Tracer tests are used extensively in the petroleum industry for the characterisation of fluid flow behaviour within reservoirs and identification of flow pathways. With the rapid development of unconventional reservoirs, tracers have been used to measure the effectiveness of hydraulic fracture stimulation, improving the understanding of fracture fluid loss and flowback efficiency. In coal seam gas reservoirs, tracer tests could also be used to investigate the connections between the producing formation and other geological units such as aquifers.

A conservative water tracer is required to be chemically inert, environmentally safe, non-toxic, and stable at reservoir conditions. It must be detectable at very low concentrations and economically practical. A group of fluorinated benzoic acids (FBAs) have been used broadly as chemically passive water tracers in several applications within petroleum. In coal seam gas reservoirs, the key question in the use of any conservative tracer is the potential for adsorption on the coal as this will lead to either retardation or loss of the tracer. Among the 18 commercially available FBAs, 2,6 difluorobenzoic acid (2,6-DFBA) has been identified to have the lowest adsorption potential as demonstrated by its octanol-water partition coefficient.

This study presents a series of batch and core-flooding experiments conducted to assess the adsorption of several water tracers on coal. The tracers selected for this study were two FBAs, one of which was 2,6-DFBA, and the ions lithium and bromide. While lithium and bromide are often regarded as conservative tracers, an obstacle to their use is the potential for the target reservoir to have an existing relatively high natural background of these ions that can make distinguishing the amended tracer challenging. In the batch experiment, an aqueous solution of the tracers in a sample of formation water was added to crushed coal. In the core-flooding experiment, an intact coal core was placed inside a temperature-controlled tri-axial pressure vessel. High pressure ISCO pumps are used to control the pressure of the system and the confining pressure to match the reservoir conditions. A solution of tracers in a sample of formation water was flooded through the core at a controlled rate. Two core floods were performed at different flow rates to fully characterize the transport properties. In the batch and core flooding tests, regular liquid samples allowed the concentration to be determined and the mass balances of these tracers to be calculated.

The measurements from the core flooding experiments were then modelled using the CXTFIT spreadsheet which provides a comprehensive suite of analytical solutions to the advection dispersion equation. Close fits to the experimental observations were obtained using this approach and the tracer’s transport properties were successfully estimated.

The results of the batch and core-flooding experiments showed that 2,6-DFBA is low to non-adsorbing to coal but the second FBA, 2,3,4,5-tetrafluorobenzoic acid (TeFBA), was found to be adsorbing during the batch experiment and retarded during the core flood. In conclusion 2,6-DFBA is suitable as a conservative water tracer for coal seam reservoirs.

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