*3.1. CO2 Leakage and Cement Reactions*

The reactive transport simulations show upward flow of CO2 through the microannulus. With the applied 20 bar overpressure, the flow rate of gaseous CO2 through the microannulus is 2.1 × 10−<sup>5</sup> kg/s. During upward flow, a fraction of the CO2 gas dissolves into the cement and caprock pore water and diffuses horizontally into the cement and the caprock, thereby lowering the pH of the pore waters. Cement minerals react with the carbonized water and start to dissolve to buffer the pH. Similar to e.g., Kutchko et al. [6], we observe inward progression of reaction zones in the horizonal direction, which is limited by diffusion (Figure 2). The reaction zones are characterized by: portlandite dissolution and calcite formation, CSH dissolution with amorphous silica and calcite precipitation, monosulfoaluminate and hydrotalcite dissolution with dolomite (max. 0.003 volume fraction), gibbsite (max. 0.0025 volume fraction) and anhydrite (max. 0.009 volume fraction) precipitation. The further

up from the reservoir level, the less advanced the horizontal progression of these zones is, since the reactions only start as soon as the upward migration of the CO2 reaches that level and the pH decreases. After 2 years of CO2 leakage, approximately 1 cm of the cement just above reservoir level is affected by horizontal CO2 diffusion and related reactions (Figure 2). The silicate minerals of the caprock do not show siginificant reactions within the simulated time. There is a minor increase of calcite in the caprock adjacent to the microannulus.

**Figure 2.** The cement mineralogy and porosity after two years of CO2 leakage are plotted for a horizontal transect across the cement sheet and the microannulus, located just above the CO2 reservoir. Cement reacts with CO2 from the microannulus on the right. Note that, even though not well visible, small amounts of dolomite, anhydrite and gibbsite precipitate in the microannulus and cement.

After the CO2 within the microannulus reached the top of the caprock, the pH is around 4.5 throughout the microannulus. In the cement affected by CO2 interactions-adjacent to the microannulus-the pH is roughly 5 and in the unaltered cement nearly 11 (Figure 3). The pH decrease causes full dissolution of portlandite in the microannulus and adjacent cement cells and gradual dissolution of the other cement phases such as CSH, with decreasing amount of dissolution from the reservoir level upwards. As a result, secondary calcite is also highest at the level close to the reseroir (Figure 3). Within the microannulus calcite that precipitated is re-dissolved due to the low pH and flow conditions which allow quick removal of dissolved species. The permeability of the microannulus is predicted to increase from 1.3 <sup>×</sup> 10−<sup>12</sup> m2 to 1.9 <sup>×</sup> 10−<sup>12</sup> m2 after 2 years of CO2 flow and corresponding cement alteration.

**Figure 3.** The background shows a schematic overview of the model with a cement sheet, the microannulus leak path and the adjacent formation rock. The pH, calcium silicate hydrate (CSH) content and calcite content after two years of CO2 leakage are plotted for the lower part of the annulus adjacent to the caprock.

#### *3.2. Microannulus Leakage Versus Natural Sealing*

Four permeability values were selected to simulate different initial leakage rates and to assess the chemical processes within a leaking or self-sealing annulus. Results are discussed after half a year of leakage simulation. The different permeabilities yield different levels of gas saturation, with a higher gas saturation for a higher permeability (Figure 4a). For all scenarios, the flow of CO2 and dissolution of CO2 in the initially water saturated microannulus result in complete dissolution of portlandite within the microannulus—which was 10% cement filled—and subsequent precipitation of calcite (Figure 4b). Above the reservoir, re-dissolution of calcite can be observed. Only for the lowest permeability of 1 <sup>×</sup> 10−<sup>13</sup> m2, CO2 gas does not reach the top of the caprock. This is due to natural sealing of the microannulus, with complete calcite clogging in the middle part of the microannulus. The front of calcite precipitation is characterised by a peak in calcium content (Figure 4c) and an increase in pH (Figure 4d). The permeability of the microannulus depends on the initial permeability and the dissolution and precipitation reactions. There is a high permeability near the reservoir where the calcite content is lowest (Figure 4e). The 1 <sup>×</sup> 10−<sup>11</sup> and 5 <sup>×</sup> 10−<sup>11</sup> m2 scenarios show only little permeability change in the upper part of the microannulus due to portlandite dissolution and calcite precipitation. The initial permeability value 1 <sup>×</sup> <sup>10</sup>−<sup>12</sup> m2 is more affected by calcite precipitation and the 1 <sup>×</sup> <sup>10</sup>−<sup>13</sup> <sup>m</sup><sup>2</sup> scenario shows complete permeability impairment due to calcite clogging.

**Figure 4.** Simulation results along the microannulus for four different initial microannulus permeabilities showing: (**a**) gas saturation, (**b**) calcite volume fraction, (**c**) dissolved calcium, (**d**) pH, and (**e**,**f**) the permeability of the high and low initial permeability scenarios respectively.
