Gas Production and Storage Using Hydrates Through the Replacement of Multicomponent Gases: A Critical Review

Abstract

With the continuous growth of global energy demand and the gradual depletion of traditional fossil energy reserves, natural gas hydrates have attracted widespread attention as a potential clean energy source due to their vast reserves and wide distribution. Although various extraction methods, including depressurization, thermal stimulation, chemical inhibitors, and displacement methods, have been proposed, there are still challenges, such as low extraction efficiency, poor sustainability, and high costs, making it difficult to achieve large-scale engineering applications. Among these, the use of gases such as CO₂ for displacement extraction of natural gas hydrates can both develop hydrate resources and sequester CO₂, achieving a win–win situation for resource development and greenhouse gas reduction. This paper provides a detailed review of the multi-gas displacement extraction technology for natural gas hydrates, systematically summarizes the latest progress in thermodynamic and kinetic studies, analyzes the technical advantages and feasibility of combining displacement methods with traditional techniques, and explores the effects of multi-gas mixtures, such as N₂, CO₂, and H₂, and their ratios on hydrate extraction efficiency. Finally, this paper summarizes the technical challenges faced by displacement extraction methods for hydrates and offers future research directions to promote the development of multi-gas displacement technology for natural gas hydrates.

1. Introduction

1.1. Natural Gas Hydrate

Natural gas hydrate is a solid, ice-like substance formed by natural gas molecules and water molecules under low-temperature and high-pressure conditions. For every 1 cubic meter of solid hydrate that decomposes, approximately 163 cubic meters of natural gas are released. The total carbon stored in global hydrate resources is roughly twice the total carbon stored in all other conventional fossil fuels, including oil, coal, and conventional natural gas [1,2]. This compound is considered a clean energy source, as it produces lower carbon dioxide emissions during combustion compared to traditional fossil fuels, helping to reduce environmental pollution and mitigate global climate change issues [1]. Predictions suggest that there are vast natural gas hydrate resources on Earth, with reserves exceeding 15 × 10¹² tons. If 17% to 20% of this resource could be exploited, it would be sufficient to provide ample energy supply for the next 200 years. Natural gas hydrate deposits are mainly distributed in two regions: one is the high-latitude land covered by permafrost, such as Siberia, Alaska, and the Canadian Archipelago; the other is the deep-sea sedimentary layers, particularly in the continental shelf and slope areas, especially in water depths ranging from 300 to 3000 m, such as the South China Sea depression in the western Pacific, the Blake Plateau in the western Atlantic, and the Arabian Sea Trench in the Indian Ocean. After nearly thirty years of global joint exploration, utilizing technologies such as seismic wave detection, seabed sampling, and deep-sea drilling, more than 230 natural gas hydrate deposits with industrial extraction potential have been identified worldwide. Figure 1 shows the geographical distribution, abundance levels, and the forms in which natural gas hydrates exist on land and in the ocean. In the figure, yellow circles (BSR) represent regions where natural gas hydrates have been discovered through the Bottom Simulating Reflector (BSR) method. BSR is an important seismic exploration marker for identifying natural gas hydrates. Red circles (by core) indicate areas where the presence of natural gas hydrates has been confirmed through core sampling. Core sampling is a direct method of obtaining underground material samples, which can definitively prove the existence of natural gas hydrates. Red squares (production) represent areas where natural gas hydrate extraction has already taken place. The figure marks several typical locations, such as Mallik in Canada, Messoyakha in Russia, and Nankai in Japan. From the map, it is evident that natural gas hydrate deposits are widely distributed across the globe. In the ocean, they are mainly concentrated along the continental margins, while on land, they are primarily found in high-latitude polar regions, such as those near the Arctic, demonstrating the immense market potential and sustainable development advantages of natural gas hydrates [3]. Natural gas hydrate can be used in various fields, such as civilian and industrial fuels, chemicals, and power generation, offering broad application prospects. It forms a complete industrial chain, from upstream exploration and development to midstream transportation and storage and downstream comprehensive utilization. Research into its extraction technology can not only accelerate energy technological innovation and industrialization but also provide significant support for global energy structure optimization, environmental protection, and climate change mitigation. With ongoing technological advancements, natural gas hydrate is expected to become an important component of the global energy system, playing a key role in the future energy transition [4].

1.2. Natural Gas Hydrate Extraction

1.2.1. Traditional Natural Gas Hydrate Extraction Methods

The extraction methods for natural gas hydrates are still in the research and testing stages. Despite facing numerous challenges, several technological solutions have been proposed. Most existing extraction methods are based on adjusting the pressure and temperature of the hydrates, shifting them from the stable hydrate zone to the dissociation zone, thereby enabling dissociation [6]. There are three main traditional methods for extracting natural gas hydrates: depressurization, thermal stimulation, and chemical inhibitor injection. The advantages and disadvantages of these three methods are shown in

Table 1. Advantages and disadvantages of different extraction methods.

Method	Advantages	Disadvantages
Depressurization Method	Technologically mature; simple to operate; no chemical additives; no negative impact on the environment	Requires prolonged depressurization, leading to increased extraction costs; relatively low extraction rate; rapid pressure changes may cause well leakage or collapse
Thermal Stimulation Method	Increase the decomposition rate of hydrates; applicable to low-temperature areas	High energy consumption; damages wellbore and reservoir; using thermal fluids may require treatment of the injected water; otherwise, it could introduce environmental contaminants
Chemical Inhibitor Injection Method	Low energy demand; adaptable to various reservoir conditions	High chemical costs; requires precise control of injection volume

[7,8,9].

The depressurization method used in the extraction of natural gas hydrates is a widely used technique. Its basic principle is to lower the pressure in the hydrate reservoir to disrupt the phase equilibrium of the hydrates, causing the dissociation of the hydrates into gas (such as CH₄) and water, thereby enabling the extraction of natural gas [10,11]. Currently, two main approaches are used to achieve depressurization: (1) the use of low-density drilling mud technology, which aims to achieve a depressurization effect; (2) the use of pump pressure technology to extract free gas and other fluids from beneath the natural gas hydrate reservoir to achieve the depressurization goal [12].

The thermal stimulation method is a technique that promotes natural gas release by heating the hydrate reservoir [13]. Its basic principle is to provide heat to the natural gas hydrate reservoir, increasing the temperature, thereby causing the dissociation of the hydrates into gas and water and releasing natural gas [14]. The implementation methods include thermal fluid injection, electric heating, and geothermal heating, among others [15].

Table 2. Merit and demerit of different thermal recovery methods.

Method	Advantages	Disadvantages
Thermal fluid injection method	Increase recovery rate; widely applicable; high heat transfer efficiency	High energy consumption; significant heat loss; complex equipment maintenance
Electric heating method	High energy efficiency; precise control; minimal equipment requirements	High equipment cost; high power consumption; poor adaptability
Geothermal heating method	Low energy consumption; minimal heat loss; wide applicability	Slow heating effect; limited improvement in yield; less effective than high-temperature heating

lists the advantages and disadvantages of different thermal extraction methods.

The chemical inhibitor injection method prevents the formation of hydrates or promotes their dissociation by injecting chemical inhibitors, thereby improving natural gas recovery efficiency [16]. This method is particularly suitable for deep-sea or polar natural gas hydrate reservoirs, as hydrates in these regions are typically stable due to temperature and pressure conditions and are not easily dissociated naturally. Chemical inhibitors can effectively control the formation and dissociation of hydrates. Common chemical inhibitors, as shown in

Table 3. Advantages and disadvantages of different chemical inhibitors.

Inhibitor	Type	Advantages	Disadvantages
Thermodynamic Hydrate Inhibitors	Alcohols and electrolytes	Effectively reduce the formation temperature of hydrates, thereby preventing their formation	It usually requires a high dosage, leading to increased costs and environmental impact
Kinetic Hydrate Inhibitors	Polymeric compound	Effectively inhibit hydrate formation at a lower dosage, suitable for long-term flow assurance	In some cases, it may not be stable enough, and its effectiveness varies with different gas compositions
Anti-agglomerants	Compounds with various chemical structures	Prevent the agglomeration of hydrate particles, thereby reducing the risk of blockage	It needs to be used in combination with other inhibitors to enhance effectiveness
Dual-function Hydrate Inhibitors	Amino acids, ionic liquids, and nanoparticles	Combining the advantages of thermodynamic and kinetic inhibition provides a more comprehensive inhibitory effect	Relatively novel, and it requires further research to determine its long-term effects and cost-effectiveness

, include thermodynamic hydrate inhibitors, kinetic hydrate inhibitors, anti-agglomerants, and dual-function hydrate inhibitors [17].

