Study on the Influence of Thermal Conductivity Characteristics of Porous Media on the Heterogeneous Distribution of Methane Hydrate

Abstract

The homogeneity of methane hydrates in marine sediments plays a significant role in determining the efficiency of gas production during exploitation processes. Revealing their distribution mechanisms is crucial for optimizing the development of gas hydrates. This work systematically investigates the evolution patterns of effective thermal conductivity (ETC) during the formation and dissociation of methane hydrate in marine sediments, focusing on their major mineral components, such as quartz sand, illite, and montmorillonite. The results reveal the influence of thermal conductivity (TC) characteristics in porous media on hydrate phase transition behavior and spatial distribution. Key findings demonstrate that the TC characteristics of porous media are one of the dominant factors controlling hydrate formation rates. High-conductivity porous media significantly accelerate hydrate formation through efficient heat transfer. The swelling characteristics of montmorillonite and its coupling effects with salt ions impair heat transfer pathways, thereby inhibiting hydrate formation. Further analysis reveals that the spatial heterogeneity in reservoir TC is the primary intrinsic mechanism responsible for the macroscopic heterogeneous distribution of hydrates. Additionally, the hydrate dissociation process disrupts solid-state thermal bridging and generates gaseous thermal barriers, causing irreversible attenuation of reservoir TC. This phenomenon exacerbates the non-uniformity of the front during dissociation and increases the risk of secondary formation during exploitation. From a novel perspective of reservoir TC heterogeneity, this study establishes mechanistic links between the thermophysical properties of porous media and the spatial distribution patterns of hydrates. This provides significant theoretical guidance for resource exploration and the safe, efficient exploitation of marine gas hydrate reservoirs.

1. Introduction

Natural gas hydrates are potential clean energy resources occurring in submarine continental slopes and permafrost sediments, which are considered one of the important alternatives to conventional fossil fuels. By conservative estimates, their carbon reserves exceed twice the total fossil fuel carbon reserves known globally [1]. With characteristics of widespread distribution, large reserves, and high energy density [2,3], this resource has become a focal point of global energy research, and the related technologies have gradually advanced from resource exploration assessment toward trial exploitation phases.

Natural gas hydrates commonly occur in reservoir systems in forms such as massive, vein/layered, nodular/concretionary, and pore-filling configurations [4,5]. Their spatial distribution shows significant heterogeneity, primarily controlled by sedimentary environments, gas migration pathways, and temperature–pressure conditions. During the exploitation processes, this heterogeneity directly influences fluid (gas–water) migration behavior within reservoirs: regions with high permeability and high hydrate saturation tend to form preferential seepage channels, which promote the preferential dissociation of hydrates; at the same time, the low-saturation zones demonstrate limited dissociation efficiency. Additionally, isolated high-saturation hydrate blocks, such as concretions and veins, are usually surrounded by poor permeability, and so, the generated gas cannot effectively converge toward production wells after dissociation. Furthermore, uneven dissociation zones caused by heterogeneous hydrate distribution can induce differential subsidence in formations and even trigger geological environmental risks, like submarine landslides. Therefore, the heterogeneous distribution of natural gas hydrates in reservoirs not only restricts resource exploitation efficiency but also influences geological environmental safety. Fundamentally, this characteristic results from the complex temporal–spatial coupling of gas migration, fluid activity, sedimentation, and geothermal conditions within reservoirs, jointly determined by formation processes and occurrence environments. Through theoretical modeling and experimental validation, Kou et al. [6] revealed the critical role of water migration during the formation of hydrate heterogeneity and established an improved kinetic model for hydrate formation. This model successfully predicted that the average formation rate of massive hydrate-bearing sediments could reach 16 times that of homogeneous sediments. The X-ray computed tomography (X-CT) results confirmed that spontaneous water flow against gravity leads to multiple hydrate morphologies and heterogeneous distributions within pores. Using a pilot-scale hydrate simulator, Wan et al. [7] compared production performance and heterogeneity evolution during the depressurization-induced dissociation in horizontal wells between gas-saturated and water-saturated hydrate reservoirs. They first categorized heterogeneity into planar and interlayer types based on spatial characteristics and proposed a dimensionless factor to quantify local heterogeneity intensity. Thus, systematically revealing hydrate heterogeneity characteristics and controlling factors becomes crucial for deepening the understanding of reservoir formation and optimizing exploitation technologies.

