• Energy Research

Clathrate hydrates are formed by water molecules and natural gas molecules under a certain temperature and pressure. Natural gas hydrates are mainly distributed in the marine sediments and the permafrost in nature. As a kind of potential clean energy, determining the structural characteristics of hydrates in sediments is the prerequisite for the safe and economic exploitation of natural gas hydrate. X‐ray computed tomography (CT) is a potential method to nondestructively detect the occurrence of natural gas hydrate. The basic physical properties such as porosity, hydrate saturation, and sediment permeability of the hydrate‐bearing sediments are analyzed by calculating the CT numbers or combining with the network model. Therefore, the X‐ray CT is used to study the hydrates produced in the laboratory and the hydrates formed in the nature by scientists all over the world. By summarizing the previous research, this article focuses on the important role of X‐ray CT in the hydrate research from the following three aspects: analyzing the basic physical properties of hydrate sediment, identifying the occurrence mode of hydrate in sediments, and measuring the hydrate formation and decomposition process. In response to the research progress and future research directions, relevant suggestions had been put forward.

Abstract Marine methane hydrates have attracted global attentions as a considerable energy resource. The permeability of hydrate reservoirs critically affects the technical and economic feasibility of hydrate exploitation as well as the efficiency of gas production. In this study, marine sediments obtained from the South China Sea were used to remold core samples. Using a core holder, the gas permeability of marine sediments with and without methane hydrates were measured by injecting methane. In this study, the values of gas permeability range from 5.2 mD to 16.7 mD. The effects of confining pressure, hydrate saturation, and initial water saturation on gas permeability of cores were analyzed. The experimental results indicated that the gas permeability decreases (increases) with increasing (decreasing) confining pressure. In addition, the increase trend of confining pressure will significantly decrease on gas permeability in case the effective stress surpassed approximately 3.5 MPa. The deformation of silty-type marine sediments caused by increased confining pressure is irreversible to a certain extent. Relatively, the initial water saturation has little effect on gas permeability. This is attributed to that the water is bounded in the marine soil when the initial water saturation is less than 60%, resulting in a small water resistance effect. In addition, the hydrate dissociation induced by depressurization under confining pressure could result in a decrease of gas permeability. The results of this work revealed the effects of methane hydrates and confining pressure on the gas permeability of marine sediments, with great significance for the methane production from marine methane hydrate reservoirs.

Abstract The abundant methane hydrates stored in marine sediments have been widely evaluated as a potential energy source. Understanding the gas and water production characteristics of methane hydrate-bearing marine sediments is critical for hydrates commercial exploitation. In this study, confining pressure was applied to simulate a sub-seafloor environment. Methane was repeatedly injected into cores to remold hydrate-bearing marine sediments with different hydrate saturation. The hydrate saturation increased from 10.6% to 21.6% as the water content increased from 8.9% to 22.2%. The results indicate that the higher the core water content, the greater the hydrate saturation and the longer the hydrate dissociation time. The gas production characteristics of hydrate-bearing sediments were severely affected by water and hydrates in pores. The results indicated that some hydrates and free gas were easily trapped by the surrounding soil, meaning that the hydrates were isolated and disconnected with the pore channels under confining pressure during depressurization. Thus, a second depressurization was conducted to achieve further gas production. For cores with different water contents, their water conversion percentage is approximately 20%. When the water content exceeded 16.7%, the water production was observed. The results of this study are meaningful for further related research and field production of marine hydrates.

Abstract Methane hydrates with underlying gas are promising for gas production commercially in case of accurate prediction and timely effective measures taken to control dynamic permeability. In a core holder, marine sediments sampled from South China Sea were used to remold hydrate muddy cores. Three consecutive gas production experimental results confirmed that the effective stress has a cumulative negative impact on the reservoir permeability. Thus, a modified Masuda permeability model considering the evolution of effective stress (0.2–5.0 MPa) and hydrate saturation (36.6%–53.1%) was proposed to calculate the dynamic permeability during depressurization. On this basis, dynamic permeability was applied to a mathematical model to study the production characteristics of hydrates with underlying gas. Compared with experimental measurements, the error of underlying gas pressure predicted by the model can be less than 1.92%. Besides, the precise trajectory of the hydrate dissociation front in the core was predicted by the model. The results indicated that the dynamic permeability is dominated by intrinsic permeability, and the effect of hydrate saturation is more significant than that of effective stress. The mathematical model could provide a simple yet practical method for estimating dynamic permeability and predicting gas production performances of hydrate reservoirs with underlying gas.

Abstract Over the years, clathrate hydrates have been investigated for its potential as an energy resource and other industrial applications. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) are two powerful NMR technologies for both molecular level and microscopic measurement, which have been applied in gas hydrate research to provide fundamental and useful information. 1H and 13C NMR spectroscopy are the most commonly applied method to study cage occupancies of guest species and crystal structures. MRI technique, on the other hand, provides microscopic insights towards the gas hydrate formation and dissociation in porous media and the study of CH4/CO2 hydrate replacement. We also reviewed the state of the art application of NMR based technology in research on the gas-liquid multiphase flow and temperature mapping within porous media. Potential improvements in NMR technology to improve the fundamental understanding towards gas hydrates is also discussed in this review article.

Abstract 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.

Carbon dioxide (CO2) emission and fresh water scarcity have become two huge global issues over the last decades, and CO2 hydrate-based technology was promoted and developed to address them. This study investigated the effects of methylcyclopentane (MCP) on the hydrate formation condition and CO2 capture efficiency. Five cases and 31 cycles experiments of different MCP/water molar ratios (ranging from 0 to 0.03) in salt solution and porous media were conducted using the isochoric method. The hydrate phase equilibrium data of CO2–MCP hydrates were measured, and meanwhile the CO2 capture efficiencies were calculated. The results indicated that the rising MCP concentrations increase both the hydrate equilibrium temperature and the gas uptake. After the comparisons of the interpolated hydrate gas uptakes at uniform hydrate phase equilibrium pressure of 3 MPa among CO2–MCP, CO2–CP (cyclopentane), and CO2–C3H8 (propane) hydrates, CO2-MCP hydrates are especially suitable for CO2 capture. It should be noted that M...