Effective CO2 sequestration monitoring using joint inversion result of seismic and electromagnetic data

Abstract:

Man-made carbon dioxide (CO₂) released into the atmosphere is a significant contributor to the greenhouse gas effect and related global warming. Sequestration of CO₂ into saline aquifers has been proposed as one of the most practical options of all geological sequestration possibilities. During CO₂ geological sequestration, monitoring is indispensable to delineate the change of CO₂ saturation and migration of CO₂ in the subsurface. Especially, monitoring of CO₂ saturation in aquifers provides useful information for determining amount of injected CO₂. Seismic inversion can provide the migration of CO₂ plume with high resolution because velocity is reduced when CO₂ replaces the pore fluid during CO₂ injection. However, the estimation of CO₂ saturation using the seismic method is difficult due to the lower sensitivity of the velocity to the saturation when the CO₂ saturation up to 20%. On the other hand, marine controlled-source EM (mCSEM) inversion is sensitive to the resistivity changes resulting from variations in CO₂ saturation, even though it has poor resolution than seismic method. In this study, we proposed an effective CO₂ sequestration monitoring method using joint inversion of seismic and mCSEM data based on a cross-gradient constraint. The method was tested with realistic CO₂ injection models in a deep brine aquifer beneath a shallow sea which is selected with consideration for the access convenience for the installation of source and receiver and an environmental safety. Resistivity images of CO₂ plume by the proposed method for different CO₂ injection stages have been significantly improved over those obtained from individual EM inversion. In addition, we could estimate a reliable CO₂ saturation by rock physics model (RPM) using the P-wave velocity and the improved resistivity. The proposed method is a basis of three-dimensional estimation of reservoir parameters such as porosity and fluid saturation, and the method can be also applied for detecting a reservoir and calculating the accurate oil and gas reserves.