Carbon dioxide (CO₂) is one of the primary greenhouse gases responsible for global climate change. As of 2022, global CO₂ emissions related to the energy sector have increased to 37.4 Gt (IEA 2024). To achieve the goal of limiting global warming to no more than 1.5 °C by the end of this century, as stipulated by the Paris Agreement, the greenhouse gas emissions must be reduced by 45% compared to current levels by 2030, as reported in the Emissions Gap Report 2022 by the United Nations Environment (UNEP 2022). It is a significant challenge, hence seeking effective CO₂ utilization and removal technologies is of great importance for mitigating climate change (Hepburn et al. 2019; Zhang et al. 2020). Carbon capture, utilization, and storage (CCUS) technology is widely recognized as a key approach to achieve net-zero CO₂ emissions, and a viable technological option for low-carbon transformation in industries that are difficult to decarbonize, such as steel and cement (Wei et al. 2021). According to International Energy Agency (IEA), in a net-zero emissions scenario, it is anticipated that a global annual CO₂ capture capacity through the implementation of CCUS technologies will reach 1.6 Gt by 2030 and increase to 7.6 Gt by 2050, making an important contribution to global carbon reduction efforts (IEA 2021). As of July 2023, there are 392 CCS facilities in various stages of development, collectively capable of capturing over 360 million metric tons of CO₂ annually. Although only 49 projects are currently operational, the success of demonstration projects and improved risk monitoring have significantly increased public acceptance of CCUS technology (GCCSI 2023).

Geological carbon sequestration (GCS) is commonly implemented by injecting CO₂ into geological bodies or structures within sedimentary basins, such as depleted oil and gas reservoirs, unmineable coal seams, and deep saline aquifers, to achieve permanent carbon storage (Ali et al. 2022; Aminu et al. 2017; Orr Jr 2009). There are four distinct storage mechanisms for GCS, namely structural trapping, residual trapping, dissolution trapping, and mineral trapping. Each with varying temporal dynamics and degrees of contribution and safety in CO₂ storage (Metz et al. 2005). Among these, mineral trapping is considered the most secure storage mechanism because it leads to the formation of stable carbonate minerals. However, sedimentary formations are composed of complex mineral constituents, such as sandstones typically containing minerals like quartz, feldspars (potassium feldspar and sodium feldspar), carbonates (calcite, siderite, and dolomite), and clays (kaolinite, illite, smectite, and chlorite), which exhibit slow dissolution and reaction kinetics with CO₂-saturated brine. Additionally, they lack the divalent metal cations necessary for forming stable carbonate minerals. These characteristics pose challenges to mineral trapping and hinder the establishment of a permanent and effective sequestration process (Jayasekara et al. 2020; De Silva and Ranjith 2012; Benson and Cole 2008).

In this scenario, the mineral carbonation of CO₂ with magnesium-rich and calcium-rich silicates offers an effective alternative for GCS. CO₂ mineralization simulates the natural weathering process of calcium-magnesium silicates. Seifritz 1990 first proposed that alkaline-ultramafic rock carbonate could sequester CO₂, this technique has been recognized as a complementary approach for CO₂ storage. It can be implemented underground (in-situ) and above ground (ex-situ) (Sanna et al. 2014). Ex-situ mineral carbonation involves reactions between captured CO₂ with alkaline materials such as steel slag (Wang et al. 2023), cement (Li et al. 2022), ground reactive rocks (Sanna and Maroto-valer 2016), and mine tailings (Oskierski et al. 2021), to achieve carbonation and produce value-added industrial products (Yadav and Mehra 2021; Liu et al. 2021; Ma et al. 2022b). However, the cost of ex-situ mineral carbonation is much higher than that of CO₂ geological storage, which hinders its large-scale development (Kelemen et al. 2019). On the other hand, in-situ mineral carbonation involves injecting CO₂ into reactive rocks such as basalt, resulting in rapid mineral carbonation/trapping and transforming CO₂ into stable carbonate minerals, greatly reducing the risk of CO₂ leakage and providing a safer, unmonitored option for permanent CO₂ sequestration (Sanna et al. 2014; Snæbjörnsdóttir et al. 2020; Ostovari et al. 2022). Whether it is in-situ or ex-situ mineralization, it is crucial to clarify the mineral dissolution and precipitation reactions along with their respective reaction rates, as these factors directly affect the safety, efficiency, and cost of CO₂ storage.

Indeed, numerous reviews on CO₂ mineralization have been published and the key details have been summarized in Table 1 (Snæbjörnsdóttir et al. 2020; Power et al. 2013; Kelemen et al. 2011; Sanna et al. 2014; Liu et al. 2021; Kim et al. 2023). However, there is a relative scarcity of reviews specifically focusing on in-situ CO₂ mineralization, with greater attention seemingly focusing on the enhancement of efficiency and the optimization of process conditions of ex-situ mineralization. Power et al. (2013) summarized the fundamental principles of mineral precipitation and dissolution, highlighting that the precipitation-dissolution process was primarily governed by supersaturation and can be influenced by factors such as temperature, pH, microbial activity, or solution composition (Geerlings and Zevenhoven 2013; Lin et al. 2022, 2024a). They also emphasized that optimizing reaction conditions for direct or indirect mineral carbonation can significantly enhance reaction efficiency. Snæbjörnsdóttir et al. (2020) extensively discussed the types and distribution of rocks suitable for in-situ mineralization techniques, proposing a set of screening criteria that consider factors such as injectivity and safety (Kelemen et al. 2011; Raza et al. 2022). This has guided site selection in in-situ mineralization projects. Besides, Sun et al. (2023) provided a comprehensive review of experimental tools and modeling methods for enhancing the characterization of changes at both the pore and core scales in mineral carbonation, contributing to the advancement of mineralization storage applications. Furthermore, existing reviews in the field of in-situ mineralization often focus on specific types of rocks (Al Kalbani et al. 2023; Rasool and Ahmad 2023). For instance, Oelkers et al. (2018) and Miller et al. (2019c) discussed the primary factors influencing olivine dissolution rates and analyzed the dynamics of its mineralization to better predict and optimize carbonation rate. Raza et al. (2022) delved into the classification and reactivity of basalt minerals, discussing the sequestration potential and emphasized its significance in the context of CO₂ storage (Rasool and Ahmad 2023).

Overall, the current state of the field lacks a systematic analysis of the factors influencing the kinetics of in-situ mineralization, and there is also an absence of comprehensive summaries on the dynamic interactions between geochemical reactions and fluid flow. These gaps pose significant limitations to the application of this technology and warrant special attention. Therefore, in this review, we aim to address these gaps by elucidating the feedback at the pore scale during mineralization reactions, discussing the kinetic mechanisms of in-situ mineral carbonation, and providing an overview of the current research status in the field of in-situ mineral carbonation technology. The goal is to explore the limitations of this technology, improve the understanding of in-situ CO₂ mineralization, and lay a theoretical foundation for achieving large-scale commercial carbon storage.

2.1 Geochemical processes

In-situ CO₂ mineralization primarily relies on the reaction between CO₂-rich solutions and mafic/ultramafic rocks. Therefore, the process necessitates the presence of CO₂ in a fluid state, which can be done in two ways. The first method involves injecting CO₂-saturated solutions (Carbfix approach) (Matter et al. 2011). Its advantage lies in the complete dissolution of CO₂ in water, facilitating an immediate reaction with metal cations released from basalt and rapid fixation. From a storage safety perspective, this method virtually eliminates leakage risks, negating the need for an impermeable caprock above the reservoir to act as a barrier. However, a notable drawback is the substantial water usage, requiring at least 10 times the amount to dissolve the gaseous CO₂. The second method involves injecting supercritical CO₂ liquid (Wallula approach) (Snæbjörnsdóttir et al. 2020), where some of the reservoir pore space is filled with supercritical CO₂, referred as residual CO₂. Over the span of hundreds to thousands of years, the residual CO₂ serves as a continuous source, dissolving into the surrounding water while maintaining a low pH. However, due to the buoyancy of supercritical liquids, ensuring storage safety necessitates the presence of a low-permeability or impermeable caprock both during and after injection, in order to prevent gas leakage before achieving complete mineralization.

