Development Of Electromagnetic Heater For Gas Flow Assurance In Marine Natural Gas Hydrate Production

Natural gas hydrate is a new type of clean energy with polar molecules.At present,it is commonly used the depressurization method for producing natural gas hydrate.It has been shown that in the early stage(or late stage)of the decomposition by the depressurization method,water and natural gas are extremely easy to regenerate hydrate because of the changes in temperature and pressure.It usually reformed in the gas production channels,wellbore,blowout preventer(BOP),choke pipeline and other parts of the reservoir causing the risk of gas production rate drop or production shutdown.Therefore,it is very important to ensure the gas circulation of natural gas hydrates during the development process and prevent the secondary production of hydrates.In order to suppress the secondary hydrate,the supply of heat can restore the gas production efficiency from the mechanism.The traditional heating method has a slow heating rate and large heat loss.In contrast,the electromagnetic wave heating method can quickly heat as needed,and it has been demonstrated its effectiveness in heavy oil.In theory,microwave is a kind of electromagnetic wave,thus,microwave heating can be used to accelerate the decomposition of gas hydrates and prevent the secondary formation of hydrates.The research has been carried out on the microwave heating system to prevent secondary natural gas hydration,which is funded by the National Key Research and Development Plan Strategic International Scientific and Technological Innovation Cooperation,Key Special Project “Joint Research on Natural Gas Hydrate Exploration and Development Technology”(SQ2018YFE020424),sub-project“Research and Development of Natural Gas Hydrate Underground Heating and Replacement Mining Equipment”.Based on the principle of the microwave heating system and the influence of microwave on natural gas hydrate,this research builds a set of natural gas hydrate indoor test bench system,which mainly includes microwave generator,the transmission system,microwave protection system,microwave heater(antenna).The microwave heater can be inserted into the reactor to simulate the heating effect of the microwave on the natural gas hydrate in the well section.The key part of the natural gas hydrate indoor test bench system is the design of antenna.Based on electromagnetics and antenna transmission theory,a microwave antenna with “H-shaped” coaxial slot is designed,and the key parameters of the coaxial slot antenna are calculated.The coaxial slot antenna consists of a solid inner conductor and a hollow outer conductor.The outer conductor is designed with multiple H-shaped cracks,and the inner and outer conductors are filled with Teflon.The frequency of the antenna is 2.45 GHz.The impedance of antenna is 50?.The diameter of the inner conductor is 9mm,as for the diameter of the outer conductor is30 mm.The length of crack is 12mm(radial)and 14mm(axial),the gap between the cracks is 35 mm,the distance between the center of a gap and the microwave input end is 16 mm,and the width of the crack is 2mm.The antenna can be easily lowered into the gas production channel to prevent secondary formation of hydrates.According to the simulation of the far-field radiation in the coaxial slot antenna designed by HFSS software,the antenna has good omnidirectional radiation.By combining the software ANSYS and HFSS to simulate the effect of secondary hydrate(diameter is 100 mm,length is 2m)heated by the antenna(length is 1m,and there are 19*4 H-shaped slots),it was found that the natural gas hydrate can be heated from-3.7? to 17.8? in 1800 s.And in 3600 s,the hydrate can be heated from-3.4? to 27.3?.Based on the microwave heating system and microwave heater as the prototype,combined with the actual South China Sea test production data,the preliminary engineering plan was designed.It was planned to arrange 4 coaxial antennas(length is 500 mm,and there are 9*4 H-shaped slots)evenly in a well section with a diameter of 500 mm and an axial interval of 90° at a distance of 200 mm from the center of the well section.Through numerical simulation analysis of HFSS software and ANSYS,it was found that the coaxial antenna can heat the natural gas hydrate from-3?to17.3? in 1800 s.