CCS Dead And Buried?
Recent tests show that infusing rock layers with CO2-saturated aqueous fluid can alter the properties of caprock, leading to the escape of the sequestered carbon back into the environment.
Back in 2005, the IPCC Working Group III Special Report on Carbon Dioxide Capture and Storage declared that the storage of naturally and industrially produced carbon dioxide in depleted hydrocarbon reservoirs and aquifers was considered an essential component of the strategy to combat the build-up of greenhouses gases in the atmosphere. It seemed like an easy solution, pump CO2 captured from nasty coal power plants and other high volume greenhouse gas sources back into the underground reservoirs that oil and gas has been extracted from. After all, those geologic formations held hydrocarbons for millions of years—now the pumped out oil fields are just sitting there, waiting to be put to use. That was until testing was done on rock from actual cap strata. It would appear that infusing rock layers with CO2-saturated aqueous fluid can alter the properties of caprock, leading to the escape of the sequestered carbon back into the environment.
Among the farfetched and even loony geoengineering schemes that have been proposed to combat the growing levels of CO2 in Earth’s atmosphere, pumping captured gas back into oil fields sounds like one of the saner proposals. Deep saline aquifers have also been proposed as subsurface burial sites for GHG. The geological sequestration of CO2 is currently under investigation in field trials at a number of sites around the world. These include Weyburn in Canada, Sleipner in the North Sea and In Salah in Algeria. But just burying the unwanted gas in used wells is not sufficient, for carbon capture and storage to succeed, the reservoirs must be able to keep the CO2 from escaping over many thousands of years.
In a letter to Nature Geoscience, P. J. Armitage, D. R. Faulkner and R. H. Worden, all from the Department of Geology and Geophysics at the University of Liverpool, report that this proposed solution may not be as foolproof as the proposers believe. It seems that Armitage et al. actually did some testing to see if things would work as advertised. Their letter, succinctly titled “Caprock corrosion,” outlines the problem with CO2 internment this way:
The success of these projects relies on the long-term integrity of low-permeability caprocks that must seal the CO2 within the reservoir. However, we find that the integrity of caprock can be altered by flowing CO2-saturated aqueous fluid. Specifically, fluid flow under laboratory conditions through siltstone caprock sampled at one of the world’s largest carbon storage projects, In Salah in Algeria, increases the permeability of the caprock by one order of magnitude under simulated reservoir conditions. The increase in permeability is caused by chemical reactions between the CO2-rich fluid and minerals commonly found in caprocks. We therefore argue that such geochemical interactions must be considered in geological carbon sequestration projects.
This scheme is a seductive one—all that nasty carbon dioxide is pumped back into the ground where it came from. Out of sight, out of mind. Unfortunately, the injection of CO2 into depleted oil fields inevitably alters the geochemical conditions of the reservoir. “Little is known about the effect of CO2 injection on the physical properties of the reservoir and caprocks,” the authors state, noting, “when CO2 has been injected into oil fields for the purpose of enhancing oil recovery, the acidic CO2-rich aqueous fluids created in the subsurface reservoir is known to interact geochemically with rock-forming minerals.”
For this sequestration scheme to work CO2 must be relatively inert once injected into the rock matrix. However, contact with water leads to solution of the CO2, the creation of carbonic acid and partial dissociation leading to acidification. Acid affects the caprock, possibly leading to leaks. Strange that no one at the IPCC thought of this before making such a proposal, being experts and all.
Injection of carbon dioxide is already being done at a number of sites around the world. To test the possible long-term outcome of such injections, Armitageet al. decided to run some tests. Again quoting from the letter:
To test the influence of CO2-saturated fluid flow on caprock integrity, we selected a typical sample from the base of the caprock sequence of the Krechba gas field at In Salah. Here, about 0.5 megatons of CO2 are being injected into the subsurface each year. The Lower Carboniferous caprock at In Salah is a 950-m-thick mudstone, of which the lower part is a thinly bedded estuarine siltstone that contains no organic material or swelling clays. Mineralogical analyses of the lower part of the caprock show the presence of the dominant silicate minerals quartz, illite and potassium feldspar, with subordinate quantities of chlorite and kaolinite, and small quantities of siderite and pyrite. Extensive characterization of the lower caprock, confirmed that our sample was representative of the lower caprock as a whole.
In their laboratory the investigators conducted tests to see if the permeability of the caprock was changed under the conditions created at an injection site. Inert argon gas was used as a test fluid to establish a permeability baseline at various pressures. Then dry CO2O2 and distilled water were subjected to the same test conditions. Finally, CO2-saturated water was run through the rock sample for 72 hours under approximate reservoir pressure.
Measurements made after the CO2-saturated water flow showed permeability increased by approximately 8 times. The investigators attribute the change to chemical reaction between the saturated water and the rock. “Mechanical effects can be discounted as the cause of the change because the permeability of the caprock was unaffected by the flow of either dry CO2 or distilled water,” they state, concluding: “The increase in permeability during the flow of CO2-saturated water has most probably been caused by geochemical processes.”
There was an interesting point made regarding the temperature in one of the depleted reservoirs that caught my attention: “Typical reservoir temperatures are about 95 °C — much higher than our room temperature experiments. A higher in situ temperature would probably increase reaction rates, so the permeability increase could occur on shorter timescales than in our experiments.” In other words, containment deterioration might progress faster than the experimental data would imply. See the letter in Nature Geoscience for more details of the testing and its implications.
The upshot of all this is that one promising carbon sequestration scheme may not be as attractive as previously thought. Carbon buried this way may, in fact, rise from its grave.