The Calculation Of Tortuosity In Methane Hydrate-bearing Porous Media

With the continuous progress and development of human society,people’s demand for energy is increasing gradually,and energy consumption is becoming more and more serious.The existing coal,oil and other energy sources can no longer meet the world’s demand for energy.As a new type of clean energy in the 21 st century,natural gas hydrate has attracted extensive attention.The formation and decomposition of hydrate in the pores will change the effective porosity,pore size distribution and pore geometry of the porous media,resulting in the change of the transmission characteristics(tortuosity,permeability,etc.)of the porous media.At present,the research on natural gas hydrate is generally combined with LBM to simulate the fluid flow in the simple two-dimensional hydrate-bearing porous media,and there are few studies on the real two-dimensional porous media and three-dimensional porous media containing hydrate.Therefore,it is of great significance to study the flow in porous media with hydrates and the effect of hydrates on the tortuosity.In this paper,hydrate generation software is developed to generate pore-filling,graincoating,and dispersing hydrates in two-dimensional porous media.Combined with the lattice Boltzmann method,the flow simulation of the two-dimensional hydrate-bearing porous media and the three-dimensional hydrate-bearing digital core are carried out,and the influence of hydrate on the tortuosity of porous media is explored.In addition,the three-dimensional hydrate conceptual occurrence models(pore-filling,grain-coating,and cementing)are established.Based on the pore and hydrate network models extracted from the conceptual models,the classification method of the hydrate occurrence form is proposed.The hydrates of the digital cores are divided.The research results show that for the tortuosity calculated by the velocity method,the tortuosity of the porous media containing the three types of hydrates has the same changing trend,and they all increase first with the increase of hydrate saturation,and then decrease with the increase of hydrate saturation.The calculation process of the velocity method is simple,without drawing streamlines.As long as the flow simulation results converge,the tortuosity value can be calculated.The normalized permeability of the three-dimensional hydrate-bearing digital core increases monotonously with the decrease of hydrate saturation,and there is a transitional stage in the middle,which is consistent with the data of previous studies.As hydrates dissociate in the pore space,the connectivity of the hydrate-bearing sediments becomes better,and the tortuosity value decreases when the hydrate saturation is 50.7%-24.8%.With the hydrates dissociate from 24.8% to complete decomposition,the tortuosity gradually increased to the initial tortuosity value.This indicates that the flow paths of hydrate-free sediments are tortuous.The study of the evolution of the hydrate topology during the hydrate decomposition process finds that with the decrease of hydrate saturation,the overall trend of the proportion of hydrate in the pores and the average hydrate radius gradually decrease.