Rapid Stress Drop Research Articles

Numerical study on the effectiveness of directional well with multiple hydraulic slots for enhanced gas recovery in deep coal seam

Coalbed methane production failures through hydraulic fracturing in the Linxing gas field in China have encountered significant setbacks, hindering efforts to enhance the recovery of deep CBM reserves in the region. To address this challenge, we investigate the potential of directional wells with multiple hydraulic slots as a stimulation technology for deep coalbed methane extraction. This study presents two numerical models to evaluate the effectiveness of this technology. The first model is a continuum damage numerical model based on the Material Point Method, which simulates large deformations and complex contact behaviors induced by hydraulic slotting in the deep coal seam. The second model is a wellbore-pore coupling model to simulate the gas extraction process. We verified both numerical models against theoretical and experimental results. We conducted a case study in the Linxing gas field using these models. The investigation assessed stress relief, gas desorption, and permeability enhancement around a large deformed hydraulic slot, as well as the gas extraction performance of a directional well with multiple slots. The results indicate that (1) The Linxing gas recovery failure may be attributed to unsuccessful hydraulic fracturing based on the history matching analysis of field production; (2) Hydraulic slotting causes gradual compaction of the slot until full closure, and results in a rapid stress drop exceeding 7 MPa in the elliptical zone surrounding the slot, which is the large plastic damage zone; (3) The enhanced permeability in plastic zone exceeds ten times the natural coal permeability, leading to total gas desorption of 556 m3 from elastic and plastic zones; (4) Performance optimization can be achieved through reducing slot length and increasing slot width when using directional well with multiple slots. In summary, this research demonstrates the efficiency of directional wells with multiple slots in enhancing gas recovery from deep coal seams.

In-situ measurement of drying and electrochemical-cycling-induced mechanical behavior and delamination of polypyrrole electrodes for energy storage applications

Delamination during the fabrication and operation of thick electrodes is one of the major instabilities that prevent their utility for high energy density electrochemical devices. In this study, we investigate the deformation and delamination of polypyrrole (pPy) film electrodes using in-situ measurements of curvature/stress. At the beginning of drying process, the water in the film exited continuously, while the film did not deform accordingly. Then, the film rapidly contracted near the end of the evaporation process and remained in its equilibrium contracted state. The film deformation was found to be caused by the evaporation of moisture near the polymer chain, whereas it was not affected by the evaporation of free water in the pores, causing the mismatch between the deformation and evaporation. Importantly, film delamination was identified in real time as the rapid stress drop corresponded to the peeling of the film from the substrate. The interfacial fracture energy was quantified to estimate the adhesion strength of the interface with various treatments. Moreover, electrochemical-cycling-induced delamination was successfully identified based on the type of delamination. We believe this study would be beneficial for both the development and operation of thick electrodes with high energy density and stability.

Localized Deformation of Lawsonite During Cold Subduction

AbstractOne of the causes of intermediate depth (50–300 km) earthquakes in cold subduction zones is proposed to be the dehydration of lawsonite, a hydrous mineral that forms a major component of cold subducting oceanic crust. However, experimental studies are limited and the deformation mechanism of lawsonite remains uncertain. Here, we conducted deformation experiments on lawsonite under seismogenic conditions appropriate to cold subduction zones at pressures from 2.5 to 6.0 GPa and temperatures from 300°C to 800°C. Deformation experiments incorporating temperature ramping, in which deformation and heating were synchronous, did not show unstable fault slip associated with dehydration. Instead, we find localized deformation accompanying a rapid stress drop prior to the initiation of dehydration, and then more gradual strain weakening during dehydration. Such microstructures as microcracks and comminution of lawsonite imply a brittle behavior for lawsonite accompanied by the partial amorphization along the fault. Our results suggest that fault slip may occur within the stability field of lawsonite under conditions suitable for the generation of intermediate depth earthquakes.