Methane hydrate is the primary constituent of natural gas hydrates, and its formation and dissociation are phase-transition processes that involve significant thermal effects. The formation process is an exothermic transformation from a disordered gas–water system to an ordered crystalline structure, whereas the dissociation is an endothermic process transitioning from a low-free-energy state to a high-free-energy state, which causes local temperature reduction. During exploitation, this endothermic effect can suppress the dissociation rate and even induce secondary hydrate formation, thereby increasing uncertainty in production operations [8]. Consequently, the heat transfer characteristics of reservoirs play a critical role in controlling hydrate formation rates and morphological distribution patterns. Variations in heat transfer behavior may lead to significant differences in occurrence modes and aggregation mechanisms of hydrates, ultimately affecting their spatial heterogeneity. The thermal conductivity (TC) characteristics of porous media within reservoir sediments are a key physical parameter to quantify the thermal conductive properties of reservoirs. Li et al. [9] used the transient plane source method to measure the effective thermal conductivity (ETC) of methane hydrate-bearing quartz sand formed in different initial water saturation conditions. Their study revealed that in partially saturated gas-rich environments, hydrate-cemented particles could significantly enhance ETC, exhibiting weak negative temperature dependence similar to crystalline materials. However, under high water saturation conditions, ETC did not show a notable increase with hydrate saturation levels. Wang et al. [10] further demonstrated that ETC positively correlates with hydrate saturation, water content, and medium TC, and during dissociation, the ETC was observed to have a slight increase due to enhanced heat transfer from gas production. Wei et al. [11] utilized point heat source instrumentation to investigate ETC variations in montmorillonite during ice melting, hydrate formation, and dissociation processes, and they found that hydrate formation increased ETC while ice melting or hydrate dissociation caused decreases. Farahani et al. [12] combined experimental measurements with pore-scale simulations to examine ETC in partially saturated silica sand under non-frozen/frozen conditions. The results revealed that ETC was controlled by particle–particle conduction and particle–fluid-particle conduction mechanisms, and influenced by pore/overburden pressure, temperature, and water/ice saturation levels. Li et al. [13] compared the ETC values of different porous media types (quartz sand, silicon carbide, and clay) with hydrates. They observed weak negative temperature correlations in quartz sand and silicon carbide, but weak positive correlations in clay systems, which indicated that the intrinsic TC of porous media governs ETC characteristics. Collectively, these studies confirm that sediment thermal properties are crucial factors that influence hydrate formation dynamics and distribution heterogeneity in reservoirs. However, specific underlying mechanisms are still unclear, which limit the accurate prediction of reservoir occurrence patterns.

In this work, considering the complex characteristics of multi-phase and multi-component natural gas hydrate reservoirs, we employed the steady-state method to systematically measure the dynamic changes in ETC during the formation and decomposition of methane hydrates within different porous media. The marine sediments from natural hydrate reservoirs and their primary components (including quartz sand, illite, and montmorillonite) were used in the study. The effects of heat transfer characteristics on the phase change behavior of hydrates in these systems are revealed. Furthermore, the TC properties of porous media governing the heterogeneous distribution of hydrates are discussed.

2. Experimental Methods

2.1. Experimental Apparatus

The experimental apparatus used in this study is shown in Figure 1, with its core component being a high-pressure reactor equipped with three temperature sensors and two pressure probes. The high-pressure reactor, enclosed by the orange dashed box in Figure 1a and detailed in Figure 1b, was fabricated from 316 stainless steel. It was designed to withstand pressures up to 25 MPa, making it suitable for methane hydrate formation experiments. The inner diameter of the reactor is 50 mm, with an internal height of 200 mm, achieving an aspect ratio greater than 4 (ensuring one-dimensional radial heat transfer conditions during experiments), resulting in an effective volume of 392.7 cm³. A heating wire with a diameter of 1.2 mm and a length of 200 mm was axially positioned inside the reactor. The temperature was controlled using a water bath (XT5201, Xutemp Temptech Co., Ltd., Hangzhou, China, with a measurement accuracy of ±0.05 K, 253.15–363.15 K). The small figure at the right side of Figure 1b is the cross-sectional distribution map of the temperature sensor in the high-pressure reactor. Temperature monitoring employed three Pt1000 thermocouples (T1(r1), T2(r2), T3(r3)) arranged radially on the same horizontal plane within the reactor, achieving a temperature measurement precision of ±0.02 K (with a coverage factor k = 2) for the real-time capture of temperature distribution. Two high-precision pressure sensors (Trafag, with a measurement accuracy of 0.1% FS, 0–25 MPa, k = 2) were connected to the inlet and outlet ports of the reactor to monitor system pressure variations. After stabilization, the pressure values at the reactor’s inlet and outlet were typically identical. However, discrepancies could arise if pores within the porous medium become blocked, thereby restricting gas mass transfer. The average of these two measurements is therefore used to represent the overall pressure condition within the reactor. During experiments, parameters including temperature, pressure, voltage, and current were automatically recorded at 3 s intervals. This apparatus has been applied in previous studies, where its measurement accuracy and reliability were validated. Further technical details can be found in the published papers [14,15].

2.2. Materials

Natural marine sediments collected from the cold seep area of the South China Sea seamount (600–800 m water depth), where natural gas hydrates have been confirmed to exist [16], were selected as experimental samples in this study. Previous investigations demonstrated that these natural marine sediments primarily consist of porous media, including quartz sand, clay minerals (dominated by montmorillonite and illite), and calcite [17,18]. To systematically investigate the influence of TC characteristics of different porous media on hydrate formation and dissociation processes, quartz sand, illite, and montmorillonite were chosen as representative reservoir porous media for the experiments. Particle size distributions of the porous media were measured using a Mastersizer 2000E laser particle size analyzer (Malvern Instruments, Malvern, UK), while their densities were determined through a real density meter VPY-30 (Quantachrome Instruments, Boynton Beach, FL, US). The sources and fundamental physical properties of the porous media materials used in the experiments are summarized in

Table 1. Sources and basic physical properties of experimental samples.

