Neng-you Wu, Chang-ling Liu, Xi-luo Hao

a Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

A B S T R A C T

This paper provides an overview of the developments in analytical and testing methods and experimental simulations on gas hydrate in China. In the laboratory, the analyses and experiments of gas hydrate can provide useful parameters for hydrate exploration and exploitation. In recent years, modern analytical instruments and techniques, including Laser Raman spectroscopy (Raman), X-ray diffraction (XRD), X-ray computed tomography (X-CT), scanning electron microscope (SEM), nuclear magnetic resonance (NMR)and high pressure differential scanning calorimetry (DSC), were applied in the study of structure, formation mechanisms, phase equilibrium, thermal physical properties and so forth of gas hydrates . The detection technology and time-domain reflectometry (TDR) technique are integrated to the experimental devices to study the physical parameters of gas hydrates, such as the acoustics, resistivity, thermal and mechanical properties. It is believed that the various analytical techniques together with the experimental simulations from large-scale to micro-scale on gas hydrate will play a significant role and provide a powerful support for future gas hydrate researches.

Keywords:

Gas hydrate

Analytical method

Experimental simulation

Detection technique

1. Introduction

Gas hydrate is an ice-like solid substance formed by the combination of low-molecular-weight gas molecules with water molecules, which occurs naturally in the permafrost and sediments of the continental slope with a water depth greater than 300 m. Two facts, namely the potential energy resources and the geo-hazards resulting from large scale slope destabilization, drive the importance of marine gas hydrate systems to both the economic society and the natural environment(Maslin M et al., 2004). Since methane hydrate constitutes the major part of natural gas hydrate, its destabilization will release methane, a potential greenhouse gas, and affect the global climate (Dickens GR et al., 1995; Maslin MA et al., 2003).Gas hydrate and its associated sediments have also become an academic focus of biogeochemical study on the deep biosphere (Wellsbury P et al., 2000).

Researches on gas hydrate experienced four milestone phases: laboratory study/scientific curiosity (1810-1934), pipe clogging, clogging prevention and control (1934-1969), resource investigation (1969-2002) and exploitation and utilization (2002-present). In China, gas hydrate study started in the middle of the 1990s, and developed significantly during the past two decades. The progress in exploration, exploitation and research of gas hydrate in China owes much to the joint effort of the China Geological Survey (CGS), Chinese Academy of Sciences (CAS), Ministry of Science and Technology (MOST), National Natural Science Foundation of China (NSFC), State Oceanic Administration (SOA), and many universities. The funded research projects cover gas hydrate’s stable condition, formation and dissociation mechanism, prospecting technology, geophysical and geochemical features, occurrence and distribution as well as their controlling factors, inventory evaluation, possible environmental impacts, analytical method and experimental simulation, and production testing on the hydrate deposits.

In May-July 2017, China successfully conducted production tests of gas hydrate in the Shenhu area, northern South China Sea. The physical properties of gas hydrate-bearing sediments are important basic parameters for the gas hydrate production test. The analyses and experimental simulations of these parameters play important roles in gas hydrate produc-tion design. The analyses on hydrate samples provide direct information about the characteristics of hydrates and hydratebearing reservoirs, as some of the properties of gas hydrate and hydrate-bearing sediments can only be investigated with natural samples, for example structure-related features, guest composition, cage occupancy, and so on. The experimental simulations provide a general view of gas hydrate production.In this paper, we provide an overview of the latest progress in analytical methods and experimental simulations for gas hydrate.

2. Gas hydrate analytical and testing techniques

The basic physical and chemical properties are helpful to understand gas hydrates both naturally occurring and artificially prepared. Traditional methods for gas hydrate analysis mainly focus on the determination of pressure, temperature and ion components, based on macroscopic and microscopic devices, such as visual autoclave and related data acquisition devices. In recent years, modern analyzing instruments and techniques, including Raman, XRD, X-CT, SEM, NMR and high pressure DSC, were applied in the study of structure,formation mechanisms, phase equilibrium, and the thermal physical properties of gas hydrates. With the development of specialized experimental devices, a series of techniques have been successfully applied to gas hydrate-related researches.Thus far, the analytical and testing technology system of gas hydrate has been established (Fig. 1). For different research subjects, different test techniques are used to obtain specific parameter information.

