Experimental of Mechanical Properties and Gas Flow of Containing-Gas Coal Under Different Unloading Speeds of Confining Pressure

AbstractBased on self-developed “Triaxial Stress Thermal-hydrological-mechanical Coal Gas Permeameter”, the experiment of the influence of different unloading speeds on mechanism and gas seepage features of containing gas coal is carried out, with the condition of initial confining pressure 6MPa and gas pressure 1.0MPa. It shows that: with the increasing of unloading speed of confining pressure, the time of coal maintaining at a stress plateau phase reduces, and it tends to 0, and the easier coal is to be fail After the start of unloading confining pressure, the time of coal maintaining in the stage of stress plateau showed a power function relation with unloading speed of confining pressure. In this study, the speed can be classified into three regions in text: the low speed region of unload confining pressure, the intermediate speed region and the high speed region. In the three speed regions, the confining pressure, when coal fails, is lowest in the low unloading speed of confining pressure, middle in the intermediate unloading speed region, and highest in the high unloading speed region

Stress and Damage Induced Gas Flow Pattern and Permeability Variation of Coal from Songzao Coalfield in Southwest China

The permeability of coal is a critical parameter in estimating the performance of coal reservoirs. Darcy’s law describes the flow pattern that the permeability has a linear relationship with the flow velocity. However, the stress induced deformation and damage can significantly influence the gas flow pattern and permeability of coal. Coals from Songzao coalfield in Chongqing, southwest China were collected for the study. The gas flow velocities under different injection gas pressures and effective stresses in the intact coal and damaged coal were tested using helium, incorporating the role of gas flow pattern on the permeability of coal. The relationships between the flow velocity and square of gas pressure gradient were discussed, which can help us to investigate the transformation conditions of gas linear flow and gas nonlinear flow in the coal. The results showed that the gas flow in the intact coal existed pseudo-initial flow rate under low effective stress. The low-velocity non-Darcy gas flow gradually occurred and the start-up pressure gradient increased in the coal as the effective stress increased. The gas flow rate in the damaged coal increased nonlinearly as the square of pressure gradient increased under low effective stress. The instability of gas flow caused by high ratio of injection gas pressure over effective stress in the damaged coal contributed to the increase of the gas flow rate. As the effective stress increased, the increase of gas flow rate in coal turned to be linear. The mechanisms of the phenomena were explored according to the experimental results. The permeability of coal was corrected based on the relationships between the flow velocity and square of gas pressure gradient, which showed advantages in accurately estimating the performance of coal reservoirs

Numerical simulation of gas migration into mining-induced fracture network in the goaf

Gas extraction practice has been proven for the clear majority of coal mines in China to be unfavorable using drill holes in the coal seam. Rather, mining-induced fractures in the goaf should be utilized for gas extraction. To study gas migration in mining-induced fractures, one mining face of 10th Mine in Pingdingshan Coalmine Group in Henan, China, has been selected as the case study for this work. By establishing the mathematical model of gas migration under the influence of coal seam mining, discrete element software UDEC and Multiphysics software COMSOL are employed to model gas migration in mining-induced fractures above the goaf. The results show that as the working face advances, the goaf overburden gradually forms a mining-induced fracture network in the shape of a trapezoid, the size of which increases with the distance of coal face advance. Compared with gas migration in the overburden matrix, the gas flow in the fracture network due to mining is far greater. The largest mining-induced fracture is located at the upper end of the trapezoidal zone, which results in the largest gas flux in the network. When drilling for gas extraction in a mining-induced fracture field, the gas concentration is reduced in the whole region during the process of gas drainage, and the rate of gas concentration drops faster in the fractured zone. It is shown that with gas drainage, the gas flow velocity in the mining-induced fracture network is faster. Keywords: Gas migration, Fractures, Mining-induced, Numerical simulatio

Failure Criteria of Gas-Infiltrated Sandy Shale Based on the Effective Stress Principle