Influence of reswollen superabsorbent polymers on the corrosion behavior of steel fibers in cracked mortars exposed to NaCl solutions

Superabsorbent polymers (SAPs) can decrease the water penetration depth and crack width in cracked cementitious materials. This suggests that SAPs may inhibit the corrosion behavior of steel fibers in cracks. In this paper, the effects of SAPs on the mechanical properties and crack pattern of mortars were respectively studied with a uniaxial compression test and a 3-point bending test. The 3D morphology of the reswollen SAPs and their influence on the water flow rate in the cracked mortars were investigated with an X-ray computed microtomography and a water flow test, respectively. Additionally, the corrosion behavior of steel fibers in the cracked mortars with and without SAPs was explored with electrochemical techniques. The results reveal that SAPs can mitigate the rapid stress-drop of mortars during failure, resulting in a reduction in the crack width on the mortar surface. The volume of the reswollen SAPs in a crack decreases when the NaCl concentration is above 0.1 mol/L, while SAPs can partially block the SAPs void and decrease the flow rate of NaCl solutions in the cracked mortars. Furthermore, in NaCl solutions (0.1 and 0.5 mol/L), SAPs can reduce the probability and rate of the corrosion of steel fibers in the cracked mortars. This is advantageous for the durability of the smart cementitious materials containing SAPs.

Temporal evolution of a shear-type rock fracture process zone (FPZ) along continuous, sequential and spontaneously well-separated laboratory instabilities—from intact rock to thick gouged fault

SUMMARY The development of shear-type fault analogues from intact rock at the laboratory scale provides a unique opportunity for investigating tectonic-scale phenomena through the lens of geophysics. The transition from rock fracture creation to laboratory fault slip must exist. We observe three spontaneously temporally well-separated mechanical instabilities attributed to the continuous evolution of a shear-type rock fracture between two artificial flaws. Their separation is validated with rapid mechanical stress drops and stabilizations, periodical acoustic emission (AE) behaviours (AE event number and AE moment release rate) and b-value drops. One instability occurs near the stress peak and corresponds to fracture incipience where fault development is mostly identified via optical observations; the other two instabilities are in the post-stress-peak domain and correspond to the fault nucleation and slip stages, respectively, with distinguishable AE releases from the fault region. The macroscale fracture has been created at the moment of rapid-stress drop for the second instability; off-fault damage, increasing gouge powder generation and slip acceleration can be identified within the fault slip stage. AE behaviour throughout fault nucleation shows a reversal of the Omori–Utsu (O–U) law. AEs attributed to the fault slip display regular O–U law decay and the distinction between the AE behaviour for fault nucleation and fault slip is pronounced. These observations and analyses can provide further understanding on the analogue relationship between a laboratory loading-induced fault and a natural fault.

Microelectromechanical Systems for Nanomechanical Testing: Displacement- and Force-Controlled Tensile Testing with Feedback Control

Background: MEMS-based nanomechanical testing has received much interests. However, it remains challenging to perform displacement- and force-controlled nanomechanical tests (e.g., stress relaxation and creep). Objective: We report a MEMS-based device for displacement- and force-controlled tensile testing of 1D nanomaterials using feedback control. Methods: The device consists of an electrostatic actuator, a load cell, and two differential capacitive sensors. A specimen is mounted between a fixed anchor on one side and a displacement sensor on the other side. Using a multi-channel capacitive readout, both specimen displacement and force (thus strain and stress) can be measured from the readout simultaneously, without the need of imaging that is often used for displacement measurement. Results: With the feedback control, both displacement- and force-controlled tensile testing can be achieved. The capability of the device is demonstrated in three representative tests of metallic nanowires – stress relaxation test, tensile test capturing rapid stress drop, and creep test. Conclusions: The reported MEMS device can be used for a range of tests where imaging for displacement measurement is not feasible, such as ex-situ tests, high-rate tests, and fatigue tests in different environments.

Deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions

We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1GPa, 400°C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500°C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust.

Microstructural evolution and tensile properties of an in-situ TiZr-based bulk metallic glass matrix composite after hot-pressing deformation in its supercooled liquid region

Hot-pressing deformation treatment was carried out at different strain rates and temperatures to improve the microstructure homogeneity of the TiZr-based bulk metallic glass matrix composites (BMGMCs) after enlargement. At a compression strain rate of 0.001/s and temperature of 653 K, the TiZr-based BMGMCs exhibited good plastic rheological properties in supercooled liquid region. After work hardening to the peak stress, steady-state flow stress was rapidly obtained, resulting in a stress drop of 223 MPa. Moreover, during this period, no rapid stress drop and fracture phenomena were observed. Compared with those of the as-cast morphology, the distribution of dendrite and glass matrix was more homogeneous after 31% thickness reduction in hot-pressing deformation. Accompanied with viscous flow of the glass matrix, the dendrites markedly aggregated together after a thickness reduction ratio of 58%. Transmission electron microscopy analysis revealed that a large amount of dislocations formed in the dendrite, and a large amount of nanocrystals precipitated from the glass matrix after hot-pressing treatment. The amount of the nanoparticles continued to increase as the amount of deformation increased. TiZr-based BMGMCs with 31% compression deformation possessed both the distinguished plastic strain (εp) of 3.95% and ultimate strength (σmax) of 1595 MPa. The stress–strain curve demonstrated that the rate of stress drop considerably slowed down after tensile strength reached a maximum due to hot-pressing deformation. The rapid expansion of the shear band was efficiently hindered through the combined effect of partial nanocrystallization of the glass matrix and deformation hardening of the dendrite. This condition further improves the total coordinated deformation capacity of TiZr-based BMGMCs.