In the extraction process of natural gas hydrate, a combination of various traditional methods is often employed to enhance extraction efficiency. For example, the combination of depressurization and thermal stimulation. By reducing pressure, the stability of the hydrate is lowered, and the heating process accelerates the decomposition of the hydrate, thereby promoting the release of natural gas and shortening the extraction cycle. The advantage of this method lies in its ability to accelerate gas release and increase extraction rates. However, the thermal stimulation process requires a large amount of energy and may cause temperature fluctuations, which can affect the stability and continuity of extraction. Additionally, prolonged depressurization may lead to instability in the hydrate reservoir, affecting the gas recovery rate [12].

Furthermore, the combination of depressurization and chemical inhibitors can also significantly improve extraction efficiency. Chemical inhibitors can suppress the re-crystallization of the hydrate during depressurization, reducing instability and accelerating hydrate decomposition at lower temperatures, thereby maintaining high gas output [10,11]. However, chemical inhibitors are expensive, and their potential environmental impact needs to be evaluated and monitored, as long-term use may pose environmental risks.

Moreover, combining thermal stimulation with chemical inhibitors can prevent the re-crystallization of the hydrate at lower temperatures during the heating process, thus maintaining extraction efficiency. Heating helps accelerate the decomposition of the hydrate, while inhibitors further stabilize the decomposition process [16]. However, this method requires a large amount of energy input, and the use of chemical inhibitors is costly. Additionally, the ratio of heat energy to inhibitor use must be carefully controlled, as excessive use may lead to environmental issues. Therefore, while coupling methods can improve extraction efficiency, their implementation must carefully consider energy consumption, costs, and environmental impact.

1.2.2. CO₂ and CH₄ Displacement Method

Traditional methods promote the natural decomposition of hydrate into natural gas by breaking the phase equilibrium of hydrates in their initial state. However, when exploring the feasibility and economic viability of various extraction technologies, the stabilizing support role of natural gas hydrates in the surrounding formation cannot be overlooked. While traditional extraction methods promote hydrate decomposition, they may also weaken the formation’s stability, potentially triggering geological disasters such as earthquakes and underwater landslides, which highlights the limitations of these extraction methods [18]. On the other hand, with the rapid development of human society and the surge in greenhouse gas emissions, this has become a major environmental issue that urgently needs to be addressed. In this context, CO₂ sequestration technology, as an effective means to promote CH₄ extraction, reduce CO₂ emissions, and alleviate the greenhouse effect, has become increasingly important. Therefore, in the exploration of natural gas hydrate extraction methods, it is also essential to actively consider how to integrate CO₂ sequestration technology to achieve a win–win situation for both energy development and environmental protection [19]. Figure 2 shows the development history of the displacement method research.

The principle of the displacement extraction method is to inject CO₂ into the submarine hydrate-bearing zone, replacing CH₄ in the hydrate with CO₂, thereby enabling the safe and efficient extraction of CH₄ hydrate while also achieving the long-term stable geological sequestration of CO₂ [20]. In addition, during the hydrate displacement extraction process, the generated CO₂ hydrate helps maintain the geological stability of the hydrate reservoir, effectively preventing formation collapse and instability caused by the phase transition and decomposition of CH₄ hydrate. In this process, CO₂ and CH₄ in the hydrate undergo a substitution reaction through physical and chemical processes, with CO₂ being fixed in the form of a hydrate, while the original CH₄ is released as gas [21]. The displacement method has strong technical adaptability and is capable of operating effectively under different temperature and pressure conditions, especially in low-temperature, high-pressure reservoirs. It offers good technical and economic feasibility and broad applicability, contributing to global emission reduction goals and the sustainable utilization of energy.

Figure 2. The development history of the research on replacement method [22,23,24,25,26,27,28,29].

Figure 2. The development history of the research on replacement method [22,23,24,25,26,27,28,29].

2. Structure and Properties of Hydrates

2.1. Crystal Structure of Hydrates

Hydrates have a crystal structure similar to that of molecular sieves, where water molecules are connected by hydrogen bonds to form a regular lattice structure. These lattice units create a cage-like structure that can encapsulate and store gas molecules. Depending on the size and quantity of the gas molecules and their interaction with water molecules, the structure of hydrates is generally classified into three types: Type I, Type II, and Type H. Due to its simplicity and stability, the most commonly found hydrate in nature is Type I, composed of smaller gas molecules such as CH₄ (which makes up more than 90%). These are commonly referred to as methane hydrates [30,31].

The cage structure of hydrates is not always completely filled, and there are certain vacancies within the cages. These vacancies allow gas molecules to enter the cages and be “encapsulated.” The stability of the hydrate and its gas storage capacity typically depend on the size, shape, and degree of matching between the vacancies in the cages and the gas molecules. During the formation of hydrates, gas molecules enter these vacancies and are surrounded by water molecules, forming a stable solid structure under specific temperature and pressure conditions [21,32]. For example, in Type I hydrates, the smaller 5⁶-cage mainly accommodates one gas molecule, while the larger 5¹²-cage can hold multiple small gas molecules. The presence of vacancies and the entry of gas molecules allow hydrates to store a large amount of gas, which is a key feature of hydrates as a medium for gas storage.

During the formation of hydrates, gas molecules are surrounded by water molecules and embedded into vacancies within the cages. Once the gas molecules enter the cages, they interact with the water molecules primarily through van der Waals forces and hydrogen bonding. Due to the small size and chemical inertness of the gas molecules, they can be stably surrounded by water molecules, forming a stable “cage-like” structure [32]. The size, type, temperature, and pressure of the gas molecules collectively determine the formation, stability, and storage capacity of hydrates. Suitable gas size and shape, low temperature, and high-pressure conditions, as well as the appropriate type and concentration of gas, facilitate the stable formation and efficient storage of hydrates. With changes in temperature and pressure, hydrates can undergo thawing or gas release, leading to the decomposition of the hydrate and the release of gas [33].

In the process of replacing methane with carbon dioxide for the extraction of natural gas hydrates, the maximum theoretical displacement efficiency is 75%. This phenomenon is primarily related to the crystalline structure of the hydrate and the interactions between gas molecules. Natural gas hydrates typically have a cage-like crystalline structure, which is mainly classified into two types: Structure I (S-I) and Structure II (S-II). In these hydrate structures, carbon dioxide molecules usually replace methane molecules. However, since carbon dioxide molecules are larger than methane molecules, carbon dioxide cannot completely replace all the methane molecules, resulting in a maximum displacement efficiency of about 75%.

In Structure I hydrates, methane molecules occupy the small cages, while carbon dioxide molecules preferentially fill the larger large cages. In Structure II hydrates, carbon dioxide molecules also preferentially occupy the larger cages, similar to the large cages in Structure I. These structural differences and the size disparity between carbon dioxide and methane molecules limit the complete replacement of methane by carbon dioxide [30,31,33].

2.2. Thermodynamic Study

Thermodynamic studies mainly focus on the formation phases of hydrates, stability regions, and the changes in free energy during the replacement process. Through thermodynamic analysis, the stability of CO₂ and CH₄ in hydrates and their replacement behavior under different temperature and pressure conditions can be predicted [34]. Additionally, research on phase equilibrium helps to determine stable operating conditions, control the thermodynamic driving forces of replacement reactions, study the dynamic processes of gas diffusion and exchange, ensure thermodynamic and kinetic stability during extraction, and provide a scientific basis for the long-term sequestration of CO₂ and environmental safety [26].

2.2.1. Thermodynamic Characteristics of Replacement Reactions

According to the basic principles of chemical thermodynamics, spontaneous reactions always proceed in the direction of decreasing Gibbs free energy. Yezdimer et al. [23] demonstrated through molecular dynamics simulations that the Gibbs free energy of CO₂ hydrate is lower than that of CH₄ hydrate. Additionally, the Gibbs free energy of the replacement reaction is negative, indicating that the reaction can proceed spontaneously [24]. From the perspective of phase change heat, the heat released during the decomposition of CO₂ hydrate (57.98 kJ/mol) is greater than the heat absorbed during the decomposition of CH₄ hydrate (54.94 kJ/mol). Therefore, the heat required for the decomposition of CH₄ hydrate can be provided by the heat released during the formation of CO₂ hydrate, and the replacement reaction does not require additional heat input from external sources [35,36].

