Mixed-Layer Illite-Smectite Illitization under Supercritical CO₂ Conditions

Abstract

The long-term safe storage of CO₂ in geological reservoirs requires the understanding of the impact of CO₂ on clay-rich sealing cap rocks. The reactivity of the mixed layer of illite-smectite was investigated to determine the reaction pathways under conditions of supercritical CO₂ (scCO₂) conditions in the context of geological CO₂ storage. A common clay (blue marl from the Guadalquivir Tertiary basin, southern Spain) was tested under brine scCO₂ conditions (100 bar and 35 °C) for 120 and 240 h. The clay sample (blue marl) contains calcite, quartz, illite, smectite, and the corresponding mixed-layer and kaolinite. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were performed. The illitization of mixed-layer illite-smectite was observed by XRD and confirmed by a variation in the content of different elements (K, Mg, Na, Ca, and Fe) of the transformation, as well as an increase in the specific surface (SSA) of the clay (36.1 to 38.1 m²/g by N₂, 14.5 to 15.4 m²/g by CO₂ adsorption). Furthermore, these reactions lead to mineral dissolution and secondary mineral formation along the CO₂–water–clay intercalations of the source rock were responsible for a change in porosity (7.8 to 7.0 nm pore size). The implications of illitisation, mineral destruction, and precipitation processes on CO₂ storage and clay layer integrity should be explored before deciding on a geological storage location.

1. Introduction

The extensive use of fossil fuels is a major source of greenhouse gas (GHG) emissions, especially carbon dioxide, and is a major contributor to global warming. The main current efforts are devoted to the development of sustainable technologies to reduce CO₂ emissions, even to achieve net zero or negative emissions [1]. The United Nations Paris Climate Agreements, signed in 2016, established a limit for global warming below 2 °C, ideally 1.5 °C above preindustrial levels [2,3,4]. The concept of zero carbon emissions implies that for every amount of CO₂ produced by human activity, the same amount must be removed from the atmosphere, either by preventing its direct emission or compensating for the existing concentration of CO₂ in the atmosphere. In this way, the concentration of CO₂ concentration in the atmosphere and the global average temperature would be prevented from increasing [5].

Carbon capture and storage (CCS), together with carbon capture and utilization (CCU) or a combination of both (carbon capture, utilization and storage, CCUS), are technologies that have a role to play in achieving the objectives outlined above [6]. Geological CO₂ Storage (GCS) underground is one of the ways to contain anthropogenic emissions included in CCS strategies. In GCS, supercritical CO₂ is injected and stored long-term [7,8].

Before injection, for geological storage to be successful, the storage location must meet three fundamentals: (1) high storage capacity, (2) feasibility for injection, and (3) safe storage over time. Safety is a critical condition because CO₂ leakage poses significant risks to other resources such as drinking groundwater, vegetation, and animal life, as well as to human health, despite the return of CO₂ to the atmosphere [8,9,10,11,12,13,14,15]. During the CO₂ storage process, the CO₂ and host and sealing rock interactions should be considered. The main concern in terms of CO₂-rock interactions in this storage stage is a possible decrease in permeability around the well that could affect injection rates and generate overpressures in the reservoir [16]. Large gas transfers characterize the injection phase through the pore space under high pressures and potentially large temperature gradients. Large temperature gradients can play a role in the interactions between CO₂ and rock, but the presence of moisture must also be considered. In terms of geochemical interactions, some reactions must be considered. These reactions include carbonate minerals, as well as sulphate and evaporite minerals, as their reaction kinetics are intense and equilibrium is reached instantaneously [17,18].

CO₂–rock interactions during storage can be studied by field and laboratory experiments, including computational simulations. Recently, these simulations have been of great interest to the scientific research community. At the beginning of this millennium, Johnson and his collaborators [19] developed the first comprehensive approach to validate the simulation of the interactions that result from the injection of CO₂ injection into the host rock.

In long-term interactions, the brine produced by the dissolution of supercritical CO₂ in water plays a significant role. Both CO₂ dissolved in brine and its acidification cause chemical interactions with the host rock, and in some proportion in the sealing rock. The importance of studying long-term interactions is to predict the reservoir’s ability to permanently trap CO₂ as a mineral phase (known as mineral trapping or mineral carbonation). In the case of cover rock, the purpose of the analysis is to evaluate changes in the permeability of the induced porosity, especially at the bottom. These geochemical interactions, which affect aluminum-silicate minerals, such as Ca-/Mg-rich silicates, feldspars, or clay minerals, are characterized by prolonged reaction rates, which in some cases require thousands of years to reach equilibrium [20,21].

