Numerical Simulation Of Natural Gas Hydrate Formation And Accumulation Under The Sea

As one of the new environment-friendly energy sources in the 21st century,natural gas hydrate has the advantages of high calorific value and abundant reserves.The total amount of hydrate in the world is about 2×10¹⁶ m³,of which 99%is distributed in the ocean,which is widely distributed in shallow strata of deep seabed.According to different causes,it is divided into biogenic gas hydrate and pyrolysis gas hydrate,in which the pyrolysis gas is more likely to form large scale and high abundance hydrate reservoir in deep water oil and gas basin.This paper attempts to study the mechanism of the formation of thermal gas hydrate reservoir by numerical simulation method.Based on the indoor experiment results about gas hydrate phase equilibrium and kinetic parameters,hydrate reaction kinetics model with mixed flux is set up.This model consides the methane supersaturation in the water and the difference of gas pressure and hydrate phase equilibrium as driving force to describe the nucleation and growth of hydrate.Making use of a part of numerical simulation software called partial-equilibrium simulation,this model can be converted into chemical reaction model that can be identified by the software,to set up numerical simulation model of natural gas hydrate formation and accumulation under the sea.The numerical simulation model can simulate the process that deep pyrolysis gas migrate into the shallow formation under the sea,and then drive the formation water to form hydrate and accumulate to reservoir.Next,it can be to study the mainly control factors under the homogeneous formation conditions and different structure types.Laboratory experiments show that the porous medium can make the phase curve of methane hydrate move to the right under the effect of capillary force;the smaller the sand particle size and the higher the clay content,the less favorable the gas-water contact and the slower the hydrate formation rate;the activation energy?Ea?and reaction frequency factor(k_fo)of the hydrate formation reaction were calculated to be 75.45-90.85 kJ/min and 8.72×10⁸-6.02×10¹¹ mol/?m²·kPa·d?respectively based on the hydrate formation data under different temperature and pressure conditions.The simulations on hydrate formation under the sea show that:1)deep methane gas migrates to the submarine hydrate stability zone with a relatively small flux for a long period of time in the absence of subsea leakage,which is conducive to the formation of large-saturation,high-abundance hydrate reservoirs;2)reservoir heterogeneity favors the retention of methane gas in the hydrate stability zone,increases the chance of contact with pore water,and reacts with the formation of hydrates.Although it is beneficial to generate large amounts of hydrate resources,the distribution of hydrates tends to be more dispersed,and in the case of low-permeability layers,it is favorable for the formation of hydrate reservoirs with uniform saturation,high abundance,and considerable reserves;3)compared with the hydrate formation characteristics under the four structural conditions of layered,anticline,mud volcano,and gas chimney,it is believed that the layered structure and the anticline structure with the fracture system are the most conducive to the formation of large-saturation,high-abundance hydrate reservoirs in a short time,and next is the gas chimney structure,while the mud volcano structure is relatively small and the hydrates are mainly concentrated near the mud crater.The research results are beneficial to enrich the research methods of gas hydrate formation and promote the development of the theory of hydrocarbon hydrate accumulation.