Influence of loading and unloading velocity of confining pressure on strength and permeability characteristics of crystalline sandstone

Zhang, Dongming; Yang, Yushun; Chu, Yapei; Zhang, Xiang; Xue, Yusheng

doi:10.1016/j.rinp.2018.04.043

Cited by 33 publications

(16 citation statements)

References 12 publications

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“…Under the conventional triaxial compression, σ 2 = σ 3 , Equations of nominal elastic modulus E and nominal Poisson's ratio µ for the deformation parameter of rocks [28,29] are formulated below by substituting σ 2 = σ 3 , into Equation (1), that is:…”

Section: Evolution Analysis Of Deformation Parametermentioning

confidence: 99%

Elastic-Plastic Threshold and Rational Unloading Level of Rocks

Guo

2019

Applied Sciences

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During loading and unloading test, various rocks manifest different stress values of elastic-plastic transformation. This study proposes to include axial pressure increment ratio in the conventional triaxial compression test to evaluate different variables (nominal elastic modulus, nominal Poisson's ratio, strain, and energy). The relationships among various factors including variables, the stress level of initial confining stress and axial pressures, were analyzed by analyzing the stress-strain plot record obtained from testing various rocks. The extreme value point of the deformation parameter, also known as the elastic-plastic threshold, was analyzed. In addition, the elastic-plastic thresholds were later used as unloading points during the unloading tests. Under the same confining condition, different rocks demonstrated different unloading levels. Furthermore, a linear correlation was observed between unloading levels and changing confining pressures, and the gradient is mainly related to the types of rocks. During the unloading tests of rocks, the rational unloading level is recommended to be no higher than the stress level at the elastic-plastic threshold under the corresponding confining pressure. Appl. Sci. 2019, 9, 3164 2 of 15 and the unloading level has no significant effect on the energy evolution process. Zhang [11] and Zhu [12] considered that the unloading stress level mitigated possible damage within rocks during the unloading damage stage. Regarding the initial axial pressure of unloading, Liu et al. [13] selected 50% of the conventional triaxial compressive strength and the corresponding confining pressure, whereas other researchers often adopted 60% and 80% [14][15][16], 70% [17][18][19][20], 80-90% [21], and 15-90% [22]. Most researchers selected 80% of the peak strength as the loaded axial pressure in unloading tests [23][24][25][26]. Details of studies discussed above can be viewed in Table 1. However, mentioned studies failed to consider the effects of the initial axial pressure during exploring the effects of unloading on rock failures.

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Section: Evolution Analysis Of Deformation Parametermentioning

confidence: 99%

Elastic-Plastic Threshold and Rational Unloading Level of Rocks

Guo

2019

Applied Sciences

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“…However, these reports focus on permeability change during single‐loading process. Few studies have investigated permeability change in the process of nonsingle loading; such works investigated the influence of different loading and unloading rates of confining pressure on the permeability of crystalline sandstone and determined the flow and convective heat transfer characteristics of fracture water in granite caused by high‐temperature hydraulic fracturing under loading and unloading confining pressure 26,27 . The permeability variations of fractured rock under complex stress path need to be further studied.…”

Section: Introductionmentioning

confidence: 99%

Experimental study of the permeability of fractured sandstone under complex stress paths

Wang

Zhang

et al. 2020

Energy Science & Engineering

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Oil and gas are main storage resources in the pores and fractures of rocks. Under a complex stress environment, the permeability evolution of the reservoir rock fracture directly affects the flow of oil and gas, which is important in oil and gas exploration and development. The permeability test of a single sample in complex stress paths was performed on intact sandstone, single‐fractured sandstone, and double‐fractured sandstone using high‐precision hydro‐mechanics‐coupled triaxial experimental equipment to study the permeability evolution of fractured sandstone. The experimental schemes of the permeability tests are as follows: (a) under increasing confining pressure; (b) under increasing liquid pressure; (c) under cyclic loading and unloading deviatoric stress; and (d) under increasing confining pressure and deviatoric stress synchronously. Results show that liquid flow on fractured sandstone can be regarded as laminar flow with low velocity. The permeability and stress sensitivity levels of the fractured sandstone are higher than those of the intact sandstone. The permeability decreases in a negative exponential manner when the confining pressure increases but increases in a positively exponential manner when the liquid pressure increases. The increase in the deviatoric stress decreases the permeability, whereas unloading increases the permeability. The entire evolution of permeability is irreversibly reduced. As the confining pressure and deviatoric stress increase synchronously, the permeability decreases, and the decrease rate tends to be stable. These results can provide an important basis for prediction of the permeability evolution of fractured sandstone and for efficient oil and gas exploitation.

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“…Some experts [16][17][18] studied the permeability of coal measured rock under cyclic loading and unloading, concluding that the unloading curve of rock did not coincide with the loading curve, forming a closed plastic hysteresis loop, and also obtained the evolution of residual strain with the cyclic times and the damage evolution of coal measured rock. Besides, the relation between confining pressure and permeability was researched [19][20][21][22][23][24][25][26]. The experiments and numerical results indicated that the relationship of confining pressure and coal permeability could be described by an exponential function.…”

Section: Introductionmentioning

confidence: 99%

Study on the Permeability Evolution and Its Formation Mechanism of Xiaojihan Aquifer Coal Seam under Plastic Flow

Guo

Liu

et al. 2020

Geofluids

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Study on permeability evolution of an aquifer coal seam in Western China is of great significance for preventing water inrush disaster and realizing water-conserving coal mining. The permeability evolution of an aquifer coal seam is related to a loading path closely under plastic flow. In this work, permeability variations of the Xiaojihan water-bearing coal seam and Longde nonwater coal seam are researched using a transient method under plastic flow. The experiment results indicated the following: (1) Under the same axial strain, the permeability, relative residual strain, and confining pressure influence coefficient of Xiaojihan coal specimens all decrease in plastic flow with the increase of loading-unloading times and confining pressure, while the permeability recovery coefficient increases during this process. (2) The permeability of Xiaojihan water-bearing coal specimens decreases with the growth of axial strain in plastic flow, resulting in the increase of relative residual strain and reinforcement of plasticity. Besides, the confining pressure influence coefficient decreases and the permeability recovery coefficient decreases slightly with the axial strain. (3) Finally, the permeability of Xiaojihan coal specimens is greater than that of Longde coal specimens, while the confining pressure influence coefficient and permeability recovery coefficient of Longde coal specimens are greater than those of Xiaojihan coal specimens. The closure rate of internal cracks of the water-bearing coal specimen is lower than that of the nonwater coal specimen, which is beneficial for water storage and transport.

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Influence of loading and unloading velocity of confining pressure on strength and permeability characteristics of crystalline sandstone

Cited by 33 publications

References 12 publications

Elastic-Plastic Threshold and Rational Unloading Level of Rocks

Elastic-Plastic Threshold and Rational Unloading Level of Rocks

Experimental study of the permeability of fractured sandstone under complex stress paths

Study on the Permeability Evolution and Its Formation Mechanism of Xiaojihan Aquifer Coal Seam under Plastic Flow

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