| Natural gas hydrate has the characteristics of huge reserves,high energy density and wide distribution.It is a kind of clean energy with high efficiency and high quality.Drilling sampling can provide the most accurate data for hydrate resource evaluation.However,in the process of hydrate coring,if the temperature and pressure of hydrate core exceed the temperature and pressure zone of hydrate,hydrate will decompose and destroy the original properties of core.The ball valve or flap valve is used to seal the core barrel,and the accumulator pre charged with high-pressure inert gas is used to maintain the pressure in the sampling barrel,so as to prevent the decomposition of hydrate samples.In this paper,the natural gas hydrate freezing sampling method is to use external cold source to freeze the hydrate sample at the bottom of the hole to a low temperature state,activating the self-protection effect of the hydrate,so that the hydrate will not decompose under low pressure,so as to obtain the high fidelity hydrate sample.The structure of the freezing sampling drilling tool is simple and the core with large diameter can be obtained.The dry ice-alcohol freezing method is one of the methods of freezing sampling,which uses the dry ice-alcohol mixture as the cold source.Through the previous freezing sampling experiments,it is found that although the core can be frozen by using this freezing method,the utilization efficiency of the freeze energy of the dry ice-alcohol mixture is low,only 10.75%,which can not make full use of the cold energy of the dry ice-alcohol mixture.In order to improve the refrigeration efficiency of the cold source,further reduce the freezing temperature of the core,and shorten the length of the sampler in structure,this paper carries out the experimental research on improving the refrigeration efficiency of the dry ice-alcohol mixture,determines the main factors affecting the refrigeration efficiency,and obtains the process parameters and technical measures to improve the refrigeration efficiency.Cold source storage process and frozen core process are the two main processes that affect the refrigeration efficiency in the frozen sampling method.To improve the refrigeration efficiency,it is necessary to improve the storage efficiency of dry icealcohol mixture and frozen core efficiency.This paper takes this as the research idea,and takes the natural gas hydrate rope bottom freezing double elastic card sampling drilling tool as the research object,designs and processes the natural gas hydrate cold source storage and sample refrigeration joint experimental device.Based on the previous experimental results,four factors are further determined,including the ratio of dry ice to alcohol,dry ice particle size,exhaust pressure and the initial temperature of dry ice-alcohol mixture,as the main factors affecting the refrigeration efficiency.Orthogonal optimization experiments of the two main processes are carried out to reveal the internal mechanism affecting the refrigeration efficiency and determine the technical parameters of the main factors affecting the refrigeration efficiency.The main freezing process parameters of freezing sampling drilling construction are analyzed and calculated.In order to improve the storage efficiency of dry ice-alcohol mixture,a cold source storage experiment is carried out.The storage efficiency of cold source plays an important role in improving the energy utilization efficiency of cold source.The effective storage time of cold source is about 120 min according to the time required for single loop of wireline coring.The effects of the ratio of dry ice to alcohol,dry ice particle size,exhaust pressure and initial temperature on the storage efficiency of cold source were determined.The results show that the cooling capacity of the cold source in the cold storage mechanism is less,and it can still provide a lot of refrigeration energy after the end of the storage process.From the analysis of the temperature,storage efficiency and exergy after the storage of the dry ice-alcohol mixture,it is found that the exhaust pressure has a significant impact on the temperature and storage efficiency of the dry ice-alcohol mixture at the end of the storage.The nano aerogel felt is used as insulation material,and the thickness of insulation layer is 11.5mm.According to the experimental results,the optimal parameters of freeze sampling dry ice-alcohol mixture are as follows: dry ice-alcohol ratio is 2.5:1.75,exhaust pressure is 0.1MPa,dry ice particle size is particle,and the initial temperature of cold source is-85 ?.After the cold source storage time of 120 min,the temperature of the mixture of dry wine and dry ice is as low as-78.85 ?,and the storage efficiency is 81.6%,which meets the technological requirements of cold source temperature and freezing energy for frozen core.In order to improve the efficiency of freezing core with dry ice-alcohol mixture,the orthogonal experiment of freezing core was carried out.According to the technological requirements of freezing sampling at the bottom of hole,the freezing experiment of simulated core is carried out on the premise that the maximum time required for core freezing at the bottom of hole is 30 min.According to the volume requirements of the refrigeration chamber in the combined experimental device of cold source storage and sample refrigeration,the ratio of dry ice to alcohol was determined to be 2.5:1.5,and the effects of dry ice particle size,exhaust pressure and initial temperature of cold source on the refrigeration efficiency of dry ice-alcohol mixture were determined.From the three aspects of freezing core temperature,freezing efficiency and exergy of dry ice-alcohol mixture,it is further analyzed that the size of dry ice particles and exhaust pressure are the key factors affecting the freezing effect of core.The more powder dry ice in the mixture of dry ice and alcohol,the more difficult it is to inject into the freezing chamber.The higher the exhaust pressure is,the lower the refrigeration efficiency of the mixture is.It was also found that the distance between the control valve plug of the cryosampler and the lower insulation of the cold storage chamber was 17.5mm,which affected the process of injecting dry ice-alcohol mixture into the cryochamber.Through comprehensive analysis of the freezing core process,the optimized ratio of dry ice-alcohol mixture is: dry ice: alcohol = 2.5:1.5,granular dry ice is selected,the initial temperature of the cold source is-85 ?,and the exhaust pressure is 0.1MPa,which can reduce the temperature of the simulated core to-59.7 ? within 30 min.Through the cold source storage experiment and frozen core experiment,using dry ice: alcohol = 2.5:1.5,using granular dry ice,the initial temperature of the cold source is-85 ?,and the exhaust pressure is 0.1MPa,the refrigeration efficiency reaches 32.35%,which is nearly 2 times higher than the previous research.Using the experimental research results of this paper,this paper analyzes and studies the technological parameters of the sampling construction of natural gas hydrate bottom hole freezing drilling,determines the main freezing technological parameters of the sampling construction,as well as the technical measures to improve the utilization rate of freezing energy,and forms a complete set of natural gas hydrate bottom hole freezing drilling sampling technology,which is important for carrying out the sea hydrate freezing sampling experiment Guiding significance. |
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