Microstructure Evolution And Mechanical Properties Of Deep Coal Under The Combined Action Of Temperature,Gas And Fracturing Fluid

The coalbed methane(CBM)reserves buried deeper than 1500 m account for more than 80% of China’s proven reserves,and the development of deep CBM will become the new normal for the exploitation of coalbed methane(ECBM)in China.The key to realize efficient deep ECBM is permeability enhancement.Compared with the shallow part,the impact of reservoir environment such as high gas pressure and temperature in deep coal seam on the effect of permeability enhancement after reservoir transformation cannot be ignored.The physical characteristics of coal after hydraulic fracturing are important indicators for evaluating the effect of reservoir permeability enhancement.Therefore,it is important to study the evolution of physical characteristics of deep coal to deepen the understanding of reservoir transformation mechanism in deep complex environment and to improve the development effect of deep ECBM.This thesis focuses on the research theme of the evolution law of physical characteristics of deep coal after fracturing,and carries out relevant experiments and theoretical analysis with the microstructure and mechanical properties of deep coal that govern the process of ECBM.The study reveals the evolution of microstructure of deep coal under the action of temperature-gas-fracturing fluid(TGF),as well as the strength,deformation and damage characteristics under true triaxial and high stress state,analyzes the energy dissipation law in the process of deformation and damage,and elaborates the microscopic deterioration mechanism of deep coal in the process of fracturing.The main results are as follows:(1)The chemical structure evolution law of deep coal under the action of TGF was obtained,and its influence on the gas adsorption properties of coal was analyzed.The aromaticity and the degree of aromatic ring condensation of coal increased and the parameters of oxygen-containing functional groups decreased after viscoelastic surfactant fracturing fluid(VES-FF)treatment.With the increase of temperature and gas pressure,the distribution of VES-FF inside the coal as well as the activation energy of reaction changed,which promoted the destruction of oxygen-containing functional groups and hydroxyl structure by VES-FF,resulting in the enhancement of gas adsorption capacity of coal and finally increasing the difficulty of ECBM.When the temperature rises above 50℃,the synergistic effect of temperature and VES-FF makes the macromolecular structure of coal develop toward the orderly direction.In contrast,the microcrystalline structure of coal is more stable,and the effects of temperature,gas pressure and VES-FF on it are weaker.(2)The evolution of pore and fracture structure of deep coal under the action of TGF and its influence on the gas adsorption and seepage characteristics of coal are revealed,and the microscopic mechanism of deep coal pore and fracture structure evolution is elucidated.There are obvious differences in the evolution laws of micro fractures and pores in deep coal: The increase of temperature and gas pressure enhances the percolation effect of VES-FF,which leads to a significant increase of fractality and fractal dimension of fracture structure of coal and increases the complexity of fracture structure,which is favorable to gas percolation and reduces the difficulty of ECBM.Under the low temperature and gas pressure conditions,the treatment of VES-FF makes the seepage pore volume of coal increase significantly and the adsorption pore volume and specific surface area decrease,while the permeability increases significantly,which is favorable to the desorption and diffusion of gas and improves the production of CBM.However,with the increase of temperature and gas pressure,the positive effect of VES-FF diminishes,and the pore volume and specific surface area of coal decrease significantly,while the adsorption pore volume and specific surface area increase significantly,which is not conducive to gas diffusion.The effect of VES-FF on the pore structure of coal samples is mainly produced through acidic dissolution and ion exchange,while temperature and gas have indirect effects by affecting the distribution and dissolution ability of VES-FF,in addition to direct effects on the pore fracture structure of coal in the form of thermal expansion and gas wedge action.(3)The strength,deformation and damage characteristics of deep coal under the action of TGF are revealed,and the energy dissipation law of the whole process of deformation and damage is described.The degradation effect of VES-FF makes the coal show ductile damage characteristics at low temperature and gas pressure,and the ductile characteristics increase with the increase of gas pressure and temperature.The increase of temperature and gas pressure enhances the deterioration of coal strength by VES-FF,which leads to the increase of coal deformation,while the lateral expansion coefficient shows the opposite trend.The total energy,elastic energy and dissipation energy of coal treated with VES-FF increased with the increase of gas pressure,decreased with the increase of temperature,and the percentage of dissipation energy decreased with the increase of gas pressure.Under the true triaxial high stress condition,the deterioration of VES-FF leads to more secondary cracks during the deformation and damage of coal samples,and this trend increases with the increase of gas pressure and decreases with the increase of temperature.(4)The mechanism of microscopic deterioration of coal during fracturing in deep reservoirs is described,and the future measures for deep ECBM are discussed.The degree of oxygen-containing functional groups and aromatic ring condensation of coal show linear increasing and linear decreasing relationships with the mechanical strength of coal,respectively.The functional groups and pore-fracture structures deteriorate the strength of coal by affecting gas adsorption and desorption,matrix expansion and water wedging,respectively.The influence of deep complex environment should be emphasized in future deep ECBM to optimize fracturing fluid formulation and fracturing process parameters to reduce reservoir damage.