A method for blowout-rate prediction for sour-gas wells, Oldenburg fields, Germany

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.