Print this page
The Study of H2S
Origin in Natural Gas Reservoirs in
Rabbani, Ahmad Reza, AmirKabir
Hydrogen sulfide
(H2S) is generally an undesirable component of natural gas. Where present, H2S
not only can critically affect the economic proportion of hydrocarbon gas in
the reservoir, but it is highly toxic and corrosive for production equipment.
Therefore the ability to predict the occurrence of H2S in undrilled
prospects would be a very useful idea in decrease of gas exploration and
production risk. A variety of discrete sources for H2S in petroleum have been
identified including: •Bacterial reduction of sulfate to H2S (BSR). The sulfate
can be from connate waters, anhydrite dissolution, injected seawater, or pyrite
oxidation by injected water. Bacterial sulfate reduction typically does NOT
result in gases containing >5% H2S. •Thermal decomposition of sulfides in kerogen and/or oil. This process typically does NOT result
in gases containing >5% H2S. •Thermochemical
reduction of sulfate to H2S (TSR). TSR is the reaction of sulfate minerals
(primarily anhydrite) and hydrocarbons (beginning at temperatures of 120-140
deg. C) to form H2S and calcium carbonate. Because anhydrite is often
associated with carbonate sequences, TSR is commonly associated with deep, hot,
carbonate reservoirs and/or source rocks. TSR is the most important process for
formation of high-H2S gases (>10% H2S). The highest concentrations of H2S
are found in deep, post-mature gases from carbonate sources where TSR is
active. In natural gas reservoirs in the south of Iran have been seen the sign
of TSR in particular in lower Dalan gas reservoir
.Significant enrichment in nitrogen, occurrence of isotopically
light CO2 and depletion in the light carbon isotope of methane, ethane and
propane in the gas reservoir that buried in more than 4km . These peculiarities
suggest that the composition of gases in this zone was modified by the process
of the Thermochemical reduction of sulfate.