The predicted fate of CO2 injected into basaltic rock.

A number of recent studies have advocated the injection of
CO2 into basaltic rocks to promote mineral carbonation due
to their abundance in divalent metal cations and their high
reactivity. This study assesses this possibility through a set
of reactive path modeling calculations. Two gas mixtures
were considered 1) pure CO2 and 2) a 75%-24.2%-0.8%
mixture of CO2-H2S-H2. It is assumed that these gases are
dissolved into a representative basaltic groundwater prior to
their injection into the subsurface. The injected water is
assumed to have a temperature of ~25 °C and is
equilibrated with ~25 bar pressure of the CO2 gas, and ~14
bar pressure of the CO2-H2S-H2 mixture. The injected fluid
will have a total CO2 concentration of ~0.8 and a pH of 3.7
for the pure CO2 gas and total CO2 concentration of ~0.42
and a pH of 4.0 for the gas mixture. Model calculations
reacted these fluids with basaltic glass, the most abundant
constituents of many basalts.
Reaction path modelling shows that 1-2 moles of basaltic
glass are needed to lower the CO2 concentration of each
liter down to natural pre-injection concentrations, but less
than 1 mole is needed for the H2S rich fluid. Major
carbonates formed were Ca-Mg-Fe-carbonate and dolomite
at pH <5, whereas ankerite and calcite formed later at
higher pH. Associated minerals at lower pH were
chalcedony, kaolinite and iron hydroxide, followed by
smectite and zeolites at higher pH. Modelling result suggest
that the first sulfur bearing phase to precipitate is elemental
sulfur, followed by greigite and mackinowite upon further
basaltic glass dissolutio