Research On The Speeding Up Mechanism Of Forming Ice Valve From The Drilling Fluid For Natural Gas Hydrate Exploration

Natural gas hydrate has been widely concerned as a potential energy resource.It also has important effects on the submarine landslide and global warming.The exploration of natural gas hydrate has been initially proceeded in China since 2002,and the exploring technology of natural gas hydrate has also been listed as one of the key research contents up to now.In recent years,the Pressure Coring System has become the essential tool for natural gas hydrate investigation.Although PCB(Pressure core barrel)has been firstly applied in DSDP(Deep Sea Drilling Project),several other pressure coring systems have been subsequently developed.However,due to the ball valve or flapper valve used to be eroded by drilling fluid or blocked by drill cuttings,thus the pressure retaining success ratio of these pressure corers based on ball or flapper valve are very low.In order to improve the pressure retaining success ratio of natural gas hydrate sampling,the pressure corer based on ice valve formed from the drilling fluid were proposed to solve the problems encountered in pressure corers based on mechanical valve.The ice valve based pressure corer has several advantages: Firstly,no moving parts were enclosed in the pressure corer,therefore no possibility of abrasion or jamming would occur,which can improve the stability of valve.Secondly,the ice valve can be formed at rough surfaces without the requirements of high quality working surface.In the end,no additional seals mechanism made of high molecule materials are needed.These advantages can be employed to overcome the shortcomings of mechanical valves while working under the harsh borehole condition.The ice valve formation time is one of the key parameters for ice valve based pressure corer.In order to reduce the ice valve formation time,the speeding up mechanism of ice valve formation from drilling fluid has been numerically and experimentally studied,and the study details and results are presented as follows:First of all,several ice-valve models were established based on the fluid-structure interaction equations,heat transfer equations,and liquid-solid phase change equations,several ice-valve models were established,and Ansys/Fluent software was employed.Therefore,the numerical methods are an effective way to investigate the speeding up mechanism of ice valve formation from drilling fluid.Study results have shown that the material of the ice valve tube has little influences on the ice valve formation time when the bottom mouth of the ice valve tube is sealed by a metal plug.Both the temperature of the cooling fluid and length of the ice valve have significantly effects on the ice valve formation time.Lower temperature benefits to reducing the ice valve formation time,and long ice valve needs more time to form the ice valve completely.Secondly,the influence of each factor on the ice valve formation time was studied in the case of that the bottom mouth of the ice valve tube is actually unsealed,which simulated the practical sampling process in field.Results indicate that the formation time remarkably increases when the bottom mouth of the ice valve tube is unsealed for the natural convection resulting from temperature difference between the center of the ice valve tube and borehole bottom.Eventually the annular space between the drill tool and the borehole would also be frozen.Thirdly,the methods by employing heat conducting inserts to accelerate the ice valve formation were proposed,and the structure design of the heat conducting inserts was optimized.Both the numerical and experimental results show that it is an effect way to employ heat conducting inserts to accelerate the formation of ice valve from drilling fluid.Comparing to the unsealed ice valve tube without heat conducting inserts,the presence of heat conducting inserts can reduce 6.5 min of ice valve formation time.Meanwhile,the material of heat conducting inserts has little effect on the ice valve formation time,and more heat conducting inserts layer benefits to reduce the formation time.In conclusion,the heat conducting inserts which are 0.9 mm thickness can accelerate the ice valve formation time faster than others.