Enhanced Mechanical Earth Modelling and Wellbore Stability Calculations Using Advanced Sonic Measurements—A Case Study of the HP/HT Kvitebjorn Field in the Norwegian North Sea

摘要

Geomechanical interpretations of inhomogeneous anisotropic materials are enhanced by three dimensional (3D) sonic measurements that provide compressional, fast shear, slow shear, and Stoneley wave slownesses in (an)isotropic drilled formations. We investigated the Kvitebjorn high pressure high temperature (HPHT) gas/condensate field for enhanced Mechanical Earth Modelling, and wellbore stability analyses using three-dimensional sonic measurements in overburden and reservoir formations. Kvitebjorn field is situated on the Norwegian continental shelf west of the Viking Graben In the HPHT Kvitebjorn field, major development challenges are the determination of the drawdown rate and the maximum depletion for optimum production while maintaining rock integrity. These challenges are directly related to the effect of changes in reservoir pressure on the in-situ formation stress state. Using advanced processing of sonic data, we estimated the amount of stress anisotropy, stress orientations, and stress magnitudes at particular reservoir intervals. These stress data together with measured in-situ minimum horizontal stress values were used to calibrate horizontal stress logs perviously estimated in the MEM. Furthermore, we constrained rock elastic properties (deformation, strength) using sonic slownesses and elastic shear moduli calculated from the sonic dataset. These data were later used to estimate rock strength and the onset of wellbore deformation. The two approaches allowed us to calculate (1) isotropic rock properties from the sonic slownesses, and (2) vertical and horizontal rock properties from the calculated shear moduli. Validity of the so-calibrated MEM was subsequently tested estimating wellbore stabilities based on the isotropic rock property data, and the vertical and horizontal rock property data sets. In each case, the model was able to accurately reproduce the events observed during drilling. The work that we present here is intended to open a new direction for geomechanical modelling: To demonstrate a new measurement allowing practical consideration of rocks as an anisotropic material, and to better constrain the effective stress ratio.