Nevertheless, more viable cap rocks are rocks in which carbonation is negligible because the impact on porosity and permeability is of significant interest to avoid risks that weaken the sealing integrity. Several authors [22,23,24,25,26] concluded that the geochemical impact and consequent porosity changes induced by CO₂ through the diffusion of cap rocks are minor and limited to the host and cap rock interface. Other authors, such as Jia and Xina [27], predicted petrophysical aspects using realistic data in pore models such as shale formations.

Knowledge of the behavior of sealing rock, and in particular clayey rock with CO₂, becomes necessary to prevent risks of leakage from stored gas. These leakages can affect the geochemistry of the environment near the storage site, potable water, and prevent CO₂ from escaping uncontrollably into the atmosphere [28,29].

This laboratory study aimed to determine the interactions between a type of clay-based sealing rock and CO₂ under supercritical conditions. Economic reasons include saving costs of transporting CO₂ from the capture plants to the storage site [8], enhanced oil recovery methodologies (EOR), and methodologies with carbon dioxide [30]. Extensive supercritical CO₂ storage projects are being developed [31,32]. Changes in the mineralogy of the sealing rock that may be susceptible to leakage of the confined gas were analyzed to ensure the efficiency and stability of geological storage.

2. Materials and Methods

2.1. Blue Marl as Sealing Rock

A common clay, blue marl, from the Guadalquivir Tertiary basin (S. Spain) was used as a sealing rock at a low CO₂ pressure and room temperature in previous studies [33,34]. In these studies, blue marl was used as a sealing rock in a secondary role in the mineral carbonation process of construction waste. Because the experimental conditions were low-injected CO₂ pressure and room temperature, the marl did not show an evident transformation in its mineralogy. Another study carried out with a similar sample by Galán and Aparicio [35] showed that in the elapsed time at high temperature and pressure, the smectite, and later the illite, did not suffer any degradation.

The marl sample contains calcite (CaCO₃), quartz (SiO₂), illite ((K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]), smectite ((Ca,Na,H)(Al,Mg,Fe,Zn)₂(Si,Al)₄O₁₀(OH)₂-xH₂O), mixed-layer illite-smectite, kaolinite (Al₂Si₂O₅(OH)₄), and a trace amount of dolomite (CaMg(CO₃)₂) and pyrite (FeS₂) (Figure 1). The semi-quantitative analysis of the minerals present in the original raw sample is shown in

Table 1. Mineralogical semi-quantification composition (wt%) of the original and treated samples after the reaction period of 120 and 240 h. σ: 3 x Estimated standard deviation = 99.7% repeatability limit, normal distribution; LOQ: limit of quantification, LOD: limit of detection, N.D.: not detected.

	Original				120 h				240 h
Minerals	wt%	±σ	LOQ	LOD	wt%	±σ	LOQ	LOD	wt%	±σ	LOQ	LOD
Calcite	35.4	0.9	1.2	0.6	38.0	1.4	1.9	0.9	39.3	1.8	2.4	1.2
Dolomite	3.3	0.6	0.8	0.4	5.3	0.8	1.1	0.6	6.3	0.9	1.2	0.6
Quartz	14.0	0.5	0.6	0.3	11.8	0.6	0.8	0.4	11.5	0.6	0.8	0.4
Plagioclase (albite)	3.9	0.5	0.6	0.3	3.0	0.5	0.7	0.3	2.9	0.5	0.6	0.3
Pyrite	0.9	0.2	0.3	0.2	0.4	0.5	0.7	0.3	0.6	0.5	0.7	0.3
Siderite	N.D.	--	--	--	1.1	0.6	0.8	0.4	1.1	0.4	0.5	0.2
Illite	22.3	1.6	2.1	1.0	24.6	3.0	4.0	2.0	25.2	3.9	5.2	2.6
Kaolinite	5.2	0.8	1.1	0.6	5.4	0.8	1.1	0.6	6.0	1.0	1.3	0.7
Smectite	10.3	1.8	2.4	1.2	8.2	1.9	2.5	1.3	5.7	1.7	2.5	1.2
I/Sm	4.5	1.0	1.3	0.6	2.2	0.6	0.8	0.4	1.4	1.1	1.5	0.7

.

