The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

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VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract Knowledge of the spacing and size of discontinuities in a rock mass is of considerable importance for the prediction of rock behaviour. The characteristics of discontinuities can be estimated using scanline surveys but the precision of the estimates must be obtained and the bias caused by linear sampling must be eliminated before they can validly be used. Initially, an expression is presented which gives the degree of confidence that can be assigned to the measured mean discontinuity spacing. A reduced form of this expression is obtained for cases where the discontinuity spacings follow the negative exponential distribution. The precision of discontinuity frequency and RQD estimates is also explained. The distribution of trace lengths produced by the intersection of planar discontinuities with a planar rock face is used to determine the distribution of trace lengths, the distribution of semi-trace lengths and the distribution of censored semi-trace lengths intersected by a randomly located scanline. Comparison of the actual and sampled distributions demonstrates the bias introduced by scanline sampling of trace lengths. Relations between the distributions can be used to produce analytical or graphical methods of estimating mean trace length from censored measurements at exposures of limited extent.

Estimation discontinuity spacing and trace length using scanline surveys

Application of solid-state fermentation to ligninolytic enzyme production

Opportunities and Challenges in Deep Mining: A Brief Review

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.

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

As coal mining is extended to greater depths, it is found that the mechanical behavior of deep coal differs from the mechanical behavior of shallow coal, resulting in a significant increase in accidents. To explore the characteristics of deformation damage and energy evolution of coal at different depths, coal samples from depths of 300, 600, 700, 850, and 1050 m at the Pingdingshan Coal Mine were acquired. The effects of depth on the in situ stress environment and physical properties of coal were considered, and triaxial compression experiments on 128 coal samples were conducted on this basis. The results show that as the depth rises up, the elastic modulus of coal gradually increases, reaching a maximum of 6.88 GPa in the range of 700–850 m. Poisson’s ratio, the strength, and the volume strain at failure increase nonlinearly, reaching a maximum of 0.48, 129.7 MPa and 0.81%, respectively, at the depth of 1050 m. With increasing depth, the elastic energy and dissipated energy increase more rapidly, and the corresponding peaks of 1050 m are 1.84 times and 3.1 times that of 300 m, respectively. The dissipated energy causes a more significant internal structural change in deep coal, thereby leading to more prominent macroscopic failure in the deep. From the perspective of energy conversion efficiency, it is mainly the accumulation of elastic energy at the pre-peak stage, followed by the release of elastic energy and a rapid increase in dissipated energy at the post-peak stage. The efficiency of accumulation and release of elastic energy decreases significantly in deep coal, while the generation efficiency of dissipated energy increases sharply. These results provide a useful reference for the safe and efficient exploitation of deep coal resources.

Deformation Damage and Energy Evolution Characteristics of Coal at Different Depths

Simultaneous recovery of coal and methane is a promising solution for making reasonable use of energy resources and preventing gas disasters. Coal mining produces an effect that enhances the permeability of coal and thus paradoxically enhances the ability to collect methane. Hence, existing knowledge about permeability enhancement needs to be improved. Therefore, a new parameter, referred to as mining-enhanced permeability, is proposed to characterize the evolution of permeability for coal and other rocks. A theoretical expression for the new parameter is deduced from the Tablet Fluid model. To analyze the mining-enhanced permeability of coal mass quantitatively, a laboratory test was conducted to simulate the stress environment of the protected layers in protected coal seam mining. At the same time, a field test was conducted at a working face of Mine No.10 at the Pingdingshan Coal Mining Group in China. The evolution of the volumetric strain and mining-enhanced permeability for both protective and protected layers were analyzed quantitatively. The loading-induced volumetric strain and permeability enhancement in the laboratory showed improvements of 1.59 and 72.26 %, respectively. The mining-induced volumetric strain and permeability enhancement also showed improvements of 0.14 and 7.77 %, respectively. The evolution of enhanced-permeability agreed with gas flow data observed in both laboratory and in situ tests. Namely, a higher mining-enhanced permeability led to high gas flow, demonstrating the plausibility of mining-enhanced permeability theory based on the Tablet Fluid model. The theory describes the impact of mining on coal permeability and quantitatively captures the evolution of coal permeability. In conclusion, the proposed theory provides a strong foundation for designing systems to simultaneously recover coal and gas.

Theoretical and experimental validation of mining-enhanced permeability for simultaneous exploitation of coal and gas

To comprehensively understand the mining-induced coal permeability change, a series of laboratory unloading experiments are conducted based on a simplifying assumption of the actual mining-induced stress evolution processes of three typical longwall mining layouts in China, i.e., non-pillar mining (NM), top-coal caving mining (TCM) and protective coal-seam mining (PCM). A theoretical expression of the mining-induced permeability change ratio (MPCR) is derived and validated by laboratory experiments and in situ observations. The mining-induced coal permeability variation under the three typical mining layouts is quantitatively analyzed using the MPCR based on the test results. The experimental results show that the mining-induced stress evolution processes of different mining layouts do have an influence on the mechanical behavior and evolution of MPCR of coal. The coal mass in the PCM simulation has the lowest stress concentration but the highest peak MPCR (approximately 4000 %), whereas the opposite trends are observed for the coal mass under NM. The results of the coal mass under TCM fall between those for PCM and NM. The evolution of the MPCR of coal under different layouts can be divided into three sections, i.e., stable increasing section, accelerated increasing section and reducing section, but the evolution processes are slightly different for the different mining layouts. A coal bed gas intensive extraction region is recommended based on the MPCR distribution of coal seams obtained by simplifying assumptions and the laboratory testing results. The presented results are also compared with existing conventional triaxial compression test results to fully comprehend the effect of actual mining-induced stress evolution on coal property tests.

Jing Xie

Papers

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

Deformation Damage and Energy Evolution Characteristics of Coal at Different Depths

Theoretical and experimental validation of mining-enhanced permeability for simultaneous exploitation of coal and gas

Mining-Induced Coal Permeability Change Under Different Mining Layouts

Changes in the structure and mechanical properties of a typical coal induced by water immersion