Fundamental characteristics of gas hydrate-bearing sediments in the Shenhu area, South China Sea

Abstract

The basic physical properties of marine natural gas hydrate deposits are important to the understanding of seabed growth conditions, occurrence regularity, and occurrence environment of natural gas hydrates. A comprehensive analysis of the core samples of drilling pressure-holding hydrate deposits at a depth of 1310 m in the Shenhu area of the South China Sea was conducted. The experimental results indicate that the particle size in the hydrate sediment samples are mainly distributed in the range from 7.81 µm to 21.72 µm, and the average particle size decreases as the depth of the burial increases. The X-ray CT analytical images and surface characteristics SEM scan images suggest that the sediment is mostly silty clay. There are a large number of bioplastics in the sediment, and the crack inside the core may be areas of hydrate formation.

Graphical abstract

Keywords

natural gas hydrate / Shenhu area / reservoirs characteristics

1 Introduction

Natural gas hydrates are one of the most promising strategic development resources [1,2], not only in terms of energy supply, but also in some industrial applications like energy storage, gas separation [3], sea water desalination [4], and transportation [5]. Although gas hydrate reservoirs may present significant energy reserves, energy recovery over the past decades has proven to be extremely challenging [6,7]. Thus far, researchers have proposed four main methods, namely thermal stimulation [8–13], depressurization [14–17], inhibitor injection, CO₂/N₂ replacement [18], and combined methods to exploit these reserves [19–21]. From the perspectives of high production efficiency, and to avoid ice blockage and hydrate re-formation [22,23], the combination of depressurization and thermal stimulation is considered as a profitable method for hydrate exploitation [21].

Basic physical properties of marine natural gas hydrate deposits are important to the study of seabed growth conditions, occurrence regularity, and environment of occurrence of natural gas hydrates. Besides, they also provide an important geophysical basis for the development of efficient and safe mining technology for natural gas hydrates [24,25]. China began to explore and evaluate natural gas hydrate resources in the South China Sea in 1999 [26]. But the first gas hydrate expedition (GMGS1) was performed in the Shenhu area in 2007 [27], where the area had been confirmed as a very promising reservoir for gas hydrate exploitation. Due to the complex geological environment of marine natural gas hydrates, natural gas hydrate reservoir evaluation is verified and calibrated by drilling sampling [1,24,25]. After years of technical research and sea trials, the China National Offshore Oil Corporation (CNOOC) successfully completed natural gas hydrate drilling sampling at a depth of 1310 m in the Shenhu area in the South China Sea, and 12 m of natural gas hydrate-rich insulation and pressure core samples were obtained from two drilling and detecting wells and three sampling holes.

To minimize the evaporation of water and maintain the sample structure, the core samples are always taken with liquid nitrogen (–196°C) insulation during storage and transportation, and stored at a low temperature of –78°C in the laboratory. Then, a series of basic physical property analyses for hydrated cores of different depths in the Shenhu area in the South China Sea were conducted, including characteristics of sediment particle size distribution and analysis of hydrate sediment microscopic surface structure, which are surface characteristics (SEM), and X-Ray CT in situ image analysis. After national offshore exploration and sampling technology of the United States and Japan [1], China has become the third country to master the core technology of marine natural gas hydrate insulation and pressure-holding sampling and mineral deposit evaluation. Obtaining the true sample (maintained in situ temperature and pressure) of marine natural gas hydrate insulation and pressure-holding is an important material basis for studying the pore characteristics and description of natural gas hydrate reservoirs. It is also an important basis for evaluating the natural gas hydrate resource and selecting the mining process.

2 Experiment

2.1 Materials

The core samples at different sites (#1A, #2A, and #2B) used in this experiment are obtained from the seabed core at a depth of 1310 m in the Shenhu area in the South China Sea where the coring location could be found in Ref. [28], as shown in Fig. 1.

2.2 Experimental apparatus

The particle size distribution, surface structure and three-dimensional skeleton structure of the hydrate-bearing sediments were investigated. The experimental apparatuses are listed in Table 1.

3 Results and discussion

3.1 Analysis of hydrate core sample particle size

In this work, according to the Standardization Administration of the People’s Republic of China (GB/T 19077.1-2008), 0.5 g of the sediment sample was placed in 1000 mL of deionized water, 0.5 mol/L of dispersant sodium hexametaphosphate was added, and the sample was completely dispersed by stirring for 1 min. A laser particle size distribution analyzer was used in this study. It was found that the position of the sediment was mostly between 7.81 and 21.72 µm as shown in Table 2. It can also be observed from Fig. 2 that the median diameter gradually decreases with increasing depth. The median diameter of the hydrate layer particles near the surface layer is relatively large, reaching nearly 21.7 µm. Below the baseline of the hydrate layer of 100 mbsf (the distance from the seabed to the upper boundary of the hydrate layer), there is a geological environment with a relatively smaller particle size, which may be conducive to the enrichment of hydrates [29].

