The Geological Controlling Factors And Prediction Of CBM Wells Production Of South Shizhuang Block

This paper takes No.3 coal reservoir of South Shizhuang Block Qinshui Basin as research object.No.3 coal reservoir is rich in coalbed methane,featuring enormous development potential.However,majority CBM wells in this region have low gas production or no production at present,so studying the geology control for the deliverability of CBM wells is of great significance to increase the production.This paper discusses the impact of various factors on productivity from geology conditions,hydrology conditions and physical properties of reservoir.Besides,the dynamic variation of reservoir parameters is simulated numerically.The main conclusions achieved are as follows:(1)The thickness of coal reservoir is generally about 6m.The depth of coal reservoir is between 600 m and 750 m.The direct argillaceous cap rocks for CBM is between 3m and 12 m.Semi-bright coal and semi-dark coal are common in the study area.Although the permeability of reservoir is low,the coal reservoir occupies the properties like high maturity,greater absorption capacity,medium pressure which are conducive to CBM development.(2)It is multi-factors that attributed to the deliverability of CBM wells in South Shizhuang block.The influence mechanism is complex and the geological factors are associated with each other.Critical desorption pressure,the ratio of critical desorption pressure and coal reservoir pressure,gas content and concept of the gas saturation have the highest correlations with gas production.High-yield wells in the study area are showing the mass,zonal distribution feature.By the influence of Sitou sealing fault,east of the study area is hydraulic retention area where groundwater is collected.The hydrodynamic condition in the region is weaker,the salinity,reservoir pressure,gas content and gas saturation are high.So the majority of the high-yield wells are located in the lower portion of the secondary folds in the western region.(3)Correlations between stress sensitivity of coal reservoir and gas content,reservoir pressure,pore volume compressibility,adsorption isotherm parameters,matrix shrinkage compressibility are established by using ARI permeability model in numerical simulation software COMET3.The correlations reveal dynamic variation of the permeability during the CBM recovery.The ratio of present permeability and initial permeability has a very significant exponential relation with the gas saturation,a better linear relation with the gas content,little linear relation with initial reservoir pressure,little power function relation with Langmuir pressure,but almost no relation with Langmuir volume,which are analyzed by the simulation results of dynamic variation of the permeability in south Shizhuang block.(4)Four well groups: ultra-high-yielding well group,high-yielding well group,middle-yielding well group and stripper well group are selected to predict CBM productivity by using COMET3.The differences of depressurization extension between ultra-high-yield well and low-yield well are studied with well interference is considered.The results show that there is a great positive correlation between the production of ultra-high-yielding well and depressurization rate,for stripper well the depressurization rate is slow and steady.