Residual CO2 saturation estimate using noble gas tracers in a single-well field test: The CO2CRC Otway project

• At the Otway site a sequence of tests were used to measure residual CO2 saturation.
• This data has been analysed to estimate the residual CO2 saturation.
• The residual saturation estimate is consistent with other tests in the sequence.

This paper presents the results of a partitioning tracer test using noble gases to measure residual CO2 saturation in the field as one part of the CO2CRC residual saturation and dissolution test sequence. Noble gas tracers were used in a sequential partitioning tracer test. The first tracer test was performed prior to CO2 injection to characterise the aquifer under single-phase conditions. The second tracer test was performed after CO2 had been driven to residual saturation. The noble gas tracers can be regarded as non-partitioning aqueous-phase tracers in the undisturbed aquifer and subsequently as water/CO2 partitioning tracers in the presence of the residual CO2 saturation.One-dimensional radial simulations are used to estimate residual saturation from the tracer tests by fitting only two independent parameters: the apparent dispersivity of each tracer in the aqueous phase and the residual CO2 saturation. This analysis method is validated by successful application to krypton (Kr) and xenon (Xe) production curves from the field data.Both tracers give the same estimate of residual saturation to within the accuracy of the method. Furthermore the estimated residual saturation from the noble gas tracer tests is consistent with independent measurements. However the analysis of the two tracer production curves indicates that Xe has higher dispersivity than Kr, despite having been injected and produced simultaneously and measured from the same samples. Finally the application of this sequential tracer test method to other field settings is discussed.