In-Situ Stress Estimation by Back Analysis Based on Wellbore Deformation with Consideration of Pore Pressure

Abstract

In oil and gas industry, wellbore stability control is paramount in an operation. It is essential to have information of the in situ stresses in well planning and prevention of wellbore failure. However, the current available measurement methods for in situ stresses in petroleum engineering are costly and often give scattering results. In this paper, a more practical displacement-based back analysis technique is proposed to determine the magnitude and orientation of the in situ stresses. The purpose is to provide an alternative tool for small operators in petroleum industry. An analytical solution is derived from displacement-stress relationship around a well in an isotropic rock with consideration of pore pressure. This method can be applied to calculate the displacement at any point around the well induced by drilling. In a reversed order, it can be used to calculate the in situ stresses from measured displacements at a number of locations on the borehole wall. For practical purpose, drained and undrained constitutive 2D models using measured diametrical deformation at different locations around a borehole wall as the input data have been developed to estimate the in situ stresses. Program codes in Matlab were written to facilitate the analysis under different conditions. An example is introduced to test the model and the program. The results validated this back-analysis approach and made a reliable estimation of the in situ stresses. The effects of pore pressure are also evaluated and are found to have significant impact on the shape of wellbore deformation. This impact differs for the drained and undrained conditions.