Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 463

A second approximation inherent in carbon sequestration models
arises from the need to estimate rock properties in the entire region of a
CO 2 storage site, over hundreds of square kilometers in areal extent and
hundreds of meters in depth, from a relatively small number of local
measurements and remote-sensing studies. One method for guessing
the rock properties in the un-sampled regions between wells consists of
representing rock formations as a set of discrete geological strata ( Figure
10.2.5 ) and assuming that these strata extend continuously in the un-
sampled regions between wells. A more elaborate geostatistical method
consists of using the properties measured at characterization wells to
generate a large number of possible rock property distributions that are
then used to predict the range of possible outcomes of CO 2 injection.
Thermodynamics and reaction kinetics within
a simulation grid block
Field-scale carbon sequestration models treat each simulation grid block
as a well-mixed reactor containing water, several mineral phases, and
eventually a CO 2 -rich phase. Key geochemical phenomena that occur
within grid blocks are summarized in Figure 10.2.6 . Parameters required
to describe these phenomena include variables that characterize the
state of the grid block such as temperature, pressure, CO 2 saturation,
and the mineralogy and porosity of the rock formation. Other required
parameters are those that describe the geochemistry of mineral-brine-
CO 2 systems: the CO 2 -brine equation of state, which tells us the relation-
ship between pressure, temperature, density, and composition. This
equation of state also needs to include the infl uence of brine salinity and
the behavior of impurities co-injected along with the CO 2 .
Field-scale carbon sequestration models must predict rates of dissolu-
tion or precipitation of solid phases in each simulation grid block. On short
time scales, the most important reactions involve the dissolution of car-
bonate minerals (primarily calcite) that occurs rapidly in response to the
acidifi cation of pore water caused by the dissolution of CO 2(sc) or supercri-
tial CO 2 . These reactions were discussed in the previous section.
The derivation of self-consistent thermodynamic databases is an
arduous task that is further complicated in the case of carbon sequestra-
tion by the coexistence of two fl uid phases and the high temperature,
pressure, and salinity in the formations of interest.