| Low permeability exists in coal seam,and recovery rate is low.Hydraulic fracturing and hydraulic displacement to enhance gas extraction technology is widely used,while taking into account the role of coal wetting and dust removal.One of the key factors affecting the fracturing or displacement effect of coal seam is the diffusion and migration characteristics of water in the fissure of coal body containing gas.By combining microfluidic experiment and numerical simulation,the direct observation of gas-water two-phase diffusion migration in microscopic cracks of coal was realized in this thesis,and the influence of different capillary numbers,viscosity ratio,density ratio and wettability on the two-phase flow morphology and displacement efficiency was studied,and the competitive diffusion flow characteristics of multiphase fluid in pores were explored.The main findings are as follows:(1)With the help of microfluidic chips,the displacement efficiency and morphology of methane water flooding under different capillary numbers were investigated.The results showed that the displacement efficiency increased with the increase of capillary number,the typical channels of the fingertip branch became wider,and the displacement form gradually changed from capillary fingertip to viscous fingertip and then to stable displacement.In the process of displacement,methane concentration does not always increase.When the wettability of the defense fluid is good,the pore pressure drops after methane breaks through the outlet,and the water at the boundary produces reflux under the action of sudden drop of pressure,resulting in a small decrease of methane concentration in the pore(2)The influence of the change of two-phase viscosity ratio on the displacement efficiency and form of methane flooding water was investigated.The results show that the water saturation in the pore decreases with the increase of the viscosity ratio,and the displacement efficiency increases.At low capillary number,fingering occurs at similar locations.With the increase of viscosity ratio,the displacement form changes from capillary finger-in to viscous finger-in.At a higher viscosity ratio,water exists in the pores in the form of strips,beads or clusters.The displacement efficiency under favorable viscosity ratio(M > 1)is greater than that under unfavorable viscosity ratio(M < 1),and the influence of capillary number on two-phase flow is consistent between favorable viscosity ratio and unfavorable viscosity ratio.(3)The influence of the change of methane density ratio on the displacement efficiency and displacement form of methane displacement water was investigated.The results showed that the displacement form of two-phase flow was not affected by the density ratio,and the distance between the finger and the outlet was not significantly different.However,as the density ratio increases,the number of breakthrough outlets in the advance branch increases,and the density ratio is positively correlated with the methane concentration.The higher the density ratio,the higher the methane saturation,and the higher the displacement efficiency.(4)The effects of wettability on the displacement efficiency and form of methane flooding water were investigated.The results show that wettability affects the displacement mode of two phases by controlling capillary force.At low capillary number,wettability has a significant effect on the displacement efficiency and form of two phases,while at high capillary number,wettability has no significant effect on the displacement efficiency.With the increase of the contact Angle,the wettability of methane is enhanced,the stability of the two-phase interface is increased,and the displacement efficiency is increased.In addition,when the contact Angle is small,the methane concentration will decrease slightly after reaching the peak value due to boundary reflux phenomenon under the condition of water wetting,but not under the condition of methane wetting. |
