Gravity monitoring of CO2 movement during sequestration: Model studies

Sequestration/enhanced oil recovery (EOR) petroleum reservoirs have relatively thin injection intervals with multiple fluid components (oil, hydrocarbon gas, brine, and carbon dioxide, or CO2), whereas brine formations usually have much thicker injection intervals and only two components (brine and CO2). Coal formations undergoing methane extraction tend to be thin (3-10 m) but shallow compared to either EOR or brine formations. Injecting CO2 into an oil reservoir decreases the bulk density in the reservoir. The spatial pattern of the change in the vertical component of gravity (G(z)) is correlated directly with the net change in reservoir density. Furthermore, time-lapse changes in the borehole G(z) clearly identify the vertical section of the reservoir where fluid saturations are changing. The CO2-brine front, on the order of 1 km within a 20-m-thick brine formation at 1900-m depth with 30% CO2 and 70% brine saturations, respectively, produced a -10-mu Gal surface gravity anomaly. Such an anomaly would be detectable in the field. The amount of CO2 in a coal-bed methane scenario did not produce a large enough surface gravity response; however, we would expect that for an industrial-size injection, the surface gravity response would be measurable. Gravity inversions in all three scenarios illustrate that the general position of density changes caused by CO2 can be recovered but not the absolute value of the change. Analysis of the spatial resolution and detectability limits shows that gravity measurements could, under certain circumstances, be used as a lower-cost alternative to seismic measurements.