Because hydraulic fractures (HF) are always normal to the least m1mmum principal stress, mapping the geometry of hydraulic fractures helps map the trajectory of the minimum principal stress direction. In addition, measuring the pressure for which both faces of a hydraulic fracture barely touch each other provides a unique way to determine the minimum principal stress magnitude. This hydraulic testing method has been adapted to measuring the pressure required to reopen mechanically a preexisting fracture of known orientation (HTPF method). Today, for crystalline rock formations, combinations of HF and HTPF tests provides a very robust method for determining the complete stress field. It supposes that stress is a linear function of the spatial coordinates of the points where measurements have been conducted. But this combination of HF and HTPF tests is not adapted to sedimentary rock masses where stresses are not a linear function of spatial coordinates. Yet when the HF method is applied in a borehole inclined to all principal stress directions it yields en-echelon fractures the geometry of which provides an efficient constraint for the maximum principal stress magnitude. Hence combination of HF an HTPF is shown to be well adapted for a complete stress determination in all rock formations.
Since the pioneering work on in situ stress determination by hydraulic fracturing (Scheidegger, 1960; Kehle, 1964; Haimson and Fairhurst, 1967, Haimson, 1978), the method has been extended and generalized. Today, hydraulic testing in boreholes, which includes both hydraulic fracturing (HF) and hydraulic testing of preexisting fractures (HTPF), provides very efficient means for determining in situ the complete stress tensor and is one of the stress determination techniques recommended by the International Society for Rock Mechanics (Haimson and Cornet, 2003).
