Triaxial Compressive Strength Tests Research Articles

Effects of CO2-saturated brine imbibition on the mechanical and seepage characteristics of Longmaxi shale

Supercritical carbon dioxide (SC– CO2) serves as a promising anhydrous fracturing fluid that has the potential to simultaneously enhance shale gas recovery and accomplish CO2 sequestration within shale reservoirs. However, the hydromechanical properties of shale are considerably affected by CO2-brine-shale interaction. This study conducted shale imbibition experiments involving subcritical/supercritical CO2 saturated brine (Sub/SC-CO2+brine). Triaxial compressive strength tests were performed to evaluate the impact of Sub/SC-CO2+brine imbibition on the mechanical properties of shale, as well as the influence of SC-CO2+brine imbibition on shale permeability. The results indicate that, after SC-CO2 brine imbibition, noticeable macroscopic cracks are formed parallel to the bedding plane. The imbibition of Sub/SC-CO2+brine results in the reduction in axial stress and Young's modulus, while simultaneously increasing the axial strain of shales. Compared to Sub-CO2+brine, SC-CO2+brine exhibits a higher imbibition pressure and a more intense geochemical reaction with shale. Due to increased compressibility sensitivity, the averaged permeability after SC-CO2+brine imbibition shows a reduction of nearly 70 % compared to that before imbibition. The insights from this research are expected to provide theoretical support for shale gas recovery and CO2 geological sequestration.

An improvement in the method of correcting indirect radial strain measurements during triaxial strength tests in rocks

The present article provides an improvement in the method to correct indirect strain measurements in triaxial compressive strength tests through axial displacement and hydraulic fluid volume change measurements. The improvement focused on reducing the parameters of the formula proposed for indirect volumetric strain in the original method, thereby facilitating the development of a simpler formula in which the radial strain depends on only two parameters: the initial volume of the rock specimen and the volume changes of the hydraulic fluid for each instant. The comparison between the improvement proposed, and original method resulted in a mean absolute difference of 0.003.•This improvement does not depend on the axial strain, unlike the original method, which requires correcting the indirect axial strain measurements before correcting the indirect radial strain measurements.•This improvement can be useful for research on the stress-strain behavior of intact rock under laboratory conditions, such as in the study of the post-peak state.

Rate effect of rocks: Insights from DEM modeling

Rocks are subjected to different loading rates at different construction stages and engineering applications. The strength of rock usually increases with loading rate. This rate dependency is one of the time-dependent behaviors of rock, whereby the micro-mechanisms are believed to be the subcritical crack growth due to stress corrosions. However, no evidence is provided yet. This study investigated rate effects of rocks through a novel implementation of Parallel-Bonded Stress Corrosion (PSC) model in Discrete Element Method (DEM). Long-term microparameters in PSC are first calibrated through creep test. Then, a series of uniaxial compressive strength, direct tensile strength, and triaxial compressive strength tests are performed, with strain rates ranging from 1×10−7/s to 1×10−3/s. Results show that the uniaxial compressive strength is highly dependent on strain rates which quantitatively matches with the experimental data. At lower strain rate, more subcritical cracks propagate due to longer stress-corrosion reaction time, resulting in a lower strength. Besides, strain rate also influences the failure patterns in post-peak, with single failure plane at lower strain rates and multiple failure planes at higher strain rates. Rate effects are also observed in direct tensile strength tests, with similar rate of increase in strength and a transition in cracking pattern, which align with the experimental data, indicating tension-induced subcritical cracking is the unified underlying micro-mechanism of rate effects for both cases. However, in triaxial compressive strength tests, rate effects become less obvious with increasing confining pressure, consistent with experimental findings, as subcritical crack growth is suppressed in shearing processes.

