Analysis of physical properties of gas hydrate-bearing unconsolidated sediment samples from the ultra-deepwater area in the South China Sea

Abstract

Marine natural gas hydrate has recently attracted global attention as a potential new clean energy source. Laboratory measurements of various physical properties of gas hydrate-bearing marine sediments can provide valuable information for developing efficient and safe extraction technology of natural gas hydrates. This study presents comprehensive measurement results and analysis of drilled hydrate-bearing sediments samples recovered from Qiongdongnan Basin in the South China Sea. The results show that the gas hydrate in the core samples is mainly methane hydrate with a methane content of approximately 95%, and the other components are ethane and carbon dioxide. The saturation of the samples fluctuates from 2%–60%, the porosity is approximately 38%–43%, and the water content is approximately 30%–50%, which indicate that high water saturation means that timely drainage should be paid attention to during hydrate extraction. In addition, the median diameter of the sediment samples is mainly distributed in the range of 15 to 34 μm, and attention should be paid to the prevention and control of sand production in the mining process. Moreover, the thermal conductivity is distributed in the range of 0.75 to 0.96 W/(m∙K) as measured by the flat plate heat source method. The relatively low thermal conductivity of hydrates at this study site indicates that a combined approach is encouraged for natural gas production technologies. It is also found that clay flakes and fine particles are attached to the surface of large particles in large numbers. Such characteristics will lead to insufficient permeability during the production process.

Graphical abstract

Keywords

natural gas hydrates / physical properties analysis / hydrate-bearing sediments

1 Introduction

Natural gas hydrates (NGHs) are a new source of clean energy, with huge reserves and high energy density. Worldwide, the research on NGHs, originally driven by scientific interests, has been gradually integrated into the national energy strategic planning of many countries. Developed countries such as the United States, Canada, Japan, and South Korea have intensified their research on NGHs in recent years. NGHs are crystalline solid compounds composed of water and guest molecules. They are usually stable at a relatively high pressure and low temperature [1]. They occur naturally in permafrost regions or under the seabed in certain areas. The associated energy is estimated to be twice as much as that stored in conventional oil and gas reservoirs. They are considered as a huge potential energy source [2–4] with promising industrial applications [5] and the role in gas isolation and storage [6] that affect the past and future climate change substantially. The characterization of the physical properties of gas hydrate reservoir has become an essential part of the trial exploitation of hydrate in the future [7].

A growing number of studies on high-resolution seismic reflection data of gas hydrate reservoirs and pressure core drilling tests have confirmed the existence of gas hydrate resources in several regions [8–11]. The previously reported research has introduced the characteristics of the bottom simulating reflector (BSR) in the Xisha Trough on the north-western continental slope of the South China Sea [9]. The hydrate core samples used in the experiment in the present paper were recovered from the Qiongdongnan Basin in the Xisha Trough area. Recent studies on sediment samples focused on the particle size distribution, hydrate structure, hydration numbers, and gas composition analysis of the sediment [12]. It is important to obtain more information on the physical properties of these sediments for further research on production techniques [13–16]. The basic principle of all existing extraction methods is to break the existing phase equilibrium of NGHs. During the extraction process, the environment where the NGHs are located changes drastically, causing their rapid decomposition and impacting the reservoir structure. Studies have shown that the physical properties of hydrate-bearing sediments are influenced by the volume fraction and spatial distribution of hydrate phases [17]. Analysis of gas hydrate samples in the laboratory can provide significant information about hydrate characteristics and hydrate-bearing reservoirs [18]. For safe and efficient natural gas extraction from hydrate-bearing sediments, it is necessary to assess the production rates and the stability of producing wells [19–21]. Some physical properties of porous media and fluid flow, including strength and deformation and seepage characteristics [22–24], can be evaluated in the laboratory [25]. The heat transfer within the reservoir has a significant influence on hydrate decomposition rates and gas production rates [26–28]. Therefore, important information on gas hydrate reservoirs can be first obtained from regional geophysical surveys and then improved by drilling results and direct geological and geochemical studies of hydrate core samples. Liu et al. also have reported that the organic matters in marine clays will affect the hydrate formation [29,30]. The studies based on the parameters of natural gas hydrate reservoirs are very important for the understanding of the fundamental characteristics of NGHs. They provide a significant geophysical basis for the efficient exploitation of NGHs in the later stage. Due to the limitation of pressure retaining drilling technology for NGH core samples, there are still few researches on in situ testing and analysis of hydrate core samples.