2.2. Methods and Characterization Procedures

The experiments were carried out in a steel reaction chamber, equipped with a pressure gauge and a thermocouple-controlled thermal heating jacket controlled by a thermocouple (Figure 2). The experimental suspensions were made by mixing 30 g of dry aggregates of different sizes of blue marl with 30 g of distilled deionized water (1:1 solid–water ratio). The pressure and temperature conditions required for the geological storage of supercritical CO₂ in deep formations must reach a pressure of 72 bar or more, with a temperature above 31 °C. CO₂, previously compressed in a pump to 100 bars to achieve the supercritical state, was then injected and maintained at constant pressure. Experimental tests were carried out at 120 h and 240 h at 35 ° C, as was studied by Martin et al. in previous studies [33,34]. After the tests, the reaction chamber was depressurized by opening the gas outlet to allow CO₂ to slowly escape. The resulting sample was collected and dried at 70 °C for 24 h. Previously, the slurries were filtered using fiberglass filters (0.7-µm medium retention) and a vacuum pump. In this way, it was possible to separate a large part of the supernatant water from the solid sample.

The mineralogical composition of the dry treated sample was determined by X-ray diffraction analysis (XRD), using a Bruker D8 Advance diffractometer with standard monochromatic Cu-Kα radiation operating at 40 kV and 30 mA. Scanning was performed with a 0.015° 2θ step size, and at 0.1 s per step from 3° to 70°. Phyllosilicate identification was carried out on oriented specimens by clay fraction suspension. Three different states of the oriented samples were prepared: air dried (AD), saturated with ethylene glycol (EG), and heating at 550 °C. Oriented ADs were prepared when the suspension was dried at room temperature. EGs (Ethylene Glycol) were prepared by saturating air-dried oriented samples in ethylene glycol vapor in a desiccator at 60 °C for 24 h. The temperature treatments were set with air-dried oriented heated at 550 °C for 2 h. These types of preparation help characterize clay minerals with the swelling property of ethylene glycol solvation swelling, such as smectites, from those without this property, such as illite, and estimate the number of smectite layers in the mixed layer of illite-smectite. Heat treatment differentiated between clay minerals that collapse at temperatures below 550 °C and those that do not [36,37,38,39]. Additionally, Rietveld refinement was realized to determine the (semi)-quantitative composition of untreated and treated powdered samples. The software used for the refinement was Profex (version 3.14.0) [40].

Thermogravimetric analysis (TG) was performed in a TG Netzsch STA 409 PC. Samples (around 150 mg) were heated in an aluminum oxide crucible under a nitrogen atmosphere at 10 °C min⁻¹ from room temperature to 1000 °C. Mass loss was measured by TG in the range of temperature 450–900 °C relative to total carbonated decomposition. The dihydroxylation of phyllosilicates in this range was considered.

Specific surface area (SSA-BET), micro- and nano-porosity were measured with an ASAP 2420 instrument (Micromeritics, Norcross, GA, USA) using adsorption of N₂ at liquid nitrogen temperature (−196 °C) and CO₂ at room temperature. Samples were outgassed for 180 min under vacuum at 150 °C to 10⁻³ Torr. The SSA was calculated using the classical Brunauer–Emmett–Teller theory (BET).

Inductively coupled plasma optical emission spectroscopy (ICP-OES), the Horiba Jobin Yvon ULTIMA 2 model instrument, was used to determine the amount of Al, Ca, Fe, K, Mg, Na, and Si in the reaction solutions of the supernatant obtained by separating the solid and liquid components by post-test filtration using fibreglass filters (0.7 µm medium retention) and a vacuum pump.

3. Results

To determine the transformation due to the interaction of scCO₂ with clay minerals in the sealing rock, the oriented aggregates prepared from the fine fraction were glycolate by solvation with ethylene glycol (EG) solvation and were subsequently examined by XRD (Figure 3). The composition of clay minerals and their evolution with scCO₂ tests and the labelling of the initial spacing positions d_00l of smectite (Sm), illite (I), and kaolinite (Ka) (d_00l ~ 17, 10 and 7.1 Å or 2θ ~ 5.1°, 8.8° and 12.4°, respectively), were included for better understanding. For illite and kaolinite, the X-ray patterns or intensity reflections were the same, with no significant variations. However, smectite and mixed-layer illite-smectite showed a decrease in intensity in the treated samples of the original.

