A Rock Physics Feasibility Workflow for 4D Seismic Monitoring of Injected CO2

Summary CCS initiatives require a program of monitoring, verifying, and accounting of injected CO2. 4D seismic reflection surveys can be used to monitor the injected CO2 over the lifespan of the project. As with traditional seismic interpretation for oil and gas exploration, seismic reflectivity data pose the problem of non-uniqueness, thus multiple forward models need to be built, first to model if there would be sufficient 4D seismic signal to make time-lapse seismic monitoring effective, and subsequently, once multiple time lapse surveys are acquired, multiple forward models should be generated to understand uncertainty around the observed signal. This work presents a rock physics modelling workflow, built on traditional quantitative interpretation techniques, to model the predicted 4D seismic signal for different CO2 injection scenarios and time steps. Here we present the Vernik-Kachanov rock physics model, which places both pores and microcracks into the average effective stress field in the solid matrix of the material, allowing for the stress sensitivity component of the rock physics model. We then account for fluid changes using Biot-Gassmann theory and published empirical and semi-theoretical fluid models for CO2-pore fluid mixing. Finally, synthetic seismic models are built using Zoeppritz equations for reflectivity and amplitude changes and time-thicknesses estimated.