Abstract To study the response characteristics of coal samples to impact load under different surrounding pressures, the self-developed magnetic drop hammer impact device was used to measure the change of ultrasonic wave velocity before and after drop hammer impact, the internal damage of coal samples was quantitatively characterized, and then, the damage factor, damage difference, and crack development direction of briquette samples were analyzed. Based on the Griffith strength theory, the relationship between stress intensity factor, crack development angle, and surrounding pressure ratio of coal samples with composite cracks was deduced. It was found that a certain surrounding pressure would slow crack expansion and reduce the amount of damage increase. The damage degree of briquette samples caused by impact load under no surrounding pressure is the largest, followed by that at the surrounding pressure ratio of 0 and 2 and that at the surrounding pressure ratio of 1 is the smallest. When the amount of damage difference of the sample is positively correlated with the number of impacts, the crack development is directional; when the amount of damage difference of the sample is nonpositively correlated with the number of impacts, the direction of crack development is random. It is therefore reasonable to predict the direction of crack expansion by the trend in the amount of damage difference. Besides, the difference in impact load between briquette and raw coal samples under different surrounding pressure ratios was discussed, and the fractal dimension of the sample surface under different surrounding pressure ratios was compared and analyzed. It is found that the fractal dimension of raw coal is larger than that of briquette, indicating that the crack development of raw coal is higher after impact; the damage amount of raw coal is larger than that of briquette, indicating that raw coal is easier to be damaged after impact.

Abstract An understanding of across-fault seals is essential for planning an injection/production strategy for a fault-bounded gas storage site. In addition, it is more likely to permit lateral leakage for a fault with sand self-juxtaposition windows. This paper is aimed at identifying the dynamic sealing behaviors of a sand self-juxtaposition fault on the geological and gas injection timescales. Banzhongbei gas storage site, China, was taken as a target area, and fault seals and hydrocarbon distributions within the original reservoirs were studied. The results showed that across-fault pressure differences of 0.085~0.146 MPa (equivalent to 41.6~71.5 m oil-column and 27.0~46.4 m gas-column heights) were supported by sand self-juxtaposition windows on the B816 fault, and the resultant absolute permeability (5.97×10−2~5.69×10−1 mD) of the fault was nearly 3~4 orders of magnitude lower than the average absolute permeability of reservoirs (1.16×102 mD). Gas composition contrasts, between the original and injection gas coupled with dynamic pressure monitoring data, indicated that lateral leakage occurred across sand self-juxtaposition windows under the condition of high across-fault pressure difference. However, the low-permeability fault showed strong negative influence on the efficiency of fluid flow in the model calculations and prolongs the timescales of pressure-difference decayed as much as 5 orders of magnitude relative to those of nonfault model calculations. These modeled dynamic sealing behaviors of sand self-juxtaposition windows may lead to a better understanding of the relative retardation of across-fault gas flow by weak sealing faults on the gas injection/production timescale.

Abstract The seismic inversion method combined with multipoint geostatistics theory has begun to receive attention, but the acquisition accuracy and calculation efficiency of 3D training image still need more optimization. This paper presents a novel method of 3D multipoint geostatistical inversion based on 2D training images directly. The 2D training image was scanned by the data template to acquire the multipoint statistical probability in 2D direction. The probability fusion method is used to fuse the 2D multipoint probability into 3D multipoint probability. The rock facies types and patterns of the simulated points are obtained by random sampling. On this basis, the elastic parameters are extracted from the statistical rock physics model, and the seismic records are convoluted. Then, the synthetic records and the actual records were compared under a given threshold. If the error exceeds the given threshold, the iterative adaptive spatial sampling method will be used to repeat the process above-mentioned, so as to ensure that the error is below the threshold. Because the 2D training image is easy to obtain and evaluate, the demand problem of 3D training image is solved. The 2D training image scanning, probability storage and access are more convenient, and the adaptive spatial sampling method is more efficient than the reject sampling, so as to ensure the operation efficiency. The model from the Stanford Center for Reservoir Forecasting is selected to test the effectiveness of this newly designed method.