Experimental Characterization and Micromechanical Modelling of Anisotropic Slates

Laboratory tests were performed in this study to examine the anisotropic physical and mechanical properties of the well-foliated Jiujiang slate. The P-wave velocity and the apparent Young’s modulus were found to increase remarkably with the foliation angle θ, and the compressive strength at any confining pressure varies in a typical U-shaped trend, with the maximum strength consistently attained at θ = 90° and the minimum strength at θ = 45°. The slate samples failed in three typical patterns relevant to the foliation angle, i.e. shear failure across foliation planes for θ ≤ 15°, sliding along foliation planes for 30° ≤ θ ≤ 60° and axial splitting along foliation planes for θ = 90°. The stress–strain curves at any given foliation angle and confining pressure display an initial nonlinear phase, a linear elastic phase, a crack initiation and growth phase, as well as a rapid stress drop phase and a residual stress phase. Based on the experimental evidences, a micromechanical damage–friction model was proposed for the foliated slate by simply modelling the foliation planes as a family of elastic interfaces and by characterizing the interaction between the foliation planes and the rock matrix with a nonlinear damage evolution law associated with the inclination angle. The proposed model was applied to predict the deformational and strength behaviours of the foliated slate under triaxial compressive conditions using the material parameters calibrated with the uniaxial and/or triaxial test data, with good agreement between the model predictions and the laboratory measurements.

Frictional and transport properties of the 2008 Wenchuan Earthquake fault zone: Implications for coseismic slip-weakening mechanisms

Abstract This paper reports on the frictional and transport properties of fault rocks collected from a surface exposure associated with the 2008 Wenchuan earthquake. The water-dampened gouges showed high-velocity frictional behavior characterized by a rapid stress drop at the start of slip, and by intermittent jumps after attaining steady state, suggesting operation of thermal pressurization (TP). A novel fluid-flow system, allowing for parallel measurements of permeability, porosity and specific storage has been developed. Strong pore fluid pressurization induced by elevated confining pressure was observed during the porosity measurements. Analogical analysis of this compaction-induced pressurization succeeded in predicting the pore pressure build-up for a faulting process. Our measurements revealed that the fault zone consists of low-permeability fault gouges (2.6 × 10− 20 m2 at 165 MPa) and high-permeability damaged-zone rocks. The fault gouges and intact country rocks act as barriers to fluid flow across the fault, whereas the damaged zone acts as a fluid conduit, hence the fault zone displays a “conduit/barrier” hydrological structure. With our lab data as input, we performed numerical modeling of coseismic slip weakening including TP and mineral decomposition. The results indicate that fluid pressurization played an important role during the Wenchuan earthquake at the exposure site, where dynamic stress reduction was strongly enhanced by increase of pore pressure due to frictional heating and smectite dehydration. Our modeling further suggests less importance of high-velocity weakening compared with weakening due to pore fluid pressurization. Taken together, our experimental and modeling results as well as the microstructure observed, all suggest that thermochemical pressurization has been an important slip-weakening mechanism during the Wenchuan earthquake rupture. The dramatic weakening predicted may explain the large coseismic displacements and rupture accelerations associated with earthquake rupture at the study site.

Dynamic damage constitutive relation of mesoscopic heterogenous brittle rock under rotation of principal stress axes

A micro-mechanics-based model is developed to investigate microcrack damage mechanism of four stages of brittle rock under rotation of the principal stress axes. They consist of linear elastic, non-linear hardening, rapid stress drop and strain softening. The frictional sliding crack model is applied to analyze microcracks nucleation, propagation and coalescence. The strain energy density factor approach is applied to determine the critical condition of microcrack nucleation, propagation and coalescence. The inelastic strain increments are formulated within the framework of thermodynamics with internal variables. Rotation of principal stress axes affect the dynamic damage constitutive relationship and the failure strength of brittle rock.

