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

Underground space utilization of coalmines in China: A review of underground water reservoir construction

The strength characteristics and progressive failure mechanism of soft rock-coal combination samples with consideration given to interface effects

Brittleness has a significant influence on rock failure under compression; however, the mechanism is rarely comprehensively discussed. This study numerically investigates the brittleness effect on microcracking behavior of crystalline rock using a grain-based model implemented into a two-dimensional particle flow code, with a focus on the discussion of how rock brittleness affects the failure mechanism. The simulated failure mode changes from tension to shear with decreasing rock brittleness, which is consistent with previous laboratory test results. As the brittleness gradually decreases in the model, the grain boundary (GB) tensile crack to shear crack ratio increases, and the corresponding fractures change from vertical or subvertical to an angle about 45° along the vertical direction. The propagation and coalescence of generated microcracks result in a transition of failure pattern from splitting to shear under uniaxial compression with a decreasing brittleness level in the rock. A transition from GB tensile crack to shear crack is also observed under direct tension when the brittleness index gradually decreases. The tension to shear transition mechanism is closely related to the relative strength of the mineral grain and mineral bonding. The relative strength of mineral and mineral bonding could be used as a parameter to characterize rock brittleness from a microscale viewpoint. 

Numerical Investigation of Brittleness Effect on Strength and Microcracking Behavior of Crystalline Rock

Understanding the mechanical properties of coal after gas and coal interaction can better guide the mining technology of gas-bearing coal seams and drainage technology of coalbed methane. In this study, triaxial compression tests of coal mass under different gas adsorption pressures are carried out to evaluate the mechanical properties, impact tendency, and brittleness of coal with different gas adsorption pressures. Results show that due to the adsorption of gas, the mechanical parameters, such as elastic modulus and strength of coal are significantly decreased. In addition to the effect of fluid pressure, the decrease of elastic energy absorbed by coal, the increase of ductile deformation and the change of molecular structure of coal particles caused by gas adsorption also significantly affect the mechanical properties of coal. The influence of gas adsorption pressure on energy input becomes gradually evident when the coal enters the stage of plastic deformation and yield failure. Then the deformation and stress bearing capacity of coal are affected, and the energy accumulation and dissipation process of coal are further affected. The gas adsorption pressure generally develops the energy absorption density and elastic energy density of three characteristic points (initial damage point, volume expansion point and peak stress point), but reduces the dissipation energy density of these points. Gas adsorption reduces the storage of deformation energy of coal, strengthens the energy dissipation, and develops the plastic failure characteristics of coal. It is not prone to appear sudden brittle failure in coal mass. Although high pressure reduces the risk of rock burst, it increases the risk of gas outburst. The chemical effects of pore/fracture fluid caused by the combined action of adsorbed and free gas lead to the further decline in coal brittleness, thus changing the deformation and failure mode of coal. 

https://link.springer.com/content/pdf/10.1007/s40948-022-00439-6.pdf

Experimental investigation of mechanical properties, impact tendency, and brittleness characteristics of coal mass under different gas adsorption pressures

Mechanisms of failure in coal samples from underground water reservoir

Mining disturbances may cause fissure water in the sandstones of a coal seam roof to link up with the coal seam, further causing changes in the moisture conditions of coal–rock systems such as immediate roofs, coal seams, and immediate floors. The varying moisture content influences the mechanical properties and failure precursor symptoms of coal–rock systems. In this study, we carried out uniaxial compressive experiments on coal–rock combinations under dry, natural moisture, and saturated conditions and measured the acoustic emission (AE) and infrared radiation (IR) data. The results show that the combinations can be classified according to the varying water absorption capacities of coal, mudstone, and siltstone with intrusion time. We classified them into a rapid water absorption stage (0 to 2 h), a slow water absorption stage (> 2 to 14 h), and a stable stage (> 14 h). At the same stage, the water absorption capacities can be ranked as coal > mudstone > siltstone. From dry to saturated, the uniaxial compressive strengths, modulus of elasticity, and peak strain of the coal–rock combination specimens is reduced by 37.5, 8.9, and 35.2% respectively. Cracks produced by coal–rock combinations were concentrated mainly on the surface of the coal portions. Before and after the peak failure, the IR temperature of the dry coal–rock combination increases first and then decreases, while the IR temperature of the natural moisture and saturated coal–rock combinations decreases first, then increases, then decreases again. From dry to saturated, the time interval and stress-level interval between the AE precursor point and IR precursor point of the specimens increases by 1.68 times and 4.5 times, respectively. These outcomes provide a useful reference for studying monitoring and early warning of coal and rock interactions and coal and rock pillar instability under the effect of water–rock interaction in engineering practice.

