scholarly journals Experimental Study on Mechanics and Permeability Properties of Water-Bearing Raw Coal Samples Under In-Situ Stress

2019 ◽  
Vol 9 (12) ◽  
pp. 2549
Author(s):  
Zhang ◽  
Wang ◽  
Wang ◽  
Zhang
Keyword(s):  
Axial Strain ◽  
Radial Strain ◽  
Quadratic Function ◽  
Confining Pressure ◽  
In Situ Stress ◽  
Peak Strain ◽  
Permeability Characteristics ◽  
Loading And Unloading ◽  
Unloading Confining Pressure

In this paper, we investigated the mechanical and permeability characteristics ofwater-bearing raw coal samples under in-situ stress, and came to some conclusions, as follows: thepeak strength and peak axial strain of samples gradually decrease with the increase of watercontent. Under the same stress condition, the higher the water content is, the lower the axial strainand radial strain will be. The peak strength and peak strain of the sample both decrease with thewater content as a quadratic function. During the post-peak loading and unloading process, withthe increase of the number of cycles of loading and unloading, the radial strain decrement andincrement of the raw coal sample gradually decrease after loading and unloading confiningpressure. The permeability of samples gradually decreases with the loading confining pressure,and the permeability of the sample gradually increases with the unloading confining pressure. Thepermeability of coal samples increases volatility with the increase of axial strain, and the fittedsample permeability and effective stress are subject to the ExpDec1 function distribution.

scholarly journals Experimental study on permeability characteristics of gas-containing raw coal under different stress conditions

2018 ◽  
Vol 5 (7) ◽  
pp. 180558 ◽  
Author(s):  
Dongming Zhang ◽  
Yushun Yang ◽  
Hao Wang ◽  
Xin Bai ◽  
Chen Ye ◽  
...  
Keyword(s):  
Flow Rate ◽  
Coal Sample ◽  
Axial Stress ◽  
Axial Strain ◽  
Confining Pressure ◽  
Gas Pressure ◽  
Permeability Characteristics ◽  
Loading And Unloading ◽  
Unloading Confining Pressure ◽  
Function Relation

The present experimental study on permeability characteristics for raw coal under different stress states is implemented by applying the triaxial self-made ‘THM coupled with servo-controlled seepage apparatus for gas-containing coal’; the result indicates that the flow rate of gas in the coal sample gradually decreases with the nonlinear loading of axial pressure and increases with the nonlinear unloading of axial stress and confining pressure. The flow rate, axial stress and confining pressure curves all satisfy the negative exponential function relation. When the sample reaches the peak intensity, the sample will be destroyed and the stress will drop rapidly; then the flow rate of the sample will increase rapidly. At this stage, the flow rate and axial strain show an oblique ‘v' pattern. The flow rate of the coal sample increases nonlinearly with the increase of gas pressure; the relation curve between flow rate and gas pressure satisfies the power function relation. Under the same confining pressure and gas pressure conditions, the larger the axial stress, the smaller the flow rate of the coal sample. Under the same axial stress and gas pressure conditions, the flow rate of the coal sample will first decrease, but then increase as the confining pressure decreases. During the post-peak loading and unloading process, the flow rate of the coal sample will decrease with the loading of confining pressure but increase with the unloading of confining pressure, and there will be an increase in wave shape with the increase in axial strain. The flow rate of each loading and unloading confining pressure is higher than that of the previous loading and unloading confining pressure. At the post-peak stage, the relation curve between the flow rate of the coal sample and the confining pressure satisfies the power function relation in the process of loading and unloading confining pressure.

scholarly journals Deformation Failure and Gas Seepage of Raw Coal in Alternate Loading and Unloading by Stages

2021 ◽  
Author(s):  
Bang-an Zhang ◽  
Yang yushun
Keyword(s):  
Axial Stress ◽  
Axial Strain ◽  
Radial Strain ◽  
Confining Pressure ◽  
Change Rate ◽  
Initial Stage ◽  
Strain Change ◽  
Loading And Unloading ◽  
Unit Stress ◽  
Unloading Confining Pressure

