Key issues in development of offshore natural gas hydrate

Abstract

As a new clean energy resource in the 21st century, natural gas hydrate is considered as one of the most promising strategic resources in the future. This paper, based on the research progress in exploitation of natural gas hydrate (NGH) in China and the world, systematically reviewed and discussed the key issues in development of natural gas hydrate. From an exploitation point of view, it is recommended that the concepts of diagenetic hydrate and non-diagenetic hydrate be introduced. The main factors to be considered are whether diagenesis, stability of rock skeleton structure, particle size and cementation mode, thus NGHs are divided into 6 levels and used unused exploitation methods according to different types. The study of the description and quantitative characterization of abundance in hydrate enrichment zone, and looking for gas hydrate dessert areas with commercial exploitation value should be enhanced. The concept of dynamic permeability and characterization of the permeability of NGH by time-varying equations should be established. The ‘Three-gas co-production’ (natural gas hydrate, shallow gas, and conventional gas) may be an effective way to achieve early commercial exploitation. Although great progress has been made in the exploitation of natural gas hydrate, there still exist enormous challenges in basic theory research, production methods, and equipment and operation modes. Only through hard and persistent exploration and innovation can natural gas hydrate be truly commercially developed on a large scale and contribute to sustainable energy supply.

Keywords

natural gas hydrate exploitation offshore / diagenetic and non-diagenetic hydrate / solid-state fluidization method / dessert in enrichment area / three-gas combined production on gas hydrate abundance

Research progress of natural gas hydrate (NGH) in the world

Natural gas hydrate, also known as ‘combustible ice’, is an ice-like, crystalline, supramolecular and cage-like compound formed by water and natural gas at a high pressure and low temperature. It is a special form of natural gas, mainly distributed in marine and terrestrial permafrost zones at water depth greater than 300 m, in which marine gas hydrate resources are global, and the amount of resources is more than 100 times that of terrestrial permafrost zones. The prominent characteristics of natural gas hydrate are wide distribution, large reserves, high density, and high calorific value. One cubic meter of natural gas hydrate can release about 164 m³ of methane gas and 0.8 m³ of water. According to estimation, the total amount of natural gas hydrate resources in the world can be converted into methane gas of about (1.8–2.1) × 10¹⁶ m³, and the reserves of organic carbon are twice as much as those of proven fossil fuels (coal, oil and natural gas) in the world. Therefore, natural gas hydrate, especially marine gas hydrate, is generally considered as an innovative clean energy resource to replace fossil energy in the 21st century [¹]. Since the 1960s, some countries such as the United States, Japan, China, Germany, South Korea and India had formulated gas hydrate exploration and exploitation research plans.

Exploitation history

The exploitation of hydrate can be traced back to 1810, when Davy accidentally discovered chlorine hydrate. However the crystal structure of hydrate was not determined until 1950s. The nearly 200 year’s research of hydrate can be roughly divided into three stages.

The first stage (1810–1934) is simple laboratory investigation. Scientists are entirely driven by a curiosity to determine that those gases can form hydrates together with water and understand the composition of hydrates in the laboratory. The most representative was Humphry Davy, a scholar of the Royal Society, who first synthesized chlorine hydrates in the laboratory in 1810. Subsequently, Berthelot Villard from France, Pauling from the United States and other scientists successfully synthesized a series of gas hydrates at the same time of scientific debate. Other gas hydrates which have been synthesized in succession caused an intense debate on the national chemical composition and physical structure. But a hundred years later, people still know little about the existence of gas hydrate in nature.

