Geophysical Signatures to Monitor Fluids and Mineralization for CO2 Sequestration in Basalts

Carbon dioxide sequestration in large reservoirs can reduce emissions of this green house gas into the atmosphere. Basalts are promising host rocks due to their volumetric extend, worldwide distribution, and recent observations that CO₂-water mixtures react with basalt minerals to precipitate as carbonate minerals, trapping the CO₂. The chemical reaction between carbonic acid and minerals rich in calcium, magnesium and iron precipitates carbonates in the pore space. This process would increase the elastic modulus and velocity of the rock. At the same time, the higher compressibility of CO₂ over water changes the elastic properties of the rock, decreasing the saturated rock bulk modulus and the P-wave velocity. Reservoirs where the rock properties change as a result of fluid or pressure changes are commonly monitored with seismic methods. Here we present experiments to study the feasibility of monitoring CO₂ migration in a reservoir and CO₂-rock reactions for a sequestration scenario in basalts. Our goal is to measure the rock's elastic response to mineralization with non-contacting ultrasonic lasers, and the effect of fluid substitution at reservoir conditions at seismic and ultrasonic frequencies. For the fluid substitution experiment we observe changes in the P- and S-wave velocities when saturating the sample with super-critical (sc) CO₂, CO₂-water mixtures and water alone for different pore and confining pressures. The bulk modulus of the rock is significantly dependent on frequency in the 2~to 10⁶~Hz range, for CO₂-water mixtures and pure water saturations. Dry and pure CO₂ (sc or gas) do not show a frequency dependence on the modulus. Moreover, the shear wave modulus is not dispersive for either fluid. The frequency dependence of the elastic parameters is related to the attenuation (1/Q) of the rock. We will show the correlation between frequency dependent moduli and attenuation data for the different elastic moduli of the rocks. Three other basalt samples were stored in a pressure chamber with a sc CO₂-water solution to study the effect of mineralization on the elastic properties of the rock. The rock elastic properties are recorded with non-contacting ultrasonic lasers at room conditions. After 15 weeks the first post-mineralization scan showed differences in the rock velocities with respect to the pre-mineralization scan. The analysis is done through coda wave interferometry and direct arrivals. The samples were inserted back into the pressure vessel for continuing mineralization and subsequent scans. Finally, we will discuss the applicability of Gassmann's equation and how the combination of mineralization together with CO₂-water mixture affects the velocity of waves in basalt rocks.