Xiaoshi Li

Structure and coalbed methane occurrence in tectonically deformed coals

Research on structure of tectonically deformed coals (TDC) is a key issue in coal and gas outburst prevention and coalbed methane (CBM) exploitation. This paper presents a summary on the research progress in TDC’s structural-genetic classification, tectonic strain influence on coal microstructure, coal porosity system, coal chemical structure and constituents, and their relationship with the excess coalbed methane. Previous studies suggested that tectonic deformation had significant influence on coal microstructure, coal super microstructure, and even chemical macromolecular structure. The main mechanisms of coal deformation are the tectonic stress degradation and polycondensation metamorphism (dynamical metamorphism). Besides, under different deformation mechanisms, the ultra- and micro-structure and chemical constituents of TDC presented distinct characteristics. Based on these achievements, we propose one possible evolutionary trend of TDC with different deformation mechanisms, and suggest that the coal and gas outburst in the TDC, especially in the mylonitic coals, may be not only controlled by geological structure, but also influenced by the tectonic stress degradation of ductile deformation. Therefore, further study on TDC should be focused on the controlling mechanism of deformation on structure and composition of coal, generation conditions and occurrence state of excess coalbed methane from deformation mechanism of coal.

Spectra response from macromolecular structure evolution of tectonically deformed coal of different deformation mechanisms

Coals with different deformation mechanisms (brittle deformation, brittle-ductile deformation, and ductile deformation) represent different ways in macromolecular structure evolution based on the metamorphism. The evolution of coal structure could affect the occurrence condition of coalbed methane (CBM) because the nanopore structure affected by macromolecular structure is the most important reservoir for CBM. This paper analyzes the evolutions and mechanisms of structure and functional group of tectonically deformed coals (TDCs) collected from Huainan-Huaibei coalfield using X-ray diffraction (XRD), Raman spectroscopy, and Fourier Transform Infrared (FTIR) spectroscopy methods. The results show that the macromolecular structure evolutions of TDC are different from the primary structure coal as a result of the different metamorphic grade and deformation mechanisms. The different deformation mechanisms variously affect the process of functional group and polycondensation of macromolecular structure. Furthermore, the tectonic deformation leads to secondary structural defects and reduces the structure stability of TDC. The coupled evolution on stacking and extension caused by the changes of secondary structural defects results from different deformation mechanisms. We consider that the changes of chemical structure and secondary structural defects are the primary reasons for the various structure evolutions of TDC compared with primary structure coal.

The effect of different deformation mechanisms on the chemical structure of anthracite coals

To study the effect of different deformation mechanisms on the chemical structure of anthracite coals and further understand the correlation between changed chemical structures and coal and gas outburst, ten groups of sub-high-temperature and sub-high-pressure deformation experiments were performed. All samples maintained primary structure, which were collected from the Qudi Mine in the southern Qinshui Basin of China. The samples were analyzed by ultimate analysis, Vitrinite Reflection (VR), Fourier Transform Infrared spectroscopy (FTIR), and Raman spectroscopy both before and after deformation experiments for contrasting. The results showed that the VR values of all samples after experiments were significantly higher than before experiments, which suggested that the metamorphism degree of anthracite coals was increased by deformation. The results also indicated that both temperature and strain rate had significant effects on the chemical structure of anthracite coals. At a high strain rate of 4×10−5 s−1, the deformation of the samples was mainly brittle in which the mechanical energy was transformed mainly into frictional energy. In this situation, all samples developed several distinct fractured surfaces and the change of chemical structures was not obvious. On the contrary, with the decrease of the strain rates, the ductile deformation was dominated and the mechanical energy was mainly transformed into strain energy, resulting in the accumulation of deformation energy confessed by increasing quantity of dislocation and creep in the coal’s interior nucleus. The absorption in the aromatic ring groups increased; otherwise the absorption in the aliphatic structures and ether oxygen groups decreased rapidly. During these experiments, CO was collected from two experimental samples. The number of aromatic rings and the structure defects within the two generated gas samples increased and the degree of molecular structure orders decreased.

Characterization of Coal Porosity for Naturally Tectonically Stressed Coals in Huaibei Coal Field, China

The enrichment of coalbed methane (CBM) and the outburst of gas in a coal mine are closely related to the nanopore structure of coal. The evolutionary characteristics of 12 coal nanopore structures under different natural deformational mechanisms (brittle and ductile deformation) are studied using a scanning electron microscope (SEM) and low-temperature nitrogen adsorption. The results indicate that there are mainly submicropores (2~5 nm) and supermicropores (<2 nm) in ductile deformed coal and mesopores (10~100 nm) and micropores (5~10 nm) in brittle deformed coal. The cumulative pore volume (V) and surface area (S) in brittle deformed coal are smaller than those in ductile deformed coal which indicates more adsorption space for gas. The coal with the smaller pores exhibits a large surface area, and coal with the larger pores exhibits a large volume for a given pore volume. We also found that the relationship between S and V turns from a positive correlation to a negative correlation when S > 4 m2/g, with pore sizes <5 nm in ductile deformed coal. The nanopore structure (<100 nm) and its distribution could be affected by macromolecular structure in two ways. Interconversion will occur among the different size nanopores especially in ductile deformed coal.

