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Backfill mining is an effective way to solve environment pollute, surface subsidence, and groundwater system damage which caused by coal mining. However, the complex underground environment may change the physical and mechanical properties of the backfill body, which poses a risk of strength failure. This paper analyzed the failure of gangue-based cemented backfill body which made up of gangue and fly ash. The results show that physicochemical reactions will generate quartz, kaolinite, and other high-strength substances; hydration reaction between the fine particles will generate hydrocalcium silicate and other C-S-H gels, they wrapped gangues as a whole, which provide a high strength of the cemented backfill body. Several experiments were carried out in order to find the reason for failure in samples under loads. The conclusion drawn as following: (1) When the load is large, the cracks extend from the surface of the samples to the interior, at the same time, the length and width of the cracks increasing obviously and connecting as net. Especially the external load exceeds the peak intensity. (2) The relationship between sample failure and pores is weak, but obvious with crack development, especially the cracks connected as a net. (3) The interface structure formed by gangue is an important source of crack development and, thus, will stimulate the development of cracks. [ABSTRACT FROM AUTHOR]

In situ leaching (ISL) is becoming the main mining practice for sandstone-type uranium deposits in China. The key to ISL technology is to aid the leaching solution in contacting the ore bed over a large range, which will induce a series of chemical reactions to extract uranium; thus, it is essential to thoroughly understand the reservoir physical properties of uranium deposits. Taking the Qianjiadian sandstone-type uranium deposits (southern Songliao Basin, China) as an example, the mineral composition and pore structure of samples in different layers were measured using X-ray diffraction (XRD), thin section analysis (TSA), low-temperature N2 adsorption (LTN2A), and mercury intrusion porosimetry (MIP), and their influences on the ISL effect were analyzed. The results show that more than 65% of the minerals in the Qianjiadian uranium deposits are felsic minerals, and the carbonate minerals, clay minerals, and augite are auxiliary minerals. The primary intergranular pores, intergranular-dissolved pores, intragranular-dissolved pores, intercrystalline pores, and microfractures are developed in uranium deposits with various lithologies to different degrees. The macropores (>1000 nm) and mesopores (100-1000 nm) of medium sandstone, argillaceous sandstone, and siltstone are well developed; in contrast, the proportions of micropores (<10 nm) and transition pores (10-100 nm) in coarse sandstone, fine sandstone, and sandy mudstone are quite high. The heterogeneity of pores in uranium deposits of different lithologies is strong and influences the mineral composition and its fabric mode. Coarse sandstone, fine sandstone, and sandy mudstone are favorable for ISL mining in Qianjiadian uranium deposits because their permeability is above the required permeability threshold of ISL. The uranium deposits with permeability below the threshold are recommended to adopt the blasting-enhanced permeability method to improve their permeability for achieving large-scale and high-efficiency ISL mining. This study can provide guidance for the selection of favorable ore beds for ISL mining and reservoir stimulation methods in low-permeability sandstone-type uranium deposits. [ABSTRACT FROM AUTHOR]

This study examined 510 Chinese students' emotional intelligence (EI) and English classroom learning anxiety at three universities in Hangzhou, People's Republic of China. Results obtained from the Trait Emotional Intelligence Questionnaire-Short Form (TEIQue-SF) and the Foreign Language Classroom Anxiety Scale (FLCAS) showed the following results: (a) More than half of the Chinese college students possessed a middle to high level of EI and at least one third experienced language anxiety in English class; (b) Moderate to relatively strong associations were found among students' EI, foreign language anxiety (FLA), English achievement, and self-rated English proficiency; (c) FLA had a significant and partial mediating effect on EI in predicting students' English achievement; and (d) FLA also significantly and partially mediated the relationship between EI and self-rated English proficiency. Results and implications are discussed in the context of the importance of emotional factors affecting the learning of English. [ABSTRACT FROM AUTHOR]

