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Role of Hydrodynamism in Compositional Heterogeneities in Acid Gas Reservoir

Bonnaud, Estelle *2; Lagneau, Vincent 1; Dessort, Daniel 2; Chiquet, Pierre 2
(1) Mines ParisTech, Fontainebleau, France. (2) TOTAL S.A., Pau, France.

Compositional heterogeneities of acid gases (H2S and CO2) have been noticed in many sour gas reservoirs. Their occurrence is an important factor of economic depreciation. Thus, the knowledge of the acid gases content and distribution is a critical parameter for the design of field development. The mechanisms to explain the presence of acid gas in a reservoir are various: thermochemical sulfate reduction, bacterial activity, thermal cracking of kerogen. On the opposite, the compositional heterogeneities of acid gas within a given reservoir are not well understood. Under conditions of pressure and temperature of typical reservoirs, acid gases are, by far, more soluble than other hydrocarbon components. A preferential leaching of H2S over time is thus possible. The paper aims at exploring the role of an active aquifer in contact with an initial high H2S-bearing gas reservoir.

The preferential leaching of H2S may be controlled by: (1) Differential solubility of gases, which can change the relative amounts of each gas near the gas-water contact (GWC); (2) Active aquifer solubilization and transport, which can export dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which can transfer the compositional anomalies farther from the gas-water contact; (4) Presence of geological heterogeneities (permeability…), 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 (a rectangular vertical section, with an active aquifer at the bottom and a gas reservoir at the top) has been simulated to quantify the H2S leaching. The results highlight the potential role of the active aquifer, which can leach acid gases and export them outside the reservoir. Then, to study the effects of geological parameters on the heterogeneities growth of H2S, additional simulations were performed on geometries closer to natural cases, including low permeability barriers. Their addition creates sharp compositional heterogeneities, with a decrease in acid gas near the contact, while H2S concentration remains unaffected in farther areas. The amount of leached H2S depends strongly on the geometry; the larger GWC area, the larger amounts leached. Finally, a small additional diffusion through the low permeability barriers can increase the total aquifer leaching.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain