Sequestration of carbon dioxide is the holy grail of "clean coal." A paper published in the Journal of Petroleum Science and Engineering casts doubt on the feasibility of carbon sequestration in underground reservoirs such as abandoned mines. The study by Christine Ehlig-Economides and Michael Economides can be found here. Their simulation studies indicate that a closed underground reservoir may only be able to hold less than 1% of its volume in injected carbon dioxide (CO2).
Published reports on the potential for sequestration fail to address the necessity of storing CO2 in a closed system. Our calculations suggest that the volume of liquid or supercritical CO2 to be disposed cannot exceed more than about 1% of pore space. This will require from 5 to 20 times more underground reservoir volume than has been envisioned by many, and it renders geologic sequestration of CO2 a profoundly non-feasible option for the management of CO2 emissions.
Clean coal technology has two elements - gasification and sequestration. Gasification involvesbreaking down the coal by a thermochemical process into a synthetic gas that burns more efficiently and would facilitate sequestration of CO2 emissions. Gasification has been developed to the point that it is being incorporated in next generation coal-fired power plants. Sequestration, on the hand, has proven to be the stumbling block.
The conclusion of this study provides some insight into why sequestration has long been discussed but never demonstrated on a scale that would make it feasible to use on most power plants in operation.
The implications of this work are profound. A simple analytical model shows immediate results very similar to those that take hours to produce with numerical simulation. Much more important, the work shows that models that assume a constant pressure outer boundary for reservoirs intended for CO2 sequestration are missing the critical point that the reservoir pressure will build up under injection at constant rate. Instead of the 1–4% of bulk volume storability factor indicated prominently in the literature, which is based on erroneous steady state modeling, our finding is that CO2 can occupy no more than 1% of the pore volume and likely as much as 100 times less.
This work has related the volume of the reservoir that would be adequate to store CO2 with the need to sustain injectivity. The two are intimately connected. In applying this to a commercial power plant the findings suggest that for a small number of wells the areal extent of the reservoir would be enormous, the size of a small US state. Conversely, for more moderate size reservoirs, still the size of Alaska's Prudhoe Bay reservoir, and with moderate permeability there would be a need for hundreds of wells. Neither of these bodes well for geological CO2 sequestration and the findings of this work clearly suggest that it is not a practical means to provide any substantive reduction in CO2 emissions, although it has been repeatedly presented as such by others.
In an interview, Michael Economides described the technical issue as follows:
"It is like putting a bicycle pump up against a wall. It would be hard to inject CO2 into a closed system without eventually producing so much pressure that it fractured the rock and allowed the carbon to migrate to other zones and possibly escape to the surface."
A project in the North Sea is often cited as a successful demonstration of the feasibility of sequestration. The authors point out serious deficiencies in that project.
“There are already some data that seem to warn of problems in the very few existing injection projects,” the authors write. They point to Sleipner, a CCS project in the North Sea, which has achieved only one-third the carbon dioxide injection volumes that would be required for a single 500-megawatt coal-fired power plant. They further note that the project has seen “significant leakage to overlying layers.”
Examining the carbon dioxide storage capacity required for one coal plant, which produces about 3 million tonnes of CO2 a year, Ehlig-Economides and Economides calculate that 30 years of storage at 1,000 pounds per square inch could require an underground aquifer with an area of 1,371 square miles, which is just under the size of the state of Rhode Island.
“Conversely,” they conclude, “for more moderate size reservoirs, still the size of Alaska’s Prudhoe Bay reservoir, and with moderate permeability there would be a need for hundreds of wells. Neither of these bodes well for geological CO2 sequestration and the findings of this work clearly suggest that it is not a practical means to provide any substantivereduction in CO2 emissions, although it has been repeatedly presented as such by others.”
Economides goes on to describe carbon sequestration as the "last refuge of the scoundrel." There have been no shortage of scoundrels in the coal industry and utilities sector.
There will no doubt be many attempts to rebut the findings of this study. However, don't be surprised if carbon storage is not a reality any time soon. The research and development costs to chase the "clean coal" holy grail of carbon sequestration should be put to better use like storage systems for energy generated from clean, renewable energy sources such as solar, wind, and geothermal power.
Climate Progress also has an excellent discussion of this study (h/t hold tight). Joe Romm notes the following:
The method is widely viewed as being decades away from commercial viability. Even then, the cost could be prohibitive: by a conservative estimate, several trillion dollars to switch to clean coal in the U.S. alone.Then there are the safety questions. One large, coal-fired plant generates the equivalent of 3 billion barrels of CO2 over a 60-year lifetime. That would require a space the size of a major oil field to contain. The pressure could cause leaks or earthquakes, says Curt M. White, who ran the U.S. Energy Dept.’s carbon sequestration group until 2005 and served as an adviser until earlier this year. “Red flags should be going up everywhere when you talk about this amount of liquid being put underground.”