Materials	Source	Particle Size (μm)	Density (g/cm3)
Montmorillonite	Nanocor Inc., Arlington Heights, IL, USA	20.86	2.07
Quartz	Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China	401.1	2.49
Illite	Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China	15.13	2.60
Natural marine sediments	Natural gas hydrate reservoirs in the South China Sea (Depth: 600–800 m)	59.53	2.45
Methane	Guangzhou Yuejia Gas Co., Ltd., Guangzhou, China	--	--
NaCl	Shanghai Aladdin Industrial Co., Ltd., Shanghai, China	--	--

. The TC values for all experimental components are listed in

Table 2. The TC values for all experimental components.

Component	TC/W·m−1·K−1
	Our Work	Previous Research
Quartz	1.06	0.769 [19,20]
Montmorillonite	0.88	0.8–1.1 [21]
Illite	0.50	0.51 [22]
Marine sediments	0.59	1.4–1.77 [23]
NaCl solution	0.46	0.59 [24]
Water	--	0.6218 [25]
Methane hydrate	--	0.49 [26]

. Deionized water with a resistivity of 18.25 mΩ·cm⁻¹ was supplied by Nanjing Ultrapure Water Technology Co., Ltd. (Nanjing, China); sodium chloride solution (purity 99.8% mass) was provided by Shanghai Aladdin Industrial Co., Ltd. (Shanghai, China); and methane gas (purity > 99.95% vol) for hydrate formation was sourced from Guangzhou Yuejia Gas Co., Ltd. (Guangzhou, China).

2.3. Experimental Procedures

(1)

Preparation of dry porous medium samples: Quartz sand, illite, and montmorillonite were dried in an oven at 110 °C for 12 h, then sealed and stored for later use.

(2)

Preparation of water/salt solution-containing porous medium samples: The water content of each porous medium sample was controlled to approximately 40 wt%. Dried porous medium samples were weighed using an electronic balance (BSA224S-CW, Sartorius, Goettingen, Germany), and the required mass of deionized water was calculated. To investigate the effect of salt on ETC, a 0.5 mol/L NaCl solution was prepared. Deionized water or salt solution was slowly added to the dried porous medium samples, followed by thorough mixing with a stirring rod to ensure uniform distribution of liquid components. The samples were sealed and left for 24 h to allow for complete absorption and swelling of the porous medium.

(3)

Sample loading: The prepared samples were loaded into a high-pressure reactor and compacted to maintain a consistent sample volume in experiments. After sealing, the reactor was immersed in a water bath with an adjustable temperature. The porosity (φ) reported in our study is the initial, effective porosity inside the reactor, prior to gas pressurization, which is expressed as:

$$\phi ={\displaystyle \frac{V_{pore}}{V}}={\displaystyle \frac{V-V_w-V_c}{V}}$$

(1)

where V_pore and V are the pore volume of the reactor and the volume of the reactor, respectively; and V_w and V_c are the volumes of water and porous media, respectively. The porous medium samples containing water/salt solutions were compacted during loading to maintain a constant sample volume, (V_w + V_c), across the different systems. This ensured that the initial φ of the porous medium systems in the experiments was constant.

(4)

Initially gas injection: Before the reaction began, the temperature of the water bath was set to 288.15 K. The system was flushed three times with pure methane gas to remove air and checked for leaks. Then, methane gas was initially injected into the high-pressure reactor until the pressure reached 10.5  MPa and was maintained at 288.15  K for 2 h under isothermal conditions. This temperature and pressure condition prevented hydrate formation and ensured full penetration of the gas into the pores of the porous media.

(5)

Hydrate formation: The water bath temperature was reduced to 277.15 K, inducing hydrate formation within the reactor and causing a pressure drop. Hydrate formation was considered complete when the inlet and outlet pressures stabilized for 3 h (fluctuation range < 0.01 MPa). In the actual sedimentary environment of the reservoir, the formation of methane hydrates may require a longer period of time than what we obtained in the experiments. The initial stage of hydrate formation was controlled by nucleation and rapid growth, with a rapid gas consumption rate and significant pressure drop. The later stage gradually changed to a slow mass transfer and diffusion control stage, which is a relatively long time on the geological time scale. The experimental simulation mainly demonstrated that most of the hydrates in the system were formed in the initial stage of hydrate formation, and the remaining unreacted part had a negligible contribution within the limited experimental time. Therefore, we used this criterion to determine the complete formation of hydrates.

(6)

Hydrate dissociation: A stepwise depressurization method was employed for hydrate dissociation. In each cycle, the exhaust valve was opened to reduce pressure to 3.5 MPa, allowing for hydrate dissociation and gas release until the pressure recovered to the phase equilibrium pressure of 3.87 MPa at this temperature [27]. This process was repeated until no further pressure recovery occurred, indicating complete hydrate dissociation.

(7)

ETC measurement: Based on one-dimensional steady-state heat transfer principles, a direct current was applied to an embedded heating wire under isothermal conditions at 277.15 K. Voltage and current data were recorded using a data acquisition system (sampling interval: 3 s). The ETC was calculated according to Fourier’s law [14].

Our previous work [14] performed multiple repeated calibration and validation tests using pure ice and agar gel as standard materials to ensure the reliability of our experimental setup and measurement methods. The relative errors in the measured ETCs of pure ice and agar gel are <5%. Due to the long duration of each experiment, we did not perform a detailed analysis of experimental uncertainties in this work, which will be explored in future work.

