The second method of direct stress measurement is hydrofracturing. In this method a section of a borehole that is free of fractures or other porosities is isolated using inflatable packers. The isolated section is then pressurized by pumping fluid into it, and the pressure of the fluid is monitored. The pressure is increased until a fracture occurs. The fluid pressure at which the fracture occurs is referred to as the breakdown pressure pb. A typical pressure–time history during hydrofracturing is illustrated in Figure 2–18. If the pump is shut off immediately and the hydraulic circuit kept closed, an instantaneous shut-in pressure (ISIP) is recorded, as illustrated. This is the pressure that is just sufficient to hold the fracture open. If the fluid pressure is dropped and then increased, the maximum pressure is the ISIP when the fracture is once again opened. Several assumptions are implicit in the interpretation of the pressure record obtained during hydrofracturing. The first is that the resultant fracture is in a vertical plane. The second is that the rock fractures in pure tension so that the stress perpendicular to the fracture is the minimum horizontal principal stress. With these assumptions the magnitude of the minimum horizontal principal stress is equal to the ISIP. Using theories for the fracture of rock, the maximum horizontal principal stress can be deduced from pb and ISIP, but with considerably less accuracy. Measurements of the minimum horizontal stress σmin as a function of depth in the Cajon Pass borehole in California are given in Figure 2–19. This borehole is adjacent to the San Andreas fault in southern California and was drilled to a depth of 3.5 km. A series of ISIP measurements were carried out using both hydrofractures and preexisting fractures, and it is assumed that these gave σmin.
In general, measurements of the vertical component of stress indicate that it is nearly equal to the weight of the overburden, that is, the lithostatic pressure. Using Equation (2–1) with ρg = 26.5 MPa km−1 the vertical component of stress σyy is also given as a function of depth in Figure 2–19. The measured stresses correlate reasonably well with 0.6 σyy. Another technique used to determine the orientation of crustal stresses is the observation of wellbore breakouts. Wellbore breakouts are the result of localized failure around a borehole in response to horizontal compression.