Dynamic Strength Of Rock Research Articles

Modelling and mechanical characteristics of PDC cutter-rock interaction by combining mixed fragmentation modes with dynamic rock strength

Polycrystalline diamond compact (PDC) bit is one of the most widely used drill bits for improving the rate of penetration in deep oil and gas well and geothermal well. However, the dynamic rock fragmentation mechanics characteristics of PDC bits are still unclearly. A coupled fragmentation mechanics model of PDC cutter-rock interaction is established by combining the mixed fragmentation modes with dynamic strength. The coupling influence laws of cutter angle, cutting depth, dynamic strength ratio, breaking modes on the horizontal force coefficient (HFC), vertical force coefficient (VFC) and specific energy are analyzed. The model of this paper can optimize cutter inclination angle, cutting depth and minimum specific energy. With the increase of the cutter inclination angle, the dynamic VFC changes into two modes. The definition of the dynamic modes depends on the dynamic strength ratio. As the cutting angle increases, the cutting force increases. The cutting force increases nonlinearly with increasing cutting depth. The specific energy of rock fragmentation increases nonlinearly with increasing cutting depth. With the increase of dynamic strength, the specific energy of rock fragmentation increases nonlinearly. When the input-energy increases, the rate of penetration response is divided into three stages. The results have important guiding significance for the PDC bit design and drilling parameters optimization to increase the rate of penetration and the efficiency of exploration and development.

Experimental Investigation of the Size Effect of Rock under Impact Load

When measuring the compressive strength of rock, size and strain rate are the two main influencing factors. To study the rock strength size effect, rock specimens with length-to-diameter ratios of 0.5, 0.6, 0.7, 0.8, 0.9 and 1 were subjected to static loading tests using the RMT rock mechanics test system and dynamic loading with the split Hopkinson pressure bar, respectively. Based on the Weibull size-effect formula, the experimental results were compared with the improved formula obtained. The results show that rock strength is influenced by size and strain rate. Both the dynamic increase factor and rock strength are proportional to strain rate. The different failure modes of rock with size variation and strain rate variation are described according to the failure process of the specimens. The same length-to-diameter ratio specimens produced more fragments with a strain rate increase. Under the same strain rate of impact, the larger the rock specimen, the finer the broken fragments. Considering the factor of strain rate in the Weibull size-effect formula, the calculated result is accurate. The improved size-effect formula could be used to better elaborate the potential mechanisms of dynamic rock strength. In the unified theoretical formula containing static and dynamic loads, the relationship of rock strength, size and strain rate is well described.

Experimental Investigation on the Energy Consumption Difference between the Dynamic Impact and the Drilling Tests of Rocks

The split Hopkinson pressure bar (SHPB) technique and various drilling tests were performed on three types of lithologies (granite and cyan and red sandstone) to understand the variation in energy consumption (EC) of the different methods used for rock crushing. The dynamic behavior of rocks was analyzed based on the dynamic stress-strain curve, the peak stress, the energy, and the failure mode for the SHPB test, while the drilling curve, the influence of the drilling pressure (DP) and the revolution speed (RS) on the drilling speed (DS) were analyzed for the drilling test. Additionally, the EC difference was compared based on the EC required to break a single unit volume of rock. The obtained results indicate that the sensitivity of the dynamic strength of rocks with different lithologies to strain rate is different and that the higher the uniaxial compressive strength is, the more sensitive it is. Additionally, the strengthening of the peak stress is more pronounced with the increase in the strain rate. The energy utilization efficiency (EUE) of the SHPB test sample has a positive correlation with the strain rate. Moreover, the typical drilling process can be divided into the initial drilling and stable drilling stages, with the DS during the stable drilling stage being lower than that during the initial drilling stage. The DP and the RS have a linear positive correlation with the DS, with the influence of the DP on the DS being more obvious. Finally, the EC during the drilling process is higher than the EC during the SHPB technique.

