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CO2-enhanced coalbed methane recovery (CO2-ECBM) has been proposed to realize energy security and low-carbon emission reduction. CO2injection into coals is a dynamic process involving the evolutions of gas kinetics, formation temperature, and mechanics. To deeply investigate the sensitivities of CO2injection parameters (pressure Pinjand temperature Tinj) and initial formation parameters (water saturation Sw0and permeability k0) on carbon exchange recovery efficiency, a thermo-hydro-mechanical coupling model was established, by considering the mutual effects of gas adsorption and migration, temperature, and effective stress on the pore-permeability evolution. Compared to the case of primary recovery, CO2-ECBM could well improve CH4production because of its superiorities of competitive adsorption, driving force, and expansion stress. Different parameters had various sensitivities on the carbon exchange behaviors; for instance, the injected CO2with a larger Pinjmigrated with a greater gas coverage area, which deteriorated the formation permeability, and the decrease of CH4content and the increase of CO2content were positively correlated with Pinj, indicating that the displacement rate of the binary gas was mainly determined by diffusion pressure. The parameter of Tinjwas not sensitive to the CH4production because of the minor impacts of thermal conduction of coals on the displacement efficiency; thus, it is important to optimize Tinjselection to balance the costs and benefits. The efficiencies of CH4recovery and CO2storage were negatively affected by larger Sw0, which indicated that the residual water decreased the relative permeability of the gas phase and hindered the migration of the binary gas, and adding surfactants was proposed to decrease the residual water saturation. The coal formation with a larger k0had more complex gas flow paths and increased the probability of carbon exchange. Finally, the economic evaluation of CO2-ECBM and the investment decision rules were also preliminarily studied. The results could provide some theoretical guidance for the engineering practice.

Water injection has been widely used to prevent gas disasters and coal dust in coal mines. Coal would be wetted and softened by water through spontaneous imbibition (SI), which is mainly influenced by the pore structure and wettability. Studying the effects of pore structure and wettability on SI is of great significance for optimizing coal seam water injection parameters. This work aims to establish a quantitative relationship between these parameters and extract critical factors controlling the SI. Mercury intrusion porosimetry (MIP), low-temperature N2adsorption (LT-N2A), Fourier transform infrared spectroscopy (FTIR), and contact-angle measurements were used to assess the pore structure and wettability of the coal samples. Low-field nuclear magnetic resonance (LF-NMR) was used to monitor water in coal. All parameters were quantified with Pearson correlation analysis, and principal component analysis (PCA) was subsequently employed to pinpoint the key factors. The results show that the SI velocity and SI ability of bituminous coal are higher than those of anthracite; bituminous coal can rapidly elute more water. The time index (n) has a strong relationship with the SI velocity, and the higher the nvalue is, the faster the water migrates in pore space. Porosity is positively proportional to SI ability, and a larger SI ability means that the sample can store more water. The wettability parameter has a higher impact on SI velocity than on SI ability. Among the wettability parameters, the contact angle is a key factor in evaluating the SI process in coal. The bituminous coal with good pore connectivity, hydrophilicity, and high porosity could have a rapid SI velocity and store more water. Oxygen-containing functional groups promote SI by enhancing water and pore surface interactions. This work can better understand the mechanism of SI and provide the theoretical basis for field application.

High-pressure air blasting is a burgeoning technique for anhydrous fracturing. A crucial prerequisite for the application of air blasting is to comprehend the distinctions between air blasting and quasi-static fracturing, as well as the change in fracture control factors with variations in blasting pressure. This research undertook experiments on coal-like material specimens with quasi-static fracturing and blasting fracturing with blast pressures ranging from 5 to 10 MPa, by utilizing a self-designed air fracturing experimental system. The fracturing mechanism and characteristics of specimens subjected to different loading conditions were analyzed using a pressure sensor, high-speed camera, and fractal theory. The following primary findings were obtained: The average fracture pressure under quasi-static loading was 3.269 MPa, exhibiting similar absolute deviations. The pressure–time curve of blast fracturing was found to undergo four distinct stages: pressure surge, pressure fallback, pressure maintenance, and pressure plummet. The duration of the pressure maintenance stage exhibited a negative correlation with the blast pressure, ceasing to exist at pressures above 9 MPa. This suggests that the dynamic impact effect gradually supplants the dominant role of the air wedge effect. Quasi-static fracturing formed single fractures, whereas blast fracturing tended to generate complex fracture networks. The increase in blast pressure could significantly enhance the complexity of the fracture network in a logarithmic fashion. The fractal dimension of quasi-static fracturing was slightly higher than that of 6 MPa blast fracturing, indicating the prevalence of quasi-static action under low blast pressure. The smash district increased exponentially as the blast pressure increased, and the average particle size decreased gradually. These investigations provide valuable insights for designing high-pressure air blasting fracturing procedures.

