Azeri-Chirag-Gunashli (ACG) is the world class, giant offshore oil field located in Azerbaijan sector of Caspian Sea with secondary drive mechanism as water and gas injection. The productivity index of comingled wells is high and average initial rate reaches 20mbd. As soon as water breaks through at producer from one of sand units, skin factor increases which leads to oil productivity reduction. Several causes of skin factor increase were investigated such as; scaling, fines migration, other mechanical damages, emulsion, relative permeability change etc. The impact of each mechanism should be well understood for better business decisions.

Impact of relative permeability change due to water breakthrough (WBT)was investigated in this paper via analytical and numerical well test methods. Impact of fluid migration from higher pressuresed sand into lower one through wellbore was also investigated. The results are aligned with the actual field observations.

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