Carbon sequestration is not uncommon in the oil industry, with the gas pumped down into reservoirs to raise the oil level and improve recovery rates.
With global greenhouse gas emissions becoming an increasing concern for scientists and governments, the technique is hoped to reduce the effects of CO2 emissions by capturing and storing them underground so that they cannot enter the atmosphere.
However, the US Department of Energy is investigating the potential of sequestration in other geological environments.
The DOE sponsored the Frio Brine Pilot Experiment in Texas, injecting 1600 tonnes of CO2 at a depth of 5053-5073 feet below the surface into a brine-rock system containing sandstone.
The injection zone is contained within a fault-bounded compartment with a top seal of 200ft of Anahuac shale.
The experiment hoped to determine the capacity of the highly porous sand stone to absorb the CO2, but the monitoring program has returned mixed results, which are being used by critics to cast doubt over the entire sequestration process.
A monitoring team from US Geological Survey said although the CO2 has so far stayed in the zone into which it was injected, there are concerns that the CO2 is reacting with the environment, dissolving the minerals that are meant to be storing it.
The survey team reported that the CO2 has turned brine in the formation from a "near-neutral" pH of 6.5 to 3.0, at which point it has become acidic enough to dissolve minerals in the formation and releasing metals.
Organic matter is also said to have entered the brine, and large amounts of carbonate minerals have been dissolved.
The report's lead author, geochemist Yousif Kharaka, has indicated that the loss of carbonates, a natural sealant, could ultimately result in the release of CO2 as well as fouled brine into overlying aquifers used for potable and irrigation water supplies.
Kharaka said he was even more troubled that the loss of carbonates could "attack" the structural integrity of cement seals used to plug abandoned oil or gas wells, leading to large amounts of CO2 escaping into the atmosphere.
Australian researchers from the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) are directly involved with the Frio Brine Pilot, and CO2CRC chief executive Peter Cook said that there were few similarities between the experimental program in Texas and Australian projects.
"Their research has shown in the field what we already know from the laboratory and theoretical studies – namely that CO2 does dissolve in water to produce acid brines," Cook said in response to the Geology report.
"This is turn can dissolve/liberate metals that are in the sand grains that make up permeable sediments in which the CO2 is injected and stored," he said.
The difference between Australian projects, such as the Otway basin pilot or the Barrow Island proposal, lies in the maturity of the geological structures used to contain the CO2 and the resulting acid brine.
"The feature of the Frio Brine project is that the sediments are 'immature'; in other words they are made up of lots of grains of fine material which is chemically fairly unstable.
"There is also a lot of iron and manganese in those sediments which can be quite easily leached out and this is what the CO2-charged water does," he said.
The Frio research indicates the need for a better understanding of subsurface reactions, but it was important not to extrapolate results from one project to another with different geological characteristics.
"By comparison, the sediments of the Otway Basin into which we will be injecting the CO2, are more mature," he said.
"For example, much of the original iron content was leached out millions of years ago and therefore there is much less in the way of metals to leach out of the sediments.
"However, there are some grains of a mineral called chlorite in the Otway Basin which will react with the CO2 to form stable carbonate minerals that will lock up the CO2 indefinitely."
Cook said the Texas and Otway Basin projects did have one important geological feature in common – geological caps.
"The rocks in which the CO2 is stored in the Frio projects, and where it will be stored in the Otway project, are overlain by one or more layers of thick fine grained sediments - mudstones - which act as a seal, preventing upward movement of gases or fluids, so it is unlikely that a problem will arise from migration of metals into shallower formations because of the presence of the seal."
To date, there have been no major incidences of carbon dioxide escaping from capped reservoirs.
