Analysis and prevention techniques for a series of accidents in the ultra-deep salt wells

The Ningjin salt field in Hebei Province is characterized by unfavorable geological conditions, including significant burial depth, a high number of interlayers, and elevated creep strength. Conducting an in-depth analysis of the series of wellbore accidents that occurred during the well Z cavity creation test can facilitate the optimization of subsequent drilling plans in this area. Drawing upon data from well opening, logging, cavity creation, and cavity measurement obtained on-site at the Ningjin Salt Cavern Construction Project, we introduce theoretical foundations such as interlayer collapse mechanisms, field vibration effects of the cavity center tube, creep-induced bottom bulging and top subsidence mechanisms in salt caverns, and casing shear-compression and tensile fracture mechanisms. These theoretical foundations are cross-verified with formula analysis, numerical simulation results, and engineering detection data to analyze the causes of wellbore accidents. Research indicates that logging resistance is associated with the rapid dissolution of thin interlayers, leading to the formation of "steep slopes" in the wellbore wall. The primary cause of central tube instability is the field vibration effect of the pipe string, followed by the longitudinal compression effect resulting from creep-induced arching at the cavity bottom. Casing damage is linked to axial tensile strain caused by creep and subsidence at the cavity top, a process further exacerbated by the field vibration effect of the central tube. The research findings offer improvement suggestions in areas such as drilling and cementing design, wellbore string and cavity stability design, as well as cavity shape control, serving as a technical reference for similar salt cavern drilling designs.