A new analysis by a team led by MIT has concluded that carbon dioxide injected into deep saline aquifers can be trapped as tiny bubbles and safely stored in the briny porous rock for centuries. The carbon dioxide eventually will dissolve in the brine and a fraction will adhere to the rock in the form of minerals such as iron and magnesium carbonates.
In a paper published in a recent issue of Water Resources Research, the team, which also includes researchers from Stanford University, Imperial College London, and Chevron Energy Technology Company, asserts that injected carbon dioxide will thus likely not flow back up to the surface and into the atmosphere, as some researchers fear.
We have shown that this is a much safer way of disposing of CO2 than previously believed, because a large portion—maybe all—of the CO2 will be trapped in small blobs in the briny aquifer. Based on experiments and on the physics of flow and transport, we know that the flow of the CO2 is subject to a safety mechanism that will prevent it from rising up to the top just beneath the geologic cap.—Ruben Juanes, MIT
Researchers have considered the possibility of sequestering CO2 beneath the Earth’s surface in at least three types of geologic formations: depleted oil and gas fields, unminable coal seams and deep saline aquifers.
The study shows that compressed carbon dioxide could be injected through a well deep underground into a natural sublayer consisting of porous rock, such as sandstone or limestone, saturated with saltwater.
The injected gas will form a plume and begin to rise through the permeable rock. Once the injection stops, the plume will continue to rise, but saltwater will close around the back of the gas plume. The saltwater and CO2 will juggle for position while flowing through the tiny pores in the rock.
Because the rock’s surface attracts water, the water will cling to the inner surface of the pores. These wet layers will swell, causing the pores to narrow and constrict the flow of carbon dioxide until the once-continuous plume of gas breaks into small bubbles or blobs, which will remain trapped in the pore space.
As it rises, the CO2 plume leaves a trail of immobile, disconnected blobs, which will remain trapped in the pore space of the rock, until they slowly dissolve and, on an even larger timescale, react with rock minerals. It is a good example of how a process that occurs at the microscopic scale affects the overall pattern of the flow at the geologic scale.—Ruben Juanes
The paper also describes how the mechanism of capillary trapping can be exploited (e.g., by controlling the injection rate or alternating water and CO2 injection) to improve the overall effectiveness of the injection project.
The work was funded by industrial affiliates of the Petroleum Research Institute at Stanford.
“Impact of relative permeability hysteresis on geological CO2 storage”; R. Juanes, E. J. Spiteri, F. M. Orr Jr., M. J. Blunt; Water Resources Research, Vol. 42, W12418, doi:10.1029/2005WR004806, 2006