Dynamic failure behavior of Jinping marble under various preloading conditions corresponding to different depths

To better understand the dynamic mechanical properties of rock at different depths, a method to simplify the stress characteristics of the rock mass surrounding a circular cavern was developed, and a series of impact tests involving high-speed photographic monitoring were performed on deep marble drilled in situ in the China Jinping Underground Laboratory at nine preloaded uniaxial stress levels (corresponding to simulated depths of 0, 100, 300, 600, 700, 900, 1200, 1500 and 1800 m). The results indicated that the strain rate sensitivity of the dynamic strength of Jinping marble decreased with the simulated depth, and obvious ductile deformation and failure characteristics occurred under high-energy dynamic loading and great depths. According to the fragmentation records, cracks clearly initiated and propagated mainly along the axial direction , and the observed failure patterns were classified into three typical modes: striped fragmentation (depth<600 m and strain rate<75/s), cylindrical surface spalling (depth≥600 m and strain rate<75/s) and comminution failure (strain rate≥75/s). Additionally, energy characteristics analysis revealed that the dissipative energy density e f and energy absorption efficiency A C exhibited significant depth differences and strain rate correlations, which corresponded well to the failure characteristics of the studied marble. Finally, an improved microcrack model considering the strain rate and simulated depth was established to characterize the dynamic mechanical behavior of the investigated marble, and the theoretically predicted strength values agreed well with the experimental values. • The dynamic failure behavior of rock under coupled static-dynamic loading conditions corresponding to different depths is revealed. • Striped fragmentation failure and a high strain rate sensitivity of the dynamic rock strength occur in shallow marble, while cylindrical surface spalling failure and a low strain rate sensitivity of the dynamic rock strength are observed at greater depths. • An improved microcrack model for dynamic rock strength prediction is established, which overcomes the limitation of previous models that lack comprehensive consideration of static preloading and strain rate conditions.

Dynamic properties and fracture damage characteristics of granite under impact load

The goal of this research is to study the dynamic properties and fracture damage laws of granite at the meso-scale under impact load. The quasi-static and dynamic mechanical properties of granite were studied through laboratory tests (Uniaxial compression, split Hopkinson pressure bar). The establishment of three-dimensional coupled numerical model of rock specimen and split Hopkinson pressure bar was realized by new coupling technology of PFC-FLAC. Three wave superposition and end stress distribution were discussed to determine the stress balance of the specimen during the loading process. The dynamic compression mechanics response characteristics and crack growth laws of granite under different strain rates were studied. The results indicated that PFC-FLAC coupling technology can solve the problems of tedious modeling process and long calculation time in the past three-dimensional discrete element model, and can better describe the dynamic mechanical behaviour of rock. There is obvious rate dependency in the dynamic strength of rock material, and the dynamic compressive strength of rock increases with the increase of strain rate. The macro fracture of rock mass is the result of the continuous development, expansion, aggregation and connection of the internal microcracks. With the increase of strain rate, the increase and cross propagation of cracks lead transformation of rock from intact to crushing destruction. Under dynamic loading, the failure process of rock is crack initiation, propagation, crossing, transfixion and rupture.

Extension of two-dimensional DDA on co-seismic slope stability analysis considering dynamic shearing strength

In this study, a new model for evaluating the dynamic shear strength of slope material has been proposed in DDA for earthquake-induced landslide simulation. The effect of fatigue and accumulated damage in evaluating the dynamic strength of rocks has been considered to judge the failure of joint contacts in the DDA block system. The accuracy of the extended DDA method is verified by simulating several examples, such as Single-Degree-Of-Freedom (SDOF) system and practical landslide model. The simulation results show that the new model considering the dynamic effect is more accurate and feasible to dynamic slope failure under seismic waves. Compared with the original DDA, the extended DDA method can be better applied to analyze earthquake-induced landslides with dynamic shearing strength considered.

Calculation of crack formation radius by modeling the explosive charge with a radial clearance

This article discusses the effect of explosion pulse on the process of cracking in a rock mass and shows the possibility of calculating the radius of a cracking zone by the theoretical value of the dynamic strength of rocks, based on their main physical and mechanical characteristics.

