Hydraulic Fracturing, Hydraulic Tests on Pre-existing Fractures, sleeve reopening tests and the analysis of en echelon fractures developed in wells inclined to the principal stress directions, have helped determine the vertical profile of the complete stress tensor in a sedimentary formation of the eastern Paris Basin. This stress profile outlines the relaxation of shear stress in a 190m thick clayey formation that results in a strongly non linear stress variation with depth. It demonstrates that today's stress field at this location does not depend on tectonic stresses but on active deformation processes that likely involve fluid-rock interactions.
If continuum mechanics is the paradigm applied for solving a geomechanical problem, then the concept of stress is of central importance. With this paradigm the so-called natural stress field, i.e. the stress field that exists before applying the perturbation of concern, must be evaluated and interpretation of borehole hydraulic tests has revealed very efficient for such evaluations. But overtime, the practice has progressively evolved from simple Hydraulic Fracturing tests (Hubbert and Willis, 1957, Kehle, 1964, Haimson, 1978) to integrated methods that rely on geophysical borehole imaging (Cornet 1993, Peska and Zoback, 1995) or on data obtained with different techniques (e.g. flat jacks, Cornet, 1996, or overcoringAsk, 2006). In this paper, hydraulic methods for evaluating the six components of the complete stress field are first briefly recalled. Then their application to the Meuse/Haute-Marne underground research laboratory (France) is discussed. It is shown how hydraulic tests results help constrain the six components of the stress field. Results are validated by comparison with borehole failure images. They raise the question on the origin of stresses in sedimentary formations and on spatial stress variations. Finally, consequences for integrating data collected at different depths in sedimentary formations are briefly discussed.