Characterization analyzes in the geomechanical behavior of travertine rock

There is a fundamental interest in studying travertine rocks, and this is to understand their structure, their geomechanical behavior and other particularities in order to guarantee their proper use in different engineering and architectural applications, and thus, evaluate the sustainability of the travertines, natural resources, the stability of slopes, the preservation of cultural heritage and the mitigation of possible anthropic risks. Travertine has petrological and mechanical properties similar to carbonates from oil fields such as those found in El Presal-Brazil, which currently contain the largest hydrocarbon reserves in the world. Given the impossibility of obtaining rock samples from this deposit to carry out the study, rocks similar to these were used. The present study specifically used samples of Lapis tiburtinus rocks, coming from the west of the city of Tivoli in Italy and these showed resistance to uniaxial and triaxial compression, and showed mechanical resistance due to increased porosity and brittleness. The investigation carried out an analysis of the geomechanical behavior travertine through an experimental program, which includes a petrological, structural, and mechanical characterization. It was determined the travertine is mainly composed of micrite and spastic calcite without the presence of grains or allochemical cements and presents high porosity of the fenetral and vulgar type. Macro and micropores were found to be chaotically distributed in the rock and have low connectivity, which demonstrates the complexity and heterogeneity of the porous structure of Roman travertine. Uniaxial and triaxial compressive strength tests were also carried out, observing a decrease in its mechanical strength due to the increase in porosity, presenting a property of brittleness in its behavior. The results were consistent and valid for this type of rock compared to other studies; determining that there is a correct and adequate operation of the triaxial cell used in the mechanical resistance tests.

Lithospheric strength and elastic properties for Mars from InSight geophysical data

Elastic moduli and unconfined compressive strength (UCS), material constants which help constrain the mechanical behaviors of rock materials, are useful for developing accurate models of geologic processes like fracturing and faulting. On Earth, UCS and elastic moduli for intact rock can be measured with uniaxial and triaxial compressive strength tests; however, when geological units are highly fractured, it is more appropriate to estimate these parameters for the rock mass, fractures included. At depth, these estimations can be obtained using seismic wave velocities where stronger more intact rock masses result in faster velocities. We use seismic wave velocities from the InSight Lander to derive the elastic moduli and UCS for the upper crust, lower crust, and mantle of Mars. These estimates could be used in developing models for lithospheric deformation on the red planet.

Research on Energy and Instability Criterion of Sandstone Hydraulic Fracturing in the Three Gorges Reservoir Area, Chongqing, China

At present, the understanding of structural failure and energy analysis of sandstone under hydrofracture is still insufficient. This time, we selected the sandstone in Chongqing Three Gorges Reservoir Area as the main analysis object to discuss and compare the characteristics of conventional uniaxial (triaxial) compression strength test and uniaxial (triaxial) compression hydraulic fracturing strength test and conduct instability analysis from the perspective of energy. Based on the mechanical characteristics and parameters about the uniaxial (triaxial) compressive strength test and uniaxial (triaxial) compressive hydrofracture strength test, we focus on the analysis of the evolution rules of the hydraulic pressure-strain curve, analyzing the criteria of crack expanding and instability by using energy principle. According to the viewpoint of fracture mechanics, the fracture morphology and failure type of sandstone under hydrofracture were discussed. The test results show that the deformation evolution rules of rock hydrofracture can be divided into four stages, including the characteristics of pore fissure water injection stage (OA), the elastic deformation stage (AB), the volume expansion stage (BC), and global rupture stage (CD). Using the P/C modulus (the ratio of hydraulic pressure and cohesive force, abbreviated as P/C), the ability of hydraulic pressure overcoming cohesive force can be evaluated during hydrofracture. Using energy variables (expressed by E k ) about crack expanding from beginning to end, the unexpanded state, critical state, unstable state, and unstable failure of crack can be estimated. There is an interaction between tensile deformation and shear deformation from the crack initial stage to crack expanding stage.

Some mechanical characterization of the Sandakan formation’s sandstone, Sabah, Malaysia

This work defines the mechanical characteristics of slightly weathered sandstone belonging to sedimentary Sandakan formation in Sandakan, Sabah. The mechanical properties of uniaxial compressive strength, shear strength (includes basic and peak friction angle and cohesion) was determined by the uniaxial compressive strength test, direct shear test, tilt test and triaxial compressive strength test. The slightly weathered very fine sandstone gave higher strength but lower in basic friction angle than slightly weathered fine sandstone. The basic friction angle for very fine and fine sandstone are 28° and 33°, respectively. The value of peak friction angle and cohesion of intact rock by the triaxial compressive strength test (52-58° and 2-4 MPa) is higher than direct shear test (16-20° and 0.25-4.00 MPa) of fine sandstone rock mass (for planar surface).

