Environmental Engineering Reference
In-Depth Information
10.4.4.3 Deep Aquifers
Deep aquifers may underlay vast areas under the continents and oceans. Such aquifers usually
contain saline water (brine) and are separated from shallower aquifers—the source of much of the
drinking water—by impermeable rock. The deep aquifers themselves consist of permeable, porous
rock, such as sedimentary shale-, lime-, or sandstone, the pores of which are saturated with brine.
Such aquifers are found at depths of 800 m or deeper. The injected CO
2
(in liquid or supercritical
phase) would dissolve in the brine as carbonic acid. In the case of limestone formation, some of
the carbonate (CO
2
3
) would dissolve into bicarbonate (HCO
3
), furthering the absorption capacity
of the reservoir and reducing the risk of leakage.
There is limited information available on the extent and CO
2
absorption capacity of deep
aquifers. In the United States, the storage capacity of subterranean aquifers ranges from 5 to 500 Gt
carbon (compared to annual emission rates from power plants of 1.7 Gt y
−
1
of carbon). Other
estimates for worldwide capacities range from 100 to 3000 Gt carbon. The problem with deep
aquifers is not so much their capacity, but their location vis-a-vis power plants, the difficulty of
drilling large diameter pipes into the overlaying strata, the cost of compressing and pumping liquid
CO
2
into the pipes, and constructing an appropriate diffuser at the end of the pipe, so that CO
2
disperses throughout the aquifer without leaking through possible permeable overlay formations.
16
Intensive research is ongoing to establishing the location and capacity of the deep aquifers for
potential sequestration of CO
2
in them.
One of the subocean floor aquifers has been used for CO
2
disposal since 1996. At the Sleipner
gas fields in the North Sea off the coast of Norway, natural gas contains 9.5% by volume CO
2
. The
CO
2
is separated by MEA absorption, compressed and injected into the Utsira undersea aquifer at a
rate of 1 Mt y
−
1
. The cost of the injection is about $15 per ton CO
2
, which compares favorably with
a tax of $50 per ton of CO
2
that the government of Norway would levy if the CO
2
were emitted
into the atmosphere. Another plan for undersea aquifer injection exists for the Natuna gas field off
the coast of Borneo in Indonesia, where natural gas contains about 70% by volume of CO
2
.
10.4.5
CO
2
Utilization
In Section 10.4.4.1 we discussed the use of flue gas CO
2
for enhanced oil or gas recovery. Other
uses would be for dry ice manufacturing, for carbonated drinks, and as a raw material for chemical
products, such as urea, methanol, or other oxygenated fuels. The problem with such propositions
is twofold: (a) most of the carbon in the product would eventually burn up or decompose back
to CO
2
and would wind up in the atmosphere; (b) the reduction of CO
2
into the useful product
requires virtually the same amount of energy as was given off when carbon oxidized into CO
2
. Also,
the present market for chemical products that could be based on CO
2
is quite limited, amounting
perhaps to less than 50-70 Mt y
−
1
, whereas the emission of CO
2
from a single 1000-MW coal-fired
power plant amounts to 6-8 Mt y
−
1
.
Let us take the example of converting CO
2
to methanol:
171 kJ mol
−
1
CO
2
(
g
)
+
3H
2
(
g
)
→
CH
3
OH
(
l
)
+
H
2
O
(
l
)
−
(10.6)
16
Pipe diameters of 50-100 cm are deemed necessary for disposing the CO
2
output of a single 1000-MW
power plant.
