Breakthroughs in Seismic Imaging of Basalts for Sequestration of Man-Made CO2

Basalt flows up to 5,000 m thick cover 168,000 sq km in the northwestern U.S. Brecciated tops and bases of individual flows form regional aquifers, and are potential sites for sequestration of gigatons of CO2 in areas where the basalts contain unpotable water and are at depths greater than 800 m. In laboratory experiments, these basalts react with formation water and supercritical CO2 to precipitate carbonates, thus adding a potential mineral trapping mechanism to the standard dissolution and hydrodynamic trapping mechanisms of most other types of CO2 sequestration reservoirs. The DOE's Big Sky Regional Carbon Sequestration Partnership has proposed a field test of capacity, integrity, and geochemical reactivity of basalt reservoirs near Wallula, Washington, and has begun surface and subsurface site characterization in preparation for drilling a characterization/injection test well.

Acquisition of surface-based seismic data is important in subsurface characterization. Seismic data are highly desirable for determining subsurface suitability of a proposed site, building conceptual and numerical reservoir models, and for time-lapse monitoring of injected CO2. Traditional surface seismic methods have had little success in obtaining usable data within basalt layers, as a consequence of severe energy scattering and interferring wave modes that degrade image quality or misplace geologic features. An innovative multicomponent (3C) 2D seismic swath experiment at the Wallula site is providing a long-sought breakthrough in improving seismic signal/noise and in imaging subsurface basalt geology. Processing of the 6.5 km, 5 line 3C swath included building an elastic wavefield model and identifying and separating seismic wave modes. The resulting P-P seismic images demonstrate a succession of unfaulted basalt layers suitable for wellbore characterization and testing. Our 3C2D seismic experiment produced successful surface-based seismic imaging of intra-basalt geology and provides a critical key for development of seismic technologies for sequestration of greenhouse CO2 in basalts.