Abstract
It is of great significance to study the porosity and permeability properties of medium and low-rank coal. The porosity and permeability in confining stress experiments were used to simulate the porosity and permeability variations of coal samples under different depth conditions. The pore structure of Baoqing coal samples is greatly affected by the confining pressure, and the pores and micro cracks are more easily compressed. Based on the experimental data of mercury intrusion porosimetry (MIP) and nitrogen adsorption (NA), the pore size distributions (PSDs) of medium and low-rank coals were studied. High mercury intrusion pressure would lead to coal matrix compression. Therefore, the pore volume calculated by MIP data was corrected by NA data. The PSDs characteristics of Jixi (JX) coal and Baoqing (BQ) coal samples are obtained from the revised pore volume, and the dominant pores of medium and low-rank coals are obtained. The results show that JX coal has higher spatial heterogeneity, connectivity and pore autocorrelation. Micro fractures have an influence on the autocorrelation and heterogeneity of coal samples, especially for BQ coal samples.
Highlights
It is of great significance to study the porosity and permeability properties of medium and low-rank coal
According to the latest prediction results, China is rich in coalbed methane resources, with the total reserve of 30.05 × 1012 m3, of which high rank accounts for 23%, medium rank accounts for 34%, and low-rank accounts for 43%3,4
Caniego et al.[16] studied the pore size distributions (PSDs) characteristics of soil using singular fractal dimensions. Gauden[14] first applied the fractal method to describe the pore structure of coal, and combined with the pore data obtained by experimental methods, quantitatively revealed the pore structure characteristics of coal
Summary
The two groups of coal blocks were collected from Dongshan Coal Mine No 7 coal seam (mining depth 840 m) in Jixi Basin and Baoqing Coal Mine No 10 coal seam (mining depth 16.44 m) in Suibin depression, respectively. The test standard referred to the method of static nitrogen adsorption capacity for the determination of specific surface area and pore size distribution of rocks of CNPC (SY/6154-1995, China). 1 and 2, it shows that both the porosity and permeability of the JX and BQ coal samples decrease significantly with the aggrandization of the confining pressures, and the variations of BQ coal samples are more obvious. When the confining pressure increased from 3.5 to 7 MPa, the porosity and permeability of BQ coal samples decreased by 25% and 66%, respectively. It shows that the stress sensitivity of BQ coal samples is stronger, and the seepage pores are impacted by confining pressure more obviously. When the relative pressure increases from 0.2 to 0.8, there is a positive correlation between the amount of nitrogen adsorption value and the relative pressure, which reveals that there is a certain amount of pore distribution in Sample ID JX-1 JX-2 JX-3 JX-4 Average BQ-1 BQ-2 BQ-3 BQ-4 Average
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