Shale instability is essentially driven by changes in stress and/or chemical alteration. However, sometimes less attention may be paid to geochemical processes. In addition, some well engineering approaches may inevitably facilitate geochemical alterations. An appropriate decision may be more difficult if shale is observed as interbred layers in a carbonate reservoir (rather than as a cap rock). In this research an exceptional approach was chosen to identify geochemical processes that induced geomechanical instability in a gas reservoir with shale interbeds in the South of Iran. Nevertheless, the problem and potential solutions were worked out corresponding to the conceptual engineering geological skills. The results showed that excessive water and HCL acid (traditionally used for well stimulation) in contact with shale interbeds could have significantly contributed in plugging of the well. Site investigations revealed that the amount of excessive (unwanted) water, to a large extent depends on the gas production rate. A systematic analysis of geochemical processes at different production rates was conducted. Mineral precipitation/dissolution of shale formation was simulated accordingly. Corresponding geomechanical interpretations were considered as key points to make an appropriate decision based on economic production rate and the likely well engineering problems.


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