The Carbfix project serves as an example to elucidate the mechanisms of in-situ mineralization, and the geochemical process is depicted in Fig. 1 (Sandalow et al. 2021; Noiriel and Daval 2017). The entire process commences with the dissolution of CO₂ in water and the transformation of the dissolved inorganic carbon (DIC) species (Eq. (1)). Under specific temperature and pressure conditions, the DIC species are strongly pH-dependent. In the presence of H⁺, the bridging bonds between cations on the mineral surfaces and silicate groups are disrupted, leading to the release of divalent metal cations (e.g., Ca²⁺, Mg²⁺, and Fe²⁺) into the solution (Eqs. (2)–(4)). Subsequently, these metal ions undergo precipitation reactions with CO₃²⁻ and HCO₃⁻ in the water, resulting in the formation of stable carbonate rock minerals such as calcite (CaCO₃), magnesite (MgCO₃), and siderite (FeCO₃), thereby sequestering free CO₂ (Eq. (5)). Meanwhile, there are dynamic interactions between changes in flow and transport properties and geometrical evolution in the system while chemical reactions are taking place. When determining the characteristics of mineral carbonation, it is essential to consider both the time-dependent dynamics and the spatialization of the processes from the atomic scale to the continuum scale. Besides, the diversity of rock minerals in the reaction products is influenced by the types of cations involved, the pH value system and the geological temperature. For instance, under high-temperature conditions (< 300 °C), dissolved calcium tends to readily precipitate calcite or aragonite once the solutions are supersaturated (determined by the ion activity product, IAP, for example [Ca²⁺] × [CO₃²⁻]); at intermediate temperatures (> 65 °C), dissolved magnesium tends to precipitate siderite and magnesite (Snæbjörnsdóttir et al. 2020). Conversely, at low temperatures (< 65 °C), the precipitation of carbonates is kinetically inhibited, resulting in the formation of relatively unstable hydrated carbonate minerals, such as hydromagnesite (Mg₅ (CO₃)₄ (OH)₂·4H₂O), dypingite (Mg₅ (CO₃)₄ (OH)₂·5H₂O), and nesquehonite (MgCO₃·3H₂O) (Turvey et al. 2018). From a thermodynamic perspective, the standard Gibbs free energy of the carbonate products is lower by 0 to 180 kJ/mol compared to that of CO₂. This indicates that the formation of carbonate minerals is an energetically favored process, wherein the system transitions from a high-energy state to a low-energy state. As a result, the generated carbonates are relatively stable and less prone to undergoing further decomposition (Snæbjörnsdóttir et al. 2020; Raza et al. 2022; Gras et al. 2020; Gadikota 2021; Turvey et al. 2018).

Schematic view of CO₂ in-situ mineralization

The rates of mineral carbonation reactions are strongly influenced by the chemical properties and reactivity of different rock types, along with the pressure and temperature during CO₂ injection (Kelemen et al. 2019, 2020; Snæbjörnsdóttir et al. 2020). Currently, mafic to ultramafic rock formations predominating in the reservoirs, rich in minerals like wollastonite (CaSiO₃), forsterite (Mg₂SiO₄), antigorite (Mg₃Si₂O₅ (OH)₄), and talc (Mg₃Si₄O₁₀ (OH)₂), both on land and in marine environments including continental flood basalts, oceanic basalts, and mantle peridotites, are deemed suitable for in-situ CO₂ mineralization. The suitability arises from the abundance of readily soluble alkaline metal ions in these geological structures (Sun et al. 2023; Kelemen et al. 2019). While current research primarily focuses on highly reactive rocks such as basalt, previous studies indicate that carbonation can also occur in other rock types, including siliciclastic reservoirs. However, the carbonation behavior in these reservoirs is intricately linked to mineral composition and heterogeneity (Mishra et al. 2023; Yang et al. 2014; Higgs et al. 2015; Kim et al. 2023). Distinct mineral compositions yield significant variations in geochemical reactions. For instance, the dissolution of minerals like feldspar and chlorite holds substantial potential for enhancing the neutralization of acidic formation water. Ca, Mg, and Fe divalent cations, released by these minerals facilitate the precipitation of carbonate minerals (Mishra et al. 2023). Heterogeneity, on the other hand, introduces spatial variation in geochemical reactions and reservoir properties, serving as impermeable barriers or baffles to CO₂ (Yang et al. 2014; Higgs et al. 2015). Although it is commonly believed that the carbonation in low-reactivity rocks like sandstone, may be limited initially, this aspect should not be disregarded, especially when considering the long-term behavior of injected CO₂.

Table 2 provides a list of the mineral amount required to sequester one ton of CO₂ and the absorption potential for different minerals. It is observed that forsterite is the most effective mineral for optimal sequestration (Raza et al. 2022; Oelkers et al. 2008). However, due to the multitude of parameters influencing mineralization reactions, it is challenging to definitively determine the type of carbonate precipitate and reaction rate during CO₂ injection. Therefore, a comprehensive assessment of various factors should be taken into account when evaluating in-situ mineralization sequestration sites.

2.2 Storage potential for in-situ CO₂ mineralization

When discussing mafic and ultramafic rocks as candidates for in-situ mineralization, their relative abundance is a crucial consideration. Basalt is abundant and widely distributed, being a major component of the oceanic crust. In contrast, peridotite is quite rare even as an outcrop and is primarily found in the mantle (NASEM 2019; Sanna et al. 2014). This distinction is significant in carbon sequestration and mineralization reactions, as the widespread distribution of basalt makes it more feasible for large-scale applications, whereas the scarcity of peridotite limits its potential for extensive use. Basalt reserves are abundant, constituting a significant portion of the global seafloor (approximately 70% of Earth's surface area) and more than 5% of terrestrial land. It is reported that the carbon storage capacity for onshore basalts globally ranges from approximately 1.0 × 10³ ~ 2.5 × 10⁵ Gt (NASEM 2019). While for submarine basalts, it is estimated to be around 8.0 × 10² ~ 1.0 × 10⁵ Gt, along the capacity for onshore peridotites is approximately 6.0 × 10⁴ ~ 6.0 × 10⁵ Gt (NASEM 2019). Therefore, as an effective complement to CCUS technology, in-situ CO₂ mineralization can expand the scope of potential storage sites, facilitating better source-sink matching and reducing overall costs. In contrast, the amount of CO₂ absorbed through basalt on continents and volcanic islands accounts for 30% of total CO₂ absorption through silicate weathering on land, highlighting substantial carbon sequestration potential (Snæbjörnsdóttir et al. 2020). Previous studies (Goldberg and Slagle 2009; Wiese et al. 2008; Snæbjörnsdóttir et al. 2014; Snæbjörnsdóttir and Gislason 2016; McGrail et al. 2006) have conducted rough assessments of carbon sequestration potential in seafloor plateau basalts, mid-ocean ridge basalts, and continental flood basalts. The results indicated that the sequestration capacity of flood basalts near Washington and British Columbia was estimated to be approximately 5.0 × 10² to 2.5 × 10³ Gt (Goldberg and Slagle 2009). In the region of Iceland, every 10 m³ of mid-ocean ridge basalt can naturally absorb over 1 metric ton of CO₂, leading to an extrapolated theoretical storage potential of 1.0 × 10⁵ to 2.5 × 10⁵ Gt for mid-ocean ridge basalts globally, which surpasses the total amount of CO₂ released from fossil fuels (Wiese et al. 2008; Snæbjörnsdóttir et al. 2014; Snæbjörnsdóttir and Gislason 2016). Continental flood basalts are primarily found in the United States and India, with the Columbia River Basalt Group in the United States estimated to have a sequestration potential of approximately 36–148 Gt (McGrail et al. 2006). Peridotite is a key constituent of serpentinite, with the largest known exposure being the Semail serpentinite, approximately 350 km in length, and 40 km in width, with an average thickness of 5 m. Within this formation, about 30% is mantle peridotite (Sanna et al. 2014). Kelemen and Matter (2008) have assessed the immense potential for in-situ mineral carbonation in the Semail serpentinite. They estimated that if all the Mg²⁺ in the mantle peridotite of the Semail serpentinite were transformed into carbonate minerals, it could sequester 7.0 × 10⁴ Gt of CO₂.

However, due to the involvement of complex physicochemical reactions (a complex function of temperature, pressure, wettability, rock properties, and fluid composition) in the in-situ mineralization process, its sequestration potential may change over time (Raza et al. 2022). Consequently, there is no specific calculation method available for accurately assessing the potential of in-situ CO₂ mineralization. Current estimation approaches typically relies on initial rock reservoir assessments (Kang et al. 2018; De Silva and Ranjith 2012). These assessments begin with volumetric methods to identify suitable rock reservoirs for in-situ mineralization storage. Subsequently, detailed characterization of mineral components and pore characteristics within the rocks is performed using techniques such as mercury intrusion (MICP) and X-ray computed tomography (X-CT) (Sun et al. 2023). Finally, the storage capacity is inferred based on Eq. 5, while the calculation method for rock reserves is shown in Eq. 6 (Bachu et al. 2007).

where, V_rock is the theoretical volume of the rock, m³; A is the reservoir area, m²; h is the reservoir thickness, m; φ is the reservoir porosity; S_w is the water saturation; V_iw and V_pw respectively refer to the volume of water injected into and produced from the reservoir during the recovery or sequestration phase, m³.

