Page 191 - Fundamentals of Gas Shale Reservoirs

MECHANICAL PROPERTIES OF GAS SHALE RESERVOIRS 171

Stress–strain (SG) Plot 0 30,000.0
30,000.0
25,000.0
25,000.0

20,000.0 Stress (psi) 20,000.0
Stress (psi) 15,000.0 15,000.0

10,000.0
10,000.0
5,000.0 5,000.0

0.0 0.0
–0.0 0.0 –0.0 0.0 0.0
Strain Strain
FIGURE 8.2 Axial stress–axial strain (left) and axial stress–radial strain (right) curves corresponding to a typical multistage triaxial test.

water from low porosity shales with appreciable amounts of formation at high closure stresses and not the failure of the
clay generally results in strengthening of the material. This core sample. Therefore, if the sample is isotropic and lami­
will introduce a significant increase in the associated strength nations are not visible, core plug orientation may not be
and stiffness parameters of the sample (Ghorbani et al., important in terms of its effect on mechanical response of
2009). In addition, drying of shales can induce high capillary the shale. This means that either vertical plugs from the
pressures, causing softer specimens to be destroyed (Horsrud whole core or horizontal plugs from sidewall cores can
et al., 1998b). Thus, for partially saturated shales such as gas be used for determining the rock mechanical properties of
shales, it is suggested that materials be cling‐filmed, wrapped the samples.
in tin foil, and then waxed, either as whole cores or as core
plugs and tested in as short a time period as possible as wax 8.2.2 True‐Triaxial Tests
is permeable to air/water on longer time frames. If the sam­
ples are preserved for lab tests, static and dynamic tests can In a conventional triaxial test, as explained in the previous
be conducted. Typically, shales dominated by mixed layer of section, the sample is subjected to an isotropic confining
illite–smectite with Young’s Modulus of 1–3 GPa and a UCS pressure during the experiment. It should be noted, however,
of 8 MPa cannot be considered for hydraulic fracturing. So that in real field condition the rocks are under three different
here triaxial tests can play an import role in describing a independent stresses known as principal stresses. These are
shale formation from the hydraulic fracturing point of view. usually the vertical stress, due to the weight of overburden
According to Josh et al. (2012), shales with higher cohesive rocks, and two horizontal stresses perpendicular to each
strength and UCS containing silts and approximately 30% other, one being maximum and the other one minimum.
clay mainly illite, which is the most thermally stable clay Simulating the rock failure using the conventional method
mineral, are often seen in gas shales reservoirs. The silty where a cylindrical sample is used for the experiment means
nature of gas shales and clay composition has an impact on that the effect of the intermediate stress is ignored in the
the mechanical properties of such shales. However, those rock’s failure response. This could have a significant effect
shales with clay content of up to 60% and smectites are usu­ on rock behavior, in particular shales which are made of VTI
ally the weakest of the common clay minerals, resulting in material where the effect of having three independent
the low Young’s modulus and weak strength parameters. stresses on the sample at different orientations could be more
Rickman et al. (2008) suggest that low modulus and high pronounced in terms of impact on its failure response. In
Poisson’s ratio shales are generally too ductile to be prospec­ order to test a sample under three independent stresses, that
tive. They have found that low Young’s modulus gas shales is, true‐triaxial stress conditions, a cube shape sample is
are those with relatively high clay content. They categorized needed. Figure 8.3 shows a view of a true‐triaxial stress cell
nonprospective shales as a group having very high clay (TTSC) which has been designed for estimation of rock
content and generally exhibiting visible laminations to the strength parameters and various petroleum‐related applica­
naked eye, whereas prospective shales in many cases are iso­ tions including hydraulic fracturing and sanding analysis.
tropic since their elastic properties in different directions are Samples of up to 300 mm can be tested using this cell and
mostly the same. Britt and Schoeffler (2009) indicated that a the magnitude of external stresses that can be applied to a
significant increase in displacement during the triaxial test 100 mm sample is approximately 15 MPa. A pore pressure
in shale core samples is due mainly to the ductility of the of 21 MPa can be applied. Figure 8.3 (right) shows the view