The second stage is the rapid exploitation of hydrate research (1934–1993), which focuses on the prediction and elimination technology of hydrate in industrial conditions [²]. At the beginning of 30s in the 20th century, it was found that the white ice-like solid was formed in pipelines, which caused great trouble to natural gas transportation. Therefore, chemists and the petroleum geologists focused their attention mainly on elimination of gas hydrate blockage in pipelines. At the same time, it was found that there existed a large number of natural gas hydrate resources in terrestrial permafrost and sea. In 1960, the Soviet Union discovered combustible ice in Siberia. In 1965, the Soviet Union first discovered gas hydrate deposits in the permafrost zone in Siberia. In 1968, the Soviet Union first discovered the gas hydrate reservoir when exploiting the Mesoyaha gas field. The first successful method for hydrate dissociation in the world was chemicals injection, which set off an unprecedented upsurge in hydrate research since the 1970s. In 1969, the United States began to implement the combustible ice survey, and included it in the national long-term plan as a strategic energy for national exploitation in 1998. Japan focused on combustible ice in 1992, and completed the investigation and evaluation of combustible ice in its surrounding sea areas. In 1979, the Deep Sea Drilling Project (DSDP) performed deep-sea drilling in the Gulf of Mexico on voyages 66 and 67, and obtained 91.24 m of gas hydrate cores from the seabed, which proved the existence of gas hydrate deposits in the seabed for the first time. In 1981, DSDP planned to use the Groma Challenger drilling vessel to extract 3-foot-long hydrate cores from the seabed. In 1992, the 146th Ocean Drilling Program (ODP) voyage obtained gas hydrate cores at Cascadia offshore platform on the western continental margin of Oregon, USA.

The third stage (1993–present), marked by the First International Hydrate Congress, is the basic formation stage of the overall exploitation and research pattern of hydrate research. Natural gas hydrate, as a potential energy source for the future of mankind, has received great attention all over the world. Major advances have been made in basic research such as hydrate formation/decomposition kinetics [³]; major breakthroughs have been made in the exploitation of new technologies such as solid-state storage of natural gas, in-depth studies on the substitution of kinetic inhibitors [⁴] for traditional thermodynamic inhibitors [⁵,⁶], and the relationship between natural gas hydrates and global environmental changes has attracted much attention. Thus the pattern of gas hydrate has been formed, based on basic research, i.e, growth of the pipeline hydrate suppression technology, exploitation of new application technologies such as solid-state storage of natural gas and water separation of gas mixtures, exploration and exploitation of natural gas hydrate resources, and capture and storage of greenhouse gases by using the hydrate method. At the same time, natural gas hydrates have been found in Siberia, the Mackenzie Delta, the North Slope, the Gulf of Mexico, the Sea of Japan, the Gulf of India, and the North Slope of the Central South China Sea. In 1998, Japan, cooperating with Canada, conducted hydrate drilling in the Mackenzie Delta in Northwest Canada, and obtained 37 m of hydrate cores at depths of 890–952 m. The drilling depth of 1150 m is the first one to study gas hydrate in permafrost zone of high latitude. Japan successfully exploited methane from deep sea combustible ice near Aichi County, Japan on March 12, 2013, becoming the first country in the world to master seabed flammable ice mining technology. Japan hopes to exploit mature technology and achieve commercial large-scale production of hydrate by 2020. The test has been performed by the Japanese economy industry under the provincial oil and gas metal mineral resources sector. Using the Earth Deep Exploration Vessel, the agency dug 330 m from the sea floor about 1000 m near the Omei Peninsula in Aichi County to reach the flammable ice layer. By extracting the water from combustible ice reservoir to reduce the layer pressure, the hydrate decomposed, the water and methane were separated, and the methane was then extracted. The whole process has taken about 4 h. The agency has continued conducting excavation tests in the area for about two weeks in order to perfect the technology. Natural gas hydrate investigation and exploration have been pushed to a new stage, and the exploitation and commercialization of natural gas hydrate has become an important goal.

Since natural gas hydrate has a vital strategic significance and great economic value, many developed and developing countries in the world included natural gas hydrate in their national key exploitation strategies in the early 1980s. Developed countries such as the United States and Japan took the lead in formulating comprehensive research and exploitation plans for natural gas hydrate, and had invested enormous human and material resources as government actions from the perspective of energy reserve strategy. From then on they had successively conducted research, resource evaluation, and fundamental applied research on natural gas hydrates in their own exclusive economic zones and international seabed areas, including reservoir formation mechanism, exploration technology, mining technology, utilization technology, and environmental impact, etc., making great progress.

Research progress in marine NGH in China

On April 14, 2005, the first discovery of natural gas hydrate carbonate specimens in China was held at the China Geological Museum, suggesting that China first discovered the world’s largest distribution area of “cold seep” carbonate rocks, with an area of about 430 km². This shows that China has abundant natural gas hydrate resources in the South China Sea, where 11 prospective natural gas hydrate resource areas in the South China Sea, such as Dongsha, Shenhu, Xisha, and Qiongdongnan, have been delineated, with a resource amount of about 6.8 × 10²–7.2 × 10¹⁰ tons of oil equivalents, which is twice the petroleum resource quantity in the South China Sea.

