Dynamic Variation Characterization Of Permeability Of Different Coal Structure Reservoir During Depletion

The No.2 coal reservoir in the Yanchuannan Coalbed Methane(CBM)Field,southeastern Ordos Basin,China,was selected to research the dynamic variation characterization of permeability of different coal structure reservoir during depletion.Property parameters of the CBM reservoir were obtained from drilling data,logging information and experimental data.Based on drainage data,production characteristics were summarized,and different drainage stages were distinguished.Dynamic variation characterization of reservoir pressure of different coal structure reservoirs was calculated by using material balance equation of CBM reservoir.Reservoir permeability was determined by means of permeability analysis model to investigate its dynamic variation characterization of different coal structure reservoirs.On the basis of control effects of effective stress and matrix shrinkage,the effects of reservoir property,tentonic characteristic and hydrogeological characteristic on dynamic variation characterization of reservoir permeability were discussed by means of the theory of CBM geology,coal geology,coal petrography and seepage flow mechanics.Drainage system in different drainage stages of different coal structure reservoirs was optimized to ensure production successfully.The key conclusions can be drawn from the study:The No.2 coal is charactesrized by special low moisture content,special low to low ash content and low volatile matter content,and is dominated by vitrinite and inertinite.The variation ranges of coal quality,coal maceral,gas content,Langmuir volume,Langmuir pressure,initial reservoir pressure,initial porosity,initial permeability are large.Additionally,the distributions of the mentioned reservoir property parameters are strongly heterogeneous in horizontal plane.The variation ranges of apparent relative density and initial reservoir temperature are small.The type of coal structure of individual well is changeable vertically and breadthwise.The types of coal structure are dominated by fragmented structure and primary structure,followed by granulated structure,and the distribution of the coal structure is strongly heterogeneous in horizontal plane.With an increase in the degree of crushing of coal strcture,Young modulus decreases remarkably,mesopore specific surface area and volume increase,micropore specific surface area and volume decrease,and continuity and flow conductivity of fracture decrease.The production characteristics of CBM wells are divided into five types as rising type,stable type,ladder-like type,fluctuant type and decay type based on gas production status.Different types of production characteristics are corresponding to different drainage stages.The rising type includes only water drainage stage.The stable type,ladder-like type and fluctuant type contain two drainage stages of water drainage and stable gas production.The decay type has water drainage stage and gas production decline stage.For the primary structure coal reservoir during depletion,three drainage stages,water drainage stage,stable gas production stage and gas production decline stage,are exist.Three variation types of reservoir pressure including gradual decline type,slow-rapid decline type and sharp decline type are observed.There are differences in the pressure drop of the three different drainage stages.Reservoir permeability continuously increases,and the variation characterization of reservoir permeability is classified into three different types,which contain gradual increase type,slow-rapid increase type and slow-sharp increase type with an increase in drainage time.The incremental reservoir permeability and the incremental reservoir permeability of unit pressure drop are different in the three different drainage stages.Primary structure coal reservoir has relatively large Young modulus,which is corresponding to relatively low stress sensitivity of reservoir permeability.Therefore,matrix shrinkage effect is dominant during depletion,resulting in an increase in reservoir permeability.Pressure drop and initial reservoir pressure are the key factors affecting the incremental reservoir permeability.The larger the pressure drop is,the larger the incremental reservoir permeability is.The incremental reservoir permeability of unit pressure drop decreases with the increasing initial reservoir pressure,and is not associated with Langmuir pressure,ash content,coal maceral,tentonic characteristic and hydrogeological characteristic.Before gas production,the height of declining working fluid level in water drainage stage of primary structure coal reservoir is added to reduce reservoir pressure and increase reservoir permeability.After gas production,the height of declining working fluid level is kept stable.The height of declining working fluid level of primary structure coal reservoir is lower than 1.5 m/d,0.5 m/d and 0.5 m/d in water drainage stage,stable gas production stage and gas production decline stage,respectively.On one hand,this can avoid the velocity sensitive effect,on the other hand,the pressure drop funnel can be radial spreaded maximumly,which increases drainage area and reservoir permeability.The purpose is to ensure high gas production.For the fragmented structure coal reservoir during depletion,there are three drainage