Lawrence Berkeley National Laboratory

As part of a small scale sequestration test (about 1500 tons of CO2) in a saline aquifer, time-lapse borehole seismic surveys were conducted to aid in characterization of subsurface CO2 distribution and material property changes induced by the injected CO2. A VSP survey demonstrated a large increase (about 75 percent) in seismic reflectivity due to CO2 injection and allowed estimation of the spatial extent of CO2 induced changes. A crosswell survey imaged a large seismic velocity decrease (up to 500 m/s) within the injection interval and provided a high resolution image of this velocity change which maps the subsurface distribution of CO2 between two wells. Numerical modeling of the seismic response uses the crosswell measurements to show that this small CO2 volume causes a large response in the seismic reflectivity. This result demonstrates that seismic detection of small CO2 volumes in saline aquifers is feasible and realistic.