Current research often neglects the time-dependent mineralization rates, leading to varied estimation methods and assessments of sequestration potential. Kang et al. (2018) analyzed the distribution of basalt reservoirs in the northern East China Sea through two-dimensional seismic and well logging data, estimating a CO₂ storage potential of 0.59 to 2.48 billion metric tons. Based on volumetric methods and mineral trapping mechanisms, Zhang et al. (2023) established an evaluation method for onshore basalt in China, estimating a potential of approximately 4.7 × 10⁴ Gt of CO₂ storage, considering direct injection and carbonated water methods. Although the latter had a smaller CO₂ storage capacity, its mineralization reaction rate was faster, hence it is preferable. The recent study by Zhang and Tutolo (2022) highlighted the substantial in-situ mineralization potential within glauconitic sandstones in Alberta Province, estimating a capacity to mineralize 150–590 Gt of CO₂ based on a reserve assessment of about 1.0 × 10³ Gt of glauconite. This notable potential arose from the naturally high proportion (20%–50%) of Fe (II) and Fe (III) in glauconite. Moreover, Ramos et al. (2023) asserted that the bottom flood basalts in the Campos Basin of Brazil exhibited notable CO₂ sequestration capabilities. Through modeling of a hypothetical reservoir above the Badejo oilfield, with an area spanning of 31 km² and a thickness of 300 m, they estimated a storage capacity of CO₂ ranging from 16 to 47 Mt. Overall, existing assessments of in-situ mineralization's sequestration capacity remain theoretical, emphasizing the need for detailed, site-specific evaluations to determine effective CO₂ storage capacities.

Unlike studies assessing carbon storage potential in sandstone formations, the current evaluation methods for in-situ mineralization sequestration lack a hierarchical and coherent system, similar to the technology-economic resource pyramid proposed by the Carbon Sequestration Leadership Forum (CSLF) (Bachu et al. 2007). While the numerical values for the storage potential presented in the aforementioned studies are promising, the current evaluation methods primarily rely on assumptions of complete conversion of calcium, iron, and magnesium into carbonate minerals (Zhang et al. 2023).Limited effort has been made to establish evaluation methods based on geochemical reaction kinetics and engineering feasibility. As a result, the calculated storage capacity often significantly exceeds the actual effective capacity, typically by orders of magnitude. Furthermore, most existing research overlooks the impact of injectivity on carbon storage, particularly in mafic/ultramafic rock formations with low porosity and permeability, where smooth injection of CO₂ may be difficult (He et al. 2024; Kim et al. 2023). Therefore, it is crucial to recognize the fact that existing potential assessments yield theoretical values that are significantly overestimated. To accurately reflect practical in-situ mineralization sequestration capacities, there is a pressing need to develop a more refined evaluation system.

2.3 Feedback between reaction and fluid flow during in-situ CO₂ mineralization

There is a dynamic interplay between chemistry, geometry, flow, and transport processes. Pore-scale feedback between geochemical reactions and fluid flow can induce changes in permeability (see Fig. 1) (Noiriel and Daval 2017). On one hand, as reaction products or secondary minerals accumulate within pore space, they occlude flow path and decrease permeability. Such negative feedback mechanisms can thus influence the flow and transport conditions near mineral interfaces, thereby reducing the mineralization rate (NASEM 2019). This negative feedback is more prone to occur in the carbonation process between CO₂-rich fluids and olivine rocks. Peuble et al. (2019) investigated the impact of CO₂ and fluid flow on olivine alteration, revealing significant decreases in permeability induced by both carbonate minerals and hydrous minerals. This substantiated the negative feedback effect of olivine alteration products on fluid flow. On the other hand, CO₂ injection typically induces rock dissolution, facilitating the opening of conduits, while the volume expansion resulting from carbonate precipitation can create significant stress differentials, potentially leading to the formation of fractures and new fluid pathways. Such a process relies on geometric evolution across various scales, spanning from the crystal surface to the pore space. The differences in reactivity between dissolution along mineral boundaries and reactions between minerals or the formation of secondary precipitates, contribute to altering flow pathways at larger scales. This positive feedback loop may enhance the permeability and expose fresh surfaces to the fluid phase, thereby promoting mineralization reactions (NASEM 2019; Noiriel and Daval 2017; Zhang et al. 2022; Rosenqvist et al. 2023). Further corroboration of this phenomenon was provided by the 3D high-resolution micro-fault scans, revealing that low-temperature carbonation led to conduit occlusion, resulting in reduced permeability (Jöns et al. 2017). Other mineralization experiments have observed the emergence of crack networks due to the precipitation of secondary minerals, leading to an increase in permeability and fluid transport properties (Lisabeth et al. 2017; Kanakiya et al. 2017). These fracture networks were jointly governed by the interaction of physical and chemical stresses, representing an outcome of chemical–mechanical coupling. Concentration gradients existing between secondary and primary pores facilitated the dynamic flow of dissolved material. This flow, coupled with the stress increment in localized secondary pores, culminated in localized strengthening. Meanwhile, the deposition of secondary minerals along the fracture surfaces could laterally inhibit dissolution and deformation, leading localized strengthening through compressive stresses that induce pore bands, thereby aiding in the propagation of near-critical fractures without requiring substantial crystalline pressure from secondary mineral growth within limited pore spaces. Thus, it allows for the gradual growth of fracture networks under near-critical stress intensities (Lisabeth et al. 2017; Kanakiya et al. 2017).

In recent years, there has been increasing attention on the importance of micro- and nano-scale characteristics within fluid-rock systems. These characteristics encompass attributes such as fluid-mineral surface energy, adsorption capacity and separation pressure. Such characteristics play a pivotal role in determining the occurrence of self-limiting negative feedback (occlusion) and the facilitation of accelerating positive feedback (fracturing) (Lambart et al. 2018; Evans et al. 2017). Nevertheless, due to the complex interplay between reaction kinetics and fluid dynamics, accurately predicting mineral dissolution and crystallization (including nucleation and crystal growth) within porous media remains a challenging task (Deng et al. 2022). Menzel et al. (2022) investigated the reasons behind sustained fluid flow during serpentinization mineralization and discovered that the ductile deformation was involved in the process of serpentinite carbonation, primarily driven by grain boundary sliding, associated with particle expansion flow and dissolution–precipitation. Moreover, lithostatic pressure promoted ductile deformation within the reactive medium and generated new pore channels. Peuble et al. (2018) also reached similar conclusions, suggesting that the dissolution–precipitation coupling during olivine carbonation was governed by nanoscale mass transfer mechanisms. These mechanisms maintained a nano-porous interconnected network near the reaction interface, ensuring sufficient permeability and promoting continuous reactions. It is proven that the feedback from external stresses, changes in rheology, and elevated pore pressure parameters enhanced fluid flow and carbonation process, due to the interdependence of pore space alterations and the intricate interplay between transport and reaction kinetics. These factors, ultimately dictate the evolution of rock porosity, permeability, and specific surface area. This relationship can be encapsulated by the Damköhler number (\(D_{{\text{a}}} = \frac{r}{vc}\)), which is defined as the ratio of the reaction rate and the diffusion mass transfer rate at the fluid/solid interface; where, r is the intrinsic reaction rate in mol/m² s, v is fluid velocity (from Darcy's law), and c is the solubility in pure water of the considered mineral (Osselin et al. 2022; Soulaine et al. 2017). For instance, when the injection rate is too low, it may irreversibly reduce permeability near the injection point, resulting in a lesser degree of mineralization. On the other hand, a higher flow rate generally enhances carbonation, as it sustains higher permeability for longer durations. However, excessively high velocities can lead to surface passivation and diminish the carbonation potential (Andreani et al. 2009; Peuble et al. 2015). Peuble et al. (2015) posited that this arises from the dual control of reaction kinetics and transport processes. At low flow velocities, the elevated concentrations of elements released during silicate dissolution indicated the dominance by chemical kinetics. Conversely, at high flow velocities, there were no changes in the fluid concentration of these elements, indicating that fluids and minerals reached instantaneously local equilibrium and that effective reaction rates were controlled by transport. Thus, it is important to control the injection rate to optimize the in-situ carbonation. Additionally, the injection rate significantly impacts fluid injectivity. At lower CO₂ injection rates, continuous injection leads to the evaporation of formation water, causing salt precipitation that can clog pores and reduce reservoir injectivity (He et al. 2022). Increasing the CO₂ injection rate can inhibit capillary backflow, effectively mitigating local salt accumulation at the injection point and preventing permeability reduction (Qin et al. 2024; Cui et al. 2023). He et al. (2024) proposed a variable injection strategy, suggesting an early transition from low to high injection rates to prevent solution backflow near the injection interface, thereby minimizing salt accumulation and its adverse effects on injectivity.

In a word, the feedback between reaction transport and fluid flow during in-situ mineralization is highly complex. A thorough and accurate understanding of the physicochemical mechanisms at the fluid–solid interface and the coupled dynamics between geochemical reactions and fluids is challenging. Therefore, developing more precise characterization techniques to elucidate this intricate process is essential.

Understanding the dynamic mechanisms governing mineral dissolution and precipitation is crucial for advancing the kinetics of mineralization reactions. It is proved that the in-situ CO₂ mineralization rate depends on factors such as the abundance of reservoir rock minerals and the rate of cation release. This interplay is highly influenced by a constellation of factors, including temperature, pressure, pH, fluid flow velocity, fluid composition, and the interfacial area of the minerals (Zhang and DePaolo 2017; Snæbjörnsdóttir et al. 2020; Matter and Kelemen 2009; Kelemen and Matter 2008).