2.4. Calculation of Hydrate Saturation

The hydrate saturation (S_h) is determined by calculating the volumetric fraction of hydrates within the pore space, expressed as:

$$S_h={\displaystyle \frac{V_h}{V_{pore}}}={\displaystyle \frac{(n_{g1}-n_{m,g}-n_{m,w})M_h}{{\rho }_h{V}_{pore}}}$$

(2)

where V_pore represents the total pore volume of the porous medium system, V_h denotes the generated hydrate volume, and S_h indicates the hydrate saturation in the pores. The variables n_g1, n_m,g, and n_m,w correspond to the initial moles of gas, residual moles of gas after hydrate formation, and moles of dissolved gas in the aqueous phase, respectively. The hydration number (N_h) is assumed to be at an ideal stoichiometric value of 5.75. M_h signifies the molar mass of methane hydrate (119.5 g/mol [1,28]), which was calculated based on the N_h. The density value ρ_h is the typical density value of type I hydrates as widely reported in the literature, which is 0.91 g/cm³ [29,30]. It was assumed that no methane was dissolved in the aqueous phase. During calculations, the moles of gas in the system at each time point were determined using the Peng–Robinson (P-R) equation of state, based on experimentally measured temperature and pressure data. Based on the measurement uncertainty of this experiment (temperature: ±0.02 K; pressure: ±0.035% reading) and the reported range of key parameters (M_h, ρ_h), an error propagation analysis was conducted. The calculated values of the S_h obtained in the experiment met the requirements for data accuracy. Detailed derivation procedures can be found in Refs. [14,31].

3. The Formation/Dissociation Processes of Methane Hydrate in Porous Media and Their ETC Measurement

3.1. Hydrate Formation Process

To investigate variations in ETC within different porous media systems and their effects on hydrate formation/dissociation processes, this study employed marine sediments from natural hydrate reservoirs and their major mineral components (quartz sand, montmorillonite, and illite) as porous media for constant-volume experiments on methane hydrate formation. System temperature, pressure, and ETC were monitored during hydrate formation. Previous studies have demonstrated that hydrate formation is co-controlled by gas diffusion and nucleation, with pore throats in porous media prone to hydrate blockage that inhibits gas diffusion, thereby reducing hydrate formation rates and saturation degrees [9]. Therefore, enhancing gas diffusion and gas–water contact are critical for improving hydrate formation efficiency. The experiment adopted a synchronous cooling method: initially, methane was injected into the high-pressure reactor until the pressure reached 10.5 MPa, which was maintained at 288.15 K for 2 h under isothermal conditions. This temperature and pressure condition prevents hydrate formation, ensuring complete gas penetration into porous media pores. This method established a relatively uniform initial gas distribution and minimized the initial permeability contrast to reduce the initial influence of mass transfer limitations on the hydrate formation. Subsequently, the water bath temperature was reduced to 277.15 K, achieving the simultaneous cooling of gas, water, and porous media within the high-pressure reactor. Under these thermodynamic conditions, which fall below the phase equilibrium conditions for methane hydrates [27], hydrate formation commenced. Taking the quartz sand system as an example (Figure 2), the evolution patterns of system temperature, pressure, and ETC during methane hydrate formation in porous media were analyzed. As shown in Figure 2, during the initial experimental stage, rapid temperature reduction caused a corresponding pressure drop from 10.5 MPa to 9.6 MPa. After temperature stabilization, continuous pressure decline occurred due to the methane consumption from hydrate formation. ETC measurements were conducted at 3 h intervals during the early hydrate formation stage and at 6 h intervals during the later stage. Transient temperature increases (fluctuation range: 0.5 K) occurred during the ETC measurements due to brief heating wire activation, accompanied by minor pressure fluctuations (±0.05 MPa). These slight disturbances did not affect hydrate formation. The experiment lasted approximately 9000 min until pressure stabilized at 3.8 MPa (±0.01 MPa fluctuation), indicating complete methane hydrate formation.

The initial system contained quartz sand with 40 wt% moisture content, yielding an ETC value of 1.16 W·m⁻¹·K⁻¹, consistent with values from the literature [14,18], confirming equipment reliability. During hydrate formation, ETC gradually increased primarily due to altered heat transfer pathways within the porous medium. Figure 3 presents a three-dimensional schematic illustrating heat transfer mechanisms in porous media systems. Initially, five heat transfer pathways existed: Path 1 (particle–particle), Path 2 (water–water), Path 3 (particle–water-particle), Path 4 (particle–gas-particle), and Path 5 (particle–gas-particle–water-particle). Given methane’s relatively low intrinsic TC compared to other components, particle–particle and particle–water-particle contacts dominated heat transfer under initial low water saturation (40 wt%). Post-hydrate formation, two new pathways emerged: particle–hydrate-particle (Path 6) and particle–hydrate-liquid-particle (Path 7). Compared to the initial particle–water-particle pathway (Path 3), hydrate cementation enhanced solid contact areas, improving ETC. However, extensive hydrate formation restricted direct particle–particle contact through cementation, reducing this pathway’s contribution. Furthermore, since the TC of methane hydrate is also significantly lower than that of ice, which is only about a quarter of the latter, when hydrate replaces some of the high-TC pore water or ice phase, the methane hydrate itself would contribute to the reduction in ETC. Consequently, as shown in Figure 2, when the system pressure stabilized, indicating complete hydrate formation, the ETC exhibited a decreasing trend. The contact patterns of different components in porous media systems, the changes in microstructure, and the material changes in each phase during the reaction process all jointly affect the overall TC result of the system. Future research can further clarify the relative contributions of each factor through more precise in situ characterization.

