Research Progress on Characteristics of Marine Natural Gas Hydrate Reservoirs

Abstract

As one of the most important future clean energy sources, natural gas hydrate (NGH) is attracting widespread attention due to the vast reserves available and high energy density. How to extract this source in a safe, efficient, and environmentally friendly manner has become the key to the commercial utilization of its resources. This paper reviews the recent advances in the study of the fundamental reservoir properties of offshore NGH, summarizing the methods and technologies for testing the sedimentary properties of reservoirs, analyzing the characteristics in reservoir mechanics, electrics, thermodynamics, and fluid dynamics, and discusses the influence of reservoir fundamental properties on NGH exploitation. The aim is to provide guidance and reference for research on the exploitation of NGH in different target exploitation areas offshore.

1. Introduction

Natural gas hydrate (NGH), commonly known as methane hydrates or combustible ice, is a type of ice-like crystalline solid composed of natural gas molecules trapped within a lattice of water molecules. Under conditions of low temperature and high pressure, water molecules form a cage-like structure through hydrogen bonding, enclosing guest molecules (such as methane, ethane, CO₂, and H₂) within their crystalline lattice to create stable hydrate formations. The most prevalent structures of NGH in natural environments are types SI and SII, primarily composed of methane hydrates. One cubic meter of NGH can yield approximately 186 cubic meters of natural gas and 0.8 cubic meters of water, indicating its high energy density. Moreover, the NGH reserves are substantial, estimated to be between 1 and 150 × 10¹⁵ cubic meters, which is twice the total conventional fossil fuel reserves and more than ten times [1,2] the total conventional natural gas reserves on Earth. Even if the estimated recoverable reserves range from 17% to 20%, these resources could meet global energy demands for over 200 years [3,4].

Due to the escalating global energy challenges and increasing environmental awareness, there has been a drive towards researching and developing efficient, clean energy sources. NGH resources are characterized by large reserves and high energy density, and are considered potent substitutes for petroleum and natural gas. NGH resources are predominantly found in deep-sea sedimentary layers and terrestrial permafrost, with approximately 27% of land and 90% of seabeds identified as potential NGH reservoirs, with around 99% of global NGH resources located in marine deposits [5]. Since the accidental discovery of NGH at the Messoyakha gas field in Siberia in the 1960s, countries worldwide have embarked on continuous exploration and development of NGH deposits. The initial attempts to produce natural gas from NGH reservoirs were conducted at the Mallik field in Canada’s Mackenzie Delta, with research led by the Geological Survey of Canada and Japan National Oil Corporation, culminating in a second production test in 2007. Subsequent trials have been conducted by Japan, the United States, and China in the Nankai Trough, Alaska, and the Shenhu area of the South China Sea, respectively. Early production trials predominantly employed methods such as thermal stimulation and depressurization, with attempts also made using inhibitor injection methods; however, depressurization has proven technically viable and more stable for production processes [6]. In 2008, the United States conducted CO₂ replacement experiments for NGH production in Alaska, exploring the potential of CO₂ exchanging with methane in reservoirs [7], providing a research direction to address geological stability during direct exploitation. Japan conducted trial exploitations in the Nankai Trough in 2013 and 2017. China conducted its first offshore trial exploitation in the Shenhu area of the South China Sea in 2017, followed by a second trial in 2020 that introduced innovative horizontal well exploitation techniques, setting records for daily gas production and total exploitation volume [8]. Despite the expanding scale and output of NGH mining trials, significant gaps remain in achieving commercial exploitation efficiency. The geological challenges and engineering hurdles must be overcome during the exploitation, particularly in maintaining the stability of hydrate reservoirs and improving exploitation efficiency, which constitute the core challenges of NGH production. The exploitation processes inevitably affect the surrounding operational temperature and pressure conditions, as well as sedimentary states, complicating the maintenance of original conditions and continuous operation. And it will potentially lead to geological hazards such as submarine landslides [9], gas leakage [10], and strata subsidence [11]. Therefore, by studying the characteristics of marine gas hydrate sediments (GHs), and analyzing the mineral composition, particle size distribution, pore structure, and mechanical properties of target exploitation areas, we can improve the understanding and assessment of NGH resource formation and distribution. This research will aid in the exploration and exploitation of NGH resources, effectively improving the development efficiency, reducing the development risks, and providing a scientific basis for the sustainable utilization of NGH.

In this paper, the recent research progress is reviewed on the fundamental physical properties of marine GHs, including summarizing the testing methods and technologies for GH basic properties, analyzing the mechanical, electrical, thermodynamic, and fluidic characteristics of GHs, exploring the impact of GH basic properties on the exploitation of NGH in various target exploitation zones of marine environments, and aiming to provide guidance and reference for the research on NGH exploitation in different target exploitation areas of marine environments.

