Numerical Simulation Of Geo-Mechanical Behavior For Hydrate Bearing Sediments Considering The Influence Of Hydrate Dissociation

Methane hydrate,a form of clean energy also called inflammable ice,has drawn a global interest as an alternative energy resource of traditional fossil energy.Rich methane hydrate recourses have been found in South China Sea,and it comes at a crucial moment for China to start the trials of field methane hydrate gas extraction.Existing trials of hydrate gas production are conducted in the gas recovery methods of depressurization,heat injection,chemical inhibition,during which hydrate dissociates and generates gas and water.The mechanical behavior of methane hydrate bearing sediment(MHBS)is influenced by the process of hydrate dissociation,which possibly induces catastrophic failures,i.e.layer collapse,sliding as well as the damage of infrastructures.Therefore,the potential geohazards caused by hydrate extraction are turning into the primary goal for the future field trials in China.It is important to consider the influence of hydrate extraction on the mechanical behavior of MHBS layer and study the failure mechanism of MHBS layer,in order to evaluate the stability of soil formation and infrastructure.Numerical simulations,field tests and laboratory tests are the most widely used methods in the investigation of MHBS mechanical behavior.Numerical simulation requires an in-deep and comprehensive understanding of the behavior of MHBS so as to build a constitutive model and a fully coupled thermo-chemo-hydro-mechanical simulator.In this research,a constitutive model based on energy dissipation theory and the concept of critical state is proposed enabling the consideration of the influence of hydrate on sediments mechanical behavior,validated by the test results of other researchers.Meanwhile,fully coupled thermo-chemo-hydro-mechanical models are built up to consider the impact of multi-physical fields such as temperature,gas pressure,water pressure and hydrate saturation on mechanical behavior of MHBS.This research focuses on the mechanical behavior change of MHBS layer during hydrate extraction and includes:1.The relationship between mechanical hydrate saturation and yield loci is built on Drucker-Prager yielding theory.Meanwhile,hardening rule related to equivalent plastic strain is also considered in this model.This model can be used to predict the mechanical behavior of MHBS though the validation of the comparison between this Drucker-Prager model simulation results and triaxial test results given by Miyazaki.2.Energy dissipation theory-based critical state constitutive model is proposed to describe the stress-strain relationship and dilatant behavior of MHBS.Compared with traditional critical state model,non-associated flow rule is derived from the differential of dissipation function.Thus,the second thermomechanics principle can be guaranteed automatically in this model.The influence of the shape and size of yield surface on the prediction of mechanical behavior is considered by introducing two parameters into the model.The limitation of traditional critical state model is eliminated that the dilatant part and contraction part on the yield surface may not be equal in this model.Return-mapping algorithm for MHBS critical state model is established.This model is validated by the test data offered by Masui.and Hyodo,which indicates that this model has the advantage of accurate prediction considering non-ellipse yield surface for MHBS.3.Based on finite element method,fully coupled thermo-chemo-hydro-mechanical simulator for hydrate extraction is established enabling the consideration of temperature,pressure and hydrate dissociation and their influences of geo-mechanical behavior on MHBS.By comparing results of this simulator with the test data offered by Masuda and the simulation results provided by TOUGH,FLAC and STARTS etc.,this model is validated for accurate prediction of hydrate dissociation during depressurization and heat injection.Moreover,simulations show that a joint depressurization and heat injection method is much more efficient than their solo method for gas production.Simulation results demonstrated that the mechanical behavior of the MHBS is influenced by the hydrate extraction method.This conclusion is important in drawing up plans for future hydrate extraction trials.4.The case studies of the influence of temperature increasing due to the well-drilling process on MHBS and the gas extraction in depressurization method are conducted in this thesis.By investigating the stress distribution and deformation of heterogeneous hydrate distributed formation,for the hydrate bearing soil layers,hydrate dissociation causes stress relaxation and volumetric contraction,which also increases the permeability in sediments layers and accelerates heat convection.The stress-strain relationship and dilatant behavior of the sediments layer without hydrate are also influenced by the soil compaction in MHBS layers.The interaction between MHBS layers and the layers without hydrate leads to heterogeneous deformation and stress distribution These results will be good references for future well bore stability analysis in field trials in South China Sea.