3.1 Mineralization reaction rate

Due to the complexities of geochemical reactions, there is currently no universally standardized quantitative formula for mineralization kinetics. Depending on the specifics of the reaction conditions, the prevailing approaches often involve the utilization of two dynamic equations, which are Shrinking Core Model (Sun et al. 2008; Wang et al. 2017, 2021a) (Eq. 7) and Avrami Model (Miller et al. 2019c; Wang et al. 2017) (Eq. 8), to calculate the rate of mineralization reactions. However, adjustment should be made for these models by including empirical realities.

where α is the extent of mineralization reaction (%, Eq. (9)); k is the apparent rate constant (min⁻¹); t is the time of mineralization reaction (min); n is an empirical constant depending on the reaction mechanism; \(P_{{\text{CO}_{2} 0}}\) and \(P_{{\text{CO}_{2} t}}\) are the partial pressure of CO₂ when the reaction starts and stops, respectively. For the scenario where n = 1, Eq. (7) elucidates a reaction governed by phase boundaries, where the particle surface serves as the reaction zone with rapid and dense heterogeneous nucleation, followed by movement of a reaction front from the surface towards the interior of the particle. When n = 2, Eq. (7) signifies a reaction constrained by diffusion, wherein the rate-determining step involves the diffusion process across a layer of product, and the reaction region diffuses through the product layer into the unreacted core of the particle.

Under the classical homogeneous nucleation theory, nucleation is facilitated through the formation of minute nuclei that traverse the energy barrier within metastable larger-volume pre-existing phases, leading to the emergence of novel phases. This energy barrier, intricately related to interfacial energy and affinity, serves as a pivotal determinant governing both the quantity and attributes of resultant particles (Abdolhosseini Qomi et al. 2022; Wallace et al. 2013). In comparison, heterogeneous nucleation is more advantageous due to its potential to exhibit lower energy barriers (Scheifele et al. 2013). In the initial stage, the formation of a metastable solid phase takes place due to the presence of a smaller nucleation barrier compared to that of inhibiting the nucleation of the stable phase. Eventually, nucleation of the stable phase occurs, either heterogeneously on or within the metastable particles, or homogeneously within the surrounding solution. This event consequently leads to the dissolution or recrystallization of the metastable phase, as commonly observed in systems like calcium carbonate (De Yoreo et al. 2015). Under ambient environmental conditions, calcite is the most stable phase, followed by aragonite, and subsequently vaterite (Declet et al. 2016). The precipitation of CaCO₃ initiates from supersaturated solutions, followed by dehydration and recrystallization into vaterite, which is the least stable polymorph. Lastly, vaterite dissolves and undergoes recrystallization into the stable calcite phase (Lin et al. 2022). In the carbonation process of magnesium-rich ores, the formation of magnesite and dolomite (CaMg (CO₃)₂) within their respective polymorphic/intermediate states is governed by sluggish kinetics, despite the thermodynamic stability. This behavior can be attributed to the following factors: (1) The carbonation process of Mg precursors is predominantly influenced by the formidable energy barrier associated with the dehydration of Mg²⁺·6H₂O cations, leading to a preferential formation of hydrated magnesium carbonate polymorphs. (2) The structural and spatial hindrance presented by the CO₃²⁻ groups within the rhombohedral arrangement of dolomite and magnesite significantly influences the system, forming the corresponding polymorphs (Santos et al. 2023). As newly formed particles at the mineral–water interface are typically amorphous and hydrated, the physicochemical properties of nuclei and surfaces undergo substantial and dynamic transformations over time and in response to hydration chemistry. To address this challenge, Jun et al. (2016) employed a time-resolved in-situ technique utilizing Small-Angle X-ray Scattering (SAXS) and Grazing Incidence Small-Angle X-ray Scattering (GISAXS) to assess nucleation kinetics, and confirming that by coupling this method with the latest development of surface characterization technology, the understanding of nucleation can be further improved.

3.2 Mineral dissolution rate

Mineral dissolution is the rate-limiting step in in-situ CO₂ mineralization sequestration processes, wherein the ability of silicate minerals to release divalent metal cations (e.g., Ca²⁺, Mg²⁺, and Fe²⁺) directly determines the rate of mineralization reactions (Zhang and DePaolo 2017; Daval 2018). Studies have shown that the dissolution rate of wollastonite was the highest, reaching values of 8.0 × 10^–9 ~ 2.0 × 10^–7 mol/m²·s at 25 °C and 1.6 × 10^–5 ~ 5.0 × 10^–4 mol/m²·s at 180 °C. However, the availability of wollastonite is limited, with a global estimated reserve of only 100 million tons. In contrast, forsterite exhibited a dissolution rate comparable to that of wollastonite when reacting with CO₂-rich fluids. Furthermore, as a principal constituent of Earth's mantle peridotites, forsterite has abundant reserves, and is thus a promising candidate for mineral carbonation sequestration (Kelemen et al. 2019; NASEM 2019). In recent years, a multitude of experiments and numerical investigations have been undertaken to elucidate the kinetics of mineral dissolution and carbonation. However, research specifically centered on in-situ mineralization is still scarce, so we summarized the outcomes of related studies, as documented in Table 3. It is evident that the dissolution rate of minerals is primarily governed by factors such as pH, temperature, and mineral surface area.

3.2.1 The effect of pH

Extensive investigations have shown that the release rates of Ca²⁺ and Mg²⁺ from magnesium-iron-bearing minerals during mineral dissolution are significantly influenced by pH. Dissolution rates exhibit a strong dependence on fluid pH, as demonstrated by Snæbjörnsdóttir et al. and illustrated in Fig. 2 (Snæbjörnsdóttir et al. 2020). Notably, aluminum-bearing minerals (such as plagioclase and basaltic glass) exhibit a U-shaped pattern in dissolution rates with increasing pH, with a minimum reaction rate near pH neutral conditions. In contrast, non-aluminous minerals like forsterite and pyroxenes typically experience a continual decline in dissolution rates as pH increases, indicating that these minerals dissolve slowly under conditions conducive to carbonate precipitation (Snæbjörnsdóttir et al. 2020; Gudbrandsson et al. 2011).

a Ca²⁺ and b Mg²⁺ release rates from mafic rocks and minerals under different pH at 25 °C. Adapted with permission from Snæbjörnsdóttir et al. (2020)

Low pH conditions are preferable for silicate dissolution, associated with increasing rates of calcium and magnesium leaching. Conversely, higher pH conditions are conducive to carbonate precipitation. Hence, a pH-swing process is engendered (Azdarpour et al. 2015). By adjusting the fluid composition, the acidity of the solution can be changed, thereby facilitating elevated mineralization rates. Research findings indicated that the mechanisms of enhancing mineral dissolution kinetics vary with the kinds of acidic substances added. Acids capable of yielding protonating ions serve to promote dissolution through ligand-mediated pathways, while non-protonating acids (such as HCl and HNO₃) expedite dissolution by intensifying overall acidity (Lu et al. 2022; Rashid et al. 2022). Inorganic ligands, including sulfates, carbonates, and phosphates, can enhance mineral dissolution rates through the formation of surface complexes. The dissolution experiments conducted by Lu et al. (2022) revealed that acids containing HCO₃⁻ or H₂PO₄⁻ could adsorb onto the surface of serpentine, forming inorganic ligands. These ligands interacted with the Mg-O bonds and facilitated the detachment of Mg²⁺ from the surface, ultimately augmenting the initial dissolution rate of serpentine. Organic ligands similarly influenced fluid-rock interactions, thereby regulating mineralization efficiency. Miller et al. (2015) investigated the impact of organic ligands such as acetate, oxalate, citrate, and malonate on mineralization reactions in scCO₂ fluid. They discovered that they could enhance the dehydration of Mg²⁺, thereby facilitating the conversion of hydro-magnesite to magnesite (Miller et al. 2018c). Reynes et al. (2023) employed 2,2'-bipyridine as a ligand to synthesize iron (II) carbonate (FeCO₃) through cationic complexation, aiming to facilitate the carbonation process of iron (II)-rich silicate minerals like ferrous olivine. Their results indicated that under conditions of pH = 11 and 80 °C, a maximum carbonation efficiency of 50% could be achieved. However, the use of high organic ligands could hinder the growth of magnesite. The results of Gautier et al. (2016) concluded that the presence of organic ligands inhibited the growth of magnesite. This inhibition primarily resulted from (1) the chelation of Mg²⁺ in solution, which reduced the solution's saturation state, and (2) the adsorption of organic ligands onto growth sites on the magnesite surface, decreasing the kinetic rate constant of magnesite growth. In pH-swing experiments aimed at regulating alkalinity, most carbonation studies employed buffered solutions of NaHCO₃ or KHCO₃ with concentrations ranging from 0.4 to 1.0 mol/L, or even as high as 8 mol/L, to maintain pH around 8. However, practical applications in sequestration site aquifers often require consideration of the natural fluid chemistry, which hasn't been extensively utilized for controlling alkalinity (Tutolo et al. 2021).