The particle size distribution of different hydrate layers is obvious, as depicted in Fig. 3. The results indicate that the particle size distribution of the hydrate layer ranges from 0.2 µm to 200 µm. An analysis of the particle size accumulation indicates that the sediments in the South China Sea mainly comprise clay (<4 µm) and fine sand grains (4–64 µm). The particle size is very fine, and the hydrate layer is distinct from the non-hydrate layer. Usually, the hydrates tend to occur in coarser particles. Based on the Shepard standard [30], the composition of each component of the sediment was obtained. As demonstrated in Fig. 4, the particle size below 40 µm reaches 85.5%, which also indicates that the soil in this area comprises argillaceous silty clay.

According to the particle size distribution data of Fig. 3, the curves of particle size distribution can be randomly divided into two parts. The ratios of the large particle size D₁₅ to the fine particle size D₈₅ from different depths are exhibited in Fig. 5. Along with the results of Root Gustafsson et al. [31], in order to prevent particle migration, the filter requirement is that the coarse particles of D₁₅ are more than 4 times that of fine particle sediment D₈₅. However, the results show that the ratio of D₁₅/D₈₅ of all samples are less than 4. Therefore, it can be concluded that the sediments in the South China Sea have a self-filtration effect, which effectively prevent sediment migration by reducing the permeability of the reservoir during the mining process, which is also of great significance for the continuous mining of hydrates. However, due to the extremely fine particle size of the sediment, such fine particles in the flow of gas and water are likely to cause blockages in the wellbore during mining. Multiple filters according to the particle size distribution are arranged to prevent silt accumulation and blockage of the wellbore.

The analysis of the natural core particle sizes provides information on particle size distribution ranges and the corresponding proportion of core particles, which directly reflects the size and concentration of the core particles. The size of the particles is of great significance for their spatial arrangement. Particle size distribution affects the spatial packing of the particles, which affects the porosity and permeability of the deposited layer. The permeability characteristics of the sediment and sand affect the process of gas transportation. Based on this information, anti-blocking measure can be taken. The analysis of existing data suggests that the core region with a double peak value is favorable for hydrate accumulation, and the proportion of each peak region may also affect the saturation distribution of hydrates. The particle size distribution range is large, and the small particles fill the pore spaces formed by the arrangement of large particles, resulting in a serious decrease in porosity. When the particles are concentrated at a location, a spherical accumulation is seen, which is similar to the ideal case. This arrangement has a porosity of about 38%. Porosity determines not only the size and amount of hydrate but also, to a certain extent, the permeability characteristics of the sediment, thus affecting the efficiencies of gas and water production. In addition, during the process of gas transportation, it is necessary to adopt sand control and anti-blocking measures corresponding to different particle size ranges for the safe and efficient transportation of natural gas without the leakage of methane gas caused by pipeline blockage. The leakage of methane gas negatively affects the marine ecological environment.

3.2 Analysis of surface structure characteristics of hydrated core samples using electron microscopy

The surface characteristics of the hydrate deposit samples from different locations were observed using SEM scanning electron microscopy. First, 10 g of the sample was taken and dried. Then, a small spoonful (about 0.01 g) of the dried sample were applied to conductive tape. This was first subjected to spraying gold treatment, and then to field emission scanning electron microscopy (Nova Nano SEM 450, FEI Co., America) to obtain a surface characteristic image of the sample, as displayed in Fig. 6.

It can be seen from Fig. 6 that the grain size characteristics of the sediments at different depths is similar, the shape of the particles is irregular, and the microstructures are flaky. The clay flakes and fine particles with particle sizes less than 5 µm account for most of the deposit, and a small number of large particles of size up to 20 µm make up the remainder. A large number of clay flakes and fine particles are adhered to the surface of large particles. Such features result in extremely low permeability characteristics. The gas hydrate core is composed of clay and silt, and the B-04 sample contains many nano-pores, which may indicate traces of diatom microorganisms and that the hydrate survival conditions have a certain symbiotic relationship with microbial accumulation. The analysis of the core composition using X-ray diffraction indicates that 80% of the core composition is quartz (SiO₂), 15.5% is carbonate (CaCO₃), and a small amount of clay minerals, feldspar, and other components make up the rest, which is similar to the results in Refs. [25,32]. Figure 7 shows the elemental composition and content of the core sediment from (#1A-03), measured by an EDS spectrometer. The purple zone in Fig. 7 stands for the energy detection area of the instrument. It can be seen that there are several elements in the mineral, some of the clay with iron-containing. The hydrate deposit layer with the presence of clay tablets is extremely fine, resulting in an extremely low permeability which is detrimental to gas-liquid flow during the process of mining for natural gas hydrate.