In more detail (Figure 4), it was observed that after 120 h of reaction, the reflection d₀₀₃ at 5.9 Å corresponding to the smectite practically collapsed. Likewise, the CO₂ interaction corresponds to the (70%) illite/(30%) smectite interstratified (002/003) with spacing close to 5.2 Å. The illitization reaction was also confirmed by a significant sharpening of the 5.0 Å peak ((004) illite). However, a new peak appears, identified as the reflection (002/003) of the interstratified illite/smectite with 40% illite [38,39]. In addition, a decrease in intensity and a shift in the low-angle peak corresponding to 001 of the smectite to lower spacing can be observed. The shift of the gravity peak of glycol gravity-peak shift from 17.1 Å (5.15°2θ) to 16.5 Å (5.32°2θ), is more characteristic of a mixed-layer I/Sm (glycol) (Figure 3).

At 240 h, a positive strengthening in the intensity of the reflection corresponding to the interstratified (40% illite) was observed, as well as a slight increase in intensity and a significant sharpening (higher symmetric peak) for the reflection of 004 of the illite (d₀₀₄ ~ 5.0 Å). This slight increase in the intensity of the illite (Figure 3) was also observed for reflection 006 (d₀₀₃ ~ 3.3 Å).

A possible dissolution and illitization process of the original smectite and mixed-layer was observed. The original mixed-layer with 70% illitic layers developed to 100% illitic layers (completed illitization) after 120 h. However, part of the original smectite began the illitization resulting in a new mixed layer (with around 40% illitic layers).

A (semi) quantitative analysis of the mineral phases present and their evolution with supercritical CO₂ treatments was performed by Rietveld refinement. Table 1 (Figures S1–S3 in the Supplementary Material) shows the (semi) quantitation results of the original (original blue marl) and treated common clay after 120 h and 240 h. A decrease in the smectite and interstratified content was observed, due to possible dissolution in acid medium (pH 3.3–3.5, estimated by Haghi et al. [41]). On the contrary, an increase in illite content was observed, along with an increase in carbonate mineral phases, calcite (CaCO₃), dolomite (CaMg(CO₃)₂), and siderite (FeCO₃); the siderite is not present in the original sample.

These results would agree with the fact that smectite in the treated samples was destroyed by dissolution, as well as the destruction or illitization of smectite in interstratified illite/smectite to illite. Additionally, a proportion of cations released in the dissolution/destruction of the mineral phases involved precipitate in the carbonate phases, chemically fixing CO₂. The precipitation of siderite (iron carbonate) was not significant. Iron cations (Fe³⁺) must have come from dissolved phases in an acid medium, such as iron-rich smectites (nontronite or beidellite type), as well as from the pseudo-decomposition of illite layers in the interstratified.

The increase in carbonate phases was confirmed by the increase in weight loss observed by thermogravimetric analysis. In these tests, the original sample (untreated) had a weight loss in the temperature range 450–900 °C of 16.9 wt% compared to the samples treated at 120 h and 240 h with losses of 20.3 wt% and 21.7 wt%, respectively (Figure 5).

In general, the alteration mechanism takes place in two steps (Figure 6):

• The mechanism of dissolution of smectite would be equivalent to that observed in other studies on the dissolution/alteration of swelling clay minerals (such as smectite and mixed-layers) by inorganic acid [42,43,44,45,46,47].
• Leaching of Al, Mg, and Fe from the octahedral and tetrahedral sheets.

As a result, the process could result in illitization of the smectite structure into the illite structure, by loss of the exchangeable cations and modification of the octahedral layer.

On the other hand, in the studies described previously [42,43,44,45,46,47], different inorganic acids were introduced into the reaction as an activating agent of the swelling clay mineral activating agent; however, in the present study, the acid is carbonic acid. The initial reaction involved the dissolution of CO₂ in water to form carbonic acid, which dissociated to form (HCO₃)⁻ and (CO₃)⁻ ions.

The increase in Ca, Na, and Mg ions obtained by ICP-OES (

Table 2. Al, Ca, Fe, K, Mg, Na, and Si in the supernatant reaction solutions by ICP-EOS.

