Seismic Monitoring at the Farnsworth CO₂-EOR Field Using Time-Lapse Elastic-Waveform Inversion of 3D-3C VSP Data

During the Development Phase of the U.S. Southwest Regional Partnership on Carbon Sequestration, supercritical CO2 was continuously injected into the deep oil-bearing Morrow B formation of the Farnsworth Unit in Texas for Enhanced Oil Recovery (EOR). The project injected approximately 94 kilotons of CO2 to study geologic carbon storage during CO2-EOR. A three-dimensional (3D) surface seismic dataset was acquired in 2013 to characterize the subsurface structures of the Farnsworth site. Following this data acquisition, the baseline and three time-lapse three-dimensional three-component (3D-3C) vertical seismic profiling (VSP) data were acquired at a narrower surface area surrounding the CO2 injection and oil/gas production wells between 2014 and 2017 for monitoring CO2 injection and migration. With these VSP datasets, we inverted for subsurface velocity models to quantitatively monitor the CO2 plume within the Morrow B formation. We first built 1D initial P-wave (V-p) and S-wave (V-s) velocity models by upscaling the sonic logs. We improved the deep region of the V-p and V-s models by incorporating the deep part of a migration velocity model derived from the 3D surface seismic data. We improved the shallow region of 3D V-p and V-s models using 3D traveltime tomography of first arrivals of VSP downgoing waves. We further improved the 3D baseline velocity models using elastic-waveform inversion (EWI) of the 3D baseline VSP upgoing data. Our advanced EWI method employs alternative tomographic and conventional gradients and total-variation-based regularization to ensure the high-fidelity updates of the 3D baseline V-p and V-s models. We then sequentially applied our 3D EWI method to the three time-lapse datasets to invert for spatiotemporal changes of V-p and V-s in the reservoir. Our inversion results reveal the volumetric changes of the time-lapse V-p and V-s models and show the evolution of the CO2 plume from the CO2 injection well to the oil/gas production wells.