Reservoir Damage Types During The Drainage Stage Of Coalbed Methane Wells And Its Prevention Measures In A Block Of The Southern Junggar Basin

There are a total of 76 coalbed methane（CBM）wells in operation in a block of the Southern Junggar basin,and 80%of them possess a gas production less than 1000 m³/d.Reservoir damage is one of the main reasons causing the low gas production in the block.Therefore,this paper systematically analyzes the main control factors of CBM well production through literature research,experiments and production data analysis,reveals the influences of geological and engineering factors on the gas production,demonstrates the reservoir damage types during the drainage stage of CBM in the block,and then propose targeted prevention measures.Main conclusions are drawn as follows:（1）The gas production of CBM wells is controlled by geological factors such as resource abundance,ground stress,coal structure,hydrodynamic conditions,as well as engineering factors including hydraulic fracturing and drainage control.However,approximately 60%of the CBM stripper wells has occurred severe reservoir damage during the drainage stage.（2）The reservoir damages in the drainage stage of CBM wells in the study area include stress sensitive damage,velocity sensitive damage,water blcoking damage and scaling damage.The drainage rate of some CBM wells is too fast and unstable,which leads to an excessive drop of reservoir pressure in the near-wellbore area,and it is difficult to replenish the fluid in the far-well area in time,resulting in a rapid increase in effective stress and severe stress sensitivity damage.Meanwhile,the high fluid flow rate causes the coal powder in the fractures to migrate under the fluid carrying,which may cause the fractures to be blocked and induces reservoir sensitivity damage.The active water fracturing fluid,which has been widely adopted in the block,is easy to generate high capillary pressure in the reservoir pores.Affected by this,part of the gas is blocked in the pores,and then the water blocking damage occurs.Through the physical simulation experiment and the analysis of the actual situation in the field,it is found that the reduction of the CO₂ partial pressure in the reservoir during the drainage stage will lead to the formation of carbonate scale,which will lead to the blocking of CBM wells and reduce the gas production.（3）According to the reservoir damage type of CBM wells in the study area,it is proposed to transform the CBM development into the integrated development of coal measure gas,constructing more stable fluid production channels in the rock formation and reducing the influence of reservoir stress sensitive damage;use low damage fracturing fluid to reduce the capillary pressure of pores,promoting the flowback of fracturing fluid and mitigating the water blocking damage;makes the coal rock powder less likely to migrate during the drainage and production process,so as to mitigate the velocity sensitive damage;adopt the acidizing treatment method and cathodic protection device to reduce the scale damage of the reservoir.In terms of drainage control,reduce the drainage rate to promote a large amount of reservoir water discharge before the desorption of adsorbed gas;insisting on low intensity drainage to keep the fluid flow rate below the critical start-up flow rate of particle migration;also,maintain high bottom hole flow pressure and casing to suppresses the formation of scale in the reservoir.