Application of tracers to measure, monitor and verify breakthrough of sequestered CO2 at the CO2CRC Otway Project, Victoria, Australia

Abstract

At the Cooperative Research Centre for Greenhouse Gas Technology's (CO2CRC) field site in the Otway Basin of Victoria, Australia, investigations into the storage of CO2-rich gas in a depleted hydrocarbon gas field have been conducted in the Waarre C reservoir. The injected gas from the nearby Buttress field contained 75mol% CO2, 21mol% CH4 with the remaining balance being a mixture of wet hydrocarbons, condensate and nitrogen. Chemical tracers (sulphur hexafluoride, SF6; krypton, Kr; perdeuterated methane, CD4) were added on the basis of literature surveys and small volume trials at the Frio II Brine experiment in Texas. The aim of the project was to measure, monitor and verify the presence of injected CO2 in a depleted gas field and that the arrival of tracers was a major component of demonstrating breakthrough of CO2 at the monitoring well, Naylor-1. The paper focuses on methods developed for the injection, recovery and analysis of samples collected at the Naylor-1 well. Results of tracer analysis compare well with other data collected (including pH and density measurements) to demonstrate breakthrough.A slip-stream injection system was designed to deliver the tracers mixed with the CO2-rich gas into the subsurface at the CRC-1 well. The tracers were added to the gas stream 17days after the start of injection (CO2 injection commenced 18th March, 2008) into the depleted natural gas field at Naylor. A U-tube system was used to retrieve the samples from the Naylor-1 monitoring well. Collected gas and formation water samples were analysed in detail for gas composition, tracers, isotopes (13C CO2 mainly) and inorganic geochemistry for the broader project. The tracer results confirm that CO2 breakthrough at the monitoring well occurred within the predicted times. However the interval between samples taken from the U-tubes was too coarse to resolve detailed differences in arrival times between the CO2 and tracers.Of the three tracers used, SF6 provided the clearest evidence of breakthrough at U-tube 2. Kr, because of its abundance in air, and its potential to be present in the subsurface, was more prone to contamination and had higher background levels prior to breakthrough. CD4 was expected to provide some more unique data based on the presence of abundant CH4 in the reservoir interval. With hindsight, larger volumes should have been injected to facilitate comparisons with the other tracers and add value to the data set. The test of CD4 however acted as a suitable proof of concept that CD4 could be used in such a high background of CH4.Further work is ongoing to generate data for partition coefficients between supercritical CO2, CH4 and water under the injection conditions.