Fig. 1. Analytical and experimental simulation system for gas hydrate study.

2.1. Structure of gas hydrate

Raman, XRD and NMR are recognized to be the three major methods for the study of the microstructure of gas hydrates. For simple gas hydrates, Raman is the most efficient way to determine the existence of hydrate, as well as the structure type, molecular composition and cage occupancy.By investigating the chemical bonds of guest molecules (such as C-C bonds, C-H bonds) and water molecules (O-H bonds),the structural information and hydrate number can be obtained. This technique is mainly used in the identification of structure, gas storage performance, formation and decomposition kinetics and in situ detection in the deep sea (Zhang X et al., 2017). Powder XRD and13C NMR are powerful methods to study the structural information of hydrate. XRD can be used to determine the hydrate crystal structure, lattice parameters, space group, cavity occupancy and hydration index parameters. Solid NMR technology is especially suitable in the structural study of multi-component gas hydrates. The technique has a high sensitivity in determining the chemical environment and dynamics of the guest molecules of different types of gas hydrate, therefore it can be used to study the structure and dynamics of gas hydrate, especially for complex and multi-component gas hydrates. Using these techniques, it is possible to study the structure information, as well as the dynamic changes in the processes of formation and dissociation, which is helpful in understanding the microscopic kinetics during the formation and dissociation of gas hydrate (Fu J et al., 2017; Meng Q et al., 2011b; Meng Q et al.,2015b).

Natural gas hydrate samples obtained in the Shenhu area in 2007, the eastern area of the Pearl River Mouth Basin in 2013 and the Qilian Mountain permafrost, have been studied using the Raman spectrometer (Liu C et al., 2015b; Liu C et al., 2017; Liu C et al., 2012b; Liu C et al., 2010a; Meng Q et al., 2011a; Meng Q et al., 2015a). The results indicate that gas hydrates obtained from the South China Sea and Pearl River Mouth Basin are both structure I hydrates and methane gas accounts more than 99% of the total gas content. In comparison, the Qilian Mountain permafrost gas hydrate is structure II,and the composition of the gas hydrates is relatively complicated (Fig. 2). Power XRD was also applied to study the gas hydrate samples obtained from the eastern area of the Pearl River Mouth Basin (Liu C et al., 2015b; Liu C et al., 2017).

2.2. Occurrence and microscopic kinetics of hydrate in sediments

The most challenging aspect of hydrate research is the shift from time-independent thermodynamics to kinetic measurements that are closely related to time. The microscopic kinetics of hydrate in sediments can be studied by means of nuclear magnetic resonance imaging (MRI) and X-CT. Using these instruments, the main factors that control the nucleation and growth of hydrate in the pore space of the sediments can be explored microscopically.

The large brightness difference between the free water and the solid hydrate is important for the effective observation of hydrate’s formation and decomposition process by MRI. In addition, the MRI signal is related to proton density. Therefore, MRI does not only distinguish different phase states in the system, but can also provide quantitative data of effective porosity, pore size distribution, fluid distribution and hydrate saturation variation of porous media. Relying on the MRI platform, Dalian University of Technology has carried out a number of research works related to hydrate formation, decomposition, fluid migration and saturation change in solution and porous media systems (Liu Y et al., 2010b; Yao L,2010). Qingdao Institute of Marine Geology has studied the kinetic processes of formation and decomposition of THF,CO2and CH4hydrate in unconsolidated porous media using MRI (Meng Q et al., 2012). Fig. 3 shows the in situ MRI images of the dissociation of THF hydrate.

Fig. 2. Summary of the laser Raman spectra of gas hydrates artificially produced and naturally occurring in different regions. PRM:Pearl River Mouth Basin, SH: Shenhu area, QMP: Qilian Mountain Permafrost (modified from Liu C et al., 2015a and Meng Q et al.,2015a).