2. Measurement Methods and Technologies for Reservoir Fundamental Properties

The fundamental property testing of marine NGH reservoirs typically involves stratigraphic exploration prediction and sample analysis, as illustrated in Figure 1. The exploration of marine strata primarily uses geological detection to analyze the information about the stratigraphic distribution, lithologic characteristics, and fault features of the reservoir. The sample analysis employs techniques in spectroscopy, crystallography, thermology, mechanics, and geology to ascertain the basic physical properties of sediments and hydrates within the reservoir samples. Marine NGH reservoir samples are obtained from seabed strata using pressure-preserving coring techniques [12,13]. Due to the limited drilling cases, high sampling costs, and the complexity of maintaining temperature and pressure, the research on measurement methods and technologies for reservoir sample properties is constrained. With the rapid advances in computer technology, emerging visualization testing and analysis methods have gained in popularity in research. These methods integrate and reconstruct data using in situ photoelectric testing technologies, allowing for the visualization of the microstructure of samples, significantly enhancing the outcomes of instrumental tests and analyses.

2.1. Geological Analysis

Geological analysis commonly employs acoustic and electrical technologies to obtain information on the stratigraphic distribution of hydrate reservoirs through geological sampling and seismic surveys, thereby characterizing the occurrence forms of hydrates. The bottom simulating reflector (BSR) is a frequently used predictive technique for NGH exploration. It utilizes the reflection of seismic waves between the hydrate layer and underlying rock layers to delineate information about the base interface of the hydrate layer, such as thickness and reflection intensity. This technique is often used to identify and locate NGH layers and assess their physical properties, stability, and the formation conditions. Additionally, geological analysis facilitates the acquisition of reservoir lithological and fault characteristics through the interpretation of core samples and seismic data. Exploratory well logging data are used to calculate the key reservoir properties such as permeability, resistivity, and hydrate saturation [14,15]. Winters et al. [16] compared the porosity measurements from core samples with well log-derived porosities at the Mount Elbert test well, finding a strong correlation that supports the overall accuracy of the well log data. However, due to the limited availability of logging data, the precise in situ assessment of hydrate porosity saturation remains challenging [7,17]. Researchers have elucidated the physical and chemical changes in hydrates under various geological conditions by studying their formation and dissociation processes [18,19,20]. Furthermore, through geological analysis, we can calculate parameters such as gas production, emissions, and accumulation [21,22], by constructing geological models to simulate the generation and migration characteristics of gases within hydrate reservoirs.

Microscopic imaging techniques, such as optical microscopy (OM) and scanning electron microscopy (SEM), are also commonly used in geological analysis to directly observe the micro-morphological features of GH rock samples. The chemical components and material content in samples are detected through X-ray fluorescence (XRF) and isotope analysis techniques. Currently, the optical microscopes used in the laboratory can achieve a resolution down to a few micrometers, while some high-precision SEMs can reach imaging at the nanometer level. Advanced techniques based on low-temperature Cryo-SEM can provide surface morphology information of hydrate samples with the highest resolution of up to 1 nanometer [23,24]. Figure 2 shows the drilling platform and pressure-preserving coring device used during the second trial exploitation in the South China Sea. Le et al. [25] utilized high-resolution optical microscopy with a pixel size of 6.5 μm and synchrotron radiation X-ray computed tomography with a voxel size of 0.9 μm to study the growth of methane hydrates at micrometer scales under excess gas conditions. They observed the growth characteristics well below the size of the pores and on time scales shorter than 1 s. This study revealed hydrate forms and growth features that cannot be detected by conventional X-ray computed tomography alone, observing hollow filamentous growth driven by capillary forces. Bai et al. [26], using SEM combined with computed tomography and specific surface area analysis, studied the micro-depositional structure of GHs, observed different pore types, and analyzed the impact of various pores on NGH enrichment, discovering that the microstructure of sediments is a critical control factor for NGH accumulation and elucidating the mechanisms of hydrate formation.

Due to the complexity of the forms of hydrate occurrences and reservoir geological conditions, geological analysis requires extensive geological data and samples, hence demanding high standards for exploration and research. Therefore, ship-borne in situ testing and analysis are poised to become the future direction of geological analysis for hydrate reservoirs. The coupling and combined use of different instruments, deployed on research vessels for real-time in situ sample analysis, will play a crucial role in guiding the exploration and development of marine NGH.

2.2. Crystallographic Analysis

Crystallographic analysis is a fundamental technique for analyzing the physical properties of materials with crystalline structures. Techniques involved in crystallographic analysis include X-ray diffraction (XRD), powder X-ray diffraction (PXRD), X-ray computed tomography (X-CT), neutron diffraction (ND), neutron powder diffraction (NPD), inelastic neutron scattering (INS), and small angle neutron scattering (SANS). XRD is a commonly used method that provides information about the distribution of sediment components, the crystal structure of NGH, lattice parameters, hydration index, and cavity occupancy rates [28,29]. XRD identifies the structural type and composition of hydrates through the positions and intensities of diffraction peaks, and studies their phase transition behaviors and thermodynamic properties [28]. X-CT, a non-destructive technique potentially used for detecting NGH, captures and projects crystal images [30]. It can determine the location of hydrates within sediments and help understand the relationship between NGH and host sediments. This technique can also ascertain local density changes, three-dimensional morphology, pore structure, porosity, and permeability during the formation and dissociation of hydrates by calculating CT numbers or using network models [31,32,33,34]. The fundamental principles of X-CT are illustrated in Figure 3 [35]. Micro-computed tomography (μCT) utilizes computational reconstruction to obtain high-resolution three-dimensional images of internal structures. Some high-resolution X-CT systems can achieve sub-micrometer spatial resolution. Paired with specially designed sample cells, these systems can image samples under high-pressure conditions of several tens of megapascals, making them suitable for studying the behavior of hydrates in extreme environments. And in recent years, it has gained popularity in hydrate research [36,37,38]. Crystallographic research has significant advantages in analyzing the internal structure and deformation of hydrates. However, due to the complex phase transitions and multiphase flow involved in the formation and decomposition of hydrates, resolving phases in multiphase fluids remains challenging. The application of more powerful characterization technologies and equipment, such as μCT and surface-enhanced Raman scattering spectroscopy (SERS), or the coupled use of various detection methods, should be considered for real-time analysis of hydrate formation, decomposition processes, and reservoir changes. Additionally, leveraging advanced computational equipment and modeling algorithms could enhance the clarity and resolution of crystal analysis, improving the accuracy of identification.

