| The No.3 coal reservoir in the south of Qinshui Basin is a typical anthracite reservoir in China,and it is also the main productive reservoir of coalbed methane development at present.How does the connectivity of pore structure directly determine the desorption,diffusion and percolation of CBM in the process of extraction,restrict the extraction efficiency of CBM surface vertical Wells,and is the key to high production of CBM Wells.In this paper,the No.3 coal reservoir in the south of Qinshui Basin is taken as the research object.The pore development characteristics of coal reservoirs in the south of Qinshui Basin were analyzed by means of mercury injection method,low temperature liquid nitrogen adsorption method,nuclear magnetic resonance relaxation method,nuclear magnetic resonance freeze-thaw method,SAXS and SEM.Through the three-dimensional visualization reconstruction of the pore structure of the coal reservoir by CT scanning,the parameters of the pore structure network in the test sample were extracted,and the development characteristics and connectivity of the pore structure of the coal reservoir were systematically studied.Mercury injection test results show that,the Micro and small pores with pore diameter < 100 nm are the main sources of pore volume,porosity of the coal reservoir in the study area is less than 7%.Low temperature liquid nitrogen adsorption experiment results show that,the highest pore content was found at the pore radius of2.5-50 nm.The NMR relaxation method is equivalent to mercury injection method in the analysis of pore diameter distribution.The results of NMR freeze-thaw test show that the pore size of the sample was concentrated in the range of 2.5~10nm.SAXS results show that the pore size is mainly distributed at 0-40 nm and the pore content is relatively high at 10-20 nm.There are a large number of poorly connected and disconnected pores in the study area,and a considerable number of semi-closed pores are developed in the coal reservoir,which is not conducive to the storage and development of coalbed methane.CT scan results showed that the coordination number of pores in the sample was concentrated at about 1,and most of the pores were dead-end pores.Both pore shape factor and throat shape factor of the sample were > 0.01,and the shape of throat section was in the transition stage from triangle to square.The throat length of thesample was short and the pore-throat ratio of the samples was large.The pore space development was uneven and the connectivity was poor.Through the establishment of the traditional geometric fractal model and thermal fractal model,the calculation shows that the fractal dimension of the measured sample is generally high.and the average value of the geometric fractal dimension and thermal fractal dimension of the seepage hole is 2.965 and 2.9891,respectively.This means that the coal has more complex pore structure and rougher pore surface.The permeability of coal reservoirs in the study area is relatively poor.The average contribution of seepage holes to the permeability calculated by thermal fractal methods is 22.29%,indicating that the seepage holes with a diameter of >100nm are the main source of permeability.The coal reservoir in southern of Qinshui Basin has transitional diffusion throughout the whole CBM development process,and Fick diffusion mainly occurs at the initial stage of production.There may be considerable Fick diffusion at the end of production.In view of the characteristics of extremely low macropore content in coal reservoirs in the south of Qinshui Basin,it is considered that seepage mainly occurs in the fracture system.Figure[27] table[12] reference[149]... |
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