Prediction of fracture development in fractured reservoirs is made difficult by the wide range of geologic conditions which may lead to development of fracture porosity and permeability. The choice of fracture characteristics that can be employed in a study of the fracturing process also is wide; reliance commonly has been placed on geometrical properties such as fracture orientation. Various other characteristics, including surface features, fracture termini, and fracture spacing, also are pertinent. Use of these features is facilitated by dividing the very broad process of fracturing into several separate but related stages: (1) initiation of fractures, (2) propagation of fractures, (3) development of fracture sets and systems, (4) intensification of fracture spacing, and (5) dilatation of fractures.

Effects to be anticipated in the first stage of this process are illustrated by laboratory deformation experiments at elevated pressures. Using a silica-cemented sandstone as test material, the writer noted that incipient fractures may occur within grains or at grain margins. Experiments suggest the possibility that cataclastic deformation contributes significantly to the failure mode at high confining pressure, even in rocks that are considered to be incompetent and ductile.

Development of an open fracture network that is sufficient to provide reservoir porosity and permeability depends on geologic conditions during later stages—specifically, the conditions between the time of fracture propagation and fracture dilatation. However, an understanding of these final events requires prior understanding of the initial stages in fracture development.

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