The capture and geological storage of CO2 is currently being investigated as a means of reducing CO2 emissions into the atmosphere. Before it can be accepted, the fate of the injected CO2 must be understood. Controlled Source Electro-Magnetic (CSEM) surveying could be used to monitor CO2 injection and migration, since the presence of CO2 causes resistivity within the reservoir to change. 1D EM modeling of potential storage sites can indicate whether CSEM monitoring would be viable. By altering the CO2 saturations in the models a range of CO2 storage scenarios can be investigated. The modeling shows that the changes in resistivity caused by the presence of CO2 are detectable by CSEM in some scenarios. However the vertical distribution of the CO2 plays a major role in determining the amount of change in the CSEM response.
CSEM surveying (Figure 1) is a powerful geophysical tool for mapping electrical resistivity in geological structures beneath the sea floor. It is increasingly being used to investigate regions of potential economic or industrial interest. CSEM imaging exploits the variation in resistivity between brine saturated sediments and gas/oil/hydrate saturated sediments, to give a better understanding of the pore fluid and water saturation. Therefore EM methods may be ideally suited to the study of CO2 storage. Geophysical methods used to monitor the migration and leakage of CO2 at current Carbon Capture and Storage (CCS) sites include seismic, gravity and borehole monitoring. Although CSEM is not currently used, it has potential advantages over other geophysical survey methods. In particular, CSEM offers the possibility of detecting and quantifying relatively low levels of CO2, information that is needed for early warning of CO2 leakage and long term site management. Unlike seismic velocity, formation resistivity is known to be sensitive to brine saturation (Sw) over a wide range of Sw. The high resistivity of immiscible CO2 (whether in its gaseous, liquid or supercritical state) means that low concentrations should be detectable; while the sensitivity of CSEM responses to formation resistivity should enable quantitative estimates of CO2 saturation. This study uses 1D forward modeling to determine the change in CSEM response at possible storage sites when CO2 is injected.
In order to determine whether a resistive target can be resolved using CSEM we compare 1D model responses from background and target models. A resistivity log is used to make a block input model for the background resistivities. Archie’s equation is then used to alter the resistivity in the section where CO2 is to be present to create a target model. The presence of CO2 within the sediments causes the bulk resistivity to increase. The amplitude of the responses are calculated for both the target and background model, and then the percentage difference between the models is determined. The responses are calculated at a range of frequencies and the frequency which produces the largest percentage difference is used. A percentage difference of at least 10% between the background and target models is required for CSEM to resolve a target.