Study of the Mechanical Properties and Damage Characteristics of Coal after Interaction with Supercritical Carbon Dioxide

Abstract

The injection of CO 2 into deep unrecoverable coal seams can serve the dual purpose of facilitating the efficient extraction of coal-bed methane (CBM) and geological storage of CO 2 . However, due to the nonhomogeneous and anisotropic nature of coal seams, which are influenced by depositional environment, geological structure, and hydrogeology, the anisotropy of the deep unrecoverable coal seams can affect the CO 2 − water−coal reaction. The mechanical properties and damage of coal after the physical and chemical reactions of supercritical CO 2 (SCO 2 )−water−coal at different seam inclinations have not been extensively studied. In this study, we analyze the mechanical properties and damage of coal before and after the reaction by utilizing noncontact full-field strain, acoustic emission, ultrasonic nondestructive testing, and nuclear magnetic techniques. The results demonstrate that the mechanical properties of the coal underwent significant changes after SCO 2 treatment, with the largest decreases in compressive strength and elastic modulus observed in the 90°coal samples at 62.7 and 67.4%, respectively. Furthermore, the strain distribution of the treated coal samples exhibited distinct characteristics of laminar dip angle. Acoustic emission analysis revealed that the damage variable D 1 of the 45°coal sample had the largest value, while the acoustic emission damage variable D 1 of the 90°coal sample had the largest increase of 99.5%. The longitudinal wave velocity of a coal sample is highest at 0°, followed by 45°, and lowest at 90°. At 90°, the maximum value of the wave velocity damage factor is 0.577. Additionally, as the total porosity of the coal increases, the maximum increase observed after testing a coal sample at 90°is 1%. Moreover, the pore structure undergoes changes during this process. The number of micropores decreases, while the number of mesopores and macropores increases. The findings of this study provide theoretical guidance for CO 2 injection into deep unrecoverable coal-bed methane.

Damage Evolution and Splitting Failure Mechanism of Layered Rock under Dynamic Pulse Load

“…The instantaneous horizontal strain contours of D-type and S-type shale during Brazilian splitting under initial state and different impact velocities are shown in Figure . Here, blue-cyan-green represents the low strain field region, and yellow-orange-red represents the high strain region …”

“…Here, bluecyan-green represents the low strain field region, and yelloworange-red represents the high strain region. 57 During the initial state splitting process, the strain cloud map of the D-type shale surface was generally a high strain area, and the stress concentration area was approximately along the central axis of the disc, showing a clear banded distribution. With the increase of impact velocity, the strain cloud diagram gradually became bent, the shape of the strain band became wider and developed to both sides, and the secondary strain band appeared on the right side of the disc at 70 m/s.…”

Study on Damage Characteristics of Hot Dry Rock by Liquid Nitrogen Cyclic Cold Shocks Based on Ultrasonic Testing

“…When the core is damaged with increased internal pores and cracks, it leads to attenuation and drift of the received wave frequency . The frequency change of ultrasonic received waves can reflect the internal damage degree of the core . In this paper, FFT was used to transform the ultrasonic waveforms from the time domain to the frequency domain for analysis of the frequency components and characteristics, as shown in Figure .…”

Acidification-Induced Micronano Mechanical Properties and Microscopic Permeability Enhancement Mechanism of Coal

“…Regardless of whether it is hydraulic fracturing, liquid nitrogen fracturing, or mining-induced disturbance, all these methods aim to physically alter the structure of the coal matrix in order to promote methane desorption, diffusion, and seepage within the coal. In addition, the use of supercritical CO 2 can alter the microporous structure and the mechanical properties of coal, increase its permeability, and induce changes in the pore structure of coal through microwave radiation. − Meanwhile, chemical methods to modify the structure of coal have also been widely used, for example, using silica nanofluids and surfactants to enhance the wettability of coal, − using a green biosurfactant to change the microstructure and wettability of coal, using ionic liquids to modify the wettability and mechanical properties of coal pore structure, and using an appropriate concentration of an alkaline solution (NaOH) to change the permeability and gas adsorption of coal . Some scholars have studied the use of acids to modify the pore channels and enhance the connectivity between pores.…”