Pore gas has a significant influence on rock strength. This study performed triaxial compression tests of gas-infiltrated sandy shale samples to investigate the strength characteristics under gas pressures of 0 and 2 MPa. The effective stress coefficient was evaluated while considering the gas and solid coupling effect, and was found to decrease with increasing confining pressure. The calculated and different assumed coefficient values (0 and 1) were applied to obtain the effective principal stress. The experimental results would serve as fundamental strength data for fitting analysis in failure criterion work. The Mohr-Coulomb, Hoek-Brown, Drucker-Prager, linear Mogi, and non-linear Mogi criteria were modified based on the effective stress principle of porous rock. In addition, the RMSE, cohesion, and internal friction angle were utilized for a quantitative criterion comparison. The results showed that the Mohr-Coulomb, Drucker-Prager, and linear Mogi failure criteria led to higher errors, whereas the Hoek-Brown criterion gave an apparent distortion as a result of the empirical strength parameters. Moreover, the non-linear Mogi criterion showed a good fit. The predicted strength was overestimated when α = 0 and underestimated when α = 1, with a more accurate strength estimated when the effective stress coefficient was calculated using the effective stress principle

Effects of Mixing Conditions on Floc Properties in Magnesium Hydroxide Continuous Coagulation Process

Magnesium hydroxide continuous coagulation process was used for treating simulated reactive orange wastewater in this study. Effects of mixing conditions and retention time on the coagulation performance and floc properties of magnesium hydroxide were based on the floc size distribution (FSD), zeta potential, and floc morphology analysis. Floc formation and growth in different reactors were also discussed. The results showed that increasing rapid mixing speed led to a decrease in the final floc size. The floc formation process was mainly carried out in a rapid mixer; a rapid mixing speed of 300 rpm was chosen according to zeta potential and removal efficiency. Reducing retention time caused a relatively small floc size in all reactors. When influent flow was 30 L/h (retention time of 2 min in rapid mixer), the average floc size reached 8.06 μm in a rapid mixer; through breakage and re-growth, the floc size remained stable in the flocculation basin. After growth, the final floc size reached 11.21 μm in a sedimentation tank. The removal efficiency of reactive orange is 89% in the magnesium hydroxide coagulation process

Strain Field Evolution and Constitutive Model of Coal considering the Effect of Beddings

AbstractIn order to study the bedding effect of coal and rock deformation, the surface deformation fields of coal and rock at different bedding angles were obtained by means of digital image correlation (DIC). By optimizing the statistical index describing the nonuniformity of strain field, the initiation and evolution characteristics of deformation localization were analyzed quantitatively. The concepts of equivalent cohesive force and equivalent tensile strength were put forward, and a transverse isotropic constitutive model of coal and rock was established. The results show that the maximum shear deformation field of uniaxial compressed coal and rock in parallel bedding is more likely to show the three-stage characteristics of uniform stage, localization stage, and failure stage. In the vertical and parallel bedding directions, the modified statistical index curve of coal and rock in the process of uniaxial compression shows typical two-stage characteristics. When loading perpendicular to bedding, the starting stress of deformation localization of coal and rock is closer to the peak strength, and when loading parallel to bedding, coal and rock are easier to show localization characteristics

Brittleness index evaluation of gas-bearing sandstone under triaxial compression conditions

Abstract Brittleness is a critical property in gas-bearing rock excavation and reservoir fracturing. This paper analyzes the three types of brittleness indices in detail, proposes new indices, and conducts relevant conventional triaxial tests for analysis. A stress–strain curve-based brittleness index would be appropriate for the evaluation of brittleness if the parameters represented unique and certain stress–strain curves. Therefore, a new brittleness index is proposed. It consists of two components: the stress variation index and the strain variation index, which reflects the strain ratio of the stress drop and the relationship between elastic and post-peak strains, combining the stress variation and strain variation in the pre- and post-peak phases. Triaxial compression tests of sandstone under different confining pressures and gas pressures were conducted, and specific stress and strain parameters were collected. Based on the comparison and analysis of various brittleness indices, it is found that the newly proposed brittleness index can be a good measure of the brittle evolution of sandstones under different confining pressures and gas pressures, and it can provide a more accurate estimate of rock brittleness than the existing indices. The index is also used to further investigate the relationship between rock brittleness and confining and gas pressures. This has important implications for the assessment of rock brittleness in practical engineering