Effects of CO2 Phase State on the Seismological Properties of Porous Materials: Implications for Seismic Monitoring of Volcanic Hazards and Sequestered Carbon

Abstract:

Currently, there is a little understanding of the influence of injected CO₂ on the rock’s geophysical signatures. Using a novel ultrasonic experimental system, we aim to measure the variations in the longitudinal (P) and transverse (S) elastic wave speeds of a CO₂ saturated porous medium at different pressures and temperatures conditions (around the CO₂ critical point). Both P- and S-wave speeds, corrected to seismic frequencies, decreased by more than 4.5% across the CO₂ gas-to-liquid and gas-to-supercritical state primarily consequently to the CO₂ density increase.

This study provides a firm basis for the use of seismological methods in monitoring sequestered CO₂ although the abruptness, and hence remote seismic detectability, depends on which phase boundary is crossed. The experiments show that over rather modest pressure and temperature ranges one can ‘tune’ the CO₂ fluid properties within the limits, of its thermodynamic behavior. Use of CO₂ as a saturant may offer a way to vary fluid properties in their measurements although care must be taken to ensure that equilibrium conditions have been attained. Further, these measurements also allow to compare observations to frequency dependent wave propagation theory. The observed wave speeds mostly agree well with those calculated at 1 MHz using the Biot theory for both CO₂ and H₂O saturated states. However, the observed and calculated wave speeds diverge above the phase transition, in some of the tests, possibly due to the kinetics of the phase transition within a porous medium. As such, aside from the direct utility in providing information on the expected seismic responses, the CO₂ provides a highly tunable fluid that can be advantageous for experimental studies.