Micro-structural evolution and their effects on physical properties in different types of tectonically deformed coals

The macromolecular structure of tectonically deformed coals(TDC)may be determined by the deformation mechanisms of coal.Alterations of the macromolecular structure change the pore structure of TDC and thereby impact physical properties such as porosity and permeability.This study focuses on structure and properties of TDC from the Huaibei and Huainan coal mining areas of southern North China.Relationships between the macromolecular structure and the pore structure of TDC were analyzed using techniques such as X-ray diffraction,high-resolution transmission electron microcopy,and the low-temperature nitrogen adsorption.The results indicated that the directional stress condition can cause the arrangement of basic structural units(BSU)more serious and closer.And,the orientation is stronger in ductile deformed coal than in brittle deformed coal.Tectonic deformation directly influences the macromolecular structure of coal and consequently results in dynamic metamorphism.Because the size of BSU in brittle deformed coal increases more slowly than in ductile deformed coal,frictional heating and stress-chemistry of shearing areas might play a more important role,locally altering coal structure under stress,in brittle deformed coal.Strain energy is more significant in increasing the ductile deformation of coal.Furthermore,mesopores account for larger percentage of the nano-scale pore volume in brittle deformed coals,while mesopores volume in ductile deformed coal diminishes rapidly along with an increase in the proportion of micropores and sub-micropores.This research also approved that the deformations of macromolecular structures change nano-scale pore structures,which are very important for gas adsorption and pervasion space for gas.Therefore,the exploration and development potential of coal bed methane is promising for reservoirs that are subjected to a certain degree of brittle deformation(such as schistose structure coal,mortar structure coal and cataclastic structure coal).It also holds promise for TDC resulting from wrinkle structure coal of low ductile deformation and later superimposed by brittle deformation.Other kinds of TDC suffering from strong brittle-ductile and ductile deformation,such as scale structure coal and mylonitic structure coal,are difficult problems to resolve.

Geochemistry of Mercury in the Permian Tectonically Deformed Coals from Peigou Mine, Xinmi Coalfield, China

As the mercury emitted from coal combustion can lead to serious environmental issues, researchers pay more attention to the content, distribution and occurrence of mercury in coal. In this paper, the content, distribution, and occurrence of mercury in the Permian tectonically deformed coals from Peigou Mine, Xinmi coalfield, Henan Province were investigated. A total of 18 bench samples were taken from No.2-1 coals seam in Peigou Mine, including 15 coal bench samples, two roofs and one floor. The mercury concentration, mineral composition, and main inorganic element content of 18 samples were determined by DMA-80 direct mercury analyzer, XRD, and XRF respectively. The results show that the mercury content ranges from 0.047 ppm to 0.643 ppm, with an average of 0.244 ppm. Though the coal seam has turned into typical tectonically deformed coal by the strong tectonic destruction and plastic deformation, the vertical distribution of mercury has remarkable heterogeneity in coal seam section. By the analysis of correlation between mercury and the main inorganic elements and the mineral composition in coal, we infer that majority of mercury mainly relates to pyrite or kaolinite.

Response of Macromolecular Structure to Deformation in Tectonically Deformed Coal

The structural evolution of tectonically deformed coals （TDC） with different deformational mechanisms and different deformational intensities are investigated in depth through X-ray diffraction （XRD） analysis on 31 samples of different metamorphic grades （R ： 0.7%-3.1%） collected from the Huaibei coalfield. The results indicated that there are different evolution characteristics between the ductile and brittle deformational coals with increasing of metamorphism and deformation. On the one hand, with the increase of metamorphism, the atomic plane spacing （d002） is decreasing at step velocity, the stacking of the BSU layer （Lc） is increasing at first and then decreasing, but the extension of the BSU layer （La） and the ratio of La/Lc are decreasing initially and then increasing. On the other hand, for the brittle deformational coal, d002 is increasing initially and then decreasing, which causes an inversion of the variation of Lc and La under the lower-middle or higher-middle metamorphism grade when the deformational intensity was increasing. In contrast, in the ductile deformational coals, d002 decreased initially and then increased, and the value of L~ decreased with the increase of deformational intensity. But the value of La increased under the lower-middle metamorphism grade and increased at first and then decreased under the higher-middle metamorphism grade. We conclude that the degradation and polycondensation of TDC macromolecular structure can be obviously impacted during the ductile deformational process, because the increase and accumulation of unit dislocation perhaps transforms the stress into strain energy. Meanwhile, the brittle deformation can transform the stress into frictional heat energy, and promote the metamorphism and degradation as well. It can be concluded that deformation is more important than metamorphism to the differential evolution of the ductile and brittle deformational coals.

