Hydrogeochemical Modeling of Hydrogen Sulfide Formation in a Carbonate Reservoir: A Case Study of the Shanul Field, Southwest Iran

Thermochemical sulfate reduction (TSR) is one of the mechanisms responsible for the generation of hydrogen sulfide in a reservoir. Recently, novel hydrogeochemical modeling approaches are developed to unravel TSR in hydrocarbon reservoirs. These modeling were developed in order to comprehend the basic hydrogeochemical mechanisms for H2S production and its controlling factors. In this paper, the modeling of the TSR process in Permian-Lower Triassic deposits was carried out using the example of the Shanul gas field in southwestern Iran. The one-dimensional diffusive mass transport model used in this study (Phreeqc) is based on equilibrium reactions for gas–water–rock interactions and kinetic reactions for sulfate reduction and methanogenesis. The simulation results show that the intensity of the TSR reaction and the volume of driven H₂S are influenced to a large extent by three factors: the mineral composition of the host reservoir rock, the pressure drop of the reservoir, and the pH of the formation water. The results highlight that, the mineral composition of the host rocks alters the intensity of TSR process. The presence of iron containing minerals may significantly inhibit the H₂S production. Modeling results show that the presence of 5% iron-containing minerals could completely remove hydrogen sulfide from the system within 20 years. Moreover, it has been observed that the change in reservoir pressure after production or injection also significantly affects the content of hydrogen sulfide in the reservoir. A pressure drop up to fifty percent (from 600 to 300 atm) of the initial reservoir pressure increases the H2S content by more than ten times (from 4×10^–4 to 4×10^–3 mol/kg (H₂O). In addition, it has been shown that changing the pH of an aqueous solution significantly changes the rate of TSR reaction. According to the simulation results a 20% decrease in water pH (From 6.5 to 5.7) will lead to an increase in H₂S concentration from 8×10^–4 to 16×10–4 mol/kg (H₂O).

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