Anhydrous calcium chloride (CaCl2) can improve the problems of long fracture time and low efficiency in fracturing hard rocks with soundless cracking agents (SCAs), but the effect of the proportion of CaCl2on the expansion properties of SCAs and its optimal ratio are less studied. In this paper, a comparative analysis of SCAs with different contents (0, 0.4, 0.8, 1.2, and 1.6 wt %) of CaCl2was performed. The macroscopic morphology, expansive pressure, hydration temperature, and potential were monitored using a static strain system and a temperature and voltage acquisition system, respectively, to investigate the effect of CaCl2on the performance of the SCA. The results showed that CaCl2could improve the hydration efficiency of the SCA. Compared with group “a” (0%), the volume expansion ratio of SCA in group “e” (1.6%) decreased by 17.9%, the initiation time of expansive pressure development and the time required for reaching the peak pressure were reduced by 96.25 and 96.48%, respectively, the peak expansive pressure was increased by 109.02%, and the time for reaching the peak temperature was reduced by 92%. The catalytic and adsorption effects of CaCl2promoted the hydration of calcium silicate hydrate (C–S–H), which shortened the “dormant” period of SCA, increased the peak expansive pressure, improved the lag of peak temperature development, and significantly improved the hydration efficiency. The maximum potential gradually decreased, the minimum potential first decreased, increased, and then decreased, and the peak expansive pressure decreased with the increasing of the maximum hydration potential. The mechanism of the effect of CaCl2on the hydration of SCA was explained according to the development of potential and catalytic principles. Considering safety and economy, the addition of 1.2% CaCl2is preferred to enhance the hydration performance of SCA. This study is important for SCA applications in industry.

The understanding of brittleness evolution for coals influenced by cyclic liquid CO2effects has great significance to evaluate the crack capacity of the matrix. A brittleness index (B) calculation model was established on the basis of the statistical damage constitutive equation and the stress–strain curves, without considering their primary curve patterns, that was mainly controlled by elastic modulus (E), compressive strength (σc), and the corresponding strain (εc). Three different rank coals were processed under the temperature effect and the temperature and adsorption coupling effect that originated from cyclic liquid CO2and then were destroyed by the uniaxial compression test. With the increasing liquid CO2parameters [cyclic number (CN) and total cyclic time (TCT)], the σcof the different rank coals decreased gradually, while the εcdid not have an apparent change tendency that was greatly influenced by the coal anisotropy (the nonuniform deformation responses to the temperature effect and the primary crack complexity) and the capacity of CO2adsorption decreasing the free surface energy. The Bvalues of lignite and bitumite showed a declining trend, while those of anthracite did not have obvious laws, because of the lack of laws for εc. The relationships between Band CN/TCT indicated that the larger the cyclic parameters, the smaller the Bvalues and the greater the coal destruction degree. The Eand σchad positive relationships with the Bcalculations, while the Poisson’s ratio (μ), damage variable (Dv), and accumulated energy (J) did not have obvious correlations with the Bvalues. The introduction of liquid CO2induced a huge number of unrecoverable deformations and fatigue damages to be generated, leading to a large plastic region and decreasing the coal brittleness. The results might be helpful to evaluate the capacity of liquid CO2fracturing and to estimate the efficiency of coalbed methane recovery.

