Research On Law Of Dynamic Flow Of Gas And Water In Micro-porosity Of Coal Mine Methane Reservoir

It is great significant to efficiently develop coal mine methane(CMM)for increase of supply of natural gas resources and reduce emissions of greenhouse gas.During the process of CMM mining,the adsorbed methane desorbed and supplied as the resource of gas when gas pressure dropped to the critical desorption pressure.At the same time,methane and water produced a miscible flow and migrate to micro-cracks through pores.At present,the research on the two-phase flow of CMM and water is mostly focused on the change to the permeability of gas-water macroscopic in the fracture.In addition,there are few studies on the gas-water flow law in the microscopic pore space.However,the changing law to fluid flow in the pores is an important reason to affect the change of coal seam permeability.Therefore,correctly understanding the microscopic gas-water flow to coal is important for the efficient development of CMM.In this paper,a research method of combining indoor experiment and numerical simulation is adopted to analyze the influence of confining pressure and displacement pressure on the microscopic gas-water flow path of coal and rock during the process of gas-water two-phase flow.The characteristics to gas-water micro-flow in different pore scales of coal and rock was also investigated.The main research results are as follows:(1)The influence of displacement pressure on the evolution of microscopic gas-water two-phase flow was obtained.There were obvious differences to effective circulation channels formed by different initial displacement pressures.During the process of rising gas pressure,free water in the macropores and mesopores was driven out,the moisture in the mesopores varied between-20.6% and 20.8%,and the moisture in the macropores varied between-14.6% and 51.1%.After the displacing,the distribution of water-bearing pores inside the coal was relatively dispersed;The mesopore moisture varied between-33.5% and-10.6%,and the macropore moisture varied between-12.8% and 9.0% when the gas pressure dropped.The water invasion was not obvious and gas pushed the water to flow along the large pores.Water in the pores with a pore size of less than 168 nm was difficult to be drive out.(2)The influence of confining pressure on the evolution to micro-flow of gas-water was revealed.After the confining pressure is applied to the coal,the stress and deformation damage increased with the increase of the pore diameter.The confining pressure can control the gas-water seepage process.The moisture in the mesopores varied between 2.3% and 46.1%,and the moisture in the macropores varied between-41.3% and 1.8% at the condition of confining pressure loading.The residual water was mainly distributed in the mesopores and the water intrusion in the coal pores was observed.The distribution of water-containing pores was relatively dispersed.The moisture in the mesopores varied between 4.0% and 20.6%,and the moisture in the macropores varied between-10.6% and 4.8% under condition of confining pressure unloading.The distribution of internal water-bearing pores had not much change during the process of displacement.Large pores will be reopened and further form new seepage channels under low confining pressure.(3)The influence of coal rock pore structure on characteristics of gas-water flow was also obtained.The gas-water interface presented a "U" shape and pushed towards the outlet during the process of gas-driven water.The curvature of the gas-water interface increased with the increase of displacement pressure.It was easy to form isolated bubbles to gas phase in large pores.For the pores connected by the small throats,low pressure cannot drive out the water.The velocity of gas flow in the connected pore throat drastically changed.However,the velocity of gas flow in the large pores smoothly changed.This work has great significance to understand the mechanism of gas-water seepage and guide the CMM production during the process of CMM mining.