Study On The Deformation Microstructures And The Microscopic Damage-healing Mechanism Of Salt Rock

With the increasing demand for energy consumption and the need for national strategic energy reserves,energy storage is becoming increasingly important,especially in the context of continuous fluctuations in international oil and gas prices caused by uncertainties such as natural disasters,politics or wars.Energy storage systems will become the key element in achieving energy sustainability that will give rise to the energy centralization,supply-demand balance and cost savings.However,China's natural gas reserves and strategic petroleum reserves(SPR)are far behind those developed countries,such as Europe and the United States.To date,various types of energy storage methods have been used and developed,one of the most important technologies is storing in underground caverns which has the advantages of cost effectiveness and so forth.For the underground energy storage systems,rock salt(so-called halite or salt rock)is commonly known as sodium chloride crystalline mineral with low porosity(<1%),low permeability(<10^-20 m²),high ductility(strain up to 30-40%),excellent water-soluble mining and good self-healing characteristics.It is likewise of interest as ideal host rock for wide applications in chemical barriers,hydrocarbon accumulations,storage of hydrocarbons(e.g.natural gas or crude oil),hydrogen,compressed air,anthropogenic carbon dioxide,radioactive nuclear waste disposal and so forth over the past few decades.In China,however,the underground salt caverns have been used as energy storage systems only since 2007-the first salt cavern gas storage in Jintan Salt Mine with less than 20 years of practical experience.There is still a long way to go in this filed.The geological structures of Chinese salt mines are dominated by lacustrine sedimentary layered salt rocks,which have complex mechanical properties and are related to the geological structures,sedimentary environment and mineral compositions.The existence of impurities and interlayers in bedded salt rocks has an important influence on the stability,tightness and safety of surrounding rocks in salt caverns.Based on the engineering background of the comprehensive utilizations of the underground salt caverns,and taking a Zhaoji salt mine,Huai'an,Jiangsu province,for an example,this thesis studied the basic physical-mechanical-chemical characteristics of layered salt rocks,on the basis of experimental analysis and theoretical research.It included the physical properties of layered salt rocks,deformation microstructures,the meso-/micro-scopic damage mechanism,fatigue damage as well as microcracks healing mechanism of damaged salt,and so on.(1)The physicochemical properties of layered salt rocks are closely related to their mineral compositions,impurity contents,temperature,pressure and other environmental conditions.In general,the interlayers often play the role of"reinforcing layers",which improves the strength and stable creep rate of layered salt rock.With the increase of confining pressure,the interlayer could inhibit the plastic transformation of salt.The shear strength of layered salt is strong,while the tensile strength is weak.The interface between interlayers is a"non-weak plane".Due to the difference of deformation resistance between salt and non-salt layers,the fatigue life of bedded salt could be reduced.Insoluble impurities can reduce the dissolution rate of salt and cause heterogeneous dissolution.The permeability of layered salt is generally 1-4 orders of magnitude higher than that of pure salt.(2)The presence of impurities or interlayers could significantly change the microstructural characteristics of bedded salt.Taking a salt mine in Zhaoji,Huai'an,Jiangsu province,as the research object,the geological structure of the saline strata in this area were well analyzed.By means of XRD,mercury injection,optical microscope and SEM testing techniques,the physical properties and deformation microstructure characteristics of layered salt rocks in this area were systematically analyzed,compared with that of the pure rock salt.Study shows that the mineral compositions of the layered salt rocks are diverse.The pure salt layer is dominated by sodium chloride.The impurities mainly include anhydrite,anhydrous glauber,calcium glauber,dolomite,quartz,clay,etc.,which accord with the lacustrine sediments.Under the influence of the plastic deformation of crystal,the mechanisms of salt rheology are mainly dislocation creep and pressure-solution creep.The porosity and permeability of layered salt rocks are very low,which belongs to low permeability medium.(3)The development and evolution of cracks in the damaged salt depends on the internal microdefect structures.The micro-tensile deformation and damage tests of the small-sized layered salt rock samples were carried out.And the microscopic mechanism of crack growth and evolution on the surface of damaged salt was studied by combining Digital Image(DI),SEM,PM and SM techniques.The results show that the development and distribution of microcracks mainly depends on the composition and distribution of mineral particles,grain size,crystal defects and weak structural planes(such as bedding planes).From the initial damage to the ultimate failure,the main cracks play a decisive role.No matter what kind of salt rock,the propagation direction of main cracks is always parallel or nearly parallel to the direction of the maximum principal stress.The meso-evolution process of salt rock damage is corresponding to the stress-strain curve.Combined with the results of microscopic analysis and the dislocation plug model,the microscopic mechanism of crack development and evolution is discussed.(4)In combination with the 3D-DIC technology,the characteristics of the damage and deformation of Zhaoji salt rocks via uniaxial compression tests were analyzed.To a certain extent,3D-DIC can eliminate the errors caused by the traditional-contacting displacement and strain measurement method.For example,the total volume strain measured by the solid extensometer is larger.However,the calculated volume strain by the virtual extensometer based on 3D-DIC is more accurate.According to the characteristics of measuring full-field deformation by 3D-DIC,a salt rock damage model is established which can be used to analyze the full-field damage variables.(5)Through the fatigue damage experimental research on rock salt with gas pressure,it is found that the gas could infiltrate the micro-cracks and pores,so as to accelerate the development of dominated microcracks due to the"wedge effect".Compared with no gas conditions,the fatigue life decreased with the increase of the confining pressure and gas pressure.The fatigue life under high pressure is markedly lower than that under low pressure.Based on the CT,the calculated damage value is linearly correlated with the fatigue life,while the fatigue life is logarithmic correlated with the gas pressure.(6)The change of permeability for the damaged salt is related to the degree of crack healing,and the degree of crack healing is related to the stress,impurities,fluids and degree of damage.The crack-healing properties of damaged salt rocks in different environments were investigated.And the microscopic characteristics of crack healing in salt rocks were analyzed from the microscopic perspective.The study shows that fluid plays an important role in the healing process of damaged salt.The healing effect with saturated brine is better than that with oil due to the diffusion effect and recrystallization effect.Generally,the oil molecules contained in the micro-cracks or pores would hinder the diffusion effect and inhibit the recrystallization process.Therefore,it would hinder the fracture healing process.The decay law of the permeability coefficient of the damaged salt with the healing time satisfies the first-order kinetics equation.The attenuation rate decreases from fast to slow,and finally reaches a stable value,i.e.the saturated healing state under the test conditions.The attenuation rate of the impurity salt,however,is slower than that of the pure salt.What's more,the greater the initial damage,the higher the energy required for the healing process,and the lower the healing efficiency.With the same degree of initial damage,the healing efficiency of bedded salt with a few impurity particles is higher than that of pure salt due to the recrystallization effect.Under the action of stresses,the salt healing process is mainly represented by the process of pressure dissolution-recrystallization.The influence of impurities or interlayers on the mechanics stability,impermeability and security of bedded salt rocks cannot be neglected.These must be attached great importance to the construction and operation periods of salt caverns.All in all,the results of this paper have certain theoretical and practical significance for the comprehensive utilization of salt caverns in China.