This study reports a comprehensive approach for analysis of increasing hydrogen sulfide (H2S) concentration after 10 years of sweet gas production from Kangan and Dalan formations (Khuff) in a gas field. Increasing H2S concentration as a toxic and corrosive component in gas phase will affect the project economics and needs immediate action in terms of production/surface processing facilities and also prediction of future gas contamination. Thus, a comprehensive study was conducted to determine the possible mechanisms behind increasing H2S concentration in some wells. Geological, engineering and field data were analyzed and all possible contributing parameters were studied in an integrated approach. All possible reasons including producing formations, water level movement, cumulative gas production, individual well gas rate, water composition/production, well completion (flow behind casing, leakage, etc.), faults, regional production, etc. were examined. Migration from a deeper gas formation was concluded to be the source of local increase in H2S concentration. To confirm the hypothesis, sampling of gas from 10 wells were performed and a geochemical survey was conducted based on sulfur isotope. Results showed a relation between increasing concentration of H2S and sulfur isotope values. Heavier values of sulfur isotope were obtained for those wells showing most increase in H2S concentration. In addition, sulfur isotope of contaminated wells were in agreement of that for deeper anhydrite formation. Results of geochemical analysis confirmed gas migration from deeper formation which was proposed by geological/engineering survey.

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