3D VSP anisotropic inversion for the Farnsworth CO2-EOR field

Abstract:

During Phase III of the U.S. Southwest Regional Partnership on Carbon Sequestration project, supercritical CO₂ has been injected into the deep oil-bearing Morrow Formation of the Farnsworth Unit in Texas for enhanced oil recovery (EOR). One baseline and three repeat 3D-3C vertical seismic profiling (VSP) surveys using well 13-10A were conducted from 2014 to 2017 for monitoring CO₂ injection and migration. The Farnsworth subsurface geologic formations are mostly vertical transversely isotropic, particularly in the deep regions around the Morrow Formation reservoir. To accurately characterize the anisotropic medium properties of the Farnsworth field, we conduct 3D anisotropic traveltime tomography and 3D anisotropic elastic-waveform inversion of the baseline VSP data to reveal subsurface spatial variations of anisotropic elastic parameters. We first build the 1D initial anisotropic models by upscaling well logs using the Schoenberg-Muir method within layers divided according to P-wave impedance. We then improve the shallow region of the model using 3D anisotropic traveltime tomography of downgoing waves in the VSP data. Finally, we perform 3D anisotropic elastic-waveform inversion of the baseline VSP data to obtain high-resolution models of P- and S-wave velocities, and three Thomsen anisotropic parameters. Our inversion result reveals spatially varying azimuth anisotropy around well 13-10A, which may be associated with the horizontal principal stress directions.