In 1995, China National Offshore Petroleum Corporation (CNOOC) successfully solved the gas hydrate blockage in Jinzhou 20-2 Gas Field (the first oil and gas self-dealer in China). Domestic experts spent seven days successfully removing the blockage. From then on, CNOOC has studied the gas hydrate control technology in the oil and gas production process and began the engineering practice.

In 2006, CNOOC, allied to the advantage team, successfully declared the first exploitation gas hydrate research project funded by the ‘863 Program.’ In 2016, CNOOC with nine teams started the National Key Research and Development Program, that is, the ‘Marine Gas Hydrate Trial Production Technology and Process,’ whose objectives are “breakthrough the bottleneck of key technologies of marine natural gas hydrate production test, form an autonomous test and production equipment, implement self-test and production of marine natural gas hydrate relying on Liwan 3-1 Deep-Sea Gas Field Group, and prepare for the co-exploitation of natural gas hydrate and oil and gas.”

In 2015, 2017, and 2019, China successively received marine gas hydrate samples and mastered the autonomous temperature and pressure preservation sampling and online test and analysis technology. In September 2015, CNOOC successfully obtained marine gas hydrate samples relying on self-developed equipment for the first time (see Fig. 1).

On May 18, 2017, the Ministry of Land and Resources of China succeeded in offshore natural gas hydrate depressurization mining based on the D90 semi-submersible drilling platform. At the same time, CNOOC, relying on independent technology and equipment, succeeded in implementing solid-state fluidization [⁷,⁸] test production of marine non-diagenetic gas hydrates with deep-water exploration vessels for the first time in the world at a water depth of 1310 m and a buried depth of 117–196 m. This technology was selected as one of the breakthrough technologies in science in 2017.

Research direction in natural gas hydrate

Many enterprises and universities conducted research on natural gas hydrate. According to the data of the 9th International Conference on Gas Hydrate (ICGH9) and the 11th International Hydrate Exploitation Congress (FIERYICE-11), the main research directions in natural gas hydrate [⁹] are as follows: (1) Reservoir basic physical properties mainly focus on the structural characteristics of natural gas hydrate, geophysical characterization of natural gas hydrate, physical characteristics of sediments containing natural gas hydrate, interface phenomena of natural gas hydrate, dynamic characteristics of natural gas hydrate, molecular simulation of natural gas hydrate, flow guarantee of natural gas hydrate production, and separation of natural gas hydrate and storage; (2) According to the latest research and exploitations of natural gas hydrate in the United States from 2017 to 2020, the exploitation direction mainly focuses on the influence of clay on the compressibility and permeability of sand during the exploitation of natural gas hydrate (Louisiana State University USA), the geomechanics and fluid coupling simulation of hydrate mine (Texas A&M University USA), and multi-scale experimental study on the flow characteristics of hydrate particles in storage during gas hydrate exploitation (University of Texas at Austin, USA); (3) Environmental protection mainly focuses on indoor simulation and field observation of ‘nuzzle’ of gas hydrate seafloor leakage (Texas A&M University, USA, US Geological Survey), characteristics of oceanic acidification and atmospheric emissions caused by methane release from gas hydrate system at the Atlantic Margin of the United States (Rochester University, USA, US Geological Survey), and characterization of gas hydrate deposits and changes in conductivity related to hydrate decomposition by using the electromagnetic method (University of California, Lawrence Berkeley National Laboratory, USA).

Gas hydrate exploration/trial production research in the world covers a wide range of depths. Although more than half a century has passed, progress has been slow for reasons such as the change of geopolitical situation, the slowdown of the world economy, the shale gas revolution in the United States, etc. However, it is still worth of the effort to make a careful review and summary only from the ‘research direction’ of gas hydrate exploitation.

Thoughts on ‘research direction’ in natural gas hydrate

From the current natural gas hydrate study directions around the world, the following conclusion can be made.

