The Reservoir Characteristics Of Deep Coal Seam In Southern Qinshui Basin

Southern Qinshui basin is the hot spot of coalbed methane(CBM)exploration,and now the Shizhuang region has started exploiting deep CBM.Compared with the shallow coal seam,the deep coal seam has poor production.The research objects of this dissertation are No.3 coal seam and No.15 coal seam.Through sample analysisand testing,well logs interpretation,and analysis of geological data of the research block and its surrounding areas,this paper studies the reservoir properties of shallow and deep coal seams,and tells the differences between them.And discuss how the sedimentary environment,hydrogeological conditions and other geological factors affect the coal reservoir properties.The results show that:(1)in the deep coal seam,the number of micro pores increase,but the mesopores and the macropores reduce;the mercury withdrawal efficiency and the average pore throat diameter decrease;the pore structure configuration deteriorates;the pore tends to be closed.Pore structure and pore morphology become worse at the depth of 1000m,and under the depth,the porosity of coal seam is less than 6%.The density of fracture changes little with depth increasing.The density is 7 to 15 per 5 cm.The permeability decreases exponentially with depth increasing,and the inflection point is 700m.The deep coalbed gas content increases above the depth of 1500m,and decline under 1500m.The variation of porosity and permeability are mainly due to the big reservoir pressure in deep coal seam.(2)Aiming at the deep coal seam in Shizhuang,according to artificial gamma,acoustic,natural gamma,caliper and the breakage of the floor,we classify the No.3 and No.15 coal structure of northern Shizhuang area.The average thickness of powdered coal of No.3 coal seam is 1 meter,which occupy 17.85%of the total coal seam.Compared with No.3 coal seam,No.15 coal seam has more powdered coal,which occupies 20.81%of the total coal seam.The powdered coal thickness first decreases and then increases when the depth enlarges.And the knee point is 1200-1300 meters.(3)The sedimentary environment of No.15 coal seam is alternative sea and river facies,but the No.3 coal seam is continental facies.And the different facies cause the vitrinite content,sulfur content,ash yield of No.15 coal seam higher than those of No.3 coal,which lead to the difference of two coal seams.(4)Coal seam water influences CBM output by affecting the coal reservoir permeability and the cracks.In Daning-Panzhuang-Fanzhuang area,the coal seam water is saturated,and well testing permeability is very high.But in the northwest and the northeast of the basin,the coal seam water is unsaturated,so the permeability is lower.The No.15 coal seam roof in Taiyuan formation in Qinshui basin is mostly composed of limestone,which interact with groundwater,and produce calcite,which filling coal pores,cracks,and affecting reservoir permeability.