Constitutive relationship of brittle rock subjected to dynamic uniaxial tensile loads with microcrack interaction effects

A micromechanical model is proposed to describe both stable and unstable damage evolution in microcrack-weakened brittle rock material subjected to dynamic uniaxial tensile loads. The basic idea of the present model is to classify the constitution relationship of rock material subjected to dynamic uniaxial tensile loads into four stages including some of the stages of linear elasticity, pre-peak nonlinear hardening, rapid stress drop, and strain softening, and to investigate their corresponding micromechanical damage mechanisms individually. Special attention is paid to the transition from structure rearrangements on microscale to the macroscopic inelastic strain, to the transition from distribution damage to localization of damage and the transition from homogeneous deformation to localization of deformation. The influence of all microcracks with different sizes and orientations are introduced into the constitutive relation by using the statistical average method. Effects of microcrack interaction on the complete stress–strain relation as well as the localization of damage for microcrack-weakened brittle rock material are analyzed by using effective medium method. Each microcrack is assumed to be embedded in an approximate effective medium that is weakened by uniformly distributed microcracks of the statistically-averaged length depending on the actual damage state. The elastic moduli of the approximate effective medium can be determined by using the dilute distribution method. Micromechanical kinetic equations for stable and unstable growth characterizing the ‘process domains’ of active microcracks are taken into account. These ‘process domains’ together with ‘open microcrack domains’ completely determine the integration domains of ensemble averaged constitutive equations relating macro-strain and macro-stress. Theoretical predictions have shown to consistent with the experimental results.

Real-time computerized tomography (CT) experiments on limestone damage evolution during unloading

The computerized tomography (CT) images and CT values for the process of unloading, micro-cracking, and dilation up to the failure of limestone specimens under different unloading levels have been obtained by using the real-time CT technique. Clear CT images and CT value of the stages from compression of micro-cavities → emergence of micro-cracks → bifurcation → development → coalescence → suddenly collapse → rapid stress drop → unloading can be observed. The CT value, equivalent to rock density in the CT scan layer, is the most important parameter describing the damage evolution process of rock. The paper reveals that the mechanical behaviors of rock under unloading are different from those under loading due to its load-path dependency. It is clear from the experiment that it is easy for failure phenomenon of rock to happen suddenly under unloading. The mechanism of damage evolution of limestone is analyzed. Moreover, a damage model based on CT value and a new failure criterion are established.

Triaxial compressive behavior of rock with mesoscopic heterogenous behavior: Strain energy density factor approach

The strain energy density factor approach is used in conjunction with a micromechanics model to investigate the condition and direction of shear failure for brittle rock subjected to triaxial compression. Moderate confinement in addition to localized deformation and damage are considered. Quantified are the effects of the various geometric and load parameters that involve the interaction of microcrack, friction and the confining pressure such that the path of the wing crack is taken into account. The influence of all microcracks with different orientations are introduced into the constitutive relation. The closed-form solution for the complete stress–strain relation of rock containing microcracks is obtained. It is shown that the complete stress–strain relationship includes linear, nonlinear hardening, rapid stress drop and strain softening effects. The theoretical results show that deviation of the direction of wing cracks from the line of the pre-existing crack decreases with increasing confinement pressure and friction coefficient. Theoretical predictions and experimental results show good agreement.

Localization of deformation and stress–strain relation for mesoscopic heterogeneous brittle rock materials under unloading

Stress redistribution induced by excavation of underground engineering and slope engineering results in the unloading zone in parts of surrounding rock masses. The mechanical behaviors of crack-weakened rock masses under unloading are different from those of crack-weakened rock masses under loading. A micromechanics-based model has been proposed for brittle rock material undergoing irreversible changes of their microscopic structures due to microcrack growth when axial stress is held constant while lateral confinement is reduced. The basic idea of the present model is to classify the constitution relation of rock material into four stages including some of the stages of linear elasticity, pre-peak nonlinear hardening, rapid stress drop, and strain softening, and to investigate their corresponding micromechanical damage mechanisms individually. Special attention is paid to the transition from structure rearrangements on microscale to the macroscopic inelastic strain, to the transition from distribution damage to localization of damage and the transition from homogeneous deformation to localization of deformation. The closed-form explicit expression for the complete stress–strain relation of rock materials containing cracks under unloading is obtained. The results show that the complete stress–strain relation and the strength of rock materials under unloading depend on the crack spacing, the fracture toughness of rock materials, orientation of the cracks, the crack half-length and the crack density parameter.