In this study, a water formation experiment using free methane gas phase was conducted in a high-pressure reactor, providing controllable conditions for investigating the heat transfer behavior during the formation of hydrates. However, in many natural submarine environments (especially diffusion-dominated systems), methane hydrates are often formed from methane dissolved in pore water rather than from the free-gas phase. Recent in situ observations have shown that in the early stage of hydrate formation, it mainly relies on dissolved methane in the liquid phase, and later turns to utilize the free methane gas. This difference in gas sources may lead to different ETC: since the TC of free gas is usually much lower than that of water or hydrates, its presence significantly reduces the overall ETC of porous media. Laboratory measurements have also confirmed that when pores are filled with free gas, the ETC of hydrate-bearing sediments is significantly lower than when pores are filled with water or hydrates. Therefore, the heat transfer parameters obtained in this experiment need to be cautious when directly extrapolated to natural reservoirs dominated by dissolved gas. Future research can further verify the conclusions of this work through experiments using a dissolved methane supply to better understand the thermodynamic and kinetic behaviors of hydrate formation under different gas source conditions.

3.2. Hydrate Dissociation Process

After hydrate formation, the dissociation process was initiated at 11,100 min of experimental duration using a stepwise depressurization method. The variations in temperature, pressure, and ETC during this process are presented in Figure 4. In the initial stage of dissociation, free gas within the system was preferentially released, causing a pressure drop. When the pressure fell below the phase equilibrium condition, hydrate dissociation commenced. Due to the endothermic nature of the dissociation reaction, the system temperature temporarily decreased but subsequently recovered to 277.15 K under the temperature-controlled water bath. Through multiple cycles of stepwise depressurization, the system eventually completed gas release and reached atmospheric pressure. To prevent temperature increases from ETC measurements from affecting the dissociation process, ETC testing was conducted only after complete dissociation, as shown in Figure 4. The results indicate that the ETC value after hydrate dissociation was lower than that during hydrate formation. This reduction was attributed to the redistribution of gas, water, and solid phases caused by hydrate dissociation, which altered interparticle contact modes and consequently modified the original heat conduction pathways.

4. Effect of ETC of Porous Media on Hydrate Formation Rate

Figure 5 presents the pressure changes during methane hydrate formation in different porous media. The results indicate significant differences in pressure drop rates and stabilization times among the systems, reflecting variations in hydrate formation rates depending on porous medium type. Generally, the hydrate formation process can be divided into three stages: (1) an initial rapid formation stage with a sharp pressure drop; (2) a slow formation stage with continuous pressure reduction; and (3) a completion stage where pressure stabilizes—a pattern consistent with previous studies [32]. In montmorillonite and marine sediments, hydrate formation proceeded slowly, continuing even after 10,000 min (≈7 days) with minimal pressure decline, requiring longer formation time and yielding limited product. Thus, the present study focused on pressure changes within 10,000 min. In contrast, the illite system reached pressure stability around 2000 min, prompting termination at 3000 min. A comparative analysis revealed that the rate of pressure drop during initial hydrate formation stages followed the order: illite > quartz sand > montmorillonite > marine sediment. The time required for complete hydrate formation was approximately 1200 min in the illite system and 9000 min in the quartz sand system, whereas in the montmorillonite and marine sediment systems, it exceeded 10,000 min. This indicates easier hydrate formation in illite.

Figure 6 illustrates the ETC changes during methane hydrate formation in different porous media. As the hydrate formed, the ETC values increased across all systems, indicating enhanced ETC due to hydrate cementation effects. Under an initial water content of 40 wt%, the initial ETC ranking (0 min) was as follows: illite > quartz sand > montmorillonite > marine sediment, consistent with prior research [33]. Comparing Figure 5 and Figure 6 reveals a positive correlation between porous medium ETC and hydrate formation rate: systems with higher ETC efficiently transferred the temperature of the water bath from the reactor walls inward, accelerating hydrate formation. Therefore, the effect of the ETC of porous media on the kinetics of hydrate formation was primarily observed in the TC characteristics of the experimental system, which influenced the rate of temperature transfer during hydrate formation. Notably, the illite system exhibited a distinct pressure plateau before rapid hydrate formation (Figure 5), corresponding to the nucleation induction period where surface water film formation or adsorption competition delayed nucleation [34,35]. Due to illite’s superior ETC, the system rapidly met nucleation conditions, enabling massive hydrate growth post-induction. In contrast, systems with lower ETC experienced simultaneous heat transfer and nucleation growth. Therefore, no obvious induction period was observed. Additionally, Figure 6 shows ETC reduction during rapid hydrate formation in montmorillonite. This phenomenon primarily stems from montmorillonite’s expansion characteristics: hydrate formation causes particle swelling, reducing pore throats and increasing tortuosity. These structural changes impeded gas diffusion and hydrate growth while trapping low-conductivity methane gas (ETC ≈ 0.023 W·m⁻¹·K⁻¹ [36]) in micropores, creating localized “thermal barriers.” Although some hydrate acted as a cementing filler, it could not fully offset porosity alterations caused by expansion, preventing efficient heat transfer pathways (e.g., “particle–hydrate–particle” bridging shown in Path 6). Consequently, macroscopic ETC decreases during hydrate formation. Similarly, as a major component of natural marine sediments, the presence of montmorillonite caused analogous ETC reduction trends in marine sediment systems during hydrate formation (Figure 6). Certainly, the permeability of porous media was a key factor that influenced hydrate formation. The processes of heat and mass transfer occurred simultaneously and mutually affected each other during the hydrate formation process. This presented a significant challenge in distinctly separating the impact of the two factors on hydrate formation. I anticipate engaging in comprehensive discussions on this topic in future research endeavors.

