Experimental development of low-frequency shear modulus measurements during flow-through CO2 induced dissolution

Time-lapse seismic monitoring is currently the primary technique available for tracking sequestered CO2 in a geologic storage reservoir away from monitoring wells. However, most work to date, in both the field and laboratory, has focused on seismic imaging of the super-critical (sc)CO2 plume as it displaces brine; rather than later phases of storage including CO2 dissolution and geochemical interactions with the rock matrix. Since GCS monitoring requires an understanding of CO2 state from injection to final storage form, investigation of the signature of later phases of interaction is useful when constraining storage security as well as evaluating reactive transport predictions. Particularly in the case of CO2-induced matrix dissolution processes, the scarcity of existing laboratory seismic datasets makes quantitative interpretation of acquired field monitoring datasets problematic. The small experimental database is also limited by the lack of low-frequency datasets, a relevant requirement since modulus changes induced by dissolution may have a strong frequency dependence due to changes in micro-crack morphology. Previous work (e.g. Vialle and Vanorio, 2011) has shown that chemical dissolution at grain contacts significantly impacts the elastic properties of reservoir rocks and creates a potentially measurable seismic signature for CO2-induced dissolution in carbonates. Since grain contacts are a key micro feature in determining the stiffness of the rock matrix, the initial dissolution appears to cause the largest change in modulus. We hypothesize that flow structures that allow the largest volume of rock to experience this initial dissolution will have the greatest softening. In this study, we present a new experimental approach for measuring changes in the low frequency seismic response during CO2-induced dissolution. Using a novel laboratory instrument (Wanamaker and Bonner, 1991) adapted with a flow-through apparatus, we measure changes in low-frequency (1-100 Hz) shear modulus and attenuation while CO2 saturated water dissolves a carbonate sample. An equivalent flow experiment was conducted utilizing x-ray micro tomography (Beamline 8.3.2, Advanced Light Source, LBNL) to image the evolution of grain morphology and geometry under similar injection conditions. These methods and results will be incorporated into analysis of planned field scale seismic monitoring experiments with the eventual goal of refining reactive transport models at GCS sites through dynamic seismic imaging of zones with measurable matrix dissolution.