Deformation Behavior and Damage-Induced Permeability Evolution of Sandy Mudstone Under Triaxial Stress

Abstract The permeability and mechanical behavior in sandy mudstone are crucial to the hazard prevention and safety mining. In this study, to investigate the evolution and characteristic of permeability and mechanical properties of mudstone during the in-site loading process, triaxial compression-seepage experiments were performed. The increase of permeability and decrease of mechanical strength gradually evaluated to the decrease of permeability and increase of mechanical strength subjected to the increase of confining stress from 5 to 15 MPa, which corresponds to the transformation from brittleness to ductility of mudstone, and the transformation threshold of 10 MPa confining stress was determined. The shear fractures across the sample at brittle regime, while shear fracture does not cross the sample or even be not generated at semibrittle and ductile state. The dynamic decrease, slight decrease, and residual response were determined in axial strain, and the divided zone increases with the increase of confining stress. The relatively higher permeability corresponds to the higher pore pressure as the increase of confining stress. The volumetric strain increases as the increase of confining stress, compared to that decrease correspond to the increase of the pore pressure, and the higher volumetric strain and the lower permeability. In addition, an improved permeability model was developed to describe the loading-based permeability behavior considering the Klinkenberg effect.

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Experimental investigation of the effect of compliant pores on reservoir rocks under hydrostatic and triaxial compression stress states

Canadian Geotechnical Journal ◽

10.1139/cgj-2018-0133 ◽

2019 ◽

Vol 56 (7) ◽

pp. 983-991

Author(s):

Hua Yu ◽

Kam Ng ◽

Dario Grana ◽

John Kaszuba ◽

Vladimir Alvarado ◽

...

Keyword(s):

Pore Pressure ◽

Triaxial Compression ◽

Matrix Material ◽

Volumetric Strain ◽

Confining Pressure ◽

Differential Pressure ◽

Sandstone Sample ◽

Stress States ◽

Rock Springs ◽

Stage 1

The presence of compliant pores in rocks is important for understanding the stress–strain behaviors under different stress conditions. This paper describes findings on the effect of compliant pores on the mechanical behavior of a reservoir sandstone under hydrostatic and triaxial compression. Laboratory experiments were conducted at reservoir temperature on Weber Sandstone samples from the Rock Springs Uplift, Wyoming. Each experiment was conducted at three sequential stages: (stage 1) increase in the confining pressure while maintaining the pore pressure, (stage 2) increase in the pore pressure while maintaining the confining pressure, and (stage 3) application of the deviatoric load to failure. The nonlinear pore pressure – volumetric strain relationship governed by compliant pores under low confining pressure changes to a linear behavior governed by stiff pores under higher confining pressure. The estimated compressibilities of the matrix material in sandstone samples are close to the typical compressibility of quartz. Because of the change in pore structures during stage 1 and stage 2 loadings, the estimated bulk compressibilities of the sandstone sample under the lowest confining pressure decrease with increasing differential pressure. The increase in crack initiation stress is limited with increasing differential pressure because of similar total crack length governed by initial compliant porosity in sandstone samples.

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The effect of pore pressure depletion and injection cycles on ultrasonic velocity and quality factor in a quartz sandstone

Geophysics ◽

10.1190/1.2424887 ◽

2007 ◽

Vol 72 (2) ◽

pp. E43-E51 ◽

Cited By ~ 4

Author(s):

P. Frempong ◽

A. Donald ◽

S. D. Butt

Keyword(s):