In recent years, the concept of in-situ pH-swing has been proposed as a means to achieve in-situ mineralization sequestration. Lowering the solution's pH through the introduction of suitable reagents to facilitate the extraction of calcium and magnesium. As the reaction progresses, the pH is subsequently elevated, thereby promoting carbonate precipitation (Azdarpour et al. 2015; Arce et al. 2017). Miao et al. (2023) demonstrated that the introduction of ammonium salt solutions (NH₄Cl, (NH₄)₂SO₄, and CH₃COONH₄) could enhance mineralization reaction rates and carbonate efficiency as a result of increased acidity of ammonium solutions, which facilitated the extraction of Ca²⁺. Subsequently, the generation of free ammonia shifted the solution to an alkaline state, thus intensifying the kinetics of carbonation. Furthermore, recent investigations by Mesfin et al. (2023) have explored the impact of cationic chloride concentrations on mineral dissolution rates. Their findings revealed that the addition of CaCl₂ or MgCl₂ enhanced the dissolution rate of basaltic glass while diminishing the dissolution rate of labradorite. This suggested that these divalent metal ions might inhibit the formation of silicon-rich activating complexes on the surface of calcium-rich plagioclase. Similar findings were also reported by Marieni et al. (2021). In the presence of seawater, the precipitation of magnesium-rich aluminum silicates led to pH buffering below 6, resulting in a reduction of mineralization efficiency.

3.2.2 The effect of temperature

Temperature is another crucial factor influencing mineral dissolution rates, often described by the Arrhenius equation to depict the temperature dependence of reaction rates (Schott et al. 2009). According to predictions, the reaction rate of olivine rock at 30 °C and 0.04 kPa \(P_{{\text{CO}}_2}\) (atmospheric \(P_{{\text{CO}}_2}\)) is approximately a million times slower than at 185 °C and 1.5 × 10⁴ kPa \(P_{{\text{CO}}_2}\) (Kelemen et al. 2011). Research conducted by Oelkers et al. (2022) indicated that the carbonation reaction of basalt was slow at 25 °C. However, at 100 °C, over 95% of the injected dissolved CO₂ in the solution would be sequestered as carbonate within five years. (Delerce et al. 2023) investigated the dissolution rates of naturally weathered basalt. Despite cation release rates being 1 to 3 orders of magnitude slower compared to unaltered basalt, mineral dissolution rates significantly increased with rising temperatures. Yadav and Mehra (2019) utilized kinetic models to forecast the potential of in-situ mineralization sequestration for wollastonite. Their findings indicated that the dissolution reaction was governed by the diffusion of various mineral ions through particles, with reaction temperature playing a crucial role in the carbonation process of wollastonite (Kashim et al. 2020). Numerous studies have investigated the dissolution rates of individual common minerals under different pH and temperature conditions. Despite the consistency observed in these findings, there is still ongoing debate regarding their applicability to natural rock-fluid systems. The general expression for mineral dissolution rates is commonly represented as Eq. 3.5 (Zhang and DePaolo 2017). However, specific minerals, such as forsterite (Declercq et al. 2023; Miller et al. 2019c; Rimstidt et al. 2012), basaltic glass (Gudbrandsson et al. 2011; Raza et al. 2022), pyroxenes (Oelkers 2001), and plagioclase (de Obeso et al. 2023; Oelkers 2001) exhibit variations in the calculation process, requiring the optimization of the approach based on experimental data fitting.

where, r is the reaction rate constant; A₀ is the temperature-independent pre-exponential factor; E_a is the activation energy; R_mineral is the mineral dissolution rates; S_r is the reaction surface area; k is the absolute kinetic rate constant, and it serves as an explicit function of pH, temperature, or other characteristics of the solution in certain cases; Q is the ionic activity product of the mineral in the solution; K is the equilibrium ionic activity product of the mineral in the solution; α is the empirical constant.

3.2.3 The effect of mechanochemistry and mineral constituents

Mechanochemical processes are also considered an important means of influencing mineralization efficiency. Chen et al. (2023) introduced a straightforward method to improve the reactivity and solubility of silicates by co-grinding with calcite, which activates serpentine. During co-grinding, there's an interfacial reaction where mutual ion exchange and calcium-magnesium substitution occur between serpentine and calcite, resulting in the formation of magnesite. Parallel findings were reported by Stillings et al. (2023), who demonstrated that comminution processes augment the CO₂ capture proficiency of minerals. This enhancement was attributed to the chemical adsorption of CO₂ into the crystalline structures during the fragmentation of rock. The energy liberated upon the rupture of chemical bonds under pressure serves as a heat source, facilitating the requisite energy demand of the reaction.

Furthermore, it is essential to consider both the chemical composition and microstructure of minerals when assessing their carbonation potential. La Plante et al. (2021) compared the carbonation potential of different minerals, revealing that the rate of carbonation and the release of calcium ions were influenced not only by the total Ca (and Mg) content but also by the Si content. This suggested that the dissolution of silicates was controlled by the atomic topology (network connectivity) of the solid reactants. Wang et al. (2021a) conducted an investigation demonstrating that irrespective of the olivine content and mineral composition, the mineral carbonation of natural silicate samples was fundamentally governed by the kinetics of olivine dissolution reactions. Hence, the computation of the effective mineral carbonation potential based on the magnesium and iron content within olivine are of paramount importance for the kinetic analysis of carbonation in natural samples. Van Noort et al. (2013) also highlighted that the increase of less reactive mineral constituents did not inherently constrain the dissolution kinetics of peridotite. Compared with the proportion of highly reactive minerals in ultramafic rocks, the microstructural such as fractures, predominantly influence the rates of peridotite dissolution and carbonation. This is due to the fluid infiltration of fluids along grain boundaries, leading to an amplification in the active surface area of reactive olivine.

3.2.4 The effect of secondary minerals

It is worth noting that the formation of secondary phases (such as amorphous silica, clays, etc.) at the native mineral-fluid interface in the form of nanoporous layers or alteration layers, as well as the presence of non-reactive phases in heterogeneous rock materials, adds complexity to the prediction of mineral dissolution. This complexity arises due to their influence on material and energy exchange at the fluid-mineral interface (Calabrese et al. 2022). Several studies have indicated that silica-rich passivation layers may form through reprecipitation on minerals, reducing the dissolution rates of rocks and carbonate precipitation by decreasing the surface area of minerals directly exposed to reacting fluids. The extent to which secondary minerals impact the dissolution rate of primary minerals usually depended on factors such as structural compatibility between the two minerals, relative differences in solubility, and the presence of interconnected porous pathways within secondary phases (Oelkers et al. 2018). The molecular dynamics simulations demonstrated by Béarat et al. (2006) revealed that the intrinsic permeability of amorphous SiO₂ allowed for the diffusion of reactants through the silica-rich layer, highlighting the dynamic complexity of the passivation layer composition. Daval et al. (2011) indicated that the passivation ability of SiO₂ coatings depended on the competition between the intrinsic dissolution rate of silicate and the restructuring (densification) rate of amorphous SiO₂. These processes affected the porosity of the passivation layer, thereby regulating its inhibitory effect on the dissolution of the underlying phase (Cailleteau et al. 2008). This also accounted for the observed differences in reactivity between olivine and wollastonite, as wollastonite dissolved more rapidly than olivine. They further noted that the extent of densification of the amorphous SiO₂ layer, and consequently its impact on ion transport, could be partially governed by the absolute dissolution rate of the mineral, attributed to the surface-specific rate of densification of the SiO₂ layer, which was correlated with the initial dissolution rate of the wollastonite (Daval et al. 2017). Sissmann et al. (2013) contended that passivation was a transient process easily influenced by external forcing, and the mineralogical and passivation characteristics of the interface layer vary with temperature. Similar conclusions were drawn by Johnson et al. (2014), who observed a reduction in the dissolution rate of magnesium olivine at 60 °C due to the formation of an amorphous SiO₂ coating on the surface. Yet, the dissolution process did not cease, as the layer became thicker over time. Another crucial factor influencing the passivation effect of the SiO₂ layer on the surface of olivine was the presence of trivalent iron (Fe (III)), generated due to the rapid oxidation of Fe²⁺ released during olivine dissolution. The interaction between Fe (III) and SiO₂ under aerobic conditions resulted in an inhibitory effect on olivine dissolution (Sissmann et al. 2013; Saldi et al. 2013, 2015). Further support for the role of Fe in determining secondary coating performance came from studies on Fe-free synthesized magnesite, where the absence of a passivation layer rich in Fe³⁺-Si prevents reaction inhibition (Li et al. 2018; Miller et al. 2018a, 2019a). Furthermore, the fluid composition played a controlling role in the performance of the passivation layer. NaCl and NaHCO₃ have been demonstrated to influence the dissolution and carbonation of magnesium olivine by inhibiting the formation of silica-rich coatings (Miller et al. 2019c).

