Brittleness Evaluation of Glutenite Based On Energy Balance and Damage Evolution

Tight glutenite reservoirs are typically characterized by highly variable lithology and permeability, low and complex porosity, and strong heterogeneity. Glutenite brittleness is an essential indicator for screening fracture targets, selecting technological parameters, and predicting the hydraulic fracturing effect of tight glutenite reservoir exploitation. Glutenite formations with high brittleness are more likely to be effectively fractured and form complex fractures. Accurate evaluation of glutenite brittleness facilitates the recovery of oil and gas in a tight glutenite reservoir. Accordingly, two brittleness indexes are proposed in this paper based on energy balance and damage evolution analysis of complete stress–strain curves to evaluate the brittleness of glutenite. Uniaxial and triaxial compression tests of glutenite specimens were carried out and the brittleness indexes were verified by comparison with other existing indexes. The relationships between the mechanical properties and brittleness of glutenite under confining pressure were analyzed based on experimental results and the effects of mechanical and structural parameters on glutenite brittleness are investigated with a numerical approach. The brittleness of glutenite increases with the increase of gravel size and/or volume content. During hydraulic fracturing design, attention should be paid to the brittleness of the matrix and the size and content of gravel. This paper provides a new perspective for glutenite brittleness evaluation from the perspectives of energy dissipation and damage evolution. Our results provide guidance for fracturing layer selection and may also facilitate field operations of tight glutenite fracturing.

Download Full-text

Experimental Investigation and Micromechanical Modeling of Elastoplastic Damage Behavior of Sandstone

Materials ◽

10.3390/ma13153414 ◽

2020 ◽

Vol 13 (15) ◽

pp. 3414

Author(s):

Chaojun Jia ◽

Qiang Zhang ◽

Susheng Wang

Keyword(s):

Damage Evolution ◽

Triaxial Compression ◽

Damage Model ◽

Confining Pressure ◽

Micromechanical Modeling ◽

Compression Tests ◽

Integration Algorithm ◽

Damage Behavior ◽

Plastic Strain Increment ◽

Elastoplastic Damage

The mechanical behavior of the sandstone at the dam site is important to the stability of the hydropower station to be built in Southwest China. A series of triaxial compression tests under different confining pressures were conducted in the laboratory. The critical stresses were determined and the relationship between the critical stress and confining pressure were analyzed. The Young’s modulus increases non-linearly with the confining pressure while the plastic strain increment Nϕ and the dilation angle ϕ showed a negative response. Scanning electron microscope (SEM) tests showed that the failure of the sandstone under compression is a coupled process of crack growth and frictional sliding. Based on the experimental results, a coupled elastoplastic damage model was proposed within the irreversible thermodynamic framework. The plastic deformation and damage evolution were described by using the micromechanical homogenization method. The plastic flow is inherently driven by the damage evolution. Furthermore, a numerical integration algorithm was developed to simulate the coupled elastoplastic damage behavior of sandstone. The main inelastic properties of the sandstone were well captured. The model will be implemented into the finite element method (FEM) to estimate the excavation damaged zones (EDZs) which can provide a reference for the design and construction of such a huge hydropower project.

Download Full-text

Damage Evolution of Coal with Inclusions Under Triaxial Compression

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-021-02436-9 ◽

2021 ◽

Author(s):

Yufeng Zhao ◽

Heinz Konietzky ◽

Martin Herbst

Keyword(s):

Damage Evolution ◽

Laboratory Tests ◽

Numerical Models ◽

Triaxial Compression ◽

Three Dimensional ◽

Confining Pressure ◽

Fracture Network ◽

Triaxial Tests ◽

Compression Tests ◽

Deformation State

AbstractAlong with the advance of the working face, coal experiences different loading stages. Laboratory tests and numerical simulations of fracture and damage evolution aim to better understand the structural stability of coal layers. Three-dimensional lab tests are performed and coal samples are reconstructed using X-ray computer tomography (CT) technique to get detailed information about damage and deformation state. Three-dimensional discrete element method (DEM)-based numerical models are generated. All models are calibrated against the results obtained from uniaxial compressive strength (UCS) tests and triaxial compression (TRX) tests performed in the laboratory. A new approach to simulate triaxial compression tests is established in this work with significant improved handling of the confinement to get realistic simulation results. Triaxial tests are simulated in 3D with the particle-based code PFC3D using a newly developed flexible wall (FW) approach. This new numerical simulation approach is validated by comparison with laboratory tests on coal samples. This approach involves an updating of the applied force on each wall element based on the flexible nature of a rubber sleeve. With the new FW approach, the influence of the composition (matrix and inclusions) of the samples on the peak strength is verified. Force chain development and crack distributions are also affected by the spatial distribution of inclusions inside the sample. Fractures propagate through the samples easily at low confining pressures. On the contrary, at high confining pressure, only a few main fractures are generated with orientation towards the side surfaces. The evolution of the internal fracture network is investigated. The development of microcracks is quantified by considering loading, confinement, and structural character of the rock samples. The majority of fractures are initiated at the boundary between matrix and inclusions, and propagate along their boundaries. The internal structure, especially the distribution of inclusions has significant influence on strength, deformation, and damage pattern.