2.3. Spectroscopic Analysis

Common spectroscopic analysis methods include infrared spectroscopy (IR), Raman spectroscopy (Raman), nuclear magnetic resonance spectroscopy (NMR), gas chromatography (GC), and gas chromatography-isotope ratio mass spectrometry (GC-IRMS). IR analysis identifies the composition and structure of GHs by studying their absorption characteristics within the infrared wavelength range [39,40]. Raman analysis determines the molecular vibrational modes and lattice structure of NGH by measuring frequency shifts in Raman scattering, which can provide information about the crystal structure and phase transition behavior of hydrates [41,42]. Some high-performance spectrometers can achieve wavelength resolution at the picometer level, which is very useful for detailed structural analysis and peak separation. Raman spectroscopy testing has a wide range of temperature and pressure conditions, and Raman spectrometers can be used for analysis without affecting the optical path. NMR spectroscopy investigates the molecular structure and dynamic behavior of NGH by detecting signals from nuclear spins [43,44], offering insights into the relative contents of different components within hydrates and their interactions with the surrounding environment. The chemical shift precision of NMR testing can reach 0.01 ppm or even higher, which is significant for the analysis of hydrate crystal structures and the study of dynamic processes. Giovannetti et al. [45] used ice water as a model to study the effects of sediments, salt (NaCl), and temperature on the formation of hydrates. They used Raman spectroscopy to study the stretching vibrations of water molecules in test liquids and ice water, comparing changes induced by sodium chloride, sediments, and temperature under various experimental conditions. Their findings suggest that the presence of a porous medium typically facilitates the formation of hydrates, and it was observed that sand grains could counteract the inhibitory effect of salt molecules. Li et al. [46] employed a low-field nuclear magnetic resonance (LF-NMR) system, as illustrated in Figure 4, combined with a gas/water two-phase system to quantitatively and visually study the heterogeneity of methane hydrate formation/dissociation in two sand samples during thermal intrusion. They discovered that the formation rate of methane hydrates negatively correlates with sediment grain size and occurs heterogeneously within porous media, while the dissociation rate is influenced by both fluid flow resistance and heat supply, accelerating with increasing quartz sand grain size in GHs due to thermal stimulation.

Spectroscopic analytical techniques play a crucial role in studying the molecular composition, structural types, and formation and dissociation kinetics of sediments and hydrates. However, these techniques impose specific requirements on the samples and involve issues of sensitivity and resolution differences in the equipment, such as the peak overlapping in IR analysis and artifact differentiation in NMR analysis. Therefore, employing multiple devices and multi-scale considerations for comprehensive analysis is essential to enhance the accuracy of the results.

2.4. Thermal Analysis

The thermal analysis methods operated by controlling temperature programs are used to record changes in the physical properties of substances with temperature variations. The thermal analysis methods used in GH research include differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), time-domain thermal reflectance (TDTA), and thermal conductivity analysis (TCA). These methods facilitate the investigation of thermal properties such as the thermal conductivity, heat capacity, heat flux density, and thermal diffusivity of GHs. DSC is one of the most commonly used thermal analysis methods. It is suitable for obtaining data on the equilibrium properties of NGH during the solid–liquid phase transitions under high pressure [47,48]. Combining thermal analysis with related heat transfer studies also deepens the understanding of the formation, dissociation, and inhibition mechanisms of GHs [48]. TGA is a thermal analysis technique that studies the regularities of sample mass changes with temperature under controlled temperature programs, and is used to investigate material thermal stability and composition [49]. TDTA employs ultrafast laser pulses to generate thermal stimuli on the surface of material samples and measures the time-temperature response of reflected light to infer the sample’s thermal conductivity. This method is useful for measuring materials with uneven thermal conductivity distributions, such as rocks and minerals. In GH research, thermal conductivity analysis mainly refers to the measurement of the thermal conductivity of hydrates and their sediments, which is a critical parameter describing the heat transfer characteristics of sedimentary reservoirs.

