the direct and
They showed a hardness range from 1 to 1.3 GPa. indirect method of UCS determination (Richard et al., 2012).
In fact, it has been indicated and proved that the UCS of
Kumar (2012) also indicated that for all shale plays, samples rocks can be reliably assessed from nanoscratch tests
with high TOC and high porosity exhibited low Young’s performed with a sharp cutter, and at a shallow depth of cut
modulus, whereas samples with low TOC, low porosity, and to prevent any significant chipping of the rock.
high carbonate content showed high Young’s modulus values. Scratch tests have been the subject of various studies
Hardness on the other hand shows negative correlation discussing the effect of rock characteristics on drilling
with porosity and clay content as well as poor correlation performance (Glowka, 1989; Nishimatsu, 1972; Deliac, 1986;
with TOC. Duc, 1974). During the scratch test, depth of cut and cutter
velocity remains constant, while magnitude and orientation of
the force acting on the cutter are measured (Detournay and
8.2.5 Scratch Tests
Defourny, 1992; Duc, 1974; Fairhurst and Lacabanne, 1957).
The uniaxial compressive strength (UCS) test is the most Two cutting mechanisms, ductile and brittle, usually occur in
conventional method in the lab to measure the strength of this type of test and depend mostly on the depth of cut
rocks. UCS plays a key role in the design of underground (Chaput, 1992; Huang and Detournay, 2008; Huang et al.,
structures, as well as ensuring the stability of the drilled 2012; Richard, 1999; Richard et al., 1998). In fact, at shallow
wells in civil, mining, and petroleum engineering. The stan­ depth of cut, ductile regime will be the dominate mechanism
dard procedure used to determine the UCS has already been while at larger depth of cut, brittle failure occurs. In the brittle
documented by ASTM (2010) and ISRM (Ulusay and regime fracture toughness controls the cutting force, whereas
Hudson, 2007). Many publications report on the application, the UCS controls the cutting force in the ductile regime
advantages, and disadvantages of the UCS test (Bieniawski, (Richard et al., 1998). Therefore, the scratch test should be
1968; Broch and Franklin, 1972; Hawkes and Mellor, 1970; performed under ductile regime. Considering the intrinsic
Hudson et al., 1972; Jaeger et al., 1976; Wawersik and specific energy associated with the cutting process, the incli­
Fairhurst, 1970). It is well known that the UCS test suffers nation of the force acting on the cutting face, and the friction
from several drawbacks as it requires cores of intact rock and coefficient mobilized across the wear flat and the nominal