The aim of this study is to estimate the initial stress in reservoirs before production using 3D calibrated geomechanical models. We propose an inverse method for estimating stress. Wellbore data can be integrated in a Mechanical Earth Model in order to estimate stresses nearby wells. It yields a first rough estimation in the whole reservoir by a simple interpolation which is not in equilibrium with the external forces and boundary conditions. From this rough stress field, the inversion aims at finding a physically acceptable stress state (i.e.: in equilibrium with the external forces and boundary conditions) that fit the local stresses wells. The forward problem is ensured by a Finite Element Analysis which is able to take into account structures such as faults, which have a significant influence on the stress magnitude and orientation. Inverse loop stops when the stress computed near wells matches the one estimated using borehole data. The uncertainties on the boundary conditions, elastic parameters and the first stress estimation are taken into account with a stochastic approach. In this study, faults are built with a volumetric representation of the core and damage zone by introducing elastic parameters variations within. This representation is possible because only small deformations are expected.
Subsoil raw materials exploitation generally induces stress changes. For instance, depletion disturbs the mechanical equilibrium, yielding a stress change in the reservoir and in the overlying geological layers (the overburden). It could have dramatic consequences such as borehole instability (Zoback et al., 2003) and fault reactivation, which can lead to unexpected oil and gas leaks or migration (Wiprut and Zoback, 2002).
Stress estimation is important during field exploitation to avoid these problems and to anticipate measures to stabilize wells using different drilling and casing techniques (Wilson et at., 1999). Depletion can be monitored and it is possible to estimate the relative stress change during the exploitation. The goal of this paper is to introduce a new approach to estimate the absolute stress in the reservoir and the overburden before exploitation.
