Search

The conventional triaxial loading tests were conducted under different saturation(50%, 80%) and confining pressures (12 MPa, 16 MPa, 20 MPa) to explore the applicability of Mohr-Coulomb(M-C) criterion, Hoek-Brown (H-B) criterion as well as Generalized Hoek-Brown(GH-B) criterion to gas hydrate-containing coal under high confining pressure condition. The stress-strain curves were analyzed, and the variation of the main principal stress with the confining pressure was fitted by using the formulas of three strength criteria. Meanwhile, the root mean square deviation RMSE and correlation coefficient were introduced to evaluate the applicability of different strength criteria for gas hydrate-containing coal. The results show that the stress-strain curves exhibit strain hardening characteristics under the condition of high confining pressure. The higher the confining pressure and saturation, the higher the peak strength. The confining pressure has a significant effect on the peak strength. M-C and GH-B criteria are proved to have good fitting effect on the test data, with higher correlation coefficient and lower RMSE value than H-B criterion. The linear fitting line between the predicted value and the test value is very close to the 1∶1 gradient line, indicating that the two criteria have good applicability to gas hydrate-containing coal. The correlation coefficient of H-B criterion is lower than M-C and GH-B criterion. The RMSE value is larger, and the applicability of H-B criterion to gas hydrate-containing coal is slightly lower than M-C criterion and GH-B criterion.

In order to obtain the applicability of the strength criterion of gas hydrate coal under high saturation, based on Drucker-Prager criterion and combined with the stress-strain curve of gas hydrate coal under conventional triaxial compression, the error between the theoretical value and the experimental value under Drucker-Prager criterion was discussed. The applicability of Drucker-Prager criterion was analyzed and compared with Mohr-Coulomb criterion. The results show that the peak strength of gas hydrate coal increases with the increase of confining pressure and saturation under the coupling effect of high saturation and confining pressure. The maximum absolute errors of strength prediction by Mohr-Coulomb criterion and Drucker-Prager criterion were 6.77% and 3.66%, respectively, indicating that the test results were in good agreement with the prediction model. However, the absolute errors of intensity prediction of Drucker-Prager criterion are in the range of 0.92%-3.66%, and the range of error fluctuation is small, indicating that Drucker-Prager criterion is more suitable for high saturation.

Coal and gas outbursts are geological disasters occurring in the process of coal mining that can cause serious casualties and economic losses, among which CH4 is the main component of coal mine gas. However, there are still many coal seams around the world that are dominated by CO2. Although the frequency of CO2 gas outburst accidents is relatively low, CO2 outbursts are very violent, notably difficult to control and highly dangerous. The application of hydration curing technology to reduce the pressure and gradient of CO2 gas in the coal can effectively reduce the occurrence of coal and CO2 outburst. Accordingly, in this paper, experimental studies on the growth kinetics of CO2 hydrate with three driving forces (2, 2.5, 3 MPa) were carried out under four different coal particle sizes (C1: 0.425–0.850 mm, C2: 0.250–0.425 mm, C3: 0.180–0.250 mm, C4: 0–0.180 mm) to obtain kinetic parameters such as gas consumption, growth rate, and heat of decomposition during the synthesis of CO2 hydrate. The results show that the hydrate nucleation time in the same particle size system does not follow the same decreasing trend with increasing driving force. Gas consumption of CO2 hydrates in the same particle size system increased with increasing driving force, and there exists a critical value regarding the effect of the driving force on CO2 hydrate generation in coal particles with the particle size. Under the same temperature conditions, increasing the driving force in the particle size system could increase the CO2 hydrate growth rate. With decreasing coal particle size and increasing driving force, the promoting effect gradually exceed the inhibiting effect, which promote CO2 hydrate formation. Through linear fitting, an equation of the average growth rate of CO2 hydrates versus the driving force for the C1-C4 systems is fitted to provide a reference to predict the average CO2 hydrate growth rate. In the same medium, with increasing driving force, more heat is required for complete decomposition, which remains relatively stable, and the heat of decomposition of CO2 hydrates is the highest in the C1 medium, indicating that the presence of CO2 hydrates in the C1 system represents the most stable state.

This study focuses on the energy variation rules of gas hydrate-bearing coal during deformation and failure, combined with the deviatoric stress-strain curves under conventional triaxial compression and the first law of thermodynamics. The results show that the elastic strain energy is dominant in the process of pre-peak deformation and failure of gas hydrate-bearing coal; while the gas hydrate-bearing coal may experience energy dissipation after the peak stress. Then, the total energy, elastic strain energy and dissipation energy all increase with increasing of the confining pressure and the saturation. Furthermore, the multiple linear regression equation depicting the viriation of saturation and confining pressure was established with the total energy of the critical failure point, with the better fitting effect. Moreover, the elastic energy accumulation at the failure point increases, thus producing the larger dissipation energy.

Accurate determination of the characteristics of coal mine gas separation products is the key for gas separation applications based on hydrate technology. Gas hydrates are synthesized from gases with two types of compositions (CO2-CH4-N2). The separation products were analyzed by in situ Raman spectroscopy. The crystal structure of the mixed-gas hydrate was determined, and the cage occupancy and hydration index were calculated based on the various vibrational modes of the molecules according to the “loose cage-tight cage” model and the Raman band area ratio combined with the van der Waals-Platteeuw model. The results show that the two mixed-gas hydrate samples both have a type I structure. Large cages of mixed-gas hydrate are mostly occupied by guest molecules, with large cage occupancies of 98.57 and 98.52%; however, small cages are not easy to occupy, with small cage occupancies of 29.93 and 33.87%. The average cage occupancies of these two hydrates are 81.41 and 82.36%, and the stability of the crystal structure of the mixed-gas hydrate in the presence of 75% CO2 is better than that of the mixed-gas hydrate in the presence of 70% CO2. The hydration indices of the two hydrate gas samples are 7.14 and 6.98, which are greater than the theoretical value of structure l.

To improve gas hydrate storage and transportation technology, ferromagnetic intermetallic compound NiMnGa particles with martensitic transformation endothermics were used to form micro/nanofluids with sodium dodecyl sulfate (SDS) to further strengthen the gas hydration process. In this work, the kinetic process of gas hydration in NiMnGa fluids with different concentrations (0, 0.1, 1, 2, and 3 wt.%) was investigated using a rotating magnetic field gas hydration separation experimental setup. The results show that the induction time of the 3 wt.% NiMnGa system was shortened by 98.3%, the gas consumption was increased by 50.5%, and the gas consumption rate was increased by 351.9% compared with the SDS system. Therefore, it is inferred from the mass transfer that NiMnGa micro/nanofluids can accelerate the formation of hydrates. [ABSTRACT FROM AUTHOR]