In research and exploration, geologists often focus on hydrates themselves, such as the origin of hydrates, the structure of hydrates, basic physical properties and the mechanical characteristics of hydrate samples.

At present, the potential resources of natural gas hydrate are huge, but how many of them have commercial exploitation value? How can the dessert enrichment zone of natural gas hydrate be characterized? How can the abundance of natural gas hydrate be quantitatively characterized? At present, not only no breakthrough is made in technology, but also little research in this field is reported in relevant journals.

Production and exploitation experts often consider the exploitation and test the production of hydrate using exploitation mode used in the traditional petroleum field. The high-pressure simulation exploitation device used in the laboratory production test of methane gas hydrate in Japan is the representative of gas hydrate exploitation simulation device all over the world. Other devices are similar. In fact, when studying the gas hydrate generated by sand filling, water filling, natural gas filling in the simulator, and cooling and pressurizing, the gas hydrate reservoir is intentionally or unintentionally studied as a conventional reservoir. From the well pattern for testing and production of natural gas hydrate exploitation by Japanese scientists, it is not difficult to find that the method is another manifestation of taking the exploitation of natural gas hydrate reservoirs as an idea of petroleum field exploitation.

When analyzing the safety and environmental risk of hydrate exploitation, attention is usually paid to the theoretical analysis and simulation experiments of greenhouse effect, geological landslide, and environmental impact caused by hydrate decomposition. However, no attention is paid to the combination with exploitation, nor to the safety and environmental risk research and simulation caused by different exploitation modes.

The exploitation of marine natural gas hydrates by using the depressurization method faces the following technical risks: If the overlying mud-sand layer is very thin, the gasified natural gas escapes upward. The flow of gas is uncertain, which may flow along the production pipeline, or spill out from the wellbore to the seabed. Will this impact the safety and ecology? Besides, if the cement of hydrate ore body is loose, it is mostly argillaceous silt. The decomposition of natural gas hydrate will produce large amount of sand. Then the problems rise. How should the sand body be dealt with after fluidization? How should the sand be controlled in the production process? Gas hydrate is gasified in formation due to pressure reduction. Does the formation form a large space without support (geological collapse)?

The deep-sea non-diagenetic gas hydrate solid-state fluidization production method faces the following technical risks: How much impact can large-scale seabed mining have on the ecosystem of seabed? After large-scale exploitation of natural gas hydrate, although some of the mud and sand are backfilled, how much impact will they have on the seabed environment due to the overlying mud and sand layer?

Discussion on key issues in marine natural gas

A new development model for NGH reservoir resources

Taking the whole geological background of gas hydrate mineral as the research object, the study on the joint exploitation of natural gas hydrate, shallow gas, and conventional petroleum should be deepened. Taking the shallow gas overlying the gas hydrate layer as the research object, it is necessary to change the harm into the benefit and to develop at the same time.

It is necessary to systematically study not only the hydrate layer itself, but also the whole geological background of gas hydrate mineral. Conventional petroleum field research includes the whole geological background of the reservoir. In the research, experiment have been conducted not only to study the reservoir, but also the water layer under the reservoir, the combination of source, reservoir and caprock, as well as their relationship in the process of petroleum exploitation. Taking the whole geological background of gas hydrate as the object, besides gas hydrate reservoir, studies should also be conducted on the thickness, cementation degree, specific gravity, and particle size distribution of mud-sand layer overlying hydrate layer, free gas layer, and free gas overlying layer. Moreover, the dynamic change relationship among layers during production process should be studied as well, including pressure field, temperature field, flow field, and logistics change.

From the gas hydrate samples found at present, it can be seen that there are free gas and shallow gas under the gas hydrate reservoir, and there are often conventional gas fields near the gas hydrate. This suggests that they may came from the same source of rocks. Taking Liwan 3-1 Gas Field as an example, from the seismic profile, free gas exists under hydrate, and they are only ten kilometers away from the conventional gas fields of Liwan 3-1. Only by using a production engineering facility system can the resources be made full use of and three kinds of gases jointly developed. Therefore, in order to study a set of engineering equipment to realize the combined exploitation of three gases, engineering design, exploitation plan and production process should be considered together. ‘Three-gas co-production’ (natural gas hydrate, shallow gas, conventional gas) may be an effective way to achieve early commercial exploitation.

