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Dynamic coal reservoir permeability impacts the production of coalbed methane (CBM) and has been extensively studied. Gas (Helium) permeability change was measured under a 4.3MPa confining stress condition for 6 anthracite coal cores from the southern Qinshui Basin. Furthermore, controlling factors of the permeability change were investigated by comprehensively analyzing the effects of gas slippage and effective stress on the permeability. Results show that during gas pressure decline: (1) the permeability initially decreases but subsequently increases, during which the rebound begins at an inlet gas pressure of about 1.9MPa (corresponding to a mean gas pressure of 1.0MPa); (2) gas slippage phenomenon appears as the mean gas pressure falls bellow 1.0MPa; (3) the permeability is approximately negatively related to effective stress; (4) the permeability decreases due to the negative effect from effective stress as the inlet gas pressure is greater than 1.9MPa; while it increases when the pressure falls below 1.9MPa because the positive effect from gas slippage is stronger than the negative effect from effective stress.

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... Yin et al 15 found that the higher the gas pressure is, the shorter the time from the beginning of unloading confining pressure to stop failure, that is, the greater the strength of the coal sample. In terms of effective stress, Li et al 16 analyzed the effects of effective stress and slippage on the gas permeability of coal and rock. Meng 17 and Chen 18 carried out the study on stress sensitivity of permeability. ...

... where, σ 1 is the maximum principal stress, σ 3 is the minimum principal stress, and σ 3 are the maximum and minimum principal stress of coal failure, respectively; M is the strength parameter corresponding to the complete shear failure of uniaxial compression, N is the influence coefficient of confining pressure on axial-bearing capacity, which shows the influence of stress state on the bearing capacity of coal sample, equation (16) shows that the maximum axial stress σ 1 that can be carried by a given coal sample is linear with the confining pressure σ 3 , denoted as: ...

... According to equation (16), the data in Table 3 are analyzed by regression, the curve is shown in Figure 4D. From this, the internal friction angle φ = 43.05°, the cohesion c = 3.4 MPa. ...

In this paper, the seepage tests of mining coal were conducted by servo‐controlled seepage apparatus. These tests consist of conventional triaxial compression seepage tests, and load and unload seepage tests. It was observed that the peak strength and corresponding axial strain of raw coal samples gradually increase with the increase of confining pressure, which conforms to the Mogi‐Coulomb strength criterion, and the internal friction angle was calculated as φ = 42.65°, and the cohesion force was c = 3.56 MPa. The ultimate strength of coal samples after load and unload test was obviously lower than that of the triaxial compression test under the same confining stress conditions, and the deviatoric stress‐permeability curves consistent with the exponential function under two stress paths. In load and unload test, the damage degree of raw coal was characterized by the permeability damage rate and the maximum permeability damage rate. The permeability of coal seam was closely related to the mining stress, it presents a nonlinearly declining as the mining stress increases, and the permeability increases nonlinearly when the mining stress was released.

... They found that the porosity drops faster than permeability with the increase of confining stress. By filling the coal with Helium gas to measure its permeability, Li et al. [12] found that the coal permeability first decreases and then increases with decreasing pore pressure. Furthermore, when the pore pressure is less than 1.9 MPa, the effective stress and gas slippage effects simultaneously control the coal permeability. ...

Deep coalbed methane (CBM) is widely distributed in China and is mainly commercially ex-ploited in the Qinshui basin. The in-situ stress and moisture content are key factors affecting the permeability of CH4-contained coal samples. Therefore, considering the coupled effects of com-pressing and infiltrating on coal permeability could be more accurate to reveal the CH4 gas seepage characteristics in CBM reservoirs. In this study, coal samples that resourced from Tunlan coalmine were employed to conduct the triaxial loading and gas seepage tests. Several findings were drawn: (1) In this triaxial test, the effect of confining stress on the permeability of gas-contained coal samples would be greater than that of axial stress. (2) The permeability ver-sus gas pressure curve of coal presents a ‘V’ shape evolution trend, in which the minimum gas permeability was obtained at a gas pressure of 1.1MPa. (3) The gas permeability of coal samples exponentially decreased with increasing moisture content. Specifically, as the moisture content increasing from 0.18% to 3.15%, the gas permeability was decreased by about 70%. These results are expected to provide a foundation for the efficient exploitation of CBM in Qinshui basin.