Download Full-text

Mechanical Behavior of Coupled Elastoplastic Damage of Clastic Sandstone of Different Burial Depths

Energies ◽

10.3390/en13071640 ◽

2020 ◽

Vol 13 (7) ◽

pp. 1640

Author(s):

Yu Zhang ◽

Lu Wang ◽

Goangseup Zi ◽

Yan Zhang

Keyword(s):

Plastic Deformation ◽

Mechanical Behavior ◽

Oil And Gas ◽

Triaxial Compression ◽

Mechanical Damage ◽

Confining Pressure ◽

Internal Variables ◽

Burial Depth ◽

Compression Tests ◽

Elastoplastic Damage

Clastic sandstone is widely distributed in oil and gas reservoirs; its internal structure has many micro-defects. Under different stress environments of burial depth, significant damage evolution and plastic deformation easily occur. A series of triaxial compression tests were performed to study the coupled elastoplastic damage mechanical behavior of clastic sandstone samples at different burial depths ranging from 581.28 m to 979.82 m. Results reveal that the stress-strain responses of clastic sandstone samples exhibit significant nonlinear and softening characteristics. The mechanical behavior is due to the coupling of plastic deformation and mechanical damage. Plastic and damage internal variables cause damage stiffness degradation and plastic flow. Considering the coupling of elastoplastic damage in the loading process, an elastoplastic damage coupling model is proposed to study the mechanical behavior of different burial depth clastic sandstones. The model can effectively describe the mechanical behavior of clastic sandstone, such as the volume compression and dilatancy transformation, plastic hardening and damage softening, which are in good agreement with the experimental results. Furthermore, the mechanical behavior of the clastic sandstone shows a dependency on the confining pressure and burial depth. The load-bearing capacity and the ability to resist deformation of the clastic sandstone are improved as the confining pressure and burial depth increase. Relevant results can provide reliable basis for the safe exploitation of oil and gas engineering.

Download Full-text

Fatigue Characteristics of Limestone under Triaxial Compression with Cyclic Loading

Advances in Civil Engineering ◽

10.1155/2018/8681529 ◽

2018 ◽

Vol 2018 ◽

pp. 1-12 ◽

Cited By ~ 3

Author(s):

Yongjie Yang ◽

Huiqiang Duan ◽

Luyi Xing ◽

Shan Ning ◽

Jiakun Lv

Keyword(s):

Cyclic Loading ◽

Damage Evolution ◽

Triaxial Compression ◽

Peak Stress ◽

Confining Pressure ◽

Primary Phase ◽

Fatigue Properties ◽

Dissipated Energy ◽

Loading Frequency ◽

Compression Tests

This paper presents an experimental investigation of the fatigue properties of limestone subjected to triaxial compression with axial cyclic loading. Tests were conducted on intact limestone samples with a loading frequency of 0.5 Hz and a confining pressure of 10 MPa. The test results show the following five points. (1) Under triaxial conditions, the axial and circumferential deformations at the failure point induced by cyclic loading are slightly larger than the corresponding deformations at the peak stress achieved by conventional compression tests. (2) The first level cyclic loading process has a strong influence on rock deformation in the primary phase during subsequent level cyclic loading. A smaller difference in stress amplitude between the two loading stress levels leads to less deformation during the latter. (3) Circumferential and volumetric changes are more sensitive to fatigue failure in terms of deformation and strain rate than axial changes. (4) The three phases of dissipated energy evolution are consistent with a sample’s deformation such that the energy dissipation characteristics reflect the fatigue damage evolution process. (5) A new damage formula is proposed that can concisely describe a rock’s zero-cycle damage and damage evolution.

