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 $(3–10m)$ 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 $(Gz)$ is correlated directly with the net change in reservoir density. Furthermore, time-lapse changes in the borehole $Gz$ clearly identify the vertical section of the reservoir where fluid saturations are changing. The $CO2$-brine front, on the order of $1km$ within a $20-m$-thick brine formation at $1900-m$ depth with 30% $CO2$ and 70% brine saturations, respectively, produced a $−10-μGal$ 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.