In order to accurately predict the production performance of coalbed methane (CBM) wells and to formulate a reasonable production system, this paper established a coal reservoir permeability model considering the influence of pulverized coal blockage. Then, on the basis of this model, the flow velocity sensitivity (FVS) experimental data of 15 groups of coal samples taken from the Baode Block, Qinshui Basin, Liulin Block, Hancheng Block, and the Huanglong Coalfield were fitted to determine the permeability models for different coal samples. On this basis, this newly established permeability model was incorporated into a previously developed CBM well performance analysis software, and production history matching was carried out on two CBM wells. Finally, the effects of the parameters of pulverized coal blockage on the permeability of coal reservoirs and the production performance of CBM wells were studied by taking the fitting parameters of CBM Well W1 as the reference. And the following research results are obtained. First, this new model considering the influence of pulverized coal blockage can quantitatively describe the variation of coal reservoir permeability with fluid velocity. In addition, this model can be incorporated into a CBM numerical simulation software or a CBM well performance analysis software to apply it in a wide range. Second, the coal reservoir permeability is less affected by pulverized coal blockage in the Baode Block, but this effect shall not be ignored in the Qinshui Basin and the Huanglong Coalfield. Third, the greater the theoretical maximum permeability damage degree (Dmax) and the permeability damage degree index (n) are, the lower the relative flow velocity (v0.5) corresponding to the critical flow velocity of pulverized coal blockage is and the more obvious the effect of pulverized coal blockage on coal reservoir permeability is. Fourth, in order to reduce the adverse effect of pulverized coal blockage on coal reservoir permeability, it is suggested to reduce the production pressure difference appropriately in the process of production, especially in the initial stage of gas production, so as to avoid severe damage to coal reservoir permeability.

Based on geological analysis of data of 16 testing wells in the Liupanshui and Zhina coalfields, the spatial distribution of coal reservoir permeability and characteristics of in-situ stress in the western Guizhou are discussed, and the control mechanism of buried depth and in-situ stress for coal reservoir permeability is obtained in this study. It is shown that the coal reservoirs have the characteristic of ultra-low and low permeability (<0.1×10-9 m2), and the permeability of coal reservoir with 0.1×10-9-1.0×10-9 m2 has considerably large proportion. The type of in-situ stress field is gradually undergoing a possible change from dynamic field in shallow layer to hydrostatic pressure field in deep layer. It has a negative power exponent relationship of coal reservoir permeability and buried depth, but the change of permeability is in accordance with in-situ stress field changed. The permeability of coal reservoir varies in different testing wells, decreasing with the increased in-situ stress and depth, and its spatial distribution has a law of “low-high-low” from SW to NE for the intensity of the stress controlled. The role of coal depth to permeability is supposed to be the in-situ stress in action essentially. The main control mechanism of coal permeability difference is that the pore and fracture tend to compress or close caused by the deformation and fragmentation of coal reservoirs under the influence of high in-situ stress in regional tectonic location of study area.

Liulin area is one of the key coalbed methane (CBM) development areas in China. Based on the regional geology analysis, the CBM composition, in-situ stress and geo-temperature field, and reservoir differences were illustrated. Additionally, the CBM well development effect, pumped layers choice, and well drilling technologies were systematically discussed. Results show that, the CH4 content, Langmuir volume and gas saturation of Shanxi Formation are higher than those of the Taiyuan Formation. In-situ stress and geo-temperature field show vertical belting, and the 700m to 850m depth are the minimum horizontal principal stress. The coal properties are different from each other, and the hydrodynamic condition contributes much to the reservoiring difference. Vertical wells which penetrate coals in Shanxi Formation show better gas rate, while in Taiyuan Formation they are of higher water rate. Double-step and multi-branch horizontal wells were adopted, resulting in the full use of coals and better gas rate. Two cycles are proposed, including the inner cycle composed of in-situ stress, geo-temperature and reservoir pressure, and the outer cycle of permeability, gas rate and water rate. During CBM production, the inner cycle stimulates the outer cycle, while the reservoir reconstruction promotes the capability release and well production system maintains production balance.

