Borehole Gravity Application to CO2 Sequestration

As of March 2009 Borehole Gravity surveys have not been used in CO2 sequestration projects. However the tool has attractive properties for monitoring the progress of CO2 injection and the chemical changes expected to occur:

The tool can measure changes of the density of large rock volumes over time. In heterogeneous carbonate reservoirs BHGM measurements are more representative of the average formation porosity properties than measurements limited to a few inches radius from the well.

The measurements can be made through casing.

The bulk rock density of a target CO2 sequestration host rock is expected to first be lowered by the CO2 gas displacing ground water. A complex series of reactions between the CO2, ground water and host rock minerals is then expected to result in changes in fluid densities and porosity. Borehole gravity could be used to monitor these changes in host rock bulk density over time through casing.

Repeat Borehole Gravity measurements immediately above the top of the host rock will change as the radius of the CO2 volume expands with continuing injection. This should allow one to differentiate between situations where the CO2 is conducted away from the well via high permeability fractures or expands as a uniform disc shaped plume.

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