Introduction to Geological Sequestration - Introduction to Carbon Capture and Sequestration - page 367

gas (e.g., mixtures of H 2 S and CO 2 ), and seasonal natural gas storage
[8.5, 8.6]. If such a formation contains various combinations of fl uids other
than water, e.g., hydrocarbons (oil and/or gas), it is called a reservoir.
For CO 2 sequestration, one would like to avoid those water reservoirs
that can be used for human consumption without much treatment. These
aquifers typically have a salinity less than 3,000 or 4,000 ppm (mg/l) total
dissolved solids (TDS). Water from these types of aquifers is often
referred to as potable groundwater (the exact defi nition depends on the
jurisdiction). In most jurisdictions, potable groundwater is protected. In
the USA, the Safe Drinking Water Act provides the legal framework for
the Environmental Protection Agency (EPA) to regulate injection into
aquifers in order to protect groundwater with TDS less than 10,000 ppm.
Formation water is defi ned as water with a much higher salinity and is
usually found at much greater depths than potable groundwater. Deep
saline aquifers, with TDS typically much greater than 10,000 ppm, are
being considered for CO 2 storage.
Deep unminable coal seams
Fractures give coal formations some permeability. If one were to magnify
these regions one would see a very large number of micropores into which
gas molecules can adsorb. Experiments have shown that these pores can
adsorb many gasses and often they contain large quantities of methane.
Interestingly, CO 2 adsorbs more strongly than methane and hence the
injection of CO 2 will displace the methane. In unminable coal formations,
we can recover the methane while permanently storing the CO 2 . This pro-
cess is called Enhanced Coalbed Methane (ECBM) recovery.
Capacity
An important question is whether we have enough capacity in geological
formations to store all the CO 2 we produce. From a practical point of view,
it is also important that these formations be “conveniently” distributed over
the planet. Table 8.3.1 shows the global capacity of the different forma-
tions in which CO 2 can be sequestered [8.5, 8.6]. If we compare these
capacities with the annual production of CO 2 of approximately 31 Gt per
year, we see that even according to the lowest estimated storage capcity,
we have suffi cient capacity to store all our CO 2 for at least 50 years.