Seismic Monitoring at the Farnsworth CO 2 -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 CO 2 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 CO 2 to study geologic carbon storage during CO 2 -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 CO 2 injection and oil/gas production wells between 2014 and 2017 for monitoring CO 2 injection and migration. With these VSP datasets, we inverted for subsurface velocity models to quantitatively monitor the CO 2 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 CO 2 plume from the CO 2 injection well to the oil/gas production wells.