To obtain an excellent heat exchange effect, it is necessary to optimize the technical processes of single-well coaxial heat exchange. Based on numerical simulation and field investigation, this study shows that a high inlet temperature can significantly improve the outlet temperature while a low inlet temperature can obtain a high heat extraction power. The outlet temperature and heat extraction power show a positive correlation binomial relation with the inlet flow rate, the thermal conductivity of cement, the thermal conductivity of reservoir and the borehole diameter, which exhibit linear negative and positive correlation relationships with the thermal conductivity of inner tube and geothermal gradient. The inlet flow rate, geothermal gradient and thermal conductivity of the inner tube are the most significant factors, followed by the thermal conductivity of reservoir and the borehole diameter, while the inlet temperature and thermal conductivity of cement are the insignificant factors. It is recommended to select the reservoirs with high geothermal gradient, good thermal conductivity and wide stable distribution, develop fixing well cement with high thermal conductivity and inner pipe materials with excellent insulation performance, reduce the inlet temperature, increase the inlet flow rate of working fluid and expand the well diameter while considering costs. • Field test of coaxial borehole heat exchanger was performed in the dry hot rock reservoir of Matouying uplift; • Flow-heat exchange model considering interactions of working fluid, heat exchanger and reservoir was established; • Heat extraction power was comprehensively influenced by process condition, material condition, and geological condition; • Suggestions for site selection and process optimization of coaxial borehole heat exchanger were proposed. [ABSTRACT FROM AUTHOR]

Abstract As a highly heterogeneous material, the porous structure of coal is usually described by fractal dimension, which generally can be obtained by gas adsorption, light scattering and image analysis methods. In this paper, the pore structure (pore volume and specific surface area) and adsorption performance (Langmuir volume and Langmuir pressure) of 12 different rank coal samples were investigated by liquid nitrogen adsorption experiment (LNA) and methane isotherm adsorption experiment. And the fractal characteristics of these coals were studied by three fractal dimensions D 1 , D 2 and D 3 acquired from LNA and small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM), respectively. Results indicate that the adsorption pore volume and specific surface area show U-shaped evolutionary trends during the coalification process. The changes of pore volume and specific surface area are dominated by the mechanical compaction and dehydration effects before the second coalification jump (R o,max = 1.3%), while they are then mainly controlled by concomitant metamorphic pores until the fourth coalification jump (R o,max = 3.7%). Langmuir volume first increases and then reaches a steady value at the third coalification jump (R o,max = 2.5%) with the increasing coal rank, however, Langmuir pressure monotonously decreases during the whole coalification process. D 1 and D 2 exhibit a dramatically increase from R o,max = 0.6% to R o,max = 2.0% and then a slowly rise when R o,max increases from 2.0% to 3.5%, which is relevant to the changes of pore structure and macromolecule structure in coal. D 3 increases at a low speed first and then increases at a high speed with the increasing coal rank, which reflects the surface morphology of pores in coal. Fractal dimension D 1 , D 2 and D 3 can be used to depict the methane adsorption properties of coal in varying metamorphism degrees. Generally, D 2 is larger than D 1 , which is because N 2 adsorption can probe more macropores but SAXS can detect more micropores. Additionally, D 1, D 2 and D 3 are all negatively related to P L. Our investigations also confirm that SAXS may be more superior to character the adsorption capacity of coal according to its highest relativity. This work clarifies the intrinsic sense and difference for characterising the surface irregularity by various methods, and provides an understanding for the experimental means relating to adsorption pores. Highlights • The volume and specific surface area of adsorption pore display U-shaped evolutionary trends during coalification. • Langmuir volume and Langmuir pressure are related to the metamorphism effect of coal. • D 1 and D 2 demonstrate discrepancy versus coal rank. • SAXS may be more superior to character the adsorption capacity of coal. [ABSTRACT FROM AUTHOR]

Closed pores, which have been confirmed to exist in coal, have a dual significance in illustrating the tremendous gas outburst content in coal mining and ascertaining the exact methane content in coal, although it is unknown if this methane can be exploited and utilized. In this paper, the principal objective is to explore the evolution and formation mechanisms of closed pores, regarding which there is a lack of knowledge and understanding. We investigated the varieties of closed pore contents in coalification, combining the small angle X-ray scattering (SAXS) and liquid nitrogen adsorption methods (LNA), and simultaneously analyzed the surface topography of closed pores by scanning electron microscopy (SEM). The closed pore volume represents a low-high-low evolution trend during coalification owing to the appearance of gas pores in the pyrogenic hydrocarbon generation stage. However, the specific surface area increases uniformly due to the increase in the number of micro-pores. The tectonic deformation effect impels the increase of closed pores, which reflected in the larger pore volume and specific surface area in strong deformation coal than that in weak deformation coal. The genesis of closed pores can be summarized as a result of heterogeneous matrix shrinkage, the tectonic compressive stress and the tectonic shear stress. Finally, this work provides a novel method to investigate the mechanism of geological CO 2 sequestration. [ABSTRACT FROM AUTHOR]