Thermal conductivity measurements involve determining the heat transfer process through spatial temperature differences, encompassing techniques for measuring heat flux density and effective thermal conductivity. The measurements of heat flux density reflect the changes at different locations or times. The heat exchange during the formation/dissociation of hydrates in vertical or horizontal directions can be analyzed, and heat losses can be calculated. Li et al. [50] analyzed the impact of reservoir heat transfer factors on hydrate dissociation by observing temperature distribution changes during the formation and dissociation of NGH. Effective thermal conductivity measurement methods include the hot-wire method, thermal ribbon method, probe method, and transient plane source method. He et al. [51] used the transient hot-wire method to measure the effective thermal conductivity of quartz sand containing hydrates, analyzing hydrate distribution through resistance changes, and studying changes in effective thermal conductivity with factors such as gas saturation, pressure, water saturation, water pressure, and temperature, with their experimental setup illustrated in Figure 5. Li et al. [52] conducted in situ measurements of the effective thermal conductivity of different water-containing sediment layers, analyzing the effects of temperature, particle size, and sediment type. They emphasized the dominant role of porous media types on the effective thermal conductivity characteristics of water-bearing sediments, noting that the intrinsic thermal conductivity of porous media controls the effective thermal conductivity characteristics of water-bearing sediments. The effective thermal conductivity values of different water-bearing sediment layers in descending order are water-containing silicon carbide sediments > water-containing quartz sand sediments > water-containing clay sediments. During the formation and dissociation of hydrates, the density of hydrate samples and the structure of porous media may change, which could lead to probe or wire damage and measurement failure during measurements using plane sources or hot-wire methods. The temperature field of the samples themselves may also be affected. Therefore, non-destructive testing methods such as TDTR or ultrasonic tomography [53,54] should be considered for analyzing temperature distributions, or existing temperature detection devices should be improved by using thermistors or point heat source methods to study the temperature field and heat transfer during the hydrate formation and dissociation processes [31,55].

2.5. Mechanical Analysis

Currently, the main methods used to study the mechanical properties of hydrated sediments are using methods such as triaxial shear tests and direct shear tests. The direct shear test, a commonly used direct testing method in laboratories, can determine the strength of hydrate-bearing sediments under large strain conditions. Zhou et al. [56] conducted a series of direct shear tests to explore the shear strength characteristics of the hydrate-bearing soil. It was shown that the shear strength of the soil can be significantly influenced by pore gas pressure, unhydrated water content, hydrate saturation, and several other factors. A shear strength criterion, which enforces the equilibrium condition of pore hydrate, is developed for hydrate-bearing soil, establishing a link between the equilibrium condition and the shear strength. A combined injection synthesis and direct shear testing system was developed and the results of the research show that the shear strength of hydrate-bearing soil is higher for the cementation habit than for the pore-filling habit, which verifies the reliability of the instrument. [57] Triaxial shear tests, conducted under low-temperature high-pressure conditions, explore the mechanical performance of GHs. These tests demonstrate a stable pattern of residual strength variation. The residual strength is minimally affected by sample uniformity, but it exhibits markedly lower residual strength at low temperatures compared to high temperatures, displaying distinct peak and softening trends. This is entirely different from the hardening trend observed in direct shear tests. Direct shear testing is a common and effective engineering method characterized by low cost, simplicity, and short duration. However, it does not allow control of the pore water pressure and drainage conditions, and the shear plane of the sample is predetermined, which typically does not align with the weak planes of the soil samples. Furthermore, the residual strength in direct shear tests is influenced by the shear rate and temperature. Increases in shear rate and decreases in temperature can enhance the strength of GHs, but the effect of shear rate on residual strength shows irregular trends, potentially leading to significant yielding and softening. The fractures in direct shear tests are greatly influenced by sample uniformity, while fractures in triaxial tests are less affected. The results of direct shear tests may not represent the true mechanical strength of GHs. Thus, triaxial shear tests are generally used for testing the mechanical properties of NGH reservoir samples. Zhao et al. [58] performed mechanical property tests on hydrate-bearing clayey–silty sediments using a triaxial shear device (as shown in Figure 6), analyzed the compressibility and shear behavior of the hydrate sediments, obtained compressibility parameters (reflecting compressibility characteristics), bonding coefficients (reflecting bonding effects), and dilatancy coefficients (reflecting filling effects), and established a constitutive model that uniformly describes the mechanical behavior of clayey and sandy sediments. Yoneda et al. [13] conducted triaxial or uniaxial compression tests on pressure core samples obtained from drilling in the eastern Nankai Trough, Japan, developed empirical equations for the triaxial compressive strength and deformation modulus as functions of NGH saturation, effectively confining pressure, porosity, and strain rate, and predicted the strength and modulus of the deformation of the sediments based on logging data.

The combined use of triaxial shear tests and various microscopic testing techniques offers a comprehensive approach to analyzing the mechanical properties and influencing factors of gas hydrate reservoir sediments. This method has become crucial in recent studies of the mechanical properties of these sediments. As previously mentioned, microscopic techniques can reveal the microstructure and composition of hydrate reservoir sediments, aiding in understanding the impact of hydrate crystals on sediment mechanical properties. Combining these techniques allows researchers to analyze the distribution of hydrate crystals within sediments, pore structure, and interactions with other minerals, leading to a more accurate assessment of sediment sample mechanical behavior and stability. Commonly used combined techniques include computer tomography combined with triaxial shear testing. Yoneda et al. [59] employed micro-focused X-ray computed tomography to analyze the large-strain mechanical behavior of hydrate-bearing sediments at different hydrate saturations (S_h = 0%, 39%, and 62%). The obtained stress–strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate-bearing sand. The quantitative analysis of localized deformation through sub-millimeter and micron-scale imaging revealed that the shear band thickness decreased with increasing hydrate saturation. Wu et al. [60] used computed tomography to observe and analyze the cementation failure behavior (morphology), deformation evolution (quantitative statistics), and localized shear deformation in different regions of the stress–strain curves. They studied the varying behaviors of hydrate cementation clusters in different regions. Figure 7 shows the CT images of the hydrate samples with axial strain increasing, where the Series C and Series D images, respectively, depict the deformation processes in uncemented and cemented regions (clustered hydrate patches).

