The purpose of this paper is to discuss some of the issues involved in the modeling of the accidental release of hydrogen sulfide (H2S) during oil and gas extraction activities. The focus is on wellhead blowouts or pipeline ruptures that lead to large releases of sour gas.

First, the types of possible source terms are reviewed, including characteristics such as rate of release, H2S content and duration of release. In particular, it is shown that most sour gas streams are less dense than is air. Second, the availability and suitability of atmospheric dispersion models is discussed. One of the most important issues is that of lift-off; that is, whether a buoyant plume that is initially on the ground will in fact lift off. For plumes that are marginally buoyant (e.g after dilution during an initial jet phase), lift-off is unlikely.

Another important issue is the choice of an appropriate airborne concentration as a Level of Concern (LOC). Quantitities such as the Emergency Response Planing Guideline (ERPG) and the LC01 (the concentration that would prove fatal to one percent of those exposed to it) are discussed. Some examples of predicted distances to these LOCs are given for severe accidents in unfavorable weather conditions.

The paper also briefly discusses the following: a) seepage of H2S from buried pipelines; b) burning plumes and releases from flare stacks; c) the reasons why "pooling" of sour gas in hollows is unlikely; d) the behavior of sour gas dispersion in complex terrain.

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