Natural gas hydrate is mainly distributed in the pores or cracks of sediments. Understanding the occurrence of gas hydrate in the sediments is important in the study of the formation mechanism, exploration and evaluation of natural gas hydrate resources. X-CT provides a possible method for the direct observation of the microscopic distribution of hydrate in the pore spaces. It is also useful for studying the internal spatial structure of gas hydrate-bearing sediments and the decomposition process of gas hydrate. The earliest X-CT study of gas hydrate in China was performed by the Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences. The kinetic processes of the formation and decomposition of methane hydrate were observed by X-CT (Jiang G et al., 2005; Pu Y et al., 2007). Wu Q et al., (2006) studied the formation and decomposition process of methane hydrate in sands using X-CT. Industrial micron scale X-CT was introduced by Dalian University of Technology and Qingdao Institute of Marine Geology to study the hydrate microscopic distribution pattern and the hydrate forming process in the laboratory (Hu G et al., 2014a; Li C et al., 2013; Wang J et al., 2015; Zhang W et al., 2016). Fig.4 shows X-CT images of the distribution of methane hydrate in porous media.

2.3. Composition and isotopic features of gas hydrate

Natural gas hydrate is mainly composed of hydrocarbon gases (C1 to C5), and non-hydrocarbon gases (e.g. N2, CO2,H2S), while methane is the dominant gas component. The composition and relative concentrations of guest molecules in natural gas hydrate are important in determining the spatial structure and characteristics of the hydrate.

The pretreatment methods of gas hydrate samples, as well as the gas collection methods and the working conditions and parameters of the instruments have been well studied to establish an analytical procedure for the determination of the gas composition of hydrates with gas chromatography (GC)and/or gas chromatography-isotope ratio mass spectrometry(GC-IRMS) (He X et al., 2013; He X et al., 2011b; He X et al., 2012). Using the above methods, the gas components and isotopic features of natural gas hydrates obtained in the Shenhu area, as well as the eastern area of the Pearl River Mouth Basin and the Qilian Mountain permafrost were studied (Dai J et al., 2017; He X et al., 2015; He X et al., 2011a; Liu C et al.,2015a; Liu C et al., 2015b; Liu C et al., 2017; Liu C et al.,2012a; Liu C et al., 2012b; Wu N et al., 2011). The carbon and hydrogen isotopic features indicated that gases dissociated from the gas hydrate samples from the Qilian Mountain permafrost are primarily oil-derived in origin, while gases dissociated from the gas hydrate samples from the Pearl River Mouth Basin and the Shenhu area are dominated by microbial CH4, generated by carbon dioxide reduction (Table 1, Fig.5). By analyzing the mineral composition, particle or pore size and water content of the sediments, in combination with the structural type of hydrate and gas component information, the degree of water conversion to hydrate and the saturation of hydrate can be calculated.

Table 1. Carbon and hydrogen isotopic composition of methane dissociated from natural gas hydrates collected from different regions (literature values are from Dai J et al., 2017; He X et al., 2015; Liu C et al., 2015a; Liu C et al., 2015b).

Fig. 3. The in situ MRI images of the dissociation process of THF hydrate with a THF to water molar ratio of 1:68 ( modified from Meng Q et al., 2012). The brighter area represents the solution generated by the dissociation of THF hydrate, while the darker area represents the solid THF hydrate. The MRI images show that the dissociation occurred first at the inner wall of the sample chamber. As time went on, solid THF hydrate would suspend in the solution, and finally decompos. The artifact showing in the images was caused by the fluid convection flow, owing to the uneven temperature.

Fig. 4. The growth and distribution of gas hydrate in the sediments collected from the South China Sea (modified from Li et al., 2016). Black:methane gas, blue-green: NaCl solution, yellow: methane hydrate, bright white: foraminifera shells, grey: sediment. (a)&(b) show the 2D section of the dried and wet sediment samples before methane hydrate formation. The NaCl solution filled the majority of the foraminifera shells.(c) shows the growth of hydrates inside the foraminifera shells and pore spaces. (d) illustrates the 3D mapping of hydrates and free water inside and outside a single foraminifer therein.

