Carbon capture and storage, or CCS, is a major component of the world’s greenhouse gas reduction strategy. Involving injecting and storing carbon dioxidein underground geologic reservoirs, the method is used at several oil and gas exploration sites worldwide to prevent the gases from entering the atmosphere.
But to significantly reduce emissions, CCS would need to operate on a massive scale, potentially sequestering upwards of 3.5 billion metric tons of CO2 each year. A new technical hurdle may mean CCS won’t be able to get anywhere near that volume.
In a paper appearing in the journal PNAS , Stanford geophysics Professor Mark Zoback and environmental Earth science Professor Steven Gorelick argue that, in many areas, carbon sequestration is likely to create pressure build-up large enough to break the reservoirs’ seals, releasing the stored CO2.
"Almost all of our current climate mitigation models assume CCS is going to be one of the primary tools we use," said Zoback. "What we’re saying is, not so fast."
Intraplate earthquakes – earthquakes that occur far from the boundaries between tectonic plates – can occur nearly anywhere in continental interiors, due to what the researchers describe as "the critically stressed nature of the Earth’s crust." Small pressure build-ups near potential faults reduce friction, increasing the likelihood of a fault slip.
It’s been known for a half-century that human activities can increase pressure to the point of inducing small temblors. In the 1960s, the injection of wastewater into a well near Denver triggered a series of small earthquakes. Last year, similar quakes were induced in Arkansas, Ohio and on the border of Colorado and New Mexico.
Reviewing field stress measurements and laboratory studies of shear displacements, Zoback and Gorelick say injection of massive quantities of CO2 would be likely to produce the same result.
Zoback has previously described wastewater-induced quakes as manageable, low-risk events. Carbon injection is unlikely to trigger large, destructive earthquakes, the professors argue, but "the implications are different if you’re trying to store carbon for thousands of years." Zoback said.
Zoback and Gorelick state that even a fault slip of a few centimeters could allow stored CO2 to reach the surface – a serious concern, since the researchers argue that carbon repositories need a leak rate of less than 1 percent every thousand years to be effective.
"The bar is much higher in this case," Zoback said.
Where to put it
The areas where CCS is already practiced successfully follow a very specific geologic profile. Ideally, the reservoirs themselves are formed from porous, weakly cemented materials that slow pressure build-up, but are isolated from the surface by an impermeable rock layer.