Research On The Dynamic Prediction Method In Coalbed Gas Wells Considering Gas Diffusion

Our country is quite abundant in coalbed methane(CBM)resource which is one of the new energy resources.The research of migration mechanism and development mechanism of CBM in our country is later than other countries.As there are few papers to investigate the output dynamic of gas and water in CBM well considering gas diffusion,it becomes increasingly critical to perform the research on change law of diffusion-seepage characteristic of CBM.Based on the analysis of storage,migration and output mechanism of CBM,this paper performed the research on adsorption,desorption,diffusion and seepage law of CBM by physical simulation experiment and theoretical derivation,and established a mathematical model which is suitable for dynamic prediction and productivity evaluation of CBM by material balance principle,in addition,using the program and actual production data to prove the model's reliability,finally completed the sensitivity analysis of parameters related to CBM wells.The major research results are as follows:(1)This paper conducted the research on diffusion mechanism of CBM,and established a mathematical model considering diffusion-seepage of CBM based on Fick first law and material balance principle,moreover,derived the capacity evaluation formula of CBM,and obtained the calculation steps to predict dynamic productivity of CBM wells as well.(2)According to physical simulation experiment,gas diffusion coefficient and gas and water relative permeability curves are measured to Hancheng coal samples in Shanxi.The relevant results are as below:The greater the injection pressure and the temperature,the greater the diffusion coefficient of coal is.The greater the degree of saturation,the smaller the diffusion coefficient of coal is.It also can be achieved that under different confining pressure,for gas and water relative permeability curve,the greater the confining pressure,the less water was flooding out and the faster gas phase relative permeability increases;but with the increase of the confining pressure,the absolute permeability of coal becomes lower.(2)It can be proved that the model and software has a high reliability through historical fitting based on the actual production data of Qinshui well 1.This paper achieved change law of productivity of CBM which was affected by different parameters through sensitivity analysis.Based on the sensitivity analysis,the paper achieved that later reservoir pressure had an index corresponding relation with time and different sensitivity parameters had an obvious effect on gas well production under different sensitivity during the production of gas well.Comprehensive analysis,it showed that big diffusion coefficient,big adjustment effect,high permeability,high original pressure,high langmuir pressure and thick coal seam can get high gas production rate.