Abstract The Bundelkhand “giant quartz reef” (BGQR) system comprises 20 major quartz reefs which run for tens of km in strike length of average width of 40 m and occurs in spatial intervals of 12–19 km in the Bundelkhand craton, North Central India. The BGQR system is distinct from quartz vein systems originating from crustal scale shearing observed in ancient as well as modern convergent tectonic settings. Fluid inclusions studied in BGQR system are intriguingly diverse although dominated by aqueous fluid which exhibit a broad range of salinity from ~0 to 28.9 wt% NaCl equivalent and temperature of homogenization range of 58 to 385°C. Primary and pseudosecondary aqueous inclusions in assemblages in grain interiors and growth zones vary randomly in their Th—salinity characteristics that preclude identification of discrete fluid events. Aqueous fluid in the BGQR system evolved through mixing of two distinct sources of fluids—a meteoric fluid and a moderate temperature—moderate salinity fluid that was possibly derived from the Bundelkhand granodiorite based on an important clue provided by hydrous mineral bearing fluid inclusions detected by Raman microspectrometry. The results of modeling with PHREEQC indicate that mixing of fluids could be a suitable mechanism in formation of these giant reefs. The available 1-dimensional diffusive transport model for deposition of silica helps in putting constraints on the time span of deposition of silica in the context of the BGQR system. The BGQR system is a possible result of shallow-crustal sources of fluid and silica and could be visualized as a “Paleoproterozoic geothermal system” in a granitic terrane.

Abstract In order to solve the problem of support crushing, rib spalling, and partial roof fall in working face crossing fault structure area, taking II1023 working face of Haizi Mine as the engineering background, the numerical simulation, theoretical analysis, and field measurement were used to simulate the rock activity law in the process of working face crossing fault. Based on this, the mechanical model of the interaction between the support and the surrounding rock in the fault area was established, and the calculation formula of the support strength required by the support under different equilibrium criteria was deduced, which provides a basis for the reasonable selection of hydraulic support. The results show that the rock stress of the working face passing through the fault zone is divided into high-stress section when it is gradually close to the fault and low-stress section after passing through the fault. In the high-stress section, the rock stress increases in front of the working face, and the distribution form is similar to the stress distribution of the narrow coal pillar. The roof easily forms a long hanging roof and slip along the fault surface, which is prone to rib spalling and partial roof fall and support crushing. The support strength is mainly related to fault dip angle, fault cohesion, support beam length, length of the main roof rock beam, and the distance between working face and fault. The reasonable support strength should be determined by the calculation results under different equilibrium criteria. When the working face advances in the high-stress section, the required support strength of the support increases with the increase of the fault dip angle and the length of the main roof rock beam and decreases with the increase of the fault cohesion, the length of support beam, and the distance between working face and fault. With the increase of the fault dip angle, the form of rotary instability changes and the probability increases. The field practice shows that during the fault crossing of the II1023 working face, the working resistance of the support shows the characteristics of high-pressure area and low-pressure area, but it does not exceed the rated working resistance of the selected support. The fully mechanized coal face passed through the fault area safely, and there were no accidents such as support crushing, rib spalling, and partial roof fall.

Abstract A novel colloidal dispersion gel (CDG) was synthesized by cross-linking polysaccharide-based hyperbranched polymer (SMHBP) with aluminum citrate. Four factors affecting gelation properties were investigated including SMHBP concentration, the weight ratio of polymer to cross-linker, NaCl concentration, and additive concentration. Environmental scanning electron microscope (ESEM) was used to observe the morphologies of the CDG to understand the cross-linked mechanism. The CDG is cross-linked by both intramolecular and intermolecular, and the formation of the CDG is dominated by the intramolecular cross-linking reaction. Core displacement experiments demonstrated that the novel CDG obtained excellent profile modification performances at a high temperature and high salinity.

Abstract In this paper, the effect of high temperature on the mechanical properties of quartz-rich sandstone was studied by true triaxial pressure experiment. The density, P-wave velocity, elastic modulus, peak stress, peak strain, and failure mode of quartz-rich sandstone before and after heat treatment were analyzed and compared. It was found that the color of quartz-rich sandstone changes greatly when heated to 500°C. In detail, the brittleness of quartz-rich sandstone gradually decreased and the ductility increased at high temperature. Although the density, P-wave velocity, and elastic modulus fluctuate, they showed a decreasing trend as a whole. In addition, the peak stress and peak strain showed an overall upward trend with the increase of temperature, and 500°C was the high-temperature failure threshold of quartz-rich sandstone. The failure mode of quartz-rich sandstone at high temperature changes from shear failure to shear-tension combined failure, and the number of cracks increases. Therefore, these research results can provide a certain reference for high-temperature underground operation.

