One of the objectives of the Aquistore project – which has seen over 393 kilotonnes of CO2 injected 3.2 km underground into a deep saline formation – has been to track the growth of the CO2 plume over time. To date, this has been accomplished by surface-based 4D seismic imaging (4 time-lapse images have been produced since injection began) and PND logging in the observation well.
4D seismic maps the gross-scale outline of the growing CO2 plume in the sandstone and brine of the Deadwood formation, whereas PND logging in the observation well provides a detailed vertical profile of CO2 saturation at the wellbore.
In mid-December, PTRC completed a crosswell survey at the Aquistore site, which offers another way of imaging the CO2 in the reservoir, but with much higher resolution. There are two wells at Aquistore – an injector and a monitoring well, located about 150 metres away from each other – and a baseline crosswell survey was done prior to the start of injection in March of 2013.
“Crosswell seismic provides the only means of obtaining detailed tomographic images of the CO2 plume in the 150 m between the injection and observation wells,” notes Dr. Don White from the Geological Survey of Canada and a member of the Aquistore Science and Engineering Research Committee. “A crosswell survey will potentially delineate variations in CO2 saturation within this zone on the scale of metres.”
A detailed understanding of how CO2 displaces brine in situ is essential for estimating storage efficiency factors, which ultimately dictate actual storage capacity in the Deadwood formation. Interpreting the data from the survey has just begun, with findings expected this summer.