IN SITU STRESS ESTIMATION METHODS AND THE INTEGRATION OF NUMERICAL MODELLING FOR STRESS RECONSTRUCTION AND FUTURE STRESS EVOLUTION
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A reliable estimation of in situ stress orientation and magnitude is necessary for determining wellbore stability in the oil and gas industry, or assessing excavation stability for mining and civil engineering projects. Methods of stress estimation in deep borehole investigations are generally limited to the use of hydraulic methods or borehole imaging techniques, which identify borehole breakouts and/or borehole deformation. However, the collection of data in a thrust regime, or in horizontally laminated ground, can be difficult and lead to unreliable results. Moreover, the back analysis of stress magnitude from both hydraulic methods and borehole imaging methods are sensitive to estimates of strength and elastic properties around the borehole. This research has shown that the magnitude of the axial stress relative to the stresses normal to the borehole axis contributes to the magnitude and distribution of maximum deviatoric stress experienced around an advancing borehole. Furthermore, an analysis of the stress path incorporating the complete stress tensor shows that the maximum deviatoric stress does not always correlate with the typical solutions for induced stresses around a circular excavation. As a result of the limitations and uncertainties associated with traditional stress estimation methods, an integrated approach to determining the stress conditions at a given site using numerical models to simulate the loading history has been evaluated. A 2-dimensional finite element model of the Paleozoic sedimentary sequence of the eastern edge of the Michigan basin has shown that a systematic incorporation of the geological and stress history can approximate a given stress profile. Numerical models are also applied for estimating the magnitude of glacially induced stress change in the upper crust for the purpose of estimating long-term stress evolution. The framework and methodology used for numerical stress reconstruction and evolution can be included at the site characterization and engineering design stages for various types of projects, including deep geologic repositories, where estimates of in situ stress and future stress change are important.