Akihiro [37], in his study of the phase equilibrium temperature and pressure of CH₄ and CO₂ hydrates below the freezing point, found that they follow the same pattern. During the replacement process, a mixed gas of CO₂ and CH₄ is generated. Goel [38], through experimental research on the formation process of CH₄ and CO₂ hydrates, plotted phase equilibrium curves for mixtures of CO₂ and CH₄ at different ratios. He found that as the CO₂ content increased, the phase equilibrium pressure of the mixed gas decreased while the phase equilibrium temperature increased. CO₂ hydrate remains stable within the temperature range of 260–270 K and the pressure range of 0.6–1.7 MPa, while CH₄ hydrate is not easily formed. Circone et al. [39] further demonstrated through experiments that, within a specific temperature and pressure range (0.1 MPa, 168–218 K), CH₄ hydrate can decompose while CO₂ hydrate remains stable. Xie et al. [40], by fitting experimental data using 10 thermodynamic models, showed that from the perspective of adsorption thermodynamics, the adsorption amount of CO₂ is larger than that of CH₄ under the same conditions, with an adsorption strength range of 16.71–35.65 kJ/mol, which is higher than CH₄’s range of 12.48–28.90 kJ/mol. This indicates that shale adsorbs CO₂ more strongly. Gas adsorption is a spontaneous and enthalpy-driven process, with a negative ΔG indicating spontaneous adsorption. Furthermore, the absolute value of ΔG for CO₂ is smaller than that for CH₄, meaning the CO₂ adsorption process is more likely to occur.

2.2.2. Stability of Hydrates and Gas Distribution Behavior

The stability and formation process of hydrates is closely related to the occupancy state of gas molecules within the hydrate crystal structure. Methane hydrate is typically stable at lower temperatures and higher pressures, while carbon dioxide hydrate has a relatively wider range of stable temperatures and pressures, and its formation conditions are more easily achieved compared to methane hydrate [26]. Additionally, the decomposition process of hydrates plays a significant role in gas replacement behavior and energy release. Therefore, a thorough study of the phase diagram and stability of hydrates is crucial for the development of these applications [41].

Ohgaki et al. [22] proposed a method for extracting natural gas hydrates using CO₂ substitution. This method involves introducing CO₂ gas into the gas–liquid hydrate three-phase system of CH₄, forming a gas–liquid hydrate equilibrium system containing both CO₂ and CH₄. Figure 3 shows the thermodynamic equilibrium curves for pure CH₄ and pure CO₂ hydrates, as well as the gas–liquid phase equilibrium curve for CO₂. The thermodynamic trends for the two natural gas hydrates can be summarized as follows: under the presence of gaseous CO₂, the formation conditions for pure CO₂ hydrate are milder than those for pure CH₄ hydrate. When gaseous CO₂ is injected into the methane hydrate layer, the unstable methane hydrate dissociates, releasing methane gas and liquid water. Subsequently, the melted water reacts with the injected CO₂ and spontaneously forms a more stable CO₂ hydrate as the product of the entire gas exchange reaction [42]. In the figure, areas A and B are located above the H₂O-hydrate-CO₂ equilibrium curve and below the H₂O-hydrate-CH₄ equilibrium curve, respectively. Thermodynamic analysis indicates that these regions suggest that, under specific temperature and pressure conditions, gaseous methane (CH₄) and CO₂ hydrate can coexist. Therefore, when CO₂ is injected into the CH₄ hydrate layer, CO₂ can be stored as a hydrate. This phenomenon reveals that CO₂ hydrate (CO₂·nH₂O) is more stable than methane hydrate (CH₄·nH₂O) [43]. Under the same environmental conditions, CO₂ can form and stabilize hydrates at lower temperatures and higher pressures, while CH₄ hydrate requires higher temperatures or lower pressures to remain stable.

The process of CO₂ replacing methane occurs above the methane hydrate phase equilibrium line to ensure that CO₂ can effectively replace methane. The reaction must take place under conditions where methane hydrate remains stable. Only above the methane hydrate phase equilibrium line does methane hydrate maintain a stable solid structure, providing a basis for CO₂ molecules to substitute into the lattice. Below the phase equilibrium line, methane hydrate will spontaneously decompose, and there will be no stable methane hydrate structure available for CO₂ to carry out the replacement reaction. At the pressure–temperature conditions above the phase equilibrium line, the process of CO₂ and water molecules forming a hydrate is thermodynamically more favorable. Higher pressure and appropriate temperature conditions help CO₂ molecules overcome energy barriers to enter the hydrate lattice and replace methane molecules. This thermodynamic advantage drives the replacement reaction towards the formation of CO₂ hydrate and the release of methane [42]. In the process of CO₂ replacing methane, if the pressure–temperature conditions fluctuate near the methane hydrate phase equilibrium line, methane hydrate may re-form, encapsulating unreacted CO₂ or blocking pore channels, thus reducing the replacement efficiency. To minimize this risk, stable conditions above the phase equilibrium line should be maintained to ensure the continuous progress of the replacement reaction [41]. Additionally, if the CO₂ concentration is too high or poorly controlled, excessive formation of CO₂ hydrate at the sediment surface may occur, creating a dense shell that hinders CO₂ diffusion and methane release, thereby reducing replacement efficiency. By properly controlling the CO₂ injection rate and reaction conditions, this issue can be avoided, maintaining a high replacement efficiency. Therefore, controlling the replacement process above the methane hydrate phase equilibrium line ensures the stable existence of methane hydrate while reducing the risks of reformation of methane hydrate and excessive formation of CO₂ hydrate.

This conclusion indicates that CO₂ hydrate has a clear thermodynamic advantage, as it not only forms more easily under natural sediment conditions but also has greater potential to replace methane hydrate [44]. The figure also presents the equilibrium data between the liquid and gas phases of CO₂, which is crucial for understanding the behavior of CO₂ in hydrates. These data can help researchers better predict the conversion efficiency of CO₂ during the replacement process of natural gas hydrates, thereby optimizing the overall process of CO₂ sequestration and natural gas extraction.

At the same time, during the exchange process, because of their smaller size and stronger polarity, CO₂ molecules can more easily be embedded into the hydrate cages, especially in Structure I hydrates, where CO₂ can effectively replace CH₄ molecules. However, the competitive entry of CO₂ and CH₄ for entry may result in changes to the stability of the hydrates. This leads to the hindrance of the displacement process, with the main limiting factors including thermodynamics, competitive adsorption, kinetics, and hydrate stability. CO₂ replacement of methane requires specific temperature and pressure conditions, and unfavorable temperature and pressure may result in low replacement efficiency. At the same time, the competitive adsorption of CO₂ and methane in the hydrate structure may affect the replacement effect, especially when methane has already been adsorbed. Kinetically, the diffusion rate of CO₂ is relatively slow, and the size and structure of the hydrate pores also influence the rate of the displacement reaction. Furthermore, excessive CO₂ displacement may alter the stability of the hydrate, causing hydrate decomposition or instability. Finally, the entire displacement process often requires a long time, especially under low temperatures and high pressure, which limits large-scale applications. Therefore, the distribution and phase behavior of gas molecules in hydrates are not only central to the formation, stability, and dissociation processes of hydrates but are also key factors in the study of hydrate utilization and its environmental impacts [45].

2.3. Kinetic Studies

Kinetic studies primarily focus on several key factors in the gas displacement process, including reaction rates, diffusion mechanisms, and dynamic changes at the interface. Although thermodynamically favorable conditions for gas displacement may exist, the actual rate and efficiency of the displacement process are still constrained by various kinetic factors. Therefore, even when thermodynamic conditions are met, the actual occurrence of the gas displacement process is still limited by these kinetic factors, requiring further experimental studies and modeling for optimization [46].

2.3.1. Gas Diffusion and Exchange Rate

The diffusion process of gas within the hydrate is crucial to the gas displacement rate. In natural gas hydrates, gas molecules diffuse into the hydrate’s cage structure, displacing the original gas molecules inside the hydrate [44]. This process involves several key steps, including the entry of gas molecules from the external environment into the hydrate interface, penetration through the solid structure of the hydrate, and exchange with the existing gas molecules.

The diffusion rate of CO₂ is typically higher than that of CH₄, which may be due to the larger size of CO₂ molecules, resulting in a more complex spatial occupation within the hydrate’s cage structure. The structure of the hydrate is typically formed by water molecules arranged in a cage-like pattern, creating pore structures to trap gas molecules [18]. Due to the larger molecular volume of CO₂, CO₂ molecules may be more likely to aggregate and interact with each other within the hydrate’s cage compared to methane molecules, thereby affecting its diffusion behavior [23].

In the gas displacement process, gas exchange primarily occurs at the gas–hydrate interface. This process refers to the interaction between gas molecules and water molecules within the hydrate, causing the gas molecules to enter the hydrate structure and potentially displace the original gas molecules [47]. During this process, a larger gas–hydrate interface area helps accelerate gas exchange. If the hydrate particles are smaller in size, the surface area per unit volume increases, leading to more contact between gas molecules and the hydrate, which in turn enhances the exchange rate. Additionally, if the hydrate structure is more loosely packed or has larger pore sizes, the diffusion rate of gas molecules within the hydrate is faster, improving the efficiency of gas exchange. Conversely, if the hydrate structure is more compact, the diffusion rate of gas molecules is restricted, and the displacement process may slow down [44].

