Effect of Residual Water in Sediments on the CO2-CH4 Replacement Process

<jats:p>CO2 replacement is a promising method of gas hydrate recovery. However, the influence of residual water in the replacement process and selections of a suitable mining area remain uncertain. To better understand this method, we examined the influence of the particle size and initial hydrate saturation on the replacement process while using the same amount of residual free water. The results showed that during the replacement process, two stages of rapid reaction and slow reaction occurred, which were manifested by the speed of pressure change in the reactor. The CO2 sequestration ratio decreased with the increase in sediment particle size and increased with the increase in initial hydrate saturation. During the replacement process, two reactions occurred: CH4 was replaced by CO2 and CO2 hydrate was formed, and the replacement amount and recovery efficiency of CH4 increased with a decrease in sediment particle size. When the sediment particle size was less than 166 μm, the CH4 recovery efficiency was significantly affected by the particle size. The replacement amount of CH4 increased with the increase in initial hydrate saturation, and the recovery efficiency decreased. This study provides a basis for selecting suitable hydrate-accumulation areas for on-site mining.</jats:p>

section_content: Natural gas hydrate is a crystalline compound similar to ice that is formed by small molecular gas and water molecules in high-pressure and low-temperature environments [1, 2]. Since natural gas hydrate is found to have a huge reservoir in seabed and permafrost regions and the major component of natural gas hydrate is methane (CH 4 ), it is considered as an unconventional energy source and has received growing attention worldwide [3, 4]. The common hydrate mining methods include depressurization, thermal stimulation, and chemical injection. All these methods recover natural gas by promoting hydrate dissociation [5, 6]. In recent years, a new method for natural gas hydrate recovery has attracted the attention of researchers as a CO 2 -CH 4 replacement. This method can replace the methane molecules in natural gas hydrates and capture CO 2 molecules in the seabed or permafrost in the form of hydrates, ultimately achieving a "win-win" situation for economic development and environmental protection [7]. 

Feasibility of the CO 2 -CH 4 replacement method has been proved and the behaviors of CO 2 -CH 4 replacement are now being intensively investigated. Ota et al. [8] studied the effects of temperatures from 272. 15-275. 15 K and found that under certain pressures, the replacement efficiency and rate increase with an increase in temperature. The replacement Energies 2023, 16, 3154 3 of 16 The results revealed the advantages of residual water for CO 2 sequestration and the side effects on CH 4 recovery efficiency, which would provide a possible path to describing the CO 2 behavior in CH 4 hydrate reservoir and lay a theoretical basis for selecting suitable hydrate-bearing sediments.
section_name: Introduction

section_name: Materials and Methods

section_content: A schematic of the experimental apparatus is shown in Figure 1. The main part of this apparatus is composed of a high-pressure reactor with an effective volume of 492 mL, an effective height of 150 mm, an inner diameter of 65 mm. The reactor is equipped with a buffer tank with an effective volume of 1117 mL. Their design pressures were 15 MPa and 20 MPa, respectively. Devices for measuring temperature and pressure were arranged in both the reaction kettle and the buffer tank. The thermometer was made of Pt100 platinum with an accuracy of ±0. 1 K. The pressure transducer had a range of 0-20 MPa and an accuracy of 0. 1% F. S. The constant-temperature water bath has an adjustment range of 270. 15-303. 15 K. The data were collected every 10 s using an Aigent34970 acquisition instrument. To simulate the environment of natural gas hydrate reservoirs, a cylindrical mold was used, and the wet sand was put into the mold for multiple beating and compaction times, and then placed in the refrigerator for 6 h to form a cylindrical sand column with a diameter of 50 mm, height of 65 mm, and effective volume of 127. 6 cm 3. 

covered were calculated by sampling the gas composition during CH4-CO2 replacements. The effect of initial hydrate saturation and particle size on replacement efficiencies were analyzed. The results revealed the advantages of residual water for CO2 sequestration and the side effects on CH4 recovery efficiency, which would provide a possible path to describing the CO2 behavior in CH4 hydrate reservoir and lay a theoretical basis for selecting suitable hydrate-bearing sediments.
section_name: Experimental Apparatus

section_content: A schematic of the experimental apparatus is shown in Figure 1. The main part of this apparatus is composed of a high-pressure reactor with an effective volume of 492 mL, an effective height of 150 mm, an inner diameter of 65 mm. The reactor is equipped with a buffer tank with an effective volume of 1117 mL. Their design pressures were 15 MPa and 20 MPa, respectively. Devices for measuring temperature and pressure were arranged in both the reaction kettle and the buffer tank. The thermometer was made of Pt100 platinum with an accuracy of ±0. 1 K. The pressure transducer had a range of 0-20 MPa and an accuracy of 0. 1% F. S. The constant-temperature water bath has an adjustment range of 270. 15-303. 15 K. The data were collected every 10 s using an Aigent34970 acquisition instrument. To simulate the environment of natural gas hydrate reservoirs, a cylindrical mold was used, and the wet sand was put into the mold for multiple beating and compaction times, and then placed in the refrigerator for 6 h to form a cylindrical sand column with a diameter of 50 mm, height of 65 mm, and effective volume of 127. 6 cm 3.
section_name: Experimental Apparatus

section_content: Methane and carbon dioxide with a mole fraction of 0. 999 were supplied by the Foshan Huate Gas Industry Corporation, and the experimental distilled water was made in the laboratory with a resistance of 18 MΩ/m. Reservoir sediments were composed of four different median particle sizes of natural river sand, the properties of which are shown in Table 1.
section_name: Experimental Materials

section_content: Methane and carbon dioxide with a mole fraction of 0. 999 were supplied by the Foshan Huate Gas Industry Corporation, and the experimental distilled water was made in the laboratory with a resistance of 18 MΩ/m. Reservoir sediments were composed of four different median particle sizes of natural river sand, the properties of which are shown in Table 1. Figure 2 shows scanning electron microscopy (SEM) images of natural sand at different magnifications. There are many voids and textures on the surface of the sand, which provide good reaction sites for the formation of hydrates [27]. Figure 2 shows scanning electron microscopy (SEM) images of natural san different magnifications. There are many voids and textures on the surface of the s which provide good reaction sites for the formation of hydrates [27].
section_name: Experimental Materials

section_content: The experimental process consisted of two steps: the preparation of thane-containing hydrate sediment materials and the replacement reaction of CO2-C
section_name: Experimental Procedures

section_content: Before the experiment, 150 g of dried natural sand was mixed with 8. 13, 12. 12, 1 or 20. 05 g of distilled water to prepare 20, 30, 40, or 50% of the initial water saturatio the wet sand. Then, the wet sand was placed into a mold, beat, and compacted. The m was placed into a refrigerator to form a cylindrical sand column at 263. 15 K for 6 h. A that, the frozen sand column was placed into the reactor. The reactor was set to a perature of 277. 15 K using the water bath. Finally, valve 11 was opened, and CH4 slowly fed into the reactor to form CH4 hydrate. The reaction was stopped when weight of residual water in the sediment was approximately 6. 2 g (the calculation pro is described in Section 2. 4. ). Tables 2 and 3 show the hydrate formation condition eight groups of experiments.
section_name: Preparation of Methane Hydrate in Porous Media

section_content: The experimental process consisted of two steps: the preparation of methane-containing hydrate sediment materials and the replacement reaction of CO 2 -CH 4.
section_name: Experimental Procedures

section_content: Before the experiment, 150 g of dried natural sand was mixed with 8. 13, 12. 12, 16. 04, or 20. 05 g of distilled water to prepare 20, 30, 40, or 50% of the initial water saturation of the wet sand. Then, the wet sand was placed into a mold, beat, and compacted. The mold was placed into a refrigerator to form a cylindrical sand column at 263. 15 K for 6 h. After that, the frozen sand column was placed into the reactor. The reactor was set to a temperature of 277. 15 K using the water bath. Finally, valve 11 was opened, and CH 4 was slowly fed into the reactor to form CH 4 hydrate. The reaction was stopped when the weight of residual water in the sediment was approximately 6. 2 g (the calculation process is described in Section 2. 4. ). Tables 2 and 3 show the hydrate formation conditions for eight groups of experiments. We selected sand with a median particle size of 101,166,279,377 µm and an initial water saturation of 30% to prepare sediments containing methane hydrate and to explore the effect of sediment size on replacement. 

Similarly, we selected sand with initial water saturations of 20, 30, 40, and 50% and a median particle size of 377 µm to prepare methane hydrate-containing sediments and study the effect of the initial hydrate saturation on displacement.
section_name: Preparation of Methane Hydrate in Porous Media

section_content: The replacement reaction process was divided into three steps:

Gaseous CO 2 injection. The remaining CH 4 in the reactor was quickly discharged, valve 11 was opened, and the reactor was purged with CO 2 for 15 s to ensure that the CH 4 gas in the reactor was completely discharged to the emptying pressure, and then valve 12 was closed. Subsequently, gaseous CO 2 was injected for one minute. When the pressure was stable at 3 MPa, valve 11 was closed. This moment was recorded as time zero of the replacement reaction. 2. 

Gas sample analysis. Gas samples were collected at time zero, and a gas chromatograph (GC9790, USA) was used to measure the gas sample components and content, record it as the content before the reaction, and then collect gas samples and measurements at time intervals. The volume of the single gas sample did not exceed 20 mL. The effects on temperature and pressure were negligible.
section_name: Replacement Reaction Process

section_content: Replacement reaction process. The replacement temperature was 277. 15 K, the replacement pressure was 3 MPa, the replacement time was set to 4 days for each set of experiments, the temperature fluctuation during the reaction process did not exceed 0. 1 K, and the pressure drop at the end of the reaction did not exceed 0. 1 MPa.
section_name: 3.

section_content: During the experiment, the skeleton structure of the sediment was almost incompressible; therefore, it can be assumed that the initial porosity in the natural sand remained unchanged, CH 4 gas was basically insoluble in water, and methane hydrate was formed uniformly in the sediment. Therefore, we believe that all the methane consumed is used to form CH 4 hydrates, and the molar ratio of methane hydrate in the sediment can be calculated by Equation (1) [28] :

where n is the CH 4 gas consumption during the formation process (mol), R is the gas constant, 8. 134 J•mol -1 •K, Z 0 and Z 1 are the compressibility factors of CH 4 gas under the corresponding temperature and pressure conditions, P 0 and P 1 are the initial and final pressures (MPa), T 0, and T 1 are the initial and final reactor temperatures (K), and V g is the volume of the gas phase in the cell, which can be determined by the volume of the sediment layer, the volume of the sediment void, and the volume of the reactor. The volume expansion caused by the phase transitions is negligible. Therefore, it can be expressed by Equation ( 2 ):

where V t is the effective volume of the reactor (mL), V s is the volume of sand added to the reactor (mL), V w is the volume occupied by the residual water (mL), and V k is the deposition void volume (mL), which can be expressed by Equation (3):

where ϕ is the initial porosity of natural sand with the corresponding particle size (%). The saturation of methane hydrate can be described by Equation ( 4 ):

where N H is the hydration number of methane. In this study, we chose 6. 0 as the hydration number of methane. M w is the molar mass of water (g/mol), and ρ H is the density of methane hydrate, 0. 925 g/cm 3.
section_name: Calculations 2.4.1. Calculation of Methane Hydrate Saturation

section_content: The recovery efficiency(η) of CH 4 is calculated using Equation ( 5 ):

where n CH is the number of moles of methane in the gas phase in the reactor (mol), n CH,0 is the number of moles of methane in the gas phase of the reactor at zero time (mol), and n CH is determined by Equation ( 6 ):

where x CH is the mole fraction of methane in the gas phase in the reactor, which is determined after analyzing the gas sample by gas chromatography; P and T represent the temperature and pressure inside the reactor one second before the gas takes, respectively. Z CH is the compression factor of CH 4. It can be calculated using the P-R equation.
section_name: Recovery Efficiency of CH 4

section_content: The weight of residual water (w r ) in the sediment can be calculated using Equation ( 7 ):

where w 0 is the weight of the water added to the sediment (g) and C w is the water conversion ratio, which can be described by Equation ( 8 ):
section_name: Weight of Residual Water in the Sediment

section_content: The moles of carbon dioxide in the gas phase of the reactor can be calculated using Equation (9) :

where x CO is the mole fraction of carbon dioxide in the gas phase in the reactor, which is determined after analyzing the gas sample by gas chromatography; P and T represent the temperature and pressure inside the reactor one second before the gas takes, respectively. Z CO is the compression factor of CO 2. It can be calculated using the P-R equation. 

The moles of carbon dioxide sequestered in sediments can be expressed by Equation (10) :

where n CO, 0 is the number of moles of carbon dioxide gas in the reactor at time zero. 

Energies 2023, 16, 3154 7 of 16

The CO 2 storage efficiency(ε) is defined as the ratio of the moles of CO 2 consumed to the total moles of CO 2 injected, which can be calculated using Equation (11) :
section_name: CO 2 Storage Efficiency

section_name: Results and Discussion

section_content: The variation in pressure inside the reactor for different particle sizes with time during the replacement reaction is shown in Figure 3. 

Energies 2023, 16, x FOR PEER REVIEW 7 of 16

where nCO,0 is the number of moles of carbon dioxide gas in the reactor at time zero. The CO2 storage efficiency(ε) is defined as the ratio of the moles of CO2 consumed to the total moles of CO2 injected, which can be calculated using Equation (11) :

(11)
section_name: Variation in Pressure during the Replacement Process

section_content: The variation in pressure inside the reactor for different particle sizes with time during the replacement reaction is shown in Figure 3. In the initial stage (the first 10 h), the pressure in the reactor drops rapidly in Experiments 2-4 because the rate of CO2 hydrate formation between the injected CO2 and the residual water in the sediment is much greater than the rate of replacement of CH4; that is, the consumption of CO2 is greater than the generation of CH4, resulting in a decrease in the total pressure in the reactor. On the contrary, it also shows that in the replacement reaction, the priority of the formation of CO2 hydrate in Experiments 2-4 is higher than that of CO2 replacement of CH4 hydrate [29]. 

