Evolution Of Overlying Coal Seams' Reservoir Properties And Its Geological And Geochemical Mechanism During Mining

Aiming at the key of “evolution of overlying coal seams' reservoir properties and its geological and geochemical mechanism during mining”,the paper studies on the Zhucang syncline in western Guizhou with multi-coalbeds area.The deformation characteristics of the overlying coal during the mining were investigated with the physical simulation measurements.The water-rock interactions of the overlying coal were also launched.After that,the evolution characteristics of pores,fractures,permeabilities and adsorption capacities of the overlying coal samples during the mining were researched systematically.Then,the geological and geochemical mechanisms on the overlying coals' reservoir properties are highly interpreted.The results of the coal mining disturbance physical simulation in the multi-coalbeds area have shown that the deformation characteristics of the overlying coal seams influenced by mining are various at different zones.The deformation of the overlying coal in the ahead and backside of the goaf are mainly expansion of the micro-fractures,while that above the goaf presents dynamic evolution from the micro-fractures expansion to coal fracture.Combined with the coal mechanics properties and the material damage theory,the coal deformation index(CDI)was presented to quantitative evaluation the coal deformation degree in various zones of the overlying coal.The results of physical simulation with the true coal also present that the thickness of the mining coal and the stratum structures have great deal with the CDI.A new deformation dynamic model of the overlying coal was established with the coal mechanics properties,coal thickness and the coal production rate,which can predict the coal deformation dynamically.The comparison of the characteristics of pores and micro-fracutres with the coal samples before and after physical simulation measurements show that there are large amount of conjugate fractures emerge from the bright coal band,which has the relatively weak mechanical properties.Besides,the pore structure changes from monotone increasing mold to bimodal distribution.With the decrease of pore diameter,the pore volume has increased,and the pore structure was optimized,especially the pores at 20 nm contribute greatly to the pore structure optimization.Because of the exploitation of coal,the pressure of the overlying coal was released,which lead to the recovery of the permeability,and the permeability recovery has a positive correlation with the original coal permeability.The Langmuir theory and mechanical balance principle were inducted,a new isothermal adsorption model,which considered the dynamic measurement volume of coal and clay mineral,the shrinkage of coal matrix and the dynamic volume of adsorbed methane,was established.This new model can describe the dynamic of the methane capacities during the isothermal adsorption procedures refinedly.The ground water from the other stratums has improved chemical activity compared with that in the homo-coal,and the chemical reaction between the coal and ground water is much more significance.After the water-rock interaction,the clay minerals and carbonate minerals in the fractures and that on the surface of the coal matrix are dissolved and corroded,the quartz can be precipitated and emigrated.The distance between the coal and the passageway for ground water is diverse,the water-rock interaction degree varies.From the near to the distant,it can be divided into dissolution-corrosion area,water-swelling area and original coal zone.For bulk coal,the water-swelling area is dominant,which lead to the depression of the coal mechanics properties,and this decreased the coal permeabilities.The geological control model on overlying coal's reservoir properties influenced by the mining mainly presents macroscopical coal deformation and microscopic pore structure adjustment,while that for geochemical control model mainly performances the reduence of coal permeabilities at various water-rock interaction degree.