A Numerical Study on the Sensitivity of DAS and Geophone Signals to a Thin CO 2 ${\mathrm{CO}}_{2}$ Plume

Abstract

One of the main concerns in ${\mathrm{CO}}_2$ monitoring during a CCUS (carbon capture, utilization, and storage) project is the detectability of changes in petrophysical properties induced by the substitution of the initial fluids by ${\mathrm{CO}}_2$. Seismic attributes are thought to have the potential to directly track changes in geophysical properties such as wave velocities. The emerging technology of distributed acoustic sensing (DAS), in addition to traditional sensors such as geophones, is bringing new perspectives in seismic acquisitions and attribute analyses. We study the sensitivity of different seismic attributes, for example, the amplitude, the instantaneous phase and frequency, to a ${\mathrm{CO}}_2$ plume with numerical simulations of wave propagation. An efficient viscoelastic wave problem solver featuring the spectral element method and memory variables is built inside the GEOS platform to calculate DAS and geophone responses in the context of a vertical seismic profile (VSP) acquisition. The quantitative comparison between the synthetic VSP records from DAS and geophones without noise shows that the DAS vertical normal strain has a comparable sensitivity to the equivalent geophone vertical velocity. The primary upgoing waves exhibit a particular footprint of the ${\mathrm{CO}}_2$ plume. The results also highlight the contribution of PS conversions to the ${\mathrm{CO}}_2$ detection. It remains a challenge to reach a high numerical resolution on the S-wave.