In this paper, experimental results for Fontainebleau sandstone are presented: skeleton behaviour through triaxial tests and interaction between pore fluid pressure and skeleton deformation. To modelize the skeleton behaviour an hypoelastic constitutive law is assumed for sandstone. Fluid-skeleton interaction is described using Biot' s coefficients which are dependant upon deviatoric stress. Examples are then given for stress distribution around a borehole.
The study of the stress-distribution around a drilled hole is of great importance in several situations, such as, while drilling, during production or injection and when calculating fracturing pressures. To that end, a good knowledge of the rheological behaviour of rocks is required. Conventional triaxial tests on Fontainebleau sandstone have been carried out with various confining pressures, under monotonous and loading-unloading sollicitations. The crack behaviour is studied through crack closure tests, and their interaction with fluids is described by IKOGOU (1897), SIBAI (1987). The results obtained proved that the non-linear behaviour is essentially due to microcracks. The crack extension is characterized by a very early start and a very pronounced orientation in the direction of loading inducing a strong anisotropic dilatancy. Loading-unloading cycles show a strong decrease in material stiffness in the direction perpendicular to loading, and zero axial permanent strains. Usual elasto-plastic constitutive laws are inefficient to describe this type of behaviour, NGUYEN (1988), SANTARELLI (1986), SHAO (1987). Within the framework of proportional loading sollicitations to structures, an orthotropic incremental model is proposed. The modulus are determined from conventional triaxial tests and their derivatives. This model is then used to determine the stresses around cavities.
The model is used to describe the behaviour of rocks whose deformation is essentially due to microcracking. We use an hypoelastic orthotropic incremental model chosen according to the crack extension mechanism (figure 1).
Characterization tests are conventional triaxial axisymetrical tests under the following confinement pressures: 5, 10, 20, 30 and 40 MPa. The tests have been conducted on specimen of 37,5 mm diameter and 75 mm height. The specimens are saturated with methanol under vacuum and are instrumented with two axial and two transversal strain gages. The test rate is έ = 1,2. 10–6 s-l. 2.3.2 Results Paths A Figure 2 shows the results obtained for a confining pressure of 10 MPa. The following comments can be made: - Dilantancy induced by microcracks decreases when confining pressure increases. - The threshold of microcrack initiation increases with the confining pressure. - The axial modulus is almost constant up to 75 % of peak strength. This shows that crack extends axially up to stresses very close to the peak strenght value. Paths B For confining pressures less than or equal to 10 MPa, permanent transverse strains appear. Beyond 10 MPa, there are no permanent strains but hysteretic loops start growing with the confining pressure, until infinite on negative unloading modulus are reached. It is therefore proposed to use secant modulus to characterize these paths.
