Methane Hydrate Exploitation Research Articles

Ice behaviors and heat transfer characteristics during the isothermal production process of methane hydrate reservoirs by depressurization

Methane hydrate is a new environmentally friendly alternative energy source in the future. During its conventional production process by depressurization, ice behaviors and heat transfer characteristics are two key factors affecting the hydrate dissociation rate. In this study, different reservoir temperatures (276.2, 277.2 and 278.2 K) and production pressures (2.3, 2.6 and 3.1 MPa) were employed to investigate the methane hydrate production process. Icing, which increases the reservoir temperature and significantly promotes the dissociation of hydrates instantaneously, is generally observed under 2.3 MPa production pressure due to the large temperature decrease by depressurization. Higher initial temperatures decrease both the formation amount and melting duration of ice, and higher production pressures can avoid the formation of ice by decreasing the temperature drop. In addition, both ice melting and hydrate dissociation are isothermal when limited by the external heat supply. During the hundreds of minutes of ice melting process, the area with ice is estimated to shrink gradually. Similarly, the dissociation rate of hydrates is controlled by the heat supply and even becomes constant when the driving force is small enough (high production pressure). The results of this study are significant for the rate control of methane hydrate exploitation.

Enhancing the gas production efficiency of depressurization-induced methane hydrate exploitation via fracturing

Most of the existing field tests of gas recovery from hydrate-bearing sediments suffer from the difficulty in pressure propagation, leading to a low productivity and energy efficiency. Fracturing has shown enormous potential in the shale gas production; thus in this study, we introduced this technique into the laboratory-scale gas hydrate production. The performance of gas production was investigated through numerically analyzing the gas hydrate dissociation behavior under different fracturing patterns. The results indicate that fracturing can significantly facilitate the pressure propagation in the hydrate sediments with a low intrinsic permeability during depressurization, thereby contributing to a better gas production performance. Fracture depth plays a critical role in promoting the gas production efficiency; the maximal enhancement ratio of average production rate by fracturing could attain 13.1%. Moreover, the contribution of reservoir’s sensible heat in hydrate dissociation is limited in the core with a low permeability; timely and sufficient heat supply is thus important for the successive gas production. The findings of this study illustrate the effects of fracturing on enhancing the gas production efficiency of depressurization-induced gas hydrate exploitation; this will provide important guidance for its potential application in the field test of marine and permafrost hydrate reservoir where low permeability is commonly encountered and an enhancing technique is much required.

Experimental analysis on thermodynamic stability and methane leakage during solid fluidization process of methane hydrate

The solid fluidization exploitation method is a new type of marine methane hydrate exploitation method. The thermodynamic stability and methane leakage are two important characteristics for evaluating solid fluidization exploitation method. In this study, the solid fluidization exploitation process was technically achieved by water injection under constant pressure. It was found that the solid fluidization process of methane hydrate can be simply divided into three stages of pressure control, concentration control and equilibrium control. The filling rate of methane hydrate cage structure affected the stability of methane hydrate, which in turn affected the decomposition of methane hydrate during methane hydrate fluidization process. In this study, with the increase of methane hydrate in the reactor, the amount of hydrate decomposition in the reactor decreased. The fluidization rate affected the time for methane concentration to reach equilibrium. For every 10 ml/min increase in fluidization rate, the amount of hydrate decomposition doubled. The amount of methane leakage during the solid fluidization process of methane hydrate was very small, and increased with the temperature of the fluidized water. When the fluidization temperature increases by 7 K, the amount of methane leakage in the hydrate containing 1 L of methane increased by about 0.28 L. This study is of great significance to the application of solid fluidization exploitation method.

