This paper describes the results of formation damage study on first horizontal well drilled and completed in Croatia. The specific problem of formation damage caused by polymer mud was evaluated in the laboratory, investigating the rock-fluid compatibility and methods for restoring the reservoir permeability around the wellbore. The source of damage was identified as a vey tough mud filtrate cake as well as the reduction of permeability inside the pore space due to mud filtrate invasion. The two stages well stimulating procedure for damage removal, consisting of soaking and injecting oxidizer, then acid, in two steps, tested in the laboratory, showed promising results.

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