solution. This report gives an overview of the available literature on the thermodynamic properties of
the CO
2
-H
2
O system and the solubility of CO
2
in aqueous solution, as well as recommendations on the
use of this data. The model for the rate of mineralisation of a feldspathic sandstone, derived earlier
(Hangx, 2005), is revised, and a new model is presented, which also predicts constraints on the
porosity-permeability evolution of a sandstone reservoir. All symbols used are summarised in Table 1.
The report will be concluded with a description of revisions to our experimental designs and
preliminary results will be presented. The first experiments aim to determine the type and rate of
several basic reactions occurring during CO
2
injection in a sandstone reservoir. One of those reactions
is that of Ca-rich feldspar, e.g. anorthite, reacting with CO
2
to form both calcite and kaolinite
CaAl
2
Si
2
O
8 (s)
+ CO
2 (g)
+ 2H
2
O
(l)
↔ CaCO
3 (s)
+ Al
2
Si
2
O
5
(OH)
4 (s)
(3)
Systematically performed experiments at various P(CO
2
), temperature, and grain size will provide
reaction rate equations, which are most likely rate-limited by the dissolution of the feldspar and
phyllosilicate phases, or the precipitation of clays, and not by the dissolution of carbon dioxide or the
precipitation of calcite.
Table 1 Symbols list
symbol, definition [units] symbol, definition [units]
a
i
activity of species i M
i
maximum amount of carbonate
precipitated as a function of i [kg]
A
s
total surface area [m
2
] N
grains
number of grains
A
grain
surface area of a grain [m
2
] N coordination number
d grain size [μm] P pressure [bar]
φ porosity R reaction rate [mol/m
2
s]
φ
fugacity coefficient ρ
i
density of species i [g/cm
3
]
k reaction rate constant [s
-1
] T absolute temperature [K]
m
i
mass of species i [kg] W vol.-% of water
m
i
molar mass of species i [g/mol] X vol.-% of anorthite
V molar volume [dm
3
/mol] Z compressibility factor
.
amount of carbonate precipitated/unit
volume/s [kg/m
3
s]
moles
i
N
number of moles of species i
2. Thermodynamic properties of CO
2
and H
2
O
Of importance to our experimental study, and
also to modelling geological sequestration of CO
2
in general, is to know the behaviour of the liquid
and/or vapour phases present in the system.
Therefore, a study has been made on the kinetic
and thermodynamic properties of both carbon
dioxide and water. Most thermodynamic
properties (Lee & Kesler, 1975; Peng &
Robinson, 1976; Kerrick & Jacobs, 1981; Bowers
& Helgeson, 1983; Duan et al., 1992a, b, 1995;
Span & Wagner, 1996; Wagner et al., 2000), as
well as the solubility of carbon dioxide in aqueous
solution (Nighswander et al., 1989; Carroll et al., 1991; King et al., 1992; Duan & Sun, 2003; Duan et
al., 2005; Portier & Rochelle, 2005), can be described by using the appropriate PVT-relations, or
Equations of State (App. 1). Phase relations can be derived from phase diagrams for H
2
O and CO
2
, as
well as the CO
2
-H
2
O system, as shown in Fig. 1. The PT conditions for the critical point, and triple
point, of both substances are given in Table 2. Beyond the critical point carbon dioxide and water
become supercritical fluids, a phase that is neither liquid, nor gas.
Table 2 The critical properties of CO
2
, and H
2
O
component CO
2
H
2
O
T
c
(°C)
30.9782
†
373.946
‡
P
c
(bar)
73.773
†
220.64
‡
ρ
c
(kg/m
3
)
467.6
†
322
‡
T
t
(°C)
-56.558
†
0.01
*
P
t
(bar)
5.1795
†
6.1173·10
-3
*
†
Span & Wagner (1996);
‡
Wagner et al. (2000);
*
Handbook of Physics and Chemistry (Lide, 2004)
In order to evaluate the solubility of CO
2
in aqueous solutions it is necessary to investigate the
behaviour of the separate pure end members. Equations of State have been widely used in the study of
solubility of gases in aqueous solutions and are often found at the basis of solubility models
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