Problems with CO2 Sequestration
- Potential Leakage and Toxicity Problems with CO2 Sequestration
31 July 2006
Cross-well seismic difference tomogram of the Frio Brine project
shows the CO2 plume.
Results from a field test on CO2 sequestration in an old brine-
filled oil reservoir suggest that the mixture of CO2 and brine
dissolves minerals in the rock walls, including carbonate, that
could lead to pathways in the rock through which the gas could
In a paper published in the July edition of Geology, the researchers
in the Frio Brine Pilot also note the potential for the mobilization
of toxic trace metals and toxic organic compounds.
The Frio Brine Pilot was the first test of closely monitored CO2
injection in a brine formation in the United States, and was funded
by the Department of Energy (DOE) National Energy Technology
Laboratory (NETL) under the leadership of the Bureau of Economic
Geology (BEG) at the Jackson School of Geosciences, The University
of Texas at Austin, with major collaboration from GEO-SEQ, a
national lab consortium led by Lawrence Berkeley National Laboratory
The researchers injected 1,600 metric tons of CO2 1,500 meters down
into a sandstone site representative of a target for large-volume
storage. The sandstones of the Oligocene Frio Formation are part of
a thick, regionally extensive sandstone trend that underlies a
concentration of industrial sources and power plants along the Gulf
Coast of the United States.
Monitoring strategy at Frio.
The team then measured and monitored the CO2 plume using a diverse
suite of technologies in three intervals: the injection zone, the
area above the injection zone, and the shallow near-surface
Each monitoring strategy used a preinjection and one or more
postinjection measurements. Wireline logging, pressure and
temperature measurement, and geochemical sampling were also
conducted during injection, and at follow-up intervals subsequent to
While the sequestration to-date has been successfulthere have been
no detected CO2 leakagesthe researchers conclude in their latest
published assessment of on-going findings and analysis that the
chemistry of the process might prove problematic.
Fluid samples obtained from the injection and observation wells
before CO2 injection showed a Na-Ca-Cltype brine with 93,000 mg/L
total dissolved solids (TDS) at near saturation with CH4 at
Following CO2 breakthrough, samples showed sharp drops in pH (6.5
5.7), pronounced increases in alkalinity (1003,000 mg/L as HCO3)
and Fe (301,100 mg/L), and significant shifts in the isotopic
compositions of H2O, dissolved inorganic carbon (DIC), and CH4.
Geochemical modeling indicates that brine pH would have dropped
lower but for the buffering by dissolution of carbonate and iron
This rapid dissolution of carbonate and other minerals could
ultimately create pathways in the rock seals or well cements for CO2
and brine leakage. Dissolution of minerals, especially iron
oxyhydroxides, could mobilize toxic trace metals and, where residual
oil or suitable organics are present, the injected CO2 could also
mobilize toxic organic compounds.
Environmental impacts could be major if large brine volumes with
mobilized toxic metals and organics migrated into potable