Rock Properties Controlling Hydraulic Fracture Propagation
Authors: Daneshy Abbas Ali 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
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Abstract
Theoretically, it is shown that the magnitude of the term E for different materials is an index of their relative fracturability; the higher this term, the more pressure needed for fracture extension. Thus, pressure needed for fracture extension. Thus, is called "fracturability index".
Experimentally, due to natural variations in samples and, thus, measured E, v, y, it is proposed to replace. The proposed to replace. The relative magnitude of this term for different rocks can be determined from only one experiment, provided all samples used have similar geometries.
A double-ring test is proposed for comparing the relative fracturability of two rocks. Thus test can be easily performed in the laboratory and is quite suitable for cylindrical cores usually obtained in the oil field. Examples are presented to show test results and method of interpretation.

Introduction
One of the unanswered questions in hydraulic fracturing is that of fracture propagation in layered rocks. Will a fracture propagation in layered rocks. Will a fracture that has extended to the boundary between two formations intersect that boundary? If it does, what is the propagation pattern and its relation to the mechanical properties of the two formations?
This paper is the first step toward an answer; it discusses the role of rock properties in fracture propagation and establishes a means of comparing the fracturability of rocks.

ENERGY BALANCE IN FRACTURE EXTENSION
The fundamental concept in fracture extension is energy balance. First expressed by Griffith, this concept can be stated as follows: the extension of a fracture always occurs in such a manner that the energy consumed in the formation of the new faces of the fracture is balanced by the reduction in strain energy resulting from the same extension.
The source of energy in hydraulic fracture extension is the pressurized treatment fluid. The magnitude of this pressure, P, is equal to the sum of two terms; one necessary to balance the least principal stress, sigma733, and the other, delta P, necessary to provide the energy for fracture extension.
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