Natural gas hydrate, as an unconventional resource, has been attracting increasing attention. Understanding the characteristics of methane hydrate formation and dissociation in porous media is important for developing gas hydrate-bearing reservoirs. This work discusses the use of low-field nuclear magnetic resonance (LF-NMR) technology to investigate the formation and dissociation of methane hydrate in the sandstone. In this work, an experimental assembly wherein methane hydrate can form and dissociate, is used to conduct LF-NMR measurements. LF-NMR, as a noninvasive measurement technology, combines the transverse relaxation time ( T 2 ) measurement with the magnetic resonance imaging (MRI). T 2 measurements can explore the characteristics of methane hydrate formation and dissociation in core samples from a pore-scale perspective. MRI can display the spatial distribution of water from a core-scale perspective. The excess-gas method and the excess-water method are successively applied to form methane hydrate, and depressurization is applied to dissociate methane hydrate in the laboratory. The characteristics of methane hydrate formation and dissociation is studied in the sandstone. Experimental results show that the signal intensity of short T 2 and long T 2 decreases simultaneously in the process of the methane hydrate formation using the excess-gas method, indicating that methane hydrate is formed in both small and large pores. When using the excess-water method, the signal intensity of long T 2 decreases, and the signal intensity of short T 2 increases in the process of the methane hydrate formation, indicating that methane hydrate is mainly formed in large pores. Methane hydrate is dissociated simultaneously in both small and large pores when using the depressurization method. Water content in small pores gradually increases. Capillary pressure causes some water to remain in the core samples following dissociation. Water content in large pores decreases initially and then increases during depressurization. In the early stages of depressurization, more water leaves large pores than is generated by hydrate dissociation. In the later stages of depressurization, less water leaves the large pores than is generated by hydrate dissociation. This study may inspire the new understanding on distribution of fluid in sediments during the process of accumulation and exploitation of natural gas hydrates.

Deep-water natural gas hydrate has high environmental risk and high technical difficulty in drilling and production. In order to promote the development of gas hydrate, we develop the assessment methods and control technologies for the operation risk during hydrate drilling and production. The work completed includes: (1) Safety analysis of conductor and wellhead for the hydrate drilling and production Establish a pipe-wellhead stability model to determine the drilling and working conditions under different working conditions. (2) Risk assessment of wellbore blockage in hydrate drilling and production Construct a wellbore multiphase flow analysis model to determine the amount of drilling inhibitor injection; obtain the location and extent of the hydrate blockage. (3) Risk assessment of wellbore instability in hydrate production Combined with hydrate formation properties, ground stress distribution and casing mechanics model, the position of the formation instability, and the damage of casing crushing is determined. (4) Gas diffusion risk assessment due to hydrate decomposition in water Study the distribution of underwater gas diffusion formed by large-area decomposition of hydrate to get the overflow flow risk. (5) Safety model and process risk assessment of hydrate drilling operations Conduct hazard identification and operation safety analysis of hydrate drilling operations, determine the risk level of each operation stage, and support the drilling operation.

Gas hydrate is a promising energy source because of its great storage under the sea. However, it is not easy to exploit as it becomes unstable and is easy to dissociate when it is disturbed and may trigger a submarine slide and cause a great impact to the safety of human beings and facilities in the sea. In this paper, changes of strength and pore pressure in a soil caused by the dissociation of gas hydrates are discussed. The shear strength of the soil containing gas hydrates decreases significantly with the dissociation of gas hydrates, which reduce the safety factor of a slope but not sufficient to cause failure. During a quick dissociation of gas hydrates in soil, gas or water in pores can hardly escape, and the released gas is greatly compressed and the pore pressure can be calculated. The strength and the pore pressure changes were considered in an infinite slope model. The result shows that only a small amount of gas hydrates has dissociated before the failure of the slope occurs. It is the increasing pore pressure rather than the reduction of soil strength that causes the failure of the slope. Parametric study was also carried out on gradient of slope, water depth and embedment depth of gas hydrates in the seabed. It was found that the gradient of a submarine slope is not an important factor to the failure of the slope while the water depth and the embedment depth affect the stability of the slope significantly.

In this paper, based on sediment with gas hydrate and flow characteristics for gas hydrate decomposition, the interaction and density difference between the phases are considered, conventional lattice Boltzmann model is modified to new lattice Boltzmann model then is applied to study flow characteristics for gas hydrate decomposition in sediment. The method is the mesoscopic model between macro and micro methods between. Modification lattice Boltzmann model is applied to carry out a complex micro-channel single-phase, multiphase flow simulation analysis, single-phase flow in porous media for gas hydrate decomposition. The results show that complex micro-channel flow field depends on the micro-channel roughness, bending degree, surface wet ability, fluid properties and other media. Single-phase flow in porous media depends on the pore diameter (saturation) and permeability of the sediment and the hydrate formation in the sediment so greatly reduces the permeability of porous media.