Approximately one-third of the world's gas fields contain ‘sour gas’, contaminated by hydrogen sulphide, (H2S)[1]. H2S is one of the most deadly hazards in the oil and gas industry, making such fields more difficult to develop, especially when they are located in the vicinity of populated areas. Conventional approaches to wellsite layout generally use conservative consequence- based criteria, which mean a large number of high-sour wells cannot be drilled due to the potential impact on population centres from an uncontrolled release of sour gas. However, adopting a risk-based approach to wellsite layout could change this.

This paper presents a case study in which a Quantitative Risk Assessment (QRA) of a high-sour exploration well was performed to quantify the risk to members of the public from the drilling operation, and ensure that risks were tolerable and As Low As Reasonably Practicable (ALARP). The QRA demonstrated that although the maximum airborne H2S concentration at the closest populated area would be 60 ppm, the risk levels were assessed as being tolerable and significantly below industry accepted risk tolerability criteria for members of the public. The QRA shows a snap-shot of what can happen but does not demonstrate how the risks are managed in order to prevent the ‘one in a million’ event from happening. Notwithstanding this, the project team identified a series of further risk reduction measures that were implemented to ensure that the risk to members of the public was ALARP.

The well was subsequently drilled successfully without incident, and the approach described in this paper implemented as the framework for future high-sour exploration drilling operations in the organisation.

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