Probing effects of thermal and chemical coupling method on decomposition of methane hydrate by molecular dynamics simulation

Methane hydrate, being a potential alternative energy source has attracted the attention of many researchers. Here, we investigated the decomposition of methane hydrate with and without alcohols at different temperatures. The results showed that the hydroxyl group of alcohol molecules close to methane hydrate tends to reorient toward methane hydrate. The formation of hydrogen bonds between alcohol and water in methane hydrate cages destabilises the (methane hydrate) structure, thus leading to its decomposition. We also studied the effects of temperature on alcohols used during methane hydrate decomposition by comparing the variations in total potential energy with temperature. We observed that 300 K is the turning point, when the temperature is lower than 300 K, the accelerating effect of alcohol on methane hydrate decomposition is more pronounced. Additionally, the effect of temperature was found to vary with alcohols. For methanol, temperature mainly affects the rate of diffusion; a high rate of diffusion accelerates the decomposition of methane hydrate. For glycerol, temperature mainly affects the interaction energy between glycerol and water, which can accelerate the decomposition of methane hydrate. Our work provides a theoretical basis for future researches into exploitation of methane hydrate by combining thermal stimulation and alcohol.

Experimental study on the permeability of methane hydrate-bearing sediments during triaxial loading

The permeability of methane hydrate (MH)-bearing sediments is closely related to the exploitation efficiency of MH deposits. During the exploitation of MHs, the stress state of hydrate-bearing reservoirs is complex, and stratum deformation may occur simultaneously. Both the change in the stress state and stratum deformation may influence the permeability of hydrate-bearing sediments. In this paper, a series of permeability measurement tests was conducted to study the influence of the effective confining pressure and triaxial compression process on the permeability of MH-bearing samples. The test results indicated that the permeability of hydrate-bearing samples was influenced by the effective confining pressure. With increasing effective confining pressure, the permeability of the pure sand sample gradually decreased. For the hydrate-bearing samples, the permeability was also influenced by the effective confining pressure. When the effective confining pressure reached a specific value, a sharp drop in the permeability of the hydrate-bearing samples occurred. The change in permeability might be caused by the increase in fine hydrate particles in the pore space. Based on the test results, the permeability of the hydrate-bearing samples was influenced by the triaxial compression process. During the shearing process, the permeability greatly increased when the axial strain reached a specific value. This phenomenon might be caused by the formation of a shear band. During the production of hydrates, controlling the change in stress applied to the hydrate particles could possibly promote production. Furthermore, the appearance of shear bands or fissures might also increase production by certain methods, such as the hydraulic fracturing process.

Effect of temperature on gas production from hydrate-bearing sediments by using a large 196-L reactor

Methane hydrate exploitation via depressurization for a reservoir with a bottom gas-rich zone was simulated using a large-sized three-dimensional hydrate simulator. A single vertical well system and a system with a combination of vertical and horizontal wells were employed to elucidate the different effects of horizontal and vertical wells on the exploitation process. The results showed that effect of promoting gas production using a horizontal well is not obvious for a high permeability reservoir. A large exploitation temperature drop may lead to ice blockage in the pipelines, and fracturing technology was introduced to solve this blockage problem. Experiments performed above and below the freezing point inferred that the decomposition of the hydrate in different areas of the reactor was not simultaneous, according to analysis of the temperature distribution of the entire reactor during the exploitation process. For exploitation above the freezing point, temperatures in the bottom of the reactor tend to maintain a low level and work against hydrate decomposition. However, for exploitation below the freezing point, the low-temperature area in the bottom of the reactor preferentially converts to ice. Heat release and salt precipitation promoted hydrate decomposition, and the decomposition rate was significantly accelerated.

Governing the transboundary risks of offshore methane hydrate exploration in the seabed and ocean floor—an analysis on international provisions and Chinese law†

Abstract Considering the fact that its existence is abundant while maintaining the ability to generate freshwater while burning, methane hydrates have been classified as sources of sustainable energy. China currently maintains an international role in developing technology meant to explore offshore methane hydrates buried under the mud of the seabed, their primary laboratory being the South China Sea. However, such a process does not come without its hazards and fatal consequences, ranging from the destruction of the flora and fauna, the general environment, and—the greatest hazard of all—the cost of human life. The United Nations Convention on the Law of the Sea (hereinafter ‘UNCLOS’), being an important international legal regime and instrument, has assigned damage control during the exploration of methane hydrates, as being the responsibilities and liability of individual sovereign states and corporations. China adopted the Deep Seabed Mining Law (hereinafter the DSM Law) on 26 February 2016, which came into force on the 1 of May 2016; a regulation providing the legal framework also for the Chinese government’s role in methane hydrate exploratory activities. This article examines the role of the DSM Law and its provisions, as well as several international documents intended to prevent transboundary environmental harm from arising, as a result of offshore methane hydrate extraction. Despite the obvious risk of harm to the environment, the DSM Law has made great strides in regulating exploratory activities so as to meet the criteria of the UNCLOS. However, this article argues that neither the UNCLOS nor the DSM Law are adequately prepared to address transboundary harm triggered by the exploitation of offshore methane hydrates. In particular, the technology of such extraction is still at an experimental stage, and potential risks remain uncertain—and even untraceable—for cross-jurisdictional claims. The article intends to seek available legal instruments or models, to overhaul the incapacity within the current governing framework, and offers suggestions supporting national and international legislative efforts towards protecting the environment during methane hydrate extraction.