The variation in pressure in Experiment 1 is opposite to that of Experiments 2-4. In the initial stage, the pressure in the reactor in Experiment 1 increased rapidly. This was because the amount of CH4 gas replaced during the reaction in the first 10 h is greater than the sum of the CO2 gas consumed in the replacement reaction and the CO2 gas consumed in the generation of CO2 hydrate with water, so the total pressure in the reactor tends to rise. The reason is that the specific surface area of the porous medium is larger when the particle size is smaller, and the area of the CO2 replacement reaction surface is larger, which means the effective area of the replacement reaction is larger, so the amount of CH4 replaced will be more. The particle sizes used in Experiment 1 was the smallest among the four experiments, which has the largest effective area of the replacement reaction [30]. When CO2 is injected, the replacement reaction occurs rapidly, In the initial stage (the first 10 h), the pressure in the reactor drops rapidly in Experiments 2-4 because the rate of CO 2 hydrate formation between the injected CO 2 and the residual water in the sediment is much greater than the rate of replacement of CH 4 ; that is, the consumption of CO 2 is greater than the generation of CH 4, resulting in a decrease in the total pressure in the reactor. On the contrary, it also shows that in the replacement reaction, the priority of the formation of CO 2 hydrate in Experiments 2-4 is higher than that of CO 2 replacement of CH 4 hydrate [29]. 

The variation in pressure in Experiment 1 is opposite to that of Experiments 2-4. In the initial stage, the pressure in the reactor in Experiment 1 increased rapidly. This was because the amount of CH 4 gas replaced during the reaction in the first 10 h is greater than the sum of the CO 2 gas consumed in the replacement reaction and the CO 2 gas consumed in the generation of CO 2 hydrate with water, so the total pressure in the reactor tends to rise. The reason is that the specific surface area of the porous medium is larger when the particle size is smaller, and the area of the CO 2 replacement reaction surface is larger, which means the effective area of the replacement reaction is larger, so the amount of CH 4 replaced will be more. The particle sizes used in Experiment 1 was the smallest among the four experiments, which has the largest effective area of the replacement reaction [30]. When CO 2 is injected, the replacement reaction occurs rapidly, and the reaction speed is faster than that of CO 2 hydrate formation, which eventually leads to a pressure increase in the initial stage of the reaction. 

In the later stage, the pressure of Experiments 1-4 showed a slow pressure drop, which can be explained as follows. First, during the replacement process, both the CO 2 -CH 4 replacement reaction on the sediment surface, and the reaction of the residual water with CO 2 to form CO 2 hydrate, occurred at the same time [31]. Both reactions form a dense CO 2 hydrate film on the sediment surface [24]. As the thickness of the CO 2 hydrate layer gradually increased, the reaction became more difficult because of the increased diffusion Energies 2023, 16, 3154 8 of 16 barrier of CO 2 molecules into the CH 4 hydrate in the sediment and CH 4 molecules passing through the CO 2 hydrate layer. Second, as the replacement process proceeds, the fugacity difference between CO 2 and CH 4 in the gas and hydrate phases decreases [20], which means that the driving force of the replacement reaction decreases; thus, the reaction becomes slower. Thus, the diffusivity of gas molecules in the sediment plays a crucial role in the displacement reaction. 

The pressure curves in the reactor over time at different initial hydrate saturations are shown in Figure 4. 

leads to a pressure increase in the initial stage of the reaction. 

In the later stage, the pressure of Experiments 1-4 showed a slow pressure drop, which can be explained as follows. First, during the replacement process, both the CO2-CH4 replacement reaction on the sediment surface, and the reaction of the residual water with CO2 to form CO2 hydrate, occurred at the same time [31]. Both reactions form a dense CO2 hydrate film on the sediment surface [24]. As the thickness of the CO2 hydrate layer gradually increased, the reaction became more difficult because of the increased diffusion barrier of CO2 molecules into the CH4 hydrate in the sediment and CH4 molecules passing through the CO2 hydrate layer. Second, as the replacement process proceeds, the fugacity difference between CO2 and CH4 in the gas and hydrate phases decreases [20], which means that the driving force of the replacement reaction decreases; thus, the reaction becomes slower. Thus, the diffusivity of gas molecules in the sediment plays a crucial role in the displacement reaction. 

