The paper is based on laboratory research and field evaluation data incorporated in a study carried out to select a suitable drill-in fluid for use in naturally fractured HTHP sour gas reservoirs. Laboratory results were verified by monitoring drilling process and initial production of a particular well located at the gas condensate field in SW Pannonian Basin, Croatia.

Production zone, naturally microfractured limestone (21% porosity, 2 md matrix permeability) at 2500–2600 m (BHSTI150°C) was left openhole. Reservoir fluid contains 53% CO2 and 800 ppm H2S.

Two aspects of a drill-in fluid evaluation were considered: optimal drilling process performance, complicated by harsh reservoir conditions; and minimizing formation damage of plugging susceptible microfracture system.

Drilling data, using the selected drill-in fluid, showed a good ROP and borehole stability. Production tests after well completion indicated no damage.

Even under extreme downhole conditions, the balance between optimal drilling performance and minimal formation damage could be achieved using described laboratory testing procedure.

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