Study On Gas Production Model And Drainage Control Method For Coalbed Methane Wells

During the production of coalbed methane(CBM)wells,the permeability of CBM reservoir constantly changes due to the dual stress sensitivity effects of hydraulic fracture and cleat system,matrix shrinkage effect,and the effect of pulverized coal fall off,migration and blockage,etc.Therefore,there are some difficulties in accurately predicting the production capacity of CBM wells and forming a reasonable drainage system.In view of these problems,firstly,based on the previous theoretical research,a multi-factor coupled CBM well productivity model considering dynamic permeability,pressure propagation process,and variable mass flow in the hydraulic fracture is established using the gas reservoir engineering and seepage theory.Afterwards the proposed model is solved and validated by numerical simulation.Secondly,the main factors influencing the productivity of CBM wells are analyzed.Finally,the multi-objective optimal drainage control method is used to design the drainage control system of the D-1 well,and the optimal drainage control scheme of the D-1 well is optimized.Research results show that: the proposed gas productivity model for CBM wells is rational,accurate and applicable to production forecasting for both low permeability and high permeability CBM reservoirs.The permeability of CBM reservoir is a key parameter affecting the productivity of CBM wells.And the peak of gas rate,the cumulative gas production of CBM wells and recovery ratio increase as the reservoir permeability increases.Pulverized coal blocking can cause serious damage to coal seams permeability.The dual stress sensitivity effect affects the production at the early stage,especially the stress sensitivity of hydraulic fracture directly leads to the rapid decrease in gas production rate of CBM wells.While the matrix shrinkage effect can significantly improve the production of CBM wells at the middle-late stage.The net thickness of coal seams,critical desorption pressure,Langmuir pressure and Langmuir volume determine the gas content of CBM reservoirs and are the basis for the productivity of CBM wells.There exists an optimal downhole pressure drop path,so that in the production of CBM there is no damage to formation but also meet the economic benefits.