Analysis of the localization of damage and the complete stress-strain relation for mesoscopic heterogeneous rock under uniaxial tensile loading

The mechanical behavior of rock under uniaxial tensile loading is different from that of rock under compressive loads. A micromechanics-based model was proposed for mesoscopic heterogeneous brittle rock undergoing irreversible changes of their microscopic structures due to microcrack growth. The complete stress-strain relation including linear elasticity, nonlinear hardening, rapid stress drop and strain softening was obtained. The influence of all microcracks with different sizes and orientations were introduced into the constitutive relation by using the probability density function describing the distribution of orientations and the probability density function describing the distribution of sizes. The influence of Weibull distribution describing the distribution of orientations and Rayleigh function describing the distribution of sizes on the constitutive relation were researched. Theoretical predictions have shown to be consistent with the experimental results.

Analysis of the localization of damage and the complete stress-strain relation for mesoscopic heterogeneous brittle rock subjected to compressive loads

A micromechanics-based model is established. The model takes the interaction among sliding cracks into account, and it is able to quantify the effect of various parameters on the localization condition of damage and deformation for brittle rock subjected to compressive loads. The closed-form explicit expression for the complete stress-strain relation of rock containing microcracks subjected to compressive loads was obtained. It is showed that the complete stress-strain relation includes linear elasticity, nonlinear hardening, rapid stress drop and strain softening. The behavior of rapid stress drop and strain softening is due to localization of deformation and damage. Theoretical predictions have shown to be consistent with the experimental results.

Analysis of the localization of deformation and the complete stress–strain relation for mesoscopic heterogeneous brittle rock under dynamic uniaxial tensile loading

Stress redistribution induced by excavation results in the tensile zone in parts of the surrounding rock mass. It is significant to analyze the localization of deformation and damage, and to study the complete stress–strain relation for mesoscopic heterogeneous rock under dynamic uniaxial tensile loading. On the basis of micromechanics, the complete stress–strain relation including linear elasticity, nonlinear hardening, rapid stress drop and strain softening is obtained. The behaviors of rapid stress drop and strain softening are due to localization of deformation and damage. The constitutive model, which analyze localization of deformation and damage, is distinct from the conventional model. Theoretical predictions have shown to consistent with the experimental results.

Viscoelastic simulation of earthquake cycle using a simple spring‐Dashpot‐mass system with a friction law

We examine the viscoelastic effect on earthquake cycle using a simple spring‐dashpot‐mass system with a laboratory‐derived friction law. The temporal change of the frictional stress is large at the early interseismic stage and gradually decreases, in contrast to the linear change in an elastic medium. This temporal change depends on the Maxwell time. The smaller relaxation time leads to the larger change of the stress accumulation rate. Such a rate change in stress accumulation may affect the seismicity at subduction zones. Moreover, we find that the smaller relaxation time produces the smaller period of earthquake cycle and that, if the system relaxation time becomes smaller than the period of earthquake cycle, the bifurcation phenomenon, where the period splits into two, occurs. It is also found that the smaller relaxation time causes the earlier occurrence of the precursory stress decrease preceding the rapid stress drop.

Characterization of low cycle fatigue damage in 9Cr–1Mo ferritic steel using X-ray diffraction technique

X-ray diffraction (XRD) technique has been used to characterize the low cycle fatigue (LCF) damage in 9Cr–1Mo ferritic steel. In this study, full-width at half-maximum (FWHM) of the XRD peak has been measured for assessing the fatigue damage. Fully reversed total-axial-strain controlled fatigue tests have been performed at ambient temperature (300 K) at strain amplitudes of ±0.25%, ±0.50% and ±0.75%. FWHM measurement of {3 1 0} plane has been carried out on specimens interrupted at different fatigue life fractions, which represent the various stages of deformation and fracture such as cyclic hardening, cyclic softening, saturation and surface crack initiation and propagation. The cyclic hardening, which occurred in the early stages of fatigue deformation, exhibited broadening of diffraction profile and a rapid increase in the FWHM at all strain amplitudes. Also, large oscillations were observed in the d(2 θ) vs. sin 2Ψ curves. With further cycling, FWHM remained almost constant in the softening and saturation stages. Finally, at the onset of rapid stress drop in the cyclic stress response and cusp formation in the compression portions of stress–strain hysteresis loops, which indicate surface crack initiation and propagation, a rapid decrease in FWHM was observed. This drop in FWHM is attributed to relieving of microstresses owing to surface crack initiation and propagation. Relieving of micro stresses has also been confirmed by a significant reduction in oscillations in the d(2 θ) vs. sin 2Ψ curves.