Stress initialization in Geomechanics using the adjoint-state method

摘要

No continuous stress field measures can be made inside the Earth's crust, so we developed a technique to estimate the possible stress field from surface stress data. Stress initialization is the aim of this work. The method presented here couples an inversion technique with a finite element model fed with local data coming from data such as in situ measurements of the stress tensor (leak-off tests, log data...). This method guarantees the stability of the initial stress field with the regional stresses thanks to modeling with a finite element (FE) software. The inversion technique consists in minimizing the difference between the FE model and the measured in situ stress by adjusting the elastic parameters of each geological unit of our model at each iteration of optimization. For this minimization problem we use a regular method which has been successfully tested on synthetic 2D models, and we developed the adjoint-state method by formulating the gradient of the error functional towards its parameters in a constraint optimization form, which is still being tested. The result of the minimization problem is a possible initial stress field which satisfies the in situ measurements.