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The study of the microstructure evolution law of coal in a natural reservoir environment for the extraction of coalbed methane mining (ECBM), especially deep ECBM, is of major significance. We treated coal samples from Qinshui Basin under combined temperature–gas–fracturing fluid conditions and analyzed the evolution of the microstructure and its fractal characteristics to study the microstructural evolution of coal under natural hydraulic fracturing conditions. In the temperature range of 303.15 to 343.15 K and the gas-pressure range of 0.5–4.5 MPa, our data demonstrate that the pore structure is more susceptible to the temperature influence, compared with microfracture. The treatment of viscoelastic surfactant fracturing fluid (VES-FF) can effectively increase the permeation pore and fracture ratio by more than 300% and reduce the adsorption pore by more than 200% through dissolution, gas wedge, and other effects, which is favorable to ECBM. At the same temperature, as the gas pressure increases, the pore decreases, whereas the fracture ratio increases. The pore fractal dimension ranges from 2.90 to 2.99, which is significantly higher than that of microfractures. The temperature has a minor effect on the fracture fractal dimension, but it causes a decrease in the pore fractal dimension. The treatment of VES-FF induces an increase in the fracture fractal dimension, implying an increase in the fractal complexity. In contrast, the pore fractal characteristics show a contrary trend. At a gas pressure of 4.5 MPa, the negative effect of VES-FF on the pore structure reaches its maximum, whereas the effect on the fracture drops to its minimum. The results document that high temperature and high gas pressure can severely limit the penetration enhancement effect of VES-FF.

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Water plays an important role in carbon dioxide (CO2) enhanced coalbed methane exploitation and CO2 geological sequestration at deep geological conditions. We performed mercury intrusion porosimetry, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction analysis on coal samples with different moisture contents after supercritical carbon dioxide (ScCO2) treatment to study the effect of different moisture contents on deep coal seam treatment by ScCO2. Using these experimental techniques, the effects of ScCO2 treatment on the microstructure of coal samples with different moisture contents were obtained. The results showed that the combination of ScCO2 and water in coal samples can cause mineral dissolution, increase the damage degree of coal structural defects, reduce the number of aromatic structures and oxygen-containing functional groups, and then lead to the expansion of the pore and fracture volume, especially the micropore volume. Moreover, with increasing the moisture content, the micropore volume of the coal samples under ScCO2 interaction with the presence of water exhibited an increasing trend. The number of oxygen-containing functional groups in the coal samples decreased. The peak position difference (G – D1) decreased first and then plateaued, and when the moisture content was in the range of 5.85–7.19%, the damage degree reached the maximum. The effect of water and ScCO2 on the dissolution of clay minerals in the coal samples was greater than that on the carbonate minerals.

The chemical structure of coal has an important influence on coalbed methane mining. Through Fourier transform infrared (FTIR) and Raman analysis of coal samples after coupling treatment of temperature, gas, and viscoelastic surfactant fracturing fluid (VES), we studied the evolution law of coal chemical structure. It was determined that the functional groups of coal decreased by approximately 30% with increasing temperature, showing an obvious temperature sensitivity. The VES fracturing fluids can destroy the functional groups of coal, and high temperature and high gas pressure conditions can seriously weaken the degree of destruction of the functional groups of coal by VES fracturing fluids but significantly enhance the destruction of the functional groups of coal by deionized water. Temperature, deionized water, and the use of VES fracturing fluid under a low temperature barely affect the macromolecular structure of coal, but when the temperature increases above 323.15 K, the coupling effect of temperature and VES fracturing fluid causes the position of the Raman peaks D1 and G for coal to move toward higher frequencies, the peak position difference to move toward lower frequencies, and the range of motion to be 10–18 cm–1. That is, the coupling effect of high temperature and VES fracturing fluid can make the macromolecular structure of coal become more ordered.