Page 55 - Well Control for Completions and Interventions

46 Well Control for Completions and Interventions

(parts per million) rather than percent. Working on wells with hydrogen
sulfide (H 2 S) requires additional precautions to protect personnel and
equipment.
The danger from H 2 S to personnel depends on its concentration and
the duration of exposure. Factors like wind strength, direction, and venti-
lation must also be taken into account. As H 2 S is denser than air, it will
accumulate in low-lying areas such as the wellhead cellar.
Human beings can tolerate very low concentrations of the gas, but
with increasing concentration the risk increases (Table 1.8).
There are several recommendations regarding exposure limits for peo-
ple working in the presence of H 2 S.
The API base their recommendations on those provided by the
Atlanta-based National Institute for Occupational Safety and Health
(NIOSH), and state that personal protection should be provided if the
work area concentration of H 2 S exceeds 10 ppm 8-hour time weighted
average (TWA), or 15 ppm as a Short Term Exposure Limit (STEL),
averaged over 15 minutes. Personal protection is not required when the
atmospheric concentration of hydrogen sulfide could not exceed 10 ppm
in the breathing zone (API RP 49). 5
In the United Kingdom, the Health and Safety Executive are rather
more conservative, setting the workplace exposure limits (WELs) as
5 ppm for an 8-hour time weighted average (TWA), and 10 ppm for a
15-minute TWA. 6

1.1.10.1 Hydrogen sulfide safety precautions
As H 2 S is toxic, it is crucial that all necessary precautions are in place and
implemented:
• Where possible, position personnel upwind of the H 2 S source.
• Blowers can be used dissipate H 2 S in sheltered locations, or on days
when there is little or no wind.
• If H 2 S well concentrations are above the prescribed threshold for pro-
tective equipment, self-contained breathing apparatus (SCBA) must be
available at the worksite, and all personnel trained in its use. Normally
this equipment is supplied by specialist vendors who will train well site
personnel and ensure the equipment is properly maintained.
• Position H 2 S detectors at any location where personnel would be at
risk in the event of a gas escape. The positioning, monitoring, and
maintenance of the H 2 S detection system is often carried out by the
vendor who supplies the breathing apparatus.