CONSEQUENCES OF FLUID LAG IN 3-DIMENSIONAL HYDRAULIC FRACTURES

Research investigations on three-dimensional (3-D) rectangular hydraul ic fracture configurations with varying degrees of fluid lag are repor ted. This paper demonstrates that a 3-D fracture model coupled with fl uid lag (a small region of reduced pressure) at the fracture tip can p redict very large excess pressure measurements for hydraulic fracture processes. Predictions of fracture propagation based on critical stres s intensity factors are extremely sensitive to the pressure profile at the tip of a propagating fracture. This strong sensitivity to the pre ssure profile at the tip of a hydraulic fracture is more strongly pron ounced in 3-D models versus 2-D models because 3-D fractures are clamp ed at the top and bottom, and pressures in the 3-D fractures that are far removed from the fracture tip have little effect on the stress int ensity factor at the fracture tip. This rationale for the excess press ure mechanism is in marked contrast to the crack tip process damage zo ne assumptions and attendant high rock fracture toughness value hypoth eses advanced in the literature. A comparison with field data is prese nted to illustrate the proposed fracture fluid pressure sensitivity ph enomenon. This paper does not attempt to calculate the length of the f luid lag region in a propagating fracture but instead attempts to show that the pressure profile at the tip of the propagating fracture play s a major role in fracture propagation, and this role is magnified in 3-D models. (C) 1997 by John Wiley & Sons, Ltd.