Page 195 - Fundamentals of Gas Shale Reservoirs

ANISOTROPY 175
wear flat area, UCS measurement can be done through the use Implementing this type of acquisitions makes it possible to
of the scratch tests. pick the first arrival time of P‐ and S‐waves velocity in vertical
Several studies have been done to reveal the application and horizontal directions, so it will be possible to obtain the
and reliability of the scratch tests in different fields of study anisotropy and Thomsen parameters in different media.
fields. Ulm and James (2011) showed that such test can be Surface seismic measurements like reflection and refrac­
used in oil well cements cured at high temperatures and pres­ tion acquisitions can be designed to use three‐component
sures. They found that increase of strength and toughness of geophones for first arrival picking of P‐ and S‐wave.
different oil well cement baseline formulations can be related Compressional and shear wave velocity models used in
to the water‐to‐binder ratio for a series of cementations mate­ processing surface seismic data in many cases do not con­
rials. Richard et al. (2012) performed nanoscratch tests under sider the anisotropic behavior of subsurface formations.
kinematic condition and concluded with a summary of exten­ Although migration is used to locate events in their exact
sive experimental testing on more than several hundred rocks locations, this method does not directly account for the char­
that this test can provide very reliable results, while it also has acteristics of anisotropic formations like shale. As deter­
many advantages over the conventional method of UCS mea­ mined by Thomsen (1986), anisotropy for nearly vertical
surements. They showed that the scratch tests can even be wave propagation is mostly governed by parameter δ, which
used to determine the UCS of different shale samples. Thus, is a complex combination of elastic parameters (Thomsen,
considering the overall limitations in reserving shale core 1986), and appears to be sensitive to the conformity of the
samples from unconventional oil and gas reservoirs, scratch contact regions between clay particles, as well as to the
tests can be a very good choice in such undesirable situations. extent of disorder in their orientation (Sayers, 2005).
However, the importance of parameter ε increases with
increasing horizontal component of the propagation path.
8.3 ANISOTROPY Event location accuracy in surface seismic reflection is
known to be fairly robust using the regularly assumed iso­
Seismic acquisition is frequently used in petroleum, mining, tropic velocity model (Tsvankin, 1996), but this can be
and civil engineering in order to estimate various geomechan­ further improved in some instances by determining ε and δ
ical and geotechnical parameters. Both surface and borehole to account for velocity anisotropy (Eisner et al., 2011). After
seismic acquisitions are used for this purpose depending on performing initial processing, the velocity model is used
the required applications. through a/the stacking and migration process, to locate the
The continuity logging method of measurements between subsurface events in their exact locations. Since, in most
boreholes has been used for detecting and characterizing the cases, the velocity is assumed to be isotropic, the model has
waves and exploring their potential applications in different to be modified to account for the anisotropic nature of the
geological environments, and is currently being used for res­ earth (Maxwell et al., 2010).
ervoir continuity in hydrocarbon fields. The concepts and
applications of this method are well documented by Liu et al.
(1992), Dresen and Ruter (1994) for coal‐seams, and by 8.3.1 Anisotropy in Gas Shale Reservoirs
Krohn (1992), Turpening et al. (1992), Parra et al. (1996) for As stated by Thomsen (1986), the anisotropy observed in
oil/gas reservoirs. The technique is now known as crosshole shales is caused by a combination of the preferred orienta­
continuity logging or crosshole seismic logging. Previous tion of clay platelets, anisotropic and other isotropic min­
applications of crosshole surveys include examples from the erals, and the preferred orientation of fissures. However,
Conoco borehole test facility, Oklahoma (Lines et al., 1992, despite the fact that the key minerals in shales are highly
1995; Liu et al., 1991) and the Gypsy test site, Oklahoma anisotropic, the overall anisotropy of the formation is weak
(Parra et al., 1996; Turpening et al., 1992). Synthetic exam­ (Maxwell et al., 2010), and can be characterized as VTI with
ples were given by Zhong and Worthington (1994) and Parra a vertical axis of symmetry (Sayers, 2005). To determine the
(1996). Lou and Crampin (1991, 1992) have provided a the­ anisotropy parameters of gas shale reservoirs, VTI parame­
oretical basis for channel wave propagation in anisotropic ters in vertical direction (TIV) need to be determined. VTI
media. To obtain information about the anisotropy of rocks parameters are five independent elastic constants that can be
located between the wells, three‐component geophones are determined as follows:
used. In this type of acquisition, numerous sources are used
to propagate shear and compressional waves in media C V 2 (8.1)
between the wells and several three‐component receivers cover 11 ph
the given interval. The offset, or interval distance, between C V 2 (8.2)
the source and the receiver boreholes depends on the type of 33 pv
measurements. The raypaths are almost horizontal in cross­ 2
hole seismic acquisitions (Cole, 1997; Hardage, 1992). C 66 V sh (8.3)