Shallow gas drilling in polar seas poses severe geological hazards, particularly unexpected eruptions that threaten platform safety and the marine environment. Accurate prediction of shallow gas occurrence and eruption risk is therefore essential for safe deep-water operations. However, previous studies seldom considered the coupled effects of gas pressure and pore-structure evolution on acoustic wave velocity, leading to deviations in hazard assessment. In this study, laboratory experiments and numerical simulations were conducted to clarify these mechanisms. Results revealed a non-monotonic relationship between porosity and P-wave velocity in shallow gas-bearing sediments: P-wave velocity decreases with increasing porosity at low porosity levels but increases beyond a critical threshold. This is attributed to changes in particle interactions and cementation that enhance the shear modulus. The inflection porosity for shallow gas (78%) highlights the diagnostic role of pore-structure evolution in predicting shallow gas distribution. A mathematical correlation between P-wave velocity and formation pressure was further established, and MP-PIC simulations showed that higher pressure coefficients significantly accelerate eruption rates, with a 0.1 increase in the pressure coefficient raising the instantaneous eruption velocity by 5.27 m³/min. Based on these findings, a quantitative evaluation method was developed to assess shallow gas hazard risk, providing engineering guidance for site selection and real-time risk prediction, and contributing to safer offshore drilling and ecological protection in polar environments.

The deep formations of the Yingqiong Basin are situated in a high-temperature and high-pressure (HTHP) environment, characterized by a narrow formation pressure window and consequently high operational risks. Accurate prediction of Equivalent Circulating Density (ECD) is crucial for wellbore stability control and well control safety during the drilling of extended-reach wells (ERWs) in this block. Existing calculation methods fail to account for the errors in well depth and true vertical depth (TVD) measurements caused by drill string buckling, which affect the ECD calculation. Therefore, to achieve precise ECD control, this study addresses the HTHP characteristics of ERWs in the Yingqiong Basin. It takes into consideration the variations in drilling fluid performance parameters, the influence of cuttings, and the well depth/TVD measurement errors induced by drill string buckling. On this basis, the traditional ECD calculation model is modified, and a set of ECD calculation models tailored to ERWs in the Yingqiong Basin is established. This model aims to meet the requirements for fine ECD control in drilling operations within the block and reduce operational risks. By comparing the error rates between the prediction results of the traditional ECD calculation model and those of the proposed model in this study, using the on-site measured ECD data from Well LD10-X-X in the Yingqiong Basin, the results demonstrate that for HTHP ERWs in the Yingqiong Basin, incorporating the well depth and TVD measurement errors caused by drill string buckling can enhance the accuracy of ECD prediction.