Abstract In this paper, the cyclic loading and unloading confining pressure tests of raw coal samples were carried out by using the "Triaxial seepage test device of thermal fluid solid of coal and rock" developed by Chongqing University. The conclusions are as follows: (1) The axial strain change rate ε1´, the radial strain change rate ε3´ and the permeability change rate k´ under unit stress state are used to represent the sensitivity of axial stress and confining pressure to deformation and permeability characteristics of samples under unit stress state. (2) At the initial stage of unloading the confining pressure, the confining pressure has a greater influence on the permeability of the sample. At the initial stage of loading confining pressure, the confining pressure has a greater influence on the radial strain of the specimen. During the subsequent loading and unloading process, the confining pressure of loading and unloading has a greater influence on the permeability of the sample, and a smaller influence on the axial strain. The loading axial stress has a greater influence on the axial strain of the sample, and a smaller influence on the permeability of the sample. (3) When the axial stress is constant, the increase range of sample permeability increases with the increase of unloading confining pressure range, and the decreasing range of sample permeability increases with the increase of loading confining pressure range, and the increase range of sample permeability under unloading confining pressure is higher than that under increasing confining pressure. (4) In the process of loading axial stress and loading confining pressure, the permeability of samples decreases nonlinearly with the increase of principal stress difference, while the permeability of samples increases nonlinearly with the decrease of principal stress difference in the process of unloading confining pressure.

scholarly journals Effect of wet-dry cycles on disintegration characteristics of clay-bearing sandstone with UF cracks

2020 ◽  
Author(s):  
Yu Wang ◽  
Qingning Qiao ◽  
Jianlin Li
Keyword(s):  
Confining Pressure ◽  
In Situ Stress ◽  
Three Gorges Reservoir Area ◽  
Test Results ◽  
Rock Samples ◽  
Pressure Experiment ◽  
Size Uniformity ◽  
In Situ Stress Field ◽  
Unloading Confining Pressure

UF cracks in rock masses commonly occur due to the unloading effect, which constantly happens after the variation of in-situ stress field or rock excavation. When undergoing periodic water fluctuation, rock mass with UF cracks is vulnerable to deterioration or even disintegration, especially for clay-bearing sandstone. To study the effect of changes in moisture on rock samples with UF cracks, clay-bearing sandstone from the Triassic Badong group in the Three Gorges Reservoir Area were chosen and investigated. The rock samples with UF cracks are obtained by conducting triaxial unloading confining pressure experiment. The effect of wet-dry cycles on the morphology properties and microstructure of the UF surface was investigated. The characteristics of particle-size uniformity from the sieve test were obtained by the calculation of RMS of particle contents. The test results show that UF cracks widen significantly and the disintegrated mass increases rapidly in the first three wet-dry cycles, while the fractal dimension of UF surface decreases sharply, but afterwards the disintegrated mass changes gently and the UF surface tends to be flat and smooth. Then, the RMS calculation of particle contents quantitatively evaluate the clay-bearing sandstone’s disintegration properties, which indicate the particle uniformity plays a key role on its disintegration mechanism. During wet-dry cycles, the tested samples tend to disintegrate more rapidly and entirely with the decrease of particle uniformity.

scholarly journals Experimental Study on Mechanical Properties and Seepage Laws of Raw Coal under Variable Loading and Unloading Rates

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-9
Author(s):  
Kangwu Feng ◽  
Kequan Wang ◽  
Dongming Zhang ◽  
Yushun Yang
Keyword(s):  
Axial Stress ◽  
Axial Strain ◽  
Radial Strain ◽  
High Sensitivity ◽  
Confining Pressure ◽  
Variable Loading ◽  
Axial Pressure ◽  
Unloading Rate ◽  
Loading And Unloading ◽  
Seepage Characteristics

This manuscript studied the effects of variable axial pressure loading rate and variable confining pressure unloading rate on the deformation behavior and seepage characteristics of raw coal under alternate loading and unloading of axial pressure and confining pressure. It believed that as axial stress increases, axial strain ε 1 decreases, radial strain ε 3 increases, and permeability k decreases, and ε 1 ′ , ε 3 ′ , and k ′ increase when confining pressure is decreases. With the loading of axial stress and the unloading of confining pressure, the variation amplitudes of ε 1 ′ , ε 3 ′ , and k ′ values reduce gradually. During axial stress loading, the rise in the amplitude of ε 1 is larger than that of ε 3 and the reduction in the amplitude of k , indicating that ε 1 is more sensitive to axial stress than ε 3 and k . During unloading of confining pressure, the increase rate of ε 3 is larger than that of ε 1 and k ; also, ε 3 showed a high sensitivity to confining pressure. In the stage of axial stress loading and confining pressure unloading, the evolution law of deformation and permeability parameters is basically consistent with the change in loading and unloading rate.

scholarly journals Hydraulic properties and energy dissipation of deep hard rock under H-M coupling and cycling loads

2019 ◽  
Vol 23 (Suppl. 3) ◽  
pp. 935-942 ◽  
Author(s):  
Cheng-Han Zhang ◽  
Shuang You ◽  
Hong-Guang Ji ◽  
Fei Li ◽  
Hong-Tao Wang
Keyword(s):  
Acoustic Emission ◽  
Osmotic Pressure ◽  
Hydraulic Properties ◽  
Coupling Effect ◽  
High Stress ◽  
Confining Pressure ◽  
In Situ Stress ◽  
Dissipated Energy ◽  
Underground Engineering ◽  
Loading And Unloading

