Quantitative Characterization of Coal Shale Pores and Fractures Based on Combined High-Pressure Mercury Pressure and Low-Temperature N2/CO2 Adsorption Methods.

Coal shale gas is an important type of shale gas. The microscopic pore size distribution and pore structure characteristics of coal shale determine the macroscopic storage and transportation of coal shale gas. In order to quantitatively characterize the microscopic pore size distribution and pore structure properties of coal shale from a multiscale perspective, the pore size distribution and pore structure of coal shale specimens with different grain sizes were quantitatively characterized using low-temperature N2/CO2 adsorption and high-pressure mercuric pressure methods, taking the coal shale of high-gas mines of the Dongbaowei Mine in the Shuangyashan Basin as the object of study. The pore size distribution fractal pattern and pore structure characteristics of coal shale were quantitatively analyzed by the joint characterization method of the coal shale pore fractal theory and pore size distribution. Relevant experimental studies found the following: (1) The specific surface area and volume of coal shale pores and fractures decrease gradually with the increase in coal shale grain size. (2) The pore size distribution of coal shale has obvious fractal characteristics at the stages of high and low mercury pressures, and the demarcation point of medium and large pores is 55 nm. (3) The difference in pore and fracture structural parameters between coal shale with different grain sizes and the original rock specimens is relatively small, and it is feasible to study the general rules of gas adsorption, desorption, diffusion, and seepage in the pores and fractures of coal shale by using coal shale shapes instead of the original rock specimens.

Adsorption-desorption characteristics of coal-bearing shale gas under three-dimensional stress state studied by low field nuclear magnetic resonance spectrum experiments.

The micro-scale gas adsorption-desorption characteristics determine the macro-scale gas transport and production behavior. To reveal the three-dimensional stress state-induced gas adsorption-desorption characteristics in coal-bearing shale reservoirs from a micro-scale perspective, the coal-bearing shale samples from the Dongbaowei Coal Mine in the Shuangyashan Basin were chosen as the research subject. Isothermal adsorption-desorption experiments under three-dimensional stress state were conducted using the low field nuclear magnetic resonance (L-NMR) T2 spectrum method to simulate the in-situ coal-bearing shale gas adsorption-desorption process. The average effective stress was used as the equivalent stress indicator for coal-bearing shale, and the integral of nuclear magnetic resonance T2 spectrum amplitude was employed as the gas characterization indicator for coal-bearing shale. A quantitative analysis was performed to examine the relationship between gas adsorption in coal-bearing shale and the average effective stress. And a quantitative analysis was performed to examine the relationship between the macroscopic and microscopic gas quantities of coal-bearing shale. Experimental findings: (1) The adsorption-desorption process of coal-bearing shale gas follows the L-F function model and the D-A-d function model respectively with respect to the amount of gas and the average effective stress. (2) There is a logarithmic relationship between the macroscopic and microscopic gas quantities of coal-bearing shale during the adsorption-desorption process. This quantitatively characterizes the differences in the curves, which may be related to the elastic-plastic deformation, damage and fracture of the micropores in coal-bearing shale, as well as the hysteresis of gas desorption and the stress field of the gas occurrence state.

Acoustic emission and fractal characteristics of red beds soft rock under water-force coupling.

