Mechanism Of Gas Diffusion-seepage In High-rank Coal And Productivity Evaluation Of Coalbed Methane Wells

The characteristics of gas diffusion-seepage in coal are the key factors affecting the production of CBM.For the problems of high rank coal CBM diffusion-seepage mechanism and the dynamic change of permeability in the process of CBM well drainage,and the productivity evaluation,etc.The research on high rank coal reservoir in southern Qinshui Basin of the gas diffusion law and its controlled mechanism in coal,porosity and permeability of coal under different stresses and its stress sensitivity,adsorption induced strain of high rank coal and its influence mechanism on permeability,and the productivity evaluation of CBM were studied by experimental research,theoretical analysis and model calculation.The diffusion and seepage mechanism of gas in coal is revealed.An improved productivity model of CBM well was established.These provide a theoretical basis for the development of CBM with high rank coal.The results are mainly as follows:(1)Based on the experiment of gas diffusion in coal,the law of gas diffusion in coal was studied,and the influence mechanism of different coal ranks,confining pressure,injected gas pressure,temperature and water saturation on the gas diffusion in coal is revealed.The diffusion coefficient of methane gas in different rank coals exhibit a trend of first dropping rapidly and then rising slowly(asymmetric "U" shape)with the metamorphic degree of coal increases.With the increase of confining pressure,diffusion coefficient of methane gas in coal dropped by the power function.The diffusion coefficient of methane gas in coal increases with the increase of injected gas pressure and obeys the change law of the Langmuir equation.With the increase of temperature,the diffusion coefficient of methane gas in coal increases logarithmically.The diffusion coefficient of methane gas in saturated coal is smaller than that of dry coal.(2)The influence of effective stress,moisture content and temperature on coal permeability and its controlled mechanism is revealed,and the evaluation method of stress sensitivity in coal reservoir is established by the experiment on porosity and permeability of coal under different stresses.Both porosity and permeability of coal decrease exponentially with the increase of effective stress.With the rise of the moisture content and temperature,the permeability damage rate and the stress sensitive coefficient go up.(3)Sorption induced strain evolution and its influence on penneability evolution under different effective stress and adsorption equilibrium pressure,and the law of adsorption induced deformations of different gases(CO2?CH4 and N2)and their influences on the coal permeability using a desorption-seepage testing system were revealed.The axial,radial and volumetric sorption induced strains increase as increasing of adsorption equilibrium pressure under a constant effective stress.The coal petrographical properties,the direction of bedding plane and adsorbed gas composition have an important influence on the adsorption induced strain of coal.With the increase of adsorption equilibrium pressure,the effective stress decreases.The permeability first decreases and then increases.The dynamic law of coal reservoir permeability in drainage is revealed.(4)The fracturing filtration property of high rank coal reservoir is evaluated by the comprehensive filtration coefficient of fracturing fluid in coal reservoir as the basic characteristic parameter.Calculation model and method for comprehensive filtration coefficient of fracturing fluid in coal reservoir with considering stress sensitivity were given.The effects of effective stress,dynamic porosity and permeability,viscosity of fracturing fluid and reservoir pressure on the comprehensive filtration coefficient of fracturing fluid were analyzed.Based on the Carman-Kozeny formula,the calculation models of the facture conductivity were established with different conditions.Conditions and the relationship between the flow conductivity and three factors:proppant size,paved layers and closure pressures were analyzed.(5)Based on dynamic law of coal reservoir permeability in CBM well drainage,an improved CBM productivity model considering "three effects"(including effective stress,coal matrix shrinkage and gas slippage effect)is derived.It is applied to 8 CBM wells in the Qinnan-East block and the results were accordant with the reality.