Sensitivity study of surface waves for CO2 storage monitoring

Abstract

Carbon dioxide capture and geological storage (CCS) is a potential technology to reduce greenhouse gas emission. Suitable techniques are essential for site characterization as well as CO2 injection and storage monitoring. A surface wave seismic method was explored in this study to investigate its feasibility for this purpose. Elastic wave responses of CO2 flooded rock were first investigated numerically in two types of rocks, carbonate rock and sandstone. It is indicated that elastic wave velocities change more significantly as there is greater difference for bulk modulus between the injected CO2 and the existing media in pore spaces. With the wave velocity and density variation ranges estimated from the rock sample study, the sensitivity of surface wave velocity was examined by perturbing parameters of the CO2 storage layer in two layered reservoir models. It is found that the surface waves are more sensitive to the changes of shear wave velocity and thickness of CO2 storage layer; but they are less sensitive to density and compressional wave velocity variations. The fundamental mode of Rayleigh waves is most sensitive to the physical parameter perturbation of the CO2 storage layer for the carbonate case. However, high frequency modes were observed to be more active for shear wave velocity and thickness variation scenarios in the sandstone reservoir simulations. The simulations demonstrate that the monitoring feasibility increases as the CO2 reservoir layer becomes thicker and the bury depth goes shallower. However, with the geological setting parameters found in existing CCS projects, it is concluded to be a challenge to detect abnormalities in a CO2 storage reservoir by comparing the shift of shear wave velocity profiles that are derived from analysis of surface wave response data. It is, therefore, proposed to consider other microtremor attributes during the development of CO2 monitoring techniques based on passive measurement of microseismicity, which is explored by some researchers.