In this study, all of the samples were recovered from the China National Offshore Oil Corporation (CNOOC) drilling expedition in the ultra-deepwater area (>1500 m) at Lingshui Sag Area (a sampling water depth of about 1700 m) of Qiongdongnan Basin in the South China Sea. The experiments were conducted on the sediment infrared thermal imaging analysis, X-ray computed tomography (CT) analyses, and scanning transmission microscopy (SEM), thermal conduction characteristics, and the mineral composition analysis, in the hope that the physical property research on these hydrate core samples could be conducive to the analysis of corresponding field data and then boost the understanding of the hydrate occurrence form and reservoir characteristics.

2 Experimental section

2.1 Sample preparation and procedures

The natural gas hydrate samples were recovered from the drilling expedition reservoir and transferred to the laboratory on the exploration ship. Then the hydrate core samples from the sampler were transferred and cut to the pressure holding device. The low temperature storage cylinder capable of retaining pressure were transported to a low temperature laboratory set at 4°C, and the pre-frozen bulk ice was placed in the water bath on the operating table for long-term temperature control around the sampler. The storage cylinder was placed on the operating table for rapid pressure relief under the low temperature condition. During pressure relief, the storage cylinder was connected to a gas collection and separation device through an external pipeline, which separated the gas and water quickly. The gas was collected by an external gas collection bag and stored. Subsequently, the storage cylinder was opened and the core sample with the liner tube was taken out for infrared temperature measurement. The sections where hydrates existed were shown as a cold spot in the infrared imaging. The sections with visible hydrates or visible anomaly low temperature were separated, sealed, and put into liquid nitrogen for storage. The remaining core samples were dissected, taken out, and filled into test tubes. The pore water and sediments were separated through a centrifuge to measure the chloride ion concentration and resistivity of the pore water. If the pore water content was low and a certain volume of pore water could be centrifuged, a quantitative diluted solution of deionized water would be added to measure the diluted chloride ion concentration. The total solution volume was obtained by drying and weighing the specimen and then the original pore water chloride ion concentration was obtained. Finally, the gas in the gas collection bag was analyzed for its components. Specifically, 1 mL of gas was taken out of the sampling bag with a syringe and injected into a gas chromatograph to analyze the gas components. The remaining gas was then tested for ignition. Figure 1 presents the procedure of operations. In addition, the core sediment physical properties were also measured after measuring the gas and water components from the decomposed hydrate.

2.2 Experimental apparatus and methods

2.2.1 Infrared thermal imaging analysis

Due to the local temperature inhomogeneity caused by the heat absorption of hydrate decomposition, the temperature field distribution at the section of the core can be obtained by using the infrared thermal imaging technology. Then the hydrate distribution area can be quickly obtained. In this study, the portable infrared thermal imager (FLIR T1010, FLIR Systems, Inc. US) was used to measure the temperature field distribution in the gas hydrate samples. Before the measurement, the hydrate core from the sampler was first cut into two lobes. In this process, there might be some heat absorption during hydrate decomposition.

2.2.2 Resistivity and ion concentration measuring instrument

In this study, the multi parameter portable measuring instrument (SG78 Mettler Toledo) was used to measure and analyze the concentration of chloride ions and electrical conductivity in the pore water of the core. All the extraction and measurement of the pore water of the NGH samples were completed in the on-site shipboard laboratory. Before the trial, due to the fact that the natural pore water content in the sample is not enough, a certain volume of centrifugal pore water could not be directly obtained. Therefore, the quantitative deionized water was used to dilute the mud, and then the chloride ion concentration and conductivity of diluted pore water was measured. At last, according to the core moisture content and quality of weighing, the original ion concentration and the resistivity of the core samples were inferred by calculating the dilution ratio.

