Study On The Dynamic Changes In Coal Reservoir Relative Permeability During Coalbed Methane(CBM)Recovery

Relative permeability to gas and water for the two phase flow is vital to the coalbed methane(CBM)reservoir primary and enhanced recovery.However,relative permeability for coal has received little attention in the past decades.In this work,a total of 36 coal samples from different underground coal mines located in the North of Qinshui Basin of China were carried out for a series of laboratory experiments,including relative permeability measurement and real-time strain monitoring,and coal petrology analyses.Basides,the dynamic model for real Coalbed Methane(CBM)producing wells in Fanzhuang and Zhengzhuang District was established to reflect the gas and water flow underground.According to the experiments and simulation results,the characteristics and controlling mechanism of dynamic change in relative permeability to gas and water,the coupling relationships between coal stress-strain,material composition,pore-fracture system and the relative permeability change,the evaluation method for favorite producing areas for CBM based on relative permeability were investigated and researched.Results are as follows.(1)The dynamic model for prediction of relative permeability to gas and water was established based on the real gas,water and pressure data.Flow regime is first identified using rate transient analysis method and new model is used to characterize the change of absolute permeability.Besides,flow material balance equation was quoted to get the water saturation.The CMG software was used to build a reasonable geology model to test the accuracy of derived relative permeability.So a systemic model for prediction of relative permeability to gas and water was established.(2)According to the real-time strain data monitored as the gas and water flow within coals,the coupling relationship between coal stress-strain and the relative permeability were revealed.Results show that the relative permeability is negatively related to the total strain of coal.Moreover,the relative permeability calculated from real producing wells under different stress background in Fanzhuang Distrcit were compared,and the results showed producing wells with low stress background have higher gas relative permeability,meaning better flow capacity in real production.(3)The controls of coal material composition and cleat distribution on the relative permeability were clarified.Results show that original cleat distribution in coal is the main path for water and gas flow,so it controls the flow mechanics in coal sample.Beside the original cleat distribution also has great impact on the hydraulic fracture in real producing wells.The controls of coal material composition reflects on the control of adsorption time,and the longer adsorption time established a negative relationship with irreducible gas saturation.(4)An evaluation system for prediction of favorite coal-bearing areas in Zhengzhuang and Fanzhuang District based on the relative permeability to gas and water was established.Through the investigation of influencing factor on gas content and relative permeability,gas rate was chosen as the final standard for evaluation.Grey relation analysis was used to analysis the weight of every parameter.