The injection of sulfate-containing seawater into an oil reservoir, for maintaining the reservoir pressure, can promote the growth of sulfate reducing bacteria and archaea near the injection wells, leading to the formation of sulfides such as hydrogen sulfide. However, intermediate sulfur species with different valence states, such as polythionates and polysulfides have been detected in several produced water samples, likely a result of phase partitioning, and chemical and microbial reactions. These sulfur species could affect the microbial communities (e.g., microbially influenced corrosion) and will impact the efficiency of souring mitigation methods. In addition, the presence of these sulfur species can result in operational, environmental, and treatment problems. Therefore, development and implementation of souring control strategies during production cycle of oil and gas reservoirs require identifying the origins, reactivity, and the partitioning behaviour of these compounds. This paper presents an overview of the known mechanisms responsible for reservoir souring and then focuses on the chemical reactions and sulfur species associated with production and consumption of hydrogen sulfide. In this work we highlight complexity of the sulfur chemistry and that the assumption that all the sulfate is reduced to hydrogen sulfide can lead to inappropriate souring management methods. The paper also reviews the detection and analysis methods used for sulfur compounds. The review demonstrates that there is a gap in the current souring models and methods due to the exclusion of key sulfur compounds and challenges in identifying and quantifying these compounds with respect to speed of analysis and sample stability.
Reservoir souring is a widespread phenomenon in reservoirs undergoing seawater injection. Sulfate in the injected seawater promotes the growth of sulfate-reducing bacteria (SRB) and archaea-generating hydrogen sulfide. However, as the reservoir fluid flows from injection well to topside facilities, reactions involving formation of different sulfur species with intermediate valence states such as elemental sulfur, sulfite, polysulfide ions, and polythionates can occur. A predictive reactive model was developed in this study to investigate the chemical reactivity of sulfur species and their partitioning behavior as a function of temperature, pressure, and pH in a seawater-flooded reservoir. The presence of sulfur species with different oxidation states impacts the amount and partitioning behavior of H 2 S and, therefore, the extent of reservoir souring. The injected sulfate is reduced to H 2 S microbially close to the injection well. The generated H 2 S partitions between phases depending on temperature, pressure, and pH. Without considering chemical reactivity and sulfur speciation, the gas phase under test separator conditions on the surface contains 1080 ppm H 2 S which is in equilibrium with the oil phase containing 295.7 ppm H 2 S and water phase with H 2 S content of 8.8 ppm. These values are higher than those obtained based on reactivity analysis, where sulfur speciation and chemical reactions are included. Under these conditions, the H 2 S content of the gas, oil, and aqueous phases are 487 ppm, 134 ppm, and 4 ppm, respectively.
A pair of related metal–organic frameworks (Zn
3
and Zn
2
Cd) developed in our group were incorporated
into Pebax
30R51 and PVDF Kynar 761 polymers to fabricate mixed matrix membranes
(MMMs). These MOFs were chosen due to the carbon dioxide molecular
sieving ability of Zn
3
, and the
slightly larger pore aperture of Zn
2
Cd that allows carbon dioxide and larger gases
to enter the pores. For Pebax-based MMMs, this work demonstrated an
over two-fold and four-and-a-half-fold increase in carbon dioxide
permeability for Zn
3
- (15 wt
%) and Zn
2
Cd-containing
(10 wt %) MMMs over the pristine polymer. Separation selectivity (CO2:N2) of 4.21 and 7.33 were observed for Zn
3
and Zn
2
Cd (10 wt %). For PVDF-based MMMs, the incorporation
of Zn
3
and Zn
2
Cd (10 wt %) increased the carbon
dioxide permeability approximately two- and three-fold. The CO2/N2 selectivity of the PVDF membranes increased
73% (1.01 to 1.86) and 68% (1.01 to 1.68) when 15 wt % Zn
3
and Zn
2
Cd were incorporated into PVDF. The improved performance
of Pebax over PVDF based MMMs is attributed to matching the permeability
of the polymer bulk phase (Pebax over PVDF) and the dispersed phase
(Zn
3
and Zn
2
Cd). The lower permeability allows
the MOF, which has slow kinetics associated with molecular sieving,
to participate in the permeation process better. With regards to Zn
3
vs Zn
2
Cd, while Zn
3
acts as a molecular sieve and Zn
2
Cd does not, we hypothesize that the faster diffusion
of carbon dioxide gas in Zn
2
Cd can outcompete the lower nitrogen gas permeability and
molecular sieving properties of Zn
3
. However, we expect that further increasing the pore aperture
would increase the permeabilities of nitrogen gas such that differences
in diffusion kinetics due to molecular size would be unimportant.
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