Lawrence Berkeley National Laboratory

The In Salah CO2 storage project in Algeria has injected over 3 million tonnes of carbon dioxide into a water filled tight sand formation. Interferometric Synthetic Aperture Radar (InSAR) range change data revealed a double lobbed pattern of deformation that has been modeled as the opening of a sub-vertical fracture, or damage, zone. The location and geometry of the linear feature were subsequently verified using a seismic reflection survey. The elastic properties of the fracture zone, including anisotropic Poisson ratio (ν), Young's (E) and shear (G) moduli, were estimated from coupled geomechanical and hydrological modeling of surface deformation and pressure variations in the injection well. The elastic moduli reflect the fracture properties after CO2 flow through the fracture zone. Thus, the seismic signature of the fracture zone could be used for monitoring the CO2 plume. Using the estimated fracture model, we built two and three dimensional models consisting of an anisotropic fracture zone embedded within an isotropic background. Finite-difference modeling of seismic shot gathers allows us to estimate the effects of scattering from the fracture zone, potentially further constraining the geomechanical model. From the seismic modeling results, we find diffracted waves, induced by the fracture zone, which behave similar to point source diffractions. This modeling is intended to guide a search for diffraction events in the 3D surface seismic field data. The modeling results indicate that using the moduli estimated from geomechanical modeling, fracture scattered events would be 100 times lower amplitude than the interface reflections, and thus would be hard to detect. While diffracted waves are observed in the field data, which may imply the need for revision of the fracture model, including shape and elastic moduli, we are not able to match the field observation with our modeled events. This work presents a frontier study on the integration of geomechanical and geophysical methods at the In Salah site as a means to test the estimate of the subsurface CO2 flooded fracture properties.