3.3 Mineralization in nanoscale water films

In-situ CO₂ mineralization sequestration follows the classical dissolution–precipitation pathway, leading to the concentration of research efforts on the reactivity of mineral carbonate reactions within CO₂-acidified aqueous phases. It is indicated that the mineralization is heavily influenced by water concentration, as water content can impact the dissolution, mass transfer, and carbonate precipitation of silicates (Lin et al. 2022; Thompson et al. 2013). Insufficient water content can drastically lower the leaching rate of cations, then decelerating or even halting the mineralization rate (Wang et al. 2021b). Furthermore, under conditions of limited water content, mineralization reactions can engender a passivation effect, leading to the accumulation of reaction products on the surface of silicate minerals. This phenomenon significantly reduces dissolution and/or carbonation rates (Mergelsberg et al. 2023). The extensive implementation of mineral carbonation sequestration often necessitates the consideration of substantial water consumption. In scenarios involving water-deficient scCO₂ injection schemes or reservoirs with less water, scCO₂ can dissolve a minor quantity of water, resulting in the formation of a wet scCO₂ fluid. This aspect appears to be unfavorable for mineral carbonation. However, recent research has revealed unexpectedly high reactivity between metal silicates and wet scCO₂ fluids. A quasi-two-dimensional confined interfacial water film, with a thickness ranging from angstroms to nanometers, forms on the mineral surface, serving as a medium for precipitation and dissolution (Placencia-Gómez et al. 2020; Abdolhosseini Qomi et al. 2022; Kerisit et al. 2012; Miller et al. 2013). As shown in Fig. 3, the crucial steps of mineralization reaction occurring within nanoconfined water films encompass: (1) water separation and nanofilms formation; (2) alterations in interfacial morphology; (3) acceleration of dissolution through H⁺-promoted and ligand-enhance; (4) formation of interfacial metal-carbonate ion pairs; (5) quasi-2D diffusion of dissolved ions and ion pair; (6) nucleation and growth of the metal carbonate phase (Abdolhosseini Qomi et al. 2022).

Molecular-scale mechanisms of CO₂ mineralization in nanoscale interfacial water films

The reasons behind the accelerated molecular reaction rates at the water-phase interface remain uncertain, potentially originating from dissolution effects or the intrinsic acid–base characteristics of the water surface (Ruiz-Lopez et al. 2020). When confined at the nanoscale within or between interfaces, its roles as a solvent and a reactant may differ significantly from its behavior in bulk water (Knight et al. 2019, 2020; Cavanaugh et al. 2019). Water adsorbed on the mineral surface adopts a highly ordered state at the molecular scale, restricting its rotational freedom. This gives rise to reduced dielectric constants and diffusion rates, thereby promoting metal complexation and facilitating the formation of stable low-dimensional surface deposits (Knight et al. 2019; Dewan et al. 2013; Fumagalli et al. 2018; Tokunaga et al. 2017). Besides, due to the extensive contact area between the water film and the mineral, the dissolution of minerals leads to a rapid increase in ion concentration, resulting in the relevant solid phase attaining a highly supersaturated state (Wood et al. 2019). On the contrary, mass translocation occurs via lateral permeation within water films, while the vertical mass transport across mineral surfaces encounters substantial hindrance due to the pronounced solubility of adjacent fluids (Placencia-Gómez et al. 2020; Tokunaga et al. 2017). Abdolhosseini Qomi et al. (2022) normalized rate data from prior studies on natural and synthetic forsterite experiments using the Avrami model. They generated graphs correlating reaction temperature and CO₂ pressure to compare the difference in mineralization mechanisms and kinetics in bulk water and wet scCO₂ fluids. The outcomes, as shown in Fig. 4, revealed higher reactivity of wet scCO₂ at lower temperatures and pressures. This heightened reactivity was attributed to the coupled dissolution of nanoscale particle precipitates and micaceous substrates (e.g., KMg₃Si₃AlO₁₀ (F, OH)₂) within nano-constrained water films, resulting in more significant dissolution compared to corresponding bulk liquid experiments.

Carbonation kinetics in water-rich versus wet supercritical CO₂ fluids, based on the CO₂ pressure–temperature plane. a The triangle and circle markers correspond to synthetic forsterite in wet scCO₂ fluid and natural forsterite in high water-to-rock ratio experiments; b–d The marker colors represent (a) time to 50% conversion (t_50%), (c) the exponent and (d) the logarithm of precipitation rate constants in first-order Avrami Model fitted to these compiled experiments. Adapted with permission from Abdolhosseini Qomi et al. (2022)

Despite the proven capability of nanoscale water films to enhance mineral carbonation rates, the distinct crystallization pathways of minerals within them remain unclear. Current research endeavors continue to focus on unraveling the stability and kinetic mechanisms of carbonate transformation within nano-confined water films. The investigation by Dasgupta et al. (2023) illuminated that nanoscale confinement significantly diminished the activation barrier for CO₂ conversion to H₂CO₃, and conversely altered the thermochemical effect from bulk aqueous to nano-confined water. Moreover, the presence of charged intermediates became more readily discernible under nanoscale confinement conditions. With the increasing of confinement, the intensified solvation effects and proton transfer enhance the thermodynamic and kinetic attributes of the reaction. Notably, the carbonation mechanism within water films appears to deviate from the conventional understanding that CO₂ must primarily dissolve in water. Lee et al. (2015) demonstrated that carbonation reactions preferentially occur at electron-rich terminal Oxygen sites adjacent to cationic vacancies, even with a thin water film. The molecular simulation outcomes of Zare et al. (2022a) indicated that, in comparison to reactions primarily mediated by bulk aqueous environments, silica surfaces in nanoscale water films actively engaged in chemical reactions through long-range hydroxyl transmission and ligand-enhanced dissolution. Within the interfacial water film, reverse proton transfer between bicarbonate and surface hydroxides promoted the formation of carbonate and the surface metal carbonate complexation, consistent with in-situ spectroscopic measurements (Zare et al. 2022b). This difference may affect the macro kinetics of interfacial carbonation. Mergelsberg et al. (2023) observed a distinctive nanoscale passivation effect within the interfacial water film based on the mineralization of basalt with wet scCO₂. Amorphous silica exists as a 2 to 3 nm thick magnesium-depleted layer on reacted forsterite particles, resulting in diminished levels of mineral dissolution. The essential understanding of the fundamental mechanisms driving surface passivation during mineralization under water-deficient conditions is crucial for ensuring the rate and efficiency of carbonation. Besides, it was found that there was a threshold for the dependence of mineralization reaction on water, and the kinetics were correlated with the thickness of the water film (Miller et al. 2020; Schaef et al. 2013; Loring et al. 2011). Loring et al. (2015) indicated that mineral carbonation occurs well below the H₂O saturation within scCO₂. By accounting for water adsorbed on the mineral surface as surface complexes, the average thickness of the water film is only 7–15 Å. Kerisit et al. (2021) proposed that when the thickness of nano-confined water film greater than 1.5 monolayers, it facilitated the growth of magnesite. This was primarily driven by sustained high supersaturation and low H₂O activity, which reduced the free energy barrier for lateral migration of Mg²⁺, enabling the diffusion processes within the water film (Placencia-Gómez et al. 2020). Furthermore, previous research (Miller et al. 2019b, 2018b; Todd Schaef et al. 2013) demonstrated that the reactivity of H₂O thin films was linked to their structural arrangement. Specifically, the rates of carbonic acid formation were associated with the concentrations of H₂O clusters exhibiting a liquid-like structure.

On the whole, laboratory studies have demonstrated the tendency for reactions to occur in water films. However, visualizing nucleation and growth processes at the nanoscale remains a challenge due to the constraints of the nano-environment. Therefore, gaining a deeper understanding of molecular-scale reaction mechanisms is crucial for enhancing the carbon mineralization efficiency.

To date, a number of geological sequestration demonstration projects have been conducted worldwide, yet only limited real cases for in-situ mineralization projects. The phased progress has only been observed in Iceland, the United States and Japan (Ali et al. 2022; Snæbjörnsdóttir et al. 2020; Lin et al. 2024a). The implementation particulars are comprehensively summarized in Table 4.

4.1 CarbFix project

The CarbFix project is situated near the Hellisheiði geothermal power plant in southwestern Iceland, which is the third-largest geothermal power station globally. It stands as the largest existing in-situ mineralization sequestration endeavor. Established in 2007, the project officially commenced operations five years later (Matter et al. 2011). Prior to this, carbonate content was measured in drill cuttings from three basaltic geothermal fields, forming the basis for determining the permeability and porosity, and CO₂ storage potential of the basalt reservoir within the Icelandic Mid-Atlantic Ridge basalt. The results indicated that every 10 m³ of basalt can naturally sequester over 1 ton of CO₂. The target reservoir is located at a depth ranging from 400 to 800 m, characterized by basaltic lithology with minor occurrences of basaltic breccia or intrusive interlayers. The horizontal and vertical permeabilities were measured at 0.3 and 1.7 μm² respectively. The formation temperature ranged from 30 to 80 °C, with the pH levels between 8.4 and 9.4 (Gislason et al. 2010; Matter et al. 2016). Though the presence of a low-permeability layer overlaying the upper section of the target reservoir, the relatively shallow depth allows for the potential upward migration of CO₂ through fractures. Therefore, the design of dissolving CO₂ in water before underground injection is considered (see Fig. 5a). In phase 1 (CarbFix1), a total of 230 tons of CO₂ were injected, comprising two separate injections of 175 tons of pure CO₂ and 73 tons of mixed gas (75 mol% CO₂, 24 mol% H₂S, and 1 mol% H₂) (Sigfusson et al. 2015; Gislason et al. 2010; Galeczka et al. 2022).