2.2.3 Particle size distribution measurement

The grain size of the sediment was measured and analyzed by using a laser granularity analyzer (Mastersizer 2000, Malvern Instruments Co. Ltd., UK). According to the Chinese National Standard (GB/T 19077.1—2008), 0.5 g sediment samples were put into 1000 mL deionized water. After adding 0.5 mol/L dispersant of sodium hexametaphosphate, the samples were stirred for 1 min to completely disperse the samples. The measured particle size ranged from 0.02 to 2000 μm, with a relative standard deviation of less than 1.5%. Each group of the sample was measured three times to take the average value.

2.2.4 Heat transfer characteristic test equipment

In this work, the heat transfer coefficient, thermal diffusivity and specific heat capacity of gas hydrate core sediments samples were measured by the Transient Plate Heat Source Technology (TPS2500S, Hot Disk, Sweden). The measured thermal conductivity of the instrument ranged from 0.005 to 500 W/(m·K). The protective layer material of the probe was polyimide film (Kapton), the resistance temperature coefficient (TCR) of the sensor material was 0.004665 K^–1, the sensor radius of the probe was 3.189 mm (at a test accuracy of ±3%). The sample only need to be kept relatively flat. The probe was placed in the sample to ensure that the sample was kept at a constant room temperature (25°C) before the experiment and during the experimental measurement to avoid temperature changes. Each group of samples ran 200 data transient test points. Finally, the three thermal physical properties of the samples were calculated.

2.2.5 Gas chromatographic analyzer

In this work, the gas chromatographic analyzer (GC7900, Shanghai Tianmei Scientific Instrument Co. Ltd., China) was used to measure the gas composition from the decomposed hydrate on the shipboard laboratory. Carrier gas was supplied by the hydrogen generator. The gas chromatograph capillary column packing was divided into non-polar columns. For the non-polar column, the peak order was mainly determined by the boiling point of the sample itself (molecular weight), the higher the boiling point, the slower the peak. According to the chromatographic peak area, the content of each component was calculated by using the normalization method. Finally, the response value of each component was corrected by the correction factor, and the impurity gas like air/N₂ was removed.

2.2.6 Measurement of SEM and X-ray diffraction (XRD)

In this study, field emission SEM (Nova Nanosem 450, FEI Co., USA) was used to analyze the microscopic surface structure and composition of hydrate samples at different depths. Before the experiment, 5 g naturally air-dried samples were taken from each group and kneaded into fine powder by hand. To prepare the powder layer with uniform distribution and appropriate density, the conductive adhesive bonding method was selected for the sample preparation under SEM in this study. The conductive tape (C tape) was pre-pasted on the sample table, and then a little marine soil powder was sprinkled on the tape. After that, the washing ears ball was used from all directions to puff away particles not firmly bonded, and then the washing ears ball was used gently to blow away the conductive adhesive tape bonding loose powder, thus making the layer a uniform layer of powder. Due to the non-conductive characteristics of marine soil powder, the powder needs to be sprayed with gold before being able to be observed in electron microscope to obtain high quality images. In addition, the powder XRD techniques (D8 ADVANCE, Bruker AXS, Germany) was used to measure the mineral constituent of the hydrate-bearing sediments. The detector is the LynxEye semiconductor one dimensional array detector. A Cu radiation was used at a wavelength of 1.5406 nm. The scanning angle was 5° with 2θ at a rate of 1.0 s/step (2θ/s). A computer software (TOPAS) based on the Rietveld quantification method was used to analyze the XRD data.