Both triaxial and direct shear tests cause irreversible damage to the samples, making it impossible to verify the accuracy and reliability of the experimental results. In particular, for naturally extracted hydrate core samples, which are typically non-replicable, there is a need to strengthen the validation and comparison between different testing methods. By standardizing test procedures and parameters, comparability between different methods can be enhanced, providing a more accurate basis for the application of test results. The mechanical performance of GHs is controlled by their internal structure, with microscopic hydrate behavior playing an increasingly significant role in macroscopic mechanical phenomena [27,36]. A measurement, combined with electrical, optical, and acoustic techniques, will aid in the integrated analysis of the mechanical and physical properties of sediments [61,62,63,64].

3. Study of Reservoir Basic Physical Properties in Target Exploitation Areas

The current global distribution of identified sources of NGH are illustrated in Figure 8. The complex origins of gases significantly influence the formation of GHs [65,66,67,68], combined with complex geological conditions and depositional environments. These are closely related to the formation mechanisms of NGH. Influenced by the GH particle size, pore size, stress strength, and surrounding fluid environment, hydrates in sediments with smaller particles and pore sizes predominantly exist as fracture fillings, known as fracture-filling hydrates, as shown in Figure 8. Due to the difficulty in filling fine-grained sediment pores, hydrates occupy variously shaped and sized fractures in multiple forms such as massive, vein, and nodular, forming hydrate reservoirs with diverse storage types [36,69,70]. In the sediments with larger particles and higher porosity, the hydrates can be easily filled with pores or cement sand grains, and exist as pore-filling hydrates. These are represented by suspension, contact, and cementation modes of occurrence (as shown in Figure 8). Different occurrence forms of hydrates impact the structural composition and physical properties of GHs, thereby affecting the exploitation process differently. Consequently, the exploration and prediction of hydrates with different storage forms should adopt varied evaluation methods.

Global NGH pilot exploitation areas are primarily located in the Malik region of the Mackenzie Delta in Canada, the North Slope of Alaska in the USA, the Qilian Mountains area of the Qinghai–Tibet Plateau in China, the Nankai Trough in Japan, and the Shenhu area in the South China Sea. In the Malik region, hydrates mainly exist in the unconsolidated loose sandstone deposits of the Tertiary Mackenzie Bay and Kugmallit sequences, distributed at depths of approximately 900–1100 m [71]. These rock layers predominantly consist of thickly bedded sandstone interbedded with gravel, and layered sandstone with siltstone. NGH predominantly forms as pore-fillings with saturation levels between 50% and 90%. Extracted core samples primarily include unconsolidated gravelly sandstone, compact sandstone, and shale, also incorporating small amounts of dolomite-cemented sandstone and low-rank coal layers. The hydrates mainly consist of Structure I methane hydrate [5,72,73]. In the North Slope of Alaska, NGH is predominantly distributed in six laterally continuous Lower Tertiary sandstone and gravel units of the Sagavanirktok Formation, covering parts of the Kuparuk River, Prudhoe Bay, and Milne Point oil fields. The lithology of the drilled samples is mainly characterized by thick sandstone, including sandstone, mudstone, and siltstone. Many wells in this area have multiple hydrate-bearing units, each ranging in thickness from 3 to 31 m (0.9 to 9.4 feet) [74,75]. The Qilian Mountains region of the Qinghai–Tibet Plateau, located at a low-latitude mountainous permafrost zone, has relatively shallow permafrost but has yielded unique NGH samples. This permafrost zone is situated along the northern edge of the Qinghai–Tibet Plateau. The drilling area’s terrain gradually slopes from higher elevations in the west and south to lower elevations in the east and north. The elevations range between 4026 and 4128 m, with permafrost depths typically between 60 and 80 m [76,77]. Hydrates are primarily found in fine siltstone and mudstone core cracks in the form of icy thin layers or fine impregnations [78]. In Japan’s Nankai Trough, hydrate reservoirs mainly contain turbidite channel-type deposits within submarine fan systems, with the primary depositional system being submarine fan turbidites [79,80], comprised of an upper muddy zone, an upper methane hydrate concentration zone (MHCZ), a silt-dominated zone, a lower MHCZ, and a hydrate-bearing zone. The upper MHCZ is alternately composed of fine sands (fine and very fine sands) and fine silts (sandy silt and clayey silt). The silt-dominated zone is similar to the upper MHCZ but with a higher proportion of silt layers. The lower MHCZ also features alternating sand and silt, with the thickest sand layers containing channel sands [81]. Sediment reservoirs in the Shenhu area of the South China Sea are primarily composed of clayey silt rich in foraminiferal and other paleontological fossils, followed by silty clay [82]. The mineral compositions of GHs in various target exploitation areas are shown in Figure 9 (based on the test results of extracted core samples). The mineralogical composition of the GHs in each of the target exploitation areas consists mainly of quartz, illite, calcite, dolomite, glauconite, sodium feldspar, potassium feldspar, pyrite, and minor amounts of amphibole and rock salt. Illite and quartz are generally present in high concentrations in the sediments, followed by glauconite and carbonate minerals, with other minerals being less abundant [83,84]. Hydrate reservoirs in the Shenhu region of China mainly consist of fine-grained clayey silt, characterized by low permeability and challenging exploitation, which contrasts significantly with the hydrate reservoirs in the USA and Canada (predominantly gravel) and Japan (generally coarse sand) [85]. Generally, sandy sediments with larger pore spaces, good connectivity, and high permeability are favorable for the flow of hydrocarbon fluids and the formation and storage of NGH. If silt-dominated sediments contain biogenic carbonates, diatom fossils, and volcanic ash, they can alter the sediment’s pore structure, connectivity, and permeability, thus improving the formation and storage environment for hydrates. Clayey sediments have smaller porosities and lower permeability, but due to the tectonic activity, differential compaction, and fluid effects, high-angle faults or fractures often exist within the sediment layers. These faults or fractures can expand the storage space of fine-grained sediments, forming various forms of fracture-filled hydrates [70,86,87].

