A Kinetic Modeling Approach to H2S Risk Assessment in the Pre-Salt Reservoirs in the Santos Basin

Summary The pre-salt play in the Santos Basin contains giant oil and gas accumulations and is considered the largest petroleum discovery in the last decade. However, the fields contain non-hydrocarbon gases (e.g., CO2 and H2S) that bring technical challenges for exploration and production. Although H2S concentrations are quite low (15 to 180 ppmv), corrosion and environmental issues can arise, with higher costs required to mitigate these problems. Consequently, it is important to identify the generation processes of these gases to improve estimates of the required investments during the initial phases of the projects. Santos Neto et al. (2013) suggested that the main H2S generation process in the pre-salt reservoirs in the Santos Basin is thermochemical sulfate reduction (TSR), mainly based on the sulfur isotopic composition of H2S, oils, and anhydrites. Additional TSR proxy analyses were conducted (compound-specific sulfur isotope analyses — CSSIA and thiadiamondoid quantification) and these subsequent results suggested that TSR may not be involved. To constrain the possible origins of H2S, we have modelled in situ TSR in low temperature carbonate reservoirs (∼100°C) using published kinetic parameters.