Study On Surface Characteristics Andmechanical Behaviors Of THF Hydrate Byatomic Force Microscope

Natural gas hydrate is regarded as one of the potential,alternative,clean energysources in the 21st century besides shale gas,coal bed gas and dense gas.The global reserves of natural gas hydrate are about twice that of conventional fossil energy.Preliminary exploration shows that the potential of natural gas hydrate resources in China is enormous.Its prospects are as high as 900×10⁸?1200×10⁸t oil equivalent,which is twice that of conventional natural gas resources in China.Among them,650×10⁸?850×10⁸t in sea area and 250×10⁸?350×10⁸t in land area.Its development and utilization are of great significance for alleviating the pressure of energy shortage,ensuring energy security and optimizing energy structure in China.In May 2017,China successfully completed gas hydrate test production project in the Shenhu Sea area of the South China Sea.In December of the same year,it was approved by the Ministry of Land and Resources as the 173th mineral species in China.In March 2020,the second trial production of natural gas in the sea area was successful,achieving a major breakthrough from exploratory trial production to experimental trial production,marking that the industrialization of natural hydrate in China is accelerating.However,the industrialization of hydrate still faces the problems of safe,efficient andcontrollable drilling and production,such as borehole instability,sand production,stratum subsidence and geological disasters,secondary hydrate plugging and so on.These problems are closely related to the surface characteristics and mechanical behavior of hydrate itself,so it is urgent to accurately characterize and characterize them in order to accurately analyze and predict the above problems.Based on this,this paper innovatively built the atomic force microscope?AFM?hydrate test system,and took THF hydrate as an example,studied the micro surface characteristics and mechanical behavior of THF hydrate,formed the AFM based hydrate surface and mechanical properties test and analysis method.The paper is divided into five chapters,the main contents are as follows:Chapter 1,the significance of studying the surface characteristics and mechanical behaviors of hydrate is clarified.The basic physical properties,structure and storage characteristics of natural gas hydrate are summarized,and then the related development and research status at home and abroad and the existing problems are discussed.Finally,the main research contents and the technical route of this paper are described.Chapter 2,the composition and working principle of the modified AFM are introduced.The information of the probe used in the experiment,the synthesis method of THF hydrate sample,and the method of AFM to test the surface morphology,liquid like layer,adhesion and mechanical parameters of THF hydrate sample are introduced.And the principle of using AFM probe and modified probe and the test results of probe shape and size are described.Chapter 3,the influence of temperature,growth interface and other factors on the surface morphology,surface roughness and crystal size of THF hydrate is studied.Then,the influence of temperature on the thickness of liquid like layer on THF hydrate surface is discussed,and the theoretical method to modify the thickness of liquid like layer is proposed.The influence of contact force,hold time,temperature and microsphere size on the adhesion of THF hydrate surface is analyzed.At last,the influence of anti polymerization agent on the surface morphology,liquid like layer and adhesion of THF hydrate was studied.Chapter 4,the mechanical properties of THF hydrate were tested with SiO₂microspheres modified probe,and the relationship of load vs.indentation depth,relationship of load vs.time under different temperature and loading rate was analyzed.Considering the capillary force of microspheres and THF hydrate,the capillary force is introduced into the mechanical analysis model.The effects of temperature,loading rate,loading times and microsphere size on the mechanical behavior of THF hydrate are discussed.Chapter 5,the conclusions and innovations are given,and the shortcomings of thepaper and the future research direction are expounded.Through the above research,the following conclusions are obtained:?1?The self built AFM can be used to study the micro and nano scale surface characteristics and mechanical behavior of hydrate effectively.Compared with the conventional AFM,this system realizes the functions of sample humidity,temperature control and probe cooling.?2?Micropores were found on the surface of THF hydrate,which are not unique to type I hydrate.These micropores may be related to dissolved gas,excess guest molecules and growth conditions.The surface morphology of THF hydrate is closely related to the growth temperature.The lower the growth temperature is,the faster the growth rate and nucleation rate are,and the smaller the grain size is.The cross section of THF hydrate grains is V-shaped,and its width and depth decrease with the decrease of growth temperature.The surface morphology and surface roughness of THF hydrate,which is confined to growth at the solid-liquid interface,are related to growth temperature,surface uplift structure,surface properties of contacting solid medium,pores and grain size.The surface roughness of THF hydrate is one order of magnitude higher than that of its contact medium,which indicates that there is a non-solid transition layer between hydrate and solid medium.?3?In the temperature range of-30? to-5?,the thickness of liquid like layer on the surface of THF hydrate is about 8nm to 111nm,and the thickness of liquid like layer increases exponentially with the increase of temperature.The capillary force produced by the liquid bridge between the hydrate and the SiO₂microspheres is the main component of their adhesion.The liquid bridge model can well explain the adhesion between THF hydrate and SiO₂microspheres and the change of adhesion.The adhesion increased with the increase of contact force,temperature and microsphere size,but increased first and then decreased with the relaxation time.The trend of first increase and then decrease may be related to the melting and secondary formation of THF hydrate.The fluctuation of THF hydrate surface also affects the adhesive force,and the convex THF hydrate surface leads to the smaller adhesive force,while the concave surface leads to the opposite.Through the liquid bridge model,it is found that the adhesion is negatively related to the contact angle between the liquid like layer and the microsphere.It can be inferred that the adhesion between THF hydrate and hydrophilic material is larger under the same conditions,while that of hydrophobic material is the opposite.?4?Anti polymerization agents?dodecylbenzenesulfonic acid and naphthylacetic acid?affected the surface morphology,liquid like layer and adhesion of THF hydrate.The anti polymerization agent can make the crystal surface of THF hydrate become rough,obviously reduce the thickness of the liquid like layer on the surface of THF hydrate,and reduce the surface tension of the liquid like layer on the surface of THF hydrate,thus reducing the adhesion between THF hydrate and SiO₂microspheres.The coarseness of hydrate grain surface and the decrease of surface tension caused by anti polymerization agent may be the mechanism of preventing hydrate grain aggregation.?5?The capillary force produced by the liquid like layer should be considered when THF hydrate was measured by modified SiO₂microspheres probe.The modified power-law rheological viscoelastic model is more suitable for the interpretation of load-indentation depth curves than Hertz elastic model.With the decrease of temperature and the increase of loading rate,the power index of THF hydrate shows a decreasing trend,but the initial Young's modulus does not show an obvious rule;the yield strength of THF hydrate shows an increasing trend from the perspective of elastoplasticity.With the increase of loading and unloading times,the maximum pressing depth increases gradually,and THF hydrate shows obvious elastic-plastic deformation.This may be related to the residual stress of THF hydrate or the ice formed by decomposing water after unloading.The measured mechanical parameters of THF hydrate,such as initial Young's modulus and power index,are greatly affected by the size and shape of the microspheres,which may be related to the size of the stress area,the size of deformation or the surface fluctuation of THF hydrate.In addition,when the external force exerted by the microspheres on THF hydrate reaches the maximum value,with the increase of holding time,THF hydrate still produces obvious deformation,which is related to the viscosity of THF hydrate,and may also be related to the phase transformation of THF hydrate under the external force.