The foundational physical properties of NGH reservoirs critically influence the hydrate accumulation and exploitation. The saturation of hydrates within the reservoirs is a vital indicator for assessing NGH accumulation. Generally, under suitable environmental conditions and ample gas molecule availability, larger GH particle sizes and greater pore spaces correlate with higher hydrate saturations. This is because hydrate formation and storage require sufficient pore space, and high porosity facilitates the formation and storage of hydrates. For instance, the reservoirs in the Malik area of the Mackenzie Delta, primarily composed of gravelly sediment, exhibit hydrate saturations ranging from 60% to 80%, reaching up to 90% [88]. In contrast, the reservoirs in the North Slope of Alaska and the Nankai Trough area in Japan, which are primarily sandy, have smaller average grain sizes with hydrate saturation ranging from 50% to 80%, with peak saturations recorded at 77.4% [16]. In the Shenhu area of the South China Sea, the reservoirs predominantly consist of muddy siltstone sediments, with notably lower hydrate saturations usually between 10% and 40%. The physical property data from various exploitation efforts are summarized in

Table 1. Physical property data of hydrate reservoirs for each exploitation.

Country	Canada	Canada	USA	USA	Japan	Japan	China	China
Location	Mackenzie delta	Mackenzie delta	North Slope in Alaska	North Slope in Alaska	Nankai trough in Japan	Nankai trough in Japan	Shenhu area in South China Sea	Shenhu area in South China Sea
Year	2002	2007	2007	2012	2013	2017	2017	2020
Teat well	Mallik 5L-38	Mallik 2L-38	Mt. Elbert1	Ignik Sikumi	AT1-P	AT1-P2 and AT1-P3	SHSC-4	SHSC-4
Well type	vertical	vertical	vertical	vertical	vertical	vertical	vertical	horizontal
Method of exploitation	Thermal stimulation	Depressurization	Depressurization and CO2 replacement	Depressurization and CO2 replacement	Depressurization	Depressurization	Depressurization	Depressurization
Drawdown duration/d	5	6	6–12 h	30	6	12 + 24	60	42
Cumulative gas production/104 m3	0.0516	1.3		2.4	124	3.5 + 20	30.9	149.86
Reason for stopping production	sanding events	sanding events	sanding events	sanding events	sanding events	weather conditions	ESP power cable failure	production off
Reservoir type	class 3	class 3	class 3	class 3	class 3	class 3	class 1	class 1
Sediment type	Sandstone	Sandstone	Sandstone	Sandstone	Sand-silt alternation	Sand-silt alternation	Clayey silt	Clayey silt
Temperature/°C	13.9	11.2	2.3–2.6	5	13.5	13.5	—	—
Pressure/Mpa	11.6	11.3	6.7	6.9	13	13	—	—
Hydrate saturation/%	60–80	60–80	65	72	50–80	60–80	31	31
Intrinsic/initial permeability/md	400/0.5–35	400/0.5–35	500–200/0.2–0.5	<1 (initial)	47–840	47–840	1.5 (Mean effective)	2.38 (Mean effective)
Porosity of sediments	—	0.02–0.04	0.4	0.4	0.4–0.5	0.3–0.5	0.33	0.37

. High saturation levels indicate a rich content of hydrates within the reservoirs, which is advantageous for exploitation. However, excessively high hydrate saturations may alter the physical properties of the reservoir rocks, such as reducing their strength, thereby impacting the stability of the reservoirs. Permeability parameters describe the fluid dynamics-related characteristics of hydrate reservoirs. They reflect the ability of fluids to flow through the rock formations. Higher permeability implies that gas can more easily flow through the rock pores, which is beneficial for hydrate exploitation. Conversely, low permeability may hinder gas flow, increasing the difficulty of exploitation.

