A scenario for the creation of H2S heterogeneities in acid gas reservoirs in contact with an active aquifer: a simulation study
Résumé
Compositional heterogeneities of H2S have been noticed in many sour gas reservoirs. Its occurrence is an important factor of economic depreciation. Thus, the knowledge of its content and distribution is a critical parameter when planning field development. The paper aims at exploring the role of an active aquifer in the creation of H2S heterogeneities in high H2S-bearing gas reservoir. Indeed, under conditions of pressure and temperature of typical reservoirs, H2S is far more soluble than hydrocarbons and other gases. A preferential leaching of H2S (e.g. versus CH4) over time is thus possible. This mechanism is controlled by: (1) Differential solubility of gases, which change the relative amounts of each gas near the gas-water contact (GWC); (2) Contact with an active aquifer, which can export the dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which transfers the compositional anomalies farther from the gas-water contact; (4) Geological parameters (type of aquifer, permeability heterogeneities) which can modify the transport scenario. To illustrate and quantify this process, we show the results of numerical simulations, performed with the two-phase transport and geochemical software Hytec. First, a very schematic reservoir with a composition considered uniformly distributed within the reservoir, has been simulated to quantify the leaching of H2S. The results highlight the potential role of the active aquifer, which can leach the gases and export them outside the reservoir. In a second phase, the effect of geological parameters on the H2S heterogeneity development was studied: additional simulations were performed on geometries closer to natural cases. The amount of leached H2S depends strongly on the geometry: the larger the GWC area, the larger the amounts leached.