Northside Study Group: Sequestering Carbon Dioxide in a Closed Underground Volume

The capture and subsequent geologic sequestration of CO₂ has been central to plans for managing CO₂ produced by the combustion of fossil fuels. The magnitude of the task is overwhelming in both physical needs and cost, and it entails several components including capture, gathering and injection. The rate of injection per well and the cumulative volume of injection in a particular geologic formation are critical elements of the process.

Published reports on the potential for sequestration fail to address the necessity of storing CO₂ in a closed system. Our calculations suggest that the volume of liquid or supercritical CO₂ to be disposed cannot exceed more than about 1% of pore space. This will require from 5 to 20 times more underground reservoir volume than has been envisioned by many, and it renders geologic sequestration of CO₂ a profoundly non-feasible option for the management of CO₂ emissions.

Material balance modeling shows that CO₂ injection in the liquid stage (larger mass) obeys an analog of the single-phase, liquid material balance, long-established in the petroleum industry for forecasting undersaturated oil recovery. The total volume that can be stored is a function of the initial reservoir pressure, the fracturing pressure of the formation or an adjoining layer, and CO₂ and water compressibility and mobility values.

Further, published injection rates, based on displacement mechanisms assuming open aquifer conditions are totally erroneous because they fail to reconcile the fundamental difference between steady state, where the injection rate is constant, and pseudo-steady state where the injection rate will undergo exponential decline if the injection pressure exceeds an allowable value. A limited aquifer indicates a far larger number of required injection wells for a given mass of CO₂ to be sequestered and/or a far larger reservoir volume than the former.