The Exploitation of Oceanic Methane Hydrate: Legal Issues and Implications for China

Abstract With the growth of global energy demand, States are actively considering the exploration for new energy. Methane hydrate is one of the world’s new energy sources with high energy density and abundant reserves, which have great strategic significance. This article focuses on three aspects, namely, project preparation, risk prevention and accident management, and addresses the risk issues arising from the exploration of methane hydrate. It is important to apply the United Nations Convention on the Law of the Sea and other treaties, as well as customary international law, while examining the rules applicable to the exploration of methane hydrate. State practice such as those of the United States, Russia, Japan, the European Union and China, are also discussed. The article puts forward some suggestions on the development of China’s methane hydrate resources. The core objective is to achieve a balanced approach to the development of environmental protection and energy development.

The seepage characteristics of methane hydrate-bearing clayey sediments under various pressure gradients

As a wide range of clean energy, methane hydrate is mostly formed in low-permeability clayey sediments. And the gas production rate from low-permeability hydrate reservoirs will be greatly influenced by the seepage characteristics. In this study, a series of seepage experiments was performed on methane hydrate-bearing clayey sediments. The results show that water flow in clayey sediments with different hydrate saturations exhibits both Non-Darcy and Darcy flow behaviors. Additionally, the minimum threshold pressure gradient (TPG) is present during water phase flow of hydrate-bearing clayey sediments, which may be not favorable for methane hydrate exploitation. The minimum TPG firstly decreases and then increases with an increase in hydrate saturation, providing theoretical guidance for the pressure gradient used in depressurization process during methane hydrate exploitation to improve gas production rate. The water permeability (Kw) and the permeability coefficient (k’) firstly increase and then decrease with an increase in hydrate saturation. In addition, the water permeability increases gradually with decreases in the minimum TPG for clayey sediments with different hydrate saturations. The relationship between minimum TPG and water permeability is described by the power function as TPGmin=2.81231×10−4×Kw−0.57767. This relationship provides the basic permeability parameters for the numerical simulation of methane hydrate exploitation.

Experimental investigation of natural gas hydrate production characteristics via novel combination modes of depressurization with water flow erosion

Depressurization is considered the most efficient method for natural gas hydrates (NGHs) exploitation. However, ice formation, hydrate reformation, and insufficient decomposition driving forces in the later stages of depressurization are the main issues to be solved. In this study, a more effective combination of depressurization with water flow erosion for the production of NGHs was investigated to promote efficient exploitation of methane hydrate (MH) by using in-situ magnetic resonance imaging. Three different MH decomposition modes were used, and water flow erosion was employed to eliminate the problem of incomplete MH decomposition in the later stages of depressurization, which is caused by insufficient driving forces and slower heat and mass transfer due to lower decomposition pressure and the protection effect of water films. The promotion of MH decomposition by water flow erosion was experimentally confirmed. Depressurization could decrease water-phase permeability in the sediment core and further optimize the water flow environment. Water flow erosion could greatly accelerate heat and mass transfer and provided extra driving force by increasing the chemical potential difference in the later stages of depressurization. In addition, the phenomenon of ice formation caused by sudden depressurization could be relieved by water flow erosion, which improved the ambient heat transfer, further changing the MH decomposition characteristics. The mutual promotion of MH decomposition by water flow erosion and depressurization was clearly demonstrated in this study.

