**3. Estimating CO2 Mineralization Potential (II): Example Modelling**

Batch reaction models were performed with the geochemical software PHREEQC v3 using the built-in phreeqc.dat thermodynamic database, allowing robust estimates of the CO2 fugacity coefficient using the Peng–Robinson EOS model [57]. CO2 solubilities are however slightly high because a Poynting (pressure) correction for the gas solubilities is not included in PHREEQC. For the Johansen Formation (e.g., 96 ◦C, 300 bar, 0.5M NaCl, from [28]) the true solubility is 1.29 mol/Kgw (using the SAFT v1 method as described in [56]), whereas the PHREEQC solubility is 1.52 mol/Kgw. Nevertheless, the high solubility does not alter the prediction of carbonatization potential as simulations were run at a constant CO2 pressure and pH rapidly approaches a value close to 5 (4.9–5.4) at calcite saturation and over a large range of CO2 pressures (100–300 bar) [10,17].

To model the kinetics of mineral reactions the rate equations presented in [10] were used, with a transition state theory (TST) based rate law for dissolution and a nucleation-growth equation for growing mineral phases. The exception was growth of dawsonite, which had to be estimated using a local-equilibrium assumption (forming at equilibrium) because of convergence problems when it was included in the kinetic assemblage. This has been demonstrated to cause an overestimate of the amount of dawsonite that forms at short time-scales, whereas the models are less sensitive at the longer time-scales [17]. Kinetic data (rate constants for dissolution, nucleation and growth, and apparent activation energies), were taken from [17] (Table 2).


**Table 2.** Kinetic data.

(1) Apparent activation energy (kJ/mol) for dissolution, listed in [46]; (2) Reaction order with respect to protons [46]; (3) Growth rate constants at 25 ◦C (mol/m2 s). pH dependencies are unknown and neglected for growth; (4) Rate coefficient at 25 ◦C and pH = 0 (mol/m<sup>2</sup> s), apparent activation energy and reaction order with respect to protons from [58]; (5) Lacks data and set to the same as dolomite [46].

The true reactive surface area (*St*) differs from the geometric values (*Si*) because of grain shape, surface roughness, and because only parts of the surface are taking part in the reaction at a given time. Roughness may increase the surface area by 1–2 orders of magnitude, whereas the fraction of the total surface that is reactive may be 1% or less. These two effects therefore partly cancel each other, but the extent of this is difficult to assess and depends on several factors. Generally, aged sediments may have orders of magnitude lower reactive surface area than activated crushed materials. The sensitivity of mineral carbonatization on the reactive surface area has earlier been demonstrated

in [10,17], and we will here simply use the geometric model (Equation (2)) and focus the sensitivity study on sedimentological features (e.g., chlorite and feldspar morphologies and mean sediment grain sizes).

$$\overline{S}\_{i} = \frac{3\mathbf{x}\_{i}\rho\_{sold}}{\overline{r}\rho\_{i}} \left[\frac{1}{\varphi} - 1\right] \tag{2}$$

For the case studies, we divided simulations into very fine sand (*r* = 0.05 mm) and medium sand (*r* = 0.2 mm), representing typical lower and upper shoreface facies of the Johansen Formation (Figure 2) [28,29]. In these simulations quartz (nucleation surface), feldspar, Fe-chlorite ooids, and rock fragments were considered to be of the same size, whereas clay particles (kaolinite, chlorite, smectite) were considered to have a mean radius of 1 μm. Sensitivity of feldspar occurrences were simulated for 4.8 wt% perthitic K-feldspar, 1 wt% lithic K-feldspar, 2 wt% perthitic Na-feldspar, and 3 wt% plagioclase. Chamosite input was 4 wt% porefill and 1 wt% ooid. The porosity was set at 25% and reservoir conditions (300 bar, 96 ◦C) were not varied between scenarios. Model sensitivity studies for temperature and nucleation growth are on-going.

#### *3.1. Carbonatization of Chlorite*

Two of the chlorite occurrences observed in the Johansen Formation, i.e., in ooids and as pore filling and grain coating cements, display very different reactivity. Because large parts of the chlorite in ooids are inside the grain and prevented from being in contact with the reactive solutions, ooid-chlorite is assumed to have about two orders of magnitude lower specific reactive surface area than the pore-filling and grain-coating chlorites. The abundance of ooids varies in the cored interval of well 31/2-3 in the Johansen Formation, but is generally not dominant relative to more accessible pore-filling, pseudomorph alterations, and grain coats. We therefore varied the fraction of ooid-chlorite from 0 to 20 vol %. Chlorite morphology is also expected to show significant lateral and stratigraphic variations. Chloritic ooids are recognised also in other potential storage formations such as the overlying Cook Formation [34] and in the Gassum Formation [35].

The simulations show that the amount of chlorite dissolved over short to medium time spans (<1000 years) very much depends on the amount that is high-reactive, i.e., the pore-filling and grain-coating chlorite (Figures 6 and 7). The time it takes to completely dissolve the ooidal chlorite is approximately 10,000 years, also in the 20% ooid-chlorite case, and it is therefore no difference in the dissolved amount at this time scale (Figure 7a). Figure 7b shows pH changes and the amount of secondary carbonates (siderite, ankerite, and dawsonite) that form in the 20% chlorite-ooid case over 100 years. Siderite is the only Fe-carbonate to form, and the amount is proportional to the amount of chlorite that dissolved (1.8 moles of siderite formed for each mole of chlorite dissolved). The short delay of four years before onset of growth (Figure 7b) was due to the nucleation induction time. As kaolinite was defined to be at equilibrium with the formation water and dawsonite formed according to the local-equilibrium assumption, dawsonite formed immediately from the dissolved CO2 and the formation water Na<sup>+</sup> and Al3+, but the growth rapidly stopped as no further Na<sup>+</sup> was supplied (Figure 7b).

**Figure 6.** Simulated dissolution of chlorite (chamosite) (e.g., Johansen Formation, 96 ◦C, 300 bar CO2); where chlorite was separated into two parts: (1) highly reactive pore filling/grain coating chlorite with large reactive surface area (1 μm grains); and (2) low-reactive ooids (125 μm aggregates where reactions are only assumed on the aggregate surface). Up to 20% chlorite in ooids was simulated.

**Figure 7.** Simulated chlorite dissolution (**a**), and corresponding secondary carbonate formation and pH (dotted curve) evolution (**b**), for 100 years of CO2-chlorite interactions with initial materials consisting of 20% chlorite as ooids, and the remaining fraction as high-reactive pore-filling or grain-coating materials.