2.4. Phase equilibrium of hydrate

For decades, the phase equilibrium of gas hydrate was studied using a high pressure-low temperature reactor. There are abundant phase equilibrium data obtained from gas hydrates with different components (Chen L et al., 2009a; Du J et al., 2011; Fan S et al., 2001; Liang D et al., 2001; Sun C et al.,2010; Sun Z et al., 2002a; Sun Z et al., 2002b; Sun Z `et al.,2001). For example, Ma Q et al., (2008) acquired hydrate equilibrium data from systems containing hydrogen, methane,ethane and ethylene, with and without THF in water. They observed that hydrogen was efficiently separated from pure methane, and gas mixtures of methane and nitrogen, by forming hydrate. Hydrate phase equilibrium data of CH4/CO2+ sodium dodecyl sulfate (SDS) aqueous solution, CH4/CO2+SDS aqueous solution+silica sand, and (CH4+C2H6+C3H8) gas mixture + SDS aqueous solution systems were obtained by Chen L et al., (2009a).

Since the traditional PVT reactors are inconvenient to operate and the data acquired often lack precision, the high pressure DSC has been increasingly applied to the study of hydrate phase equilibrium in recent years. Using this technique,the measurement of heat flux during hydrate phase change can be conducted under high pressure. Consequently, the thermal decomposition equilibrium data and specific heat under different experimental conditions can be obtained. Based on this technique, Chen Q et al. (2012) studied the phase equilibrium of nitrogen hydrates and methane hydrates in the pore water and porous media systems using core samples drilled from the South China Sea. The results showed that the hydrate phase equilibrium point is about 2 K lower than that in pure water with a pressure range from 10 to 30 MPa. However, the hydrate phase equilibrium under specific geological conditions at sea requires further study in the future.

3. Experimental simulations concerning gas hydrate

Since natural gas hydrates commonly occur in unconsolidated sediments, practical and effective techniques are required to be available for porous media. The basic physical parameters for the exploration and exploitation of gas hydrate reservoirs include acoustics, resistivity, mechanics, thermal physical characteristics, permeability etc. Due to the difficulty of data acquisition under actual geological conditions,experimental simulation is the most effective way to study these physical parameters. Integrated with various detection techniques, special simulation devices have been developed to experimentally study the physical parameters of gas hydrate reservoirs.

3.1. The acoustics characteristics of gas hydrate

The formation and dissociation of hydrate in a sediment core can be determined by the change in frequency of an acoustic wave, which can also help indirectly determine the relationship between hydrate concentration, pore space, confining pressure and saturation degree of gas and water. The distribution of gas hydrate in sediments affects the acoustic characteristics, which closely relates to the exploration and resource estimation of gas hydrate.

The Qingdao Institute of Marine Geology has established a series of experimental devices to study the acoustic characteristics of hydrate-bearing sediments. Integrated with ultrasound and TDR detection technique, acoustic parameters and the saturation of hydrate sediments can be measured during the formation and decomposition of gas hydrate in consolidated and unconsolidated sediments. Ren SR et al. (2010) conducted experiments to measure the acoustic P-wave velocity of sand packs with the formation of methane hydrate under simulated subsea sediment conditions. The results indicated that the acoustic velocity and amplitude of the acoustic waves increased with the formation of hydrate. Similar experimental simulations were conducted with the formation of gas hydrate in various devices, and the relationship between the acoustic wave velocity and hydrate saturation was studied by many researchers (Bu Q et al., 2017a; Bu Q et al., 2017b; Bu Q et al.,2017c; Gu Y et al., 2006; Li F et al., 2011; Liang J et al.,2009). Fig. 6 shows the change of acoustic velocities during the formation of gas hydrate.

使用上述的5種焊接材料分別焊接R1胎體基材與45鋼母材，用微機控制萬能材料試驗機進(jìn)行剪切試驗，試驗結(jié)果見表2。

Fig. 5. Relationship between δ13C and δD of CH4 dissociated from natural gas hydrates collected from different regions.

Fig. 6. Acoustic velocities and the corresponding hydrate morphology during hydrate formation (modified from Hu et al., 2014b). In the first stage of hydrate formation, the dominant hydrate cementing morphology causes a significant increase in velocity; in the second stage, the dominant floating hydrate morphology causes a slow increase in velocities; after hydrate saturation was higher than 60%, the dominant hydrate cementing morphology brought large increases in both Vp and Vs.

