The International Energy Agency (IEA) Weyburn project is an international project that is
studying the feasibility of long-term geological storage of carbon dioxide (CO2), allied to an
enhanced oil recovery operation, by Encana, in the Weyburn oilfield, south-eastern
Saskatchewan, Canada. CO2 is being injected into the oil reservoir to improve oil production,
whilst at the same time the process should lead to long term geological storage of large volumes
of CO2. Soil gas studies are being undertaken as part of an EU-funded component of this project,
with the primary objectives of measuring the natural background concentrations and to ascertain
if there is a leak of CO2, or associated tracer gases, as a direct result of the solvent flood
presently occurring at the Encana Weyburn oil field. This report describes the results from three
sampling periods conducted between July of 2001 and October of 2003.
Sampling of the large 360 point grid above the injection area over the last three years showed
CO2, O2 and CO2 flux values in the range of natural soils, and these observed levels can be
explained by standard metabolic pathways that normally occur in the shallow soil horizon. The
spatial anomaly distributions of these gases are reasonably reproducible from year to year and
season to season, despite the fact that the range of values vary from high concentrations in the
hot wet summer of 2001 to the low values found in the fall of 2002 and 2003. In contrast to
these biologically active species, the statistical and spatial distribution of radon and thoron is
very similar from one sampling season to the next. This provides support for the idea that
leakage is not taking place, as one would expect to see high radon during the periods of high CO2
if the latter was acting as carrier for the more trace former gas. Hydrocarbon values were found
to be within normal ranges in October of 2003, however the previous two seasons showed
elevated values which were not fully expected. In fact ethylene and propane show a statistical
distribution over the three years which is quite similar to that of CO2, whereas methane and
ethane have relatively constant values except for more concentrated outliers during the first and
second seasons. As the heavier hydrocarbons do not normally originate via shallow biological
reactions it is difficult to reconcile these results with those of the other gases.
A comparison of the grid data with that of the background site, located in similar surface
geology but outside the Weyburn oil field, shows a very similar statistical distribution for all
monitored parameters. This result supports the interpretation that the observed gas
concentrations are not due to deep leakage. In particular, both sites have a very similar CH4 /
(C2H6 + C3H8) ratio. This value is low and in the range of thermo-catalytic regime, however the
fact that such a value was found both within and outside the oil field implies that the origin of
these gases are not necessarily from a deep oil reservoir.