Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 472

( Figure 10.2.2 ) a hundred-fold increase in the concentration of dissolved
organic molecules (such as acetate, CH 3 CO 2 − ) was detected in the vicinity
of the CO 2 plume [10.12]. These organic molecules may have been solu-
bilized by the geochemical changes associated with the injection of CO 2 .
The presence of organic molecules in geological formations may
infl uence the fate of injected CO 2 in several ways. In the short term, dis-
solved organics could adsorb at the CO 2 -brine interface, decreasing the
CO 2 -brine interfacial tension. Less soluble organic molecules could form
hydrophobic coatings on mineral surfaces, altering the wetting properties
of rock formations [10.13, 10,14]. As discussed in Section 9.4, the capil-
lary pressure P c associated with CO 2 invasion in porous rocks is strongly
dependent on the CO 2 -brine interfacial tension and the wettability of the
pore walls by brine vs. CO 2 . In short, organic molecules could modify the
mineral-brine-CO 2 multiphase fl ow properties in ways that could either
enhance or decrease CO 2 storage effi ciency. In the longer term, dis-
solved organic molecules are known to either catalyze or inhibit mineral
weathering reactions such as those that were described in Section 9.8
[10.15]. This will either accelerate or slow down the permanent trapping
of CO 2 as mineral carbonates.
In the case of shale formations, the potential role of organic matter is
evidenced by the existence of signifi cant quantities of organic grains
(roughly 0.5 to 13% vol.) in these formations. A detailed view of one of
these organic grains is shown in the inset in Figure 10.2.10 (another
organic grain is shown in purple in Figure 10.2.11 ). The electron micros-
copy study highlighted in Figure 10.2.10 revealed that the largest pores
in the clayshale samples under investigation were associated with
organic grains. This fi nding indicates that organic matter could dispro-
portionately affect fl uid fl ow properties in this type of sample, because
the largest pores are also the ones where fl uid fl ow is most likely to take
place.
Finally, microbiological effects are thought to play several potential
roles in GCS. Few microorganisms are able to survive in the presence of
supercritical CO 2 , but at least one microorganism has been found to
survive in the extremely harsh conditions (high P, T, salinity, and CO 2
concentration) of GCS sites [10.16]. Other such microorganisms are likely
to be discovered in the future. At the carbon sequestration pilot site at
Ketzin, Germany, microorganism numbers and activity were found to
rebound relatively rapidly (within a few months) after the passage of the
CO 2 plume at an observation well [10.17]. One potential impact of