Page 524 - Petrophysics

492 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

been proposed for the origin of the regional fractures, ranging from
plate tectonics to cyclic loading/unloading of rocks associated with
earth tides. As in the case of tectonic fractures, small-scale variation
in regional fracture orientation of up to f20" can result from strength
anisotropies in reservoir rocks due to sedimentary features such as across
bedding.
Contractional fractures: These types of fracture result from bulk volume
reduction of the rock. Desiccation fractures may result from shrinkage
upon loss of fluid in subaerial drying. Mud cracks are the most common
fractures of this type. Syneresis fractures result from bulk volume
reduction within the sediments by sub-aqueous or surface dewatering.
Dewatering and volume reduction of clays or of a gel or a colloidal
suspension can result in syneresis fractures. Desiccation and syneresis
fractures can be either tensile or extension fractures and are initiated
by internal body forces. The fractures tend to be closely spaced and
regular and isotropically distributed in three dimensions. Syneresis
fractures have been observed in limestone, dolomites, shales, and
sandstones [7].
Thermal contractional fractures may result from contraction of hot
rock as it cools. Depending on the depth of burial, they may be either
tensile or extension fractures. The generation of thermal fractures is
predicted on the existence of a thermal gradient within the reservoir
rock material. A classic example of thermally induced fracture is the
columnar jointing observed in igneous rocks.
Fractures may also result from mineral changes in the rock, especially
in carbonates and clay constituents in sedimentary rocks. Phase changes
such as the chemical change from calcite to dolomite result in
changes in bulk volume, and this leads to complex fracture patterns
(Figure 8.2).
It is clear from the above discussion that the complex stress/strain
distribution in reservoir rocks results in complex fracture patterns.
Fracture patterns corresponding to different geological systems have key
characteristics that can be used to classlfy and index natural fracture
networks observed in outcrops and subsurface samples (Figures 8.3
and 8.4).

ENGINEERING CLASSIFICATION OF NATURALLY
FRACTURED RESERVOIRS

Fractures may have either a positive or a negative impact on
fluid flow, depending on whether they are open or sealed as a