2.3.2. Reaction Mechanisms in the Displacement Process

The process of CO₂ displacing methane hydrate is a complex and dynamic multi-step reaction that involves competition, diffusion, and the reorganization of the hydrate structure between gas molecules. In this process, CO₂ molecules compete with methane molecules for occupancy of the hydrate’s empty cages. This process is not merely a simple gas exchange but also involves a series of changes related to the hydrate structure. A schematic diagram of the two CH₄ hydrate replacement mechanisms is shown in Figure 4 [46].

The CO₂-CH₄ hydrate replacement process can be divided into three steps: First, CO₂ contacts the surface layer of the CH₄ hydrate. As CO₂ enters the hydrate surface, the cage-like structure of the methane hydrate begins to break down, causing a portion of the methane gas to rapidly decompose and be released. At this point, methane gas escapes from the hydrate, and the structure of the surface layer of the hydrate is disrupted. As the displacement process progresses, CO₂ and CH₄ quickly combine with surrounding free water molecules, forming a mixed hydrate layer. Finally, CO₂ gas slowly diffuses into the interior of the hydrate and continues to displace the methane molecules within the hydrate structure. As CO₂ penetrates deeper into the hydrate, methane molecules originally occupying the internal cages are gradually replaced, and methane gas escapes outward, completing the displacement process. This stage of diffusion is relatively slow and depends on factors such as temperature, pressure, and the pore structure of the hydrate [20].

2.3.3. Kinetic Characteristics of the Displacement Reaction

Studies have shown that the CO₂-CH₄ displacement process is a complex dual mechanism significantly influenced by the occupancy of the hydrate cages. Bai et al. [48] conducted an in-depth study of this process using molecular dynamics (MD) simulations, revealing that the chemical potential changes of CO₂ molecules lead to the decomposition of methane hydrate and, by disturbing the structure of the hydrate, cause CH₄ molecules to be released from the cage-like structure. Nakano et al. [49] found experimentally that CO₂ diffuses faster than methane (CH₄) in the hydrate, proving that the CO₂-CH₄ hydrate displacement method is kinetically feasible. Tung et al. [50] pointed out that liquid CO₂ can directly replace methane (CH₄) in the hydrate without the need for the decomposition process. Other studies [51] explored the microscopic behavior of CO₂ and methane hydrates during the displacement reaction by using high-pressure reactors under different temperature and pressure conditions combined with molecular dynamics simulations, as shown in Figure 5. Figure 5A,B show the process of CO₂ replacing methane hydrate with and without free water. Figure 5C demonstrates that during the free water replacement process, the number of hydrogen bonds gradually decreases, and the decomposition rate of methane hydrate and the formation rate of CO₂ hydrate show different slopes at each stage. The results indicate that within a certain temperature and pressure range, as temperature and pressure increase, the displacement efficiency gradually improves.

Researchers have also conducted in-depth studies on the formation and decomposition mechanisms of hydrates using visualization techniques such as neutron diffraction, Raman spectroscopy, and nuclear magnetic resonance (NMR). For example, Uchida et al. [52] used Raman spectroscopy to confirm that the CO₂-CH₄ hydrate displacement reaction primarily occurs at the surface layer of the CH₄ hydrate. The results showed that during the displacement process, the concentration of methane (CH₄) gas in the gas phase gradually increased over time, but the rate of increase slowed down as the reaction progressed. This indicates that as the displacement reaction advances, CH₄ molecules in the hydrate are gradually replaced by CO₂, leading to a stabilization in the concentration of CH₄ gas. Kuang et al. [53] studied the microstructural evolution of CO₂ hydrates in porous media using NMR technology, revealing the structural changes of CO₂ hydrates in the pore space and their interactions with the surrounding environment under different conditions. Additionally, by analyzing the variation in pore water volume distribution with depth in sand layers under different storage conditions (as shown in Figure 6), they found differences in the formation locations of hydrates in porous media, confirming that CO₂ hydrates nucleate randomly in porous media and preferentially form in pore spaces. Their research provides an important theoretical basis for understanding the behavior of CO₂ hydrates in underground storage and the development of natural gas hydrates. Farahani et al. [54] compared the formation and decomposition of CH₄ hydrates in synthetic and natural sediment samples. Through experimental analysis, they found that CH₄ hydrates in natural sediments exhibited different kinetic characteristics during formation and decomposition compared to synthetic samples. This discovery provides new insights into the stability of natural gas hydrates, storage conditions, and gas displacement phenomena, presenting both challenges and opportunities for the exploitation and utilization of hydrates.

In the practical process of CO₂ displacement for natural gas hydrate exploitation, thermodynamics and kinetics work together in a complementary manner. Thermodynamic studies provide the phase equilibrium conditions and driving forces for the displacement process, while kinetic studies reveal the reaction rates, diffusion mechanisms, and influencing factors of the process. By combining thermodynamic and kinetic models, researchers can optimize the various operating conditions involved in the extraction process to achieve more efficient gas displacement. In terms of temperature and pressure control, thermodynamic models can determine the optimal temperature and pressure ranges to promote the CO₂ displacement of methane. Kinetic simulations, on the other hand, can help researchers predict the reaction rates and efficiency under different temperature and pressure conditions, thereby identifying the most suitable operating conditions.

3. Combined Enhancement of Displacement Method and Traditional Methods

The displacement method, as a novel natural gas extraction technology, holds significant potential in improving extraction efficiency, reducing greenhouse gas emissions, and promoting sustainable extraction. However, its high cost, technical complexity, and environmental risks are key challenges that need to be addressed for its commercialization [46]. Therefore, researchers worldwide have conducted extensive studies on enhancing the CO₂-CH₄ displacement process, exploring various aspects from the macro-level displacement process to the micro-level displacement mechanisms. The aim is to find an efficient enhancement method that comprehensively considers factors such as economic cost, environmental protection, and process complexity.

Table 4. A comparison of the impacts of different methods on the combined natural gas extraction.

Extraction Method Combination	Advantages	Disadvantages	Suitable Geological Conditions	Key Factors Affecting Extraction
Displacement Method Combined with Pressure Reduction Method	Increase extraction rate; sustainable gas release; reduce the risk of hydrate re-crystallization	High cost of gas injection; extraction efficiency limited by reservoir characteristics	Higher bottom pressure, better permeability, and porosity	Reservoir temperature and pressure; gas injection rate and gas selection
Displacement Method Combined with Thermal Stimulation Method	Thermal energy promotes hydrate dissociation, enhancing displacement effectiveness	Thermal stimulation method may cause potential damage to the reservoir; high energy consumption and relatively high cost	The hydrate layer at a lower temperature (0 °C to 10 °C) is relatively thick and evenly distributed	Reservoir temperature and thermal response characteristics; heat injection methods and temperature control; thermal stability and structure of the reservoir
Displacement Method Combined with Chemical Inhibitor Method	Improve the long-term stability of natural gas production; prevent hydrate recrystallization	Inhibitors may increase environmental risks; chemical inhibitors are expensive and could negatively impact extraction costs	Mid- to high-saturation hydrate reservoirs under low-temperature and high-pressure conditions	Selection and injection concentration of chemical inhibitors; cost and environmental friendliness of inhibitors; synergistic effect of inhibitors and displacement gases

presents a comparison of the impacts of the displacement method combined with various traditional methods of natural gas extraction.

3.1. Combined Extraction of Displacement Method and Depressurization Method

The traditional depressurization method has been applied in natural gas hydrate extraction to some extent. However, during the depressurization process, the dissociation of hydrates is an endothermic reaction, leading to a gradual decrease in methane production and a continuous drop in reservoir temperature. At the same time, a large amount of water is produced during depressurization, which not only further reduces gas production efficiency but may also cause a series of safety issues. This presents numerous challenges for the practical application of this method [21]. In contrast, the displacement method promotes hydrate dissociation by injecting external gases (such as CO₂) to replace methane molecules in the hydrate. Combining depressurization with the displacement method can fully leverage the synergistic effects of both, significantly improving methane release efficiency [48,55].

The advantage of this combined method is that the depressurization process creates the necessary low-pressure environment for the injection of displacement gases, ensuring that the displacement gas can effectively penetrate the hydrate layer. Meanwhile, the displacement method accelerates the dissociation of the hydrate through gas exchange, greatly enhancing the release rate of CH₄ [47]. Specifically, when CO₂ is used as the displacement gas, it can undergo a substitution reaction with methane in the hydrate, which not only helps improve methane recovery but also enables the sequestration of greenhouse gases, thus offering significant environmental benefits.

