Page 160 - Applied Petroleum Geomechanics

Basic rock fracture mechanics 153

consists of both intact rock blocks and discontinuities. An intact rock and a
rock mass have significant different geomechanical behaviors (Peng and
Zhang, 2007). Nearly every rock property is determined to some extent on
the discontinuities and the fluids they contain. The success of many ap-
plications such as efficient recovery from fractured reservoirs, hazardous
waste disposal, and geothermal energy extraction depends on a thorough
understanding of fracture behaviors. Many petroleum reservoirs are situated
in fractured porous formations, in which discontinuities have significantly
different mechanical properties that control the behaviors of the reservoirs.
The discontinuities can be the single most important factor governing the
deformability, strength, and permeability of the rock mass. Moreover, a
particularly large and persistent discontinuity could critically affect the
stability of any underground excavation (Hudson and Harrison, 1997). For
these reasons, it is necessary to understand the geomechanical properties of
discontinuities and to know how the discontinuities affect rock behaviors.
There are many types of discontinuities in rock formations. Bedding, a
discontinuity, is one of the most common geologic phenomena in sedi-
mentary rocks. The bedding plane or interface between two layers in the
sedimentary rock mass represents geologically the transitive interface from
one sedimentary environment to another, and it also represents an inter-
ruption of sedimentary planes. A bedding plane generally consists of plant
detritus, mica, and other weak minerals. Therefore, it is usually treated
mechanically as a weak plane, as shown in Fig. 4.11.
The other discontinuities include unconformity planes and tectonic
structural planes, such as faults, fractures, and joints induced by tectonic
activities. Fig. 4.12 shows steep bedding planes and a fault in the Arbuckle

Figure 4.11 Thin bedding planes in the Marcellus shale outcrop.