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2018 AAPG International Conference and Exhibition

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Acid Gas (CO2 and H2S) Risk Assessment at the Basin Scale


The presence of acid gases such as hydrogen sulfide (H2S) and/or carbon dioxide (CO2) in Exploration & Production drastically impacts the economics and human risks of a project. Being able to predict the presence of these gases in an early phase of an E&P project is thus very important to manage the risk and help the decision making. On one hand, the highest CO2 contents (>40vol. %) are only explained by inorganic sources, either from a mantle or crust origin. A new model integrated within TemisFlowTM software has been developed to manage both fluxes of CO2 entering into the sediments in contact with the basement, and CO2 produced by metamorphic reactions occurring in specific CO2-prone sediments often present in the hot deep synrift facies. Once generated, the migration of CO2 is modelled by advection both in gas phase and dissolved in the formation water. Dissolution equilibrium follows Duan et al. (2003) equation taking into account the effect of formation waters salinity, temperature and pressure. This allows to simulate basin-scale CO2 distribution either in a gas phase or dissolved in the brine, through the geological times. On the other hand, high concentrations (>10vol. %) of H2S, result from Thermochemical Sulfate Reduction (TSR) of petroleum. TSR occurs in specific high temperature H2S-prone carbonate rich reservoirs, and is very detrimental to the quality and the volume of hydrocarbon resources. To answer the H2S risk assessment, a dedicated model has been developed. It is based on a mass balance equation according to the overall chemical reaction which has been first proposed by Uteyev (2011). The H2S generated is related to H/C ratio of the hydrocarbon and depends strongly on sulfate minerals texture and distribution in the basin. The model allows to simulate H2S and CO2 quantities, as well as their distribution in a gas phase or dissolved in the basinal brine. The volume of dissolved sulfate, precipitated calcite, and the porosity evolution related to the TSR reaction is also assessed. The management of the CO2 risk will be illustrated by cases studies inspired by (1) basin model of the Brazilian passive margins of the Central Segment of the South Atlantic Ocean basin for mantellic CO2, (2) hot sedimentary basin of the South China Sea for metamorphic CO2. The H2S risk will be illustrated by a 3D basin model of Devonian carbonate reservoirs of the Nisku and Leduc formations (Alberta) that experienced TSR and may contain up to 30% of H2S.