Zhao et al. [56] proposed an alternative method for depressurization extraction. Experiments showed that after 2 h of depressurization dissociation, CO₂ was used to displace the hydrates. Compared to non-depressurization extraction, the total displacement rate after combined depressurization increased to over 30%. Lee et al. [57], using a triaxial compression testing apparatus and applying the method of depressurization followed by displacement, found that the methane recovery rate could be increased from 35% with pure CO₂ replacement to around 60%. They also studied the impact of the depressurization process and CO₂ injection on the stability of natural gas hydrate reservoir structure. The results indicated that, during the dissociation of hydrates, as the dissociation ratio of CH₄ hydrates gradually increased, the structural strength of the hydrates significantly decreased. This phenomenon suggests that the stability of the hydrates weakens with methane release, causing the reservoir structure to gradually become looser, which increases the risk of potential reservoir collapse or gas leakage during extraction. Further analysis revealed that when the dissociation ratio of CH₄ hydrates reaches 20%, subsequent CO₂ displacement can achieve the optimal displacement efficiency.

Chen et al. [58] employed a “cross-flow method” for depressurization combined with displacement using two wells. The experimental results indicated that the inlet pressure, outlet pressure, and confining pressure all had significant effects on production efficiency. Higher inlet pressure facilitated the effective injection of CO₂ into the porous medium (as shown in Figure 7a), promoting the formation of CO₂ hydrates and enhancing displacement extraction. Additionally, when the outlet pressure is higher than the equilibrium pressure of CO₂, depressurization can disrupt the structure of methane hydrates, thus increasing the hydrate displacement rate. The experiments also showed that the impact of outlet pressure was greater than that of confining pressure (Figure 7b). In this study, the utilization efficiency ranged from 27.2% to 46.6% (Figure 7c), reflecting that improving CO₂ utilization efficiency through depressurization is feasible, with outlet pressure being one of the most important parameters determining utilization efficiency.

In contrast to the extraction sequence mentioned above, Yang et al. [59] adopted a method where CO₂ displacement is carried out for a period of time before depressurization. In the initial stage, CO₂ is injected to displace the hydrates, which increases the concentration of CO₂ in the hydrates. Then, by reducing the pressure, the driving force for hydrate dissociation is enhanced, further promoting the CH₄/CO₂ displacement reaction, ultimately achieving a total displacement rate of 80 mol%. The advantage of this extraction sequence is that it can utilize CO₂ more efficiently, not only increasing gas production but also effectively reducing the energy required for methane hydrate dissociation, making the overall extraction process more energy-efficient and environmentally friendly.

Moreover, the combined method of depressurization and displacement can address the issues of slow methane release and uneven gas diffusion that occur in single depressurization methods. By properly adjusting the depressurization rate and the amount of displacement gas injected, the methane release process can be optimized, avoiding the reformation of hydrates or localized freezing caused by excessive depressurization or improper gas injection, thereby improving the stability and safety of the extraction process. This makes it a safer method for natural gas hydrate extraction [60].

3.2. Combined Extraction of Displacement and Thermal Stimulation Methods

The thermal stimulation method is simple to operate, technically mature, and widely applicable. However, when used alone for natural gas extraction, the thermal stimulation method can be costly and may lead to reservoir instability, potentially triggering geological hazards. Therefore, in practical applications, thermal stimulation is often combined with the displacement method. This combination can improve methane recovery rates while reducing the energy consumption and reservoir instability risks associated with thermal stimulation [61,62].

Zhang et al. [62] studied the CH₄ substitution rate from the perspective of combined thermal stimulation and displacement methods for natural gas extraction. They found that the methane recovery efficiency (CRE) during the hydrate substitution process significantly increased from the original 10–50% to 21–63%. Tupsakhare et al. [63] conducted heating stimulation experiments using a gas mixture of 85% CO₂ and 15% N₂ with heating powers of 100 W, 50 W, and 20 W. Under a 100 W heating power, about 60% of the methane gas could be extracted from the hydrate. In another study, Tupsakhare et al. combined CO₂/N₂ displacement with thermal stimulation to enhance hydrate recovery. The results showed that, compared to the recovery rate of 26.5% from thermal stimulation alone, the recovery efficiency of natural gas using the combined thermal stimulation and displacement method (68.8%) was significantly higher.

The efficiency of heat transfer varies significantly depending on the choice of heat source. When using an external heat source for hydrate extraction, its efficiency is generally lower than that of internal heat sources. The main reason for this is that heat tends to dissipate in the non-hydrate regions outside the hydrate layer [21]. Fan et al. [64] conducted experiments, as shown in Figure 8a, to explore three different heat injection modes: Incremental Heating Injection for Enhanced Replacement Recovery (IHIR), Decremental Heating Injection for Enhanced Replacement Recovery (DHIR), and Constant Heating Injection for Enhanced Replacement Recovery (CHIR). The changes in CH₄ recovery rates under these modes are presented in Figure 8b. The results show that incremental heat injection can lead to more CH₄ dissociation and replacement, achieving the highest CH₄ recovery rate (17.0%) and the highest gas mole fraction (13.5%) while also reducing the energy consumption for gas separation and purification.

The combined use of thermal stimulation and displacement methods can play a crucial role in enhancing CH₄ recovery and CO₂ sequestration. However, ensuring the stability and safety of the operation through effective monitoring and adjustments remains a challenge for technical implementation. Additionally, in practical applications, optimizing the parameters of thermal stimulation and displacement to balance methane recovery and CO₂ sequestration is still an important area of research.

3.3. Combined Extraction of Displacement and Chemical Inhibitor Methods

The chemical inhibitor method has the advantages of accelerating the decomposition of natural gas hydrates and improving methane recovery efficiency. However, its drawbacks cannot be overlooked. The main issues include poor selectivity of the inhibitors, environmental pollution risks, high costs, and potential adverse effects on the formation and stability of hydrates, which is why it is rarely used alone in natural gas hydrate extraction. In recent years, some researchers have combined chemical additives with the CO₂-CH₄ hydrate displacement method to explore joint extraction technologies. Tetra-n-butylammonium bromide (TBAB), a commonly used hydrate promoter, has been applied to enhance CO₂-CH₄ displacement research. It effectively lowers the phase equilibrium conditions of CO₂ hydrate, thereby improving the displacement capacity of CO₂ for CH₄ hydrate and significantly enhancing the displacement efficiency [65]. Some studies also indicate that [66] chemical inhibitors can effectively improve CO₂’s displacement efficiency for CH₄ hydrate, optimizing the extraction process. However, simultaneous decomposition and displacement significantly reduce displacement efficiency. Therefore, the optimal approach is to first add chemical inhibitors to decompose CH₄ hydrate, releasing most of the CH₄ gas, and then inject CO₂ for displacement, which can significantly improve displacement efficiency. In addition to thermodynamic inhibitors, researchers have also explored hydrate promoters and anti-agglomerants. Heydari and Peyvandi’s study [67], which used biological surfactants to replace natural gas hydrates, showed that biological surfactants not only effectively promoted the formation of methane hydrates but also significantly improved CO₂ displacement efficiency. Specifically, the displacement ratio increased by 72.6%, and displacement kinetics improved by 39%. Although the biological surfactants were added during the hydrate formation phase, the study also indicated that their addition altered the hydrate morphology and had a positive impact on continuous displacement reactions.

Currently, this method is still in the early stages of research, with many scholars, both domestically and internationally, conducting related studies. The displacement potential and economic benefits remain unclear. In addition, issues such as the selection and optimization of inhibitors, environmental impacts, and their long-term effectiveness and stability continue to be major constraints for large-scale applications. Therefore, how to better simulate and understand these coupled processes, optimize process parameters, and achieve the best extraction results remains a significant challenge in technological progress.

Overall, the combined application of the displacement method and traditional extraction methods provides an innovative solution for natural gas extraction. This combination not only significantly optimizes extraction efficiency but also provides strong support for the sustainable development of energy. As shown in

Table 5. Combination of the displacement method with three traditional methods.