As the distribution of hydrate in the reactor is hard to control, it is difficult to accurately establish the acoustic response characteristics of hydrate under an actual physical model. For the same reason, the effects of different hydrate types on the acoustic properties of the underwater complexes are not yet fully understood.

3.2. The resistivity characteristics of gas hydrate

The measurement of gas hydrate resistivity is based on the decrease in resistance caused by water consumption, which can be used to judge the formation process of hydrate. In the process of hydrate formation, the change in resistivity can visually explain the formation and distribution of hydrate in the pore spaces of sediments.

Chen Q et al. (2008, 2009b) studied the electrical properties of CO2hydrate-bearing sediments, mainly focusing on the formation and dissociation processes in a porous medium. The results showed that the porous medium system’s impedance was mainly influenced by the concentration and activity of the ions produced from CO2ionization in water. Based on the standard and modified Archie equation, Wang et al. (2011;2010) studied the saturation of gas hydrate from resistivity data. The results demonstrated that gas hydrate saturation estimated from the isotropic resistivity of site SH2 in the Shenhu area had an average value of 24% with a maximum value of 44%, which coincided well with the values estimated from chloride anomalies. Chen Y et al. (2013b) studied the relationship between gas hydrate saturation and resistivity in the sediments from the South China Sea. The results showed that the resistivity was correlated with the distribution pattern of the gas hydrate. In addition, a variety of studies (Li S et al.,2010b; Li S et al., 2010c; Ren SR et al., 2010; Zhou X et al.,2007) were conducted to investigate the resistivity of gas hydrates during the formation and decomposition processes in porous media. Most of these studies were limited to the testing of resistivity parameters using a fixed frequency/DC power source as the excitation source. However, Xing L et al.(2015) and Jin X et al. (2016) reported that the electrical properties of hydrate-bearing sediments could be affected by many factors. The resistivity data obtained from different frequencies, different types of excitation sources and their inverse hydrate saturation are still not well-established.

3.3. The thermal physical characteristics of gas hydrate

The stability of gas hydrate is sensitive to temperature.Therefore, the thermal parameters of natural gas hydrate including thermal conductivity, thermal diffusivity and specific heat capacity are the basic physical data for gas hydrate exploitation and hydrate application technology.

Huang D and Fan S (2004; 2005) measured the thermal conductivity of methane hydrate formed from SDS solution and gas hydrate-bearing sand, respectively. Li D et al. (2010a)determined the thermal conductivity of methane hydrate in the dissociation self-preservation zone below freezing point. The data revealed that the thermal conductivity increased with increasing temperature, due to the increase of the ice fraction.Chen Q et al. (2013a) measured the thermal conductivity and saturation of natural gas hydrate in marine sediments. The results showed that when the hydrate saturation increased from 0 to 49%, the thermal conductivity increased first, and then decreased. Zhao J et al. (2012a) measured the thermal conductivity during the dissociation of gas hydrate in the gas hydratebearing sediments with a closed reactor and evaluated the effect of heat transfer.

3.4. The mechanical characteristics of gas hydrate

Experimental simulation of the mechanical properties of hydrate-bearing sediments is essential in the basic stability analysis of hydrate reservoirs. The mechanical properties and strength indexes of hydrate-bearing sediments were obtained by measuring the stress, pore pressure, axial deformation, torsional shear deformation and volume change of the specimens during the experiment.

Previous work studied the mechanical properties of hydrate (Li L et al., 2012a; Li Y et al., 2012d; Liu F et al., 2013;Liu L et al., 2016; Liu L et al., 2015c; Liu W et al., 2014; Shi Y et al., 2015; Sun X et al., 2012; Sun Z et al., 2013a; Sun Z et al., 2013b; Yan R et al., 2012; Yu F et al., 2011; Zhang et al., 2018) from different aspects, including sample preparation methods, temperature and pressure conditions, shear rate etc. These studies made great progress in understanding the mechanical properties of hydrate-bearing sediments.However, the lack of standard triaxial shear test methods and different testing processes make it difficult to model the strength parameters.

3.5. The geochemical response to gas hydrate formation and dissociation

The formation and dissociation of gas hydrate inevitably causes change in geochemical conditions, including ion and dissolved hydrocarbon concentrations, isotopic fractionation,aerobic/anaerobic oxidation of methane, formation of authigenic minerals and so on.

