Factors Causing The Coalbed Methane And Water Production Difference In Shizhuangnan Block,Southern Qinshui Basin,North China

Coalbed methane production wells have characteristics of a large number of low gas production wells and big gas production differences in Shizhuangnan block.To investigate the causes of this big gas production differences has significant impacts on future coalbed methane development of low gas production wells.Based on the data of well logging,hydraulic fracturing and drainage,the current study has identified the coal texture,calculated the shale content(measuring volume)and equivalent drainage radius,analyzed the process of hydraulic fracturing,predicted the dynamic changes of coal permeability and summarized drainage rule.Combined with grey correlation analysis,the causes of gas production difference are recognized.(1)High shale content(>30%)of direct coalbed roof/floor has obviously controlling effect on sealing of coalbed methane and isolating external water from other aquifer.Without external water,the bigger equivalent drainage radius often means further pressure drop funnel,more desorbed gas volume and higher gas production.The equivalent drainage radius has great difference between different areas,resulting from different gas content and water saturation.(2)The area where coal has thicker granulated coal generally has much higher gas content in coal reservoir.The coal seam that has thicker cataclastic coal and undeformed coal that at the same time located in the lower part of coal often is more favorable for fluid flow and can decrease the possibility of coal blockage,and larger pressure drop.Moderate vertical hydraulic fracture contributes to gas-water separation and fluid flow,however horizontal hydraulic fracture is much easier to close as the effect of upper formation pressure and bad for fluid flow.The drainage system comparison between adjacent high and low gas production wells indicates that bottom hole pressure decreasing slowly before producing gas and keeping stable after producing gas is favorable for the extension of equivalent drainage radius and protection of reservoir permeability.This drainage system can help get higher gas production.(3)Coal texture,the process of hydraulic fracturing,the dynamic change of coal permeability and drainage system have very close relationship among them.The vertical variation characteristics of coal texture can affect the process of hydraulic fracturing and further the effect of effect of coal reservoir reconstruction.Also,the change curve of hydraulic fracturing parameters can reflect the vertical variation of coal texture in some degree.The controlling of drainage system can affect the dynamic change of coal reservoir permeability and further the extension of equivalent drainage radius.The coal fracture permeability generally decreases quickly in the beginning and keeps stable in the later time.The coal reservoir permeability generally decreases quickly in the beginning and recovers to the initial conditions as the effect of coal matrix shrinkage.The high percentage of granulated coal can be much easier to block the seepage channel with coal power,and thus affecting the coal permeability and equivalent drainage radius.Therefore,various factors is affected by each other and control the process of production together,finally decide the gas production.