An individual risk assessment framework for high-pressure natural gas wells with hydrogen sulphide, applied to a case study in China

Accidents of high-pressure sour gas wells at the well drilling and completion stages may cause serious consequences when poisonous H₂S gas diffuses with the diversity of terrain and wind etc., especially in China, where most of sour gas wells are located in mountainous areas with complicated geological conditions. This paper focuses on an assessment framework to explicitly calculate the individual risk of high-pressure sour gas wells at the well drilling and completion stages. The method is based on accident probability analysis by fault tree simulation as well as accident consequence analysis through building reasonable accident scenarios for H₂S gas diffusion related to death probability within a given distance. The death probability curves of different wind directions and corresponding minimum proximity distances to the surrounding buildings could be provided by probabilistic risk analysis. Finally the individual risk contours could be mapped by concentration interpolation, and be compared with acceptable risk level. The case study in Chinese Sichuan-Chongqing region shows that the minimum proximity distances of different directions range from 600 to 1200m locating between individual risk values of 10^-4 and 10^-6. The zones within 1.2-km radius and northwest area deserve more attentions in the design and deployment of the protective measures.