PNNL-13501 - Pacific Northwest National Laboratory

Modeling the Sequestration of CO2 in Deep Geological Formations
Study Control Number: PN00068/1475
K. Prasad Saripalli, Ashok Chilakapati, Pete McGrail
The Department of Energy has identified carbon management as a critical component of its mission. This project is
developing the analytical and numerical tools needed by DOE to evaluate management alternatives involving carbon
sequestration in deep geologic formations.
Project Description
The purpose of this project is to develop analytical and
numerical models to evaluate carbon sequestration in
deep geologic formations. Specifically, these tools are
needed to evaluate issues such as the capacity of the
geologic formation around a carbon injector, the relative
apportioning of carbon mass among the three states
(hydrodynamic, dissolved, and mineral phase), the longterm
fate of sequestered CO2, the potential for CO2
release, and the effect of formation plugging on CO2
injection. Equations governing the radial injection of CO2
into deep saline aquifers, axisymmetric flow of CO2
around the injector, and eventual buoyancy-driven
floating with simultaneous dissolution were formulated.
Two models were prepared: the semi-analytical model
PNLCARB and the numerical model STOMP-CO2. The
models effectively simulate deep-well injection of waterimmiscible,
gaseous, or supercritical CO2. The models
were used to investigate the effect of pertinent fluid and
reservoir and operational characteristics on the deep-well
injection of CO2. The results indicated that the injected
CO2 phase initially grows as a bubble radically outward,
dissolves partially in the formation waters, eventually
floats toward the top due to buoyancy, and settles near the
top confining layer. Formation permeability, porosity, the
rate and pressure of injection, and the rate of CO2
dissolution influenced the growth and ultimate
distribution of the CO2 phase. The first-generation tools
developed by this project will help to evaluate and
optimize methods for sequestering carbon in deep
geologic repositories.
Introduction
Geological sequestration of CO2 has been recognized as
an important strategy for reducing the CO2 concentration
in the atmosphere. However, economically viable
technologies for geological sequestration and models
describing the same are in their infancy. This project
focused on modeling the injection, sequestration, fate, and
escape of CO2 in geological formations. The three
mechanisms of CO2 sequestration are hydrodynamic,
dissolved, and mineral phase trapping. These
mechanisms can influence one another; for example, in
addition to providing a carbon sink, mineral trapping can
significantly alter hydrogeologic properties (such as
secondary porosity and permeability) and hence, the
efficiency and capacity for hydrodynamic and dissolved
trapping. Dissolved-phase trapping, a strong function of
pressure, salinity, and temperature, can similarly
influence the other two processes. As such, modeling
geological sequestration requires an accurate
representation of the individual processes and their
interaction. Following are the primary processes that
need to be considered in modeling geological
sequestration: 1) multiphase, radial injection of CO2 and
the growth of its area of review around the injector, 2)
buoyancy-driven migration of CO2 toward the top aquifer
confining layer, 3) dissolution of CO2 during injection
and vertical migration, and the resulting aqueous
speciation of carbon, 4) carbon mass exchange via
precipitation and dissolution of minerals through the
interaction of dissolved and gaseous phase CO2 with the
formation, and 5) changes in hydrogeological properties
due to mineral trapping and the resulting formation
damage and injectivity decline. The reservoir-scale
numerical codes typically do not model the above
processes around an injection well with adequate
mechanistic detail. Over the past several years, our team
developed significant expertise in many of these
multiphase and reactive transport processes, and their
modeling for deep-well waste injection applications. This
experience can be combined to develop effective
modeling tools for geological sequestration of CO2.
Approach
Two mathematical models were developed for the
injection, sequestration, distribution, and long-term fate,
including release to the atmosphere, of CO2. One of them
is PNLCARB1.0, a semi-analytical model for modeling
the injection and sequestration around a single injection
well. Governing equations were derived for the radial
Earth System Science 233