Experimental investigation into the dissociation of methane hydrate near ice-freezing point induced by depressurization and the concomitant metastable phases

Methane hydrate (MH) has been viewed as an important potential energy resource, drawing attention of its efficient exploitation. An intriguing phenomenon that gas hydrates dissociate in a slow rate and can preserve for long periods of time out of the stable region was considered as a performance of hydrates metastability. A mechanism that supercooled water can lead to the retarded dissociation of hydrates was proposed by researchers, which will affect the gas production efficiency and sediment safety during the exploitation of MH. In this work, experiments on MH dissociation and production of methane gas from porous MH sediment near ice-freezing point induced by depressurization were performed to obtain the evidence of the metastability of the gas hydrate system. A series of experiments at different production pressures, depressurization rates and initial water saturations were carried out to study the effects of supercooled water on gas production performance. The results show that the existence of supercooled water below the freezing point of water would retard the dissociation of MH, by which the gas production rate was slowed down. The duration of this metastable state of the system was dependent on the induction time of the ice formation during the MH dissociation process. The formation of ice would promote gas production from the sediment, while the formation was observed as a stochastic process at 2.0 MPa and 2.2 MPa. The method employed should be adjustable and amendable to investigate the metastability of the gas - liquid - gas hydrate system and the related effects on gas production during depressurization-induced MH dissociation process below ice-freezing point in pilot scale.

A novel method to enhance methane hydrate exploitation efficiency via forming impermeable overlying CO2 hydrate cap

To enhance the exploitation efficiency of natural gas hydrate by decreasing the yield of water, a novel “reservoir reformation” concept is proposed that involves the reformation of a natural gas hydrate reservoir by constructing an artificial impermeable overlying CO2 hydrate cap. The feasibility of this concept has been demonstrated in this laboratory-scale experiment. After reformation by injecting CO2 emulsion into the permeable overburden, a confined environment with an impermeable CO2 hydrate cap is successfully constructed for depressurization operation. The cap can maintain mechanical stability until the end of production process. With the protection provided by the artificial CO2 hydrate cap, the production efficiency was greatly improved to 83.3% and the water yield is remarkably decreased. Moreover, the optimal CO2/H2O volume ratio of the emulsion for forming the desired CO2 hydrate cap was confirmed to be 1:1. The formation of CO2 hydrate cap can also protect the geological stability of depleted methane hydrate zones and seal a large amount of CO2, which is of both energetic and environmental significance; however, intensive and extensive research should be conducted in the future.

A novel method to greatly increase methane hydrate exploitation efficiency via forming impermeable overlying CO2 cap

Aiming at increasing the exploitation efficiency of natural gas hydrate via decreasing the yield of water, a new idea “reservoir reformation” is proposed in this work, i.e., reforming natural gas hydrate reservoir by constructing an artificial impermeable overlying CO2 hydrate cap. Fine feasibility of this idea has been proved in this laboratory scale work. After reformation by CO2 injection to the permeable overburden, a confined environment with an impermeable CO2 hydrate cap is constructed successfully for depressurization operation. The cap can maintain mechanical stability for enough long time until the finish of production process. With the protection of the artificial CO2 hydrate cap, the production efficiency is greatly improved and the water yield is decreased remarkably. This work is of energy and environment double significance although lots of deeper and wider work should be done in future.

Natural gas hydrate exploitation by CO2/H2 continuous Injection-Production mode

CO2 replacement is considered as a promising method for the simultaneous development of natural gas hydrate and CO2 sequestration. The addition of small molecular gases, such as N2 and H2, into the injected gas can increase the gas recovery ratio and prevent CO2 liquefaction. Based on previous studies, this work presents methane hydrate exploitation using the CO2/H2 continuous injection-production mode. The mechanism combines gas sweep with CH4/CO2 replacement. A series of experiments were carried out to optimize the injected gas composition and flow rate, which have a significant effect on the rate of CH4 hydrate decomposition, amount of CO2 sequestration, and cost. The compositions of the injected gases had little effect on the recovery rate when a relatively higher flow rate was employed. A balance between CH4 production and CO2 sequestration was established when the CO2 mole fraction was slightly <74%, and the temperature of the reservoir did not decline throughout the whole process. The injection rate affected both the displacement efficiency and the production cost. Moreover, through these experiments, we discovered a more economically feasible and productive measure for regulating the injection rate.