stages containing water drainage stage,stable gas production stage and gas production decline stage,and five variation types of reservoir pressure including gradual decline type,slow-rapid decline type,slow-rapid-slow decline type,rapid-slow decline type and rapid-slow-rapid decline type.There are differences in the pressure drop of the three different drainage stages.The change of reservoir permeability is characterized by three different cases: continuously increase,decerase first and then incrase and continuously decrease.In the first case,the variation characterization of reservoir permeability has three different types with an increase in drainage time,i.e.,gradual increase type,slow-rapid-slow increase type and slow-rapid-slow-rapid increase type.In the second case,the variation characterization of reservoir permeability is slow decrease-rapid increase-slow increase type with an increase in drainage time.In the third case,the variation characterization of reservoir permeability is divided into two different types as slow-rapid decrease type and rapid-slow-rapid decrease type with an increase in drainage time.The incremental reservoir permeability and the incremental reservoir permeability of unit pressure drop are different in the three different drainage stages.With respect to fragmented structure coal reservoir,the leading role of effective stress effect or matrix shrinkage effect during depletion is simultaneously controlled by initial reservoir pressure and Langmuir pressure.When initial reservoir pressure is higher than 8 MPa and Langmuir pressure is lower than 3 MPa,effective stress effect is predominant,resulting in a decrease in reservoir permeability.The reduced reservoir permeability is closely associated with pressure drop and Langmuir pressure,and increases with an increase in pressure drop.The larger the Langmuir pressure,the smaller the reduced reservoir permeability of unit pressure drop.When initial reservoir pressure is lower than 8 MPa or Langmuir pressure is higher than 3 MPa,matrix shrinkage effect is predominant,resulting in an increase in reservoir permeability.The incremental reservoir permeability is closely associated with pressure drop,and increases with pressure drop decreases.By constrast,ash content,coal maceral,tentonic characteristic and hydrogeological characteristic have no effect on the variation of reservoir permeability of unit pressure drop.When reservoir permeability increases during depletion,before gas production,the height of declining working fluid level in water drainage stage of fragmented structure coal reservoir is added to reduce reservoir pressure and increase reservoir permeability.After gas production,the height of declining working fluid level is kept stable.The height of declining working fluid level of fragmented structure coal reservoir is lower than 1.0 m/d,0.3 m/d and 0.3 m/d in water drainage stage,stable gas production stage and gas production decline stage,respectively.On one hand,this can avoid the velocity sensitive effect,on the other hand,the pressure drop funnel can be radial spreaded maximumly,which increases drainage area and reservoir permeability.The purpose is to ensure high gas production.When reservoir permeability decreases during depletion,before gas production,the height of declining working fluid level in water drainage stage of fragmented structure coal reservoir is added to reduce reservoir pressure.After gas production,the reason controlling high gas production in fragmented structure coal reservoir is unknown and needs further study.For the granulated structure coal reservoir during depletion,two drainage stages including water drainage and stable gas production are observed.Reservoir pressure declines gradually,and there are differences in the pressure drop of the two different drainage stages.Reservoir permeability continuously decreases,and its variation characterization is gradual decrease type with an increase in drainage time.The incremental reservoir permeability and the incremental reservoir permeability of unit pressure drop are different in the two different drainage stages.Granulated structure coal reservoir has relatively small Young modulus,which means relatively high stress sensitivity of reservoir permeability.Therefore,effective stress effect is dominant during depletion,resulting in a decrease in reservoir permeability.There is an increasing tendency in the reduced reservoir permeability with pressure drop increases.The reduced reservoir permeability of unit pressure drop decreases with an increase in Langmuir pressure and ash content.Additinally,The reduced reservoir permeability of unit pressure drop in the stagnant zone is larger than that in the recharge zone.Before gas production,the height of declining working fluid level in water drainage stage of granulated structure coal reservoir is added to reduce reservoir pressure.After gas production,the height of declining working fluid level in granulated structure coal reservoir is kept stable and lower than 0.5 m/d and 0.1 m/d in water drainage stage and stable gas production stage,respectively.The velocity sensitive effect can be avoided,and the pressure drop funnel can be radial spreaded maximumly,increasing drainage area and retarding the effect of the reduced reservoir permeability on gas production.The purpose is also to ensure high gas production.