Shallow gas is a potential danger as well as a potential resource, which should be taken as an important research object. (So far, almost no study on shallow gas has been reported.) Shallow gas (free gas) is often found in the lower part of deep-sea gas hydrate reservoirs. Marine hydrate storage areas are at the water depth of more than 300 m and 1000 m under the seabed. Seismic inversion data of a gas hydrate well in the South China Sea show that shallow gas and hydrate coexist. It is suggested that systematic research on shallow gas should be strengthened, including the reservoir-forming mechanism of shallow gas, the occurrence state, the relationship with upper hydrate layer, the physical properties of the mud-sand layer attached to it, the influence of shallow gas exploitation on the structure of the adjacent layer, whether hydrate will be formed in the process of exploitation, the impact on the environment in the process of exploitation of shallow gas, the overlying hydrate in the process of exploitation of shallow gas, the layer relationship, i.e., the engineering scheme of shallow gas and hydrate co-exploitation.

New classification of gas hydrate reservoirs

Gas hydrate reservoirs can be divided into diagenetic and non-diagenetic reservoirs which can even be subdivided into six grades, from the exploitation point of view. At present, there are many classification methods according to gas hydrate occurrence mode, geological environment, and gas source (see Table 1). Research have been conducted on corresponding environmental protection measures for different exploitation modes.

In 2014, at the 9th World Conference on Gas Hydrate Exploitation, held in Heidelabad, India, Shouwei ZHOU proposed for the first time that natural gas hydrates could be divided into diagenetic gas hydrates and non-diagenetic gas hydrates. At the 11th World Hydrate Exploitation Conference held in Chengdu, China, in 2018, natural gas hydrates were further subdivided into six levels.

From the point of exploitation, it is suggested that the concepts of diagenetic hydrate and non-diagenetic hydrate be introduced and natural gas hydrate be subdivided into six grades, taking the stability of rock skeleton as the main basis, focusing on how to describe the main factors considered after gasification, such as diagenesis, stability of rock skeleton structure, particle size and cementation mode.

The Mesoyaha hydrate reservoir in Russia has a depth of 700–800 m, a thickness of 84 m, and a porosity of 16%–38%. The only commercial hydrate production well has been producing intermittently for 17 years. It is exploited by depressurization and assisted injection of thermodynamic inhibitors (CH₃OH, CaCl₂). It has free gas reservoir and is easy to depressurize. With the synchronous exploitation of conventional oil fields, hydrate deposits have lasted for 17 years (intermittently) and no formation collapse has been reported. The analysis shows that the hydrate layer belongs to a relatively stable sandstone structure and the first class hydrate deposit.

Japan’s offshore gas hydrates have been successfully tested by using the depressurization method. The median size of samples is 133.2 microns, accounting for 72.3%. Sand production is serious, but it can still support short-term exploitation. Marine gas hydrate reservoirs in Japan can be regarded as the transitional type between diagenesis and non-diagenesis, and as the third grade.

In South China Sea, the size of hydrate samples mainly distributed between 0.221 and 174.55 microns, and the size distribution below 40 μm reaches 83.25% (see Table 2).

This hydrate layer is basically rock-free. Once gas hydrate reservoir is gasified, it will become water and sand mud. Fine silt (Particle size distribution below 40 μm reaches 83.25%) is difficult to control. The minimum particle size required by international sand control technology is 44 micron. If the reservoir is completely rock-free, it can be considered as the fifth hydrate reservoir.

At present, the research on environmental protection measures is often limited to the laboratory. Different types of natural gas hydrates need to adopt different exploitation modes [¹⁰] after simulated hydrate formation and re-gasification (see Table 3). Research should be conducted on mining technology [¹¹–¹³], equipment, and environmental protection.

Description and quantitative characterization of gas hydrate enrichment area and gas hydrate abundance

Our future major work is to increase the research on the description of gas hydrate enrichment area, the quantitative characterization of gas hydrate abundance, and search for gas hydrate dessert area with commercial exploitation value.

Under the condition of abundant formation, gas hydrate has a large number of cracks and veins, which constitute a large space for hydrate occurrence, as shown in Fig. 2.

