Field Cases for Back-Analysis of Maximum Horizontal Stress of Oil Fields from Breakout Formations

ABSTRACT: Efficient in-situ stress prediction in oil fields is a vital step in wellbore stability and drilling optimization. In many cases, the lack of accurate prediction of in-situ stresses is the key issue in borehole instability problems; consequently, drilling issues with high severity often occur, leading to non-productive time periods and significant increases in drilling costs. Three-dimensional geomechanical models have improved the prediction of the maximum horizontal stress of the fields from the formed breakouts. This 3D model is based on poroelastic constitutive law in conjunction with Mogi-Coulomb shear failure criterion. However, the classic Mohr-Coulomb failure criterion has been also used to verify the effect of the intermediate principal stress on the back analysis of the maximum horizontal stress. In all the field cases, the Mogi-Coulomb failure criterion was able to accurately predict the maximum horizontal stress that was well compatible with the actual field. 1. INTRODUCTION When a well is drilled, the removal of rock mass will result in a redistribution of stress around the wellbore. The redistribution of stresses near the wellbore may lead to shear or tensile failure at the borehole wall, resulting in wellbore instability. These changes in stress distribution are functions of the magnitude of the far-field in-situ stresses, their orientation relative to the wellbore, rock properties, formation fluid pore pressure, and the annular mud pressure inside the wellbore. Far-field stresses are natural earth stresses prior to drilling, away from the wellbore, and are not affected by the borehole. In contrast, wellbore stresses are near-wellbore stresses and are affected and controlled by far-field stresses, mud density, and the corresponding equivalent circulating density (ECD). Far-field stresses are normally described in cartesian coordinates, with one vertical stress σV and two horizontal stresses orthogonal to each other. They are known as minimum horizontal stress σh and maximum horizontal stress σH. The wellbore stresses are described in cylindrical coordinates, with one radial stress σr and two orthogonal stresses, which are the axial stress σz and the tangential or hoop stress σθ, as illustrated in Figure 1.