Abstract: Overthrust nappe structure usually has strong tectonic stress and abundant bedding and cracks, and borehole instability has plagued its drilling operation. It is necessary to determine the in-situ stresses distribution before research on wellbore stress state and wellbore stability. In this study, a new method for study on the in-situ stress distribution of overthrust nappe structure is proposed. Based on in-situ stress measurement, adopting the constrained optimization finite element method (COFEM), 3D in-situ stress of this structure is numerically simulated and verified by field formation leak-off test data. The feature of shale hydration and the relation between the rock mechanical properties and water content are analyzed from the experiment of the shale hydration. Based on the behavior of shale hydration and the failure criterion of rocks, the model for stress distribution of the water sensitive shale borehole surrounding beds was developed and the software for shale borehole stability is programmed. The software can be used to research on the collapsing pressure, fracture pressure, the safe drilling fluid density window and the shale collapse cycle. This study is useful for the mechanism analysis of shale wellbore instability in the overthrust nappe structure and the related countermeasures arrangement.

Key words: overthrust nappe structure, in-situ stress, COFEM, shale hydration, wellbole stability, collapse cycle