With the integration of Raman, CCD sensors and a deepsea environment simulation system, the changes in concentration of dissolved ions and methane during the hydrate synthesis/decomposition process in hydrate-bearing sediments were studied (Hao Y et al., 2017; Ren H et al., 2013; Tian Z et al., 2014). These studies found that the concentrations of major ions correlated with methane gas consumption. After the formation of gas hydrate, temperature was the dominant factor that affected the concentration of dissolved methane. The aerobic oxidation of methane and the isotopic fractionation during the process were also investigated (Li J et al., 2017a). The results showed that the composition of methane, ethane and propane decreased during aerobic consumption, and carbon and hydrogen isotopic features of hydrocarbons showed a tendency towards enrichment.

3.6. The experimental simulation for gas hydrate exploitation

Since natural gas hydrate has been regarded as a potential energy source, the development of feasible methods for the commercial production of natural gas hydrate is essential.Several methods have been proposed for the production of natural gas hydrate, including depressurization, thermal stimulation, inhibitor injection, CO2replacement and so on. Several experimental simulations have been performed to study the feasibility of these exploitation methods.

Yang et al., (2012) carried out hydrate depressurization experiments in a mid-scale reactor to study the temperature change and gas production. The results showed that the temperature decreased sharply almost everywhere during the initial stage of dissociation. However, the dissociation rate of hydrate was not uniform, being controlled by heat transfer. To study the relationship between the production behavior and the size of the hydrate reservoir, Li and Zhang, (2011) and Li et al., (2012b; 2012c) compared the gas production behaviors during the dissociation of methane hydrate by depressurization in two hydrate simulators with different effective volumes (5.8 L vs. 117.8 L). The results indicated that the progress of gas production in the two simulators could be divided into three stages: the free gas production stage, mixed gas (free gas and gas dissociated from the hydrate) production stage and gas production from hydrate dissociation stage.

Pang W et al., (2009) employed the hot water stimulation method to study the kinetic behaviors of methane hydrate dissociation in a 10 L reactor. They found that the melting of ice would significantly hinder the dissociation of gas hydrate.Furthermore, the microwave heating method was used to stimulate the dissociation of gas hydrate by Li D et al.,(2008). Compared with traditional stimulating methods, the efficiency of the microwave heating method is much higher under the same heating power.

The replacement of CH4in natural gas hydrates using CO2in different phase states was reviewed by Zhao J et al.,(2012b). It was emphasized that a CO2emulsion could provide superior performance in the replacement of CH4.Yuan Q et al., (2012) conducted an experimental simulation to study the favorable conditions for methane recovery from gas hydrate using gaseous carbon dioxide. The replacement mechanism was assumed to include CH4hydrate dissociation and mixed hydrate reformation.

4. Conclusions

This paper summarizes the latest developments of analytical methods and experimental simulations of gas hydrate in China. The modern analytical techniques and the experimental simulations provide important chemical and physical properties of gas hydrate and have played significant roles in gas hydrate research. However, as a new mineral species, gas hydrate is poorly studied compared with other common minerals. Therefore, the analytical techniques and simulation experiments need to be studied systematically. Prospective future work will mainly focus on: (1) Modern analytical instruments and technologies will be introduced into different research fields of hydrate to form a new analytical system. The measurements change from a single parameter to multiple parameters. (2) Development of micro-scale devices integrated with modern analytical technologies (i.e. MRI and X-CT) to form visualization techniques. It is believed to be useful for obtaining the physical properties of gas hydrate-bearing sediment.(3) Development of large-scale hydrate simulation devices is necessary to conduct experiments in an environment much closer to that of the actual hydrate accumulation. Consequently, the acquired physical parameters of hydrate are more suitable for practical application. However, the huge volume of large scale simulation equipment will bring some difficulties in experimental operation, safety and be time-consuming. Hence, appropriate-sized devices are required to be used for different experimental purposes.

Acknowledgement

This study was financially supported by the Taishan Scholar Special Experts Project; the Open Fund of the Qingdao National Laboratory for Marine Science and Technology(QNLM2016ORP0203).