Groundwater has a significant influence on the mechanical properties of surrounding rock. Aiming at the large deformation of surrounding rock of red layer soft rock tunnel affected by groundwater, the uniaxial graded loading tests were carried out on red beds soft rock with different water content. The failure process of the specimen was monitored by acoustic emission (AE) and the crack evolution law was analyzed, and the scanning electron microscopy (SEM) was used to compare the microstructure of the specimens before and after immersion. Combined with fractal theory, the monofractal and multifractal characteristics of AE ringing count during the loading process of red beds soft rock were analyzed. The results show that, with the gradual increase of water content, the AE ringing count before the yield stage gradually decreased, and the corresponding cumulative ringing count at the same time gradually decreased, and the decrease was large in the early stage of immersion, and decreased in the later stage. The cumulative ringing curve gradually slowed down, the internal crack appeared earlier, the cumulative ringing curve stepped significantly, the AE signal amplitude gradually weakened, and the bandwidth of each frequency band gradually decreased. The failure of red beds soft rock with different water content is dominated by shear crack, and with the gradual increase of water content, the proportion of shear crack increases gradually, and the AE b value decreases gradually. With the gradual increase of the relative peak strength, the correlation dimension D of red beds soft rock with different water content increases first and then decreases. At 80% of the relative peak strength, the correlation dimension D reaches its maximum value and then drops sharply until it is maintained at a relatively low level, and the correlation dimension D gradually decreases with the water content. The fitting correlation coefficients of different water content (lnC(r), lnr) are all above 0.9, indicating that the AE ringing count of water-bearing red beds soft rock has fractal characteristics, and the higher the correlation coefficient, the higher the self-similarity of AE ringing count sequence. As the weight q gradually increases, the generalized fractal dimension D(q) gradually decreases. When q ≠ 0, under the condition of the same q, D(q) presents a trend of first increasing and then decreasing. The multifractal characteristics of AE ringing count of red beds soft rock with different water content is inverted 'U' shape. From the natural state to immerse 1 d, the ∆α gradually increases, and from 1 to 7 d, the ∆α gradually decreases, where Δα = αmax - αmin represents the spectral width of the multifractal spectrum. When saturation is not reached, ∆f < 0 indicates that the number of cracks in the specimen is small, when saturation is reached, ∆f > 0 indicates that a large number of cracks are generated inside the specimen and macro cracks are formed, where Δf = f (αmax) - f (αmin) represents the frequency relationship between different signals of different sizes. This research can provide a reliable theoretical basis for the construction and maintenance of large deformation of water-rich soft rock tunnel excavation, and have certain engineering significance.

Analysis of Effective Pyrolysis Zone and Heat Loss in Oil Shale Reservoir with Random Fractures.

Given the unclear variation law of the effective pyrolysis zone in the process of in situ heat injection mining of oil shale, the actual pyrolysis effect cannot be accurately judged. In this paper, considering the influence of two different random fractures, the thermal-fluid-solid coupling mechanical model of oil shale in situ heat injection mining is established. The effective pyrolysis zone, steam injection pressure, and temperature-affected zone of the roof and floor rocks in the process of in situ heat injection mining of oil shale are analyzed. The results showed that interconnected and high-density fractures are important channels for superheated steam seepage, which is conducive to the efficient penetration of superheated steam in oil shale reservoirs. There are multiple pyrolysis paths in oil shale reservoirs in bedding fractures, while oil shale reservoirs in hydraulic fractures are uniformly pyrolyzed by a high-temperature network formed by superheated steam. When the bedding fracture model and the hydraulic fracture model are injected with heat for 233 and 90 days, the oil shale reservoir reaches the effective pyrolysis temperature, and the pyrolysis efficiency of the hydraulic fracture network is 2.59 times that of the bedding fracture network. The average temperature of the affected area of the overlying and overlying strata is 471.98 and 467.02 °C, respectively. When in situ heat injection mining of oil shale is carried out, it is necessary to adjust the heat injection time reasonably and keep the hydraulic fracture away from the upper and lower boundaries of the oil shale reservoir to avoid the heat dissipation of superheated steam.

Macroscopic fracture mechanism of coal body and evolution characteristics analysis of impact force in deep coal and gas outburst.

