Self-Potential Monitoring for Geologic CO2 Sequestration

To appraise the utility of geophysical techniques for monitoring CO2 injected into aquifers, we carried out numerical simulations of an aquifer system underlying a portion of Tokyo Bay and calculated the temporal changes in geophysical observables caused by changing underground conditions as computed by the reservoir simulation. We used the STAR general-purpose reservoir simulator with the CO2SQS equation-of-state package (Pritchett, 2005) which treats three fluid phases (liquid- and gaseous-phase CO2 and an aqueous liquid phase) to calculate the evolution of reservoir conditions, and then used various “geophysical postprocessors” to calculate the resulting temporal changes in the earth-surface distributions of microgravity, apparent resistivity (from either DC or MT surveys), seismic observables and electrical self-potential (SP). The applicability of any particular method is likely to be highly site-specific, but these calculations indicate that none of these techniques should be ruled out altogether. In case of SP, CO2 injection does not create large electric signals through electrokinetic coupling within the saline aquifer owing to small coupling coefficients under the high salinity conditions. However, if a substantial pressure disturbance is induced to shallower levels where the interface between shallower fresh- and deeper saline-waters (which works as the boundary between regions of differing streaming potential coefficient) is present, obvious SP changes can appear on the ground surface. Continuous and/or repeat SP measurements are thought to be a promising geophysical technique to monitor pressure changes in shallower levels than the saline aquifer where CO2 is injected. In addition to SP measurements in a relatively wide area like covering the horizontal extent of CO2 plume, SP monitoring in a local area around a deep well is thought to be worthwhile from a different angle. SP anomalies of negative polarity are frequently observed near deep wells. These anomalies appear to be caused by an underground electrochemical mechanism similar to a galvanic cell: the metallic well casing acts as a vertical electronic conductor connecting regions of differing redox potential. Electrons flow upward though the casing from a deeper reducing environment to a shallower oxidizing environment, and simultaneously a compensating vertical flow of ions is induced in the surrounding formation to maintain charge neutrality. If the redox potential in the deeper region is then increased by injecting an oxidizing substance, the difference in redox potential between the shallower and deeper regions will be reduced, resulting in an SP increase near the wellhead. We have been monitoring earth-surface SP during gas injection tests at various sites in Japan. When air was injected into a 100-meter well within the Sumikawa geothermal field, a remarkable simultaneous increase in SP centered on the wellhead was observed. A small but unmistakable SP increase also took place near the wellhead when CO2 was slowly injected, which we believe was caused by local pH reduction at depth resulting from dissolution of the injected CO2 in the aquifer fluid.