Method	Experimental Conditions	Advantages	CH4 Recovery Rate	Literature Source
Combined Pressure Reduction and Substitution Method	Experiments conducted using a customized high-pressure flow-through apparatus at different methane hydrate dissociation levels (0%, 20%, 40%, 60%, 80%, 100%)	The mechanical properties of methane hydrate-bearing sediments were considered to provide a basis for economically safe extraction; experimental studies were conducted to investigate the effects of various factors on mechanical properties and methane recovery rate	35.4–63.3%	Lee et al. [57]
Pressure Reduction-Assisted CO2 Substitution Method	Design of a one-dimensional experimental setup to simulate the interface between horizontal wells, investigating the impact of different pressures (inlet pressure, outlet pressure) on CO2 substitution behavior. The experimental temperature is 275 K, and the methane hydrate saturation is 32%	By combining the advantages of CO2 substitution and pressure reduction, production efficiency is improved, and risks are reduced; the impact of pressure parameters on natural gas extraction was studied, providing theoretical support for further research and application	27.2–46.6%	Chen et al. [58]
Pressure Reduction-Assisted CO2 Substitution Method	Study of the depressurization-assisted CO2 substitution process by varying initial hydrate dissociation ratio (0%, 50%, 100%), substitution period (1, 4, 7 days), and CO2 injection flow rate	The issues of weakened geomechanical strength of methane hydrate-bearing sediments caused by pressure reduction alone and the slow production rate during substitution were addressed; methane production and CO2 sequestration efficiency were improved	Through depressurization-assisted substitution, the amount of CO2 stored in the sediment can be greater than the amount of CH4 produced, with approximately 92% of the initial methane being replaced by CO2	Choi et al. [68]
Combined CH4/CO2 Substitution and Thermal Stimulation Method	Experiments conducted under different methane hydrate saturations, substitution zones, and freezing point conditions	The diffusion rate of CO2 was increased through thermal stimulation, overcoming the diffusion limitation in the CO2 substitution process alone; the methane substitution percentage, CO2 storage efficiency, and energy efficiency under different conditions were analyzed and discussed	64.63%	Zhang et al. [62]
Combined CH4/CO2 Substitution and Thermal Stimulation Method	Experiments conducted in a large-scale hydrate vessel (LSHV) with heating rates of 20, 50, and 100 W	The effect of temperature on N2 capture was studied, and it was found that N2 is selectively captured in hydrate cages at temperatures below 12 °C	At a heating rate of 100 W, the mole number of methane during thermal stimulation is 8.5; during thermal stimulation with CO2 substitution, it is 16; and during thermal stimulation with CO2 + N2 substitution, it is 20	Tupsakhare et al. [63]
Combined Thermal Stimulation and CH4/CO2 Substitution Method with Nanoparticle Addition	Experiments conducted in a high-pressure stainless steel reactor under different pressures (40 bar, 45 bar) and temperatures (5.5 °C, 8 °C, 10 °C)	Without the need for vacuum extraction, this method can effectively increase methane recovery and CO2 storage efficiency; the optimal experimental conditions (45 bar and 8 °C) were determined	The recovery rate of CH4 increased from 19.8% to 51.9%	Adibi et al. [69]
Inhibitor-Assisted Substitution Method (Using Methanol Solution)	Using an automated core flooding system to simulate and monitor fluid flow and studying the effects of different inhibitors on CH4 hydrate dissociation and CO2 substitution by varying the injected fluid	The CO2 substitution method and thermodynamic hydrate inhibitor technology were combined to replace the simple CH4 hydrate substitution process	Under the experimental conditions, the methane recovery rate exceeds 92%	Khlebnikov et al. [66]
Bio-Surfactant-Assisted Method (Using Rhamnolipid)	Studying the effect of different concentrations of rhamnolipids on the kinetics of methane hydrate formation and comparing it with the chemical surfactant SDS	Compared to the chemical surfactant SDS, it significantly reduces the induction time and total time; improves gas consumption and increases the kinetic growth rate of the hydrate; it can enhance the substitution rate and CO2 storage capacity	Injecting rhamnolipid increased the substitution percentage by approximately 72.6%	Heydari et al. [67]

, the combination of the displacement method with three traditional methods demonstrates complementary advantages in several aspects, further enhancing the overall extraction efficiency.

The combination of CO₂ injection, depressurization, thermal stimulation, and chemical inhibitors has shown significant potential in natural gas hydrate extraction, improving recovery rates (such as sustained methane production in the South China Sea pilot project) and carbon sequestration efficiency, contributing to the low-carbon transition. However, its technical depth and scaling face multiple challenges: the CO₂ injection and depressurization combination is limited by reservoir permeability differences, with low-permeability gas fields prone to insufficient fluidity and potential underground water contamination risks; combined with thermal stimulation, although it enhances gas flow (as seen in the Russia Mezoyakha gas field), the high-temperature environment accelerates equipment aging, increases maintenance costs, and irreversible changes in reservoir porosity threaten long-term stability [1]; when used with chemical inhibitors (such as ethylene glycol), although it can control hydrate dissociation risks (as seen in the South China Sea basin project in Japan), long-term use poses underground water contamination risks and relies on costly monitoring systems. On the economic side, high initial investments (equipment and reservoir modification) and specialized operation and maintenance costs far exceed the financial capacity of small and medium-sized enterprises, while the imperfect carbon trading market and fragmented policies further weaken revenue stability. Despite significant technical advantages, the large-scale application still needs to overcome core challenges such as geological adaptability, equipment durability, and environmental risks. In the future, AI should be used to optimize technical parameters, low-cost materials should be developed, and a global carbon market should be promoted, along with tax incentives and international standards coordination at the policy level, to balance energy efficiency and climate goals [4].

4. Multicomponent Gas Displacement Process

The CO₂-CH₄ hydrate replacement has become a widely studied field in academia. Previous studies have shown that up to 68% of the methane (CH₄) captured in hydrates can be replaced by carbon dioxide (CO₂) for recovery [20,70]. The main reasons for the low efficiency of CO₂ replacement in natural gas hydrate extraction are as follows: (1) The methane (CH₄) molecules in natural gas hydrates are relatively small and can be accommodated in the “large cages” and “small cages” of the hydrate structure. In contrast, CO₂ molecules, which are slightly larger and have a different molecular structure compared to CH₄, can only replace methane molecules in the large cages and are unable to effectively replace methane in the small cages. (2) There are strong van der Waals forces between methane molecules and hydrate cages, which makes the methane molecules tightly bound within the hydrate. (3) During the CO₂ replacement process, when CO₂ molecules enter the hydrate structure, especially at the hydrate surface, CO₂ forms a mixed hydrate layer with some methane molecules. This leads to difficulty in CO₂ entering the internal structure of the hydrate, limiting the exchange between CO₂ and methane and significantly reducing the replacement efficiency. Therefore, finding small molecular gases that can replace CH₄ in the small cages, increasing the hydrate phase equilibrium pressure, and enhancing gas transfer are key factors in improving the efficiency of CO₂ replacement for natural gas hydrate extraction.

4.1. CO₂-H₂ Mixture Replacement to Improve CH₄ Recovery Rate

The use of a CO₂ and H₂ mixture can improve the replacement efficiency [28]. The addition of H₂ does not lead to additional occupation of the hydrate cages. The reason hydrogen can facilitate the replacement process is primarily because H₂ molecules, being smaller, diffuse easily within the hydrate system. The introduction of H₂ reduces the partial pressure of methane in the gas phase, which helps to destabilize the methane hydrate, promoting its dissociation rather than directly replacing methane through gas exchange. Additionally, H₂ can enhance the mass transfer process, aiding in the diffusion of methane gas within the medium [71]. The CO₂-H₂ methane extraction technology uses mixed gas injection to enhance recovery and carbon sequestration. CO₂ displaces methane, while H₂ boosts reservoir permeability [28]. Benefits include hydrogen’s high diffusivity for improved gas flow and catalytic carbon utilization (e.g., methane dry reforming). Key challenges include the following: hydrogen flammability risks, complex gas separation (membrane/cryogenic methods), and hydrogen embrittlement under high pressure. No large-scale applications exist yet, with insights drawn from hydrogen storage and CO₂-EOR. Heterogeneous reservoirs risk uneven gas distribution and efficiency drops. High costs dominate, alongside safety expenses and uncertain carbon revenue. Future success hinges on affordable green hydrogen, hydrogen-resistant materials, and integrated “production-extraction-sequestration” systems to balance feasibility and economics in low-carbon transitions.

This finding aligns with the research by Ding et al. [27], who used a simulated CO₂/H₂ mixed gas integrated gasification combined cycle (IGCC) to replace CH₄ in methane hydrates and monitored the changes in CH₄, CO₂, and H₂ gas concentrations using a gas chromatograph (GC). Three key points (hydrate phase “A” point, gas–hydrate interface “B” point, and gas phase “C” point) were selected for Raman detection (as shown in Figure 9). As shown in Figure 9a, the experiment formed sI CH₄ hydrate. During the continuous injection of the CO₂/H₂ gas mixture into the reactor, the Raman spectra changes at the hydrate phase point (Figure 9c,d) indicated the formation of CO₂ and CH₄ hydrates, suggesting that the replacement occurs only between CO₂ and CH₄ molecules. No H₂ signal was detected in the figures, as shown in Figure 9b, indicating that H₂ was not encapsulated in the hydrate cages. The experiment demonstrated that the replacement process occurs in two steps: first, the dissociation of CH₄ hydrate, followed by the formation of CO₂ hydrate. Furthermore, the CH₄ recovery rate through CH₄-CO₂/H₂ replacement was greater than 71%, significantly higher than the CH₄-CO₂ replacement (50%). Notably, H₂ does not compete with CH₄ for occupying hydrate cages but rather plays a facilitative role in the CO₂-CH₄ replacement process.