5. Effect of ETC of Porous Media on Hydrate Saturation During Formation

Figure 7 illustrates the variation in methane hydrate saturation (S_h) during formation processes within different porous media systems. Generally, S_h increases progressively with hydrate formation in all systems and stabilizes after pressure stabilization, corresponding to the pressure changes shown in Figure 5. As observed in Figure 7, the illite system exhibits stepwise growth: after an initial plateau phase (corresponding to the nucleation induction period), S_h rises rapidly, followed by another plateau phase, indicating distinct stage-wise characteristics in hydrate formation. The montmorillonite system transitioned from rapid initial formation to a slow growth phase, which correlated with its swelling properties. Following hydrate formation, montmorillonite particles swelled, reducing pore space and impeding gas diffusion, thereby further delaying hydrate formation. Natural marine sediments demonstrated similar trends, transitioning from rapid initial formation to slower stages, influenced by their expansive properties and complex composition. The influence of mass transfer within porous media on hydrate formation was predominantly observed during the completion stage, whereas the influence of heat transfer within porous media primarily manifested during the initial stage.

In this work, a total water content of 40 wt% was used to ensure that all systems were initialized with the same mass fraction of water. However, this value did not represent the available free-water content for hydrate formation in the porous media used in this work, especially for montmorillonite. Montmorillonite can retain a significant amount of water as interlayer hydration water and strongly adsorbed water, which is thermodynamically unavailable for clathrate hydrate formation. Therefore, the hydrate saturation shown in Figure 7 is not related to the total water content but to the effective free-water content. The final hydrate saturation levels were ranked as follows: quartz sand > montmorillonite ≈ illite > marine sediments. Quartz sand achieved the highest hydrate saturation due to its larger particle size (as shown in Table 1) and more favorable pore structure for gas storage and migration. Although the amount of free water forming hydrates in montmorillonite was less than that observed in the illite system, they had similar particle sizes and exhibited comparable final saturation levels. Although marine sediments had larger particle sizes, their complex composition and salt content inhibited hydrate formation. A comprehensive analysis of Figure 5, Figure 6 and Figure 7 leads to the conclusion that the ETC of porous media primarily affected the rate of hydrate formation, while exerting minimal influence on the final hydrate quantity.

6. Effect of Salt Ions on the ETC and Hydrate Formation Within Natural Marine Sediment

Marine natural gas hydrate reservoirs exist in saline porous water environments, where salt ions exert significant influences on their formation and decomposition [37]. Previous studies have confirmed that the presence of salt reduces the ETC of porous media systems under conditions of low water saturation [38]. Meanwhile, our previous studies confirmed that the salt ions have a delayed effect on the change in ETC caused by the swelling of porous media. During the phase change process, the ETC of natural marine sediments was significantly influenced by the combined effects of salt ions and montmorillonite components [18]. To investigate the effect of salt ions on the ETC and hydrate formation within a natural marine sediment system, this study conducted methane hydrate formation experiments using a salt-containing solution introduced into a montmorillonite system. The main salt ions in seawater are Na⁺ and Cl⁻, with a salinity of approximately 3.5 wt% [39,40]. Therefore, a 0.5 mol/L NaCl solution was used as the salt-containing solution, intended to mirror a marine system, for the experiment [41,42,43]. The pressure and ETC variations during the experimental process are shown in Figure 8. In Figure 8a, the pressure drop value in the montmorillonite system containing the NaCl solution (approximately from 10 MPa to 9 MPa) was lower than that in the montmorillonite system (about from 10 MPa to 5.5 MPa). This indicates that the presence of salt ions significantly reduced the hydrate formation rate within the montmorillonite system. Furthermore, Figure 8a shows that the pressure decline trend in the montmorillonite system containing the NaCl solution was similar to that in natural marine sediment systems. This suggests that the hydrate formation in marine hydrate reservoirs is influenced by the combined effects of salt ions and montmorillonite. Meanwhile, as illustrated in Figure 8b, the addition of salt ions decreased the ETC of the montmorillonite system. This phenomenon is attributed to two primary mechanisms: first, the hydration effect of salt ions, which binds water molecules into hydration layers and thereby reduces liquid heat transfer efficiency; second, the inhibition of hydrate formation, which diminishes the “particle–hydrate–particle” heat transfer pathway created by hydrate cementation (i.e., Path 6 in Figure 3). Although the ETC of the montmorillonite system slightly decreased with the addition of salt ions, this effect was relatively small and could be considered negligible. Therefore, the influence of salt ions on the formation of hydrates in sediments was mainly due to the inhibition of hydrate cage formation caused by the hydration effect of salt ions [44] and the seepage changes caused by the inhibition of montmorillonite swelling by salt ions. The role of salt ions in hydrate formation within sediments is predominantly governed by mass transfer [45].

