Probabilistic approach for risk assessment of offshore hydrate wellbore during test production

Abstract

The problem of wellbore risk due to complicated phase variation in coupled seepage in the process of hydrate production is one of a major obstacle restricting its commercial production. In response to this problem, a hybrid methodology for evaluating the risk of offshore hydrate test production wellbore is proposed using self-developed Particle Filter (PF) model, which realizes risk update through measurement data based on physical model. In this study, the physical model is constructed by a multi-field coupling model and the corresponding algorithm. The monitoring data is generated by Monte Carlo simulation to illustrate the method application. Then the probability prediction curve of the shear failure of the sediment around the wellbore can be calculated based on the PF. Finally, a novel dynamic risk matrix is developed to evaluate the risk of the current hydrate wellbore. A case study using geological parameters of hydrate sediments in the Shenhu area of the South China Sea was presented to illustrate the application of the proposed framework. Results indicated that the remaining safe production time at 90 days is between 210 days and 280 days, with a pressure drop of 3.0 MPa and 0.75 as the threshold. Subsequently, a dynamic risk matrix diagram of the risk of the hydrate wellbore is proposed based on Event Sequence Diagram (ESD) and Bayesian Network (BN). The result shows that the risk of abandonment caused by landslides should be prevented in the early stage, and the workover risk caused by damage to the wellbore structure should be paid special attention to in the middle and late stages. The proposed methodology can provide applied technology for wellbore risk management in the process of hydrate test production.