Figure 10a shows the phase equilibrium conditions of hydrates formed by mixtures of H₂ + CH₄ + CO₂ at different ratios. The injection ratio of H₂ has a significant impact on the phase equilibrium of the H₂ + CH₄ + CO₂ ternary hydrate. As the H₂ ratio increases, the dissociation kinetics of the hydrate become more favorable, but the separation barrier for CO₂ also increases. In contrast, a lower H₂ ratio favors the formation of the hydrate but may lead to a lower CH₄ yield [72].

The study by Sun et al. [73] shows that when the mole fraction of CO₂ in the gas is low, the impact of the mixed gas on the hydrate structure is more significant. Additionally, as the hydrogen content increases, the dissociation rate of CH₄ hydrates also shows an increasing trend. Under appropriate temperature and pressure conditions, the methane replacement rate can reach over 90% [74]. Wang et al. [28] conducted experiments with different gas compositions (mole ratios of CO₂ and H₂), during which they monitored parameters such as the mole ratio of CO₂ and H₂ during the gas exchange process, methane replacement rate, and CO₂ sequestration ratio. The study indicates that there is a balance point between the mole fractions of CO₂ and H₂: too high a mole fraction of CO₂ promotes CO₂ sequestration but sacrifices gas yield, while a higher mole fraction of H₂ favors gas yield but is not conducive to CO₂ sequestration. A balance in CO₂ substitution can be achieved within a CO₂ mole fraction range of 55% to 72%, as shown in Figure 10b. Xu et al. [75] conducted replacement experiments under specific temperature and pressure conditions and analyzed the composition of released gases using gas chromatography (GC), measuring the CH₄ replacement efficiency during the process. The results show that there are significant differences in the final composition and replacement efficiency of CH₄ in experiments with different gas mixtures. The order of replacement efficiency is as follows: CO₂/H₂ > CO₂/N₂ > pure CO₂. Xie et al. [76] clarified the impact of gas partial pressure on hydrate replacement in CH₄-rich systems. The experimental results, for the first time, show that in a CO₂/CH₄/H₂ ternary gas system, even if the partial pressure of CH₄ exceeds the phase equilibrium pressure of CH₄ hydrates, H₂ can promote the premature dissociation of CH₄ hydrates. The effects of different H₂ and CO₂ gas mixing ratios on the extraction efficiency of pure hydrate phases and hydrate-sediment mixed phases are detailed in

Table 6. The effect of the CO₂ + H₂ mixed gas replacement method on the enhancement of the natural gas hydrate recovery rate.

Mixed Gas Ratio (CO2/H2)	Hydrate Medium	Temperature	Pressure	CH4 Recovery Rate	Literature Source
0.72/0.28	Sandstone + brine	275.6 K	5.0 MPa	28.0%	Wang et al. [28]
0.55/0.45				47.0%
0.36/0.64				25.0%
0.18/0.82				70.0%	Xu et al. [75]
0.4/0.6	Pure water	274.0 K	4.5 MPa	78%
0.601/0.399		274.2 K	6.0 MPa	32%	Sun et al. [73]
0.74/0.26	Quartzite + brine	276.0 K	3.6 MPa	41.4~52.4%
0.74/0.26	Quartz sand + brine	276.0 K	3.7 MPa	30.0~50.0%
0.4/0.6	Quartz sand + brine	276.0 K	3.7 MPa	40.0~75.0%	Sun et al. [73]
0.22/0.78		276.0 K	3.7 MPa	12.0~88.0%

.

4.2. CO₂ and N₂ Mixture Displacement to Improve CH₄ Recovery Rate

The application of CO₂ and N₂ mixed gas displacement in methane (CH₄) extraction is considered an effective method to significantly improve the methane recovery rate from natural gas hydrates. The CO₂ and nitrogen gas mixed methane extraction technology involves the simultaneous injection of CO₂ and N₂, which work synergistically to displace adsorbed methane and reduce pressure, achieving both enhanced recovery and carbon sequestration. Its advantage lies in the ability to adjust the gas mixture ratio to suit geological conditions (such as permeability and temperature) while reducing the risk of secondary hydrate formation. However, there are significant technical challenges: the gas mixture ratio needs precise control to avoid pore blockage or decreased efficiency, adsorption separation technologies (such as pressure swing adsorption) have limited adaptability to complex formations, and high-pressure corrosive environments exacerbate equipment wear and increase maintenance costs. In practical applications, while experiences from CO₂-enhanced oil recovery (such as Shengli Oilfield) and nitrogen-assisted extraction can be referenced, large-scale cases remain scarce, and heterogeneous formations are prone to gas retention or micro-seismic risks. Economically, the high initial investment and operational energy consumption depend on carbon trading revenues. However, small and medium-sized enterprises face challenges due to financial pressure and fragmented policies, especially in regions with underdeveloped carbon markets. Future progress will require technological optimization (such as AI control and low-cost catalysts), policy coordination (global carbon pricing), and industry chain integration (gas fields—chemicals—sequestration) to help balance resource development and emission reduction goals in the energy transition. Research by Park et al. [25] shows that when flue gas is used to replace methane (CH₄) in Type I hydrates, the displacement efficiency can reach 85%, while the displacement efficiency of pure CO₂ gas is only 64%. Raman spectroscopy results indicate that approximately 23% of CH₄ in the hydrate cages is replaced by N₂, while 62% is replaced by CO₂. This suggests that CH₄ can be displaced by N₂, and the occupation of N₂ effectively improves the replacement efficiency. Koh et al. [42] summarized that N₂, as a smaller guest molecule, can form type II hydrates on its own. Different concentrations of mixed gases, pure CO₂, and N₂ lead to the formation of different Type I and Type II hydrate structures. Since CO₂ molecules are larger, the addition of N₂ gas can significantly improve the displacement efficiency (as shown in Figure 11). When pure CO₂ gas is used to replace methane in hydrates, CO₂ molecules are thermodynamically unstable in the small Type I cages (5¹²). Therefore, displacement mainly occurs in the larger Type I cages (5¹²6²), and methane molecules in the small cages are not replaced.

After the introduction of N₂, N₂, as a smaller molecule, tends to occupy the small cages, which allows N₂ and CO₂ to work together to achieve the displacement process in both large and small cages. This interaction not only enhances methane recovery but also effectively improves production efficiency [77]. Research by Pandey et al. [78] also indicates that the use of a CO₂ and N₂ mixed gas can further increase methane recovery. Furthermore, as the concentration of CO₂ in the CO₂ + N₂ mixture increases, the methane recovery rate also rises, while the stability of the mixed CH₄-CO₂ hydrate is enhanced.

Specifically, the essential mechanism of this process lies in the competition between N₂ and CO₂ gases for entry into the molecular cage structures of the hydrate. Due to the different molecular behaviors of N₂ and CO₂ during hydrate formation, N₂ gas can influence the dissolution of CO₂ and the conditions for hydrate formation. When N₂ competes with CO₂ for entry into the hydrate cages, it alters the thermodynamic stability of the hydrate, making the temperature and pressure conditions for hydrate formation more stringent than those for pure CO₂ hydrates.

Various researchers have conducted studies on the impact of different CO₂ and N₂ mixing ratios on methane (CH₄) hydrate recovery rates, and the results are shown in

Table 7. The effect of the CO₂ + N₂ mixed gas displacement method on the improvement of the natural gas hydrate recovery rate.

Gas Mixture Ratio (CO2/N2)	Hydrate Medium	Temperature	Pressure	CH4 Recovery Rate	Literature Source
0.1/0.9	Porous silica + water	274.0 K	11.5/14.6/18.6 MPa	77%/80%/79%	Lee et al. [79]
0.2/0.8			13.7 MPa	80%
0.6/0.4	Pure water		4.5 MPa	73.4%	Xu et al. [75]
0.146/0.854	Silica sand + water	273.3 K	4.2 MPa	53.3%	Yang et al. [59]
0.28/0.72	Pure water + SDS solution	284.3K	9.0 MPa	13.2%	Niu et al. [80]
0.5/0.5	Pure water	273.9 K	5.0/6.7 MPa	8.3%/17.7%	Zhou et al. [81]
0.75/0.25		274.0 K	2.6/3.2/3.5 MPa	9.5%/12.6%/17.9%

. Lee et al. [79] primarily used Differential Scanning Calorimetry (DSC) and Pressure–Volume–Temperature (PVT) experimental methods to investigate the replacement process of methane hydrate (CH₄·nH₂O) under different gas mixing systems, particularly the displacement process with CO₂ and N₂ mixed gases. The study found that the injection of mixed gases effectively promotes the dissociation of methane hydrates, enabling methane recovery while also trapping CO₂ as CO₂ hydrates or mixed hydrates. Additionally, the gas mixture ratio significantly influences the displacement efficiency, with different gas combinations exhibiting varying replacement effects. During the displacement process, the original pure methane hydrate gradually transforms into a mixed hydrate composed of CH₄, CO₂, and N₂. This change leads to increased fluctuations in heat flow, thereby affecting the thermodynamic and kinetic properties of the hydrate.