With the increase of mining depth and intensity, coal and gas outburst dynamic disasters occur frequently. In order to deeply study the macroscopic fracture mechanism of coal body and evolution characteristics analysis of impact force, taking the outburst coal seam of Pingmei No. 11 Coal Mine and Sunjiawan coal seam of Hengda Coal Mine as the research objects, the simulation roadway test system of self-developed true triaxial coal and gas outburst is applied to carry out the simulation test of deep coal and gas outburst with buried depths of 1000 m, 1200 m, 1400 m and 1600 m. During the test, the overlying strata stress is simulated by axial compression, the surrounding rock stress is simulated by confining pressure, the gas pressure is simulated by pore pressure, the impact force and acoustic emission monitoring technology are introduced, and the coal seam gas pressure is simulated by mixture pressure of 45% CO2 and 55% N2. From the viewpoint of fracture mechanics, the crack propagation mechanism of coal in the outburst launching area is discussed, the evolution characteristics of impact force and gas pressure are analyzed, and the influence law between acoustic emission signal and impact force is revealed. From the viewpoint of energy conversion, the transformation character of gas internal energy to impact kinetic energy (gas pressure to impact force) are analyzed. The results show that the generation of I-type crack is a prerequisite for outburst catastrophe. With the crack propagation, I-type and II-type cracks intersect and penetrate, resulting in internal structural damage and skeleton instability of coal. Gas wrapped fragmentized coal body thrown, outburst occurs. There is obvious negative pressure in the roadway after outburst. The occurrence of negative pressure is greatly affected by the physical and mechanical properties of coal, ground stress and gas pressure. Impact kinetic energy is mainly provided by gas internal energy. Part of the gas pressure is converted into impact force. The strength and duration of the impact force are determined by the gas pressure. Under the condition of deep working conditions (high ground stress and low gas pressure), the propagation of impact force in the roadway is more hindered. Both impact force and acoustic emission signals can monitor the occurrence of outburst. The peak point of acoustic emission ringing count is earlier than the impact force. The acoustic emission signal can monitor the outburst hazard earlier. The impact force can more specifically reflect the coal fracture.

Gas pressure evolution characteristics of deep true triaxial coal and gas outburst based on acoustic emission monitoring.

Coal and gas outburst is one of the geological disasters that seriously threaten the safety of coal mines production. In recent years, with the increase of mining depth, outbursts become frequent. To further explore the occurrence mechanism of deep coal and gas outburst, a self-developed true triaxial coal and gas outburst simulation device was used to simulate the coal and gas outburst at different depths. The results show that with the increase of simulation depth, the critical gas pressure of outburst gradually decreases, and the unit outburst intensity increases sharply. The gas threshold of deep coal and gas outburst is lower. During the incubation and excitation, gas pressure has three special variation rules, namely self-increasing characteristic, stage and instantaneous. In the early incubation, acoustic emission (AE) energy is at a low level, low energy frequency is dominant; in the later incubation, AE energy increases greatly, high energy frequency is dominant. From the perspective of AE energy, a quantitative index that reflects the danger of coal and gas outburst in the incubation is defined, which provides a scientific reference for the prediction and prevention of disasters of coal and gas outburst in deep mining.

Analytical Hysteretic Behavior of Square Concrete-Filled Steel Tube Pier Columns under Alternate Sulfate Corrosion and Freeze-Thaw Cycles.

The hysteretic behavior of square concrete-filled steel tube (CFST) stub columns subjected to sulfate corrosion and freeze-thaw cycle is examined by numerical investigation. The constitutive model of steel considered the Bauschinger effect, and compression (tension) damage coefficient was also adopted for the constitutive model of core concrete. The experimental results are used to verify the finite element (FE) model, which could accurately predict the hysteretic behaviors of the CFST piers. Then, the effects of the yield strength of steel, compressive strength of concrete, steel ratio, axial compression ratio, and alternation time on ultimate horizontal load are evaluated by a parametric study. The results showed that the yield strength of steel and the steel ratio have a positive effect of hysteretic behavior. The compressive strength of concrete and alternation time significantly decreased the unloading stiffness which causes the pinching phenomenon. The yield strength of steel, compressive strength of concrete, and alternation time of environmental factors (corrosion-freeze-thaw cycles) has no obvious effect on the initial stiffness, while the steel ratio has a remarkable effect. The ultimate horizontal load increases with the increasing steel ratio, yield strength of steel and compressive strength of concrete. Meanwhile, the decrement of alternation time led to the increase of ultimate horizontal load. This suggests that the confinement coefficient and alternation time are the two main factors that impact the ultimate horizontal load. A formula which considers the reduction coefficient for the ultimate horizontal load of the CFST columns subjected to sulfate corrosion and freeze-thaw cycles is proposed. The formulae can accurately predict the ultimate horizontal load with mean value of 1.022 and standard deviation of 0.003.