[1] Understanding the elastic behavior of basalt is important to seismically monitor volcanoes, subsea basalts, and carbon sequestration in basalt. We estimate the elastic properties of basalt samples from the Snake River Plain, Idaho, at ultrasonic (0.8 MHz) and seismic (2-300 Hz) frequencies. To test the sensitivity of seismic waves to the fluid content in the pore structure, measurements are performed at three saturation conditions: saturated with liquid CO 2 , water, and dry. When CO 2 replaces water, the P-wave velocity drops, on average, by 10%. Vesicles and cracks, observed in the rock microstructure, control the relaxation of pore-fluid pressures in the rock as a wave propagates. The bulk and shear moduli of basalts saturated with liquid CO 2 are not frequency dependent, suggesting that fluid pore pressures are in equilibrium between 2 Hz and 0.8 MHz. However, when samples are water saturated, the bulk modulus of the rock is frequency dependent. Modeling with Gassmann's equations predicts the measured saturated rock bulk modulus for all fluids for frequencies below 20 Hz but underpredicts the water-saturated basalt bulk modulus for frequencies greater than 20 Hz. The most likely reason is that the pore-fluid pressures are unrelaxed. Instead, the ultrasonic frequency rock moduli are modeled with high-frequency elastic theories of squirt flow and Kuster-Toksöz (KT). Although KT's model is based on idealized pore shapes, a combination of spheres (vesicles) and penny-shaped cracks (fractures) interpreted and quantified from petrographical data predicts the ultrasonic dry and saturated rock moduli for the measured basalts.Citation: Adam, L., and T. Otheim (2013), Elastic laboratory measurements and modeling of saturated basalts, J. Geophys.

[1] The chemical interaction between carbon dioxide, water, and basalt is a common process in the earth, which results in the dissolution of primary minerals that later precipitate as alteration minerals. This occurs naturally in volcanic settings, but more recently basalts have been suggested as reservoirs for sequestration of anthropogenic CO 2 . In both the natural and man-made cases, rock-fluid reactions lead to the precipitation of carbonates. Here, we quantify changes in ultrasonic wave speeds, associated with changes in the frame of whole-rock basalts, as CO 2 and basalt react. After 30 weeks of reactions and carbonate precipitation, the ultrasonic wave speed in dry basalt samples increases between 4% and 20% and permeability is reduced by up to an order of magnitude. However, porosity decreases only by 2% to 3%. The correlation between significant changes in wave speed and permeability indicates that a precipitate is developing in fractures and compliant pores. Thin sections, XRF-loss on ignition, and water chemistry confirm this observation. This means time-lapse seismic monitoring of a CO 2 -water-basalt system cannot assume invariance of the rock frame, as typically done in fluid substitution models. We conclude that secondary mineral precipitation causes a measurable change in the velocities of elastic waves in basalt-water-CO 2 systems, suggesting that seismic waves could be used to remotely monitor future CO 2 injection sites. Although monitoring these reactions in the field with seismic waves might be complicated due to the heterogeneous nature of basalt, quantifying the elastic velocity changes associated with rock alteration in a controlled laboratory experiment forms an important step toward field-scale seismic monitoring.Citation: Adam, L., K. van Wijk, T. Otheim, and M. Batzle (2013), Changes in elastic wave velocity and rock microstructure due to basalt-CO 2 -water reactions,

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Geological sequestration of carbon dioxide in deep reservoirs may provide a large-scale option for reducing the emissions of this gas into the atmosphere. The effectiveness of sequestration depends on the storage capacity and stability of the reservoir and risk of leakage into the overburden. Reservoir rocks can react with a [Formula: see text]-water mixture, potentially resulting in the precipitation of minerals in the available matrix pore space and within pre-existing fractures. This induced mineralization may form internal seals that could help mitigate the leakage of [Formula: see text] into the overburden. For basaltic host rocks, carbonic acid partially dissolves minerals in the host rock, such as the calcium plagioclase mineral, freeing various cations (e.g., [Formula: see text] and [Formula: see text]) for later precipitation as carbonate cements (Gislason et al., 2010).