Discing behavior and mechanism of cores extracted from Songke-2 well at depths below 4,500 m

In situ capture of crack network evolution is extremely difficult but necessary in fundamental research on mining mechanics and coalbed methane extraction. Systematic field monitoring of the mining-induced crack networks near three mining faces with vertical depths of 700 m, 850 m, and 1100 m in the Pingdingshan coal mine in China was conducted. By utilizing fractal geometry and an algorithm that predicts the connectivity rate, the changes in the fractal dimension and connectivity of the mining-induced crack network in these coal-rock masses with mining face advance were determined. The results indicate that the evolution of the crack network can be divided into three stages: a slow linear growth stage, an exponential growth stage, and a stable growth stage. As depth increases, the crack connectivity in a coal mass increases. The ranges of the mining influence zones at mining faces with vertical depths of 700 m, 850 m and 1100 m are approximately 50 m, 60 m and 75 m, respectively. The distances between the maximum mining pressures and the three corresponding coal mining faces are 15 m, 18 m and 25 m, respectively. At a depth of 1100 m, the crack connectivity of the coal-rock mass is 1.36 times that at a depth of 850 m and 1.58 times that at a depth of 700 m. A high crack connectivity can easily arise in thousand-meter-deep mines, which can lead to increased gas emissions and pose risks to production safety at the mining operation face.

Fractal evolution and connectivity characteristics of mining-induced crack networks in coal masses at different depths

The mechanism of microwave rock breaking and its potential application to rock-breaking technology in drilling

It has become an inevitable trend of human development to seek resources from the deep underground. However, rock encountered in deep underground engineering is usually in an anisotropic stress state (σ1>σ>σ3) due to the influences of geological structures and engineering disturbances. It is therefore essential to study the mechanical, seepage, and dynamic disaster behaviors of deep rock under true triaxial stress to ensure the safe operation of deep rock engineering and the efficient exploitation of deep resources. In recent years, experimental techniques and research on true triaxial rock mechanics have achieved fruitful results that have promoted the rapid development of deep rock mechanics; thus, it is necessary to systematically review and summarize these developments. This work first introduced several typical true triaxial testing apparatus and then reviewed the corresponding research progress on rock deformation, strength, failure mode, brittleness, and energy as well as the 3D volumetric fracturing (dynamic disaster) properties of deep rocks under true triaxial stress. Then, several commonly used true triaxial rock strength criteria and their applicability, the permeability characteristics and mathematical models of deep reservoir rocks, and the disaster-causing processes and mechanisms of disturbed volumetric fracturing (rockburst, compound dynamic disasters) in deep rock engineering were described. This work may provide an essential reference for addressing the true triaxial rock mechanics issues involved in deep rock engineering, especially regarding the stability of surrounding rock at depth, disaster prevention and control, and oil and gas exploitation. 

Experimental study on the mechanical and failure behaviors of deep rock subjected to true triaxial stress: A review

During the development of hot dry rock, the research on thermal fatigue damage caused by thermal shock of cold and heat cycles is the basis that ensures the long-term utilization of geothermal resources, but there are not enough relevant studies at present. Based on this, the thermal damage tests of granite at different temperatures (250, 350, 450 °C) and quenching cycles (1, 5, 10, 15 cycles) were carried out. Preliminary reveals the damage mechanism and heat transfer law of the quenching cycle effect on hot dry rock. The results show that with the increase of temperature and cycles, the uneven thermal expansion of minerals and the thermal shock caused by quenching promote the crack development of granite, resulting in the decrease of P-wave velocity, thermal conductivity and uniaxial compressive strength of granite. Meanwhile, the COMSOL was used to simulate the heat transfer of hot dry rock under different heat treatment conditions. It concluded that the increase in the number of quenching cycles reduced the heat transfer capacity of the granite, especially more than 10 quenching cycles, which also reflects that the thermal fatigue damage leads to a longer time for the temperature recovery of the hot dry rock mass. In addition, the three-dimensional nonlinear fitting relationship among thermal conductivity, temperature and cycle number was established for the first time, which can better reveal the change rule of thermal conductivity after quenching thermal fatigue effect of hot dry rock. The research results provide theoretical support for hot dry rock reservoir reconstruction and production efficiency evaluation.

Changes in the thermodynamic properties of alkaline granite after cyclic quenching following high temperature action

During deep mining, the excavation disturbance stress path is the domination factor for the stability of the surrounding rock mass as well as the ground pressure. One of the important parameters of the stress path is the loading or unloading rate of the disturbed rock or coal, which depended on the mining rate. To achieve a well understanding of the mining rate and its effect on the coal behavior, a preliminary case investigation of the mechanical properties of the coal at the various mining rates in both the laboratory scale and field scale was performed. Based on the uniaxial compression test and the digital image correlation (DIC) method, the mechanical behavior of the coal samples, such as the evolution of the strength, surface deformation, crack propagation, and elastic strain energy of the coal under the various loading rates were analyzed. A threshold range of the loading rate has been observed. The uniaxial compressive strength (UCS) and releasable elastic strain energy (Ue) increase with increasing loading rate when the loading rate is below the threshold. Otherwise, the UCS and Ue may decrease with the loading rate. Under the low loading rate (≤0.05 mm/min), the tensile deformation of the original defects could result in crack coalescence, whereas failure of the coal matrix is the key contributor to the crack coalescence under the high loading rate (greater than0.05 mm/min). Afterwards, with the consideration of the bearing capacity (UCS) and energy release of the mining-disturbed coal mass (Ue), a power exponential relationship between the mining rate (MR) in the field and the critical loading rate (vc) in the laboratory was proposed. The application potential of the formulas was then validated against the field monitored data. Finally, based on the critical loading rate, the released strain energy, and the monitored pressure on the roof supports, a reasonable mining rate MR for the Ji15-31030 working face was determined to be approximately 3 m/d.

Bengao Yang

Papers

Mechanical behavior of coal under different mining rates: A case study from laboratory experiments to field testing

The mechanism of microwave rock breaking and its potential application to rock-breaking technology in drilling

Exploration of weakening mechanism of uniaxial compressive strength of deep sandstone under microwave irradiation

Experimental investigation on the gas flow characteristics of coal samples with different fracture network complexities

Fluid flow characteristics of cross-fractures with two branch fractures of different roughness controlled by fractal dimension: An experimental study