Fragmentation Characteristics of Seafloor Massive Sulfides: A Coupled Fluid-Particle Flow Simulation

The research on the fragmentation mechanism of seabed minerals under high ambient pressure significantly contributes to the exploitation of seafloor massive sulfides (SMS). In this paper, the uniaxial compressive strength (UCS) test and triaxial compressive strength (TCS) test were carried out on two kinds of SMS samples to obtain the key mechanical properties of minerals, including cohesion, internal friction angle, compressive strength, and elastic modulus. Then, based on these mechanical parameters, the fluid-solid coupling cutting model of two SMS samples at high ambient pressure is established by using the coupling method of discrete elements and smooth particles. A mixed-bond model is selected, and the microscopic parameters are determined by a repeated calibration process. Meanwhile, the cutting force and debris information are monitored and collected in real time during the whole cutting process. The results show that under different confining pressure environments, the model shows the transformation of minerals from brittleness to ductility. The cutting force increases with the increasing ambient pressure. Due to the fluid pressure, the crushing mechanism tends to shear failure, which is more likely to produce mud and finer fragments.

A Binary Medium Model for Frozen Silty Sand Simplified by Breakage Parameter

In order to investigate the strength-deformation characteristics of frozen silty sand, the triaxial compressive strength tests of saturated frozen silty sand under different fine particle contents were carried out, and the binary medium theory was introduced to interpret the stress-strain relationship. Due to the characteristics of the existing binary medium model with many parameters and complicated determination method, a simplified binary medium model based on breakage parameter is proposed. The derived model was verified by the triaxial tests of frozen silty sand. The results show that the stress-strain relationship can be divided into three stages with the increase of axial strain, namely, linear elastic deformation stage, plastic deformation stage, and strain softening stage. All three stages can be well explained by the transformation theory of bonded element and frictional element with the binary medium model. In the linear elastic deformation stage, the external stress is mainly borne by the bonded element. In the plastic deformation stage, the stress sharing ratio of the bonded element decreases and that of the frictional element increases. In the strain softening stage, the stress sharing ratio of the bonded element decreases rapidly, while that of the frictional element increases rapidly. Under the same confining pressure, both deviator stress and the maximum values of bulk expansion decrease, while the shear strength decreases linearly with the increase of fine particle content. By comparing the measured deviator stress in triaxial test with the calculated values of binary medium constitutive model simplified by breakage parameter, the proposed model can better simulate the stress-strain relationship of frozen silty sand. The results of the study can provide some theoretical reference for the constitutive model of seasonal frozen soil.

A validated multiscale model linking microstructural features of fired clay brick to its macroscopic multiaxial strength

Given the popularity of fired clay bricks in increasingly taller buildings, as well as the large variety of raw materials, additives, tempers, and production technology, microstructure-based modeling of the brick strength is essential. This paper aims at linking the microstructural features of bricks, i.e. the volume, shape, and size of mineral phases, pores, and the glassy binding matrix in between, to the multiaxial failure behavior of bricks. Therefore, a continuum micromechanics multiscale model, developed originally for stiffness and thermal conductivity upscaling, is adopted and complemented with a Mohr–Coulomb failure criterion at the microscale. By micromechanics-based downscaling of uniaxial brick strength tests, quantitative insights into the strength of the binding matrix are obtained for the first time. After successful nanoindentation-based validation of the identified micro-strength, the model is used for predicting the macroscopic multiaxial brick strength, which in turn is successfully validated against independent bi- and triaxial compressive strength test results.

Prediction Models of Shear Parameters and Dynamic Creep Instability for Asphalt Mixture under Different High Temperatures.

This study mainly investigates the prediction models of shear parameters and dynamic creep instability for asphalt mixture under different high temperatures to reveal the instability mechanism of the rutting for asphalt pavement. Cohesive force c and internal friction angle φ in the shear strength parameters for asphalt mixture were obtained by the triaxial compressive strength test. Then, through analyzing the influence of different temperatures on parameters c and φ, the prediction models of shear strength parameters related to temperature were developed. Meanwhile, the corresponding forecast model related to confining pressure and shear strength parameters was obtained by simplifying the calculation method of shear stress level on the failure surface under cyclic loading. Thus, the relationship of shear stress level with temperature was established. Furthermore, the cyclic time FN of dynamic creep instability at 60 °C was obtained by the triaxial dynamic creep test, and the effects of confining pressure and shear stress level were considered. Results showed that FN decreases exponentially with the increase in stress levels under the same confining pressure and increases with the increase in confining pressure. The ratio between shear stress level and corresponding shear strength under the same confining pressure was introduced; thus, the relationship curve of FN with shear stress level can eliminate the effect of different confining pressures. The instability prediction model of FN for asphalt mixture was established using exponential model fitting analysis, and the rationality of the model was verified. Finally, the change rule of the parameters in the instability prediction model was investigated by further changing the temperature, and the instability forecast model in the range of high temperature for the same gradation mixture was established by the interpolation calculation.

