25th International Meeting on Organic Geochemistry IMOG 2011

P-356
Controls on the kinetics of thermochemical sulfate reduction
Geoffrey Ellis 1 , Tongwei Zhang 2 , Qisheng Ma 3 , Alon Amrani 4 , Yongchun Tang 3
1 U. S. Geological Survey, Denver, United States of America, 2 Bureau of Economic Geology, Austin, United
States of America, 3 Power, Environmental, Energy Research Institute, Covina, United States of America,
4 Geological Survey of Israel, Jerusalem, Israel (corresponding author:gsellis@usgs.gov)
Published estimates of the kinetic parameters
describing the rate of thermochemical sulfate
reduction (TSR) vary widely and are inconsistent with
geologic observations. Recent experimental and
theoretical evidence indicates that TSR in natural
environments probably involves a two-stage reaction
mechanism. The initial stage involves the reduction
of a reactive sulfate species to hydrogen sulfide (H2S)
in the absence of reduced sulfur. The second stage
involves a much more rapid catalyzed reaction once a
threshold partial pressure of H2S has been attained.
Extensive experimental evidence shows that aqueous
CaSO4 and free sulfate ions (SO4 2- ) are not readily
reduced by TSR. Aqueous geochemical modeling of
typical reservoir brines shows that the dominant
reactive sulfate species at common reservoir
temperatures (100-200°C) is aqueous MgSO4 contact
ion-pairs (CIP‘s). However, when aqueous solutions
containing Mg and SO4 are heated to temperatures
>200°C used in laboratory simulations, the
precipitation of magnesium hydroxide sulfate hydrate
precludes the formation of MgSO4 CIP‘s and bisulfate
(HSO4 - ) becomes the predominant sulfate species.
Ab initio quantum chemical calculations indicate that
the kinetic parameters for the uncatalyzed reduction
of HSO4 - and MgSO4 CIP‘s are equivalent (Ea~56
kcal mol -1 and Af~1.5e+13 sec -1 ). Moreover,
experimental simulations of the initial TSR reaction
involving HSO4 - and several different crude oils
provide a range of activation energies from 55.3 to
58.9 kcal mol -1 and frequency factors from 5e+16 to
1e+17 sec -1 . The variation in the kinetic parameters
observed for different oil types appears to be related
to the concentration of labile sulfur compounds in
whole crude oil. Once the threshold concentration of
H2S has been exceeded, the rate of TSR increases
significantly, and detailed experimental work shows
that the activation energy for the catalyzed reaction is
directly proportional to the concentration of H2S. This
relation can be expressed as:
Ea = -0.066(pH2S) + 58.01
where Ea is the activation energy in kcal mol -1 and
pH2S is the partial pressure of H2S in psi (see figure).
Recalculation of the kinetics of sulfate reduction using
two previously published datasets (Kiyosu, Chem
Geol., 30: 47-56; Goldhaber and Orr, in Geochemical
transformations of sedimentary sulfur, Vairavamurthy
and Schoonen, eds., p. 612), and taking into account
the effect of HSO4 - concentration, produces results
that generally agree with our observations. These
findings lay the foundation for a predictive model of
TSR in geologic environments; however, accurate
determination of the H2S-generation potential likely
requires some knowledge of the chemistry of
petroleum present, the local water chemistry, and the
thermal history of the reservoir.
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