Parameters Evolution During Gas Drainage And Numerical Simulation Of Boreholes Optimization

The control over gas disasters in Chinese coal mines is in severe situations.To prevent the occurrence of gas disasters,gas extraction is strongly advocated.Extracting gas from boreholes is the most commonly used technology.Gas extraction can reduce gas pressure and gas content in coal seams as well as gas emission quantity from coal mines,and then prevent the occurrence of gas disasters.At the same time,the extracted gas(coalbed methane(CBM))can also be used as a clean energy.However,as coal seams are porous media and considering the occurrence state of coal seams,the flow law of gas is extremely complex.Particularly for some water-bearing coal seams,the gas flow law is more complex due to the co-existence of three phases,i.e.gas,liquid,and solid.To further explore the evolution characteristics of gas parameters in gas extraction from boreholes in water-and gas-bearing coal seams,a series of research methods were combined,including theoretical analysis,experimental measurement,and numerical simulation.By using these methods,the adsorption and desorption experiments of water-bearing coal samples,depressurizing extraction experiments of water-and gas-bearing coal samples,and physical simulation experiments of gas extraction under conditions of different initial geostresses and gas pressures were conducted.Through these experiments,we studied the spatial-temporal evolution laws of gas pressure and permeability of coal masses during the gas extraction from boreholes.In addition,we established a thermal-hydro-mechanical(THM)coupling model for gas-bearing coal masses including stress field,seepage field,and temperature field and considering the influences of water.The model was established based on knowledge of various disciplines such as linear wet-thermoelastic hypothesis,rock mechanics,seepage mechanics,and heat transfer and combining stress equilibrium equation,continuity equations of water and gas,and energy equation.Then,the simulation data of COMSOL Multiphysics and the experimental data were compared to verify the reliability of the model.Finally,the cross-measure boreholes in floor gas extraction roadway were taken as the engineering calculation case.On this basis,the spatial-temporal evolution of gas pressure in the gas extraction process and the outburst elimination scopes under different initial conditions were analyzed by using the multi-physical field coupling analysis software COMSOL Multiphysics.Simulation analysis was also carried out to explore the influences of the distance,layout,and dip angle of cross-measure boreholes in floor roadways on the outburst elimination attributed to gas extraction.Then,the borehole layout was optimized.The following conclusions were mainly obtained:(1)The high-pressure variable-temperature adsorption and desorption experimental system in the laboratory was used to carry out adsorption and desorption experiments on the water-bearing coal samples.By analyzing the adsorption isotherms,desorption amount of gas,and desorption velocity of gas,the influences of water on the adsorption and desorption of gas were determined.The physical simulation experiments on the depressurizing extraction of coal samples with different water contents were conducted to analyze the change characteristics of various parameters including the gas flow,permeability,and deformation in the depressurizing extraction of gas.(2)The physical simulation experiments of gas extraction were performed using the multi-field coupled physical simulation experimental system for CBM extraction.By using theoretical analysis method,the gas migration law around the boreholes and the spatial-temporal evolution of permeability during the gas extraction were explored and the relation between the gas pressure and the permeability was analyzed.(3)A dynamic model for the porosity of matrix was established considering the squeezing effect caused by changes in stress and gas pressure,the expansion and deformation effect caused by adsorption,and the thermal expansion and deformation effect due to temperature change.Based on factors including the fracture width,length of matrix block,fractures,and deformation,a permeability model of fractures was deduced according to the triaxial stress state of coal seams.We also inferred the total strain of unsaturated porous media on the basis of considering stress-induced strain,thermal strain,wet strain,strain caused by the compression of gas pressure on coal masses,and strain resulting from adsorption and desorption.The stress field equation was constructed by combining the geometric equation and introducing the stress equilibrium equation.In addition,the seepage field equation for gas-water two phases was built based on the dynamic effective permeability model of gas,relative permeability of water,Darcy's law,Fick diffusion law,and gas-water two-phase continuity equation.The temperature field equation of water-bearing coal seams was also constructed based on the change in the thermal strain energy,wet strain energy,heat conduction,heat transfer,convection,and work of fluids.By building the above equations,we finally constructed the THM coupling model for gas-bearing coal masses considering the influences of water and incorporated the model in the COMSOL Multiphysics.On this basis,the numerical simulation was carried out on the conditions of keeping the geometric model,boundary conditions,initial conditions,and parameter values same with those in the physical simulation experiments.The values of gas pressure at each measuring point obtained in the physical simulation experiments were compared with the corresponding values computed through numerical simulation.The comparison verified that the constructed THM coupling model is reliable and feasible.(4)The research took Zhongmacun Coal Mine in Jiaozuo,Henan Province,China as the research object and the cross-measure boreholes in floor gas extraction roadway as engineering calculation case.By using the multi-physical field coupling analysis software COMSOL Multiphysics,the spatial-temporal evolution of gas pressure in the gas extraction process and the outburst elimination scopes were analyzed.In addition,the influences of different initial conditions of the coal seam on the gas extraction efficiency were also analyzed.Owing to borehole distance and layout directly influence the efficiency of gas extraction from boreholes,scientifically and reasonably setting the borehole parameters is a precondition for improving the efficiency of gas extraction.By calculating the effective extraction radius,considering the mutual influences of boreholes,and conducting numerical simulation,the optimal distance,dip angle,and layout of boreholes were determined based on the outburst elimination scopes.In addition,the gas extraction effects before and after the optimization were compared.