Deep coal-rock geomechanics for wellbore stability and drilling-fluid design in the Benxi–Shanxi Formations, Ordos Basin, China

Deep coal-rock intervals in the Ordos Basin host substantial unconventional gas resources but commonly pose severe drilling risks due to poor borehole integrity. This study integrates regional geological characterisation with engineering observations to characterise the geological and geomechanical attributes of deep coal-rocks from the Benxi–Shanxi formations and to develop a targeted drilling-fluid strategy for wellbore stability. Deep coal-rocks are generally characterised by high thermal maturity, complex mineral assemblages, well-developed cleats/fractures, low mechanical strength, and pronounced stress sensitivity. The primary reservoir space is provided by a pore–fracture system dominated by micropores and cleat/fracture networks, which also facilitates drilling-fluid invasion and mechanical degradation. This study propose that wellbore instability in these deep coal-rocks is mainly governed by a brittle–shear composite failure, driven by the coupled effects of chemical weakening associated with fluid invasion and localised stress concentration around the borehole. To mitigate collapse, a drilling-fluid system is designed and validated, emphasising suitable mud density, multi-stage plugging, and strong inhibition. Laboratory evaluations demonstrate that the combined action of key plugging agents (G314 and G308) with inhibitors effectively seals multi-scale fractures, suppresses clay hydration, and builds a resilient filter cake, thereby markedly improving wellbore stability. These results provide practical guidance and technical support for safe and efficient drilling in deep coal-rock reservoirs of the Ordos Basin.