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 fora wide range of practical applications in both thegeophysical as well as geomechanical domain. Forinstance, stress magnitudes and orientations can be verydifferent in anisotropic media as compared to isotropicmedia. Hooke’s law can be used to calculate stressesalong the wellbore, but in order to do so the elasticproperties of the formation are required. In the case ofTransverse Isotropic (TI) formations such as shales, thenumber of independent elastic coefficients is five. Acommonly used source for information on these fiveparameters is comprised of borehole sonic data.Nevertheless, even in a deviated well the number ofborehole sonic measurements is at most four, thusnecessitating the integration of single-well boreholesonic data with relevant data coming from analternative source.A case study is presented where sonic data from asingle well were combined with offset well data inorder to fully characterize the overburden shale in theNorth Sea Eldfisk field. The results were subsequentlyused to determine the minimum horizontal stress profilealong the well. This stress profile was compared withfracture gradients derived using more conventionalmodels such as Eaton’s and M&K. It was concludedthat stress profiles on the basis of a full anisotropicmodel provide a natural fit with wellbore stabilityobservations without requiring boost factors or theintroduction of unjustifiable tectonic strains.