Leilei Si

Research on the Composition and Distribution of Organic Sulfur in Coal.

The structure and distribution of organic sulfur in coals of different rank and different sulfur content were studied by combining mild organic solvent extraction with XPS technology. The XPS results have shown that the distribution of organic sulfur in coal is related to the degree of metamorphism of coal. Namely, thiophenic sulfur content is reduced with decreasing metamorphic degree; sulfonic acid content rises with decreasing metamorphic degree; the contents of sulfate sulfur, sulfoxide and sulfone are rarely related with metamorphic degree. The solvent extraction and GC/MS test results have also shown that the composition and structure of free and soluble organic sulfur small molecules in coal is closely related to the metamorphic degree of coal. The free organic sulfur small molecules in coal of low metamorphic degree are mainly composed of aliphatic sulfides, while those in coal of medium and high metamorphic degree are mainly composed of thiophenes. Besides, the degree of aromatization of organic sulfur small molecules rises with increasing degree of coalification.

Evolution of Coal Permeability with Cleat Deformation and Variable Klinkenberg Effect

The characteristics of the gas flow in reservoir have a great impact on exploiting coalbed methane (CBM), so many researchers have carried out the experiments to test the coal sample permeability in the laboratory. The Klinkenberg effect is an important factor in the apparent permeability which is obtained in the laboratory, and it also be recognized as a constant value for a specific gas. From the principle of the Klinkenberg effect, the Klinkenberg coefficient is closely related to the width of the gas flowing path. The coal cleat width changes because of the compressibility and sorption-induced strain features. Therefore, the Klinkenberg coefficient can not be treated as a constant. By using the cubic conceptual model of coal, the deformation behaviors of the coal matrix and fracture are analyzed in this paper, and the influential factors of the Klinkenberg coefficient are obtained. The theoretical equation of methane’s Klinkenberg coefficient was also established. The evolution equation of the cleat width is derived by coupling the effective stress and gas sorption, and the Klinkenberg coefficient model is also rewritten. Using the parameters of the coal sample, some results are obtained. The Klinkenberg coefficient increases with the increase in the pore pressure because of the sorption-induced strain at the constant effective stress; The Klinkenberg coefficient varies with the increase in the pore pressure because of the competition between the stress–strain and sorption-induced strain at the constant mean stress; The Klinkenberg coefficient increases with the increase in the mean stress at a constant pore pressure. The results improve the understanding of the Klinkenberg effect for the gas flow in a coalbed and provide theoretical guidance for CBM exploitation.

Improved Porosity and Permeability Models with Coal Matrix Block Deformation Effect

Coal permeability is an important parameter in coalbed methane (CBM) exploration and greenhouse gas storage. A reasonable theoretical permeability model is helpful for analysing the influential factors of gas flowing in a coalbed. As an unconventional reservoir, the unique feature of a coal structure deformation determines the state of gas seepage. The matrix block and fracture change at the same time due to changes in the effective stress and adsorption; the porosity and permeability also change. Thus, the matrix block deformation must be ignored in the theoretical model. Based on the cubic model, we analysed the characteristics of matrix block deformation and fracture deformation. The new models were developed with the change in matrix block width a. We compared the new models with other models, such as the Palmer–Manson (P–M) model and the Shi–Durucan (S–D) model, and used a constant confining stress. By matching the experimental data, our model matches quite well and accurately predicts the evolution of permeability. The sorption-induced strain coefficient f differs between the strongly adsorbing gases and weakly adsorbing gases because the matrix block deformation is more sensitive for the weakly adsorbing gases and the coefficient f is larger. The cubic relationship between porosity and permeability overlooks the importance of the matrix block deformation. In our model, the matrix block deformation suppresses the permeability ratio growth. With a constant confining stress, the weight of the matrix block deformation for the strongly adsorbing gases is larger than that for weakly adsorbing gases. The weight values increase as the pore pressure increases. It can be concluded that the matrix block deformation is an important phenomenon for researching coal permeability and can be crucial for the prediction of CBM production due to the change in permeability.

