When interpreting micro-hydraulic fracturing data for in-situ stress determinations, the value of the breakdown pressure (on a first or subsequent injection cycle) is usually used in conjunction with the instantaneous shut-in pressure. This tacitly assumes that the fracture initiates at this breakdown pressure level. However, there exists abundant evidence suggesting that the crack is propagating well before the maximum borehole pressure is reached. This paper investigates the initiation phenomenon using a newly developed numerical approach, the Body Force Technique.
The Body Force Technique is derived from the Boundary Integral Method and allows a precise evaluation of the stress state in the vicinity of a fracture as well as its aperture. It enables evaluation of the stress intensity factor under quasi-static conditions. The coupling with seepage phenomenon around the borehole and through the fracture faces is accomplished using a Line Source Flow Distribution Method allowing incorporation of transient pore pressure effects. The approach is extremely efficient and requires a minimum of computing time.
The proposed numerical method, limited to two-dimensional analyses so far, has been used to investigate the sensitivity of the fracture initiation process to (1) the preexisting crack size, (2) the pumping rate, (3) the pumping time, (4) the in-situ stresses, (5) the formation permeability, and (6) the fluid viscosity.
It has been shown that all of the above parameters play an important role and that the breakdown pressure corresponds to the initiation phenomenon only under very restricted conditions. The Body Force Technique may allow for better interpretation of stress measurement data and pumping test data.
Breakdown pressure is commonly defined as the peak pressure attained when fluid is injected into a borehole until fracture occurs. Since breakdown is usually accompanied by a sudden pressure drop, it is reasonable to assume that this corresponds to the onset of unstable fracture propagation.
The most commonly used criteria for fracture initiation resulting from borehole pressurization, derived from the theory of elasticity, is written as (Haimson and Fairhurst, 1970): (Mathematic Equation)(Available in full paper)
Experimental results, however, often indicate large discrepancies between the value of sT and the tensile strength magnitudes s t determined by other testing technique (Hardy and Jayaraman, 1961). To overcome this drawback, another criterion has often been proposed, based on the theory of linear elastic fracture mechanics (Hardy, 1973; Roegiers, 1975; Nishimatsu et al., 1973; Rumel and Winter, 1981) i.e.,
(Mathematic equation)(Available in full paper)
The stress intensity factor, KI, is not only a function of the stress conditions as in the case of sT,max, but is also a function of the geometry (hole radius and crack size). This criterion is in harmony with the fact that hydraulic fracture can usually be considered as Mode I failures. However, this criterion also predicts some data scattering, since KIC-value are sensitive to crack size. Pore pressure and applied external load (Schmidt and Huddle, 1971; Schmidt, 1976; Abou-Sayed, 1978; McLennan, 1980). However, the sensitivity is moderated by increasing the confining pressure.
Recent experiments have revealed that the breakdown phenomenon is far more complex than currently accepted theories indicates.