The pressure curves in the reactor over time at different initial hydrate saturations are shown in Figure 4. Figure 4 shows that the pressure changes during the displacement process in Experiments 5-8 dropped sharply in the initial stage of the reaction and decreased slowly in the later stage. It also shows that the replacement reaction mainly occurred on the surface of the sediment. The CO2 hydrate layer formed in the initial stage was thinner and the reaction speed was faster, resulting in a large pressure drop. However, as the reaction progressed, the CO2 hydrate film formed around the sediment surrounded the entire sediment surface, and its thickness continued to increase, which eventually led to an increase in the channel resistance for CO2 molecules to enter the sediment to participate in the reaction; thus, the pressure curve in the later period did not decrease significantly. Figure 4 shows that the pressure changes during the displacement process in Experiments 5-8 dropped sharply in the initial stage of the reaction and decreased slowly in the later stage. It also shows that the replacement reaction mainly occurred on the surface of the sediment. The CO 2 hydrate layer formed in the initial stage was thinner and the reaction speed was faster, resulting in a large pressure drop. However, as the reaction progressed, the CO 2 hydrate film formed around the sediment surrounded the entire sediment surface, and its thickness continued to increase, which eventually led to an increase in the channel resistance for CO 2 molecules to enter the sediment to participate in the reaction; thus, the pressure curve in the later period did not decrease significantly. As the residual water quality was approximately equal in all experiments, it can be assumed that the amount of CO 2 dissolved in the residual water was the same. Therefore, the influence of residual water on the results was consistent. As shown in Figure 5, there is little difference in the storage efficiency of CO 2 for different particle sizes, and the smaller the particle size, the greater the storage efficiency. There are two possible explanations: one is that the smaller the particle size, the greater the porosity, and the relatively smaller the resistance for CO 2 molecules to enter the sediment to participate in the reaction [32] ; second, the smaller the particle size, the larger the specific surface area, and with more CO 2 hydrates generated on the surface, more CO 2 needs to be consumed. As a result, the storage efficiency would increase. As the residual water quality was approximately equal in all experiments, it can be assumed that the amount of CO2 dissolved in the residual water was the same. Therefore, the influence of residual water on the results was consistent. As shown in Figure 5, there is little difference in the storage efficiency of CO2 for different particle sizes, and the smaller the particle size, the greater the storage efficiency. There are two possible explanations: one is that the smaller the particle size, the greater the porosity, and the relatively smaller the resistance for CO2 molecules to enter the sediment to participate in the reaction [32] ; second, the smaller the particle size, the larger the specific surface area, and with more CO2 hydrates generated on the surface, more CO2 needs to be consumed. As a result, the storage efficiency would increase. As the residual water quality was approximately equal in all experiments, it can be assumed that the amount of CO2 dissolved in the residual water was the same. Therefore, the influence of residual water on the results was consistent. As shown in Figure 5, there is little difference in the storage efficiency of CO2 for different particle sizes, and the smaller the particle size, the greater the storage efficiency. There are two possible explanations: one is that the smaller the particle size, the greater the porosity, and the relatively smaller the resistance for CO2 molecules to enter the sediment to participate in the reaction [32] ; second, the smaller the particle size, the larger the specific surface area, and with more CO2 hydrates generated on the surface, more CO2 needs to be consumed. As a result, the storage efficiency would increase. Figure 6 shows that the CO 2 storage efficiency of Experiments 5-7 is not significantly different, because the formation of CH 4 hydrate in the sediment is grain-coating, and the CO 2 consumption at this time is only affected by the hydrate saturation. As the hydrate saturation decreases, the permeability of the sediment increases, which is conducive to the diffusion of CO 2 into the sediment. However, when the hydrate saturation is greater than 35%, CH 4 hydrate exists in the form of pore-filling [33, 34], whose permeability is greatly increased compared with grain-coating; thus, the CO 2 storage rate in Experiment 8 is significantly increased. However, it also shows that the influence of hydrate morphology difference on gas permeability is greater than that of hydrate saturation on gas permeability.
section_name: Variation in Pressure during the Replacement Process

section_name: CO2 Storage Efficiency

section_name: CO 2 Storage Efficiency

section_content: Figures 7 and 8 show the variation in the total gas content in the reactor and the gas phase, respectively, during the replacement process of Experiments 1-4. 

than 35%, CH4 hydrate exists in the form of pore-filling [33, 34], whose permeability is greatly increased compared with grain-coating; thus, the CO2 storage rate in Experiment 8 is significantly increased. However, it also shows that the influence of hydrate morphology difference on gas permeability is greater than that of hydrate saturation on gas permeability.
section_name: Variation in CO 2 and CH 4 in the Gas Phase

section_content: Figures 7 and 8 show the variation in the total gas content in the reactor and the gas phase, respectively, during the replacement process of Experiments 1-4. Calculated by Equation ( 1 ), the reduction in gas in the reactors for Experiments 1-4 were 0. 0118, 0. 0286, 0. 0268, and 0. 0256 mol, respectively. These values represent the amount of gas consumed by the newly formed CO2 hydrates during the replacement process. The variation in the total gas content in the reactor of Experiment 1 was the smallest within the same replacement time, but the content of the gas component CH4 increased the most, reaching 0. 042 mol. From this, it can be concluded that the cumulative replacement of CH4 and the consumed CO2 are not significantly different, which means that CO2 alone is less involved in the process of forming CO2 hydrate and mainly participates in the process of replacing CH4, which is beneficial to replacement mining. 

The gas content in Experiments 2-4 changed significantly with time, indicating that CO2 was mainly involved in the formation of CO2 hydrate, while the CO2-CH4 replacement reaction was not dominant, so the cumulative replacement amount of CH4 was less than that in Experiment 1. The replaced CH4 in Experiments 2-4 was 0. 022, 0. 017, and 0. 015 mol, respectively. Under the same conditions, the replaced CH4 increased with a decrease in the sediment particle size. This can be explained as follows. First, owing to the formation of vein-like hydrates in fine sediments [35, 36], the effective area of the interface between CO2 gas and hydrates is large, while the hydrates formed by coarse sediments are pore-filled hydrates, and the effective area of the reaction is small. Second, with smaller particle size, there is greater permeability, making it easier for CO2 to enter the sediment for replacement reactions. 

