Study On Microstructure Evolution And Increasing Permeability Law Of Coal Under Supercritical CO₂

In view of the situation that the permeability of the most high gas coal seam is poor and the effect of coalbed methane extraction is not ideal by strengthening extraction in low permeability coal seam,combining the strong diffusion,dissolving and increasing permeability of pore medium and other unique advantages of super-critical carbon dioxide,the method of improving the recovery of coalbed methane is put forward,which utilizes the super-critical carbon dioxide to improve the permeability of low permeability coal seam and promote seepage,desorption and diffusion of coalbed methane.The micro-structure evolution and the increasing permeability law of low permeability coal seam are studied under the action of super-critical carbon dioxide by the method of combining the macro-micro experiment,the theoretical analysis and numerical simulation,the main research work and achievements are as follows:(1)The triaxial loading experimental device that can increase permeability by super-critical carbon dioxide is self-made,the macro increasing permeability experiment of coal is carried out after different conditions of super-critical carbon dioxide.The results show: the increasing permeability effect on the low permeability coal seam by super-critical carbon dioxide is remarkable,the permeability of coal has been increased by one order of magnitude than that before the increasing permeability experiment and after the action of non-supercritical carbon dioxide.The permeability change law of coal is obtained after the action of super-critical carbon dioxide with different volume stress,pore pressure and temperature: the permeability of coal shows the negative exponential change law with the increase of volume stress;With the increase of pore pressure,the permeability of coal presents the positive exponential change law;The permeability gets improvement after the action of super-critical carbon dioxide with constant pore pressure and different temperature,but in the super-critical temperature range of carbon dioxide,the permeability of coal shows the decreasing change law with the increase of temperature.In the super-critical state,the pore pressure of carbon dioxide plays the main control function on the increasing permeability effect.(2)The micro-scanning imaging experiment of coal is carried out on the basis of macro increasing permeability experiment,the micro pore-fissure evolution law and the influence law of which on the macro permeability are obtained after the action of super-critical carbon dioxide: the micro porosity is ten times as high as before theincreasing permeability experiment;With the increase of pore pressure,the honeycomb pores gradually evolve and form the connected network channel,the fractal dimension of pores increases gradually,the porosity presents positive exponential change law;The porosity of coal increases significantly after the action of super-critical carbon dioxide with the constant pore pressure and different temperature,but the porosity shows decreasing change law with the increase of temperature in the super-critical state;the macro permeability of coal presents positive exponential change law with the increase of micro porosity after the action of super-critical carbon dioxide;The macro and micro experiment results are consistent: with the increasing permeability action by the super-critical carbon dioxide,the evolution of micro pore-fissure is promoted and the permeability of low permeability coal seam is improved.(3)According to the evolution law of coal micro pore-fissure after the action of super-critical carbon dioxide,the dynamic evolution model of porosity is established considering the effects of volume stress,pore pressure,temperature and the dissolution-increasing permeability action of super-critical carbon dioxide.The dynamic evolution model of permeability is constructed combining the relation between porosity and permeability.The damage variable is defined according to the change of micro porosity,the damage function is obtained considering the effects of volume stress,pore pressure,temperature and the dissolution-increasing permeability action of super-critical carbon dioxide.The control equation of coal deformation field is established,the seepage field equation of super-critical carbon dioxide transporting in low permeability coal seam is established,the temperature field is also established.(4)The 3D solid model with the random distribution of pore-fissure is built according to the micro pore-fissure evolution law of increasing permeability coal,the model can reflect the true micro structure of coal,utilizing the field variable subroutine provided by abaqus software,combining the python script and the secondary development function of subroutine,the numerical simulation of increasing permeability law in single hole and double hole injected of super-critical carbon dioxide is carried out in low permeability coal seam under different injection conditions.The results show: after the injection of super-critical carbon dioxide,the porosity and permeability coefficient of the coal around the injection hole are both improved by one order of magnitude more than before gas injection;With the increase of pore pressure,theevolution rate of micro pore-fissure increases,damage degree and permeability coefficient increase;The micro porosity of coal improves after the injection of different temperature super-critical carbon dioxide,but with the increase of temperature,the pore-fissure evolution rate of coal slows down,the increasing degree of damage and permeability coefficient decreases.The increasing permeability effect in double hole injection of super-critical carbon dioxide is better than the single hole,choosing the appropriate spacing between the holes is beneficial to the evolution and perforation of micro pore-fissure,which can improve the effect of increasing permeability.