Experimental determination of elastic anisotropy and biot's constant in a saudi arabian reservoir sandstone

Rock mechanical triaxial and hydrostatic tests were conducted on 1.5-inch diameter sandstone samples obtained from four wells. A total of 23 different depth intervals were tested, at pressures representing the in-situ reservoir conditions. Static elastic moduli (Young's modulus and Poisson's ratio) and static Biot's constant were determined from the results of the triaxial tests. Dynamic elastic constants of selected rock samples were determined from ultrasonic compressional and shear wave transmission data obtained under hydrostatic stress conditions through vertical, horizontal and 45-degree-oriented samples. Porosity and permeability measurements were also made on corresponding sister samples. Wave velocities and the computed dynamic moduli indicated the existence of stress-induced elastic anisotropy, which was found to decrease with increasing confining pressure. Biot's constant values in planes parallel and perpendicular to the bedding were found to be nearly the same, indicating that the anisotropy in the wave velocities and dynamic elastic moduli does not translate directly into anisotropy in the Biot's constant. However, the Biot's constant in both planes decreased significantly with hydrostatic stress, indicating a stress-dependency. The static Biot's constant obtained from triaxial tests also decreased with increase in axial stress, demonstrating a significant axial stress-dependency as well. The results were found to be in general agreement with the literature.