Comparison of carbon-injection methods. a The CarbFix method with the dissolution of CO₂ in water during injection into a basaltic reservoir and b The Wallula method with liquid CO₂ was injected into basalts. Adapted with permission from Snæbjörnsdóttir et al. (2020)

Quantifying the rates of CO₂ supply and confirming the presence of precipitated metal phases with identifiable elemental signals pose significant challenges (Gunnarsson et al. 2018). Therefore, monitoring the extent and flux of carbonate and other secondary mineral precipitation solely based on dissolved elemental concentrations is a complex task. it is generally not a common practice to determine carbon sequestration levels by measuring CO₂ solubility and analyzing mineral composition post-mineralization reaction during field trials. Instead, the quantity of stored CO₂ is typically ascertained through the isotopic tracing method. Tracers employed for in-situ mineralization can be classified into reactive and non-reactive categories. Reactive tracers often involve the use of calcium and magnesium isotopes, which are less influenced by degassing effects (Harrison et al. 2021). Non-reactive tracers frequently utilize carbon isotopes, which effectively differentiate between organic carbon (OC, δ¹³C values less than 15%) and inorganic carbon (IC, δ¹³C values greater than 10%). In the case of the CarbFix project, ¹⁴C was introduced into the injected CO₂ to monitor its transport and reactivity. The amounts of dissolved inorganic carbon (DIC), ¹⁴C isotope, and other non-reactive tracers injected were determined, and the difference between these quantities and those measured downstream in monitoring wells was used to calculate the amount of carbon mineralized. Additionally, a mass balance approach was employed to quantitatively assess the fate of the injected CO₂. It was observed that nearly all DIC was consumed from injection well to monitoring well. Consequently, it is estimated that over 95% of the injected CO₂ had mineralized in less than two years, thereby paving the way for progress into the second phase (CarbFix2) of the project (Matter et al. 2016; Pogge von Strandmann et al. 2019; Snæbjörnsdóttir et al. 2017).

For CarbFix2, the injection rate was significantly increased compared to previous phase. From June 2014 to July 2015, a total of 4526 tons of dissolved CO₂ and 2536 tons of dissolved H₂S mixed fluid were injected. The target basaltic layer was deeper and at a higher temperature, approximately 250 °C. The monitoring revealed that a significant portion of the injected CO₂ began to convert into carbonate minerals within a few months. According to Gunnarsson et al. (2018), 1 ton of CO₂ would generate 0.84 m³ of calcite, and 1 ton of H₂S would produce 0.70 m³ of pyrite. If all the gases injected at the CarbFix2 site from June 2014 to December 2017 were to precipitate in the form of calcite and pyrite, the total volume of these minerals would be approximately 2.9 × 10⁴ m³. This accounts to only 0.005% of the Hellisheiði reservoir's capacity (which is 6 × 10⁸ m³), highlighting the significant carbon sequestration potential. Additionally, the operating cost of Carbfix is less than 25 US dollars/t CO₂, making it competitive with carbon allowance prices in the ETS market and cheaper than other CCS methods. The primary energy requirement for the project is the energy needed to pressurize water containing CO₂ to 2.5 MPa at 25 °C, which is approximately 75 kWh (CARBFIX 2022). Currently, this CO₂ fixation plant has been operating smoothly, with a total injection of over 80,000 tons of CO₂ (CARBFIX 2022).

Although the successful operation of the Carbfix project is encouraging, its success is attributed to specific factors, such as favorable geological conditions, abundant freshwater resources, and well-developed geothermal power plant infrastructure. Therefore, future projects should comprehensively consider the geological structure, mineral composition and structure of the basalt reservoirs, and hydrological characteristics of the target site to ensure feasibility. Furthermore, the CO₂ injection method of this project requires a significant amount of water, which may not be suitable for all basalt reservoirs (Marieni et al. 2021). Alternative strategies, such as seawater instead of freshwater, or exploring the mineralization process of wet CO₂ fluid described in Sect. 3.3, are effective solutions to this issue, but require further research. Based on this, the project team is also conducting foundational research on dissolving CO₂ in seawater before injection, to expand the applicability of this technology in water-scarce regions, coastal, and nearshore areas (CARBFIX 2022). Since September 2021, CarbFix has partnered with Climeworks to conduct experiments involving the annual injection of up to 4,000 tons of CO₂ into shallower basalt reservoirs, building on the expansion of direct air capture capabilities. Plans are underway to construct a cross-border carbon transportation and storage hub in Iceland, enabling the maritime transport of CO₂ from other regions for injection into the CarbFix site. The goal is to increase the sequestration capacity to 3.0 × 10⁵ tons of CO₂ per year, with a gradual expansion to 3 million tons per year by 2030 (CARBFIX 2022). It is estimated that the investment for this project ranges from 220 to 250 million euros, with an estimated annual revenue of approximately 25 to 45 million euros at full operational capacity.

4.2 Wallula project

In 2009, the Wallula pilot project was initiated in eastern Washington, USA. It is a part of the U.S. Department of Energy's Regional Carbon Sequestration Partnership Initiative, with an estimated total cost of approximately 2.2 billion US dollars before project implementation. It marked the world's first endeavor to conduct in-situ mineralization sequestration using scCO₂ in a flood basalt formation, at a depth of 800 to 900 m (Fig. 5b) (McGrail et al. 2017a, 2014a). The flood basalt formation is composed of multiple successive units of lava flows. A regional aquifer is presented in the distribution area, characterized by ample porosity and lateral connectivity. The internal fluid flow within this formation can extend to kilometer-scale distances (Zakharova et al. 2012; McGrail et al. 2006), rendering it suitable for CO₂ injection for sequestration. Furthermore, seismic exploration results indicated the absence of fault structures or fractures in the Wallula region, providing a relatively stable geological condition for storage activities. Above the Columbia River basalt formation, there are impermeable sedimentary layers and basaltic layers that serve as caprocks. These caprocks can impede the flow of CO₂, or at the very least, slow down its flow rate, providing ample time for carbonation to occur, and securely immobilizing the CO₂ (Zakharova et al. 2012).

The CO₂ injection operations for this project were completed within a span of 25 days (from July to August 2013), with an approximate daily injection rate of 40 tons of CO₂, totaling 1000 tons injected. Post-injection well analyses revealed that the CO₂ was located at the top of the basalt flow layer and did not exhibit any upward movement. Additionally, testing of the shallow soil gases around the injection well detected no leaks, indicating a successful sequestration process. After two years of continuous monitoring following the injection, researchers retrieved cores from the injection zone and subjected them to detailed physical and chemical analyses. Nodules found within the entirety of the cores were identified as the carbonate mineral siderite, containing calcium, iron, magnesium, and manganese. Further carbon isotope analysis revealed that these nodules chemically differed from naturally occurring carbonates in the basalt, yet exhibited a pronounced correlation with the isotopic signature of the injected CO₂, providing additional evidence of carbonation reactions occurring with the injected CO₂ (McGrail et al. 2014b, 2017b). White et al. (2020) conducted an analysis based on hydrological modeling to assess the changes in near-field, wellbore, and reservoir conditions before and after CO₂ injection in this project. Their findings indicated that approximately 60% of the injected CO₂ was sequestered through mineralization within two years, with the generated carbonates occupying around 4% of the available reservoir pore space. Additionally, Polites et al. (2022) observed the precipitation of calcite and amorphous silica in sidewall cores. They pointed out the co-occurrence of carbonates after CO₂ injection, emphasizing the crucial significance of clarifying changes in carbonate nucleation composition for predicting carbonate stability. This insight aided in estimating the rates, timing, and reaction pathway relationships of mineral dissolution and precipitation.

Compared to Carbfix, Wallula project primarily employs conventional post-injection monitoring methods, including in-well monitoring and sampling analysis. During the injection and sequestration process, subsurface temperature and pressure are closely monitored to promptly understand the injection conditions, and fluid samples are collected for hydrological analysis. However, the monitoring data obtained from these methods cannot estimate the percentage of injected CO₂ that has undergone carbonation. To achieve better monitoring results, more innovative monitoring methods should be proposed in combination with practical conditions.