Pore Pressure ◽

Effective Stress ◽

Axial Strain ◽

Fluid Pressure ◽

Porous Rocks ◽

Confining Stress ◽

Viscoelastic Deformation ◽

Experimental Conditions ◽

Pore Pressures ◽

Pressure Depletion

Passing seismic waves generate transient pore-pressure changes that influence the velocity and attenuation characteristics of porous rocks. Compressional ultrasonic wave velocities [Formula: see text] and quality factors [Formula: see text] in a quartz sandstone were measured under cycled pore pressure and uniaxial strain conditions during a laboratory simulated injection and depletion process. The objectives were to study the influence of cyclical loading on the acoustic characteristics of a reservoir sandstone and to evaluate the potential to estimate pore-fluid pressure from acoustic measurements. The values of [Formula: see text] and [Formula: see text] were confirmed to increase with effective stress increase, but it was also observed that [Formula: see text] and [Formula: see text] increased with increasing pore pressure at constant effective stress. The effective stress coefficient [Formula: see text] was found to be less thanone and dependent on the pore pressure, confining stress, and load. At low pore pressures, [Formula: see text] approached one and reduced nonlinearly at high pore pressures. The change in [Formula: see text] and [Formula: see text] with respect to pore pressure was more pronounced at low versus high pore pressures. However, the [Formula: see text] variation with pore pressure followed a three-parameter exponential rise to a maximum limit whereas [Formula: see text] had no clear limit and followed a two-parameter exponential growth. Axial strain measurements during the pore-pressure depletion and injection cycles indicated progressive viscoelastic deformation in the rock. This resulted in an increased influence on [Formula: see text] and [Formula: see text] with increasing pore-pressure cycling. The value [Formula: see text] was more sensitive in responding to the loading cycle and changes in pore pressures than [Formula: see text]; thus, [Formula: see text] may be a better indicator for time-lapse reservoir monitoring than [Formula: see text]. However, under the experimental conditions, [Formula: see text] was unstable and difficult to measure at low effective stress.

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The Law of Effective Stress for Rocks in Light of Results of Laboratory Experiments / Prawo Naprężeń Efektywnych Dla Skał W Świetle Wyników Badań Laboratoryjnych

Archives of Mining Sciences ◽

10.2478/v10267-012-0068-4 ◽

2012 ◽

Vol 57 (4) ◽

pp. 1027-1044 ◽

Cited By ~ 1

Author(s):

Andrzej Nowakowski

Keyword(s):

Pore Pressure ◽

Effective Stress ◽

Compression Test ◽

Triaxial Compression ◽

Volumetric Strain ◽

Pore Fluid ◽

Effective Pressure ◽

Test Results ◽

Strength Limit ◽

The Law

Abstract This paper presents the results of laboratory tests carried out in order to formulate effective stress law. The law was sought for two different cases: first - when rock was treated as a porous Biot medium (Biot, 1941; Nur & Byerlee, 1971) and second - when the law was formulated according to definition of Robin (1973) developed by Gustkiewicz (1990) and Nowakowski (2007). In the first case coefficents (4) and (5) of the Biot equation (3) were were determined on the basis of compressibility test, in the second one effective pressure equation (9) and effective pressure value (11) were found on the basis of results of so called individual triaxial compression test (see Kovari et al., 1983) according to the methodology given by Nowakowski (2007). On the basis of Biot coefficients set of values was found that volumetric strain of the pore space described by a coefficient (5) was not dependent on the type of pore fluid and the pore pressure of only, while in case of volumetric strain of total rock described by coefficient (4) both the structure and texture of rock were important. The individual triaxial compression test results showed that for tested rock an effective pressure equation was a linear function of pore pressure as (15). The so called Rebinder effect (Rehbinder & Lichtman, 1957) might cause, that the α coefficient in equation (15) could assume values greater than one. This happened particularly in the case when the porous fluid was non-inert carbon dioxide. In case of inert pore fluid like kerosene the test results suggested that the a coefficient in equation (15) decreased while the differential strength limit was increasing. This might be caused by, so called, dillatancy strengthening (see Zoback & Byerlee, 1975). Another considered important parameter of the equation (15) was the value of the effective press p'. The results showed that the value of this parameter was practically independend on the pore fluid type. This conclusion was contrary to previous research (see, for example, Gustkiewicz et al., 2003 and Gustkiewicz, 1990) so these results should be treated with caution. There are no doubts, however, over p' increasing simultaneously with increase in Rσ1-σ3. Basically, the differential strength limit of the specimen is greater the greater is confining pressure applied to it. Thus, higher Rσ1-σ3 values are accompanied by higher p'.

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A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

Processes ◽

10.3390/pr6080127 ◽

2018 ◽

Vol 6 (8) ◽

pp. 127 ◽

Cited By ~ 6

Author(s):

Dongjie Xue ◽

Jie Zhou ◽

Yintong Liu ◽

Sishuai Zhang

Keyword(s):

Axial Strain ◽

Triaxial Compression ◽

Sudden Change ◽

Volumetric Strain ◽

Confining Pressure ◽

Percolation Model ◽

Triaxial Tests ◽

Transition Behavior ◽

Orthogonal Experiments ◽

Stress Dependent

Modeling the coupled evolution of strain and CH4 seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH4 seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH4 seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH4 seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH4 seepage in coal.

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Mechanical Behavior and Permeability Evolution of Coal under Different Mining-Induced Stress Conditions and Gas Pressures

Energies ◽

10.3390/en13112677 ◽

2020 ◽

Vol 13 (11) ◽

pp. 2677

Author(s):

Zetian Zhang ◽

Ru Zhang ◽

Zhiguo Cao ◽

Mingzhong Gao ◽

Yong Zhang ◽

...

