Page 196 - Geology of Carbonate Reservoirs

FRACTURES AND FRACTURED RESERVOIRS 177

of the fractures may be closed by cements, gouge, or compaction. In such cases, one
fracture set — the one oriented parallel to maximum stress under present - day condi-
tions — will remain open. The open set will, of course, have the greatest infl uence on
reservoir behavior. Field development in fractured reservoirs requires knowledge

of the extent to which fracture porosity and permeability influence the reservoir. At
one end of the spectrum, fractures contribute the bulk of reservoir porosity and
permeability; while at the other end, they may be impediments to flow because they

have been filled with gouge or mineral cements.
7.1.1 Definition of Fractures

Fractures are defined as naturally occurring macroscopic planar discontinuities in
rock due to deformation or physical diagenesis (Nelson, 2001 ). Fractures may form
as the result of brittle failure under differential stress but ductile fractures also
occur. Most fractured reservoirs, especially in carbonates, are brittle fractures. Brittle
failure means that rocks failed by rupture after the application of differential stresses
exceeded their elastic limit. Rocks behave as brittle materials as long as they are
subjected to deformation within the brittle domain , which is characterized by com-

paratively low temperatures and confining pressures such as those at the earth ’ s
surface and at relatively shallow burial depths. Silica - cemented sandstones, dolos-
tones, limestones, some chalks, and cherts are brittle at earth surface temperatures
and pressures. At higher temperatures and confining pressures ductile behavior

dominates and buckling or flowage may result instead of fracturing. Ductile and

plastic behavior (high strain under low to moderate stress) can occur at low tem-

peratures and confining pressures in rocks such as shale, mudrocks, salt, evaporites,
marls, and clay - rich chalks. An example of ductile behavior can be observed by
putting soft metal wire such as copper or gold under extension stress. The wire
gradually lengthens, stretches (necks - down), and finally fails. Ductile fractures occur

under higher confining pressures or high temperatures and they commonly occur
as bands of deformed rock rather than distinct, sharply defined “ cracks ” that we

usually visualize in association with the word fracture. Finally, plastic behavior is
characterized by large amounts of strain produced by limited stress. An example of

this behavior is the flowage of “ silly putty ” on a tabletop. Plastic behavior is not
associated with brittle failure or fracturing, but rather it is the style of deformation
that characterizes similar folds and material “ flowage. ”

Fractured reservoirs are, according to Nelson (2001) , “ reservoirs in which natural

fractures have, or are predicted to have, a significant effect on fl uid flow either in

the form of increased reservoir permeability and/or reserves or increased permea-
bility anisotropy. ” He emphasizes that the words “ predicted to ” are especially
important because the data needed to conduct quantitative analyses on fractured
reservoirs must be collected early in the life of the reservoir in order to take advan-
tage of and manage the “ significant effects ” of the fractures.

7.1.2 Types of Fractures
Two main variables determine which types of fractures will form: stress character-
istics and material behavior. Stress is defined as force acting on an area and strain

is deformation due to stress. In the case of fractures, strain is failure by rupture