Investigation On The Gas Flow Properties In Coal Seam And CBM Extraction Simulation Based On 2D DFN Modeling

Coal is a complex porous medium which contains fractures and pores.As the main migration channel of coalbed methane,the geometrical characteristics and spatial distribution of fractures have an important influence on the migration of gas and control the engineering effect of gas extraction.The flow of gas involves the dynamic fluid-solid coupling effect.In this paper,a two-dimensional discrete fracture network model is generated by Matalb programming to characterize coal seam fractures.By means of theoretical analysis and numerical simulation,the influence of fracture structure characteristics on the permeability of coal seam is systematically discussed.The distribution characteristics of stress and permeability of coal rock around the gas extraction borehole are studied.In this paper,a fluid-solid coupling mathematical model of coalbed methane migration is established,and combined with the fracture network model,the numerical simulation of gas extraction is carried out in COMSOL Multiphysics.In this way,the spatial and temporal evolution rules of gas pressure and gas velocity are obtained.The main research contents and conclusions are as follows:(1)Based on Monte Carlo method,a two-dimensional discrete fracture network model generation program was developed in Matlab,and the generated fracture network model was processed in Matlab and exported as an image;the fracture network image is imported into COMSOL Multiphysics by using the image function and converted into the computational domain to distinguish the fracture and coal matrix.(2)By controlling a single variable,fracture network models with different geometric parameters and distribution rules of these parameters were generated in Matlab.Combined with the finite element analysis tool,COMSOL Multiphysics,the influences of the distribution rules of different geometric parameters,fracture types and geometric parameters on the permeability of coal seam were discussed.The results show that the permeability of coal seam increases with the increase of fracture density,fracture trace length and fracture width;the permeability of coal seam increases with the decrease of the Angle between fracture and gas pressure gradient;large size fracture has a dominant contribution to coal permeability;when the fracture geometric parameters obey the exponential distribution,the coal rock permeability is the largest,while when the fracture geometric parameters obey the uniform distribution,the coal rock permeability is relatively the smallest.(3)According to the previous researchers’ achievements of the stress distribution around the borehole while hydraulic fracturing,the permeability model is established to distinguish the fractured zone,plastic zone and elastic zone around gas extraction boreholes by introducing the stress-permeability empirical model;based on the engineering geological parameters of Chongqing Yuyang coal mine and the established permeability model,the effects of various geological parameters and construction parameters on the stress and permeability of coal rock around the borehole are studied.(4)By generating a two-dimensional discrete fracture network of coal seam in line with engineering geological characteristics and establishing an uncoupled model of gas seepage in coal seam,the numerical simulation of gas extraction in COMSOL Multiphysics is conducted and the gas flow properties are analyzed.The results show that with the passage of time,the gas pressure around the borehole gradually decreases,and the farther away from the center of the borehole,the slower the pressure decreases.This phenomenon leads to the formation of a pressure drop cone around the borehole;the gas velocity in fracture is much higher than that in coal matrix,and the effective radius of gas extraction increases first and then decreases,which reflects the phenomenon that gas extraction is easy first and then difficult in practical engineering.(5)Based on the basic theories of elasticity mechanics and seepage mechanics,gas adsorption and pore pressure,a fluid-solid coupling mathematical model of coal seam gas migration was established.Combined with the fracture network model generated in the simulation of gas extraction without taking fluid-solid coupling effect into consideration,the numerical simulation of gas extraction with taking fluid-solid coupling effect into consideration was carried out in COMSOL Multiphysics.The results show that the decrease rate of gas pressure slows down with the increase of extraction time,which indicates that the difficulty of extraction is easy at the beginning;with the increase of extraction time,the gas flow velocity decreases gradually,and the gas flow velocity far away from the drilling area is obviously lower than the gas flow velocity near the borehole.(6)By comparing the results of gas extraction simulation with and without taking fluid-solid coupling effect into consideration,it can be concluded that when the solid deformation field is not considered,the gas pressure in coal seam decreases faster,and the gas pressure is generally lower than that when the fluid-solid coupling effect is considered;at the same extraction time,the gas velocity with fluid-solid coupling effect is higher than that without coupling effect;under the influence of the solid mechanical field,the gas velocity distribution in the reservoir is obviously different from that in the single field numerical simulation,which can be reflected by the phenomenon that gas velocity in coal seam changes more smoothly when fluid-solid coupling effect wasn’t taken into consideration.