Effects of moisture conditions on mechanical properties and AE and IR characteristics in coal–rock combinations

https://onlinelibrary.wiley.com/doi/epdf/10.1002/ese3.581

Study on the movement characteristics of the overlying stratum and surrounding rock control in ultraclose coal seams: A case study

The invention relates to a grouting anchor cable advanced supporting method, belongs to the field of advanced supporting and aims at solving the problems of insufficient supporting strength and slow supporting speed in the prior art and roof breakage and large worker labor intensity caused by repeated supporting. The advanced supporting method disclosed by the invention comprises the following steps: step 1, determining a mining roadway (1) to be supported on a current working face (3); step 2, determining an advanced supporting distance L according to the advanced supporting pressure distribution law and the engineering geological condition of the mining roadway (1); step 3, adding grouting anchor cable advanced supporting to a roof of the mining roadway (1) to form a roof supporting structure. In a coal mining process, the working face (3) is propelled forwards, the grouting anchor cable advanced supporting is simultaneously and continuously added in the propelling direction, and theadvanced supporting distance is maintained in a safe range. The grouting anchor cable advanced supporting method disclosed by the invention can ensure safe mining and simultaneously improve supporting strength and supporting efficiency and reduce worker labor intensity.

Grouting anchor cable advanced supporting method

© 2020 The Authors. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd. Hydraulic fracturing forms complex hydraulic fracture networks (HFNs) in shale reservoirs and significantly improves the permeability of shale reservoirs. Although rock brittleness is a major factor in determining whether a shale reservoir can be fractured, the relationship between HFNs and rock brittleness remains unclear. To investigate this relationship in a shale reservoir with bedding planes, this paper presents a series of hydraulic fracturing simulations based on a hydromechanically coupled discrete element model. In addition, we analyzed the sensitivity of the difference in rock brittleness to bedding-plane angle and density. The parameters used in the model were verified by comparing the simulated results with experimental results and a theoretical equation. The results showed that breakdown pressure and injection pressure increased with increasing rock brittleness. The tensile hydraulic fracture of a shale reservoir (THFSR) was always the most abundant type of hydraulic fracture (HF)—almost 2.5 times the sum of the other three types of HFs. The distribution of areas with higher fluid pressure deviated from the direction of the maximum principal stress when the angle between the bedding plane and maximum principal stress directions was large. Upon increasing this bedding-plane angle, the breakdown pressure and rock brittleness index first decreased and then increased. However, regardless of bedding angle, the relative proportions of the various types of HFs remained essentially constant, and the seepage area expanded in the direction of the maximum principal stress. Increased bedding-plane density resulted in a gradual increase in the total number of HFs, with significantly fewer of the THFSR type, and the large seepage areas connected with each other. This study thus provides useful information for preparing strategies for hydraulic fracturing. Disciplines Engineering | Science and Technology Studies Publication Details Chong, Z., Yao, Q., Li, X., Zhu, L. & Tang, C. (2020). Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding-planes. Energy Science and Engineering, This journal article is available at Research Online: https://ro.uow.edu.au/eispapers1/3878

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Liu Zhu

Papers

Mechanisms of failure in coal samples from underground water reservoir

Effects of moisture conditions on mechanical properties and AE and IR characteristics in coal–rock combinations

Study on the movement characteristics of the overlying stratum and surrounding rock control in ultraclose coal seams: A case study

Grouting anchor cable advanced supporting method

Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding‐planes