scholarly journals Effect of Intermediate Principal Stress on the Strength, Deformation, and Permeability of Sandstone

Energies ◽  
2018 ◽  
Vol 11 (10) ◽  
pp. 2694 ◽  
Author(s):  
Zhenlong Song ◽  
Minghui Li ◽  
Guangzhi Yin ◽  
Pathegama Ranjith ◽  
Dongming Zhang ◽  
...  

Although the mechanical behaviors and flow aspects of sandstone have been previously investigated, studies of the effect of the intermediate principal stress (σ2) on the strength, deformation, and permeability of sandstone are lacking. In this work, the mechanical behaviors and permeability of sandstone under true triaxial stress conditions were investigated using a newly developed true triaxial geophysical apparatus. The experimental results showed that with increasing σ2, the peak strength, octahedral effective normal stress, and octahedral effective shear stress of the sandstone increased, and the rate of increase decreased. This is because a larger intermediate principal stress coefficient b has an inhibitory effect on rock strength. In our study, as the ratio of σ2/σ3 increased, the specimen entered compressive strain in the σ2 direction during the first stress drop. The stress and strain path deviations occur during rock failure. The amount of deviation increased as the σ2 increased before the peak stress. This phenomenon indicates that elastic mechanics are not suitable for understanding this sandstone rock during its failure. The permeability evolution of the sandstone under true triaxial stress conditions was measured and analyzed to investigate the effect of σ2. During the complete true triaxial stress-strain experiments, the variation we found in gas seepage velocity could be divided into two stages. Before the first pressure drop, the gas seepage velocity was mainly affected by volume strain. After the first pressure drop, the seepage velocity was affected by the deviator strain, which can change the seepage channels.

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-9 ◽  
Author(s):  
Gang Wang ◽  
Pengfei Wang ◽  
Yangyang Guo ◽  
Wenxin Li

To study the effects of shear stress on the mechanical and seepage properties of gas-bearing coal in three-dimensional stress conditions, a novel true triaxial apparatus (TTA) was developed for the rigid loading of the major principal stress σ1 and the intermediate principal stress σ2, and for the flexible loading of the minor principal stress σ3. Both the upper and lateral pressure heads do not interfere with each other when loading σ1 and σ2. The control and measurement of gas flow sealed effectively in coal samples were achieved by using a gas seepage system. The TTA was used to perform a series of experiments on shear seepage in coal samples under true triaxial stress conditions. The experimental results about coal’s shear failure modes, shear stress-shear displacement curve, and permeability-shear displacement curve all showed that the TTA with its better accuracy and reliability had advantages in studying the effects of both the intermediate principal stress and shear deformation on the mechanical properties of coal samples and on the characteristics of gas seepage. Thus, it could provide a new test means for further studies on shear-induced seepage in the gas-solid coupling coal.


2019 ◽  
Vol 68 ◽  
pp. 102908 ◽  
Author(s):  
Chao Liu ◽  
Guangzhi Yin ◽  
Minghui Li ◽  
Bozhi Deng ◽  
Zhenlong Song ◽  
...  

2011 ◽  
Vol 51 (1) ◽  
pp. 567
Author(s):  
Ahmadreza Younessi ◽  
Vamegh Rasouli

Sand production prediction is becoming a regular study in reservoirs with weak or unconsolidated sands. Three main approaches for sanding prediction are analytical, numerical and experimental methods. Laboratory experiments have proven to provide more realistic results, with these being used to understand sanding mechanisms and validate analytical and numerical methods. A large number of experimental studies have been carried out by researchers worldwide—most of which have been performed on cylindrical-shape samples under uniaxial (i.e. σ1 ≠ 0, σ2 = σ3 = 0) or triaxial (i.e. σ1 ≠ 0, σ2 = σ3 ≠ 0) stress conditions. In general, a sanding experiment under true-triaxial stresses (i.e. σ1 ≠ σ2 ≠ σ3 ≠ 0) is more realistic in simulating downhole conditions. This stress condition can be simulated in the laboratory on a cubic sample. The first part of this paper provides a comprehensive but brief literature review on past sanding laboratory experiments. This will be followed by the introducition of a unique true-triaxial stress cell (TTSC) which was modified and used for sanding simulations in the laboratory. The applied modifications will be illustrated and the test procedure described. The sample preparation for testing synthetic samples will be explained and some preliminary results obtained will be presented.


Author(s):  
Prasoon Garg ◽  
Bhardwaj Pandit ◽  
Brijes Mishra ◽  
G.L. Sivakumar Babu

Mining at greater depths can lead to stress-induced failure, especially in areas of high horizontal in-situ stress. The induced stresses around the opening are known to be in a poly-axial stress state where, σ_1≠ σ_2≠ σ_3 with special case of σ_3= 0 and σ_1, σ_2 ≠ 0 at its boundary. The conventional triaxial testing does not represent the actual in-situ strength of the rock in regions of high horizontal stress, as it ignores the influence of intermediate principal stress (σ_2). The typical poly-axial testing (biaxial and true-triaxial tests) of intact rock mostly requires sophisticated and expensive loading systems. This study investigated the mechanical behavior of intact rock under a poly-axial stress state using a simple and cost-effective design. The apparatus consists of biaxial frame and a confining device. The biaxial frame has two platens that apply equal stress in both directions (σ_1=σ_2) on a 50.8 mm cubical specimen when placed inside the uniaxial loading device. The confining device performed separate biaxial tests under constant intermediate principal stress (σ_2 = constant) and true-triaxial tests when used along with the biaxial frame. This study then compared the failure modes and peak strength of Berea Sandstone specimens with other biaxial/triaxial devices to validate the design of the poly-axial apparatus. We also performed uniaxial tests on both standard cylindrical samples and prismatic specimen of different slenderness ratios. These tests provided a complete understanding of the failure mode transition from standard uniaxial compressive tests to triaxial stress conditions on cubical specimen. Additionally, this study determined best-fitted strength envelopes for biaxial and triaxial stress state. Based on regression analysis, we found a quadratic polynomial to be a good fit to biaxial strength envelope. For true-triaxial strength envelope, we found the 3D failure criterion by Nadai (1950) to be a good fit with R^2 of 0.964


2019 ◽  
Vol 354 ◽  
pp. 601-614 ◽  
Author(s):  
Yufei Chen ◽  
Changbao Jiang ◽  
Guangzhi Yin ◽  
Dongming Zhang ◽  
Huilin Xing ◽  
...  

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