Study On Microstructure And Water Yield Of Sandstone Aquifer With Weak Cemented Roof In Jurassic Coalfield

Jurassic coal field in China has extensive distribution scope large reserves small exploitation difficulty and huge potential for exploitation.However coal seam roof in this era is mostly weak cementation stratum which has short diagenesis time low intensity weak cementation and easy softening disintegration when water easily causes roof water inrush accident during coal mining process.Studying physical mechanics characteristics and water rich rule of weak cementation coal seam roof has become urgent task for coal production.In this paper,taking the weak cemented stratum of 2-1 coal roof in Tonggelang mine field as the research object,the macroscopic physical and mechanical characteristics of the weakly cemented stratum are studied by physical.and mechanical experiments;the mineral composition,particle contact relationship,cementation type and pore characteristics of the weakly cemented formation are studied by using the rock microstructure experiment;the main evaluation methods of water abundance of 2-1 coal roof sandstone aquifer are selected According to the control factors,the evaluation index model of water yield is established,and the water yield of direct and indirect water filled aquifers of 2-1 coal seam weakly cemented roof sandstone is evaluated by zoning;finally,The micro pore structure of weakly cemented sandstone aquifer is classified and its influence on aquifer water yield is studied.The main achievements of this paper are as follows:(1)The density of weakly cemented sandstone is between 2.05 and 2.23g/cm3,the uniaxial compressive strength is between 10～30MPa,the tensile strength is between 0.5 and 1.0 MPa,the elastic modulus is between 3.00 and 7.00 GPa,and the cohesion is between 2.0 and 5.0 MPa.Compared with conventional sandstone,it has the characteristics of low density,low mechanical strength and great difference after macroscopic failure.The Jurassic sandstone basically does not disintegrate after soaking in water,while the Cretaceous sandstone disintegrates partially after soaking in water.(2)The mineral composition of weakly cemented sandstone is mainly composed of quartz and feldspar,as well as calcite,dolomite,analcite,pyrite,biotite,etc.these minerals constitute the skeleton particles of weakly cemented sandstone,while the cement of weakly cemented sandstone is mainly composed of clay minerals such as chlorite,illite and kaolinite;the particle contact of weakly cemented sandstone is mainly linear contact,and that of Jurassic weakly cemented sandstone is mainly linear contact The results show that the sorting property of weak Cemented Sandstone in Cretaceous system is medium,and the overall roundness of them is poor.Most of the cements are argillaceous cementation,followed by calcareous cementation,pore cementation and contact cementation.The number of intergranular pores in weakly cemented sandstone is large,the porosity is in the range of 15-30%,and the distribution of large pore throat is concentrated,and the permeability is good The wave time difference(AC)value is used to predict the porosity and permeability parameters in the mine field.(3)Based on the study of the meso-structure of weakly cemented sandstone,six main controlling factors affecting the water richness of aquifer are selected,which are equivalent thickness of sandstone,sand-mud interlayer number,sand-mud ratio,core recovery,porosity and permeability.The subjective and objective weighting method of analytic hierarchy process and coefficient of variation method is used to weight the main controlling factors.The water richness evaluation of 2-1 coal roof direct water-filled aquifer and indirect water-filled aquifer is carried out by using the powerful multi-source information fusion function of GIS,and the evaluation results are verified by the thematic map of unit water inflow.the results show that the trends of the two are roughly the same,and the evaluation results are reliable.(4)Some parameters of meso-pore structure are extracted by PCAS software,and the meso-pore structure of weakly cemented sandstone is divided into three types.The weighted average value of the pore characteristic parameters is calculated and compared with the unit water inflow of the corresponding boreholes.The results show that the larger the pore weighted fractal dimension of the aquifer in the same horizon is,the smaller the displacement pressure is,and the larger the unit water gushing of the corresponding borehole is.The stronger the water-rich.