Abstract Waterproof coal pillar in the underground reservoir is affected significantly by inclination loads and water erosion. Comprehensive understanding of the influence of moisture content and inclination angle on the mechanics properties and microfracture behavior of coal specimens plays an important role for revealing the instability mechanism of waterproof coal pillar. In this paper, a new modified combined compression and shear test (MC-CAST) system was firstly developed for achieving the specimen inclined loading and then used to carry out the inclined uniaxial compression test of water-bearing coal specimens at the inclination angle of 0°, 5°, 10°, and 15° and the moisture content of 0%, 2.42%, 5.53%, 7.55%, and 10.08%. The mechanical and microfracture behaviors of water-bearing coal specimens were analyzed and discussed by AE techniques and SEM. Research results indicate that ① the peak shear stress, peak strength, and elastic modulus of coals showed obvious moisture content weakening effect, and the weakening effect was more significant at larger inclination angle; ② the peak strength and elastic modulus decreased nonlinearly as inclination angle increased, of which decreasing amplitude was greater at high moisture content than that at low moisture content; ③ the peak shear stress gradually increased with the increasing inclination angle for 0%-7.55% moisture content, while it increased first and then decreased, and reached the maximum value at 10° inclination angle for 10.08% moisture content; ④ the stress thresholds of crack closure, initiation, and damage of coal specimens decreased gradually with the increase of moisture content and inclination angle, but the ratio of above three thresholds to their corresponding peak strength was basically unchanged; and ⑤ the erosion-stripping effect of bound water and pore pressure effect of free water, respectively, caused initial damage and secondary superposition damage of coal specimens before and after inclination loading test. Combining with the promotion effect of additional shear stress on crack initiation, propagation, and coalescence, the coupling effect of the above three factors was the fundamental reason for the strength deterioration of water-bearing coals under inclined loading. The research results can provide an important basis for the strength and stability evaluation of waterproof coal pillars of underground reservoirs in ecologically fragile areas.

Abstract The sediment bedding direction and loading-unloading have significant effect on permeability of coal sample. The coal samples taken from Zhangcun Coal Mine in Shanxi were used to measure coal permeability with different sediment beddings under the effect of loading and unloading by the triaxial coal-rock seepage experimental apparatus. The influence of gas pressure, loading-unloading, and sediment bedding direction on the permeability of coal samples was analyzed, and the functional relationship among these parameters was recovered by fitting. The results show that the permeability of all vertical, dip, and horizontal bedding coal samples decreases exponentially with the increase of effective stress during stress loading, while the permeability of three kinds of bedding coal samples increases exponentially with the decrease of effective stress during stress unloading. Under the same gas pressure, the fracture space of coal samples with vertical and dip bedding is more likely to be compressed and closed at the initial stage of loading, resulting in great decrease of the permeability. In the initial stage of unloading, the microcracks and natural beddings in coals gradually expand and connect. Due to the well development of fractures, the permeability of vertical bedding coal samples increases greatly, while the permeability of dip and horizontal bedding coal samples increases slightly. In the loading process, the permeability of coal sample is vertical bedding > dip bedding > horizontal bedding in order. For the tested samples, the permeability of vertical bedding coal samples is 1.3 to 2.8 times that of dip or horizontal bedding coal samples. In the unloading process, the permeability of the vertical bedding coal samples was 2.8 to 3.3 times that of the dip or horizontal bedding coal samples.

Abstract To evaluate the stability of a prestressed railway embankment (PRE), approximate solutions for the additional normal and shear stresses on an arbitrary potential sliding surface of a PRE are first derived using the elastic theory, by which the propagation effect of additional stresses due to the prestressing loads is appropriately considered. Then, a limit equilibrium slice method incorporating the additional stress effect, equivalent interslice forces, track weight, and train axle load but avoiding repeated iteration calculations is developed to analyze the PRE stability. The additional stress solutions are also introduced to modify three commonly used conventional slope stability analysis methods, and comparative analyses were performed to investigate the applicability of the proposed method. The results showed that the differences between the safety factor (Fs) of a PRE evaluated using the proposed and modified methods were within 2% in this study and the values were less than those yielded by the conventional methods because of the partial negative effect of the additional normal and shear stresses on a few slices around the lateral pressure plate (LPP). Besides, the adaptive genetic algorithm was introduced to search the critical sliding surface of the PRE. Finally, the effect of reinforcement pressure on the stability of a PRE was investigated, where the results indicated that the Fs of the PRE increased from 1.323 to 1.626 with an increase of the reinforcement pressure from 0 to 200 kPa, while the PRE gradually varies from potential integral instability to potential local instability. The accurate and efficient determination of the additional stresses on the sliding surface is important in analyzing the PRE stability, and the developed method can provide an easy-to-use and relatively safer stability analysis method for the design of a PRE.