Research investigations on three-dimensional (3-D) rectangular hydraulic fracture configurations with varying degrees of fluid lag are reported. The interpretations deduced from the computed stress intensity factors for a wide spectrum of fracture slenderness ratios and effective minimum in situ stresses are dramatically different from those derived from plane-strain crack simulations. In particular, this paper demonstrates that a 3-D fracture model coupled with fluid lag at the fracture tip can predict very large excess pressure measurements for hydraulic fracture processes. In addition, this paper demonstrates that 3-D predictions of fracture propagation based upon critical stress intensity factors are extremely sensitive to the pressure profile at the tip of a propagation fracture and that plane-strain models of fracture propagation will underestimate the fracture pressures required for propagation. This rationale for the excess pressure mechanism is in marked contrast to the crack tip process damage zone assumptions and attendant high rock fracture toughness value hypotheses advanced in the literature. Selected model result comparisons with experimental data are presented to provide additional validation of the proposed 3-D rectangular geometry and fluid lag-fracture fluid pressure sensitivity phenomenon.

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