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.