Description
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.