Investigations of alleged CO2 leakage in Weyburn, Canada in the context of longer term surface gas monitoring

The Weyburn oilfield in SE Saskatchewan, Canada has been in production for more than 50 years. A CO2 flood was started in 2000 to enhance oil recovery. The gas is piped from a coal gasification plant across the US border in North Dakota. In addition to boosting oil production it is expected that about 30 million tonnes of CO2 will be permanently stored in the reservoir, at a depth of about 1400 m, by the end of the 30 year lifetime of the project. Near surface gas monitoring has been carried out from the early stages of CO2 injection. It forms part of the IEAGHG Weyburn-Midale research project and has received additional support from EU-funded research projects and UK national funding. The first surface gas measurements were made in July 2001, with subsequent surveys in the autumn of that year and then each autumn from 2002 to 2005. Repeat surveys were carried out in 2011 when new methods were trialed and specific investigations were made at a site of alleged CO2 leakage. The results show no evidence for any leakage of the injected CO2. They are consistent with seasonal variations in biogenic CO2 produced in the shallow subsurface. The results of the 2011 and earlier studies will be used to help provide guidance on best practice for near-surface monitoring at CO2 storage sites.