Effect of Clay on the Kinetics of CO₂ Hydrate Relevant for Carbon Capture and Sequestration

Immediate actions are imperative to mitigate global warming and climate change, potentially involving the storage of anthropogenic CO₂ in geological and oceanic reservoirs. Oceanic sequestration, particularly beneath the seabed, holds promise for long-term CO₂ storage, crucial for achieving carbon neutrality across scientific and industrial sectors. Effective subsea CO₂ sequestration hinges on macroscopic factors such as salinity, sediment porosity, types of sediments, and additives. Special attention is warranted in exploring CO₂ sequestration in solid gas hydrate form within subsea sediments, focusing on chemical interactions and environmental impacts at the pore scale during hydrate formation and growth. Experimental studies conducted at 274.65 K temperature and 4.5 MPa injection pressure in seawater using different clay media in a continuous stirred tank reactor (CSTR) revealed significant insights. Bentonite clay, in particular, demonstrated substantial enhancement in CO₂ hydrate conversion kinetics compared to clay-free conditions. A notable improvement of nearly 200% in gas uptake and a twofold increase in hydrate conversion were observed in bentonite clay environments. This research underscores the critical role of understanding chemical interactions among CO₂, hydrate-bearing sediments, clay, and marine environments in facilitating large-scale CO₂ storage beneath the ocean floor.

CO₂ sequestration in clay-dominated systems is considered the most effective technique for sequestrating gigatonnes (Gt) of CO₂ beneath oceans (Kennedy and Wagner 2011; Agrawal et al. 2023). CO₂ sequestration in oceans as hydrates can serve a purpose well as oceans cover 2/3^rd of the earth's surface. Oceans have the potential to sequestrate gigatonnes of CO₂ as gas hydrates or liquid pools (Boyd et al. 2019; Bertram et al. 2021). The sequestrated CO₂ will form an ice-like crystalline structure known as a gas hydrate inside the pores of the subsea sediments (Kumar and Sangwai 2023a; Han et al. 2024). Oceans beyond 1000m depth already have large deposits of methane hydrate in subsea sediments within hydrate stability zones. Likewise, CO₂ hydrate can also be stored with methane hydrates in the subsea sediments. Subsea sediment beyond 500m sea depth can form CO₂ hydrate within the interstitial space of sediment (Feng et al. 2023; Kumar and Sangwai 2023b). In one estimate, CO₂ hydrate can store ∼120-160 m³ of anthropogenic CO₂ in 1 m³ of hydrate structure at standard temperature and pressure (Sun and Englezos 2014). Another advantage of studying clay-dominated systems is that clay is already present in subsea sediments owing to river terrestrial erosion. The clays are usually present in subsea sediments’ upper layer (10-100 m from the seafloor). The subsea sediments usually consist of silt, sand, and clay as dominating constituents (Kumari et al. 2021). Key factors influencing the hydrate formation kinetics include sediment composition, water salinity, and the presence of hydrate formation additives (Ma et al. 2016; Kumar and Sangwai 2023b; Park et al. 2023). Understanding the chemical interactions between CO₂, water, and sediment minerals is crucial for optimizing CO₂ uptake and hydrate stability (Sun et al. 2021; Han et al. 2024). The influence of clay particles on hydrate formation and decomposition in sediments exhibits considerable variability, primarily owing to their low gas permeability and the fluctuating water activity within clay pores, which correlates with water content (Zhang et al. 2022). Studies indicate that the stability of hydrates in sediments can be notably influenced by external components such as bentonite, which possesses a relatively large surface area. Bentonite mainly consists of aluminosilicate. The bentonite can be further classified as sodium bentonite (swelling) and calcium bentonite (nonswelling) depending on their swelling characteristics (Magzoub et al. 2020; Kumar and Sangwai 2023b). The swelling tendencies of the bentonite clay depend on the ratio of calcium and sodium ions in the clay composition. Since bentonite is mostly a swelling type of clay, it swells multiple times more than its original volume in water (Magzoub et al. 2020). Water within clay is typically categorized into bound water and free water. Bound water refers to water within the electric double layer, whereas free water encompasses water outside this layer. Free water can further be subdivided into capillary water and gravitational water. Research and experimental studies have explored various aspects of subsea hydrate-based sequestration, including the kinetics of hydrate formation, the impact of different sediment types, and the behavior of hydrates under varying environmental conditions. Technologies such as continuous stirred tank reactors and in situ monitoring systems are employed to study and optimize the process. Challenges associated with subsea hydrate-based sequestration include ensuring the long-term stability of stored CO₂, predicting the behavior of hydrates over geological timescales, and assessing potential environmental impacts. Despite these challenges, the method offers significant potential as a complementary approach to terrestrial and other oceanic carbon capture and storage techniques, contributing to global efforts to achieve carbon neutrality and mitigate climate change. While the effects of different clay types on hydrate formation and decomposition have been widely studied, a definitive and systematic method for determining CO₂ hydrate formation kinetics in clay and their impact on hydrate formation and dissociation is still lacking. This study focuses on bentonite, a fine clay with a large surface area suitable for CO₂ sequestration. Isothermal hydrate formation experiments were conducted with bentonite clay and seawater to investigate this further and explore CO₂ sequestration potential in clay-dominated regions, especially subsea sediment. In this article, 7 wt.% bentonite is used for the kinetic study of gas hydrates with seawater and sodium dodecyl sulfate (SDS) systems. The study of bentonite and seawater systems will improve our understanding of the effect of clay on CO₂ hydrate formation kinetics in a subsea environment.