The behavior of rock changes dramatically with increasing depth, as it does for most other materials subjected to increasingly large compressive stresses. The major feature of this change is the transition from dominance of the fracture energy or toughness Kc to control of fracture growth by the confining stresses. Among the results are: quasi-static vs. dynamic fracture propagation and the potential for ductile vs. brittle behavior of the rock response. Shear-induced fracturing becomes dominant at greater depths, except when thermal or hydraulic processes intervene to produce extensive tensile fracturing. Emphasis is given to hydraulic fracturing in this review, because of its many practical applications, but other important mechanisms are also discussed, such as slippage, branching, poro-elastic and thermal effects. Finally, some major unresolved issues in this area are defined for future work: primary among those are the mapping of (created) underground fracture geometries and the effective use of wellbore measurements to predict or determine such geometries under a variety of representative rock conditions..
There is a vast and diverse body of literature on "Rock Mechanics"; this includes much early work (e.g. Jaegar & Cook, 1975), a flurry of activity in the 1970's (e.g. Sikarsie, 1973, Cowin & Carroll, 1976), mainly associated with mining and petroleum, and a continuous stream of work published, for example, in U.S. Rock Mechanics Symposia and Proceedings of the Society of Petroleum Engineers (SPE). In addition, there is a substantial body of literature on "Fracture Mechanics" (e.g. Liebowitz, 1972, Sih, 1977, Erdogan, 1976) which continues to expand in connection with many activities in structural mechanics: most of this work can be found in the Proceedings of the American Society of Mechanical Engineers (ASME). This literature has focused mainly on fracture propagation under effectively tensile (plus shear) states of stress and has not dealt with the role of substantial compressive stress - although many of the mathematical results can easily be extended to cover such conditions (e.g. Keat et al., 1988). A major thrust of our work over the past ten years has been to judiciously develop and implement capabilities for adequate analysis and design of underground engineering activities, incorporating reasonable physical assumptions and effective mathematical models. The main body of our research work is contained in reports of the MIT UFRAC project (Cleary, 1987), a number of which have been published (e.g. Cleary et al., 1983, 1984). A major application of this work has been to improvement of field operations involved in hydraulic fracturing (e.g. Cleary et al., 1986, 1988); this work has been conducted for the Gas Research Institute (GRI), for the purpose of generating or improving the U.S. gas supply, but the resulting technology can be applied to many other areas of endeavor - including oil production, geothermal energy and mining. Although the focus of our work has been on practical applications, we have had to develop a number of basic analytical/ computational tools and a detailed understanding of the physics involved in underground fracturing.
