Study On The Mechanism Of Natural Gas Hydrate Production By Radial Well Depressurization

Natural gas hydrate is a clean and efficient potential alternative energy source.The oil equivalent of natural gas hydrate resources in the South China Sea is estimated to exceed 60 billion tons.The development of hydrate resources is of great significance to ensure national energy security,but the low gas production efficiency has seriously affected the industrial development of hydrates.Radial well technology is considered an important technical means to achieving high-efficiency development of hydrates.Studying the mechanism of depressurization and exploitation of natural gas hydrates in radial wells can provide a theoretical basis for the field application of this technology.In this paper,laboratory experiments and numerical simulation methods are used to study the productivity and geomechanical responses of hydrate extraction by radial well depressurization.The main results are as follows:(1)A device set was designed to simulate hydrate extraction,and the difference between hydrate extraction by vertical and radial wells depressurization was compared.When extracting hydrate with a radial well,the pressure and temperature in the reactor drop significantly,which is beneficial to the decomposition of hydrate and the production of decomposed gas.At the end of the simulation,the cumulative gas and water production of radial wells are significantly higher than those of vertical wells.(2)The open-source program HydrateResSim was improved,and a 3D model of hydrate reservoir mining by radial well depressurization was established.The influence of well completion parameters and stratum physical properties on the productivity of hydrate reservoirs is studied.The stimulation ability and mechanism of radial wells for hydrate production are revealed.The research provides a theoretical basis for the design of hydrate production by depressurization in radial wells.The study determined that the hydrate decomposition rate and the recovery rate are positively correlated with the total length of the branch;the high permeability of the reservoir and the low permeability of the caprock,as well as the high permeability in the horizontal direction and the low permeability in the vertical direction,are conducive to the efficient development of hydrate by radial wells.(3)Experiments and numerical simulations of hydrate production by depressurization show that the main stimulation mechanisms of hydrate production by radial well depressurization are as follows: promoting the propagation of pressure drop and expanding the hydrate decomposition front;providing channels with high conductivity,and increasing the drainage area;reducing the phase equilibrium temperature of hydrate and increasing the sensible heat available for hydrate decomposition in the formation;enhancing the heat convection and heat conduction intensity inside the formation to provide more heat for hydrate decomposition.(4)A thermal-hydro-mechanical-chemical coupling model was established by using CMG STARS.Mechanical responses during hydrate production with radial well depressurization are simulated,and the influence of production pressure and mode is analyzed.The conclusions are as follows: for hydrates exploiting with radial wells,formation subsidence is quite severe,and it happens mainly in the early stages of production;Hydrate mining by radial wells leads to a slight inclination angle of the seabed,and geological catastrophes are unlikely to occur;The influence of formation subsidence on wellbore and wellhead stability should be considered in hydrate production design;increasing production pressure or using intermittent production can reduce the mechanical response intensity during hydrate production but significantly reduce gas production efficiency.The research in this paper provides a theoretical basis for applying radial well technology in hydrate mining.