Through the comparison and comprehensive analysis of Figures 7 and 8, it can be concluded that the reaction shows a trend of decreasing total gas content as the reaction proceeds, during which CO2 is decreasing, CH4 is increasing, and the decrease in CO2 is greater than the increase in CH4. Therefore, in the replacement reaction, CO2 does not simply replace CH4 in equal amounts but also forms new CO2 hydrates due to the combination of CO2 and residual water. 

Figure 9 shows the variation in the total gas content in the reactor over time during the replacement process of Experiments 5-8. Calculated by Equation ( 1 ), the reduction in gas in the reactors for Experiments 1-4 were 0. 0118, 0. 0286, 0. 0268, and 0. 0256 mol, respectively. These values represent the amount of gas consumed by the newly formed CO 2 hydrates during the replacement process. The variation in the total gas content in the reactor of Experiment 1 was the smallest within the same replacement time, but the content of the gas component CH 4 increased the most, reaching 0. 042 mol. From this, it can be concluded that the cumulative replacement of CH 4 and the consumed CO 2 are not significantly different, which means that CO 2 alone is less involved in the process of forming CO 2 hydrate and mainly participates in the process of replacing CH 4, which is beneficial to replacement mining. 

The gas content in Experiments 2-4 changed significantly with time, indicating that CO 2 was mainly involved in the formation of CO 2 hydrate, while the CO 2 -CH 4 replacement reaction was not dominant, so the cumulative replacement amount of CH 4 was less than that in Experiment 1. The replaced CH 4 in Experiments 2-4 was 0. 022, 0. 017, and 0. 015 mol, respectively. Under the same conditions, the replaced CH 4 increased with a decrease in the sediment particle size. 

This can be explained as follows. First, owing to the formation of vein-like hydrates in fine sediments [35, 36], the effective area of the interface between CO 2 gas and hydrates is large, while the hydrates formed by coarse sediments are pore-filled hydrates, and the effective area of the reaction is small. Second, with smaller particle size, there is greater permeability, making it easier for CO 2 to enter the sediment for replacement reactions. 

Through the comparison and comprehensive analysis of Figures 7 and 8, it can be concluded that the reaction shows a trend of decreasing total gas content as the reaction proceeds, during which CO 2 is decreasing, CH 4 is increasing, and the decrease in CO 2 is greater than the increase in CH 4. Therefore, in the replacement reaction, CO 2 does not simply replace CH 4 in equal amounts but also forms new CO 2 hydrates due to the combination of CO 2 and residual water. 

Figure 9 shows the variation in the total gas content in the reactor over time during the replacement process of Experiments 5-8. This can be explained as follows. First, owing to the formation of vein-like hydrates in fine sediments [35, 36], the effective area of the interface between CO2 gas and hydrates is large, while the hydrates formed by coarse sediments are pore-filled hydrates, and the effective area of the reaction is small. Second, with smaller particle size, there is greater permeability, making it easier for CO2 to enter the sediment for replacement reactions. 

Through the comparison and comprehensive analysis of Figures 7 and 8, it can be concluded that the reaction shows a trend of decreasing total gas content as the reaction proceeds, during which CO2 is decreasing, CH4 is increasing, and the decrease in CO2 is greater than the increase in CH4. Therefore, in the replacement reaction, CO2 does not simply replace CH4 in equal amounts but also forms new CO2 hydrates due to the combination of CO2 and residual water. 

Figure 9 shows the variation in the total gas content in the reactor over time during the replacement process of Experiments 5-8. Calculated by Equation (1), the reductions in gas in the reactor for Experiments 5-8 are 0. 038, 0. 0256, 0. 0183, and 0. 0321 mol, respectively, which is the amount of gas con- Calculated by Equation (1), the reductions in gas in the reactor for Experiments 5-8 are 0. 038, 0. 0256, 0. 0183, and 0. 0321 mol, respectively, which is the amount of gas consumed by the newly formed CO 2 hydrate during the replacement process. As shown in Figure 9, with an increase in the initial hydrate saturation, the reduction range of the total gas content in the reactor first decreased, and when the initial hydrate saturation reached a certain value, the reduction range of the total gas content increased. This can be explained by the fact that hydrate saturation and morphology control the CO 2 flow into the sediment and the effective reaction area. This is because they directly control the relative gas permeability in CH 4 hydrate-containing sediments. The difference in the hydrate morphology affects the relative permeability of the gas and the contact interface of the CO 2 gas hydrates. According to experimental records [34], when the initial hydrate saturation is less than 35%, CH 4 hydrate exists in the form of a grain-coating (mainly nucleated on the sediment surface; it grows along the sediment surface [37] ). When the initial hydrate saturation is greater than 35%, and CH 4 hydrate exists in the form of pore-filling (mainly nucleating in the pores between the sediments [35, 38], hydrate crystals are suspended in the pores and not in contact with the sediments, and there is no interaction). Based on this, it can be inferred that the hydrate formed in Experiments 5-7 is grain-coated, and the hydrate structure has the same effect on the permeability, so the permeability of the sediment will be determined by the hydrate saturation. In Experiment 5-7, the hydrate saturation gradually increases, so the permeability of the sediment is gradually decreasing [39]. This hinders the occurrence of the replacement reaction, and finally leads to a decrease in the reduction range of the total gas content in the reactor. Although the hydrate saturation in Experiment 8 was the highest, the gas relative permeability in the sediment was improved due to the formation of pore-filling hydrate, so the final gas reduction increased. 

