| Natural gas hydrates(NGH)are mainly present in sediments and terrestrial permafrost zones,and are recognized as clean energy sources that can replace conventional oil and gas resources in the future.NGH is easily decomposed,can only be stable in the low temperature and high pressure conditions.To obtain high-fidelity hydrate samples is the key to the exploration and exploitation of NGH.Currently,the primary methods used for NGH core sampling are pressurepreservation techniques.The NGH pressure-preservation technique uses a high-pressure ball valve or a flapper valve,operated by a torsional spring,to seal the pressure barrel.At the same time,the initial temperature of the hydrate is maintained by the use of an insulating material or by means of electronic refrigeration.Less than 60% of gas-hydrate cores can be retrieved by the existing pressure preservation technique because of mechanical valve sealing failures.These samplers can only provide a small diameter core,due to the ball-valve structure.The pressurepreservation techniques(?168mm)can only have ?50mm NGH-sample,can not fully meet the geological test analysis requirements.Therefore,we propose the hole-bottom freezing technique to solve the problems of the NGH pressure-preservation core-sampling technique.The technique uses an external cold-source to freeze the hydrate core.This technique reduces the critical decomposition pressure of the hydrate by cooling,and activates the self-protection effect of the hydrate to suppress dissociation and enables the obtaining of the refrigerated hydrate core.The marine hydrate is present in the loose layer of 0-1500 m below the seafloor of 300-4000 m.The non-wireline coring takes a long time to have the NGH-sample and the efficiency of the frozen NGH-sample is low,which can cause the hydrate decomposition.Therefore,this paper proposes the organic combination of the wire-line sampling method and the hole-bottom freezing sampling method.To carry out cold source optimization research,applied to the hydrate drilling sampling.First,this paper analyzes the hole-bottom frozen target temperature of the frozen NGHsample,guides the selection of the cold source and the design of the sampler.According to the temperature range(-31 ? ~-2 ?)of NGH self-protection effect,combined with the decomposition characteristics of NGH at low temperature.The decomposition characteristics of hydrate at different low temperature conditions under normal pressure were simulated by TOUGH + HYDRATE software.The decomposition of the hydrate at-30?~-5? was carried out.Finally,the hole-bottom frozen target temperature of the frozen NGH-sample was-30?~-15 ?.Combining the hole-bottom frozen target temperature and the current hole-bottom freezing sampling method,the key question is how to improve the freezing efficiency of the sample.The sampling technique uses dry ice as the refrigerant.Dry ice and alcohol are mixed in the mixing chamber of the sampler to form cryogenic alcohol as a cold source.However,due to the size limitations of the sampler itself,the small contact area of the alcohol with dry ice in the mixing chamber results in a low mixing efficiency of alcohol with dry ice.It is difficult to form a cryogenic alcohol that can meet the requirements of a frozen hydrate core in the mixing chamber of the sampler.To obtain a cryogenic alcohol with sufficient freezing energy,three mixing chamber structures(normal mechanism,multilayer-shelf mechanism,perforated-pipe mechanism)were designed for improving the mixing efficiency of alcohol and dry ice by increasing the contact area in the mixing chamber.Alcohol and dry ice mixed test results show that the lowest temperature of the cryogenic alcohol obtained from the perforated tube structure is-30.3?.And then carried out frozen sample test,the results show that-30.3? cryogenic alcohol can not meet the needs of frozen samples.It is therefore proposed to further prefer a cold source to rapidly freeze the larger hydrate sample to the target temperature range.According to the principle of cold source preferred,the alcohol-dry ice mixture,liquid nitrogen and liquid nitrogen-alcohol mixture are used as cold source.According to the two processes of hole-bottom frozen sampling,that is,the hole-bottom stored process of the cold source and the hole-bottom freezing process,the numerical simulation and experimental study of the three cold sources were carried out to determine the best cold source.Hole-bottom stored process of the cold source: ANSYS-Fluent was used to simulate the holebottom stored process.The