Influence of combination forms of intact sub-layer and tectonically deformed sub-layer of coal on the gas drainage performance of boreholes: a numerical study

High concentration and large flow flux of gas drainage from underground coal seams is the precondition of reducing emission and large-scale use of gas.However,the layered occurrence of coal seams with tectonically deformed sub-layers and intact sub-layers makes it difficult to effectively drain gas through commonly designed boreholes.In this study,the gas drainage performance in coal seams with different combinations of tectonically deformed sub-layers and intact sub-layers was numerically analyzed.The analysis results show that the gas drainage curve changes from a single-stage line to a dual-stage curve as the permeability ratios of Zone II(kII)and Zone I(kI)increase,raising the difficulty in gas drainage.Furthermore,a dual-system pressure decay model based on the first-order kinetic model was developed to describe the dual-stage characteristics of pressure decay curves with different permeability ratios.In the end,the simulation results were verified with reference to in-situ drainage data from literature.The research results are helpful for mines,especially those with layered coal seams comprising tectonically deformed sub-layers and intact sub-layers,to choose appropriate gas drainage methods and develop the original drainage designs for achieving better gas drainage performance.

Geological modeling of coalbed methane reservoirs in the tectonically deformed coal seam group in the Dahebian block,western Guizhou,China

The widely spread Carboniferous-Permian coal seam group in southern China has great potential for coalbed methane resources,but the extensively developed tectonically deformed coal seriously restricts its development.Taking the Dahebian block in western Guizhou as the study area,the geological model of coalbed methane reservoirs in the tectonically deformed coal seam group was established,and the spatial distribution pattern of model parameters was clarified by clustering algorithms and factor analysis.The facies model suggests that the main coal body structures in Nos.1,4,and 7 coal seams are cataclastic coal and granulated coal,whereas the No.11 coal seam is dominated by granulated coal,which has larger thicknesses and spreads more continuously.The in situ permeability of primary undeformed coal,cataclastic coal,granulated coal,and mylonitized coal reservoirs are 0.333 mD,0.931 mD,0.146 mD,and 0.099 mD,respectively,according to the production performance analysis method.The property model constructed by facies-controlled modeling reveals that Nos.1,4,and 7 coal seams have a wider high-permeability area,but the gas content is lower;the high-permeability area in the No.11 coal seam is more limited,but the gas content is higher.The results of the self-organizing map neural network and K-means clustering indicate that the geological model can be divided into 6 clusters,the model parameter characteristics of the 6 clusters are summarized by data analysis in combination with 6 factors extracted by factor analysis,and the application of data analysis results in multi-layer coalbed methane co-development is presented.This study provides ideas for the geological modeling in the tectonically deformed coal seam group and its data analysis.

Structural Controls on Coalbed Methane Reservoirs in Faer Coal Mine, Southwest China

Guizhou （贵州） Province, Southwest China, is rich in coalbed methane （CBM） resources, wherein its geological structure is complicated. We discuss the occurrence characteristics of CBM based on CBM borehole test data and geological setting. In combination with the analysis of the regional tectonics, macro-and micro-scopic geological structures and pore size distributions, the structural controls on CBM reservoirs were further discussed from the aspects involving tectonic evolution, structural features, and deformation of coal. The results show that the CBM enrichment was mainly controlled by the regional tectonic subsidence and weak structural deformation on coal reservoirs after coal formation. The Yangmeishu （杨梅树） syncline and topography are the main controlling factors to the current distribution pattern of CBM, which is higher in the north than the south and trending toward the NE direction. Normal faults and fractures can be divided into open, closed, and occluded types. The open type reduces both gas content and methane concentration of nearby coal seams. The closed type causes the decrease of gas content, while methane concentration is still high. The occluded type fail to reduces gas content, and even results in the enrichment of CBM in small areas near fault. Moderate brittle deformation can improve the pore structure and development of structural fracture of coal reservoir.Cataclastic structural coals with well-developed fractures and relatively uniformly distributed pore structures are common in the Faer （发耳） coal mine, which are favorable for the production of CBM.