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Chen X.,CAS Institute of Mechanics | Zhang X.,CAS Institute of Mechanics | Lu X.,CAS Institute of Mechanics | Wei W.,Petrochina | Shi Y.,Marine Environmental and Engineering Geological Survey
Acta Mechanica Sinica/Lixue Xuebao | Year: 2016

Gas hydrate (GH) dissociates owing to thermal injection or pressure reduction from the well in gas/oil or GH exploitation. GH dissociation leads to, for example, decreases in soil strength, engineering failures such as wellbore instabilities, and marine landslides. The FLAC3D software was used to analyze the deformation of the soil stratum and vertical wells with GH dissociation. The effects of Young’s modulus, internal friction angle, cohesion of the GH layer after dissociation, and the thickness of the GH layer on the deformation of soils were studied. It is shown that the maximum displacement in the whole soil stratum occurs at the interface between the GH layer and the overlayer. The deformation of the soil stratum and wells increases with decreases in the modulus, internal friction angle, and cohesion after GH dissociation. The increase in thickness of the GH layer enlarges the deformation of the soil stratum and wells with GH dissociation. The hydrostatic pressure increases the settlement of the soil stratum, while constraining horizontal displacement. The interaction between two wells becomes significant when the affected zone around each well exceeds half the length of the GH dissociation zone. © 2016 The Chinese Society of Theoretical and Applied Mechanics; Institute of Mechanics, Chinese Academy of Sciences and Springer-Verlag Berlin Heidelberg


Zhang X.H.,CAS Institute of Mechanics | Lu X.B.,CAS Institute of Mechanics | Hong S.Y.,Marine Environmental and Engineering Geological Survey | Zhen X.,Marine Environmental and Engineering Geological Survey
Proceedings of the ISOPE Ocean Mining Symposium | Year: 2013

Thermal conduction and phase transformation are physical-chemical processes during the dissociation of gas hydrate-bearing sediments. Heat transfer leads to the expansion of hydrate dissociation front and the weakening of soils accompanied by the seepage of fluids and the deformation of sediments. As a consequence, ground failure may occur which can damage engineering structures and lead to environmental disasters. Hydrate dissociation in sediments is investigated using tetrahydrofuran (THF) hydrate sediments under various thermal strengths, and the time-dependent development of a hydrate-dissociation front is elucidated. An axial-symmetrical theoretical model and a numerical method are proposed based on experimental observations and analysis of the physical processes. Numerical and experimental results for evolution of the hydrate-dissociation front are in good agreement. Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE).


Zhang X.H.,CAS Institute of Mechanics | Lu X.B.,CAS Institute of Mechanics | Shi Y.H.,Marine Environmental and Engineering Geological Survey | Xia Z.,Marine Environmental and Engineering Geological Survey | Liu W.T.,Marine Environmental and Engineering Geological Survey
Ocean Engineering | Year: 2015

Gas hydrate (GH) is a kind of solid energy resource with huge reserve. Stratum instability (such as marine landslide) may be caused by hydrate dissociation due to the softening of stratum and the build-up of excess pore pressure. In this paper, a centrifuge experiment was conducted to study the evolution of stratum instability during hydrate dissociation. The hydrate dissociation zone expands from the heating source in hydrate bearing sediment (HBS). The pore fluid pressure increases significantly accompanying the slow seepage and soil layer's softening and deformation. Large horizontal displacements and vertical displacements can be observed and increase with the expansion of hydrate dissociation zone. The results can be used for the verification of numerical simulations and a reference for engineering design. © 2015 Elsevier Ltd. All rights reserved.


Zhang X.H.,CAS Institute of Mechanics | Lu X.B.,CAS Institute of Mechanics | Shi Y.H.,Marine Environmental and Engineering Geological Survey | Xia Z.,Marine Environmental and Engineering Geological Survey
Marine and Petroleum Geology | Year: 2015

The mechanical properties are important for the evaluation of stratum deformation and instability in hydrate exploitation. By using an integrated test apparatus for synthesis of hydrate sediment and tri-axial tests, a series of compression tests is conducted on the silty clay containing tetrahydrofuran hydrate (SCTH) similar to that of South China Sea. The stress-strain curves and Mohr circles are obtained at different degree of hydrate saturations and confining pressures. The silty clay containing tetrahydrofuran hydrate shows typical ductile behavior, the shear strength increases linearly with the increase of hydrate saturation and confining pressure. The secant modulus increases with the increase of hydrate saturation. The shear strength is analyzed based on the Drucker-Prager criterion, Mohr-Coulomb criterion, Lade-Duncan criterion, while the secant modulus is analyzed using the mixed law of composite materials. It is shown that the strength can be well described by the Drucker-Prager criterion and Mohr-Coulomb criterion, and the secant modulus is close to the harmonic average of the modulus of soil skeleton and hydrate in SCTH. © 2015 Elsevier Ltd.

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