Download Full-text

Study on mechanical properties and damage evolution of carbonaceous shale under triaxial compression with acoustic emission

International Journal of Damage Mechanics ◽

10.1177/1056789521991193 ◽

2021 ◽

pp. 105678952199119

Author(s):

Kai Yang ◽

Qixiang Yan ◽

Chuan Zhang ◽

Wang Wu ◽

Fei Wan

Keyword(s):

Mechanical Properties ◽

Acoustic Emission ◽

Water Content ◽

Damage Evolution ◽

Damage Assessment ◽

Triaxial Compression ◽

Confining Pressure ◽

Damage Variable ◽

Natural State ◽

Carbonaceous Shale

To explore the mechanical properties and damage evolution characteristics of carbonaceous shale with different confining pressures and water-bearing conditions, triaxial compression tests accompanied by simultaneous acoustic emission (AE) monitoring were conducted on carbonaceous shale rock specimens. The AE characteristics of carbonaceous shale were investigated, a damage assessment method based on Shannon entropy of AE was further proposed. The results suggest that the mechanical properties of carbonaceous shale intensify with increasing confining pressure and degrade with increasing water content. Moisture in rocks does not only weaken the cohesion but also reduce the internal friction angle of carbonaceous shale. It is observed that AE activities mainly occur in the post-peak stage and the strong AE activities of saturated carbonaceous shale specimens appear at a lower normalized stress level than that of natural-state specimens. The maximum AE counts and AE energy increase with water content while decrease with confining pressure. Both confining pressure and water content induce changes in the proportions of AE dominant frequency bands, but the changes caused by confining pressure are more significant than those caused by water content. The results also indicate that AE entropy can serve as an applicable index for rock damage assessment. The damage evolution process of carbonaceous shale can be divided into two main stages, including the stable damage development stage and the damage acceleration stage. The damage variable increases slowly accompanied by a few AE activities at the first stage, which is followed by a rapid growth along with intense acoustic emission activities at the damage acceleration stage. Moreover, there is a sharp rise in the damage evolution curve for the natural-state specimen at the damage acceleration stage, while the damage variable develops slowly for the saturated-state specimen.

Download Full-text

Damage evolution and recrystallization enhancement of frozen loess

International Journal of Damage Mechanics ◽

10.1177/1056789517731138 ◽

2017 ◽

Vol 27 (8) ◽

pp. 1131-1155 ◽

Cited By ~ 21

Author(s):

Zhiwei Zhou ◽

Wei Ma ◽

Shujuan Zhang ◽

Cong Cai ◽

Yanhu Mu ◽

...

Keyword(s):

Mechanical Properties ◽

Strain Rate ◽

Damage Evolution ◽

Deformation Process ◽

Triaxial Compression ◽

Confining Pressure ◽

Recrystallization Process ◽

Frozen Soils ◽

Time Deformation ◽

Pressure Melting

A series of multistage triaxial compression, creep, and stress relaxation tests were conducted on frozen loess at the temperature of −6℃ in order to study the damage evolution and recrystallization enhancement of mechanical properties during deformation process. The effect of strain rate, confining pressure, and hydrostatic stress history in the degradation laws of mechanical properties is investigated further. The strain rate has a significant influence on the stress–strain curve which dominates the evolution trend of mechanical properties. The mechanical behaviors (strength, stiffness, and viscosity) of frozen loess all exhibit evident response for the consolidation and pressure melting phenomenon caused by the confining pressure. The multistage loading tests under different hydrostatic stresses are capable of differentiating the development characteristics of mechanical properties during axial loading and hydrostatic compression process, respectively. The testing results indicated that the recrystallization of the ice particle in the frozen soils is an important microscopic factor for enhancement behaviors of mechanical parameters during the deformation process. This strengthening degree of mechanical properties is determined by temperature, duration time, deformation degree, and stress state during the recrystallization process. The phase transformation led by pressure melting and ice recrystallization is a nonnegligible changing pattern of frozen soils microstructure, which has apparent role in the damage evolution of mechanical properties.

Download Full-text

Triaxial Testing of Geosynthetics Reinforced Tailings with Different Reinforced Layers

Materials ◽

10.3390/ma13081943 ◽

2020 ◽

Vol 13 (8) ◽

pp. 1943

Author(s):

Fu Yi ◽

Changbo Du

Keyword(s):

Triaxial Compression ◽

Friction Angle ◽

Confining Pressure ◽

Test Results ◽

Compression Tests ◽

Strength Index ◽

Stress Strain ◽

High Confining Pressure ◽

Zhang Model ◽

Triaxial Compression Tests

To evaluate the shear properties of geotextile-reinforced tailings, triaxial compression tests were performed on geogrids and geotextiles with zero, one, two, and four reinforced layers. The stress–strain characteristics and reinforcement effects of the reinforced tailings with different layers were analyzed. According to the test results, the geogrid stress–strain curves show hardening characteristics, whereas the geotextile stress–strain curves have strain-softening properties. With more reinforced layers, the hardening or softening characteristics become more prominent. We demonstrate that the stress–strain curves of geogrids and geotextile reinforced tailings under different reinforced layers can be fitted by the Duncan–Zhang model, which indicates that the pseudo-cohesion of shear strength index increases linearly whereas the friction angle remains primarily unchanged with the increase in reinforced layers. In addition, we observed that, although the strength of the reinforced tailings increases substantially, the reinforcement effect is more significant at a low confining pressure than at a high confining pressure. On the contrary, the triaxial specimen strength decreases with the increase in the number of reinforced layers. Our findings can provide valuable input toward the design and application of reinforced engineering.

