Search

To study the anisotropy of gas seepage in coal seams with low permeability,coal samples were prepared respectively in the parallel bedding direction and the vertical stratification direction of the same coal rock. Tri-axial servo seepage tests on gascontained coal were detected by using a self-developed apparatus. It is found that the permeability of coal samples in the parallel is far greater than that of coal samples in the vertical. With the increase of axial pressure,the permeability of coal samples in the parallel reaches its minimum at the yield point before the peak failure,while the permeability of coal samples in the vertical gets its minimum at the initial period of the elastic stage. The permeability of both the parallel and the vertical nonlinearly increases after the peak failure.

Coal seam water injection is the most basic and effective dust control technology used on the coal mining face. Numerical simulations are helpful for predicting the humidity range of coal seam water injection. The results can provide guidance for the field design of coal seam water injection process parameters. In order to understand the influence of coal seam water injection pressure and water injection time on the coal seam wetting effect, this paper uses the Fluent 17.0 software system to study the process parameters of the coal seam water injection seepage process under different conditions. It is found that in the process of coal seam water injection, with the increase in water injection pressure and the prolongation of water injection time, there is a specific threshold value for the change in coal seam permeability. Only when the water injection pressure and time increase to the threshold value will the permeability of the coal seam be significantly enhanced and the wetting effect improved. The pressure threshold of the mine is 15 MPa–20 MPa, and the time threshold is the first 42 h. [ABSTRACT FROM AUTHOR]

The exploration of Hot Dry Rock (HDR) geothermal energy is essential to fulfill the energy demands of the increasing population. Investigating the physical and mechanical properties of heated rock under different cooling methods has significant implications for the exploitation of HDR. In this study, ultrasonic testing, uniaxial strength compression experiments, Brazilian splitting tests, nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were conducted on heated granite after different cooling methods, including cooling in air, cooling in water, cooling in liquid nitrogen, and cycle cooling in liquid nitrogen. The results demonstrated that the density, P-wave velocity (Vp), uniaxial compressive strength (UCS), tensile strength (σt), and elastic modulus (E) of heated granite tend to decrease as the cooling rate increases. Notably, heated granite subjected to cyclic liquid nitrogen cooling exhibits a more pronounced decline in physical and mechanical properties and a higher degree of damage. Furthermore, the cooling treatments also lead to an increase in rock pore size and porosity. At a faster cooling rate, the fracture surfaces of the granite transition from smooth to rough, suggesting enhanced fracture propagation and complexity. These findings provide critical theoretical insights into optimizing stimulation performance strategies for HDR exploitation. [ABSTRACT FROM AUTHOR]

Acidified CO2 fracturing is a viable method for increasing production in deep, tight sandstone reservoirs. However, the potential mechanism of changes in pore structure and mechanical properties of sandstone under acidified CO2 supercritical composite is not clear. Understanding this mechanism is important for the study of crack initiation and extension in tight sandstone reservoirs. This study utilizes sandstone samples from the Xujiahe Formation reservoir in Rongchang District as experimental specimens. The primary focus is to analyze the changes in pore structure and mechanical properties of these samples after acidification–supercritical CO2 composite action. Nuclear magnetic resonance (NMR) and uniaxial compression tests are employed as the main investigative techniques. The results show that there was a physicochemical synergy between the acidification–supercritical CO2 composite effect; the crack initial stress, damage stress, and peak stress of the sandstone after 16 MPa supercritical CO2 acidification treatment were reduced by 20%, 49.5%, and 49.8%, respectively; the crack volumetric strain accelerated and the sandstone evolved from brittle to ductile damage; and the larger pore space and microcracks of the sandstone increased significantly after the treatment, which can be attributed to the gradual dissolution of intergranular cement leading to the formation of new pores connected to the existing pore network. The change mechanism of sandstone after acidification–supercritical CO2 compound treatment is also proposed. [ABSTRACT FROM AUTHOR]

