| Rapid economic development is inseparable from the continuous help of energy.Under the background of energy security,environmental problems and “double carbon”,it is urgent to seek new energy resources to optimize the energy consumption structure.Natural gas hydrate has many advantages,such as large resource reserves,wide distribution,high energy density,no pollution,etc.,and is considered one of the most potential alternative energy sources in the21 st century.Sediments containing natural gas hydrate are generally called hydrate-bearing sediments,and are referred to as energy soils for short.The exploitation process of hydrate is a dynamic process of gas-liquid two-phase continuous migration,in which the stress field evolution represented by the strength deformation of energy soils may cause many engineering geological disasters such as wellbore instability,hole collapse,platform overturning and submarine landslide.The evolution of the seepage field,represented by the permeability of energy soil,is related to the exploitation value of the reservoir and directly affects the specific exploitation scheme.In this paper,a series of experiments and theoretical studies have been carried out closely around the concrete engineering practice of hydrate production in the South China Sea,focusing on the separation of stress and seepage in the current research of energy soils,and focusing on the difficulty of describing the cross-particle size of constitutive model for energy soils.The main work and achievements are as follows:(1)Based on the high-pressure and low-temperature hydrate triaxial apparatus,a “shearseepage integrated test system” with multiple functions,such as triaxial shear,triaxial seepage and shear-seepage coupling,was formed by adding a constant-pressure infusion pump as the upper source of seepage,a back-pressure valve as the lower source,a balance and various sensors.The test results show that the system can realize various tests such as single stress field,single seepage field,non-shear stress-seepage coupling and shear stress-seepage coupling.(2)The marine clay collected from hydrate-bearing areas in the South China Sea and quartz sand were used to remold clayey-silty deep-sea energy soils.A series of consolidateddrained triaxial tests were carried out to obtain the strength and deformation characteristics of energy soils,with hydrate saturation,effective confining pressure,clay content and fine particle content as conventional factors,and overconsolidation ratio,pore pressure,shear rate and temperature as unconventional factors.The results show that different factors have different effects on the mechanical behavior of energy soils,and there are coupling effects between them.The microscopic shearing mechanism is the evolution of particle morphology and particle interaction,and it can be further summarized as “three crushing+one rotation”,that is,hydrate particle crushing,matrix sand particle crushing,cementation crushing and particle rotation.(3)The water-phase seepage tests of energy soils without considering the shear effect were carried out by using the “shear-seepage integrated test system”,and the influence laws of effective stress fluctuation,hydrate saturation and seepage pressure difference on the permeability of energy soils were explored.The results show that the increasing hydrate saturation and effective stress will lead to a decrease in permeability,and this decrease mainly occurs at the initial stage of the increase of saturation and effective stress.The mechanism of permeability reduction is the blockage of the seepage channel by hydrate particles and the extrusion of the seepage channel by effective stress.Due to the irrecoverability of plastic strain,the decrease of permeability caused by increasing stress is irreversible.The seepage pressure difference can promote permeability,and its mechanism lies in the decrease of effective stress caused by increasing seepage pressure difference.(4)Based on strength deformation and seepage tests in a single field,triaxial shear and water seepage tests of energy soils were carried out at the same time,and the “shear-seepage coupling characteristics” of clayey-silty deep-sea energy soils were further explored.The results show that the strength and stiffness of energy soils are significantly reduced under seepage,and the strain-softening trend is enhanced.There is an obvious correspondence between stress-axial strain-volumetric strain-permeability curves.In the hardening stage,because the sample is compressed(the seepage channel narrows),the volumetric strain shows shear contraction,and the permeability decreases.In the softening stage,the particles are broken and new seepage channels such as shear bands are produced,and the volumetric strain shows shear dilatation,and the permeability increases.According to the full stress-axial strainvolumetric strain-permeability curves,the typical shear process(stress-strain curve)can be divided into four stages,namely,initial compaction stage,plastic development stage,plastic expansion and fracture development stage,plastic failure and rapid expansion stage.(5)According to the experimental results and previous research conclusions,the triaxial compression and shear characteristics of energy soils were summarized in this paper.Then,in the framework of unified hardening model for clay and sand(CSUH model),the compressibility parameter related to hydrate saturation was constructed to describe the compressibility of energy soils.The cementation factor and dilatancy factor were introduced to reflect cohesive strength and dilatancy effect respectively.By using a state parameter to reflect the shear state of energy soils,and combining with the non-associated flow law,an elastoplastic constitutive model which can uniformly describe the clayey and sandy energy soils was established.The results show that the established constitutive model can uniformly describe the shear and compression characteristics of clayey fine-grained and sandy coarse-grained energy soils.It can effectively reflect the mechanical properties of energy soils such as strain hardening and softening,shear contraction and dilatation,and has a good prediction effect on the influence of hydrate saturation and effective confining pressure. |