Evaluation Method of Production Pressure Differential in Deep Carbonate Reservoirs: A Case Study in Tarim Basin, Northwest China

Deep and even ultra-deep petroleum resources play a gradually increasing and important role with the worldwide continuous advancement of oil and gas exploration and development. In China, the deep carbonate reservoirs in the Tarim Basin are regarded as the key development areas due to their huge reserves. However, due to the unreasonable design of production pressure differential, some production wells suffered from severe borehole collapse and tubing blockage. Therefore, the main purpose of this paper is to optimize a more practical method for predicting the critical production pressure differential. The commonly used analytical methods with different failure criteria for predicting production pressure differential were summarized. Furthermore, their advantages and disadvantages were analyzed. A new numerical model is established based on the finite element theory in order to make the prediction of production pressure differential more accurate. Additionally, both analytical and numerical methods were applied to evaluating the production pressure differential of deep carbonate reservoirs in the Tarim Basin, and the results were discussed compared with field data. In addition, a series of laboratory tests, including porosity and permeability measurements, electron microscope scanning, XRD for mineral analysis, uniaxial and triaxial compressive strength test, etc., were carried out by using the collected carbonate cores from formations deeper than 7000 m to obtain the input parameters of the simulation such as the rock properties. The experimental results showed that the carbonate rocks exhibited a remarkable brittleness and post-peak strain softening. The calculation results revealed that the Mogi-Coulomb criterion is slightly conservative; however, it is more suitable than other criteria to evaluate pressure differential. Furthermore, it has been confirmed by the field data that the finite element numerical method can not only reveal the instability mechanism of the wellbore but also predict the critical production pressure differential accurately. Unfortunately, the on-site operators sometimes require a more convenient way, such as an analytical method, to figure out the pressure differential, even though the evaluation of the numerical method is more accurate. Therefore, the discussion in this paper can provide a basis for the operators to determine the production pressure differential flexibly.

A Method to Correct Indirect Strain Measurements in Laboratory Uniaxial and Triaxial Compressive Strength Tests

When carrying out uniaxial, standard triaxial, or true triaxial compressive tests on rock, the strain response of the specimens can be measured directly or indirectly. The most commonly used devices are strain gauges (local direct measurements) and displacement sensors (global indirect measurements). Strain gauges are glued to specimens and directly measure electrical resistance. Displacement sensors, typically linear variable differential transformers (LVDTs), indirectly measure displacements. The former is more precise at its smaller measurement scale, but may not work under high temperatures or when a strain gauge detaches due cracking under stresses close to the peak or in the post-peak portion of the stress–strain curve. It was observed that the strain responses recorded using strain gauges and displacement sensors tend to differ, which is why a corrective approach was proposed to recover stress–strain curves based on indirect measurements. This study describes an approach to correct platen-to-platen displacement measurements based on energy calculations to derive axial strain and to obtain radial strain using modified computations based on the volume fluid change within the Hoek’s cell in triaxial testing. The study concludes that part of the displacement recorded by LVDTs can be associated with displacement in the plate sample contacts, but the component of energy transmitted to the samples can be estimated at different levels of load, so that these indirect measurements can be corrected. For the case of triaxial tests, the volumetric or radial strain can be computed based on the volume change in the Hoek’s cell. This approach is useful for correcting indirect strain measurements. It can also serve as a starting point for the development of corrections for application to data from other test types, such as for true triaxial tests. The study aims to ultimately contribute to a better understanding of strain measurements while conducting compressive tests on rock.

