An ultra-deep exploration well has been planned to be drilled in the Tarim Basin, northwest of China, whose Ordovician target zone is at a depth of over 7500 m, and a graben developed there. Extensive bedding planes and natural fractures developed in the target formation according to seismic analysis and image logging data from several reference wells, indicating a high risk of borehole breakout or other complications while drilling through the fragmentized Ordovician zone. To prevent potential borehole breakout and associated nonproductive time during drilling the well, this paper presents an integrated geomechanics approach that combines theoretical analyses and numerical simulations by collecting and investigating geological, seismic, logging, and drilling data. The pore pressure, collapse pressure, fracture pressure were estimated and a safe mud weight window was provided for drilling in the Ordovician graben formation. The fracture development degree was evaluated and the result shows micro-fractures may be developed in the target formation. The factors affecting wellbore stability were analyzed by establishing a discrete element model. The wellbore stability of the studied well was well controlled that no severe drilling complexities were encountered during drilling in the target formation by using the integrated geomechanics approach.
Drilling in the graben formation may encounter complexities such as borehole collapse, block falling, and leakage because of the great depth, complex lithology, highly fractured reservoirs, and the stress condition of the field (Chen et al., 2020), resulting in frequently backfilling and sidetracking and plenty of non-productive time according to the drilling processes of several wells drilled in Shunbei field in the Tarim Basin, China. To understand the borehole collapse mechanism and ensure drilling in graben formation successfully, it is necessary to figure out the pressure profile, especially the collapse pressure, the fracture development degree, and the factors affecting wellbore stability in the graben formation.