The surface characteristics of natural core particles determine the surface roughness of the particles while the size and morphology of the particles determine the spatial structure of the particle accumulation. The surface roughness of the particles determines the wetting characteristics of the particles, the contact angle of the water with the particles, and the specific surface energy. Besides, it also affects the adhesion of water to the surface of the particles, thereby affecting the structure of the hydrate. Previous study [29] has shown that coarse particles contribute to high saturation hydrate formation. The morphological properties of the particles affect their accumulation structure, which affects the pore characteristics of the sediment. Hydrates exist in pores. Therefore, porosity directly affects hydrate reserves. The influence of porosity on hydrate reserves can be determined via a resource assessment of the reserves. Leakage of gas when the hydrate is abnormally decomposed and other problems can be detected.

3.3 Analysis of pore characteristics of hydrate core samples

The pore saturation analysis of hydrate deposit samples from different locations was performed via X-ray CT. In situ X-ray CT scans (SMX-225CTX-SV, Shimadzu Co., Japan) were performed on samples with plastic drums with a spatial resolution of 40 µm. Before conducting the X-ray CT tests, the pressure of the in situ condition was maintained when core samples recovered from the seafloor with the help of sampler. Then, the hydrate core samples were transferred to the liquid nitrogen environment. Hydrate samples decompose very slowly under the self-protective effect, which also should be highly conducive to the X-ray scanning of microstructure of sediment core samples. The X-ray microstructure results for samples from different drilling positions are presented in Fig. 8.

The selected CT images show that marine soil cores exhibit a homogeneous whole and the main structure characteristics of the core are silty clay minerals with a few cracks. Therefore, it is predicted that there may be no massive or flaky natural gas hydrates in the seabed area. A fourth type of hydrate occurrence structure was detected in these scans, the scattered hydrate, which can be predicted to be distributed in the dense pores of these cores. To see the pore distribution of the natural core, the resolution of the CT equipment was adjusted to the limit of 2 µm. However, the pore distribution could still not be resolved. The pore sizes of the natural sediment were found to be extremely small, which caused difficulties in the research on the pores of the natural seabed sediment. For sample #1A-02, some portions were broken due to the evaporation of water. However, it can be observed from the image that there are choroidal cracks, large cracks, and pores in the natural seabed sediment, making them possible places where natural gas hydrate is likely to exist. Water evaporation, core fracture, and other factors also affect the internal structure of natural seabed sediments. This distortion of the sediments affects the experimental results, making it difficult to judge the possible areas with hydrates.

The CT visualization of natural cores represents the true spatial structure of the core and accurately extracts the density information of the different layers of the core. This information assists in the effective analysis of the accumulation degree, geological line information, and fracture distribution in rock layers of different depths. In loose sediments, hydrates are mostly distributed in flaky or choroidal forms. In dense rock formations, hydrates are mostly scattered within the pores of the rocks. The permeability of such rock formations is low, and the mode of hydrate formation leads to a low saturation, which is not conducive to the exploitation of gas hydrates. In the core of hydrate, due to the large density difference between the concentrated hydrate and the core, the CT equipment can directly realize the visualization of the location of the hydrates in the core, achieving the rapid detection of the hydrates. The volume content of the hydrate can also be directly calculated, which can then allow the determination of the saturation of the hydrate. In addition, by performing pore network extraction on the CT images, the permeability characteristics of the hydrate core can be obtained. The hydrate saturation and core permeability are important parameters for safe and efficient mining. The degree of saturation determines the amount of resources required for commercial mining and the economics of commercial mining. The permeability characteristics directly affect the migration characteristics of gas, water, and sand during the process of gas extraction, thereby affecting the efficiency of gas extraction. The permeability also determines the escape and leakage characteristics of methane gas, which have serious impacts on the seabed ecological environment and atmospheric environment. Table 3 shows the representative calculation results for porosity in all hydrate cores. The porosity of the hydrate core is about 43% and the moisture content of the hydrate core is 31.908%.

4 Conclusions

In this paper, laboratory experiments were conducted to investigate the physical properties of gas hydrate-bearing sediments from the South China Sea. The main conclusions are as follows:

The median diameter of the core samples in the Shenhu area in the South China Sea decreases gradually as burial depth increases, most of which are between 8 and 22 µm. The particle size distribution is the same at different depths, and the particle sizes mainly lie between 0.221 µm and 174.55 µm. The amount of particles below 40 µm is 85.5%.

There may be no massive or flaky natural gas hydrates in the seabed area, and the hydrates exhibit a fourth type of occurrence structure where scattered hydrate is distributed in the dense pores of the core.

The mineral composition of this region is mainly 80% quartz (SiO₂) and 15.5% carbonate (CaCO₃). The hydrate deposit layer is an argillaceous silt type mineral, and the particle size in this deposit is extremely fine, which is not conducive to the flow and infiltration of gas-water.

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