The matrix permeability is only 0.6–0.9 mD. The natural gas saturation calculated by logging is 30%–45%. From the perspective of exploitation, gas hydrates enriched in crevices and veins play a key role in the contribution of production. It is suggested that more studies should be conducted on the description methods of gas hydrate enrichment areas, and the quantitative characterization method of gas hydrate abundance should be established according to different types of gas hydrates, pore structure, and basic physical properties of gas hydrates [¹⁴–¹⁷], based on which, the desserts in gas hydrate enrichment areas with commercial exploitation value are found.

Natural gas hydrates (NGH) abundance is currently calculated according to the method of conventional oil and gas. Figure 3 shows the abundance of typical deep-water gas field, tight gas, shale gas, and NGH in China. The NGH abundance in the South China Sea region is less than half of the shale gas and is an order of magnitude lower than the conventional deep-water fields. The distribution of NGH is different and the saturation of the same structure changes rapidly. The samples taken from the South China Sea were collected from two sampling holes 30 m apart. The saturation on the plane of samples varies from about 30% to 60%. Longitudinally, the logging curves and actual sampling demonstrate that the saturation changes from 37.26% to 54.75%, which can be observed apparently in Fig. 4.

Dynamic permeability and characterization of permeability of NGH

The concept of dynamic permeability and characterization of the permeability of NGH should be established by using time-varying equations. The dissociation process of NGH varies with the pore structure and phase state in the pore space. The dissociated gas and water of NGH will be transported and produced under the influence of heterogeneous temperature field, pressure field, and pore characteristics, which ultimately affect the efficiency of NGH exploitation [¹³]. The permeability is in an unsteadily changeable process owing to pore structure changes caused by gas hydrate dissociation. At present, permeability is set as a constant in many practices based on the concept of permeability in conventional oil and gas. As the value of permeability is taken as a constant in the initial state, the permeability of NGH is in the range of 0.1–1 mD, which ignores the variation of permeability caused by structural changes of NGH sediments. Currently, many researches are based on visualization techniques to obtain three-dimensional digital core analysis of natural gas hydrate deposits and pore network structure of hydrate-bearing under steady-state conditions, and to construct the pore network models of hydrate deposits with different saturations and porosities, which could be used in numerical simulation for gas and water flows inside the sediments. The premise of this study is that core permeability is a constant value, and the existing relevant studies are mainly focused on the steady-state conditions. There are still no relevant reports on the two-phase migration characteristics of gas and water during the hydrate dissociation process with changeable phase states, pore structures, temperature fields, and pressure fields.

The definition of NGH dynamic permeability is established by using a gas-water two-phase flow model.

The two fluids are assumed to be incompressible, but the viscous force is negligible compared with the capillary force. The extracted network structures of gas-water flow is predicted using the approach to Valvatne and Blunt. In previous work [18], the feasibility and accuracy of the model were verified in hydrate-bearing sediments.

The permeability K is calculated by using the pore network model based on Darcy’ Law, as expressed in Eq. (1), in which it is assumed that the fluid flow in each phase is independent.

(1)K={\displaystyle \frac{{{\displaystyle \mu }}_p{{\displaystyle q}}_{\mathrm{t}\mathrm{s}\mathrm{p}}L}{A\mathrm{\Delta }P}},

where K is the absolute permeability, μm²; μ_p is the viscosity of p-phase, mPa·s; q_tsp is the total single-phase flow rate by an imposed pressure drop DP across its length L(cm), cm³/s; and A is the cross-sectional area of the model, cm².

With hydrate formation in hydrate-bearing sediments, the cross-sectional areal A is calculated by using the cylindrical model, as expressed in Eq. (2).

(2)A(S_{\mathrm{h}})=A_0(1+S_{\mathrm{h}}^{0\mathrm{.5}}).

The pore surface area A(S_h) increases as hydrate grows in the center of the pore. Just before hydrate fills the pore, the surface area is twice its original value. In the cylindrical model, A₀ is the initial area of cross-sectional pore in the pore network model; and S_h is the hydrate saturation.

Conclusions

Although great progress has been made in the exploitation of marine natural gas hydrate, there still exist great challenges in basic theoretical research, production methods, equipment, operation modes and other aspects. Only through hard and persistent exploration and innovation, can it be possible to realize large-scale commercial exploitation and make contributions to the sustainable energy supply of mankind.

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