VARIATION OF THE INTRINSIC ROCK PROPERTIES ON HOEK-BROWN FAILURE CRITERION PARAMETERS

The Hoek-Brown (H-B) criterion is one of the most commonly used rock failure criteria in recent years. This criterion includes a constant parameter called mi which is a fundamental parameter for estimating rock strength. Due to the importance of the mi parameter in the H-B criterion, it is necessary to conduct comprehensive studies on various aspects of the effect of this parameter on the behavior of rocks. Therefore, in this study, using numerical simulation of the Triaxial Compressive Strength (TCS) tests in PFC-2D code, the effects of microscopic properties of different rocks on the H-B parameter mi have been studied. Based on the results of this study, it was found that the effects of micro-parameters on the H-B parameter mi can be different depending on the type of rock, however this parameter has an inverse relationship to the micro-parameters of bond tensile strength and bond fraction of the rocks. Also, the mi parameter increases with an increase in the micro-parameters of the friction coefficient, the friction angle, the particle contact modulus, and the contact stiffness ratio of rocks.

NUMERICAL AND EXPERIMENTAL RESEARCH OF ROCK FAILURE MECHANISM AND ITS DEPENDENCE ON mi HOEK-BROWN FAILURE CRITERION

Rock failure mechanism is one of the most important issues in rock mechanics engineering which plays a key role in the stability analysis of various structures. Therefore, different failure criteria have been proposed to understand the failure mechanism of rocks. One of the most commonly used rock failure criteria is the Hoek-Brown criterion, in which there is a parameter called mi, which is very important to the response provided by this criterion. Due to the importance of conducting extensive studies on this parameter, in this current research, by performing a series of experimental triaxial compressive strength test and numerical simulating in PFC-2D code, the effect of the Hoek-Brown constant mi on the failure mechanism and crack growth of different rocks has been studied. Based on the results of this study, it was found that the effect of parameter mi on the failure mechanism of different rocks varied according to the type of rocks, and the greatest effect of this parameter was on the peak strength of rocks. In addition, it was found that under higher lateral pressures, there are less destructive cracks in rocks, and as a result, they show more ductile behaviour.

Improving the mechanical characteristics of well cement using botryoid hybrid nano-carbon materials with proper dispersion

As a key barrier to ensure zonal isolation and wellbore integrity, well cement is required to have good flexibility and higher strength to withstand increasingly hostile downhole conditions. Nano-carbon materials are promising fillers to endow significant improvement in mechanical properties, anti-crack abilities, and durability of well cement. In this study, botryoid hybrid nano-carbon materials (BHNCMs) are introduced into wellbore cement at different concentrations (0.5% to 3% by weight of cement) for the first to formulate more flexible wellbore cement composites. The density, rheological properties, thickening time, fluid loss, and free fluid of the slurries of neat cement and cement composites were measured to evaluate the impact of the addition of BHNCMs on the workabilities of neat cement slurry. The triaxial compressive strength tests, which simulate more realistic downhole stress conditions, were performed to investigate the mechanical properties of hardened BHNCM cement composites. The results show that BHNCMs only need a simple preparation method to be effectively dispersed due to their self-assembled botryoid structure, which help manage the problem of agglomeration of nanocarbon materials. The addition of BHNCMs significantly improves the flexibility of oil well cement and enhances its strength. The optimal concentration of BHNCMs for successful improvement of well cement properties is found to be 1% by weight of cement (BWOC). At the optimum concentration, the compressive strength and flexibility of neat cement improve by 20%. The SEM-based microstructural analysis of cement composites showed various mechanical property enhancement mechanisms (filling effect, nano-core effect, bridge effect, and pinning effect) of BHNCMs.

Hydro-mechanical properties of a low-clay shale with supercritical CO2 imbibition

The mechanical properties and permeability of shales after supercritical carbon dioxide (SC-CO2) imbibition are vital to shale gas production and CO2 sequestration in shale reservoirs. In this study, triaxial compressive strength tests, steady state permeability tests, and CT scanning tests are performed on shale samples with 30 days of SC-CO2 imbibition. The results show that minerals dissolution and precipitation occur during SC-CO2 imbibition. The porosity calculated by the digital core increases by 19.190%. The axial stresses of samples with or without SC-CO2 imbibition increase with increasing confining pressure. When the confining pressure changes between 5 and 20 MPa, the reductions range from 6 to 13% for strength and from 11.26 to 16.47% for Young’s modulus, respectively. The strength of the samples with or without imbibition agrees well with the Mohr–Coulomb criterion. The values of cohesion and internal friction angles are very close which indicates that shale samples with SC-CO2 imbibition can still refer to the failure criterion for samples without CO2 imbibition. The permeability of the sample after SC-CO2 imbibition increases exponentially with the increase in injection pressure and decreases exponentially with an increase of effective pressure. The permeability of the sample before and after imbibition is more sensitive to the change of pore pressure than confining pressure. SC-CO2 imbibition enhances the increase of permeability. The ratio of the shale permeability after imbibition to that before imbibition varies from the minimum of 42 times to the maximum of 827 times. The reduction of strength and increasing of permeability are beneficial for shale gas recovery.