When gas flows in low permeability porous media, the effect of gas slippage cannot be ignored. It takes many times to obtain gas permeability under different pore pressure of low permeability core with conventional method by linear fitting to get intrinsic permeability and slip factor. Based on the relationship between intrinsic permeability and slip factor, a new unsteady gas permeability experiment method was proposed. The principle of the new method is to fit the inlet pressure decay curve tested and calculated that can be obtained by solving unsteady gas flow equation considering slippage effect according to least square curve fitting, then the wanted intrinsic permeability and slip factor can be acquired and verified through conventional experiment. Compared with steady state experiment, this new unsteady method is more correct, and it greatly shortens the time, and simplifies the experiment process.

In China the coal seam commonly belongs to the hypotonic reservoir, especially the neopaleozoic coal seam with permeability smaller than 1 millidarcy approximately accounts for 70 percent, among which the coal-bed methane resource with the burial 1000 m in depth amounts to 2.95 myriad hundred million tons, accounting for 53 percent of coal resources amount. Large amount of coal-bed methane resources keeps in storage under the depth of 1000 m. Under such a depth in the deep mining, the pressure of the coal-bed is considerably high, and the porous media is regarded to be highly compacted. Considering the gas seepage in this compaction porous medium, the gas slippage effect is remarkable. The researches on gas slippage effects has the important significance to reveal the gas seepage mechanism through the hypotoniccoal reservoir. The experimental studies are carried out to reveal the seepage laws of Tiefa, Huaxing and Jincheng coal samples, to analyze the impact of confining pressure and pore pressure on the gas slippage effect of the hypotonic coal samples, and to get new theoretical formula for the gas slippage effects of the hypotonic coal samples. A foundation is established to understand the internal structure of coal and coal-bed methane slippage flow in coal seams. The experiment results have theoretically significant values to carry out industrialized exploitation of coal-bed methane in hypotonic coal reservoir.

Effective stress law of all kinds of coal samples, including steam coal, fat coal, corking coal, thin coal and anthracite, under pore pressure of gas, is experimentally studied using a newly developed test machine. These samples are taken from Coal Mines in Wuda, Hebi, Yanzhou, Yangquan, Qingshui, and Gujiao in China. The experiment results show that, under pore pressure of gas, the tested coal samples comply with Biot’s effective stress law,
$${\sigma '}_{ij} = {\sigma}_{ij} - {\alpha}p{\delta}_{ij}$$
where the Biot’s coefficient a is not a constant, and is bilinear function of volumetric stress (T) and pore pressure (p), that is,
$$\alpha = a_1 + a_2 \Theta + a_3 p + a_4 \Theta p$$
We define four areas according to the numerical feature of a, that is, functionless area of pore pressure, normal function area, fracturing function area, and quasi-soil function area. The effective stress law of coal mass introduced by this paper is a constitutive equation in the study of coupled solid and fluid. This has significance in the drainage and outburst of methane in coal seam.

In this paper, second of the two-part series, pressure-permeability trend established in the laboratory was compared with modeled results using two analytical models, Shi and Durucan and Palmer et al, developed to predict changes in permeability of coalbed methane reservoirs with continued production. The experimental results did not match the modeled results very well for either of the two models. Also, there appeared to be several uncertainties, primarily the constancy of cleat compressibility and estimation of certain input parameters. A modification of the Shi and Durucan model was then proposed to increase the impact of the shrinkage effect on permeability. With this modification, the measured and modeled results matched perfectly, with the modeled permeability of coal increasing continuously, just as observed in the laboratory and supported by field data, and the rate of increase accelerating at low pressures. The phenomenon was explained by analyzing the permeability as a function of reservoir pressure.