An integrated geomechanics approach to evaluate and manage wellbore stability in a deep graben formation in Tarim Basin

An ultra-deep exploration well has been drilled in the Tarim Basin, northwest of China, with an Ordovician target zone is at a depth of over 7500 m. The Ordovician formation is within a graben structure. Extensive bedding planes and natural fractures develop in the target formation according to seismic analysis and image logging data from several reference wells, indicating a high risk of borehole instability or other complications while drilling through the fragmentized Ordovician zone. To mitigate potential borehole instability problems and associated nonproductive time during drilling the well, an integrated geomechanical study is conducted that combines geological, seismic, logging, and drilling data. The pore pressure, collapse pressure, and fracture pressure of the well are estimated and a safe mud window is provided to guide drilling operations in the Ordovician graben formation. A fracture development index is also calculated to locate the fracture distribution zones. The factors affecting wellbore stability are analyzed by establishing a discrete element model. With the guidance of this study, the well was successfully drilled and the stability of the wellbore was well controlled during drilling in the fractured formation.