In the Shenhu area of the South China Sea, GH overall porosity is relatively uniform, predominantly consisting of micro- to nanometer pores, with pore channels appearing as sheet-like and curved lamellar structures [19], and pore diameters ranging from approximately 500 nm to 20 µm, constituting the primary connectivity spaces. Some larger pores exist due to foraminifera, with occasional fractures observed. Overall porosity ranges from 33% to 55%, with capillary pressures between 0.57 and 1.10 MPa [85], and effective permeability ranging from 0.2 to 40 millidarcies [89,90,91,92]. Generally, sandy sediments have relatively good permeability, reaching tens or even thousands of millidarcies. However, such sandy layers are typically scarce, as exemplified by the Nankai Trough layers tested in Japan. Most often, the sediment comprises very fine-grained muddy silt with low permeability, sometimes less than a few millidarcies. This characteristic is common across most of the globally explored NGH reservoirs, including the Shenhu area in the South China Sea [93,94].

4. Study on the Reservoir Characteristics Measurement and the Influence on NGH Exploitation

4.1. Mechanical Properties

The mechanical properties of GHs are critical for analyzing the potential geological hazards and the stability of mining wells during exploitation processes. The presence of NGH significantly alters the mechanical characteristics of sediments [95]. The mechanical behavior of GHs is primarily determined by factors such as hydrate saturation, effective confining pressure, sediment composition, and temperature. For GHs with lower hydrate saturations, the impact of hydrate saturation on their strength is minimal. As saturation increases, the strength and stiffness of hydrate-bearing sedimentary reservoirs significantly improve, accompanied by increased expansiveness, primarily due to the filling and cementing effects of hydrates. An increase in hydrate saturation reduces the sediment compaction processes while enhancing expansion processes, thereby influencing the mechanical properties of the reservoirs [96]. Furthermore, as hydrate saturation increases, hydrate particles fill the interstitial spaces between the grains, enhancing the interactions among hydrate particles, which can increase the tensile and shear strength of the reservoirs and enhance hydrate stability. Increased effective confining pressure leads to stronger interlocking and bonding between sand and hydrate particles, augmenting the internal friction of the sediment, while also restricting the free movement of sand and hydrate particles, significantly enhancing the mechanical strength of GHs [97]. The relationship between the strength of GHs and hydrate saturation as well as the effective confining pressure are illustrated in Figure 10. The composition of the sediment also has a significant impact on the mechanical performance of hydrates. The proportion of particles in the sediment, their size distribution, and shape can affect the mechanical properties of hydrates. A higher proportion of larger particles increases the strength of the hydrate–sediment composite, while round sand particles enhance the cohesion of the hydrate–sediment composite [98]. An increased proportion of fine particles promotes the formation of larger hydrate cement clusters, thereby strengthening the sedimentary reservoirs [99]. Additionally, changes in temperature and pore pressure also affect the mechanical performance of the hydrate–sediment composite; reductions in temperature and increases in pore pressure enhance the stiffness and strength of the sedimentary reservoirs [96,97,100].

4.2. Electrical Properties

The volumetric fraction and spatial distribution of sediment components can be characterized by evaluating their overall electrical and electromagnetic properties [106]. Sensitive electromagnetic measurement techniques can complement seismological studies and are used to determine the saturation and distribution of NGH in natural environments [107,108,109,110]. The conductivity of GHs is predominantly controlled by the movement of hydrated ions within the pore water and around the mineral surfaces within the electrical double layers, particularly for sediments with high specific surface areas [111]. The resistivity of GHs is influenced by the combined effects of hydrate microstructure and pore architecture. The microstructure of hydrates depends on the distribution and migration of gas and water within the pores, which in turn is controlled by the characteristics of the pore structure [19,39]. Chen et al. [112] utilized X-ray computed tomography (X-CT) and resistivity measurement techniques to study the response characteristics of the resistivity of hydrate-bearing sediments. Their results indicated that when hydrate saturation is below 20%, resistivity is primarily controlled by the salt exclusion effect, and the impact of hydrate content is not significant. However, when hydrate saturation exceeds 20%, the increase in hydrates leads to enhanced pore blockage, causing significant variations in resistivity. Through resistivity–saturation relationship models established based on electromagnetic testing techniques, we can analyze the anomalous resistivity responses in GHs and apply these findings for calculating and evaluating the saturation of hydrate reservoirs.

