Investigation On The Characteristics Of Porosity/Permeability Variation And Gas Production In Underground Coal Gasification Process

Underground coal gasification(UCG)is an important method to realize green and clean development and utilization of coal resources.The variation in characteristics of the pore structure and seepage of coal during the gasification process has an important influence on the site selection,production control,and numerical simulation of UCG.Reliable numerical simulation of UCG is of great significance for formulating a reliable development plan for UCG.In this thesis,thermodynamic characteristics and pore structure evolution of coal during the gasification process are firstly investigated through thermogravimetric analysis and scanning electron microscopy.Then,based on high-heat treatment experiments and porosity and permeability tests of columnar coal samples,the porosity and permeability variation of coal during the gasification process is characterized,and the prediction model of the porosity and permeability of gasified coal is established.On this basis,the physical and chemical processes of UCG are analyzed,and a field-scale3-D numerical simulation model of UCG is established by the CMG-STARS simulator.Finally,the variation of porosity and permeability during coal gasification is analyzed,gasification chamber evolution and gas production of UCG is characterized,and the sensitive factors affecting UCG productivity are analyzed.Research results show that the porosity and permeability of coal are significantly increased by pyrolysis to make coal into coke with a complex pore structure.The coke is rapidly gasified and burned after contacting agents,and then the combustion front gradually advances.The gasification chamber quickly develops into a spherical shape with the injection point as the center and turns into a double spherical shape with the retraction of the injection point,and finally becomes an ellipsoid shape.After ignition,the flow rate of syngas promptly reaches the peak,increases slowly after two significant fluctuations,and then decreases slightly with the completion of injection point retreat.The maximum syngas flow rate is 9.11×10~4 m~3/d,with reasonable composition and an average calorific value of 15.89 MJ/m~3.The L-CRIP is safer and more efficient.Water injection can significantly boom gas production.With the increase of oxygen injection,gas production increases,but gas yield per ton of oxygen first increases and then decreases.The optimal water-oxygen ratio is 2:1,and the optimal injection point retreat rate is 0.5 m/d.The research results obtained in the thesis are of guiding significance to the formulation of the UCG field test development plan.