Geophysical Monitoring for the Frio Pilot CO2 Injection Test

ABSTRACT The Frio Pilot test involves injection of approximately 3000 tons of CO2 into the brine-saturated Frio formation at a depth of approximately 1500 m at a test site located northeast of Houston. Interpretation of 3-D seismic coupled with petrophysical analyses and other geologic data showed that the test site is located in a small fault block off the flank of a salt dome. The CO2 is injected into a 10 m thick sand layer in an interval of alternating sand and shale layers overlain by the 75 m thick Anahuac shale. Well logs in the new well provide data to confirm test site stratigraphy as well as data needed for interpretation of geophysical monitoring measurements. Geophysical monitoring, which is augmented by hydrologic pressure measurements and geochemical sampling, involves time-lapse measurements, incorporating both surface and borehole techniques. A vertical seismic profiling (VSP) survey was designed for both monitoring and imaging the structure in the injection volume, and involved 8 explosive shot points at 100 - 1500 m offsets. An 80 level receiver string with 240 3-component sensors was used. Crosswell surveys involved P- and S-wave seismic and electromagnetic (EM) measurements (between steel-cased wells) at 1.5 m spacing over a 75 m interval. EM measurements were at 50 and 80Hz, and an orbital-vibrator seismic source provided seismic data in the 150Hz frequency range. Joint interpretation of crosswell seismic and EM with appropriate rock physics models can potentially provide quantitative information on CO2 saturation between boreholes.