The permeability of deep rock is closely related to the stability and safety of underground engineering. The rocks in deep stratum are mostly with high stress and high osmotic pressure. Therefore, it is necessary to consider the coupling effect between porewater pressure and in situ stress on rock mass. A series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling conditions are carried out to studied the mechanical and hydraulic properties of granite in the depth of 1300 m to 1500 m. Especially, the effect of the disturbance on the permeability of fractured rocks are investigated by unloaded the confining pressure. Tests results presented that the stress-strain curves of deep granite showed typical brittle characteristics. The principal stress of granite exhibited a linear relationship under the high confining pressure of 34-40 MPa and high osmotic pressure of 13-15 MPa. Dissipated energy of the rock decreased to a relatively low level after 2-3 loading cycles and then slowly increased. Permeability showed a decreasing trend as the loading and unloading cycles increase. Finally, acoustic emission technology was used to monitor the fracture evolution in rocks, the acoustic emission signal released as the fractures develop and energy dissipated. The results would provide basic data for the exploitation and excavation in the deep galleries.

scholarly journals Failure and Seepage Characteristics of Gas-Bearing Coal under Accelerated Loading and Unloading Conditions

2019 ◽  
Vol 9 (23) ◽  
pp. 5141
Author(s):  
Zhang ◽  
Wang ◽  
Du ◽  
Lou ◽  
Wang
Keyword(s):  
Confining Pressure ◽  
In Situ Stress ◽  
Permeability Evolution ◽  
Permeability Model ◽  
Significant Control ◽  
Gas Dynamic ◽  
Working Face ◽  
Loading And Unloading ◽  
Seepage Characteristics ◽  
Gas Bearing

In actual mining situations, the advancing speed of the working face is usually accelerated, which may affect the failure and seepage characteristics of gas-bearing coal, and may even induce dynamic disasters. In order to discover the effects of such accelerated advancement of the working face, an experimental study on the failure and seepage characteristics of gas-bearing coal under accelerated loading and unloading conditions was carried out in this work. The results showed that the energy release was more violent and impactful under accelerated loading and unloading paths. The time required for the failure of the sample was significantly shortened. After being destroyed, the breakup of the sample was more severe, and the magnitude of the permeability was greater. Accordingly, the acceleration of the loading and unloading had significant control effects on the failure and permeability of coal and it showed a significant danger of inducing coal and gas dynamic disasters. Meanwhile, the degree of influence of the acceleration on the coal decreased with an increase in the gas pressure and increased significantly with an increase in the initial confining pressure. It was found that for a deep high-gas mine, the accelerated advancement of the working face under a high in situ stress condition would greatly increase the risk of coal and gas dynamic disasters. Then, the permeability evolution model of gas-bearing coal in consideration of changes in the loading and unloading rates was theoretically established in this work, and this permeability model was validated by experimental data. The permeability model was found to be relatively reasonable. In summary, the effects of accelerated loading and unloading on the failure and seepage characteristics of gas-bearing coal were obtained through a combination of experimental and theoretical studies, and the intrinsic relationship between the accelerated advancement of the working face and the occurrence of coal and gas dynamic disasters was discovered in this work.

scholarly journals Effect of Stress and Moisture Content on Permeability of Gas-Saturated Raw Coal

Geofluids ◽  
2020 ◽  
Vol 2020 ◽  
pp. 1-13
Author(s):  
Junhui Wang ◽  
Zhijun Wan ◽  
Yi Wang ◽  
Zhixiang Liu ◽  
Sifei Liu ◽  
...  
Keyword(s):  
Moisture Content ◽  
Coalbed Methane ◽  
Confining Pressure ◽  
Porous Matrix ◽  
In Situ Stress ◽  
Permeability Evolution ◽  
Test Range ◽  
Moisture Contents ◽  
Volumetric Stress

Hydraulic fracturing and premining gas drainage are important to safe mining and coalbed methane extraction. These technical processes cause the redistribution of in-situ stress and the regional variation of moisture contents within the affected zone. Therefore, we investigated the coupled effect of variable stresses (from 9 MPa to 27 MPa) and moisture contents (from 0.22% to 4.00%) on the permeability evolution of gas-saturated raw coal. The results show that (1) the relationship between the mean effective stress and the permeability can be described by a power function according to the permeability evolution model of the porous matrix established in this study. Besides, the influence mechanisms of moisture on fitting coefficients in the function were analyzed. (2) The permeability decreases with the increase of in-situ stress (e.g., confining pressure or volumetric stress) in a negative exponential manner. (3) The curves of permeability variations with moisture content are not always linear, and the permeability is more sensitive to the moisture content than the volumetric stress in the test range. Moreover, the sensitivity of permeability varies in different regions. These results would be beneficial for permeability prediction and surface well parameters design.