Experimental Study on the Mechanical Properties of CH4 and CO2 Hydrate Remodeling Cores in Qilian Mountain

The CH4-CO2 replacement method has attracted global attention as a new promising method for methane hydrate exploitation. In the replacement process, the mechanical stabilities of CH4 and CO2 hydrate-bearing sediments have become problems requiring attention. In this paper, considering the hydrate characteristics and burial conditions of hydrate-bearing cores, sediments matrices were formed by a mixture of kaolin clay and quartz sand, and an experimental study was focused on the failure strength of CH4 and CO2 hydrate-bearing sediments under different conditions to verify the mechanical reliability of CH4-CO2 replacement in permafrost-associated natural gas deposits. A series of triaxial shear tests were conducted on the CH4 and CO2 hydrate-bearing sediments under temperatures of −20, −10, and −5 °C, confining pressures of 2.5, 3.75, 5, 7.5, and 10 MPa, and a strain rate of 1.0 mm/min. The results indicated that the failure strength of the CO2 hydrate-bearing sediments was higher than that of the CH4 hydrate-bearing sediments under different confining pressures and temperatures; the failure strength of the CH4 and CO2 hydrate-bearing sediments increased with an increase in confining pressure at a low confining pressure state. Besides that, the failure strength of all hydrate-bearing sediments decreased with an increase in temperature; all the failure strengths of the CO2 hydrate-bearing sediments were higher than those of the CH4 hydrate-bearing sediments in different sediment matrices. The experiments proved that the hydrate-bearing sediments would be more stable than that before CH4-CO2 replacement.

DEM simulations of methane hydrate exploitation by thermal recovery and depressurization methods

Methane hydrate (MH, also called fiery ice) exists in forms of pore filling, cementing and load-bearing skeleton in the methane hydrate bearing sediment (MHBS) and affects its mechanical behavior greatly. To study the changes of macro-scale and micro-scale mechanical behaviors of MHBS during exploitation by thermal recovery and depressurization methods, a novel 2D thermo-hydro-mechanical bonded contact model was proposed and implemented into a platform of distinct element method (DEM), PFC2D. MHBS samples were first biaxially compressed to different deviator stress levels to model different in-situ stress conditions. With the deviator stress maintained at constant, the temperature was then raised to simulate the thermal recovery process or the pore water pressure (i.e. confining pressure for MH bond) was decreased to simulate the depressurization process. DEM simulation results showed that: during exploitation, the axial strain increased with the increase of temperature (in the thermal recovery method) or decrease of pore water pressure (in the depressurization method); sample collapsed during MH dissociation if the deviator stress applied was larger than the compression strength of a pure host sand sample; sample experienced volume contraction but its void ratio was slightly larger than the pure host sand sample at the same axial strain throughout the test. By comparison with the laboratory test results, the new model was validated to be capable of reproducing the exploitation process by thermal recovery and depressurization methods. In addition, some micro-scale parameters, such as contact distribution, bond distribution, and averaged pure rotation rate, were also analyzed to investigate their relationships with the macroscopic responses.

Measurements of the changing wave velocities of sand during the formation and dissociation of disseminated methane hydrate

AbstractThe formation and dissociation of methane hydrate within sediment can lead to large changes in wave velocities, which provide valuable insights into the processes involved in hydrate formation. These are of practical importance in geophysical characterization, as well as developing strategies for the future exploitation of methane hydrates. This paper presents changes in wave velocity, measured during hydrate formation, and subsequent dissociation, using the resonant column apparatus. Hydrate was formed under “drained” and “undrained” conditions. Drained specimens had free access to methane during formation, while for undrained specimens, methane content was fixed. Hydrate formation and dissociation were induced by changing the specimen temperature under constant effective stress. In excess of 20 determinations of shear wave and flexural wave velocity were carried out over a 9 h period, both during hydrate formation and dissociation. This time was sufficient to record almost all of the changes in wave velocity within a specimen. The exothermic nature of hydrate formation was clearly seen in the form of spikes in temperature measured at the base of the specimens. For all specimens, the relationship between wave velocity and degree of hydrate saturation was nonlinear and significantly different during formation and dissociation. The patterns observed suggest that hydrate morphology not only is important in controlling the ultimate wave velocities, at the end of formation, but has a significant impact on the rates of change of wave velocities during formation and dissociation. A conceptual model is presented to explain differences in observed behavior during formation and dissociation.

