Studies On The Static-dynamic Mechanical Behaviors And Characters Of Hydrate-bearing Sediments

Natural gas hydrate is a new kind of unconventional energy,due to the characters of high value of energy density,wide range of distributions and huge amount of resources.With the increasing of energy demands and the decreasing of fossil fuels,the world focuses their attentions on gas hydrate.Besides,gas hydrate,a critical role in global carbon cycle,has great influence on climate and environment evolution around the world.So many countries in world,including America,Japan,Canada,Germany,Korea,India and China,plan their development programs on hydrate.However,there are still numerous challenges to exploit and recovery gas hydrate from nature.The safety concerns,including the processes of wellbore collapse,sand production,seafloor settlement,induced earthquake and methane leakage,are one of the critical challenges.In order to analyze and evaluate these safety-related processes,we must have a comprehensive understanding on the mechanical properties of hydrate system.Therefore,the tools of mechanical tests,micro-structure observations and theoretical analysis were comprehensively used in this thesis to study the mechanical properties of hydrate system.Firstly,the static and dynamic properties of hydrate-bearing sediments were obtained by customized mechanical devices.Then,the micro-structures of hydratebearing sediments were observed by CT scan.Finally,the relation between static and dynamic mechanical parameters was derived for field applications.Based on the above research ideas,the thesis includes six chapters and the contents of each chapter are as following:The first chapter,the characteristics and development history of natural gas hydrate were briefly introduced,and the energy and environmental significances were emphasized.After that,the research purpose and significance were illustrated.Then,the domestic and foreign research status of the mechanical properties of hydrate-bearing sediments were overviewed,and the difficulties and problems faced with the mechanical properties of hydrate-bearing sediments were analyzed.Based on these,the main contents,targets and technology route were introduced.The second chapter,this chapter mainly studied the static mechanical properties of hydrate-bearing sediments under large strain,using the customized direct shear apparatus.When the hydrate formation and decomposition in specimens were simulated in laboratory,the effects of geological environmental facts on static mechanical properties of hydrate-bearing sediments,such as strength,cohesion and friction angles were evaluated.The third chapter,this chapter mainly studied the dynamic mechanical properties of hydrate-bearing sediments under small strain,using the customized resonant column and wave measurement apparatuses.The influences of hydrate status and formation conditions on wave velocities(modulus)and attenuations(damping ratios)of hydrate reservoirs were simulated and compared.The fourth chapter,this chapter mainly observed the 3D micro-structures of hydratebearing sediments under laboratory conditions,using the customized X-ray CT apparatus.The observations of the micro-structures under various laboratory conditions were linked to the evolution of the static and dynamic mechanical properties of hydrate-bearing sediments.The fifth chapter,this chapter mainly simplified the governing equation between the hydrate saturation and the wave velocity of hydrate-bearing sediments based on the rock physics model,as well as the governing equation between the hydrate saturation and the shear strength of hydrate-bearing sediments based on the Mohr-Coulomb criterion.The wave velocity based prediction model on the strengths of hydrate reservoirs was derived according to the above two governing equations,and was clarified by the coupled data of field sonic loggings and in-situ strength tests.The sixth chapter,the main conclusions of this thesis were summarized,and the shortcomings and future research direction were illustrated.According to the above research,the following conclusions are drawn:(1)the static mechanical properties of hydrate-bearing sediments were dominated by hydrate morphology,which was further related to the skeleton and hydrate distribution in sediments.Under the high hydrate saturation,the peak and residual strengths of specimens were bigger,and the behaviors of shear-dilation and brittle-failure were occurred easier.The increments of peak and residual strengths resulted from the cohesion and friction angles,respectively.With the increasing of hydrate saturation,the cohesion of sandy and silty specimens represented the linear and exponential rising-trends,respectively.Under the same variations of hydrate saturation,the friction angles of sandy specimens increased first,and then fluctuated within a certain range,while those of silty specimens increased and followed by decreasing.(2)with longer decomposition time,the strengths of hydrate-bearing sediments weakened exponentially until close to those of hydrate-free sediments.Under the identical conditions in laboratory,the specimens with higher initial saturation could maintain higher decomposition strengths,and the silty specimens suffered greater strength losses than those of sandy specimens.With the limited tests,the effects of sediment skeletons and decomposition methods on strength changes of specimens were not obvious.(3)the dynamic mechanical properties of hydrate-bearing sediments also highly relied on the status of porous media.The resonant column tests concluded that the modulus and damping ratios both increased with hydrate saturation.The abnormal increasing of damping ratios in hydrate-bearing sediments may relate to the unconverted water around the particles,which caused the extra attenuation of energy between surface propagation.In addition,with the hydrate saturation increasing,the stress-sensitive constant shown that the hydrate played a dominant role in dynamic mechanical properties of specimens,rather than the stress.Yet,the stiffness degradation curves of specimens shown that the normalized modulus of specimens with high hydrate saturation weakened faster under identical strain variation.Besides,the wave measurement tests proved further that the dynamic mechanical properties of specimens also obviously changed with hydrate formation and decomposition.The wave velocity and energy attenuation both increased with hydrate saturation,whereas the poisson’s ratios decreased.(4)the micro-structure and hydrate distribution,governing the mechanical behaviors of specimens,were effected by various factors.The pressure-temperature path,including frozen history,dominated the water migration during hydrate formation.The hydrate,formed by the cooling and then pressuring path,preferred to nucleation at the core boundary and growing toward the core center,because of the exothermic effect during hydrate formation and the effective heat dissipation at boundary.The existence of fine particles in coarse specimens was benefit for suppressing the water migration.Because the conversion rate between water and hydrate limited by the heat and mass transportation,the hydrate distribution and amount in fine specimens under various water contents were different.Furthermore,the mixing order of specimens would change the initial morphology of different phases and the final distribution of hydrate.(5)the wave velocity based shear strength prediction model =(1-(1 [?(6? ?6)8)/?? ] + ?2,derived from the theories of rock physics model and Mohr-Coulomb criterion,was good for the application in hydrate reservoirs.With the help of the common used field sonic logging data,the formation wave velocity and effective stress,the shear strengths could be precisely and easily calculated through the proposed semi-empirical formula.Considering the parameter changes with different hydrate morphologies,it could be extended to various hydrate reservoirs.This approach served great benefits for the shear strength evaluation on hydrate reservoirs,especially when the in-situ strength tests were not available in field.