Study On The Dynamic Change Of Gas Permeability And Quantitative Method Of Discharge And Production Of Coalbed Methane Reservoir

Permeability and drainage production system development are the key factors affecting coalbed methane(CBM)production.To quantitatively characterize the change law of coal reservoir permeability and optimize the drainage production system,the physical simulation and numerical simulation was adopted to quantitatively evaluate the influence of matrix shrinkage and slip-off effect on coal permeability.The conceptual model of single well drainage was established.The quantitative optimization model of step-by-step pressure drop drainage and production in different drainage stages of CBM wells was constructed and the corresponding program was compiled.Combined with the actual CBM drainage and production data,the differences between the adopted optimization system and the actual drainage and production system were compared.The results show that the matrix shrinkage effect is the main effect of permeability improvement,up to 5 times the Klingberg effect.When the drainage intensity is large,the gas emergence time is early,but the pressure drop effect of the coal in-plane and longitudinal is poor,the pressure drop funnel is concentrated in the wellbore accessories with limited influence range and the pressure drop effect of drainage is poor.When the drainage intensity is small,it shows the opposite trend.With the increase of porosity and compression coefficient of coal,the expansion rate of pressure drop radius decreases gradually,and there is a good logarithmic decline relationship.With the increase of permeability,the swelling rate of pressure drop radius increases linearly.With the increase of the extraction intensity,the pressure drop radius expansion rate shows a logarithmic increasing trend.The permeability,gas saturation,and Langmuir pressure are positively correlated with the expansion rate of desorption radius,while porosity,compression coefficient,and Langmuir volume are negatively correlated with the expansion rate of desorption radius.The permeability has a linear positive correlation with desorption boundary pressure drop rate,while desorption boundary pressure drop rate has an exponential negative correlation with compression coefficient and porosity.With the increase of the maximum control pressure drop,the change of cumulative gas production of CBM wells under the optimal control strategy also shows a trend of first increase and then decrease,and the reasonable value of the maximum control pressure drop is 0.06 MPa/d.Coalbed methane well simulation results show that when the optimal drainage strategy is used,the time of gas emergence and the time of gas production peak is significantly earlier.Moreover,the pressure drop transmission range is larger,the average daily gas output is also higher than the actual production,the production process is relatively stable.The fluctuation of daily gas production value is small,and the overall gas production curve changes more gently.