7. Effect of ETC of Porous Media on Hydrate Dissociation

To analyze the influence of porous media ETC on hydrate dissociation, this study compared percentage differences in ETC before and after hydrate formation/dissociation within different porous media systems, as shown in

Table 3. Measured ETC values from experiments on methane hydrate formation and dissociation in porous media.

Porous Media	λ0/W·(m·K)−1	λform/W·(m·K)−1	Δλf/%	λdis/W·(m·K)−1	Δλd/%
Quartz	1.16	1.45	+25.00	1.37	−5.52
Illite	2.20	2.90	+31.82	1.75	−39.66
Montmorillonite	1.22	1.11	−9.02	1.07	−3.60
Montmorillonite + NaCl	0.86	0.90	+4.65	0.85	−5.56
Marine sediments	1.08	1.00	−7.41	1.07	+7.00

. Δλ_f represents the percentage difference in ETC before and after hydrate formation, calculated as $\Delta {\lambda }_{\mathrm{f}}={\displaystyle \frac{{\lambda }_{\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}}-{\lambda }_0}{{\lambda }_0}}\times 100\%$, while Δλ_d indicates the percentage variation in ETC before and after hydrate dissociation, expressed as $\Delta {\lambda }_{\mathrm{d}}={\displaystyle \frac{{\lambda }_{\mathrm{d}\mathrm{i}\mathrm{s}}-{\lambda }_{\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}}}{{\lambda }_{\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}}}}\times 100\%$. Here, λ₀ denotes the initial system ETC before hydrate formation; λ_form is the ETC after hydrate formation (at 10,000 min experimental duration); and λ_dis represents the ETC after complete hydrate dissociation. The results indicate that the effect of hydrate formation on system ETC was dependent on the type of porous medium. In quartz sand and illite systems, hydrate formation enhanced the system ETC by over 20%, primarily attributed to cementation effects creating efficient heat transfer pathways, consistent with previous findings [14]. The montmorillonite system exhibited negative growth in ETC after hydrate formation due to swelling effects. Notably, salt ions could suppress montmorillonite swelling, reversing ETC trends to positive growth in saline–montmorillonite systems. For natural marine sediments containing both montmorillonite and salt ions, although hydrate formation slightly reduced ETC, this adverse effect was significantly weaker than in pure montmorillonite systems without salt content.

As shown in Table 3, all porous media systems exhibited decreasing ETC trends after hydrate dissociation. Hydrate dissociation induced phase changes that disrupted continuous solid-bridge heat transfer mechanisms (Path 6), while post-dissociation water and gas migration failed to restore original tight solid contacts (Path 1). Consequently, even after complete dissociation, the system could not recover the high-efficiency thermal conduction framework formed during hydrate cementation, reverting to less efficient “particle–water–particle” or “particle–gas–particle”-dominated heat transfer modes. Additionally, substantial methane gas, generated during dissociation, accumulated at pore throats or particle contact points, forming localized continuous or isolated bubbles [46]. Given the extremely low ETC of gases (approximately 0.023 W·m⁻¹·K⁻¹ [36]), these regions created significant thermal barriers impeding heat transfer along solid frameworks. Marine sediments showed increased ETC post-dissociation due to complex compositional factors, including salinity and multi-mineral coexistence. In conclusion, reservoir ETC undergoes dynamic attenuation during hydrate exploitation, directly affecting heat transfer efficiency and dissociation rates. This phenomenon may be particularly pronounced in low-permeability, clay-rich marine reservoirs where ETC reduction could intensify local self-preservation effects, thereby constraining exploitation efficiency.

8. The Influence Mechanism of ETC on the Heterogeneous Distribution of Hydrates in Marine Reservoirs

Marine natural gas hydrates primarily exhibit five occurrence modes in reservoirs: pore-filling, grain-cementing, grain-enveloping, patchy, and hybrid types [47]. Their macroscopic distributions display heterogeneous characteristics such as pore filling, massive/noddle, and naturally fractured [48]. Different occurrence modes directly alter pore structures, influence gas–liquid seepage behavior and mechanical strength of reservoirs, thereby constraining exploitation efficiency and formation stability. P-wave distributions and resistivity profiles from sections crossing the boreholes drilled for the 2017 production test in the Nankai Trough reveal a heterogeneous distribution of methane hydrate (Figure 9a [49]). Therefore, revealing the mechanism of hydrate heterogeneity is critical for achieving safe and efficient development.