In this study, the micro-pores and fractures of six coal samples with different metamorphic degrees ( R o,m = 0.75–2.80%) were analysed using field emission scanning electron microscopy (FESEM). The micro-fractures and pores were quantitatively classified based on fractal theory. Our results show that pores in coal matrix can be categorised as either micro-pores or micro-fractures according to fractal theory. Micro-pores exhibit round or elliptical shapes, whereas micro-fractures are bent and exhibit irregular shapes, which are connected with other micro-fractures via round or elliptical channels. With increasing metamorphic degrees, the distribution of the total surface porosity of micro-pores and micro-fractures is characterised by an inverted “U” shape. The total surface porosity is at its greatest during the high-rank bituminous coal stage. However, the correlation between coal metamorphic degree and surface densities of micro-pores and micro-fractures varies. During the low- and medium-rank stages, the surface frequency and the average width of micro-pores and fractures increase with increasing coal rank. During the high-rank coal stage, with increasing coal rank, the surface frequency of micro-pores and fractures rapidly increases but as a whole is lower than those during the low- and medium-rank stages. The average width of micro-fractures decreases with increasing average surface frequency in a logarithmic relationship; in contrast, the average width of micro-pores increases logarithmically with increasing average surface frequency. [ABSTRACT FROM AUTHOR]

The injection of supercritical CO 2 into coal seam alters the mechanical properties of the coal, which should be paid close attention when considering security issues during CO 2 -Enhanced CoalBed Methane recovery (CO 2 -ECBM). In this study, the Qinshui Basin is considered as the research object, and mechanical parameters, such as the peak strength, elastic modulus, and Poisson's ratio of anthracites under the coupled influences of CO 2 saturation pressure, CO 2 saturation time, and water content were investigated. The results indicate a decrease in the peak strength and elastic modulus and an increase in the Poisson's ratio of the CO 2 -saturated coal. Increasing the CO 2 saturation pressure, extending the CO 2 saturation time, and increasing the water content contributes to the softening of the coal. The modified Langmuir equation and stretched exponential (SE) equation are used to accurately predict the peak strength and elastic modulus reduction with CO 2 saturation pressure and CO 2 saturation time, respectively. The failure patterns change from brittle shear failure for natural coal and transitional shear failure for supercritical CO 2 -saturated coal to the ductile non-dilatant barreling for the supercritical CO 2 +water-saturated coal. The softening effect of coal can be attributed to the chemical and physical reactions between coal and CO 2 , including the surface energy reduction of CO 2 -bearing coal, extraction and plasticization reactions, differential adsorption swelling and swelling stress, and dissolution of carbonate solution for the mineral filling pore/fracture. In view of the geo-mechanical changes caused by CO 2 to coal, the optimization of the targeted reservoir is pivotal for the successful implementation of CO 2 -ECBM. [ABSTRACT FROM AUTHOR]

The permeability of coal reservoir is dynamically changed during CO 2 enhance coalbed methane recovery projects (CO 2 -ECBM) and is controlled by the two main influencing factors: effective stress and CO 2 adsorption. Six coal core samples parallel and vertical bedding plane directions were drilled, which were then adopted to conduct the permeability tests under conditions of cyclic loading/unloading and supercritical CO 2 saturation. Results show that the permeability is depended on the coal rank, the middle rank coal possesses the highest initial permeability because of its moderate compaction and largely generated cleats during the coalification. The permeability is anisotropic, manifesting in the permeability in parallel bedding plane direction is larger than that in the vertical bedding plane direction. The stress sensitivity coefficient and IPLRi in parallel bedding plane direction exceed that in vertical bedding plane direction, however, PALR exhibits the opposite law. Hereat, the fractures parallel to bedding plane are susceptible to the stress change and the cleats vertical to bedding plane are sensitive to the ScCO 2 adsorption. In view of the irreversible permeability damage by the stress change and CO 2 adsorption, experiments in this paper confirms that prefracturing can dramatically decrease the IPLRi , stress sensitivity coefficient and PALR of coal seams, which may be the potential and effective measure to enhence the injectivity of CO 2 in coal seams. [ABSTRACT FROM AUTHOR]