Keyword(s):

Triaxial Compression ◽

Volumetric Strain ◽

Coal Mass ◽

Gas Pressure ◽

Stress Conditions ◽

Permeability Evolution ◽

Compression Tests ◽

Induced Stress ◽

Conventional Triaxial Compression ◽

Gas Pressures

The gas permeability and mechanical properties of coal, which are seriously influenced by mining-induced stress evolution and gas pressure conditions, are key issues in coal mining and enhanced coalbed methane recovery. To obtain a comprehensive understanding of the effects of mining-induced stress conditions and gas pressures on the mechanical behavior and permeability evolution of coal, a series of mining-induced stress unloading experiments at different gas pressures were conducted. The test results are compared with the results of conventional triaxial compression tests also conducted at different gas pressures, and the different mechanisms between these two methods were theoretically analyzed. The test results show that under the same mining-induced stress conditions, the strength of the coal mass decreases with increasing gas pressure, while the absolute deformation of the coal mass increases. Under real mining-induced stress conditions, the volumetric strain of the coal mass remains negative, which means that the volume of the coal mass continues to increase. The volumetric strain corresponding to the peak stress of the coal mass increases with gas pressure in the same mining layout simulation. However, in conventional triaxial compression tests, the coal mass volume continues to decrease and in a compressional state, and there is no obvious deformation stage that occurs during the mining-induced stress unloading tests. The theoretical and experimental analyses show that mining-induced stress unloading and gas pressure changes greatly impact the deformation, failure mechanism and permeability enhancement of coal.

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Experimental Study on Evolution of Rock Damage Rule in the Condition of Conventional Three Axis Compression

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.670-671.432 ◽

2014 ◽

Vol 670-671 ◽

pp. 432-436

Author(s):

Bing He

Keyword(s):

Damage Mechanics ◽

Axial Strain ◽

Triaxial Compression ◽

Volumetric Strain ◽

Continuum Damage Mechanics ◽

Continuum Damage ◽

Conventional Triaxial Compression ◽

Axis Compression ◽

Plastic Volumetric Strain ◽

Damage Rule

Based on continuum damage mechanics, by defining the initial and critical damage value, because of the plastic volumetric strain of the specimen, we established the calculation methods of the rock specimen’s damage value. Through the conventional triaxial compression test of the shale, we find that damage value D changes as axial strain changes during compression, and summarizes its variation.

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Experimental Research into the Evolution of Permeability of Sandstone under Triaxial Compression

Energies ◽

10.3390/en13195065 ◽

2020 ◽

Vol 13 (19) ◽

pp. 5065

Author(s):

Liming Zhang ◽

Shengqun Jiang ◽

Jin Yu

Keyword(s):

Axial Strain ◽

Triaxial Compression ◽

Testing Machine ◽

Volumetric Strain ◽

Confining Pressure ◽

Rate Of Change ◽

Peak Strength ◽

Sudden Increase ◽

Seepage Pressure ◽

Confining Pressures

Failure tests on sandstone specimens were conducted under different confining pressures and seepage pressures by using an MTS triaxial rock testing machine to elucidate the corresponding correlations of permeability and characteristic stress with confining pressure and pore pressure during deformation. The results indicate that permeability first decreases and presents two trends, i.e., a V-shaped increase and an S-shaped trend during the non-linear deformation stage. The greater the seepage pressure, the greater the initial permeability and the more obvious the V-shaped trend in the permeability. As the confining pressure was increased, the trend in the permeability gradually changed from V- to S-shaped. Compared with the case at a high confining pressure, the decrease of permeability occurred more quickly, the rate of change becomes greater, and the sudden increase observed in the permeability happened earlier under lower confining pressures. Within the range tested, confining pressure exerted a greater effect on the permeability than the seepage pressure. In comparison with the axial strain, volumetric strain better reflected changes in permeability during compaction and dilation of sandstone. The ratio of crack initiation stress to peak strength ranged from 0.37 to 0.50, while the ratio of dilation stress to peak strength changed from 0.58 to 0.72. Permeabilities calculated based on Darcy and non-Darcy flow changed within the same interval, while the change in permeability was different.

