The Study Of Coalbed Methane Enrichment Rule And Productivity Dominating Parameters In The Gujiao Area,xishan Coalfield

In this dissertation,the coal mining geologic and coalbed methane wells data in the Gujiao area,Xishan coalfield,were collected.The field geologic mapping and geologic logging were conducted.The coal composition,vitrinite reflectance(Ro),isothermal methane adsorption,low temperature nitrogen adsorption,static porosity and permeability,dynamic porosity and permeability and nuclear magnetic resonance(NMR)of 17 coal block samples,which were collected from the Nos.2 and 8 coal seams in the Gujiao area,were measured.Furthermore,the chemical properties of 30 water samples collected from 30 coalbed methane wells were tested.Geologic data and testing results were used to analyze the geologic characteristics of the Nos.2 and 8 coal seams,and which was combined with gas distribution to research coalbed methane distribution models in the Gujiao area.On these bases,the main parameters affecting gas production in different coalbed methane distribution models were analyzed.The main research conclusions are as follows:(1)The Gujiao area was divided into 4 coalbed methane distribution models,and they are shallow buried gentle monocline model(SGM),deep buried gentle monocline model(DGM),steep monocline model(SMM)and complicate fold model(CFM).In the CFM and SMM,the gas content and saturation of the main coal seams are heavily low.Compared with the CFM and DGM,gas resource is much more enriched in the SGM and DGM.In the SGM,large buried depth and better preservation condition are helpful to enrich gas,however,small buried depth and weak hydrodynamic environment areconducive to enrich gas in the DGM.(2)Dimensionless gas-producing rate was combined with isothermal adsorption/desorption process and gas productivity to determine stable gas-producing stage,and of which the ambit is 0.65 in the Gujiao area.Gas productivity was classified into five ranks(gas production of > 1000 m3/d,500~1000 m3/d,300~500 m3/d,0~300 m3/d and 0 are extreme high production wells,high production wells,medium production wells,low production wells and dry wells,respectively)according to the distribution of stable gas production.(3)The compressibility and permeability stability increase,but damage rate of permeability decreases with increasing metamorphic grade of coal.Gas coal,fat coal and coking coal are trend to deform and generate powders which will block fractures,because are relative soft coals.About these coals,the utilization of straight well hydro-fracturing developing technology should be careful.(4)During coalbed methane draining process,the free and dissolved gases in macropores and fractures are firstly produced,and adsorbed gas in mesopore began to desorb even when the reservoir pressure drops a litter,but those in micropores starts to desorb only when the reservoir pressure drops to a relative small value.Resistivity logging and key pressures during hydro-fracturing process can reflect the coal reservoir permeability before and after hydro-fracturing process,respectively.The evaluating model of permeability by resistivity was established,and the feasibility of evaluating fracturing effects by Pb-Ps and Ps-Pd was explored.(5)In the Gujiao area,the gas productivity-dominating parameters are totally different in different coalbed methane distribution models.In the SGM,original reservoir permeability and hydro-fracturing effect are the most important parameters controlling gas production.Resource content dominates gas production in the DGM.In the SMM,there is no water has been produced and the gas production is heavily low.High water-draining content and lowproduction pressure differential are the most important parameters improving gas production in the CFM.