Download Full-text

Microcracking and the failure of polycrystalline ice under triaxial compression

Journal of Glaciology ◽

10.3189/s0022143000009606 ◽

1992 ◽

Vol 38 (128) ◽

pp. 65-76 ◽

Cited By ~ 1

Author(s):

P. Kalifa ◽

G. Ouillon ◽

P. Duval

Keyword(s):

Triaxial Compression ◽

Peak Stress ◽

Confining Pressure ◽

Triaxial Tests ◽

Strain History ◽

Compression Tests ◽

Brittle Transition ◽

Polycrystalline Ice ◽

Strain Components

AbstractTriaxial and uniaxial compression tests have been carried out at –10°C on granular ice in order to study the role of microcracking on failure in the ductile-brittle transition zone. In the triaxial tests, the effect of confining pressure and strain rate on the crack population, as well as on strength and strain at the peak stress, was investigated. In the uniaxial tests, we measured the evolution of elastic and non-elastic components of deformation with the stress-strain history. The concept of effective stress, with a single scalar damage variable, was used to calculate the effect of microcracking on the strain components.

Download Full-text

Mechanical Properties of Bentonite-Sand Mixtures Under Triaxial Stress Condition

MRS Proceedings ◽

10.1557/proc-353-307 ◽

1994 ◽

Vol 353 ◽

Cited By ~ 1

Author(s):

M. Umedera ◽

A. Fujiwara ◽

N. Yasufuku ◽

M. Hyodo ◽

H. Murata

Keyword(s):

Mechanical Properties ◽

Shear Strength ◽

Water Content ◽

Triaxial Compression ◽

Confining Pressure ◽

Strength Properties ◽

Compression Tests ◽

Optimum Water Content ◽

Triaxial Compression Tests ◽

Optimum Water

AbstractA series of triaxial compression tests is being conducted under the drained condition on bentonite and sand mixtures, known as buffer, in saturated and optimum water content states to clarify the mechanical properties of the buffer.It was found that the mechanical properties of bentonite and sand mixtures are strongly influenced by water and bentonite contents: shear strength in a saturated state is less than that in an optimum water content state; shear strength decreases rapidly with increasing bentonite content. Strength properties are much dependent on confining pressure.

Download Full-text

Experimental Study on Shear Strength Considering Initial Void Ratio and Plasticity Index

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.405-408.63 ◽

2013 ◽

Vol 405-408 ◽

pp. 63-67

Author(s):

Xing Chen Wang ◽

Ri Qing Xu ◽

Jian Feng Zhu

Keyword(s):

Shear Strength ◽

Void Ratio ◽

Triaxial Compression ◽

Confining Pressure ◽

Deviatoric Stress ◽

Plasticity Index ◽

Test Results ◽

Compression Tests ◽

Functional Relationships ◽

Initial Void Ratio

A series of drained triaxial compression tests under different conditions were performed to quantitatively study the influence of the initial void ratio and plasticity index on the shear strength of remolded saturated clays. The test results show that both the peak stress friction angle and peak deviatoric stress decrease with increasing initial void ratio and plasticity index of the soil under the same confining pressure; whereas, they increase with increasing confining pressure of the soil under the same initial void ratio and plasticity index. A new synthesized physical parameter λ, which simultaneously represent both the type and the condition of remolded saturated clays, is defined based on the test results in this work. The functional relationships among the parameters φd and peak deviatoric stress in Mohr-Coulomb equation and the parameter λ are established to develop a modified Mohr-Coulomb equation by considering physical properties of soil. In this equation, only two input parameters, i.e., λ and the confine pressure, are needed to predict the shear strength of the soil. In order to check the accuracy of the proposed equation, laboratory tests were conducted to evaluate against the predicted results. The results show that the peak shear strength of remolded saturated clays can be well described by the proposed equation. Key words: shear strength; Mohr-Coulomb equation; remolded saturated clays; initial void ratio; plasticity index.

Download Full-text