Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 381

Section 1
Introduction

In Chapter 8, we provided an overview of geological CO 2 sequestration.
The idea is to select our geological formations with care so we can
ensure that the CO 2 stays at those locations as we planned. At the
molecular level, we need to understand how CO 2 is interacting with
rocks. With the broad outline and overview of geological carbon seques-
tration presented in the last chapter as a foundation, we are in a position
now to address key issues and questions surrounding the physical and
chemical processes related to sequestration:

• Within the wide variety of geological formations on earth, what are the

characteristics of the formations that are most promising for seques-
tering CO 2 , i.e., which rock formations ensure that the CO 2 does not
reach the surface?

• What will be the physical properties of CO 2 (e.g., density and viscosity)

within these formations at a given depth?

• How will CO 2 fl ow within these formations over time?

Some of these questions are very similar to those discussed in the
carbon capture chapters. A CO 2 molecule may not notice the difference
between adsorption in an adsorbent material versus in a rock, or
whether it diffuses through a membrane or within a geological forma-
tion. Indeed as we will see, the concepts to describe transport in a
geological formation are similar to those we have previously described —
similar, but not identical. In our design of a membrane or adsorbent, we
could safely assume that the small structure for which we obtained a
molecular level understanding is representative of the entire mass of
material in our separation unit at the power plant. In geological seques-
tration, however, our CO 2 plume can stretch tens of kilometers around
the injection site. On this scale, assuming the earth is a perfect, homo-
geneous substance without any defects does not make any sense. Over
such a large scale, the subsurface will be heterogeneous and we need
to understand how the heterogeneities infl uence the behavior of the
CO 2 plume. We will also see that at such a large scale a molecular