Figure 10 shows the variation in the gas component content in Experiments 5-8 with time. 

(mainly nucleating in the pores between the sediments [35, 38], hydrate crystals are suspended in the pores and not in contact with the sediments, and there is no interaction). Based on this, it can be inferred that the hydrate formed in Experiments 5-7 is grain-coated, and the hydrate structure has the same effect on the permeability, so the permeability of the sediment will be determined by the hydrate saturation. In Experiment 5-7, the hydrate saturation gradually increases, so the permeability of the sediment is gradually decreasing [39]. This hinders the occurrence of the replacement reaction, and finally leads to a decrease in the reduction range of the total gas content in the reactor. Although the hydrate saturation in Experiment 8 was the highest, the gas relative permeability in the sediment was improved due to the formation of pore-filling hydrate, so the final gas reduction increased. 

Figure 10 shows the variation in the gas component content in Experiments 5-8 with time. Calculated from Equation (1), the accumulated amounts of CH4 replaced in Experiments 5-8 were 0. 007, 0. 015, 0. 024, and 0. 028 mol, respectively. It can be concluded that under the same conditions, with an increase in the initial hydrate saturation, more CH4 was replaced. This is because with higher hydrate saturation, there are more CH4 hydrates in the sediment, which results in a larger interface area between CO2 gas and the hydrate. This implies that there is a larger effective area for the reaction. The results demonstrate that in addition to the exchange of CO2 and CH4 molecules, the replacement process will also be accompanied by the process of CO2 combined with residual water to Calculated from Equation ( 1 ), the accumulated amounts of CH 4 replaced in Experiments 5-8 were 0. 007, 0. 015, 0. 024, and 0. 028 mol, respectively. It can be concluded that under the same conditions, with an increase in the initial hydrate saturation, more CH 4 was replaced. This is because with higher hydrate saturation, there are more CH 4 hydrates in the sediment, which results in a larger interface area between CO 2 gas and the hydrate. This implies that there is a larger effective area for the reaction. The results demonstrate that in addition to the exchange of CO 2 and CH 4 molecules, the replacement process will also be accompanied by the process of CO 2 combined with residual water to form new CO 2 hydrates. Compared to the exchange of CH 4 molecules, CO 2 molecules form CO 2 hydrates more easily at the gas-liquid interface.
section_name: Variation in CO2 and CH4 in the Gas Phase

section_content: The calculated recovery efficiency and replacement amount of CH 4 for Experiments 1-4 is shown in Figure 11. 

The replacement amount and recovery efficiency of CH 4 increases with a decrease in particle size. However, the median particle sizes of 166, 279, and 377 µm had little effect on the recovery efficiency, whereas the recovery efficiency of the median particle size of 101 µm was much greater than that of other particle sizes. A possible reason for this is that according to the microscopic distribution of hydrate formation, CH 4 forms pore-filling hydrates in sediments with larger particle sizes, whereas it forms vein-like hydrates in sediments with smaller particle sizes. The surface area of vein-like hydrates was larger than that of pore-filling hydrates, and the interface area between CO 2 gas and CH 4 hydrates was larger during the replacement process. In addition, the porosity and permeability of the sediment increased as the particle size decreased, and the diffusion of CO 2 gas in the sediment improved accordingly. Therefore, the replacement and recovery efficiencies of CH 4 were higher. form CO2 hydrates more easily at the gas-liquid interface.
section_name: CH 4 Recovery Efficiency

section_content: The calculated recovery efficiency and replacement amount of CH4 for Experiments 1-4 is shown in Figure 11. The replacement amount and recovery efficiency of CH4 increases with a decrease in particle size. However, the median particle sizes of 166, 279, and 377 μm had little effect on the recovery efficiency, whereas the recovery efficiency of the median particle size of 101 μm was much greater than that of other particle sizes. A possible reason for this is that according to the microscopic distribution of hydrate formation, CH4 forms pore-filling hydrates in sediments with larger particle sizes, whereas it forms vein-like hydrates in sediments with smaller particle sizes. The surface area of vein-like hydrates was larger than that of pore-filling hydrates, and the interface area between CO2 gas and CH4 hydrates was larger during the replacement process. In addition, the porosity and permeability of the sediment increased as the particle size decreased, and the diffusion of CO2 gas in the sediment improved accordingly. Therefore, the replacement and recovery efficiencies of CH4 were higher. 