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Experimental Study on the Deformation and Permeability Characteristics of Raw Coal under the Coupling Effect of Confining Pressure and Pore Pressure

Advances in Materials Science and Engineering ◽

10.1155/2021/9983975 ◽

2021 ◽

Vol 2021 ◽

pp. 1-8

Author(s):

Kangwu Feng ◽

Kequan Wang ◽

Yushun Yang

Keyword(s):

Elastic Modulus ◽

Pore Pressure ◽

Coupling Effect ◽

Axial Strain ◽

Confining Pressure ◽

Gas Pressure ◽

Peak Strength ◽

Permeability Evolution ◽

Permeability Characteristics ◽

Deformation Properties

The effects of confining pressure and pore pressure on the deformation and permeability characteristics of raw coal are studied experimentally. The deformation properties of raw coal by fracture and its permeability evolution laws under the coupling effect of confining pressure and pore pressure were further studied using a tri-axial servo-controlled seepage system for thermo-fluid-solid coupling of methane-bearing coal. The effects of confining pressure and gas pressure on the strength, elastic modulus, and permeability of raw coal were also analyzed. From the results, it was observed that rise in the confining pressure results in reduction of the initial permeability of raw coal and simultaneously increase its strength which results in higher axial deformation upon failure. Rise in gas pressure would increase the permeability and axial strain of raw coal on the whole and reduce its peak strength. Permeability first decreased and then increased during the loading of deviator stress, following a “V-shaped” change pattern. The results of sensitivity analysis indicated that confining pressure more significantly affected the peak strength and elastic modulus than gas pressure, while the gas pressure more significantly affected the permeability of the material than its confining pressure.

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Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

Geofluids ◽

10.1155/2019/1796380 ◽

2019 ◽

Vol 2019 ◽

pp. 1-14

Author(s):

Lu Shi ◽

Zhijiao Zeng ◽

Zhiming Fang ◽

Xiaochun Li

Keyword(s):

Turning Point ◽

Triaxial Compression ◽

Volumetric Strain ◽

Ductile Failure ◽

Confining Pressure ◽

Small Strain ◽

Permeability Evolution ◽

The Maximum Principle ◽

Confining Pressures ◽

Onset Of Dilatancy

Injecting CO2 into a reservoir disturbs the geostress field, which leads to variations in the permeability of caprock and affects its sealing performance. In this paper, the evolution characteristics of the permeability of Yingcheng mudstone were experimentally studied during deviatoric compression under different confining pressures. As the confining pressure increased, the strength of the mudstone increased bilinearly, the angle between the fault and the maximum principle stress increased, and the fault became flatter. During compression, the permeability of mudstone first decreased and then increased and the turning point of the permeability was between the onset of dilatancy and the turning point of volumetric strain; when the fault formed, the permeability increased sharply and the fault-induced increment was reduced exponentially with increasing confining pressure. In addition, the mudstone transformed to the ductile failure mode when the effective confining pressure was greater than 35 MPa, which means that the permeability did not jump within a small strain. Finally, a practical strain-based model of permeability evolution that separately considers compaction and dilatancy was proposed, and the predicted permeability values were in good agreement with the experimental results. This study revealed the effect of confining pressure on permeability evolution during compression and can help evaluate the sealing ability of mudstone caprock.

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Porosity and Permeability Evolution with Deviatoric Stress of Reservoir Sandstone: Insights from Triaxial Compression Tests and In Situ Compression CT

Geofluids ◽

10.1155/2020/6611079 ◽

2020 ◽

Vol 2020 ◽

pp. 1-16

Author(s):

Cong Hu ◽

Franck Agostini ◽

Yonggang Jia

Keyword(s):

Triaxial Compression ◽

Deviatoric Stress ◽

Permeability Evolution ◽

Compression Tests ◽

Confining Stress ◽

Porosity Reduction ◽

Porosity And Permeability ◽

Deviatoric Stresses ◽

Triaxial Compression Tests

Porosity and permeability are the two most important characteristics of underground gas storage in sandstone reservoirs. Injection of gas into reservoir rocks will cause rock deformation. The deformation will influence the porosity and permeability properties of the rocks. We investigate the evolution of these two properties of storage sandstone by triaxial compression tests and a uniaxial in situ compression CT test. As the deviatoric stress increases, the sandstone is compressed firstly (porosity reduction) and then dilates (porosity enhancement). With the increase in confining stress, the occurrence of volumetric dilation will be delayed. Trapped porosity of this sandstone at different deviatoric stresses is very small (0.122%-0.115%) which indicates that nearly all pores are connected. During the compression stage, the decrease in permeability is related to compression of pores and microcracks. During the volumetric dilation stage, it is related to increase in tortuosity. This interpretation can be confirmed by observations of in situ compression CT. The permeability evolution estimated by pore network modeling is consistent with macroscopic testing results.

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