Laboratory evaluation of K0 for overconsolidated clays

1991 ◽  
Vol 28 (5) ◽  
pp. 650-659 ◽  
Author(s):  
Vinod K. Garga ◽  
Mahbubul A. Khan
Keyword(s):  
Axial Stress ◽  
Axial Strain ◽  
Stress Path ◽  
Horizontal Stress ◽  
In Situ Stress ◽  
Laboratory Method ◽  
Laboratory Evaluation ◽  
Triaxial Apparatus ◽  
Overconsolidated Clays

Most of the laboratory testing methods available for the evaluation of in situ horizontal stresses are applicable to normally consolidated or lightly overconsolidated clays. This paper describes a new laboratory method for the determination of in situ horizontal stresses of heavily overconsolidated clays using a stress-path triaxial apparatus. The proposed method is based on the concept that if the radial stress exceeds the in situ horizontal stress, while maintaining the axial stress constant and equal to the in situ vertical effective stress, only then will the sample experience significant axial strain. The results obtained for undisturbed samples of an overconsolidated clay crust are found to be in agreement with some available methods. For verification of the applicability of the proposed method, K0 was determined for artificially prepared samples that had been subjected to known stress paths simulating field stress history. Key words: K0, overconsolidation, in situ stress, in situ test, clay crust, laboratory test.

Research on Stress Sensitivity of Low Permeability Sand Stone Reservoir

2011 ◽  
Vol 317-319 ◽  
pp. 2432-2435
Author(s):  
Yu Xue Sun ◽  
Fei Yao ◽  
Jing Yuan Zhao
Keyword(s):  
Plastic Deformation ◽  
In Situ Stress ◽  
Stress Sensitivity ◽  
Oil Field ◽  
Low Permeability ◽  
High Permeability ◽  
Permeability Changes ◽  
Loading And Unloading ◽  
Sandstone Reservoir

In the process of low-permeability sandstone reservoir exploitation, stress sensitivity takes place with the effective stress rises gradually, which will cause permeability decline. Allowing to the condition of in-situ stress, the study and experiment on the rock core in Jilin oil field Fuxin326 oil layer are presented. The experimental results show that the stress sensitivity of this oil layer is small; the regularity of permeability changes is in accordance with exponential function. The stress sensitivity of high permeability core is larger than that of low permeability core. Moreover, experimental and theoretical analysis shows that low permeability core has a larger permeability loss than high permeability core in loading and unloading process where elastic plastic deformation of rock will happen, which is the major reason that permeability loss can not return completely.

scholarly journals Experimental Study on Seepage Characteristics of Fractured Rock Mass under Different Stress Conditions

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Ma Haifeng ◽  
Yao Fanfan ◽  
Niu Xin’gang ◽  
Guo Jia ◽  
Li Yingming ◽  
...  
Keyword(s):  
Mechanical Behavior ◽  
Permeability Coefficient ◽  
Fractured Rock ◽  
Axial Strain ◽  
Strain Curve ◽  
Confining Pressure ◽  
Strain Effect ◽  
Permeability Evolution ◽  
Hoop Strain ◽  
Permeability Characteristics

In order to obtain the mechanical behavior and permeability characteristics of coal under the coupling action of stress and seepage, permeability tests under different confining pressures in the process of deformation and destruction of briquette coal were carried out using the electrohydraulic servo system of rock mechanics. The stress-strain and permeability evolution curves of briquette coal during the whole deformation process were obtained. The mechanical behavior and permeability coefficient evolution response characteristics of briquette coal under stress-seepage coupling are well reflected. Research shows that stress-axial strain curve and the stress-circumferential strain curve have the same change trend, the hoop strain and axial strain effect on the permeability variation law of basic consistent, and the permeability coefficient with the increase of confining pressure and decreases, and the higher the confining pressure, the lower the permeability coefficient, the confining pressure increases rate under the same conditions, and the permeability coefficient corresponding to high confining pressure is far less than that corresponding to low confining pressure. The confining pressure influences the permeability of the briquette by affecting its dilatancy behavior. With the increase of the confining pressure, the permeability of the sample decreases, and the permeability coefficient decreases with the increase of the confining pressure at the initial stage, showing a logarithmic function. After failure, briquette samples show a power function change rule, and the greater the confining pressure is, the more obvious the permeability coefficient decreases.

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