2.2.7 X-ray CT measurement

The CT visualization imaging of natural cores visualizes the spatial structure of cores, presenting the accurate density information of different lamellae of cores. The instrument can be employed to analyze the degree of accumulation, geological grain information, and fracture distribution of rock layers at different depths effectively. In this study, X-ray CT was used for pore saturation analysis of hydrate sediment samples at different locations and in situ X-ray CT scans (SMX-225CTX-SV, Shimadzu Co., Japan) with a resolution of 40 μm were performed on samples with plastic barrels. All hydrate samples were separated by a sampler and then transferred to a CT sample chamber with liquid nitrogen cryopreservation for measurement.

3 Results and discussion

3.1 Infrared thermal imaging analysis of hydrate core samples

The depth of the ultra-deepwater is defined as the water depth of over 1500 m. In this study, the seabed depth of the core geological sampling is approximately 1700 m. The sample depth is defined as the depth below the marine mud line. An in situ section of the sample cores was performed. Figure 2 shows the infrared images of the core samples of four different depths (123.7–130.0 m below seafloor (mbsf)). The sediments containing large blocks of white hydrates were found in the samples at a sampling depth of 123.7–124.4 mbsf. Based on the images, it is known that the temperature of the entire sample is relatively low. The temperature at the sampling point was –1.9°C, and the lowest temperature in the imaging area was –3.1°C. After the hydrate block was ignited, the bright yellow flame region temperature reached 89°C. In the sample at a sampling depth of 127.0 mbsf, the infrared image was entirely black due to the low temperature of the entire marine sediments caused by the hydrate decomposition. The infrared images in Fig. 2 also indicate that the distribution type of hydrate in the high saturation area at depths of 123–124 mbsf is mainly block filling type at the Lingshui Sag area in Qiongdongnan Basin in the South China Sea.

3.2 Gas hydrate saturation estimation by measuring the resistivity and ion concentration

3.2.1 Measurement of the resistivity and ion concentration of the pore water

The electrical conductivity of pore water and decomposed water was measured using a portable electric resistance instrument and compared. A portable chloride ion meter was used to measure chloride ion concentration in pore water for a total of 17 samples. Table 1 shows that chloride ion concentration in the decomposed water of the samples fluctuates between 1.40 × 10⁴–4.0 × 10⁴ mg/L. The electrical conductivity varies between 35 and 85 ms/cm.

3.2.2 Estimation of gas hydrate saturation

In an open system, the excluded ions diffuse away sometime after the formation of gas hydrate. Therefore, the water samples recovered from a gas hydrate zone after gas hydrate dissociation during drilling have a lower salinity than those from zones that do not contain gas hydrate. Therefore, the saturation of hydrate can be estimated according to this principle. As the saturation of hydrate increases, the dilution proportion of pore water increases after hydrate decomposition. Based on this, in this study, the gas hydrate saturation S was estimated by Eq. (1) from the calculation of chloride ion anomalies [31].

(1)S={\displaystyle \frac{\mathrm{1}}{{\rho }_{\mathrm{h}}}}(\mathrm{1}-{\displaystyle \frac{C{l}_{\mathrm{pw}}}{C{l}_{\mathrm{sw}}}}),

where {\rho }_{\mathrm{h}} = 0.9 represents the density of pure gas hydrates, C{l}_{\mathrm{pw}} denotes the measured chloride ion concentration in pore water samples, and C{l}_{\mathrm{sw}} denotes the baseline chloride ion concentration in normal pore water.

The saturation was estimated based on these results. The measurement and estimation results are listed in Table 1. In the estimation of sample hydrate saturation, samples from the depths of 97.2 mbsf and 100.7 mbsf were used as the baseline chloride ion concentration. In fact, based on the field logging data and gas composition analysis of the sampled cores, it can be found that there are two hydrate enrichment horizons from 0 to 180 mbsf in this area. No hydrates were found to expose on the seafloor surface at 0–1 mbsf. The field data analysis indicates that there is a low saturation hydrate layer at 10–90 mbsf, while there is a massive hydrate distribution layer at 100–180 mbsf. Therefore, two baseline chloride ion concentrations were given for these two hydrate layers, respectively. According to the results, the hydrate saturation of the samples fluctuates between 2% and 60%. The sample at a sampling depth of 151.7 m had the highest estimated hydrate saturation of 54.75%.