RETRACTED: A global survey of gas hydrate development and reserves: Specifically in the marine field

Gas hydrates, also known as methane hydrates, are formed due to the high hydraulic pressures present under the cold seabed over long periods of time. Gas hydrates are mainly composed of methane produced in the seabed by bacteria in the use of the remains of animals and plants as food. Often appearing as translucent or opaque ice, gas hydrates can be separated into water and methane gas, which can be burned at normal temperatures and pressures, giving this substance the nickname “combustible ice.” As global oil reserves continue to be depleted, scientists are regarding methane hydrates as a new energy source that is very likely to replace oil in the 21st century. According to reports by the United States Geological Survey, the potential natural gas energy that can be recovered from global methane hydrate formations is two times the amount of fossil fuel energy available to the world. Therefore, many countries that are deeply engaged in the development of gas hydrates, such as the United States, Japan, Canada, China, India, and Taiwan, hope that this new energy source can become a substitute for more conventional petroleum sources. Japan—the first country to develop methane hydrates—will be ready for commercial mass production in the eastern Japanese Nankai Trough prior to 2018, according to Japan׳s Methane Hydrate R&D Program-MH 21. However, the exploitation of methane hydrates in terrestrial permafrost requires less technical risk and costs. Joint explorations in areas of Alaska by the United States, Japan, and Canada will enter the preparation phase for commercial output as early as 2015. In Taiwan, cooperation with Germany and the United States has led to methane hydrate exploration and the initiation of drilling sampling in the South China Sea that is expected to be completed in 2016, with commercial production ready as soon as 2026.

The economics of exploiting gas hydrates

We investigate the optimal exploitation of methane hydrates, a recent discovery of methane resources under the sea floor, mainly located along the continental margins. Combustion of methane (releasing CO2) and leakage through blow-outs (releasing CH4) contribute to the accumulation of greenhouse gases. A second externality arises since removing solid gas hydrates from the sea bottom destabilizes continental margins and thus increases the risk of marine earthquakes. We show that in such a model three regimes can occur: i) resource exploitation will be stopped in finite time, and some of the resource will stay in situ, ii) the resource will be used up completely in finite time, and iii) the resource will be exhausted in infinite time. We also show how to internalize the externalities by policy instruments.

A simple distinct element modeling of the mechanical behavior of methane hydrate-bearing sediments in deep seabed

The study on the mechanical behavior of methane hydrate-bearing sediments (HBS) in deep seabed is of great significance for the safe exploitation of methane hydrate in the future. Recent studies have shown that the mechanical behavior of HBS is significantly influenced by methane hydrate since it leads to cementation among soil grains. For better understanding its microscopic mechanical mechanism, this paper presents a simple numerical model of HBS using the distinct element method (DEM). First, a set of tests on two bonded aluminum rods were performed under different loading paths with a specially designed apparatus. Then, a simple bond contact model was proposed based on the experimental data and implemented into our two-dimensional DEM code, NS2D. Finally, a series of drained biaxial compression tests under different confining stresses on HBS samples with different bond strengths, which are used to represent different methane hydrate saturations \((S_{\mathrm{MH}})\), were carried out with this code. By comparing the results of numerical simulations with the experimental data obtained from triaxial compression tests, the study shows that the DEM incorporating the new bond contact model is capable of capturing the main mechanical characteristics of HBS such as the strain softening and dilation. And it can also capture that (a) the peak shear strength increases as \(S_{\mathrm{MH}}\) or the confining stress increases, while the dilation increases as \(S_{\mathrm{MH}}\) increases or the confining stress decreases; (b) both the cohesion and friction angle increase with the increasing of \(S_{\mathrm{MH}}\), but the influence of \(S_{\mathrm{MH}}\) on the cohesion is much more significant than on the friction angle.