Hydraulic Fracture Propagation And The Interpretation Of Pressure-Time Records For In-Situ Stress Determinations

Following the stress measurement carried out as part of the Rangely earthquake control experiment (Raleigh et al, 1972; and Haimson, 1973), numerous investigators began making in-situ stress measurements using the hydraulic fracturing technique. Haimson (1977) and McGarr and Gay (1977) review much of this work. Because the reliability of these measurements depends upon understanding the fracture initiation and extension process, we consider several aspects of hydraulic fracture propagation relevant to use of the technique for the determination of in-situ stress. Hubbert and Willis (1957) showed that the pressure to hydraulically fracture a wellbore depends on the tensile strength of the rock and tectonic stresses. A number of authors (Scheidegger, 1962; Kehle, 1964; and Haimson and Fairhurst, 1971) subsequently pointed out that the pressure-time records of a hydraulic fracturing operation could be used to compute the tectonic stresses if the tensile strength of the rock were known. Hubbert and Willis showed that a vertical fracture should form in a vertical wellbore at the azimuth of the maximum compressive horizontal principal stress, SHmax(it was presumed that one principal stress, Sv, is due only to the weight of the overburden and is vertical). Pb is presumably indicated by an abrupt drop in borehole pressure and is thus termed the breakdown pressure (see Figure 1). Because the fracture should propagate perpendicular to the direction of least compression when SHmin Sv, SHmin is presumed to be equal to the instantaneous shut-in pressure (ISIP, see Figure 1). Interpretation of shut-in pressures in cases when SHmin Sv is discussed at length below. When pre-existing fractures are present Abou-Sayed et al (1977) suggest that it is possible to estimate SHmax when the length of such fractures are known. Rather than use pre-existing fractures, it has been shown that appropriate steps may be taken to overcome their presence.