Identification Of Coal Structures With The Aid Of Geophysical Logs

In this paper,multiple regular experiments including coal composition,vitrinite reflectance and scanning electron microscope were conducted.Furthermore,innovative experiments including low temperature N2 adsorption,low pressure CO2 adsorption,low field nuclear magnetic resonance,uniaxial / triaxial compression test,acoustic emission tests,resisivity and acoustic speed test were constructed.The evlolution and controlling mechanism of pore system of coal structure samples,the electric-acoustic response were studied,the model of idenfication of coal structures using logging data was built,and favorable development area were predicted as well.For better understanding,the detailed results and acheivements are as follows:(1)Based on the experiment analysis on pore system of coal structure samples,the evolution mechanism of coal pore structure of undeformed and deformed coals were investigated.Results show that micropore and mesopore volume can be obtained from low pressure CO2 adsorption and low temperature N2 adsorption,respectively.The low field nuclear magnetic resonance results can characterize the pore proportion of each coal structure sample.The undeformed coal is dominated by micropores.The micropore proportion in the cataclastic coal decreases and the mesopore proportion increases.The micropore and mesopore proportions in the granulated coal decrease,while the macropre proportion increases.(2)The dynamic model of stress-strain,acoustic emission,resistivity and acoustic speed of coal structure samples under uniaxial and triaxial compression was clarified.With the increasing deformation of coals,from the undeformed coal to the granulated coals,the Young's modulus and compressive strength decrease,and acoustic emission account decreases,while the resistivity increases in the order of magnitude.(3)Based on microscopic and macroscopic observation of coal structure samples,the natural gamma,deep lateral resistivity,acoustic time and density logging curves were employed to build a qualitative identification model.With the increasing deformation of coals,the amplitude of the natural gamma and density decreases,while that of the deep lateral resistivity and acoustic time increases.The crossplots of densitiy-natural gamma and acoustic time-deep later resistivity were analyzed to build a quantitative identification with a vertical 0.1 m resolution.(4)The favorable exploration and development area of coalbed methan in the southern Qinshui Basin was clarified.The triangle digram with the undeformed coal,cataclastic coal and granulated coal based on the thickness proportion was used to classify the coal reservoir into four main types.The study area was classified into four main coal structure area,including Area I,the undeformed coal dominated area;Area II,the cataclastic coal dominated area;Area III,the granulated coal dominated area;Area IV,the transitional area.Area II has the most cataclastic coal with the most interconnected fractures,which improves the permeability of coal reservoir and is beneficial to hydraulic fracturing.