OVERBURDEN CHARACTERIZATION FOR GEOMECHANICS AND GEOPHYSICAL APPLICATIONS IN THE ELDFISK FIELD: A NORTH SEA CASE STUDY

摘要

The presence of elastic anisotropy has implications for a wide range of practical applications in both the geophysical as well as geomechanical domain. For instance, stress magnitudes and orientations can be very different in anisotropic media as compared to isotropic media. Hooke’s law can be used to calculate stresses along the wellbore, but in order to do so the elastic properties of the formation are required. In the case of Transverse Isotropic (TI) formations such as shales, the number of independent elastic coefficients is five. A commonly used source for information on these five parameters is comprised of borehole sonic data. Nevertheless, even in a deviated well the number of borehole sonic measurements is at most four, thus necessitating the integration of single-well borehole sonic data with relevant data coming from an alternative source. A case study is presented where sonic data from a single well were combined with offset well data in order to fully characterize the overburden shale in the North Sea Eldfisk field. The results were subsequently used to determine the minimum horizontal stress profile along the well. This stress profile was compared with fracture gradients derived using more conventional models such as Eaton’s and M&K. It was concluded that stress profiles on the basis of a full anisotropic model provide a natural fit with wellbore stability observations without requiring boost factors or the introduction of unjustifiable tectonic strains.