This paper reviews field observations of the hydraulic fractur- ing process, in particular multiple fracturing and high fracture pres- sures. The limitations of existing concepts in fracture mechanics and hy- draulic fracturing are reviewed in light of typical features of fractur- ing observed in various materials under wide range of conditions. A new concept of fracture toughness is presented and its implications to hydraulic fracturing are discussed.

Critical evaluation of hydraulic fracture (HF) treatments over the past several years has exposed frequent discrepancies between field results and theoretical predictions. The most obvious is the discrepancy between field measurements and theoretical simulations of net fracture pressure.

Several potential mechanisms have been proposed to explain this discrep- ancy (Palmer & Veatch 1987, Warpinskt 1987, Shlyapobersky 1985, Cleary et al. 1991). These mechanisms can be divided into three major groups: (1) higher than predicted frictional fluid pressure drop along a single frac- ture caused by tortuosity of the flow channel, enhanced crosslinked gel fluid viscosity, enhanced fluid leak-off, and/or turbulent flow, etc.; (2) creation of a multiple set of parallel evenly growing fractures with much larger flow resistance; and (3) increase of tip net fracture pres- sure caused by a fluid "nonpenetrating" region, rock dilatancy, elevated apparent fracture toughness, or other tip extension mechanisms that have not been identified at the present time.

This paper reviews the foundations of fracture mechanics and shows that the assumption of fracture toughness as a material constant is very limi- ted and not applicable for modeling HF growth. The process zone observed around quasi-equilibrium fractures explains the increase of fracture toughness in laboratory tests. This fact justifies the use of fracture toughness as a parameter for field calibration of HF models and necessi- tates a theory able to predict the fracture toughness in the field. An existing theory that considers the propagation of a crack together with an evolving process zone (crack layer) is discussed and applied to field observations and modeling of hydraulic fractures. The implications of this theory for HF containment and field operations are outlined.

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