Dynamic Evaluation Of Coal Reservoir And Optimization Of Drainage Strategy In The Shizhuangnan Block

Coalbed methane(CBM),an important unconventional resource,has been developed commercially in the Qinshui Basin.This study takes the CBM wells of the Shizhuangnan Block to make in-depth research on the geology and development of CBM.Firstly,the CBM production law is analyzed based on the static geological conditions;secondly,the reservoir dynamic characteristics are evaluated from the aspects of reservoir pressure and permeability;thirdly,the propagation characteristics of the pressure drop funnel are described based on the classical material balance equation and pressure profiles;finally,the quantitative optimization model of CBM production strategy is established according to the seepage theory.The main results are shown as follows:From the perspectives of natural faults,compatibility between in-situ stress and lithology,and coupling relationships between reservoir physical properties and production strategy,the reasons for great differences of productivity in the CBM wells are analyzed.An analysis process is also proposed to provide a theoretical support for rational production of these wells.A prediction model for average pressure of coal reservoir is established from a dynamic perspective according to the CBM material balance equation.Compared with the traditional models,the dynamic change of equivalent drainage area and the self-regulation effect of coal reservoir are taken into account,which makes the calculation results more accurate.In addition,the dynamic characteristics of reservoir permeability are evaluated based on the effective stress and matrix shrinkage effects.In the single-phase water flow and gas desorption stages,the demarcating pressures are defined to determine the dynamic change degree of the reservoir permeability,and the coal reservoir types are further classified.It is found that there is a significant corresponding relationship between the coal reservoir types and CBM production.The pressure drop types of CBM wells in the block are divided into ?rapidly drop type?,?medium-term stability type?,?slowly drop type? and ?fluctuation type?,corresponding to ?rise type?,?rebound type?,?decline type? and ?fluctuation type? based on reservoir permeability,respectively.A semi-analytical mathematical model was built through combining the pressure profiles and material balance equations,which can use to describe the reservoir pressure propagation during the CBM production.This model considers the influence of hydraulic fractures.Based on the model,the dynamic behavior of the pressure drop funnel can be accurately described in the early production stage and the inter-well pressure interference boundary can be obtained in the later production stage.This model provides an innovative method to predict the damage boundary and guide further reconstruction for the CBM wells with poor production.The optimal drainage strategy model for CBM wells in different drainage stages was established based on seepage theory and the dynamic behavior of permeability,allowing the maximum drop rate of bottom hole flowing pressure,cumulative water production,and maximum daily gas production to be obtained in different production stages.The model has been verified by numerical simulation,and thus the optimized production strategies should be adopted in the CBM production process to ensure production wells produce efficiently.Otherwise,the coal reservoir could be irreversibly damaged and expansion of the pressure drop funnel should be limited.The model provides guidance and theoretical basis of production strategy optimization for high and stable production of CBM wells.