A steady increase in carbon dioxide (CO2) emissions has been documented since the 1950s, primarily attributed to human activities, such as the burning of fossil fuels (Department of Energy and Climate Change, 2016; Edenhofer et al., 2014; McCarthy et al., 2001). To mitigate the potential global temperature increase beyond 1.5°C compared to preindustrial levels, it is imperative to transition from fossil fuels to renewable energy sources and implement technologies for capturing and storing excess CO2 (Intergovernmental Panel on Climate Change [IPCC], 2018).

Geothermal energy, a renewable energy source, has been successfully harnessed for heat and electricity generation in various parts of the world. However, geothermal reservoirs, often located in volcanic regions, present a significant challenge due to the release of naturally occurring volcanic gases, including CO2 and H2S, during production (Fridriksson et al., 2016). Although CO2 emissions from geothermal power plants are approximately 20 times lower than those from coal-burning plants of equivalent size, addressing all greenhouse gas emissions is essential to establish geothermal energy as a truly clean and renewable source (Aradóttir et al., 2015). In the case of Indonesia, which possesses an estimated 40% of the world's geothermal resources, a scientifically informed mitigation strategy could unlock the potential for vast carbon-neutral energy sources (Suharmanto et al., 2015).

Equation (1) suggests that H+ ions must be consumed to favor the formation of solid carbonates, enabling carbon sequestration. Mafic rocks, such as basalts, possess a high neutralization capacity, providing alkaline earth metal cations upon dissolution, which can further contribute to carbonate mineral formation (Matter et al., 2007).

Importantly, geothermal energy can be harnessed from a wider array of rock systems beyond basalt. For instance, Indonesia, rich in geothermal energy resources (Sawin et al., 2017), features reservoir systems composed of andesitic rocks (Boedihardi et al., 1993; Purnomo and Pichler, 2014). Andesite is found in various parts of the world, including Central and South America and multiple locations in Australasia. Nevertheless, research on carbon sequestration in rock types other than basalt remains relatively limited. A recent study on reactive transport modeling of the Kizildere geothermal field in Turkey indicated a prevalence of CO2 solubility trapping and a lack of trapping through carbonate mineralization in metamorphic schist and marble host rocks at 220°C (Erol et al., 2023). The limited CO2 mineralization in this case can be attributed to (i) the scarcity of divalent cations, such as Mg2+, Ca2+, and Fe2+, in the chemical system and (ii) fluid-rock interaction that is primarily controlled by K–Al–Si-containing minerals, for example, muscovite. Batch reactor experiments with rhyolite rock and supercritical CO2 at temperatures ranging from 150 to 170°C and pressures of 35 MPa led to the dissolution of primarily calcite and K-feldspar, and precipitation of calcite and ankerite (Na et al., 2015). Similarly, fractured granite rock samples were used in water–CO2–rock interaction core flooding experiments in a study on CO2-based enhanced geothermal systems at 200°C. The results indicated the predominant dissolution of feldspars and the precipitation of mainly calcite and dolomite (Wu et al., 2021). However, other experiments on CO2-saturated granite geothermal systems at 250°C showed the dissolution of K-feldspar, oligoclase, and epidote, along with the precipitation of smectite (Lo Ré et al., 2014). Carbonate precipitation was only observed upon cooling and degassing, suggesting that longer time scales are necessary for its formation (Lo Ré et al., 2014). Certain studies reported on reactions between acidic fluids and various volcanic glass samples, primarily from Iceland (Wolff-Boenisch et al., 2004, 2006), which revealed a slower dissolution rate for glasses with higher silica content. Consequently, some expected trends could also be proposed regarding the dissolution behavior of rocks that have a less amorphous, more crystalline structure. The dissolution rates in these studies were used to determine calcium release rates, enabling interpretations and estimations of the CO2 consumption capacity of the samples. Nevertheless, these studies primarily focused on ambient temperature and a pH value of 4 or higher, and the samples used were mainly volcanic glass or individual mineral samples, as opposed to rock samples containing crystalline minerals. A recent study presented geochemical modeling results for CO2 injection in the Ungaran geothermal field in Java Island, Indonesia, using data on rock mineralogy and geothermal water chemistry (Utomo and Güleç, 2021). The latter study represented one of the first modeling efforts conducted on intermediate volcanic rock (andesite) geothermal reservoir, and indicated promising results for carbon trapping through mineralization. However, this study concentrated solely on modeling, and has yet to be validated by experimental data.

The primary objectives of this study are twofold: (i) to investigate the geochemical reactions occurring between andesitic rocks and representative geothermal reservoir fluids in Sumatra Island, Indonesia, and (ii) to assess the implications of these fluid–rock interactions for potential implementation of carbon storage technology involving mineral formation through the reinjection of CO2 dissolved in fluids into andesite geothermal reservoirs, in order to assist in determining whether this is a viable sequestration method if basaltic reservoirs are not available within the relevant geographic area.

This study is exclusively focused on andesitic rock types for potential carbon sequestration. Note that an assessment of the field-scale potential of carbon sequestration would also need to examine other factors, such as rock permeability, particularly of the overlying layers, to ensure that the system has some structural and residual carbon trapping potential as well.