3.3 Analysis of water content, specific gravity, and particle size characteristics of hydrate-bearing sediments

Water content and specific gravity represent the basic physical properties of the marine natural core. Water saturation in the core pore has substantial effects on hydrate saturation. Therefore, it is a significant input for assessing the pattern of hydrate mineral deposit occurrence and resource quantity. With excessively low water saturation, there is insufficient water to form adequate reserves of hydrate-bearing sediments with methane gas for practical exploitation. Meanwhile, excessively high-water saturation hampers gas from diffusing into the pores and reduces the gas volume during the production process. Moreover, it also affects gas migration in the production process. Gas production will be accompanied by a large amount of water production. Therefore, an appropriate amount of water content is a key factor for hydrate concentration and viable exploitation. The specific gravity of the natural core represents the inherent density property of core particles. It is an important determinant of the movement characteristics of the particles in the water production stage during methane hydrate-bearing sediments production. The particle density determines the critical settling velocity and movement pattern of particles in fluid flow. Thus, it has implications for addressing the issues of blockage of gas transport pipeline and gas migration. Therefore, specific gravity property is an important parameter in pipeline transport flow safety studies. The water content test was conducted using 30 g of each sample. The experimental results are tabulated in Table 2. The results indicate that the water content of the samples is about 30%–50% and the specific gravity is about 2.49–2.72. The results are similar to those in Ref. [11].

The particle size of hydrate-bearing sediments at different locations was analyzed by a laser particle size analyzer. With the laser particle size distribution (PSD) instrument (Mastersizer 2000, Malvern Instruments Co. Ltd., UK), the data in Table 3 were obtained. Based on the data in Table 3, the median diameter and PSD of sediments at different sampling depths are demonstrated in Fig. 3. The particle size ranges from 0.4 to 300 μm, as shown in Fig. 3(a). It is obviously observed that the median diameter of the sediments changes slightly with the depth, and the diameter was mostly within the range of 15–35 μm (seen Fig. 3(b)). The median diameter of sediments between the deepth of 0.1–12.3 m near the surface and of the sampling depth of 123.7–172 mbsf is relatively large, reaching more than 20 μm. The median diameter reaches a maximum of 34.112 μm at a sampling depth of 0.8 mbsf. The median diameter of the sediments at the sampling depth of 27.2–100.7 m is relatively small, in the range of approximately 15.0–18.0 μm, indicating that a large area of the geological environment with relatively smaller particle size may exist at the depth.

3.4 Analysis of thermal characteristics of hydrate-bearing sediments

The heat transfer properties of natural cores are important basic physical properties of hydrate-bearing sediments. Research shows that the effective heat transfer is an important control mechanism for hydrate, and the latent heat of the sediment layer and the effective heat exchange with the surrounding strata are decisive factors to ensure the continuous and efficient extraction of hydrates. In contrast, the thermal conductivity of natural cores directly determines the thermal conductivity of cores, which, in turn, affects the heat transfer during hydrate decomposition, having significant implications on the production efficiency and economic value of hydrate-bearing sediments. In addition, the core thermal properties determine the decomposition interface.

