Geophysical monitoring of subsurface CO2

Abstract

Geological storage of CO2 will require accurate, high-resolution geophysical monitoring to map the subsurface flow paths and the phase state of the fluids. Existing and potential monitoring methods include seismic (both surface and borehole), electromagnetics, gravity, and well logging. Seismic methods are expected be the main form of monitoring as it is cost effective and covers the whole area of interest. When used in a time-lapse sense, areal mapping of the injected CO2 is possible with existing technology. However, improvements in resolution are required to detect leakage and identify preferential flow paths. Multi-component seismic acquisition may be necessary to discriminate between changes in saturation and pressure within the CO2 reservoir. Passive monitoring of induced microseismic events may prove to be an effective way to evaluate the effects from the injection of CO2 on the integrity of the cap rock and provide early warning of fracturing or fault reactivation. Supporting geophysical methods will be needed for accurate quantitative interpretation of the seismic data. The geological setting and the availability of wells and pre-existing baseline data will have a profound influence on the choice of any additional geophysical methods. Electromagnetic methods have lower resolution than seismic, but are far more sensitive to changes in fluid saturation and may detect anomalies that are too subtle to interpret from seismic data alone. Gravity surveys are relatively cheap to acquire on land, but improvements in measurement sensitivity and constrained inversion methods are required before gravity can be of value for monitoring subsurface CO2 in the supercritical state.