High-rank Coalbed Methane Flow Stage Division And Drainage System Optimization

The change of gas permeability during development of coalbed methane(CBM)causes the variation of gas production from CBM wells.Stress sensitivity,matrix shrinkage,two-phase flow and migration of coal fines are the main problems that restrict gas production.The optimization of production strategies is the most direct way to solve this problem.However,the impact of coal fines is not considered in the current production scheme and the study on dynamic performance of CBM wells.In this paper,the pore structure characteristics of coal reservoir,the occurrence characteristics of gas in each stage,the flow characteristics and the production mechanism are analyzed,and the single-phase water flow model considering stress sensitivity is deduced.Then,on the basis of considering the effect of effective stress effect,matrix shrinkage effect,two-phase flow process and pulverized coal migration on the permeability in the drainage and production process,the production stage division method was proposed,and the drainage and production system was optimized in different production stages through numerical simulation method.In the end,the study carried out numerical simulation research through an example,fitted historical data,and verified the optimization method of production scheduling system obtained in the study.The conclusions are as follows:In this paper,the characteristics of pore structure of coal seams,gas storage,gas flow and production mechanism are analyzed.Considering stress sensitivity,a flow model of the single water phase is derived.Based on this model,I find that controlling pressure during production cannot rapidly expand the pressure drop.Then,considering the effects of effective stress,matrix shrinkage,two-phase flow process and pulverized coal migration on the permeability during production,a method to divide production stages is proposed.Production strategy is was optimized on different production stages using reservoir simulation method.Finally,a reservoir simulation study on a filed case is conducted,After history match and validation for the optimized production strategy,the following conclusions are obtained:(1)migration of coal fines has a great impact on coal permeability,and has a significant impact on gas production.Optimization of production strategy can help migration of coal fines,As a result,the formation permeability is improved and the damage of impact of coal fines to coal seams and CBM production is reduced.(2)According to the effects of stress sensitivity,matrix shrinkage,two-phase flow and migration of coal fines on gas permeability.I divide the production stage of the CBM wells into five stages:?flowback of fracturing fluid,?removal of coal fines?controlling BHP to prevent desorption,?dewatering and gas production,?Waterless gas production stage.(3)When the water production rate is low(i.e.8,coal fines cannot be effectively carried out of the formation,which reduces permeability and gas production,However,a very high water production rate i.e.20)will cause denudation of a large amount of coal powder and will block the formation,resulting in a decrease of permeability.Besides,gas production will decline fast after reaching the peak.When water production rate is moderate(i.e.12 and 15),coal fines can be effectively carried out of the formation,Simultaneously it increases the permeability of the formation without generating additional pulverized coal.Therefore,the permeability of coal seam is kept at a high level,and the high production of gas Wells can last for a long time.(4)The mechanism of pressure control for production improvement is not to expand the pressure drop rapidly.It is aimed to control coal fines migration.Removing coal fines at a moderate and continuous rate can improve the permeability of coal seam.The numerical simulation results show that keeping the pressure above the desorption pressure is superior to traditional pressure control schemes.If initial water production rate is 15 and produce water at 12 after pressure control,then the highest gas production can be acquired.(5)The Case study shows that when Well X has a high water production rate,pulverized coal can block the formation.After history matching,if the pressure of Well X is kept above desorption pressure,a higher gas production rate is acquired compared with the history production data.