Analysis of 87Sr/86Sr ratios and modelling of formation water, injection water and produced water
compositions from the CO2CRC Otway Research Facility in Victoria, Australia are used to test tracer
behaviour and response in push-pull experiments. Such experiments are an essential pre-requisite to
understanding the controls imposed by reservoir heterogeneities on CO2 dissolution rates which may
be an important stabilising mechanism for geological carbon storage. The experiments (Otway stage
2B extension in 2014) comprised two sequential tests in which ~ 100 tonnes of CO2-saturated water
was injected with combinations of Sr and Br or Li and Fluorescein tracers, each injection being followed
by two staged extractions of ~ 10 tons and a final extraction of ~ 50 tons all spaced at ~ 10 day
intervals. Analysis of the 87Sr/86Sr ratios of the produced fluids from the first injection, spiked with
SrCl2 and NaBr, is consistent with Sr behaving conservatively. This contrasts with previous interpretations
in which Br was argued to have behaved conservatively while Sr, which dilutes ~ three times as
fast as Br, was thought to be lost to a mineral phase. Such Sr-loss cannot explain the evolution of
87Sr/86Sr ratios. The analysis of 87Sr/86Sr ratios in the waters produced after the second injection episode,
spiked with LiCl and Fluorescein tracers, allows calculation of the fractions of the formation waters
and the injection waters from both tests 1 and 2. The Sr, Li and SO4 tracers (the later formed by oxidation
of formation sulphide) all indicate similar rates of dilution that is consistent with conservative
behaviour. The results of the two injection episodes with spaced extractions are compared with two
subsequent push-pull injections in which the produced waters, spiked with methanol, were extracted
continuously. These continuous extraction experiments exhibited significantly less dilution over the
same range of produced to injected water volumes (upto only ~ 0.6) than the earlier experiments with
spaced extractions. This implies that some process related to the pauses in extraction enhances mixing
of injected and formation waters. Achieving the objective of using push-pull experiments to assess
reservoir heterogeneities and CO2 dissolution rates will require better assessment of the various tracers
to establish which behave conservatively followed a proper understanding of the causes of the variations
in mixing as fluids are extracted from the formations.This study was supported by the UK Natural Environment Research council Highlight grant
NE/N015908/1. It is a contribution to GeoCquest, a BHP-supported collaborative project of the University
of Melbourne (Australia), the University of Cambridge (UK) and Stanford University (USA),
aimed at developing a better understanding of small-scale heterogeneity and its influence on CO2 trapping
mechanisms. The authors would like to thank CO2CRC Ltd. for giving access to data from the
CO2CRC’s Otway Research Facility