Geophysics plays two important roles in geological storage of CO2. Firstly, geophysics is used to map the underground formations that the CO2 will be injected into, allowing quantitative estimates of the amount of CO2 that can be stored at a site to be made. Secondly, it is used for monitoring the site, for example for detecting leakage from the underground to the atmosphere, during and after injection. Reflection seismic imaging is the best geophysical method for mapping the CO2 reservoir while other methods also play an important role in monitoring the injected CO2. Injection of CO2 into the reservoir will reduce the gas content of the pore fluid and increase the pressure in the reservoir, thus, lowering the seismic velocity of the reservoir formation. Experience from large scale injection of CO2 at Sleipner, Norway, shows that the CO2 distribution in the injection reservoir can be mapped using active source 3D time-lapse seismic methods from the surface. Even small amounts of CO2 can be mapped in the injection reservoir if borehole seismic methods are used.
3D surface seismic surveys highly effective in mapping the spatial extent of the CO2 in the injection reservoir, but are expensive and time consuming to perform, especially on land, and borehole methods only provide data in the vicinity of the boreholes. Cheaper and faster methods are required for monitoring large volumes. Three such options for monitoring that are under study at the Geophysics program are (1) using the micro-earthquakes that may be generated as CO2 is injected, (2) the use of ambient noise for detecting unexpected changes in the sub-surface conditions, and (3) installation of semi-permanent receivers and seismic sources at key locations. Monitoring of micro-earthquakes in the injection reservoir to track the CO2 gives the added benefit of that the caprock can be monitored simultaneously on the same seismic network. The first two monitoring strategies are passive seismic monitoring methods, implying no sources are required, and, therefore, relatively inexpensive once the network has been installed. However, significant research is needed to determine the viability of the methods, especially the use of ambient noise for sub-surface monitoring.
Contact: Chris Juhlin