FTIR and Raman Spectral Research on Metamorphism and Deformation of Coal

Under different metamorphic environments, coal will form different types of tectonically deformed coal (TDC) by tectonic stress and even the macromolecular structure can be changed. The structure and composition evolution of TDC have been investigated in details using Fourier transform infrared spectroscopy and Raman spectroscopy. The ductile deformation can generate strain energy via increase of dislocation in molecular structure of TDC, and it can exert an obvious influence on degradation and polycondensation. The brittle deformation can generate frictional heat energy and promote the metamorphism and degradation, but less effect on polycondensation. Furthermore, degradation affects the structural evolution of coal in lower metamorphic stage primarily, whereas polycondensation is the most important controlling factor in higher metamorphic stage. Tectonic deformation can produce secondary structural defects in macromolecular structure of TDC. Under the control of metamorphism and deformation, the small molecules which break and fall off from the macromolecular structure of TDC are replenished and embedded into the secondary structural defects preferentially and form aromatic rings by polycondensation. These processes improved the stability of macromolecular structure greatly. It is easier for ductile deformation to induce secondary structural defects than in brittle deformation.

Structural Characteristics and Physical Properties of Tectonically Deformed Coals

Different mechanisms of deformation could make different influence on inner structure and physical properties of tectonically deformed coal (TDC) reservoirs. This paper discusses the relationship between macromolecular structure and physical properties of the Huaibei-Huainan coal mine areas in southern North China. The macromolecular structure and pore characteristics are systematically investigated by using techniques such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and low-temperature nitrogen adsorption method. The results suggest that under the directional stress, basic structural units (BSU) arrangement is closer, and the orientation becomes stronger from brittle deformed coal to ductile deformed coal. Structural deformation directly influences the macromolecular structure of coal, which results in changes of pore structure. The nanoscale pores of the cataclastic coal structure caused by the brittle deformation are mainly mesopores, and the proportion of mesopores volume in ductile deformed coal diminishes rapidly. So the exploration and development potential of coalbed gas are good in reservoirs such as schistose structure coal, mortar structure coal and cataclastic structure coal. It also holds promise for a certain degree of brittle deformation and wrinkle structure coal of low ductile deformation or later superimposed by brittle deformation.

A New Parameter as an Indicator of the Degree of Deformation of Coals

AbstractThe deformation of coal is effected by thermal effect, pressures and tectonic stress, and the tectonic stress is the principal influence factor. However, the proposition of a useful quantitative index that responds to the degree of deformation of coals quantitatively or semi-quantitatively has been a long-debated issue. The vitrinite reflectance ellipsoid, that is, the reflectance indication surface (RIS) ellipsoid is considered to be a strain ellipsoid that reflects the sum of the strain increment caused by stress in the process of coalification. It has been used to describe the degree of deformation of the coal, but the effect of the anisotropy on the RIS ellipsoid has not yet been considered with regards to non-structural factors. In this paper, Wei’s parameter (ε) is proposed to express the deformation degree of the strain ellipsoid based on considering the combined influence of thermal effect, pressure and tectonic stress. The equation is as follows: ε=√[(ε1-ε0)2+(ε2-ε0)2+(ε3-ε0)2]/3, where ε1=ln Rmax, ε2=ln Rint, ε3=ln Rmin, and ε0=(ε1+ε2+ε3)/3. Wei’s parameter represents the distance from the surface to the spindle of the RIS logarithm ellipsoid; thus, the degree of deformation of the strain ellipsoid is indicated quantitatively. The formula itself, meanwhile, represents the absolute value of the degree of relative deformation and is consequently suitable for any type of deformation of the strain ellipsoid. Wei’s parameter makes it possible to compare degrees of deformation among different deformation types of the strain ellipsoid. This equation has been tested in four types of coal: highly metamorphic but weakly deformed coal of the southern Qinshui Basin, highly metamorphic and strongly deformed coal from the Tianhushan coal mining area of Fujian, and medium metamorphic and weakly or strongly deformed coal from the Huaibei Coalfield. The results of Wei’s parameters are consistent with the actual deformation degrees of the coal reservoirs determined by other methods, which supports the effectiveness of this method. In addition, Wei’s parameter is an important complement to the indicators of the degrees of deformation of coals, which possess certain theoretical significance and practical values.

Characterization of Coal Reservoirs in Two Major Coal Fields in Northern China: Implications for Coalbed Methane Development

Based on the macroscopic and microscopic observation of coal structure, the vitrinite reflectance analysis, and the mercury injection testing of coal samples collected from Huaibei coalfield and Qinshui basin, the characterization of coal reservoir and its restriction on the development of coalbed methane are studied. The results indicate that coal reservoir in study area can be classified as five types according to the coal metamorphism and deformation degrees, and they are respectively high grade metamorphic and medium deformational to strongly deformation coal (I), high grade metamorphic and comparatively weakly deformational coal (II), medium grade metamorphic and comparatively strongly deformational coal (III), medium grade metamorphic and comparatively weakly deformational coal (IV), and low grade metamorphic and strongly deformational coal (V). Furthermore, the type II and type IV coal reservoirs are favorable for the development of the coalbed methane because of the well absorptive capability and good permeability. Thus, southern part of Qinshui basin and south-central of Huaibei coal field are potential areas for coalbed methane exploration and development.