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CO2CRC Otway Project - Soil Gas Monitoring 2007 - 2012

Schacht, Ulrike
CO2CRC, University of Adelaide, Adelaide, SA, Australia.

Soil gas monitoring has been and still is forming an important part of studies into the geological storage of carbon dioxide (CO2) It is one of the sentinel monitoring systems to detect possible leakage of CO2 from the storage reservoirs at the CO2CRC Otway Project site located in in the Port Campbell Embayment of the Otway Basin, Victoria, SE Australia. Between 2007 and 2012 two baseline and four soil gas assurance monitoring surveys were conducted. This study evaluated existing vadose zone soil gases over and around the CO2CRC Otway Project site for both anthropogenic CO2, and y natural CO2 accumulations. In 2008 65,000 tonnes of naturally occurring CO2 were injected into the storage formation (Waarre C reservoir) at a depth of 2000 metres.

Carbon dioxide, methane and helium were the main gases analysed. For all surveys presented, soil gas chemistry results are typical of organic matter decomposition as the main source for the CO2. Helium concentrations, 13C/12CCO2 isotopes (CH4) also support a modern organic source. A deep subsurface source, which would indicate deep gas migration to surface, is not apparent.

A simple statistical analysis was applied to the data, to determine if the overall character of the observations has altered significantly over the timespan of the various surveys

A parameter in the analyses that turns out to be very significant is location. While the sites at which samples are taken are not exactly the same each year, we can pick out closest sites in the two years and see if the concentrations have altered. This is a better analysis because it takes account of the possibility that some of the variations in the data are spatial as well as temporal. A map of sampling sites showing concentration difference (pre- versus post injection) indicates that in each year post-injection, there were locations where the CO2 had increased (compared to the single reliable pre-injection survey in 2008). However, there is no consistency in these locations, and none show changes in 13C in the sense expected for deep CO2 at the site. Part of the effect is probably due to limestone weathering but part seems to be genuine large natural variability. The soil gas monitoring work in Otway thus helps to characterize the extent of natural variation and hence allows the interpretation of potential real vs false alarm signals.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012