In a CO₂ and N₂ mixed gas environment, when the nitrogen (N₂) proportion is relatively low, the methane (CH₄) displacement recovery rate is very low. However, when the N₂ proportion exceeds 50%, the methane displacement recovery rate increases to 12.2%, while the CO₂ hydrate sequestration rate reaches 42.8%. In contrast, when pure liquid CO₂ is used for natural gas hydrate extraction, the CO₂ sequestration rate is only 22.1%, significantly lower than the effectiveness of the CO₂ and N₂ mixed gas displacement method [59,75,79,80,81].

Experimental results and the equilibrium pressure data of the hydrate at different N₂ ratios (as shown in Figure 12) indicate that when CO₂ and N₂ mixed gases are used for natural gas hydrate extraction, the equilibrium pressure of the hydrate increases with the increase in the N₂ proportion. At higher N₂ concentrations, the hydrate requires higher pressure to maintain its stability. The stability of CH₄ hydrate decreases, making it more prone to replacement reactions, and the recovery rate significantly improves [72].

Since sI hydrates are widely present in natural gas hydrates, most studies focus on the replacement reactions in sI hydrates. However, sII hydrates have also been found in nature in certain regions, and the replacement mechanism of sII hydrates is significantly different from that of sI hydrates. Seo et al. promoted the replacement reaction of sII (C₃H₈ + CH₄) hydrate by externally injecting a CO₂/N₂ (50:50) mixed gas. The study showed that the replacement degree of C₃H₈ + CH₄ hydrate was about 54%. This method not only achieved a higher gas recovery rate than the pure CO₂ replacement method but also maintained the crystal structure of the hydrate after the replacement reaction (as shown in Figure 13b), indicating that the replacement process did not cause any structural changes in the sII hydrate. At the same time, according to the Raman spectrum of the CO₂ and N₂ molecular vibration modes in the C₃H₈ + CH₄ hydrate (Figure 13a), it was observed that both CO₂ and N₂ molecules occupied the cages of the sII hydrate. In Figure 13c, the reduction in C₃H₈ and CH₄ amounts in the initial C₃H₈ + CH₄ hydrate and the increase in CO₂ and N₂ amounts in the replaced C₃H₈ + CH₄ hydrate are similar, further confirming that CH₄ in the small cages is replaced by N₂, while C₃H₈ in the large cages is mainly replaced by CO₂ [82].

Overall, although a higher proportion of N₂ helps improve the displacement efficiency and CO₂ sequestration rate, it also introduces new challenges, such as the formation of multiple hydrates and the encapsulation effect, which may adversely affect the methane displacement recovery rate. Therefore, when using mixed gas displacement methods for natural gas hydrate extraction, it is necessary to comprehensively consider factors such as gas ratios, displacement efficiency, and the formation and encapsulation effects of hydrates to achieve optimal recovery and CO₂ sequestration outcomes.

4.3. The Displacement of CH₄ Recovery Is Improved by Using CO₂ Mixed with N₂ and H₂

When using a CO₂ and N₂ mixed gas for natural gas hydrate displacement, it can promote the dissociation of hydrates under lower pressure conditions, theoretically improving gas displacement efficiency. However, in practical operations, the displacement effect of the mixed gas has not fully achieved the expected recovery rate, indicating that some factors during the actual dissociation process may have affected the enhancement of displacement efficiency.

The main reason for this phenomenon is that the formation of CO₂ hydrates and N₂ hydrates adversely affects the recovery of internal natural gas hydrates, primarily methane hydrates. Specifically, CO₂ and N₂ interact with the hydrate structure during the displacement process, forming CO₂ hydrates and N₂ hydrates. These newly formed hydrate structures can encapsulate the original methane hydrate particles, hindering their dissociation and release, thus impacting the methane recovery efficiency [29].

Therefore, under different pressure and temperature conditions, Chaturvedi et al. [83] studied the effects of CO₂, N₂, and H₂ combinations on hydrate formation and methane recovery. At higher N₂ concentrations, even with the introduction of a small amount of H₂, the methane recovery rate in natural gas hydrates could still be increased to 67–69 mol% (Figure 14a–c). When the H₂ concentration increased from 0.1 mol% to 1 mol%, the methane recovery rate further improved to 71 mol%, and the presence of hydrogen at high pressure promoted natural gas hydrate formation. Previous studies have paid less attention to the synergistic effect of H₂ in the presence of N₂, and a possible explanation has been proposed: small gas molecules like N₂ occupy smaller hydrate cages, preventing CO₂ from entering larger cages and thus hindering hydrate formation. However, H₂ competes with N₂ for the smaller cages, allowing CO₂ to fill the larger cages for a longer time until the hydrate channels close. This theory, however, still requires further verification. Although the hydrogen promoter increased the hydrate yield and expanded the hydrate formation region, its effect remained limited at high N₂ concentrations. Further research indicated that when the H₂ mol% exceeded 1 mol%, the methane recovery rate no longer increased significantly (Figure 14d–g), possibly because H₂ occupies hydrate cages, hindering further natural gas hydrate formation.

The mechanism can be further illustrated in Figure 15. The addition of hydrogen helps optimize the hydrate formation process of the CO₂ + N₂ mixed gas, and H₂ helps increase the rate of hydrate formation, thereby accelerating CO₂ storage and conversion and optimizing the gas displacement effect for methane. However, if the hydrogen concentration is excessively high, it could disrupt the stability of CO₂ hydrates. Elevated hydrogen levels may compete with CO₂ for hydrate formation, thereby hindering the formation of CO₂ hydrates and diminishing its ability to replace methane effectively. Moreover, hydrogen itself does not have a strong ability to form hydrates, and its hydrate formation rate is much lower than that of CO₂, and excessively high hydrogen concentrations may lead to adverse changes in the structure and thermodynamic properties of the hydrates, affecting the overall stability of the reservoir. Therefore, to ensure reservoir stability and maximize methane recovery, it is essential to strictly control the hydrogen proportion when injecting CO₂ + N₂ + H₂ mixed gas [27].

5. Conclusions

Displacement reactions, as an important method for the production and storage of multi-component gases from hydrates, have made significant progress in basic research and laboratory stages, demonstrating their broad application potential in areas such as energy storage, gas separation, and environmental protection. Storing gases in hydrates not only significantly increases the gas storage density but also enables efficient gas release and reuse under low-temperature and normal-pressure conditions.

However, despite some success in laboratory studies of hydrate displacement reactions, several challenges remain in practical applications. First, the efficiency and reaction rate of the displacement reaction are still limited, and the thermodynamic issues and optimization of reaction conditions need further research. Second, the selective displacement and stability of multi-component gases need to be addressed through additional theoretical and experimental work.

To further improve the technical efficiency of methane displacement with mixed gases and achieve sustainable development, the following research directions need to be explored:

Improving the accuracy of experimental devices and numerical models: Future work should focus on developing more accurate experimental equipment and numerical models that are closer to real extraction environments, validating the performance and effectiveness of different gas mixtures.

In-depth study of the internal mechanisms of the displacement process: Future research should use high-resolution experiments and theoretical simulations to clarify the thermodynamic and kinetic characteristics of different reaction paths, providing a theoretical basis for optimizing displacement efficiency.

Optimizing the gas mixture ratio and injection strategy: For the injection of CO₂, N₂, and H₂ mixed gases, optimizing the ratio and injection strategy to achieve the best methane recovery rate and CO₂ storage efficiency is a key area for future research. Additionally, the synergistic effects of salt additives and gases should be considered, exploring the combined impact of CO₂+N₂+salt additives on reservoir stability, recovery rate, and CO₂ sequestration efficiency.

Improving CO₂ sequestration and stability: Future research should focus on the long-term safety of CO₂ sequestration, the storage capacity of reservoirs, and potential leakage pathways to ensure the environmental safety of the technology.

Enhancing the dual utilization benefits of CO₂ and methane: Combining the extraction of CO₂ and CH₄ hydrates with long-term CO₂ sequestration through integrated energy recovery and greenhouse gas emission reduction measures can enhance the overall benefits of natural gas hydrate extraction. The secondary sequestration method combining high-temperature CO₂ with deep geothermal energy can improve CO₂ sequestration rates while potentially enhancing methane recovery, promoting the development of this technology in a green and energy-efficient direction.