Hydraulically fractured shale formations could become repositories for permanent geologic carbon sequestration. Zhiyuan Tao and Andres Clarens, University of Virginia, Charlottesville, USA, propose a computational method for estimating the CO₂ sequestration capacity of a fractured shale formation. Their method is applied to the Marcellus shale, a 600-mile-long formation in Pennsylvania, USA, and based on historical and projected CH₄ production along with published data and models for CH₄/CO₂ sorption equilibria and kinetics.

The network of cracks produced by the fracking process could be filled up with CO₂ before the depleted wells are sealed. This would not require building any new infrastructure to sequester CO₂. Shales have a low permeability, but they can adsorb an appreciable amount of CO₂ on fracture surfaces. The mass transfer kinetic results indicate that injection of CO₂ would proceed several times faster than production of CH₄.

According to the researchers, the Marcellus shale alone could store between 10.4 and 18.4 Gt of CO₂ between now and 2030. This represents more than 50 % of total CO₂ emissions of the USA from stationary sources over the same period. Other shale formations with comparable pressure/temperature conditions, such as Haynesville (underlies large parts of southwestern Arkansas, northwest Louisiana, and East Texas) and Barnett (Bend Arch-Fort Worth Basin, Texas), could provide additional storage capacity.

• Estimating the Carbon Sequestration Capacity of Shale Formations Using Methane Production Rates,
  Zhiyuan Tao, Andres Clarens,
  Environm. Sci. Technol. 2013.
  DOI: 10.1021/es401221j