Aqueous phase tracers for monitoring fluid mixing in geological reservoirs: Results from two field studies: Fid-Bau Portal

Aqueous phase tracers for monitoring fluid mixing in geological reservoirs: Results from two field studies

Black, Jay R. / Vu, Hong P. / Haese, Ralf R.

Highlights•Soluble aqueous phase tracers are evaluated in two field studies related to CCS.•Fluorescein acted non-conservatively, adsorption to minerals being the likely cause.•Sr²⁺ acted non-conservatively, potentially due to exchange with minerals.•Br⁻ and Li⁺ exhibited conservative behaviour as soluble aqueous phase tracers.

AbstractDissolved species are widely used to determine the degree of fluid mixing and fluid-rock reactions in geological reservoirs where differences are observed in groundwater chemistry. Likewise these components can be added artificially to injection waters to act as aqueous phase tracers in field tests. Here we report results from two field studies where aqueous phase tracers were applied. The first field site was located in the Darling Basin of NSW where new exploration wells were drilled to determine if suitable CO2 storage reservoirs could be identified. In this field study fluorescein was continuously added to a drill mud used during the drilling of an exploration well. After drilling the well, a wireline tool was used to collect formation water from different depths in the well and the residual fluorescein concentration was used to determine contamination of formation waters by drill mud. The composition of dissolved solids in the in situ formation water samples were close to 100% of those found in the final drill mud composition, whereas, c. 70–83% of the drill mud fluorescein concentration was observed in the in situ water samples. This indicated significant drill mud contamination of samples collected by the wireline tool, and potential non-conservative behavior of the fluorescein. The second field site was located at the CO2CRC Otway field site near Nirranda South in Victoria, Australia. Here, a range of aqueous phase tracers (fluorescein, Na⁺, Cl⁻, Br⁻, Sr²⁺ and Li⁺) were added to a CO2-saturated injection water and analysed in the back-produced water of a push-pull experiment (Otway 2B Extension) in order to determine the degree of mixing between injection and formation water. Fluorescein behaved in a non-conservative manner under the mild acidic conditions (in situ pH=4.5), most likely due to adsorption to mineral surfaces. Scatter in the Na⁺ and Cl⁻ concentrations made their interpretation problematic, while some loss of Sr²⁺ is inferred possibly involving exchange with minerals in the reservoir. In contrast, Br⁻ and Li⁺ showed conservative behavior and are recommended for future tracer studies in reservoirs where pH is variable.