In view of the toxicity of H2S, sour-gas production has to be carefully planned for safe operation. For the determination of safe evacuation distances in the unlikely event of an accident, well-blowout conditions had to be investigated. A methodology for predicting blowout rates from sour-gas wells in the south Oldenburg area in Germany is presented. Flow under sonic conditions, wellbore dynamics, reservoir capacities and their interdependencies control the fluid movement. Generally the blowout rates have been calculated by superimposing reservoir and tubing performance curves (based on steady-state correlations). This approach predicts lower short-term transient rates than actually occurs. A finite-element pipe-rupture model for ultrashort-term transient rate calculations is used. A coupling mechanism between these results and a reservoir simulator with a steady-state vertical-flow-performance correlation is then defined. A time-based switching criterion is determined iteratively for each situation when the flow goes subsonic, and traditional well outflow models can be used.
Sour-gas wells are mission critical in their operation, primarily because of the potential harmful effects on inhalation by humans. H2S is a secondary irritant and a chemical asphyxiant whose effect on mucous membranes is overshadowed by systemic effects resulting from absorption. Exposure may result in pulmonary edema, hemorrhage, and tissue necrosis. Despite a low human odor threshold (<1ppm), H2S is considered to be an insidious poison because our sense of smell rapidly fatigues and therefore fails to provide a good warning of gas concentration. The Occupational Health & Safety Admin. permissible exposure limit is 10 ppm, which is 30 times lower than the immediately dangerous to life and health limit.