The thermal conductivity of hydrate sediment particles at different locations was analyzed using thermal conductivity apparatus for at least six samples. The heat transfer characteristics of the sediments are mainly affected by their effective density and composition [32]. The heat transfer pathways include particles to particles, particles to the liquid phase and then to particles, and pathways through the liquid that fills the pores. The heat transfer characteristics of gas hydrate cores were measured using the hot disc for the flat plate heat source method [33]. The experimental results summarized in Table 4 provide data for developing hydrate extraction methods. The analysis shows that the thermal conductivity of the 2019 expedition samples ranges from 0.75 to 0.96 W/(m·K). Compared with the thermal properties of other foreign gas hydrates in test production reservoirs, both the sediment thermal conductivity and the heat conduction during hydrate decomposition are slow, which can easily cause the low decomposition rate, and sometime even secondary hydrate generation when using depressurization methods during production. Table 5 provides the thermal conductivity of some different materials [27]. The thermal conductivity measurements of sediments from Ulleung Basin, South Korea in the laboratory carried out by Kim et al. were used as a reference [34]. It is observed that the thermal conductivity of the 2019 expedition samples is slightly higher than that of silica, sand, and air. The results justify the use of silica sand to simulate the hydrate-bearing sediment layer in the laboratory [35–37].

3.5 Gas composition analysis of hydrate samples

The pressurized gas hydrate samples were decomposed in situ for collecting the decomposed gas. A portable gas chromatograph was used to detect hydrate decomposition gas components online for a total of six samples, and the results are presented in Table 6. The analysis suggests that the main components of the gas are methane, ethane, and carbon dioxide. In all samples, the content of methane exceeds 80%, reaching 95.83% at 67.2 m. The content of ethane fluctuates between 4% and 16% and that of carbon dioxide is generally lower than 4%.

3.6 Analysis of mineral composition of marine sediments

The gas hydrate-bearing samples were measured using XRD to investigate the mineral component of the sediments. It is known that the XRD data can assist to identify the crystalline phase of different minerals of mixed phases in soil. The mineral components of the sediment were calculated based on the XRD data. The sediments were sampled at a depth of 6.20–151.70 mbsf and were identified petrographically using an XRD analyzer. Figure 4 manifests the results of the XRD spectra analysis of the sediments. From the XRD spectra results, it can be noticed that with the increase of the depth of the seabed, the variety of mineral components in the sediments gradually decreases. The main mineral components are quartz, mica, calcite, illite, imvite, and etc.

In addition, the chemical composition and the proportions of each component were also analyzed. The results were exhibited in Fig. 5. The minerals consist mainly of quartz, clinochlore, sericite, sodium feldspar, calcium carbonate, and illite with a mass fraction of 23%, 17%, 15%, 14%, 10%, and 10%, respectively. The other components may be non-crystalline phase materials or minerals of lower content. Figure 5(b) shows the results of the chemical analysis of the sediments. It can be observed that the sediments mainly consist of silicon dioxide, silicon trioxide, calcium oxide, iron trioxide, potassium oxide, magnesium oxide, sodium oxide, sulfur trioxide, and titanium dioxide with a mass fraction of 54%, 15%, 5%, 5%, 4%, 3%, 2%, 2%, and 1%, respectively, along with some other chemicals.

3.7 CT pore characterization of hydrate particles

For loose siltstone formations, hydrates are mostly distributed in sheets or veins in large quantities. Meanwhile, for more dense rock formations, hydrates are mostly scattered in rock pores, which have a lower permeability and lower saturation due to the hydrate occurrence pattern, which is not conducive to hydrate production. For hydrate-bearing cores, because the concentrated and enriched hydrates have a large density difference with the core, the use of CT equipment can directly visualize the hydrate occurrence location in the core and realize rapid hydrate detection. Meanwhile, the volume content of the hydrate can be directly calculated, and then the hydrate saturation can be obtained. In addition, the use of CT images for pore network representation can generate information about the permeability of hydrate cores. The saturation of hydrates and permeability of hydrate cores are important parameters for safe and efficient hydrate production. Therefore, the use of CT for core visualization is an important tool for comprehensive core characterization.