4.3. Thermal Properties

In the actual hydrate reservoirs, the presence of natural gas, water, hydrates, and mineral particles creates a multiphase coupled system that complicates the heat transfer process, making it challenging to accurately measure and predict the thermal conductivity parameters of the reservoirs. Numerous computational models for heat transfer in GHs have been developed, yet there is still a need for extensive experimental data to further calibrate and optimize these model parameters. Various experimental studies have revealed the complex interactions between porosity, effective stress, grain size, and the pore spaces filled by fluids and hydrates [113,114,115]. For GHs, their effective thermal conductivity largely depends on the heat transfer processes between particles, including the following: (a) conduction along the mineral; (b) particle–fluid–particle conduction across the fluid near contacts; (c) particle-to-particle conduction across contacts; (d) fluid convection within large pores; (e) particle–fluid conduction; and (f) conduction along the pore fluid within the pore space (hydrostatic and advection pore fluid), as shown in Figure 11 [116,117]. Experimental studies, such as those by Waite et al. [118], have found that with increasing solid hydrate content, the effective thermal conductivity of sand and methane hydrate mixtures initially increases and then decreases, reaching a maximum when the solid hydrate proportion is 33%. The increase in thermal conductivity is due to hydrates in the pores enhancing particle contacts, thereby strengthening heat transfer [116]. As hydrates fill the pores and gradually replace sand grains, the overall thermal conductivity decreases because the thermal conductivity of hydrates is lower than that of mineral particles [116,119]. Furthermore, confining the stress and phase transition processes also affects the thermal conductivity of hydrate-bearing sediments. Effectively confining stress can increase the number and quality of particle contacts, raising the thermal conductivity; during hydrate dissociation, the migration of water improves contact quality, also increasing thermal conductivity [120]. Thus, the effective thermal conductivity of GHs under multiphase coupling conditions in hydrate reservoirs is not only related to the thermal conductivity of the components, but should also consider contributions from sediment particles and pore fluids.

4.4. Fluid Dynamics Characteristics

Permeability is commonly used to measure the ability of a porous medium to allow fluid passage. The microstructure of the porous medium not only controls the permeability and fluid flow paths, but also influences the saturation and aggregation distribution of the NGH [111]. Besides the mineral composition, shape, and arrangement of sediment particles, as well as the surface area and tortuosity of the pores, the presence of NGH adds complexity to the sedimentary reservoirs. The spatial distribution of hydrates alters the size, shape, and interconnectivity of pores, thereby changing the sediments’ permeability [113]. Numerous experimental studies at macroscopic and microscopic scales have investigated fluid flow phenomena in hydrate-bearing porous sediments, exploring the relationship between permeability and the liquid phase flow patterns within GHs. The presence of hydrates affects the permeability by reducing the pore size and altering pore shape. When hydrates form within pore spaces, the fluid transport pathways are diminished, correspondingly reducing permeability [121,122]. Wang et al. [123] combined pore network models with X-ray computed tomography (X-CT) to study the impact of grain size and porosity on the permeability of porous media, finding that larger grain sizes correspond to higher porosity and greater absolute permeability. Fine grains, typically smaller than 100 μm, often coexist with NGH-bearing sediments and are easily mobilized by the flow of water and/or gas during the formation of gas hydrate. The migration of fine grains, involving the generation, movement, and retention of solid particles in porous media, inevitably leads to a reduction in permeability [124]. Furthermore, under identical water saturation conditions in reactors, higher porosity results in greater relative water permeability and lower gas permeability [114]. The research by Jin et al. [125] showed that the absolute permeability of sediments is directly proportional to the number of continuous pores in the vertical direction. The ratio of horizontal to vertical channel numbers also affects absolute permeability [126]. Seol et al. have studied the impact of hydrate saturation on the permeability of sedimentary layers [127]. They found that hydrates initially form in porous media at low saturations, and when hydrate saturation is below 40%, the impact on sediment permeability is minimal. However, when hydrate saturation exceeds 40%, the permeability of the reservoir sediments significantly decreases.

5. Conclusions and Future Directions

Current laboratory tests, field observations, and sample analyses have extensively studied the physical properties of submarine GHs, revealing critical characteristics such as porosity, permeability, thermal performance, mechanical properties, and the composition and structure of sediments. These studies aid in better understanding how various sediment properties impact hydrate formation and exploitation.

Gas hydrates exist in solid form within reservoirs in marine environments and are present in multiple occurrence forms, complicating the description of reservoir physical properties. The extraction process of NGH involves complex interactions among multiple physical fields within the reservoir, including gas–water two-phase flow, reservoir thermodynamics, phase transition dynamics, and geomechanical deformations, posing significant challenges to the study of GH characteristics. Research on GH properties will increasingly focus on the dynamic changes in fundamental physical property parameters related to the flow field, such as relative permeability and capillary forces of gas–water; dynamic changes in the thermal properties of hydrate-bearing sediments related to the temperature field, including the endothermic nature of hydrate decomposition; studies on the kinetics of hydrate formation/decomposition related to the chemical field, especially considering equations pertinent to porous media; and the mechanical properties of and dynamic changes in hydrate-bearing sediments related to the mechanical field. Thus, further elucidating the coupling relationships among these physical fields, obtaining key coupled parameters and their dynamic characteristics, and advancing foundational research on the physical properties of marine GHs are critical pathways to guide the rational and efficient extraction of hydrates.

Complex environmental and geological conditions can easily trigger multiphase flow obstacles and control risks of safe pressure in hydrate extractions, making the foundational research on the physical properties of marine GHs crucial for the safe regulation of NGH resource exploitation and geological risk management. Integrating stratigraphic exploration predictions with sample analyses, and establishing a bridge between “deep-sea in-situ laboratories” and “onshore simulation experimental systems”, the creation of “in-situ experimental stations” that examine the interplay between natural gas hydrate extraction processes and deep-sea environmental changes will likely be a future trend in GHs foundational physical property research. This approach will aid in uncovering the complex environmental dynamics of and overall macroscopic changes in the hydrate system, further advancing hydrate extraction technologies, reducing geological risks in engineering, and addressing the challenges of sustainable development in the hydrate industry.