Influence of the Pore Geometry Structure on the Evolution of Gas Permeability

In order to investigate the effect of pore geometry structure on the gas permeability, 3 permeability models with different pore shapes were constructed, considering the sorption-induced deformation, adsorption molecular layer and variable Klinkenberg’s effect. The effect of pore geometry structure on the effective pore radius, Klinkenberg’s factor and permeability was analyzed under 3 different conditions, including constant effective stress conditions, constant pore pressure conditions and constant mean stress conditions. Results showed that, under constant effective stress conditions, the spherical pores show a greater effect on the effective radius, followed by the cylindrical pores and the slit pores. Under the constant pore pressure conditions and the constant mean stress conditions, the effective pore radius is more sensitive to the slit pores, followed by the cylindrical pores and the spherical pores. Under 3 different conditions, Klinkenberg’s factor is more sensitive to the slit pores, followed by the cylindrical pores and the spherical pores. Moreover, the permeability evolution with different pore geometry structures shows similar characteristics with effective pore radius, indicating that the effective radius dominates the permeability difference in different pore geometry structures. Furthermore, a numerical model was proposed to investigate the permeability evolution in the reservoir conditions. In the initial extraction stage, the permeability is increased with the rising slit pores. However, in the later stage, the spherical pores show more notable improvement effect on the permeability. Then, the effective pore radius and Klinkenberg’s factor were analyzed to reveal the influence mechanism of different pore geometry structures, indicating that the effective pore radius dominates the permeability difference, while Klinkenberg’s effect plays more significant role for the permeability trends and shows notable improvement effect on the permeability.

SOM’s Effect on Coal Spontaneous Combustion and Its Inhibition Efficiency

ABSTRACT As an important part of coal, the existence of soluble organic matters (SOMs) has its own impact on the number of active groups within coal and its pore and fracture structure, which have influence on both spontaneous combustion and inhibitory effect of coal. But some in-depth researches are still lacking on this matter. On the basis of testing the composition of SOMs and analyzing the change of coal’s pore and fracture structure, we conducted an oxidation experiment on model compounds stimulating SOMs, and coal’s spontaneous combustion inhibition experiment before and after extraction. By testing characterization parameters like oxygen consumption rate, activation energy, inhibition rate, and the crossing point temperature, we analyzed how SOMs make a difference on spontaneous combustion and inhibitory effect of coal. It shows that, after the extraction of SOMs, both the amount and rate of oxygen consumption of coal goes down, while the activation energy of coal ascends after extraction. Once being added with model compounds stimulating SOMs, coal’s oxidation degree increases. Between 50°C and 140°C, the inhibition rates of ZHCM, ZHYM, and CTM range from 50.7% to 71.2%, 40.1% to 52.0%, and 18.1% to 27.6%, respectively, and residual coals’ inhibition rates surpass raw coals’ inhibition rates. The crossing point temperature of ZHCM reached 122.5°C, 35.2°C higher compared to that of YM. This shows that adding inhibitors after extract can have a strong effect on restraining coal’s spontaneous combustion. Through free radical reaction and oxidation reaction, SOMs would release the heat to hasten the coal’s oxidation process. After extraction, coal’s permeability increases, which makes it easier for an inhibitor to seep into coal, thereby restraining the intensity of coal’s oxidation.

Identification of Primary CO in Coal Seam Based on Oxygen Isotope Method

ABSTRACT Analysis on the source of CO is very important for prevention of coal spontaneous combustion. In this article, a method of identifying primary CO in coal seams based on an oxygen isotope method is proposed through research. Tests on the oxygen isotopes (δ18O) of CO were generated in coal oxidization reaction and coal pyrolysis reaction. It is found that δ18O of CO will reduce with rising temperature in both reactions, with δ18O of CO being 15–20‰ in air atmosphere and that being 25–28‰ in argon atmosphere in the temperature range of 130–220°C, confirming that the sources of oxygen atoms of CO vary with different atmospheric conditions. Through collecting and testing a number of gas samples from the coal seam and roof in the coal mine, their δ18O of CO is found to be 35.2–37.3‰. The result would improve the prevention of coal spontaneous combustion.