The Characterization, Genesis And Evolution Of Nanopores In Coals And Shales

As the exploration, exploitation and commercial value of unconventional oil and gas in North America, the research of unconventional reservoir attracts increasing interests. The characteristics of pores are important parameters for reservoir evaluation. The characterization of pore system in unconventional reservoirs is important for the evaluation of the gas bearing evaluation. Nanopores are the most important pore type in unconventional reservoir and they control the major storage space of tight gas, shale gas and coalbed methane. The study of pore systems in coal and shale is not only substantial for the exploration and exploitation of unconventional oil and gas, but also for the theory of structure of coal and kerogen and the theory of genesis and evolution of nanopores. There are three kinds of characterizing methods for coals and shales:image analysis, intrusive method and nonintrusive method. Intrusive methods are the mainstream methods. Image analysis is mainly used to qualitative observations of pores. In this study, a series of coals with different maturation, thermal simulated coals and the three potential gas shales in south China （Dalong Shale, Longmaxi Shale and Hetang Shale） were used for the studies of atomic force microscopy （AFM）, dual-beam field emission scanning microscopy-focused ion beam （FESEM-FIB） and N₂ gas adsorption. Some other experiments such as organic petrology, total organic carbon, X-ray diffraction were also conducted for these samples. This study is focused on characterization of pore system in coals and shales. The differences and advantages of image analysis and N₂ adsorption are compared, and suggestions are given for better use of these two methods. The genesis of nanopores in coals and shales are discussed.The characteristics of nanopores in coals and shales are revealed by AFM and FESEM-FIB observation. There are two major types of pores in coals and shales:primary pores and secondary pores. Primary pores are classified into intraparticle pores, interparticle pores and matrix pores. Secondary pores are classified into organic matter thermogenic pores, micro-fracture, clay transformation pores, dissolved pores, and intercrystalline pores. Interparticle pores, dissolved pores and micro-fractures are mainlyμm scale. Matrix pores and organic matter thermogenic pores control the nanopore system in coals and shales. The pores in clay aggregates, which have wide pore size distribution in nanoscale and slit shape, are the main part of matrix pores. Secondary organic matter pores contain gas-genetic pores and intermolecular pores. Gas-genetic pores are usually in nanoscale and elliptical shaped. They mainly developed alone in organic matters, but in high and over maturation stage or coals could also develop in group and appear as irregular shape. Intermolecular pores are angstrom-scaled voids between super-molecules and basically developed in coals.Super fine grinding technology and confidence interval statistics method were used to reduce the error of AFM data in sample preparation stage and data collecting stage. Open resource software Gwyddion was used to build AFM-Gwyddion method which could rebuild the surface tomography of coals in 2D and 3D. Parameters of surface characteristics such as average surface roughness （Ra）, root mean square roughness （Rq）, surface skewness （Rsk） and coefficient of kurtosis （Rku） and parameters of pore system which containing pore number, areal porosity, minimal pore size, maximal pore size, pore area and form factor were acquired by AFM-Gwyddion method. The results shows that the surface roughness （Ra, Rq） of coal series has the tendency of "reduce-increase-decrease" with increasing thermal maturation. Surface roughness of the coal series are mainly influence by the maceral composition （and the responding skeleton structure of super-molecule） at low maturation stage and characteristics of nanopores at the high maturation stage. There are three major stages of pore evolution in the coal series:declining of μm scale pores, developing of nanoproes and declining of nanopores, corresponding to the saltus of bitumous coal to anthracite.SEM-PCAS method was build base upon image processing software Particles （Pores） and Cracks Analysis System （PCAS）. This method was used to process and analysis SEM images of the three potential gas shales in Yangtze region. Regular pore parameters were acquired, such as pore size and pore area. Probability entropy, form factor, and fractal dimension are introduced to the image analysis of pores. The most important advantage of SEM-PCAS method is the ability to study pores with different occurrence. By system study of different types of pores in shales, gas-genetic pores in organic matters was found to be small （<60 run） and regular shaped （form factor> 0.6）. The ratio of gas-genetic of Longmaxi Shale is 85.3%, which is the highest in three potential gas shales in south China. The high ratio of gas-genetic pores decide the relative small pore size （61.1 nm in average） and relative large form factor （0.67 in average）. The low ratio of gas-genetic pores in Hetang Shale and Dalong Shale suggest the destruction of gas-genetic pores and may explain the low gas content of Hetang Shale.N₂ adsorption experiment was utilized to acquire the pore size distribution, pore volume and BET surface area data of the three series of samples. The features and advantages of AFM-Gwyddion method, SEM-PCAS method and N₂ adsorption experiment were compared. The most important advantage of AFM is the wide range of characterizing pore sizes which contains from micropores to macropores. Based on the capability of composition distinguishing, the FESEM-FIB can study on each pore type separately. N₂ adsorption can acquire a large set of data which are convenient for comparison. As the features and advantages of the three kinds of experiments, AFM-Gwyddion method is suggested to characterize the pores in coal. The combination of SEM-PCAS method and N₂ adsorption is better for characterization of pores in shales.By summarizing the results of experiments in this study and previous studies, organic matter characteristics （total organic carbon content, maceral type and thermal maturation）, mineral composition （quartz content, clay minerals content and carbonate minerals content） and diagenesis are confirmed as the major control factors on pore development. Thermal maturation and maceral type control the genesis and evolution of thermogenic pores in organic matters. Total organic carbon content decides the upper limit of thermogenic pores. Clay minerals and organic matters are usually mixed, controlling the micropore and mesopore porosity. In general, pores in coals are controlled by thermos maturation while the characteristics of organic matter and clay minerals control the pores in shales.