In order to study the damage evolution and splitting failure characteristics of shale with different bedding orientations under dynamic pulse load, different impact velocities (40, 50, 60, 70 m/s) are selected to carry out dynamic pulse load on D-type and S-type shale samples. Ultrasonic testing, Brazilian splitting test [combined with digital image correlation technology and acoustic emission (AE) monitoring], and nuclear magnetic resonance are carried out on shale samples before and after impact. The results show that the P-wave velocity damage coefficient and splitting mechanic parameters of D-type shale show exponential attenuation, while those of S-type shale show linear attenuation. With the increase of impact velocity, the time-domain spectra of D-type and S-type shale show different degrees of waveform distortion. The frequency-domain spectra present a change characteristic of “high-frequency decrease, low-frequency increase”, the total energy of energy spectra decreases, and the main frequency band shifts from medium frequency sub-band (fourth to sixth) to low frequency sub-band (especially first). The AE characteristics of D-type shale are peak distribution with obvious brittle fracture characteristics, while S-type shale is uniformly distributed with progressive failure characteristics. The horizontal strain cloud map shows a band-like stress concentration distribution. Before and after impact, the total porosity of D-type shale decreases, while that of S-type shale increases. The fractal dimension of mesopores generally increases, while the fractal dimension of macropores remained basically unchanged. The existence of the bedding surface is the main reason for the difference in fracture mechanism of different types of shale, and the weakening of the bedding surface caused by impact load is an important factor that complicates the complexity of this difference.

Pulsating hydraulic fracturing has been applied in industry to increase gas productivity or clear fluid channel blockage, such as coalbed methane recovery. It produces cyclic hydraulic pressure to generate fatigue damages in coal causing deterioration of the mechanical strength. However, difficulties still exist in the quantification of the coal deterioration in the lab. This work provides a way of measuring the internal events induced by the cyclic hydraulic pressure based on the acoustic emission (AE) test and three-dimensional (3D) X-ray microscopy (XRM). It gives a method to quantify the damage in coal with the multifractal dimension and digital image analysis. The results show that the cyclic hydraulic pressure has a great impact on the fracturing behaviors of coal. As the reduction of initial crack pressure is proportional to the pretreated pulse pressure, the AE sensors receive more cracking signals during the whole procedure. For conventional hydraulic fracturing, the AE counts concentrate at the initial crack point with a big pressure drop. However, due to the influence of cyclic hydraulic load, changes exist in the AE count distribution. It represents that the fracture strength has been changed by the cyclic hydraulic loading. The average frequency-rise angle (AF-RA) eigenvalue provides the proportions of tensile and shear failures. It shows that the proportions of tensile and shear failures change from 56.38 and 43.62 to 41.79 and 58.21%, respectively. To observe the internal cracks of coal, the 3D XRM has been applied to directly illustrate the spatial distribution of created cracks. More cracks have been seen in fractured coal samples after higher pulse pressure loading. The multifractal analysis indicates that the AE spectra and computed tomography (CT) images can be characterized by the multifractal dimension. Dqmonotonically decreases with the increased qvalue. The relationship of f(α)-α is presented by the inverted U shape. Δfand Δα provide a quantitative analysis of pore structure heterogeneity. It shows that the heterogeneity of pore structure decreases with the increase of pretreated pulse pressures. The change of pore structure heterogeneity has been validated based on the AE localization and the CT images. With the increase of the pulse peak pressure, more microcracks emerge and evenly distribute around the borehole in coal. This work provides an experimental method to quantify the fatigue damage induced by the cyclic hydraulic pressure which is very significant for the investigation of the pulsating hydraulic fracturing.

The low-temperature oxidation of coal during coalbed methane extraction is inevitable. To study the evolution of the CH4adsorption and desorption characteristics in coal masses during the low-temperature oxidation of coal, starting from the evolution of physical and chemical adsorption of CH4in coal masses during the low-temperature oxidation of coal and combining it with the evolution of free radicals, we constructed a physical model of CH4adsorption and desorption in the coalbed methane extraction process. This model provides the theoretical basis for improving the efficiency and quantity of coalbed methane extraction. Our results indicate that during coalbed methane extraction, the mesopores, macropores, and overall porosity increase with the rise in oxidation temperature. However, the number of micropores first increases and then decreases during the process, leading to the CH4physical adsorption capacity showing a trend of first increasing and then decreasing with increasing oxidation temperature; however, the number of −COOH groups shows the opposite trend to that of the number of micropores, resulting in the CH4chemical adsorption capacity first decreasing and then increasing with the increase of the oxidation temperature; meanwhile, the free radical content increases gradually with the increasing oxidation temperature, leading to the continuous consumption of O2adsorbed on the coal surface and the reinforcement of the CH4adsorption capacity. To maximize coalbed methane extraction efficiency, it is necessary to take measures to avoid the low-temperature oxidation of coal at the initial stage.