Effects of CO ₂ phase state on the seismological properties of porous materials: implications for seismic monitoring of volcanic hazards and sequestered carbon

Abstract

Time-lapse seismic monitoring is a key component in the geological sequestration of greenhouse gases. Currently, a lack of understanding of the influence of injected CO 2 on rocks’ geophysical signatures is reported. Using a novel ultrasonic experimental system, we measured the variations in the longitudinal (P) and transverse (S) elastic wave speeds of a CO 2 -saturated porous medium at different pressure and temperature conditions around the CO 2 critical point. The results show that both P- and S-wave speeds, corrected to seismic frequencies, decreased by more than 4.5% across the CO 2 gas to liquid and gas to supercritical states, primarily as a consequence of CO 2 density increase. This study provides a-20 firm basis for the use of seismological methods in monitoring sequestered CO 2 ; although the abruptness, and hence remote seismic detectability, depends on which phase boundary is crossed. Further, these measurements also allow us to compare observations to frequency-dependent wave propagation theory. The observed wave speeds mostly align with those calculated at 1 MHz using the Biot theory for both CO 2 and H 2 O 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 CO 2 provides a highly tunable fluid that can be advantageous for experimental studies.

Joint Inversion of Geophysical Data for Geologic Carbon Sequestration Monitoring: A Differentiable Physics‐Informed Neural Network Model

“…In this case, the value 0.4 is selected as the threshold based on visual inspection of the results. The lack of sensitivity of the time‐lapse seismic response to fluid changes is also observed in other studies (Gasperikova et al., 2022; Schmitt et al., 2022). Figure 9 shows the inverted porosity and CO 2 saturations obtained using seismic data alone.…”

“…Time‐lapse (4‐D) seismic acquisition is one of the monitoring techniques widely deployed in GCS projects, where a series of seismic surveys are sequentially acquired to quantify changes in reservoir properties (e.g., elastic properties, fluid pressure and saturation) during and after CO 2 injection (Caspari et al., 2011; A. Chadwick et al., 2010; Egorov et al., 2017; Glubokovskikh et al., 2020; Grude et al., 2013; B. Li & Li, 2021; Rubino et al., 2011). However, in some cases, depending on the rock stiffness, the time‐lapse seismic response might be insensitive to fluid changes (Gasperikova et al., 2022; Schmitt et al., 2022). To address this issue, non‐seismic monitoring techniques have been investigated and successfully applied to GCS.…”

Geostatistical Inversion for Subsurface Characterization Using Stein Variational Gradient Descent with Autoencoder Neural Network: An Application to Geologic Carbon Sequestration

“…We assume that fluid properties remain constant and do not vary with reservoir temperature and pressure. However, for more reliable inversion results, it is recommended to develop a dedicated rock physics model that incorporates the effects of pressure and temperature on fluids, particularly for dynamic CO 2 monitoring (Schmitt et al, 2022).…”

Geostatistical Inversion for Subsurface Characterization Using Stein Variational Gradient Descent With Autoencoder Neural Network: An Application to Geologic Carbon Sequestration

“…We assume that fluid properties remain constant and do not vary with reservoir temperature and pressure. However, for more reliable inversion results, it is recommended to develop a dedicated rock physics model that incorporates the effects of pressure and temperature on fluids, particularly for dynamic CO 2 monitoring (Schmitt et al., 2022).…”