“Every time you place a sensor thousands of feet down, it requires drilling a well bore that, depending on depth and diameter, can cost between 5 [million] and 10 million dollars,” says Ken Humphreys, of the FutureGen Alliance. Humphreys says less-expensive systems such as acoustic sensors that monitor the movement of carbon dioxide from the surface are currently under development.
As engineers develop new technologies for carbon capture and sequestration, technical setbacks may be inevitable. AEP, the utility company that began pumping 2 percent of its carbon dioxide emissions underground on October 1, had hoped to begin sequestration earlier, but the project was delayed when sensors showed higher-than-anticipated moisture content in the carbon dioxide. If the liquefied gas contains too much water, carbonic acid can form, corroding the steel pipes used to transport it underground.
To bring the water content down to a safe level, AEP said it would have to further cool the carbon dioxide to remove water through precipitation before pumping it underground. Additional testing, however, revealed that the moisture content had been misread and was actually within safe levels.
“There are definitely teething pains in getting it up and running,” says Gary Spitznogle of AEP. “It’s just the nature of a new process. Not everything works right in the first iteration.”
The cap-and-trade legislation now making its way through Congress could help hasten solutions to many of the technical issues that CCS still faces. But one of the biggest remaining questions is whether sufficient reservoirs exist to store all of the carbon dioxide that may be captured.
The best-studied storage deposits are former oil and gas reservoirs capped by layers of nonporous rock that kept the petrochemicals locked deep underground for millions of years. Yet of an estimated 3,947 gigatonnes of carbon dioxide storage capacity under the U.S., only 1 percent consists of depleted natural gas and oil reservoirs. The vast majority of capacity–3,630 gigatonnes–consists of deep saline formations that have received less scrutiny.
“We’re at the place where there is no problem doing millions of tonnes a year, but to solve the climate problem we need to do billons of tonnes or gigatonnes a year, and at that scale, storage becomes a real issue,” Herzog says.
Majer, of Lawrence Berkeley National Laboratory, says small-scale tests such as AEP’s pilot project will go a long way toward determining the viability of storage in saline aquifers. “We don’t know all the answers yet, but we pretty much know how to get the answers,” he says. “And who knows, the answer may still be, it ain’t gonna work.”