CFD Simulations of H_2S-Rich Plumes from Oil/Gas Well Blowouts

摘要

Oil and gas exploration wells under high pressure present the risk of blowout, which is a sudden, unplanned, and potentially dangerous eruption of fluids (natural gas and oil) from well drilling. If not ignited, the large volume of natural gas (and liquid) released can form a cloud or plume that can contain hydrogen sulfide (H_2S) as high as 90 percent by volume. Vaporization of high molecular weight hydrocarbons can result in heavier-than-air gas mixtures that disperse close to the ground and resist dilution, yielding H_2S concentrations well above the National Institute for Occupational Safety and Health (NIOSH) immediately dangerous to life or health (IDLH) threshold of 100 parts per million by volume, possibly over considerable distances downwind of the release. The dispersion of potential blowout releases was modeled on behalf of industrial clients to determine distances downwind from a well site that could be endangered. The need to model blowout releases in rugged, mountainous terrain cast uncertainties on the applicability of traditional dense gas models such as SLAB and DEGADIS. As such, the dispersion of blowout releases was simulated using the computational fluid dynamics (CFD) model PANACHE, which is recognized as an alternative dense gas dispersion model in the U.S. EPA's Appendix W modeling guidelines. The complex, two-phase flow in the near-source region was simplified to determine worst-case release characteristics for far field modeling. CFD simulations extending several kilometers downwind have been conducted for varying meteorological conditions. The simulations have exhibited some near-field influence of dense gas dispersion, with releases following topographic features. The level of excess gas density is generally predicted to dissipate rapidly, however, possibly from terrain-induced turbulent mixing. Even so, elevated concentrations of H_2S have the potential to persist for several kilometers downwind of source areas.