Element (mg/L)	Original	120 h	240 h
Al	<0.002	≤0.015	≤0.015
Ca	7.83 ± 0.16	46.1 ± 0.4	33.2 ± 0.9
Fe	0.004 ± 0.001	0.39 ± 0.07	0.67 ± 0.04
K	21.3 ± 0.9	10.1 ± 0.6	11.1 ± 0.6
Mg	21.1 ± 0.3	53.1 ± 0.5	72.1 ± 0.5
Na	19.2 ± 0.7	38.8 ± 0.5	41.5 ± 0.3
Si	2.50 ± 0.11	2.19 ± 0.09	3.46 ± 0.09

) as a function of time with the original marl would be related to the exchange of interlayer cations from smectite to illite, as observed by the reduction of K (typical interlayer cation of illite). However, Ca also comes from a slight dissolution of the calcite in the marl.

The leaching of the octahedral cations caused a relative increase in the Fe and Mg ion content in the reaction solutions. The results were non-conclusive for Al (below the quantification limit). However, authors previously referred to noted that leaching of the octahedral cations occurred in the order Mg, Fe, and Al [42,43,45,47].

After interaction with CO₂ or carbonic acid later, the specific surface area of BET (BET-SSA) and the pore volume increased (

Table 3. Surface area, pore volume, and pore size by N₂-BET.

	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
Original	36.11	0.070	7.79
120 h	37.88	0.073	7.68
240 h	38.10	0.077	6.99

, and more details are in the Supplementary Materials file). After 120 h of reaction with CO₂, a significant increase in BET-SSA was observed, which maintained a slight increase over time until 240 h. The tendency of the pore volume also increased with time. On the other hand, the pore size decreases with the reaction time. Consequently, it appears that the marls in interaction with CO₂ undergo an increase in the specific surface area probably due to acid attack, increasing the pore volume, creating a new porosity of a lower average size than the pre-test porosity. The change in morphology could be caused by the modification of the smectite structure promoted by an acid attack. The reaction was primarily caused by the exchange of interlayer cations during the reaction and structural modification in the octahedral layer by the leaching of ions.

The N₂ adsorption-desorption isotherms of the pre- and post-test samples show identical patterns, with no significant change in the pore shape. However, the adsorbed amount of N₂ was slightly lower after 240 h of testing. This may be due to the reduction in pore size and the possible physical absorption of CO₂ in carbonation tests.

The specific surface area by CO₂, total pore volume, and total pore area increased directly with the reaction time, while the average pore area decreased again in size (smaller pore area) (

Table 4. CO₂ adsorption summarized report.

		Original	120 h	240 h
BET Surface Area (m2/g)		14.5298	15.1392	15.4480
Total Volume in Pores (cm3/g)	≤1.066 nm	0.00233	0.00259	0.00287
Area in Pores (m2/g)	>1.066 nm	8.953	8.769	8.787
Total Area in Pores (m2/g)	≥0.367 nm	16.758	17.176	18.363

). Both the N₂ and CO₂ adsorption tests seem to show an increase in porosity in the marls, which is smaller in all pore sizes than before the treatments.

The adsorption of CO₂ isotherms showed a higher amount of adsorbed gas in treated samples, which was higher with a longer reaction time (Figure 7). This behavior could be related to the increase of smaller pore size (meso- to micro-pore).

4. Discussion

For geological CO₂ storage (GCS), possible CO₂ leakage pathways impact three risk category areas: containment, site performance, and public perception [48]. Therefore, it is important to understand the effects of mineralogical variability and heterogeneity on the geochemical response of the geological storage system to the injection of CO₂. The results of previous studies conclude that different mineralogical compositions significantly influence the patterns of geochemical and geophysical reactions, and hence the geological properties of CO₂ due to injection [14,49,50,51].

Since the main confining properties of the sealing rock are closely related to the content of the clayey material, it is crucial to understand the reactivity of these minerals in interaction with scCO₂ in wet conditions to assess the performance and safety of the geological storage system. Few studies have shown relevant experimental results and modelling models for illitization phenomena under CO₂ storage conditions for clay rocks [52,53]. This work explores these implications in common carbonate-rich clay such as marl, called blue marl, as well as assesses impacts on the sealing capacity of cap rock.

