For years, the idea of capturing and sequestering carbon dioxide emissions from large numbers of new or retrofitted fossil-fired plants ran up against a big unknown: is there anywhere enough CO2 storage capacity in the US to make a real difference? Now, there is something very close to an answer.
The most detailed study yet on the potential for long-term CO2 storage in the US has concluded there would be sufficient room to store at least 100 years’ worth of CO2 emissions from coal-fired and natural gas-fired plant in 11 geographically diverse deep saline aquifers.
“This result suggests that the large-scale implementation of carbon capture and sequestration (CCS) is a geologically viable climate-change mitigation option in the US over the next century,” said the study, which was conducted by researchers at the Massachusetts Institute of Technology and funded by the US Department of Energy and others.
“One of the motivations for doing the study that there was such a wide range of estimates out there” regarding the national capacity for storing CO2, Ruben Juanes, the leader of the study and the ARCO Associate Professor in Energy Studies at MIT’s Department of Civil and Environmental Engineering, said in an interview.
“We also wanted to bring new methodologies into capacity estimation” to assess optimal CO2 injection rates to prevent the fracturing of the surrounding rock, and to gauge the ability of the rock to trap the CO2 indefinitely.
“One of the key findings of the study–something that hadn’t been discussed before–is that the capacity is not a fixed number,” said MIT graduate student Michael Szulczewski. Instead, he said, the capacity is “a function of how long a period over which you are willing to inject the CO2”–the longer the period, the more gradual the increase in pressure, and the higher the available capacity.
The research team used complicated models to assess the capacity of 20 arrays of injection wells in 11 deep saline aquifers to receive CO2 that has been captured from plant flue gases, compressed into a supercritical fluid an then injected in deep wells.
To estimate the demand for CO2 storage, the researchers assumed that the rate of CO2 production “will increase linearly, reach a maximum, and then decrease linearly with equal and opposite slope until returning to the current rate,” the study said.
“While future CO2 production trends will likely be complex, we use this simple model because it captures the essential features expected in future trends: an increase in the rate of production as energy demand grows and fossil fuels continue to supply the energy, and then a decrease as low-emissions energy sources begin to replace fossil fuels,” it said.
As noted, the 11 deep saline aquifers studied in the most detail are scattered, and include sites in North Dakota, Wyoming, Utah, New Mexico, and Texas; other sites are in Alabama, Florida and Georgia, Maryland, Michigan, Indiana, and Illinois (see map).
Asked whether sedimentary basins in centers of coal-fired generation like West Virginia and Pennsylvania were found lacking, Szulczewski said they were not. “It was the absence of publicly available data” in those states that led researchers to focus on the 11 aquifers they cited, he said.
“We certainly didn’t investigate every potential sedimentary basin, and even in the ones we looked at we didn’t investigate all areas within them,” Szulczewski said, adding that the study’s capacity estimates are likely conservative.
–Housley Carr