ESTIMATION OF ANISOTROPY OF THE BAKKEN SHALE FROM ULTRASONIC VELOCITY MEASUREMENTS

In hydraulic fracture design, mechanical properties of shale rocks are important parameters to determine and need further studies to predict more efficiently fracture growth. It is known that fracture development is affected by a combination of in-situ field conditions: local stress, rock properties, and so on. It has been found that shale reservoirs are heterogeneous and anisotropic due to presence of clay minerals, and layered microstructure. Furthermore, it has been demonstrated that factors such as mineral composition, porosity and kerogen content vary considerably and more importantly in the direction perpendicular to bedding than horizontally along with the stratification. In this study, we used the three plugs approach for characterizing transversely isotropic velocity model of Bakken Shale using ultrasonic measurements on three adjacent plugs: vertical 0⁰, horizontal 90⁰ and 45⁰. The compressional (Vp) and shear wave (Vs) velocities measured at different orientations with respect to layering, are used to identify the complete tensor of the elastic moduli, and to evaluate the velocity anisotropy as a function of increasing effective stress. For the tested organic-rich shale core plug, the measured velocity values exhibit strong anisotropy and particularly for Vp. Seismic velocities follow an increasing trend as the effective stress increases. The anisotropy decreases somewhat with increasing consolidation, and with the closing of preexisting fractures and microcracks. The reduction of anisotropy is more evident for the P-wave because it decreases when the vertical effective stress increases from 10 to 50 MPa. Results of our study contributed at investigating the different factors that affect anisotropy in Bakken shales which will help identify new approach to improve shale modeling and impact of kerogen on brittleness.