Ecofriendly viscoelastic surfactant fracturing fluids (VESFFs) can effectively promote the connection of fractures. These can modify the fracture surface and change the flow capacity at the pressure-holding stage, which affects coalbed methane exploitation. In this study, deionized water (DW) and three common VESFFs which, namely, are the cationic surfactant fracturing fluid (CSFF), cationic–zwitterionic surfactant fracturing fluid (CZSFF), and anionic–zwitterionic surfactant fracturing fluid (AZSFF), were used to modify coal fractures for the first time. To analyze the flow characteristics of coal fractures, a triaxial servo seepage test device was used to conduct the test. The variations in fracture surface morphology were analyzed by a three-dimensional optical topography scanner, an environmental scanning electron microscope, and an X-ray diffractometer. The degree of flow capacity improvement following modification by the VESFF decreased in the following order: CSFF > AZSFF > CZSFF > DW. At a high confining stress (6.1 MPa), DW treatment of the coal fracture decreased the flow rate by 26% and hydraulic aperture by 11.16%. The CSFF modification of the coal fracture increased the flow rate by 18% and the hydraulic aperture by 6.43%. A new friction factor model with the Reynolds number and relative roughness as the variables was proposed, which describes the flow characteristics of the gas from laminar to turbulent flow. VESFFs affected the flow capacity of coal fractures in five aspects: mineral dissolution, mineral precipitation, electrostatic removal of coal particles, reduced degree of fracture engagement, and increased effective fracture connectivity.
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.
CO2 geological storage (CGS) is considered to be an important technology for achieving carbon peak and carbon neutralization goals. Injecting CO2 into deep unminable coal seams can achieve both CGS and enhance coalbed methane (ECBM) production. Therefore, the deep unminable coal seams are considered as promising geological reservoirs. CO2 exists in a supercritical CO2 (ScCO2) when it was injected into deep unminable coal seams. The injection of ScCO2 can induce changes in the seepage characteristics and microstructure of deep water-bearing coal seams. In this study, typical bituminous coal from Shenmu, Shanxi Province was used to investigate the effects of ScCO2 on seepage characteristics, pore characteristics, and mineral composition through experiments such as seepage tests, low-temperature liquid nitrogen adsorption, and X-ray diffraction (XRD). The results indicate that ScCO2 treatment of dry and saturated coal samples caused a significant increase in clay mineral content due to the dissolution of carbonates, leading to the conversion of adsorption pores to seepage pores and an improvement in seepage pore connectivity. Therefore, the Brunauer-Emmett-Teller (BET) specific surface area and pore volume of the two coal samples both decreased after ScCO2 treatment. Moreover, the permeability of dry and saturated coal samples increased by 191.53% and 231.71% at 10 MPa effective stress respectively. In semi-saturated coal samples, a large amount of dolomite dissolved, leading to the precipitation of Ca2+ and CO32- to form calcite. This caused pore throats to clog and macropores to divide. The results show that the pore volume and average pore size of coal samples decrease, while the specific surface area increases after ScCO2 treatment, providing more space for gas adsorption. However, the pore changes also reduced the permeability of the coal samples by 32.21% and 7.72% at effective stresses of 3 MPa and 10 MPa, respectively. The results enhance our understanding of carbon sequestration through ScCO2 injection into water-bearing bituminous coal seams.
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