Junqian Li

Physical characterization of the pore-fracture system in coals, Northeastern China

Usually mercury intrusion/extrusion curves reflect the characteristics of open pore-fracture system of coals. Using this method, the nature of the open pore-fractures of 18 Chinese coals (varying in vitrinite reflectance from 0.65 to 1.76%Ro, max) was studied. A quantitative evaluation method for micropore (0-0.1 μm, in diameter), mesopore (0.1-1 μm), macropore (1-10 μm) and fracture (>10 μm) within coals was established by the mercury intrusion curves. The method was verified by the fractal geometry theory. Moreover, three Types (I, II and III) of the open pore-fracture systems of coals were analyzed by combining mercury withdrawal efficiency and permeability with pore size distribution. Results show that (a) Type I coals are characterized by abundant open micropores (mean 62.8 vol. %) and rarely open fractures, which leads to a large minable potential but very low production rate for coalbed methane (CBM); (b) Type II coals have low minable potential and high production rate for CBM, mainly because of the distribution of a few micropores (mean 35.5 vol. %) and a large number fractures (mean 19.7 vol. %) in coals; (c) Type III coals are the most appropriate to exploit CBM due to the existence of optimal open pore-fracture system (micropores, mean 58.5 vol. %; fractures, mean 15.4 vol. %) within coals.

Multi-component segmentation of X-ray computed tomography (CT) image using multi-Otsu thresholding algorithm and scanning electron microscopy:

X-ray computed tomography is an efficient method for quantitatively estimating the characteristics and heterogeneity of shales in three dimensions. A threshold is commonly used to separate pore-fractures from the background image. However, few studies have focused on the multi-component segmentation of computed tomography images. To obtain the distribution characteristics of different components in three dimensions, a segmentation method was proposed that combines a multi-Otsu thresholding algorithm with scanning electron microscopy. The gray value distributions of different components were first determined using this method. Then, the shale components were divided into several groups based on these gray values. The threshold of each component group was determined using the multi-Otsu thresholding algorithm. The computed tomography image stacks of two shale samples were processed using this segmentation method, and another computed tomography image stack was used to verify the method. The results showed that (1) the multi-component segmentation method can successfully segment computed tomography image stacks using the calculated values determined by computed tomography, which agree well with the measured values obtained from X-ray diffraction, total organic carbon, and porosity tests in the laboratory; (2) samples with similar provenances and mineral compositions have the same gray value distribution in the back scattering scanning electron microscopy and computed tomography images; (3) this method is superior in both the effectiveness and efficiency of the computed tomography image stack segmentation of samples according to the gray value distribution, as determined by samples with similar provenances and mineral compositions.

Quantitative Evaluation on the Elastic Property of Oil-Bearing Mudstone/Shale from a Chinese Continental Basin

This study aims at quantitatively evaluating the impact of material compositions on the elastic property (static Youngs modulus) of oil-bearing mudstone and shale (MS). Six samples (two mudstone and four shale) from a Chinese continental basin (Bohai Bay basin) were investigated under different hydrostatic triaxial compression conditions. The primary findings of this study include the following: (a) the elastic property of MS rocks are sensitive to the hydrostatic confinement condition. In response, the MS Youngs modulus gradually increases with increasing confining stress. (b) The mineralogical compositions impacting the MS Youngs modulus primarily include quartz, plagioclase, calcite and clay. Overall, the total content of siliceous (calcareous) minerals has a positive (negative) effect on Youngs modulus. Clay also contributes to the decrease of Youngs modulus. (c) Due to its relatively low stiffness and strength, organic matter has a substantially negative impact on Youngs modulus. (d) MS reservoirs that contain a high siliceous mineral content will be favorable to hydraulic fracturing. Finally, an empirical model evaluating MS Youngs modulus was established by coupling Youngs modulus with material compositions; the model shows a good fit with the experimental results. Thus, the MS Youngs modulus is comprehensively affected by multiple constituents found in MS rocks and can be expressed as a function of material composition.

Geological factors on gas entrapment mechanism and prediction of coalbed methane of the no. 6 coal seam in the Jungar coalfield, northeast Ordos Basin, China

Low rank coal reservoirs of the no. 6 coal seam in the Jungar coalfield are characterised by low gas content but favourable porosity and permeability for recovery. The accumulation and enrichment of CBM is favoured by the presence of multiple (five seams) and thick (12.7–40.4 m) seams, adequate permeability (3.6–26 mD) and significant abundance of coal resource (5.44 × 10 10 tonnes), but hindered by low observed gas content (0.01–1.5 m 3 /t), which is shown to result from both shallow burial depth and high permeability of the seams. However, the sheer magnitude of the no. 6 coal resource offsets this shortcoming of low gas content and makes this a good prospect for exploration and exploitation. Conditions are most favourable in the southwest coalfield where a monoclinal structure and favourable hydrodynamic conditions have prevented gas escape. Areas of the no. 6 coal seam buried under the CH 4 weathering line ( > 860 m), with larger coal thickness ( > 10 m), and higher gas content ( > 1.2 m 3 /t) have the greatest potential for CBM enrichment.