Background Injecting hot flue gas produced and discharged by gas-fired power plants into deep coal seams where gas is difficult to extract enjoys dual benefits: gas production growth and geologic CO₂ sequestration. However, there is an urgent need to determine the impact of the heat-carrying property of hot flue gas on the stability of coal reservoirs. To address this issue, the key is to clarify the fracturing evolution mechanisms of eroded coals under the temperature effect of the CO₂-H₂O system.

Methods Using the independently built platform of CO₂-H₂O-coal interactions that consider the temperature-pressure coupling effect, this study preprocessed coal specimens through uniaxial loading, triaxial loading, and Brazilian splitting under different temperatures. Based on the results of mechanical strength tests of the coal specimens, this study investigated the nonlinear deterioration pattern of coal strength under different temperatures of the CO₂-H₂O system. By comparing the acoustic emission (AE) and non-contact, full-field strain digital image correlation (DIC) monitoring results during the loading processes, this study analyzed the impacts of the temperature of the CO₂-H₂O system on the fracturing evolution of eroded coals. In combination with scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS), this study explored the competitive degradation effect arising from thermal fracturing and mineral dissolution under different erosion temperatures.

Results and Conclusions  The results indicate that with an increase in the erosion temperature, the progressive fracturing stage of coal specimens subjected to uniaxial loading shifted from pre-peak stress to post-peak stress. Meanwhile, the competitive degradation effect induced by thermal fracturing and mineral dissolution led to a nonlinear decrease in the peak strength of the coal specimen. For the coal specimens subjected to splitting, mineral dissolution led to significantly decreased tensile strength as the erosion temperature increased, while thermal fracturing did not significantly aggravate their tensile strength degradation. In the case of low-temperature erosion of the coal specimens, mineral dissolution resulted in microscopic defects, which were interconnected to form numerous new fractures during specimen loading. As a result, both the frequency of abrupt changes in the AE ringing count and cumulative AE ringing counts increased. In the case of high-temperature erosion, thermal fracturing resulted in numerous connected fractures in the coal specimens, leading to a significant decrease in both the frequency of abrupt changes in AE ringing counts and the cumulative AE ringing counts. Accordingly, the specimen fracturing gradually shifted from rapid and sudden tensile fracturing to progressive and slow shear fracturing. As the erosion temperature rose, the macroscopic fracturing morphology of the coal specimens subjected to uniaxial loading underwent flaky separation, the separation of shear blocks, and mixed fracturing, sequentially. This led to a more complex distribution of macroscopic fractures and more complete fracturing. Meanwhile, numerous microscopic defects were formed within the eroded specimen, promoting shear fracturing. However, tensile fracturing remained predominant in the case of peak stress. Concurrently, the zones lagging behind featured strain concentration and complex strain fields. For the coal specimens subjected to splitting, the macroscopic fracturing morphology gradually evolved from central fracturing to more tortuous non-central rupture. Furthermore, the coal specimens showed complex tensile strain distributions, and the tensile strain concentration areas enlarged and deflected toward one side of the specimens. The mineral dissolution effect can produce only dissolution pits and holes inside the coal specimens, while large-scale fractures can be formed in a relatively long erosion time. Under high-temperature erosion, the mineral dissolution effect greatly decreased. In contrast, the thermal fracturing effect resulted in numerous new fractures on a larger scale, leading to significantly decreased mechanical strength of coals. The results of this study can provide a theoretical reference for the hot flue gas-enhanced gas production growth and the stability evaluation of reservoirs during geologic CO₂ sequestration.