Stress field and safe drilling mud weights in the Shizigou salt system of Yingxiongling, Qaidam Basin, China

Abstract In this study, we calculate stress perturbations around the salt diapir in the Shizigou salt system of Qaidam Basin, China, by using a three-dimensional finite element model. Our goal is to identify safe mud weight windows around the salt in this complex region. We establish the salt-system geomechanical model by integrating seismic and logging data and inputting rock material properties and regional background stress field. The results show that the presence of the gypsum salt layer and structural decoupling lead to variations in principal stress directions ( $$\:{\sigma\:}_{2}$$ and $$\:{\sigma\:}_{3}$$ ) above and below the salt. Stress perturbations around salt rocks, or mechanical compaction effects from the salt rocks, are closely related to their geometric shapes and regional background stress field. Mismatches between the hydrostatic drilling fluid (mud) pressure and formation pressure can induce the drilling accidents. Therefore, based on model stress results, formation pore fluid pressure, and the Drucker–Prager plastic criterion, we conduct a drilling risk assessment. We find that drilling through large-scale concave salt structures is relatively safe with wide safe drilling windows, but at other locations, safe drilling windows are narrow. The stricter condition (wellbore collapse width less than 120°) reduces the range of the safe drilling window at salt convexities by 60%. Our study, through the establishment of a geomechanical model, provides practical insights for reducing drilling risks and designing drilling mud weights and routes.