Comparative study on uniaxial and triaxial strength of plastic concrete containing nano silica

Plastic concrete is used in the manufacture of cut off wall and its mechanical properties is important for performance of dam. In this paper, in order to investigate the durability and mechanical properties of plastic concrete, uniaxial and triaxial compressive strength tests were performed on plastic concrete specimens in different ages (7, 14, 28, 56 and 90 days) and different percentage of replacement cement by nano silica (0, 1.5, 3, 4.5 and 6%) and compressive strength, the stress–strain behavior, elastic modulus, shear strength parameters of plastic concrete have been evaluated. According to the result, as the age of the specimen increases, elastic modulus, compressive strength and triaxial strength of the plastic concrete enhances. Furthermore, although by increasing the amount of nano silica the strength parameters of plastic concrete are improved and workability of concrete is reduced and the amount of elastic modulus of the specimen is almost constant. The results of shear strength parameters also show that the addition of up to 3% of nano silica increases the cohesion and internal friction angle decreases and with over 3% nano silica this trend is reversed and cohesion decreases and friction angle increases.

The Mechanical and Microstructural Properties of Composite Structures Made of a Cement-Tailing Backfill and Rock Core

In underground metal mines that use sublevel or stage open-stope and backfilling mining methods (SSOBMMs), there is a special structure around which both sides of the rock pillar are wrapped by backfill. As a permanent part of an underground mine, how much can backfill improve the rock pillar’s compressive strength? What is the difference in the mechanical properties between the special structure and the signal rock? To explore these questions, a composite structure made of a cement-tailing backfill (CTB) and rock core (RC) was designed. Uniaxial and triaxial compressive strength tests and scanning electron microscope (SEM) were used to research the mechanical properties, failure process, failure characteristics, and microstructure characteristics of the cement-tailing backfill and rock core (CTB-RC) specimens. It was found that the full stress–strain curve of the CTB-RC specimen under triaxial compressive strength (TCS) test had two times the stress increases reaching a lower peak deviator stress two times after the RC was destroyed. The CTB can reduce the destruction and slow down the deformation speed of the inner rock cor (IRC). It can also prevent rigid slip of the IRC after it is damaged and maintain the stability and integrity of the overall structure. The findings of this study can provide some basic knowledge on the mechanical properties of the CTB-RB and provide theoretical guidance for the optimization direction of the width of the rock pillar and the room in mines using SSOBMMs.

Comparative Analyses Concerning Triaxial Compressive Yield Criteria of Coal with the Presence of Pore Water

The least absolute deviation is used as a metric to analyze the applicability of five yield criteria, to describe the yield characteristics of coal based on triaxial compressive strength tests on natural, water-saturated, and seepage coal samples with the presence of pore water. The results show that the strength of coal exhibits nonlinear characteristics with the increase of confining pressure, which the linear Coulomb criterion fails to authentically describe. Although the parabolic Mohr criterion can describe the nonlinearity feature more decently than the linear yield criterion, the fitting error is significant, and the uniaxial compressive strength of coal is overestimated. The Hoek-Brown criterion, quadratic polynomial criterion, and exponential criterion yield decent fitting quality for the coal rock. In particular, the exponential strength criterion can accurately reflect the actual uniaxial compressive strength of the rock. However, the differential principle yield stress for an infinite confining pressure calculated from the exponential strength criterion is lower than the measured value. Furthermore, by employing effective stress principle to analyze the yield criteria for the saturated and seepage coal samples, one can find that the quadratic polynomial criterion and the exponential criterion can also reflect the changes of yield characteristics during the fluid-solid coupling triaxial compression test.