The calculated recovery efficiency of CH4 hydrate for Experiments 5-8 is shown in Figure 12. The calculated recovery efficiency of CH 4 hydrate for Experiments 5-8 is shown in Figure 12. The replacement of CH4 increases with an increase in the initial hydrate saturation, whereas the recovery efficiency decreases with an increase in the initial water saturation. It is possible that the recovery efficiency is mainly controlled by CO2 permeation into the interface between CH4 and CO2 hydrate. The CH4 hydrate attached to the surface of the sediment increases with an increase in the initial hydrate saturation; therefore, the higher the hydrate saturation, the more CH4 is replaced. However, as the reaction progresses, the hydrate film layer becomes thicker, and the resistance of CO2 molecules to penetrate the sediment and participate in the replacement reaction increases, thereby reducing the recovery efficiency. When the hydrate saturation was greater than 35%, the amount of replaced CH4 was more significantly affected by hydrate saturation. When the hydrate saturation was less than 35%, the amount of replaced CH4 was less affected by hydrate saturation. This is because when the hydrate saturation is greater than 35%, CH4 hydrate exists in the form of pore-filling, whereas when the hydrate saturation is less than 35%, it exists in the form of grain-coating. The permeability of pore-filling is known to be greater than that of grain-coating [35]. Therefore, it is easier for CO2 to permeate the sediment and react to replace more CH4.
section_name: CH4 Recovery Efficiency

section_content: Based on the consistency of the amount of residual water for all the experiments, the effects of sediment particle size and initial hydrate saturation on CO2-CH4 displacement The replacement of CH 4 increases with an increase in the initial hydrate saturation, whereas the recovery efficiency decreases with an increase in the initial water saturation. It is possible that the recovery efficiency is mainly controlled by CO 2 permeation into the interface between CH 4 and CO 2 hydrate. The CH 4 hydrate attached to the surface of the sediment increases with an increase in the initial hydrate saturation; therefore, the higher the hydrate saturation, the more CH 4 is replaced. However, as the reaction progresses, the hydrate film layer becomes thicker, and the resistance of CO 2 molecules to penetrate the sediment and participate in the replacement reaction increases, thereby reducing the recovery efficiency. When the hydrate saturation was greater than 35%, the amount of replaced CH 4 was more significantly affected by hydrate saturation. When the hydrate saturation was less than 35%, the amount of replaced CH 4 was less affected by hydrate saturation. This is because when the hydrate saturation is greater than 35%, CH 4 hydrate exists in the form of pore-filling, whereas when the hydrate saturation is less than 35%, it exists in the form of grain-coating. The permeability of pore-filling is known to be greater than that of grain-coating [35]. Therefore, it is easier for CO 2 to permeate the sediment and react to replace more CH 4.
section_name: Conclusions

section_content: Based on the consistency of the amount of residual water for all the experiments, the effects of sediment particle size and initial hydrate saturation on CO 2 -CH 4 displacement were discussed using a self-built displacement experimental device. Eight experiments were conducted to examine the effect of displacement under different conditions. The experimental results show that during the replacement process, two stages of rapid reaction and slow reaction occurred, which were manifested by the speed of the pressure change in the reactor. The CO 2 sequestration ratio decreased with the increase in sediment particle size and increased with the increase in initial hydrate saturation. During the replacement process, two reactions occurred: CH 4 was replaced by CO 2 and CO 2 hydrate was formed, and the replacement amount and recovery efficiency of CH 4 increased with a decrease in sediment particle size. When the sediment particle size was less than 166 µm, the CH 4 recovery efficiency was significantly affected by the particle size. When the particle size was greater than 166 µm, the CH 4 recovery efficiency was not significantly different. The replacement amount of CH 4 increased with the increase in initial hydrate saturation, while the recovery efficiency decreased. When the initial water saturation in the sediment was greater than 35%, the recovery of CH 4 was significantly affected by the initial hydrate saturation. When the initial water saturation in the sediment was less than 35%, the CH 4 recovery was less strongly affected by the initial hydrate saturation. 

In conclusion, in CO 2 -CH 4 replacement production, selecting a storage area with high hydrate saturation, small particle size, and water saturation greater than 35%, will be beneficial to the CO 2 storage efficiency and the amount of CH 4 replaced.
section_name: Conclusions

section_name: Acknowledgments:

section_content: The authors thank Shi-jun Liu for his help in the operation of GC and Jia-mei Mo for her help in the operation of SEM.
section_name: Acknowledgments:

section_content: Funding: This research was funded by the Guangdong Provincial Science and Technology Plan Project ( 2021A0505030053 ), Guangdong Provincial Special Fund Project for Promoting Economic Development ( GDME-2022D043 ) and the Special project for marine economy development of Guangdong (six marine industries) ( GDNRC[2022]46 ).

section_name: Data Availability Statement:

section_content: The authors confirm that the data supporting the findings of this study are available within the article.
section_name: Data Availability Statement:

section_name: Conflicts of Interest:

section_content: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
section_name: Conflicts of Interest: