Chemical Constraints Deduced from Modeling H2S Formation in Carbonate Reservoirs
Ronald K. Stoessell
H2S generation in carbonate reservoirs has been modeled with the Fortran-PC computer program REACT using a quasi-equilibrium approach over a temperature and pressure range of 100° to 200°C and 300 to 1,000 bars. The model assumed thermodynamic equilibrium governed all chemical interactions with the exception of sulfate and hydrogen sulfide reactions with organic carbon molecules. For these reactions, an algorithm in REACT modified the equilibrium constants such that true equilibrium was gradually approached with increasing temperature.
A gas phase
, deficient in hydrogen sulfide, was predicted from thermodynamic
equilibrium through the addition of organic molecules in the absence of certain
minerals: sulfates and iron-bearing oxides and carbonates. Hydrogen sulfide was
generated (in this study) by reducing the sulfate in anhydrite while oxidizing
organic carbon and hydrogen. Calcite precipitated due to the released calcium
and CO2. H2S was removed by iron sulfide precipitation
through the destruction of iron-bearing oxides and carbonates. In the absence of
an iron source, but with excess anhydrite, thermodynamics predicts the
continuing destruction of organic molecules with H2S staying in the
aqueous fluid. Only by shifting the equilibrium constants controlling the
stability of the or anic molecules by 5 to 8 orders of
magnitude
did a gas
phase
(with up to 10 mole % H2S) form in the presence of excess sulfate.
Results will be reported on current modeling efforts to determine the general conditions under which the presence of iron-bearing, sulfide-free minerals can be used as indicators of the absence of H2S.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.