Sour gas wells, gathering systems and gas processing plants share one hazard above all others: hydrogen sulfide (H2S). Many of these sites are in hilly or remote country, a long way from the nearest public weather station, which is exactly where one wind reading tells you the least.
Why H2S is different
OSHA describes H2S, or sour gas, as colorless, highly toxic and extremely flammable. It is heavier than air and may travel along the ground or collect in low-lying areas. (OSHA eTool)
- Immediately dangerous. The NIOSH IDLH value is 100 ppm. Death can result from exposures of 700 to 1,000 ppm. (OSHA eTool)
- Smell is no warning. Above about 100 ppm, H2S rapidly deadens the sense of smell. (OSHA)
- Wells can change. OSHA notes that sweet wells have been known to turn sour overnight. (OSHA eTool)
- Burning it makes another hazard. Burning H2S produces sulfur dioxide. (OSHA)
- It can reach the public. EPA’s report to Congress found that an accidental H2S release from a well could have severe consequences because of its toxicity and its potential to travel significant distances downwind. (EPA)
Why terrain-aware wind matters for H2S
A gas that hugs the ground goes where the wind and the land take it: down drainages, along creek bottoms and into the low spots where homes, roads and lease crews often are.
ALOHA on its own cannot show that. ALOHA takes one wind speed and one direction, holds them constant downwind, and assumes the ground under the cloud is flat. NOAA notes that it does not account for wind turning with slopes, hills and valleys. (NOAA) So an ALOHA threat zone by itself gives you no picture of how the terrain is steering the air that carries the gas.
LR-x® fills that gap. It runs the same ALOHA model and, on the same map, draws the U.S. Army Research Laboratory 3D wind field. LR-x® maps the wind over the actual terrain, so you can see how air is moving through valleys and low ground (channeled along a valley, slowed in a draw, speeding over a ridge) and judge where a heavier-than-air gas like H2S may flow and collect. LR-x® does not predict where gas will pool; it gives you the terrain-steered wind beside the ALOHA threat zone so you can make that call. The wind field does not change ALOHA’s calculation. How the 3D wind field model works.
Know the limits. In very light, thermally driven winds, such as calm, cold nights, any diagnostic wind model is at its weakest, and a heavy gas will tend to drain toward low ground. On those nights, use the terrain on the LR-x® map to judge where low ground around the release could let the gas collect.
What LR-x® adds to an H2S contingency plan
- H2S release modeling. LR-x® models hydrogen sulfide releases. It runs the ALOHA plume model, shows the H2S threat zones on the map and updates them as the weather changes. ALOHA (NOAA/EPA) models both neutrally buoyant and heavy gas dispersion. (NOAA)
- Any chemical ALOHA can model. ALOHA’s chemical library covers over 500 pure chemicals and some common chemical solutions, a subset of NOAA’s CAMEO Chemicals database. (NOAA ALOHA Technical Documentation)
- Three additional forecasts, about every 2 minutes, to see a wind shift before it pushes gas toward a road or residence.
- Portable weather on scene. At a remote site with no nearby public station, LR-x® can use a portable sensor you set up on scene, such as a Coastal Environmental Systems WEATHERPAK®.
- KML export. LR-x® exports the real-time plume model, the real-time 3D wind field model and the three forecast models as KML files you can use in ArcGIS, alongside lease maps, roads and residences.
- What-if scenarios for planning radius-of-exposure discussions, drills and training against real terrain.
OSHA points operators to API RP 49 for drilling and well servicing operations involving H2S. LR-x® supports that kind of planning; it does not replace gas detection, monitoring or the operator’s contingency plan.
Who uses it
Operator safety and emergency response staff, well control and production teams, rural fire districts and hazmat teams, county emergency management and the LEPC.
See also: refineries, chemical plants and all facility types.