Assessing drilling mud and technical fluid contamination in rock core and brine samples intended for microbiological monitoring at the CO2 storage site in Ketzin using fluorescent dye tracers

The CO₂SINK project in Ketzin represents a field laboratory for the storage of CO₂ in a 650-m deep saline aquifer. The project is accompanied by a microbiological monitoring programme to characterise the composition and activity of the autochthonous microbial community in rock and brine samples and their changes in response to CO₂ storage. A prerequisite of these studies is the acquisition of samples free of contamination from microorganisms and organic and inorganic components. Drilling mud and technical fluids are the main sources of contamination. This study describes the application of the fluorescent dye tracers fluorescein and rhodamine B as contamination controls for rock core and brine samples. Fluorescein was added to drilling mud that was used during the coring phase of the Ketzin wells Ktzi 200, 201 and 202. In addition, total organic carbon (TOC) concentrations, reflecting the carboxymethyl cellulose (CMC) component of the drilling mud, were determined to verify the tracer results. The fluorescence and TOC analyses revealed that drilling mud filtrate penetrated the outer 20 mm of mildly permeable sandstone cores. Rhodamine B was added to brines that were used to displace the drilling mud and to flush the wells after completion. The tracer monitoring during the discharge of drilling mud and displacement brines from the wells during hydraulic tests and nitrogen lifts enabled the quantification of reservoir fluid quality. After the production of 140–190 m³ (16–21 borehole volumes) of fluid, the drilling mud concentration was reduced to about 0.05%. The use of fluorescein emerged as a field-capable, sensitive and reliable method during the sampling of rock core and formation brine samples.