Time-Lapse Active Source Seismic Characterization of a Leaky CO\u3csub\u3e2\u3c/sub\u3e Reservoir: Little Grand Wash Fault, Utah

A carbon capture and sequestration (CCS) approach requires economical methods to monitor reservoir CO2 flow paths through time. I explore the use of an inexpensive surface seismic approach to monitor the time-varying response of a leaky CO2 reservoir. My site is located in east central Utah, where the Little Grand Wash fault provides a natural analogue for a failed sequestration site. This fault and related anticlinal trap provides a conduit to collect and deliver CO2 from shallow reservoir depths to the atmosphere. Elevated soil CO2 flux measurements, outgassing at the Crystal Geyser, and travertine deposits provide the surface expression of CO2 seeps along and near the fault. Borehole and past geophysical data provide a structural and stratigraphic framework for the site. Through historic and new water temperature data, I identify and characterize eruption cycles at the Crystal Geyser. I show that the frequency and duration of eruptions changes through time, and I observe an overall increase in eruption duration. With a new seismic monitoring approach, I show that a surface-based accelerated weight drop source into a stationary geophone spread is repeatable and appropriate for time-lapse seismic studies to monitor reservoir changes. I show repeated surface and body wave measurements with a 30-hour time-lapse dataset. I model seismic velocity changes with changing CO2 saturation within the main Navajo Sandstone reservoir. My models show that during initial saturations, seismically resolvable reservoir changes are possible to monitor. However, I show that a critically saturated reservoir, like that along the Little Grand Wash fault, shows travel time or amplitude changes that are below the resolving capabilities of my surface-based seismic system. While my surface based seismic approach is not appropriate for monitoring CO2 changes at my field site, this same approach could be used to monitor CO2 changes during initial CCS injection where a larger seismic response would be expected