The heterogeneous distribution of hydrates is controlled by factors including sedimentary environments, gas migration pathways, and temperature–pressure conditions. Previous studies proposed a water migration-driven “nucleation–growth–migration” sequence for heterogeneity formation, emphasizing the regulatory role of gas–water movement within pores [6,38]. The experimental results obtained from X-ray Computed Tomography (X-CT) images show the heterogeneous hydrate distribution and various hydrate morphologies in porous media (Figure 9b [50]). Building upon this foundation, our research focused on elucidating the effect mechanism of ETC heterogeneity in reservoirs on hydrate spatial distribution.

Marine sediments are inherently heterogeneous mixtures composed of minerals (quartz, illite, montmorillonite, etc.), pore fluids (saline seawater), and organic matter, with spatially intrinsic differences in ETC [46,51]. Experimental findings revealed that regions dominated by quartz and illite formed efficient heat transfer channels due to high ETC values, facilitating rapid hydrate formation. In contrast, areas rich in expansive clays, like montmorillonite, exhibited lower ETC; in this case, hydrate formation induced particle swelling, disrupted particle contact patterns, and created gaseous “thermal barriers,” resulting in inefficient heat transfer channels. Salinity ions (e.g., Na⁺, Cl⁻) in pore fluids exerted dual effects: they reduced the ETC of pore fluid through hydration and weakened the low-conductivity effect of montmorillonite by inhibiting its swelling. Consequently, high/low ETC channels formed by porous media, combined with salinity ion effects, collectively governed reservoir ETC heterogeneity.

To clarify the control mechanism of ETC heterogeneity on hydrate phase transition behavior, we illustrate the formation and decomposition processes of hydrates in sediment layers composed of quartz sand, illite, and montmorillonite (Figure 10). During hydrate formation, when the ambient temperature dropped below the hydrate phase equilibrium temperature (T_f), the illite layers with high ETC rapidly and uniformly transferred heat, enabling the system to quickly reach and maintain thermodynamic equilibrium conditions for hydrate formation. This triggered large-scale, continuous nucleation across multiple hydrate morphologies. Quartz sand layers exhibited slower heat transfer, causing delayed nucleation where hydrates mainly manifested as grain-cementing and grain-enveloping types. Montmorillonite, with poor ETC and high expandability, predominantly formed pore-filling hydrates. These distinctions created differentiated hydrate occurrence modes and growth sequences across porous media regions, ultimately leading to macroscopic hydrate heterogeneity within sediment layers.

During hydrate decomposition, the endothermic reaction generates a low-temperature (T_d) front that propagates from the decomposition interface into the sediment layer. Illite layers efficiently dissipated T_d due to superior ETC, maintaining system thermal stability. Quartz sand layers exhibited moderate heat transfer rates, where localized cooling may induce secondary hydrate formation. Montmorillonite layers, with minimal ETC, impeded T_d diffusion and readily promoted extensive secondary hydrate generation. This further intensified spatial heterogeneity in gas–water seepage during decomposition.

In summary, the heterogeneous distribution of marine reservoir porous media caused spatial variations in ETC, establishing a spatiotemporal competition pattern for hydrate nucleation. Coupled with gas–water migration control within pores, this ultimately generated hydrate heterogeneity. Such heterogeneity not only affected formation dynamics but also complicated decomposition processes, potentially triggering secondary formation, uneven seepage, and reservoir instability—thereby posing significant challenges to exploitation efficiency and operational safety.

9. Conclusions

This study systematically investigated the control mechanisms of TC characteristics of porous media on methane hydrate formation/decomposition behaviors and their spatial distribution patterns. Through the analysis of ETC evolution during hydrate formation in natural marine sediments and their dominant porous media components (quartz sand, illite, and montmorillonite), we found that spatial heterogeneity in reservoir ETC constitutes a key intrinsic mechanism governing the macroscopic heterogeneous distribution of gas hydrates. The following conclusions can be drawn:

(1)

TC properties of porous media serve as one of the principal controlling factors for hydrate formation rates. Porous media with high ETC facilitate efficient heat transfer, thereby significantly accelerating the kinetics of hydrate formation; whereas the swelling characteristics of montmorillonite and its coupling effects with salt ions alter pore structures and heat transfer pathways, representing critical mechanisms that inhibit formation and cause ETC reduction.

(2)

The spatial heterogeneity in the reservoir’s ETC represents the intrinsic cause of macroscopic heterogeneity in hydrate distribution. In porous media with high-conductivity, hydrate cementation further enhances heat transfer efficiency, forming preferential thermal conduits that promote hydrate enrichment. Conversely, in low-conductivity porous media, impeded heat transfer and slow growth kinetics lead to dispersed hydrate occurrences.

(3)

Hydrate decomposition irreversibly diminishes the reservoir’s ETC. This process disrupts the efficient solid-state thermal bridging established by hydrates, while the released gas trapped in the pores creates “thermal barriers”, resulting in system-wide ETC attenuation. These phenomena indicate a dynamic weakening of the reservoir’s heat transfer capacity during hydrate exploitation processes, potentially exacerbating front instability and localized secondary hydrate formation risks, thereby compromising sustained production efficiency.

From the novel perspective of “reservoir ETC heterogeneity”, this research establishes a mechanistic linkage between the thermal properties of porous media and the spatial distribution patterns of hydrates. It elucidates the thermally controlled origins of hydrate heterogeneity. These findings provide a critical scientific foundation for the precise exploration and safe, efficient development of marine natural gas hydrate resources.