Page 193 - Fundamentals of Gas Shale Reservoirs

MECHANICAL PROPERTIES OF GAS SHALE RESERVOIRS 173
According to them weak shale has moderate porosity (~20%) acquiring enough information about the mechanical prop­
and low velocities which increase by 10–15% with increasing erties of shale. However, due to mechanical and chemical
stress. They indicated that the dynamic stiffness is three to instability of shales, retrieving larger core samples for con­
five times the static stiffness for 100% water saturated shales. ventional mechanical testing will be barely possible. Hence,
They also showed that the velocity of low porosity shale is a new approach in the determination of the mechanical
almost two times higher than that of the weak shale which parameters of shale is really necessary in order to obtain the
increases by approximately 5–8% with increasing isotropic required information from small‐scale sample of shale.
stress. Kuila et al. (2011) indicated that a hard shale shows Nanoindentation testing (NIT) has been developed in the
significant anisotropy in wave propagation but this anisot­ past few decades. The testing method uses a high‐resolution
ropy for the P‐wave is much higher than that of the S‐wave. electronic instrument to measure applied loads and value of
They attributed this to low clay content and the laminated displacement of an indenter (Hay and Pharr, 2000). The aim
nature of the shale. In addition, it is well known that water or of NITs is to calculate Young’s modulus and hardness of the
gas saturation has a significant effect on the P‐wave velocity sample from load‐displacement recording. A small size
in a porous media. This impact will be increased as the sample (~mm) and a very small load are used in this type of
degree of saturation increases. However, this straightforward test, so indention area will be as small as a few nanometers.
concept may not be valid for gas shale reservoirs since there Depth of penetration as well as area of indentation is also
are publications reporting different effects of gas saturation recorded using known geometry of an indenter tip. This
in shale (Hsu and Nelson, 1993; Lashkaripour and Passaris, technique is widely used to determine mechanical properties
1993; Rozkho, 2010). The possible reason for this might be of metals, ceramics, polymers, etc. In conventional uniaxial
the kerogen type, mineralogy, heterogeneity of the shale as or triaxial tests performed to obtain Young’s modulus and
well as its pore structure. Hence, it would not be rational to mechanical properties of different samples, a relatively large
draw any conclusions about the mechanical parameters of sample (~cm) and high load is required to get accurate
shale based on the behavior of the P‐wave velocity. Ghorbani results. However, in NITs, a very small sample and very low
et al. (2009) pointed out the significant increase in P‐wave force (~mN) are used to calculate mechanical properties of
and S‐wave velocity of partially saturated shale samples. the samples. Thus, this technique can be a very good choice
Naturally, the S‐wave velocity will not be affected by fluids, for determination of/determining the mechanical properties
but because the rigidity of the shale increases as the fluid of shale samples since retrieving large samples from this
saturation decreases, even S‐wave velocities will change. type of formation is barely possible.
Nagra (2002), who did studies on shale gas reservoirs, con­ Abousleiman et al. (2007) used NITs on 10 Woodford
cluded that as the water content decreases, different shales outcrop shale samples to calculate elastic properties. X‐ray
show different behavior. For instance, with low water diffraction (XRD) mineralogy data of these samples showed
content, shale seems to be a ductile–brittle material with quartz content varying from 27 to 53 vol%, clay content
some strain hardening. At very low water content, opalinus ranging from 18 to 43 vol%, kerogen content varying from
clay will be entirely brittle material with strength reduction 11 to 18 wt%, and porosity ranging from 0.16 to 0.19
toward its residual strength. In terms of stiffness, Young’s fraction volume. Indentations were done both parallel and
modulus significantly increases as the saturation decreases. perpendicular to the bedding plane. They exhibited low
Considering the situation described earlier, care must be range of Young’s moduli because the sample might be weak­
taken when assessing gas shale reservoirs containing differ­ ened due to chemical weathering at shallow depth.
ent amounts of clays. This is mainly because of the negative Zeszotarski et al. (2004) used nanoindentation technique to
impact of clays on the saturation, strength, and P‐wave and measure mechanical properties of kerogen. Elastic prop­
S‐wave velocity. It is still possible to relate the variation erties were measured both parallel and perpendicular to the
in P‐ and S‐wave velocity to stiffness and strength of gas bedding plane.
shale reservoir rocks, but care must still be taken (e.g., Samples used in their study showed total organic car­
Gray, 2010). bonate (TOC) content of 22 wt% and maximum temperature
of 421°C. They indicated that Young’s modulus of shale has
an inverse relationship with TOC content. They measured
8.2.4 Nanoindentation Tests on Gas Shale Plays
hardness and indentation modulus on three orthogonal faces
Shale is widely recognized as one of the most heterogeneous of shale sample cube and reported no evidence of anisotropy
and complicated materials. Due to the variation of its com­ in the mechanical properties of kerogen. Ahmadov et al.
position, prediction of its elastic properties is a very hard (2009) performed nanoindentation measurements on a
task to accomplish. Having information about the mechanical sample from organic‐rich formation from 3800 m depth
properties of shale is critical as it can be helpful for drilling showing kerogen content in the range of 7–21 vol%. All
stability design and seismic interpretation (Ahmadov, 2011). imaging and measurements were done orthogonal to the
Besides, hydraulic fracture will be significantly improved by horizontal bedding plane. They calculated Young’s modulus