4.3 Nagaoka project

The Nagaoka project represents Japan's inaugural carbon sequestration pilot initiative, located in the Nagaoka oil and gas field in southern Niigata, and the well configuration at the reservoir depth is shown in Fig. 6a. The target reservoir is situated at a depth of approximately 1,100 m with a thickness of 60 m. The formation exhibits a subsurface water temperature of 48 °C and a pressure of 10.8 MPa, while the injected CO₂ is in a supercritical state. Based on well testing and logging data, the reservoir has been classified into five zones, with Zone 2 (with a thickness of 12 m and porosity of 22.5%) serving as the primary CO₂ sequestration layer. The caprock consists of non-permeable Pleistocene shallow marine formations (White et al. 2006; Mito et al. 2008).

a Plane view of well locations at the top layer of Zone 2 and b Reservoir simulation results for CO₂ storage in the three phases. Adapted with permission from Mito et al. (2008)

Between July 2003 and January 2005, approximately 20 to 40 tons of high-concentration CO₂ (99.9%) were injected daily into the target Haizume formation. Over 17 months, the total CO₂ injection reached 10,400 tons, with a sequestration cost of approximately 44 to 95 US dollars/t CO₂. The project concluded in 2007, with subsequent efforts primarily focused on reservoir monitoring. Mito et al. (2008) demonstrated through reservoir modeling that mineral trapping likely occurred in the early stages of CO₂ sequestration, as shown in Fig. 6b. Additionally, a geochemical TOUGHREACT model established by Yamamoto et al. (2017) indicated that due to pressure-driven processes during injection, reservoir water was displaced, facilitating the gradual diffusion of CO₂. Then it promoted the dissolution of rock minerals (such as plagioclase and chlorite) within the Haizume formation, leading to the generation of more stable carbonate precipitates. It also provided conclusive evidence for the existence of the mineralization mechanism. Building on these findings, Japan officially initiated its CCS demonstration project in 2008, conducting additional geological and engineering assessments.

For the key parameter of CO₂ saturation, which is crucial for predicting CO₂ sequestration amount, the challenge lies in accurately obtaining the CO₂ distribution at well points, particularly after injection, as measuring CO₂ saturation in cased wells presents significant difficulties. The Nagaoka project in Japan was the first in the world to conduct more than 40 in-well geophysical logging sessions after CO₂ injection, setting a precedent for using multiple logging sessions to identify CO₂ changes before and after injection and at different stages (Ma et al. 2022a). The execution of this project demonstrated the feasibility of in-situ mineralization in siliceous reservoirs. However, appropriate porosity and permeability are prerequisites for CO₂ sequestration, as low permeability may result in low storage capacity and hinder CO₂ injection (Kirmani et al. 2024). Strictly speaking, this project does not fall under the category of purely in-situ mineralization sequestration but rather constitutes an onshore saline aquifer storage initiative within siliciclastic reservoirs. However, subsequent advancements in the project have confirmed the presence of in-situ mineral carbonation processes. Therefore, the successful implementation of the aforementioned projects serves as a valuable reference for the advancement of in-situ mineralization.

4.4 Tencent CarbonX Program

In 2023, Tencent launched the CarbonX Program, aiming to support the technological development of CCUS in China, by identifying the next generation of cutting-edge low-carbon technologies (Tencent 2023). A total of 30 leading CCUS projects receiving the fund, anticipated to scale up by 2030. One of these projects focuses on the research and application of rapid CO₂ mineralization in basalt formations. In addition, Tencent collaborated with Carbfix to establish the first demonstration project for CO₂ in-situ mineralization in Asia since 2022. Initial stages such as preliminary research and site selection have been completed, and Leizhou Peninsula of Guangdong Province, China is selected as the sequestration site for further investigation after comparing the distribution and reservoir properties of basalt formations. The basalt formations in the Leizhou Peninsula were formed by mantle plume volcanic activity over millions of years. These formations encompass a vast area (approximately 3,000 square kilometers), primarily composed of alkali olivine and tholeiite. They have undergone low degrees of alteration, exhibit good porosity and permeability, and possess a certain potential for mineral carbonation (Li et al. 2023). The target reservoir is situated at a depth of approximately 300 to 500 m, with suitable porosity and permeability, making it conducive to CO₂ injection. Li et al. (2023) have calculated the theoretical storage potential based on the mechanisms of volcanic rock mineralization, taking factors such as the distribution area, average thickness, and mineral composition of the basalt formations into consideration. The results indicated that the total volume of basalt formations on the Leizhou Peninsula is approximately 257 km³. The theoretical sequestration potential of CO₂ in this region falls between 19 to 459 billion tons. Furthermore, Carbfix is concurrently developing a containerized injection system design, which is scheduled to commence in 2024, involving the injection and sequestration of thousands of tons of CO₂ as part of the carbon fixation demonstration (CARBFIX 2022).

To sup up, in-situ CO₂ mineralization sequestration offers the following advantages in reducing CO₂ emission: (1) The naturally widespread minerals are suitable for in-situ mineralization, presenting considerable potential for CO₂ storage. (2) The geochemical reactions occur rapidly, enabling the conversion of most CO₂ into solid carbonate minerals within a short period. This ensures rapid, permanent, and safe CO₂ sequestration, reducing reliance on reservoir cap conditions. (3) The injected CO₂ can achieve a stable mineral sequestration state relatively faster, avoiding the need for long-term safety monitoring of CO₂ sequestration, thereby reducing sequestration costs. The estimated cost for CO₂ storge in basalt ranges from 20 to 30 US dollars per ton of CO₂.

Nevertheless, several challenges need to be addressed:

1. (1)

   Although the mechanism of in-situ mineralization reactions has been extensively studied, the relationship between geochemical reaction characteristics and storage rates and efficiency remains unclear. It is essential to focus on the effects of various factors on mineralization outcomes, clarifying the formation mechanisms of secondary mineral diversity, and revealing the chemical kinetics of mineral dissolution and reprecipitation at different scales to further enhance mineralization rates and efficiency.

2. (2)

   Current evaluation methods for in-situ mineralization sequestration potential are relatively simplistic and approximate, often overestimating the actual effective capacity. Establishing a comprehensive and reliable system for selecting target reservoirs and evaluating CO₂ storage capacity, considering CO₂ injectability, is imperative.

3. (3)

   There are two existing forms of CO₂ injection: saturated solution and supercritical fluid, each with distinct advantages and disadvantages. Detailed design of CO₂ injection schemes, along with technical, economic, and leakage risk assessments, tailored to different types of target reservoirs, is necessary to identify the most suitable implementation plan.

4. (4)

   Source-sink matching is a critical for CCUS projects. Matching CO₂ emission sources and suitable reservoirs may be challenged by long distances and high costs of pipeline infrastructure. When integrated with direct air capture (DAC), this technology becomes a negative carbon technology, which is beneficial for carbon dioxide removal (CDR). This approach not only effectively removes CO₂ from the atmosphere but also permanently sequesters it through mineralization reactions, thereby significantly enhancing the sustainability and efficacy of CCUS projects.

5. (5)

   CO₂ in-situ mineralization sequestration involves complex multi-process, multi-spatial, and long-term procedures. The safety of storage and its potential negative environmental impacts remain significant public concerns. Therefore, it is crucial to conduct safety and environmental monitoring and impact assessments, ensuring real-time detection of CO₂ leakage and timely adjustments to ensure project safety.

In-situ mineralization sequestration technology aims to permanently sequester CO₂ from emission sources into reactive rocks, avoiding the steps and costs associated with large-scale mining, transportation, and handling of solid reactants. When integrated with DAC technology, it can play a significant role in achieving carbon neutrality. However, due to the complexity of interactions between reaction transport and fluid flow, accurately measuring the reaction kinetics of carbon mineralization is challenging. Mineral dissolution is the rate-limiting step in the in-situ CO₂ mineralization, where the rate of mineralization reactions is directly influenced by the capacity of silicate minerals to release divalent metal cations. This dependence correlates closely with factors such as temperature, pressure, pH levels, fluid composition, and the contact surface area of the minerals. Moreover, water content significantly impacts the dissolution and mass transfer of silicate minerals, as well as the precipitation of carbonate minerals. Recent studies have also emphasized the significance of molecular-scale mechanisms in CO₂ mineralization within nanoscale interfacial water films, highlighting the critical nature of these interactions.

The potential for in-situ mineralization sequestration is immense. Undeniably, accurately controlling the optimal conditions and key parameters for mineralization reactions presents significant challenges. Enhancing the rate and efficiency of mineralization reactions remains a primary focus for future work. The low porosity and permeability of mafic and ultramafic rocks pose considerable challenges for CO₂ injection and sequestration, thereby limiting the effectiveness of in-situ mineralization. Reliable assessment methods for storage capacity, considering reservoir injectability, need to be developed. The approach of CO₂ injection should align with the geological conditions of the sequestration site. Supercritical CO₂ can be injected if the reservoir is sufficiently deep with excellent caprock conditions. Otherwise, CO₂-saturated solutions are preferable for shallower depths with fractures or faults. In addition to conventional physico-chemical detection methods, further development of precise numerical simulations is required to deepen the understanding of multi-scale fluid-rock reaction kinetics and migration patterns, thereby enhancing visual observation and monitoring techniques.