In this study, the X-ray CT scanning results of the experiments on samples at a depth of 172 mbsf recovered from the 2019 expedition are displayed in Fig. 6. It can be observed that some of the samples had fractured due to water evaporation. However, vein-like fractures, large fractures, and holes can be observed in the images of natural seabed sediments. It should be noted that the hydrate sediment samples in this study were scanned at atmospheric pressure under liquid nitrogen cooling. Therefore, the pore water in the cracks in the scanned images will freeze the core samples. Besides, the fine dispersed hydrates in the pores may not be easily distinguished from the ice. Those black area and small holes in the images are possible spaces for the occurrence of gas hydrates [38]. Water evaporation and core fractures can also influence the internal structure of natural seabed sediments, affecting the experimental results and posing difficulties to determining the possible areas of hydrate presence.

3.8 Surface structural characterization of hydrate particles by electron microscopy

The surface properties of natural core particles determine the surface roughness of the particles, the morphological size of the particles and their properties, the contact angle of water, and the specific surface energy, which, in turn, affects the attachment of water to the particle surface and thus the occurrence structure of hydrates. It is shown that rough particles contribute to the generation of highly saturated hydrates. In turn, the sphericity and morphological properties of the particles affect the accumulation structure of the particles and the pore characteristics of the sedimentary layer where hydrates occur. Therefore, the size of the porosity directly affects the resource assessment of hydrate-bearing sediments and the gas leakage during anomalous decomposition of gas hydrates.

Pore saturation analysis of hydrate sediment samples at different locations was conducted using SEM for at least six samples. The surface characteristic images of the samples at different depth were obtained as shown in Fig. 7. It can be observed that the sediment particle size distribution is similar at different depths, the particle shape is irregular, and the microstructure is flaky, with clay flakes and fine particles mostly less than 5 μm in size, with a few large particles up to 10 μm in size. Large particles have many clay flakes and fine particles attached to the surface. Such characteristics will cause extremely low-permeability characteristics.

4 Conclusions

In this study, a comprehensive geotechnical analysis of marine gas hydrate-bearing sediment cores from the ultra-deepwater area at Lingshui Sag area of Qiongdongnan Basin in the South China Sea were conducted. The physical properties of the sediment samples in the targeted development area, including soil index properties associated with sediment particles, ion analysis of pore water, gas composition analysis, X-ray CT-scan data, surface microstructural features, thermal transport properties, and mineral compositions analysis were measured and discussed in detail. The dominating conclusions obtained were summarized as follows:

The gas hydrate in the core samples is mainly methane hydrate with a methane content of approximately 95%. The other components are ethane and carbon dioxide. The hydrate saturation of the samples fluctuates between 2% and 60% according to the estimation of chloride concentration difference.

The core sediment thermal conductivity ranges from 0.75 to 0.96 W/(m·K). The thermal conductivity of hydrates at the study site is relatively low, suggesting that it may be difficult to use only thermal stimulation for gas hydrate extraction. The more accepted method nowadays is the pressure reduction method, but the process of pressure reduction is associated with ice generation and secondary hydrate generation problems. Therefore, a combined approach is encouraged for natural gas production technologies.

The gas hydrate samples were analyzed by field scanning electron microscopy to observe the structure of the sample particle surface. It can be observed that the sediment particle size distribution is similar at different depths, the particle shape is irregular, and the microstructure is flaky, with most of the clay flakes and fine particles less than 5 μm in size, and a small number of large particles reaching 10 μm in size. Clay flakes and fine particles are attached to the surface of large particles in large numbers. Such characteristics will lead to insufficient permeability during the production process. In addition, the median diameter is less than 40 μm. Therefore, attention should be paid to the prevention and control of sand production in the mining process.

Water content characterizes the basic physical properties of marine natural cores. The water content of the samples is approximately 30%–50%, and the water saturation in the pore space of the core affects the hydrate saturation substantially and is therefore important for hydrate deposit generation patterns and resource assessment. With an excessively low water saturation, there is no sufficient water to form adequate reserves of hydrate-bearing sediments with methane gas for viable exploitation. Meanwhile, excessively high-water saturation hampers gas from diffusing into the pores and reduces the gas volume during the production process. The gas production will be accompanied by a large amount of water production. Therefore, appropriate water content is a key factor for concentrated hydrate enrichment and reasonable production.

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