During the excavation of tunnels, the principal stress orientation changes, with a significant impact on the stress distribution, mechanical properties, and plastic zone evolution of rocks surrounding tunnels, causing severe deformation control, and monitoring problems in the stability of tunnels. Currently, biaxial compression tests were conducted to explore the influence of principal stress orientation on mechanical properties of rocks surrounding tunnels. The analytical solution of stress and the model of plastic zone of rocks considering the principal stress orientation and the distance from the excavation boundary were established to reveal the failure mechanism of surrounding rock under different principal stress orientations. With an increase in the angle between the principal stress orientation and the long axis of tunnel, the maximum tangential stress around the tunnel gradually changed to the minimum horizontal principal stress direction, and its value gradually increased, leading to quicker failure of surrounding rocks, and reducing the strength enhancement effect of the same section size of the tunnel. However, the increase in the angle reduced the damage range and the range of the plastic zone around the tunnel and caused the plastic zone to gradually approach the bottom and roof. The maximum depth of the plastic zone remained parallel to or nearly parallel to the minimum horizontal principal stress direction. When the principal stress orientation was kept constant, the maximum depth of the plastic zone shifted to the minimum horizontal principal stress direction with an increase in the vertical principal stress. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is an urgent and constantly perplexing problem with coal resource extraction, threatening coal mine safe and sustainable production severely. Its mechanism and the participation of gas in coal breaking are still unclear. To explore this problem, in this paper, gas desorption-diffusion regularity of bituminous coal with different particle sizes and its influence on outburst-coal breaking were investigated through mercury intrusion porosimetry (MIP) tests, isothermal adsorption tests, and desorption-diffusion tests for coal particles with different sizes. The results indicated that the cumulative diffusion amount ( Q t ) and rate ( Q t / Q ∞ ), the effective diffusion coefficient ( D ′ ), and the kinetic diffusion parameter (υ) decreased as particle size increased. That meant gas was easier to desorb and diffuse from the smaller coal blocks, consequently making coal break into more tiny particles and accelerating gas desorption. As a result, a positive feedback effect that coal breaks continuously and gas releases rapidly and abundantly was formed in a short time when outbursts started, which caused gas release in quantities and promoted the occurrence of outbursts. The findings of this study enhance our understanding of the mechanism of gas participating in coal fragmentation during outbursts, which are significantly conducive to the prevention and control of coal mine disasters and sustainable production of coal resources. [ABSTRACT FROM AUTHOR]

In this study, X‐ray diffraction, N2 adsorption (N2A), and mercury intrusion (MI) experiments were used to investigate the influence of acid treatment on pore structure and fractal characterization of tight sandstones. The results showed that acid treatment generated a certain number of ink‐bottle pores in fine sandstone, aggravated the ink‐bottle effect in the sandy mudstone, and transformed some smaller pores into larger ones. After the acid treatment, both the pore volume in the range of 2–11 nm and 0.271–8 μm for the fine sandstone and the entire pore size range for the sandy mudstone significantly increased. The dissolution of sandstone cement causes the fine sandstone particles to fall off and fill the pores; the porosity increased at first but then decreased with acid treatment time. The fractal dimension obtained using the Frenkel‐Halsey‐Hill model was positively correlated with acid treatment time. However, the total fractal dimensions obtained by MI tests showed different changes with acid treatment time in fine sandstone and sandy mudstone. These results provide good guiding significance for reservoir acidification stimulation. [ABSTRACT FROM AUTHOR]

Design and analysis of low-phase-noise low quadrature error V-band subharmonically injection-locked quadrature voltage-controlled oscillator (QVCO) and quadrature frequency-locked loop (FLL) are presented in this paper using 90-nm CMOS process. A modified self-injection technique is employed in the QVCO to reduce quadrature error and phase noise. A transformer coupled topology is adopted to widen the locking range (LR) of the subharmonically injection-locked QVCO. With a subharmonic number of 3, the proposed subharmonically injection-locked QVCO demonstrates an overall LR of 3.5 GHz, a phase noise of −126.8 dBc/Hz at 1-MHz offset, a phase error of 0.32°, and an amplitude error of 0.26 dB. Moreover, the quadrature FLL is developed using the QVCO, and the subharmonic number can be up to 32 since the control voltage of the QVCO is adaptively aligned to overcome the LR. The operation frequency of the subharmonically injection-locked quadrature FLL is from 49 to 51.4 GHz. The measured amplitude and phase errors are 0.5 dB and 2.5°, respectively. Between 15 °C and 50 °C, the measured minimum phase noise at 1-MHz offset and jitter integrated from 1 kHz to 40 MHz are lower than −106 dBc/Hz and 110 fs, respectively. Compared to the prior art, the proposed $V$ -band subharmonically injection-locked quadrature QVCO and quadrature FLL feature excellent performance and good robustness. [ABSTRACT FROM AUTHOR]

During the mining of coalbed methane, the temperature, overburden pressure, and pore pressure increase with the depth, leading to a decline in permeability and making the mining more difficult. Many researches are devoted to get more accurately changing mechanism of permeability by measuring sorption-induced strain. However, the presently huge facilities and complex process make the test quite inconvenient. Therefore, based on common laboratory equipment, we have developed an optical observation device with the function of multi-field coupling, which is a pioneered design in China. It realises the contrast experiment of the sorption-induced strain under free absorption inflation and restrained strain with axial direction. Through carbon dioxide and methane adsorption experiments with this device, the pressure - strain curve and time - strain curve is obtained, from which we observe that coal is more sensitive to CO2 adsorption than methane adsorption. The Langmuir equation can satisfactorily simulate experimental data. The adsorption experiment can be divided into three phases: rapid, slow and stable. Adsorption-induced strain of CO2 is two to four times than that of methane. Through those experiments, we have verified the previous research results and proved the feasibility of the device. [ABSTRACT FROM AUTHOR]