Study On The Stability Characteristic Of Gas Hydrate In Clayey-silt Samples During Freezing Sampling Process

By 2020,China's external dependence on crude oil and natural gas had reached73.5% and 41.7%,respectively.The energy situation is very serious,so it is urgent to explore alternatives for conventional oil and gas resources.Moreover,the world energy supply is developing in the direction of clean,efficient,and low-carbon.Deepening the development and application technology of clean and low-carbon energy and improving the green energy system are the key strategies to being carbon neutral.Natural gas hydrate is a kind of unconventional clean energy,which is widely buried and had huge reserves.Natural gas hydrate resources are abundant in China's sea area.In the Shenhu area of the South China Sea,the first and second natural gas hydrate production tests had been completed in July 2017 and March 2020,respectively,and the commercial development of natural gas hydrate was gradually moved towards.The sampling and core analysis of natural gas hydrate can realize the grasping of reservoir information in detail and delineation of natural gas hydrate mining area,which plays an important role in resource reserve prediction and evaluation of production technology.Based on the stability of Natural gas hydrate at low temperature and high pressure,pressure-maintaining sampling devices are mainly adopted to obtain natural gas hydrate cores abroad.However,due to the complex environment of the hole-bottom and harsh requirements for mechanical seal,the ratio of core acquisition was low and the success rate was not high in pressure-maintaining sampling.For hydrate exploration implemented in the South China Sea,China has rented foreign hydrate coring tools at a high price.The fidelity sampling for marine natural gas hydrate drilling in the sea area is still a key technical difficulty to be solved urgently in China.Based on the theory of the phase equilibrium of gas hydrate,to achieve fidelity sampling,the freezing sampling technique reduces the critical dissociation pressure of natural gas hydrate and prolongs the stability time of the sample during lifting by freezing the sample at the bottom of a well,as well as inhibits hydrate dissociation based on the unique self-preservation effect of natural gas hydrate.Although the drilling tool development and refrigeration efficiency theory for the freezing sampling technique of natural gas hydrate is becoming increasingly perfect,the research on the stability of clayey-silt hydrate during sampling is still insufficient.According to the changing characteristics of temperature and pressure environments during the sampling process,this paper used clayey-silt hydrate samples from the South China Sea as the main subjects and carried out the corresponding theoretical and experimental research on the stability of the clayey-silt hydrate samples during sampling.To verify the self-preservation effect of clayey-silt hydrate at low temperatures and analyze the influencing mechanisms of temperature and pressure on its stability,the dissociation of clayey-silt hydrates at low temperatures were investigated from the core scale by numerical simulation using Tough + Hydrate code based on the reservoir parameters of clayey-silt hydrate in the South China Sea.It is found that the methane hydrate dissociated slowly at low temperatures,and the time required for complete dissociation increased with the decrease in temperature.Samples with similar reservoir properties of clayey-silt were prepared,methane hydrate was synthesized in the samples,and their stability was tested under different low-temperature conditions.Consequently,the self-preservation effect of methane hydrate in clayey-silt was verified.Additionally,the effects of temperature and pressure on the stability of clayey-silt hydrate were explored,revealing that stable low-temperature condition was favorable for the selfpreservation effect of methane hydrate in clayey-silt samples,and the stability of hydrate was positively correlated with dissociation pressure.Furthermore,through numerical simulation and experimental study,the stability mechanism of clayey-silt hydrates at low temperature was analyzed,and the target freezing temperature of freezing sampling for natural gas hydrate was preliminarily determined.During sample lifting,the temperature and pressure of clayey-silt hydrate will go through the metastable state of pure methane hydrate.To investigate the stability characteristics of clayey-silt hydrate at the metastable state,the dissociation enthalpy of clayey-silt hydrate was calculated using the Clausius-Clapeyron equation.It was found that the temperature and pressure required for the stability of clayey-silt hydrate were lower than those under the corresponding phase equilibrium conditions.When clayey-silt hydrate was in the metastable range,the dissociation is extremely slow.Moreover,the dissociation characteristics of methane hydrate at the metastable state were analyzed by numerical simulation in molecular dynamics using Materials studio,and the mechanisms of self-preservation effect and metastable state of the hydrate were revealed from the micro-level.The stability characteristics of clayey-silt hydrate at the metastable state could increase the stability time of hydrate samples during lifting.It is difficult to observe the phase transition of water and hydrate dissociation in micro-pores of clayey-silt.The condensation rule of supercooled water in clayey-silt was monitored at low temperature using low-field nuclear magnetic resonance(NMR).The transition rule of supercooled water during hydrate dissociation and its effect on dissociation rate were analyzed.During the freezing sampling of natural gas hydrate,accurate control of freezing temperature and the effect of the phase transition of supercooled water on hydrate stability can effectively inhibit hydrate dissociation and improve the fidelity of in situ samples.In conclusion,through the above theoretical analysis,numerical simulation,and experimental study,the self-preservation mechanism of hydrate during freezing sampling,the stability characteristics of clayey-silt hydrate at the metastable state,and the influencing mechanisms of supercooled water formation and condensation on hydrate stability were explored in depth.While further improving the deep theory of freezing sampling technique for natural gas hydrate,it supports and guides the design and construction technique of drilling tools for freezing sampling.In addition,the dissociation characteristics of natural gas hydrate at the metastable state are of great significance in the efficient exploitation of hydrate.The rapid formation of hydrate in clayey-silt and their extremely high self-preservation effect has huge potential application value in gas storage and transportation.