simulation results showed that the liquid nitrogen loss was 52% and the alcohol-dry ice mixture lost 0.9 %,temperature rise of alcohol-liquid nitrogen mixture is 5?.At the same time,three kinds of cold source storage experiments were carried out.The results showed that the liquid nitrogen was completely vaporized during the 2-hour storage process,and the alcohol-dry ice mixture lost 0.018%,and temperature rise of alcohol-liquid nitrogen mixture is 22?.Based on the numerical simulation and experimental results,it can be seen that the storage effect of alcohol-dry ice mixture is the best.Hole-bottom freezing process:ANSYS-Fluent was used to simulate hole-bottom freezing process.The simulation results showed that the sample can be frozen from 2? to-48.7? during the 30 min sampling time.The alcohol-dry ice mixture can freeze the sample to-24.3? in the same time.The alcohol-liquid nitrogen mixture can freeze the sample to-7.5?.The Holebottom freezing experimental of the three cold sources results showed that the freezing effect of liquid nitrogen was zero due to the unstable physical properties.Under the same conditions,the simulated sample can be frozen to an average temperature of-24? by adjusting the amount of dry ice in the alcohol-dry ice mixture.The simulated sample can be frozen to-2? by the alcohol-liquid nitrogen mixture.Based on the numerical simulation and experimental results,it can be seen that the freezing effect of alcohol-dry ice mixture is the best.The simulation and experimental results of the two processes,and the numerical simulation results of the frozen NGH-sample show that the minimum temperature of the alcohol-dry ice mixture can reach-110? and can be kept in the hole(5?)for 120 min.The alcohol-dry ice mixture can freeze the sample from 2? to-42.56? in 7 min.The sample temperature can be maintained within the target temperature range for 30 min.The mixture utilizes the sublimation endothermic properties of dry ice and long-lasting cryopreservation,allowing rapid freezing of the sample to the target low temperature range.Thus the alcohol-dry ice mixture was determined as a cold source for the hole-bottom freezing sampling technique.The use of alcohol-dry ice mixture as a cold source,combined with the wire-line sampling technology and hole-bottom freezing sampling technology,designed the wire-line hole-bottom freezing sampler.The sampler diameter is ?127mm,the sample diameter is ?50mm,the sample length is 2000 mm,the total length of the sampler is 6878 mm.The sampler includes fishing mechanism,positioning mechanism,suspension mechanism,cold source injection control mechanism,single action mechanism,adjustment mechanism,cold source storage mechanism,freezing mechanism and sampling mechanism.The cold source storage mechanism,the cold source injection control mechanism and the freezing mechanism are the three most important institutions.Respectively,to achieve the hole-bottom storage of the cold source,control the cold source from the cold storage mechanism into the freezing mechanism,freezing the sample of the three main functions of the sample.Design and calculate the overall structure and key technical parameters of the sampler.There are ten key steps involved in the process of the wireline hole-bottom freezing sampling technology: sampler debugging,preparation of ultra-low temperature cold source,cold storage precooling,cold source filling,drilling,hole-bottom storage of cold source,cold source injecting,hole-bottom freezing sample,frozen sample acquisition,sample storage and analysis.Conducted a drilling test,the test results show that sampler can meet the design requirements.This paper analyzes the characteristics of the wire-line hole-bottom freezing sampling technology.Combining the target temperature range of the hole-bottom frozen NGH,the study on the efficiency of the hole-bottom frozen NGH was put forward for the first time.The efficiency of hole-bottom frozen sample is improved by the preferred cold source,and the frozen sample is realized by the cold source.Based on the hole-bottom freezing sampling method and wire coring method,the wire-line hole-bottom freezing sampler are designed and the drilling tests results showed that the sampler can meet the design requirements.In this paper,the key technical parameters and the structure of the sampler are put forward.The process flow of the frozen sampling are summarized,which provides theoretical support and technical reserve for t the exploration and exploitation of NGH. |
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