Abstract
It is of great benefit to study the material and structural heterogeneity of coal for better understanding the coalbed methane (CBM) storage and enrichment. In this paper, multi-scale X-ray computed tomography (CT), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) at multi scales were conducted to thoroughly study the material distribution, heterogeneity, pore development, porosity and permeability of coal. It is suitable and reasonable to divide the testing samples into three structural categories by average density and heterogeneity degree, and the meso structure in the three categories accords with the morphology on SEM images. The pore size distribution and pore development of each subsample cannot be correspondingly related to their respective structure category or morphology due to different observation scales, while the macro pore size development, accumulated macro pore volume and macro pores porosity accord with the meso structure category and morphology information by CT and SEM at the same scale very well. Given the effect of macro pores on permeability and the contribution of micro pores to CBM storage capacity, reservoirs with developed micro pores and macro pores may be the most suitable coal reservoir for CBM exploitation.
Highlights
In recent years, coalbed methane (CBM) has become a potential alternative energy to fossil fuels for its clean property, high calorific value and large storage in the field of energy exploitation
Previous studies on coal structure mainly concentrate on characteristics of pore structure, such as pore volume (PV), pore specific surface area (PSSA), pore size distribution (PSD), fractal dimension and so on,[3,4,5,6,7] few of the studies have attempted to characterize the entire coal structure with coal matrix, clay minerals and pore distribution all taken into consideration
The X-ray computed tomography (CT) scanning, scanning electron microscope (SEM) scanning and mercury intrusion porosimetry (MIP) testing are important meso and micro observation methods for porous material, and they are different in applications due to respective precisions and observation scopes
Summary
In engineering practice, CBM production and composition from individual stacked coal seams in a multi-zone completed gas well vary greatly.[15]. By using X-ray CT scanning, coal skeleton deformation and pores/fractures evolving in rock materials under different testing conditions were extensively obtained.[16,17,18,19] Besides, on the basis of threshold segmentation and 3-D reconstructions, the qualitative visualization and pore structure quantitative characterization can be performed, too.[20,21] As for scanning electron microscopy (SEM), it works in larger magnification and can accurately shows more detailed material composition and structural characterization information, it is widely used in the morphology study of rock and soil materials.[22,23] The mercury intrusion porosimetry (MIP) is extensively used to characterize the pore characteristics of porous media, such as PV, PSD, and PSSA.[24,25,26,27] It is worth noting that these methods have excellent observation on meso and micro structure, there are still some differences in their application due to respective precision and scope. The result consistency at different scales was further discussed
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