The first result of this study carried out under conditions close to those found in CO₂ geological storage systems, evidenced a process of illitization of smectite and mixed-layer illite–smectite that gradually transforms into illite under acidic conditions (carbonic acid). The illitization process of the smectite layers was observed by XRD.

The illitization process of the smectite layers was observed by XRD. The illitization process could be due to the dissolution/decomposition of the smectite layers that release Al³⁺, Mg²⁺ and Fe^2+/3+ ions from the octahedral sheets, as shown by the ICP-EOS results. The decrease in the K content would be in opposition to the illite precipitation.

An important implication of the integrity of clayey seal rock caused by the illitization process of mixed-layer illite–smectite is the increase in porosity due to the alteration of the structures of the smectite layer [52,53], which is classically associated with a decrease in particle volume and loss of water molecules. Structurally, the smectite layers reduce their size in the basal direction, from 12–15 Å to the size of the illite (10 Å) by loss of hydration and change of exchange cations in the interlayer. A significant volume of illitization could result in the formation of microcracks that could interconnect over time and cause the undesirable leakage of gas from the confining region to materialize. CO₂ injected into a moist environment promotes the generation of carbonic acid and bicarbonate anions in solution, reducing the pH to between 3–5 [22,25,54]. Reduced pH causes the weathering of aluminosilicates in the rock, such as smectites. It also causes the dissolution of carbonate minerals, such as calcite in the marl studied.

The dissolution of primary minerals and the consequent contribution of cations (Fe^2+/3+, Ca²⁺, Mg²⁺, Na⁺, and Al³⁺) buffer the pH and can achieve alkaline solution values (around 7–8) [24]. These free cations, combined with carbonic anions, precipitate in secondary carbonate minerals, mainly Ca²⁺, Mg²⁺ and/or Fe²⁺ (+4 wt% calcite, +3 wt% dolomite and +1 wt% siderite, Table 1), chemically fixing a proportion of CO₂ [10,14,24,25,26].

On the one hand, the dissolution of the primary minerals of the rock would increase porosity. However, secondary precipitation of carbonates would cause a reduction in porosity [10,22,24,26,55]. Aljamann et al. [56] showed that an increase in illite content in shale formations was related to an increase in surface area and pore capacity, allowing additional space for CO₂ absorption. From a sealing efficiency point of view, an increase in porosity will weaken the sealing capacity of the capped rock, while a decrease in porosity will improve the sealing capacity of a geological storage site [57].

Finally, it should be considered that clayey rocks have a heterogeneous distribution of constituent minerals, as well as the presence of veins. Carbonate and dissolution-susceptible mineral veins of considerable volume may be a potential risk for failure and leakage of stored gas.

5. Conclusions

Changes in smectite and mixed-layer illite/smectite minerals from clayey rocks used as cap rock were studied under geological CO₂ storage.

The high CO₂ pressure used in geological storage induces irreversible changes in the structure of clay mineral particles in the clayey cap rock. Under carbonic acid conditions, promoted by the dissolution of CO₂ in water, a combined dissolution and illitization of smectite was confirmed by changes in exchangeable cations determined by ICP-EOS (from 21.3 to 11.1 mg/L for K, typical in illite). Additionally, a proportion of cations released in the dissolution/destruction of the involved mineral phases precipitate in the carbonate phases, calcite, dolomite and siderite, chemically fixing CO₂. Acidic solutions had a significant impact on both the dissolution of mixed-layer illite—smectite and carbonate minerals and the precipitation of secondary minerals.

The multipoint BET technique was used to obtain geometrical properties of the pores, such as the specific surface area (from 36.1 to 38.1 m²/g by N₂ and from 14.5 to 15.4 m²/g by CO₂ adsorption test) and the pore volume (slightly decrease from 0.070 to 0.077 cm³/g), which were affected by mineral dissolution and precipitation. These changes were indicative that CO₂ interaction can affect rock reactivity over a long storage time.

Slight porosity in changes (from 7.8 to 7.0 nm, pore size) are related to the alteration, dissolution, and secondary precipitation of the constituent minerals in the clay-sealing rock. Together with the possible heterogeneity of the mineral composition of the cap rock, the integrity of the cap rock is risked due to the massive dissolution of minerals, encouraging the interconnection of undesirable leakage pathways of the stored CO₂.

Therefore, knowledge of the interaction between CO₂ and sealing rock and its implications for GCS requires extensive studies to ensure long-term safety.