| Due to the special diagenetic environment and groundwater migration conditions,the Jurassic weak cemented sandstone is of a complex unconventional water-bearing rock series,which brings many serious hydrogeological problems to the coal mine water prevention and control in the western region.In this paper,based on traditional geology and advanced analytical techniques,the lithology and sedimentary mechanism of the Jurassic weak cemented sandstone in the Hami mining area are studied.Through the seepage experiment,laboratory test and theoretical analysis,the relationship between permeability and microscopic pore structure was studied,and the permeability evolution mechanism of weakly cemented rock was expounded by the microscopic view.This paper has achieved the following innovations:(1)Through 15 groups seepage experiments of weak cemented sandstone,the seepage curves present a process of attenuation,stabilization and sudden increase.The process of seepage experiments can be divided into three stages: saturated seepage stage,stable seepage stage and seepage mutation stage.The saturated seepage stage is related to the saturated water absorption characters of the dry rock sample,and the permeability of the second hours decreases to 0.1 times of the initial value.Through dynamic water disturbance seepage test,YP-2,YP-15 and YP-21 rock samples show sudden increaseing phenomenon,and the highest degree of sudden change of permeability can reach 10 times.(2)Based on the micro-pore structure tests,the influence of micro-pore structure change on permeability were analyzed.The permeability raise with the increase of porosity and mercury removal rate,but there was no obvious correlation between them;the average pore throat radius has a strong linear positive correlation with the permeability;The linear correlation between pore throat distribution frequency and permeability is very low.After seepage mutation,the average pore throat radius of sandstone increases,the throat-pore ratio decreases,that of withno seepage mutation is opposite.The change trend of throat-pore ratio is a main reference index for judging the mutation of seepage.(3)The distribution and provenances of soluble salts and kaolinite in the pores of weak cemented sandstone were analyzed by water quality analysis and SEM.The mechanism of chemical and physical effects of water rock on microscopic pore structure and permeability were further studied.It was found that the dissolution of microcrystalline salts,ion-adsorbed salts,associated mineral ions of authigenic kaolinite and precipitated salts resulted in the transformation of tiny throats to Mediumsized throats;the migration and exudation of kaolinite improved the radius and smoothness of throats,and the number of medium and large pore throats were increasing.The lost of soluble salt and the kaolinite is the main factor of the permeability enhancing.(4)Based on the influence degree and correlation of microscopic pore structure characteristic parameters on permeability,the micro-pore structure straight capillary pipe seepage model is constructed,and the permeability theoretical equation of weak cemented sandstone of steady seepage stage and seepage transition stage is proposed.According to the seepage theory equation,the permeability is directly proportional to the porosity and the square of average pore throat radius,and the evaluation result of the seepage theory equation is basically consistent with the measured value.Based on the dialectical analysis of macroscopic seepage,mesoscopic indoor seepage and microscopic seepage theory,the necessary conditions and mechanism of macroscopic seepage permeability change are demonstrated from microscopic view,and the constructive concepts of water-retaining mining in weak cemented sandstone aquifer are expounded from microscopic and microscopic view. |
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