AN ANALYSIS OF THE INFLUENCE OF THE PRESSURIZATION RATE ON THE BOREHOLE BREAKDOWN PRESSURE

This paper investigates the dependence of breakdown pressure, the crit ical pressure at which tensile failure of the rock is initiated by inj ecting fluid in a borehole, on the rate of pressurization. The mathema tical model explicitly accounts for the existence of micro-cracks at t he borehole wall that trigger the failure process. Breakdown, in this context, occurs when the stress intensity factor of a critically orien ted micro-crack reaches the rock toughness. The model is presently res tricted to low-permeability/low-porosity rocks. By considering one-dim ensional lubrication flow in the crack coupled with the non-local elas tic response of the crack, the evolution of the net pressure, crack op ening and stress intensity factor is obtained as functions of the pres surization rate. The relation between breakdown pressure and pressuriz ation rate in the case of zero initial net pressure is shown to be con trolled by only one dimensionless number: the ratio between the initia l width of the unstressed micro-crack and the induced elastic opening at failure. It is further shown that (i) the fluid pressure in the ear ly stages of the pressurization history drops in the crack and that ca vitation can occur, and (ii) local back-flow in the crack takes place. The dependence of breakdown pressure, P-b, on the pressurization rate , A, is determined as well as the range of A, where p(b) varies signif icantly. The lower and pseudo upper bounds of this range of pressuriza tion rate correspond to limiting regimes of slow and pseudo fast press urization. (C) 1997 Elsevier Science Ltd.