Breakthroughs in seismic and borehole characterization of Basalt sequestration targets

Abstract

Abstract Mafic continental flood basalts form a globally important, but under-characterized CO 2 sequestration target. The Columbia River Basalt Group (CRBG) in the northwestern U.S. is up to 5 km thick and covers over 168,000 km 2 . In India, flood basalts are 3 km thick and cover greater than 500,000 km 2 . Laboratory experiments demonstrate that the CRBG and other basalts react with formation water and super critical (sc) CO 2 to precipitate carbonates, thus adding a potential mineral trapping mechanism to the standard trapping mechanisms of most other types of CO 2 sequestration reservoirs. Brecciated tops of individual basalt flows in the CRBG form regional aquifers that locally have greater than 30% porosity and three Darcies of permeability. Porous flow tops are potential sites for sequestration of gigatons of sc CO 2 in areas where the basalts contain unpotable water and are at depths greater than 800 m. In this paper we report on the U.S. DOE Big Sky Regional Carbon Sequestration Partnership surface seismic and borehole geophysical characterization that supports a field test of capacity, integrity, and geochemical reactivity of CRBG reservoirs in eastern Washington, U.S.A. Traditional surface seismic methods have had little success in imaging basalt features in on-shore areas where the basalt is thinly covered by sediment. Processing of the experimental 6.5 km, 5 line 3C seismic swath included constructing an elastic wavefield model, identifying and separating seismic wave modes, and processing the swath as a single 2D line. Important findings include: (1) a wide variety of shear wave energy modes swamp the P-wave seismic records; (2) except at very short geophone offsets, ground roll overprints P-wave signal; and (3) because of extreme velocity contrasts, P-wave events are refracted at incidence angles greater than 7–15 degrees. Subsequent removal of S-wave and other noise during processing resulted in tremendous improvement in image quality. The application of wireline logging to onshore basalts is underexploited. Full waveform sonic logs and resistivity-based image logs acquired in the 1250 m basalt pilot borehole provide powerful tools for evaluating geomechanics and lithofacies. The azimuth of the fast shear wave is parallel to S H and records the changes through geologic time in basalt flow and tectonic stress tensors. Combined with image log data, azimuthal S-wave data provide a borehole technique for assessing basalt emplacement and cooling history that is related to the development of reservoirs and seals, as well as the orientation of tectonic stresses and fracture systems that could affect CO 2 transport or containment. Reservoir and seal properties are controlled by basalt lithofacies, and rescaled P- and S-wave slowness curves, integrated with image logs, provide a tool for improved recognition of subsurface lithofacies.