The transient intrusion effect of high energy fluid on the rock failure around the wellbore during the dynamic stimulation process

Dynamic wellbore stimulation technologies, such as high-energy gas fracturing, electrical pulse fracturing, hydraulic pulsation fracturing, are generally designed to multi-fracture the near-wellbore region through generating high energy pulse with intermediate or high loading rate. During the stimulating process, the dynamic strength and failure mode of saturated rocks around the wellbore can be largely affected by the transient intrusion of high energy fluid. Accordingly, an experimental method was developed to provide a method to apply the tailored pulse loading in the central borehole of the cylindrical rock samples. Three groups (Saturated + Center hole exposed (S-CHE), Saturated + Center hole wall isolated (S-CHI), Dry + Center hole wall isolated (D-CHI)) tests were carried out. The borehole fluid pressure-time curves and the failure modes of rock samples were tracked, analyzed and compared for all of the experiments. Experimental results demonstrate that: The relationship between rock dynamic strength and loading rate is monotonic direct proportionality. The dynamic strength of saturated rock shows a much stronger strain rate dependence than that of the dry rock. At the same loading rate, fluid penetration can largely weaken the dynamic strength of rock samples. The stress pulse generates a complex crushed zone around the wellbore, while the fluid penetration tends to produce radial multi-fractures. These phenomenons were explained using the micromechanics-based macroscopic damage theory, which is capable of obtaining the relationship between the fluid intrusion and the fracture deformation in the rock dynamic failure process.

Dynamic Characteristics of Deep Dolomite Under One-Dimensional Static and Dynamic Loads

The failure characteristics of rock subjected to impact disturbance under one-dimensional static axial compression are helpful for studying the problems of pillar instability and rock burst in deep, high geostress surrounding rock under blasting disturbances. Improved split Hopkinson pressure bar equipment was used for one-dimensional dynamic–static combined impact tests of deep-seated dolomite specimens under axial compression levels of 0, 12, 24, and 36 MPa. The experimental results demonstrate that the dolomite specimens exhibit strong brittleness. The dynamic strength always maintains a strong positive correlation with the strain rate when the axial compression is fixed; when the strain rate is close, the dynamic elasticity modulus and peak strength of the specimens first increase and then decrease with the increase in axial compression, and the peak value appears at 24 MPa. The impact resistance of specimens can be enhanced when the axial compression is 12 or 24 MPa, but when it increases to 36 MPa, the damage inside the specimen begins to cause damage to the dynamic rock strength. Prior to the rock macroscopic failure, the axial static load changes the rock structure state, and it can store strain energy or cause irreversible damage.

The Effect of Dynamic Stress Cycling on the Compressive Strength of Rocks

AbstractQuasi‐static rock strength is a nonconservative property, as fatigue during cyclic loading reduces the macroscopic strength. When strain rate under compressive loading increases above a lithology‐specific threshold, the primary failure mechanism transitions from localized failure along discrete fractures to distributed fracturing. However, the role of load path under high strain rate conditions has not been explored in any detail. We examine the effect of rapid stress cycles on the dynamic compressive strength of Westerly Granite using a modified split Hopkinson pressure bar approach and explore the implications of our results for the formation of pulverized fault zone rocks. Under cyclic loading conditions, the compressive strength can be reduced by a factor of 2, demonstrating that, like the quasi‐static strength, the dynamic rock strength is also a nonconservative property. Therefore, traditional high strain rate experimental approaches utilizing simple load paths may overestimate strength when rocks are subjected to complex load paths.

Insights into the influence of fluid imbibition on dynamic mechanics of tight shale

Fluid imbibition is a recognized strong phenomenon in tight shale that could greatly influences the dynamic failure behaviors. An experimental study was carried out to account for the influences of three typical drilling fluids (Air, Oil-based, and Water-based) on dynamic mechanics of tight shale. The collected samples were separated into three groups by the treatment of the distilled water saturation, white oil saturation, and drying, respectively. The dynamic properties involving Young's modulus, Compression strength, and specific energy consuming were experimentally measured by using Split Hopkinson Pressure Bar (SHPB). Results confirm that the fluid imbibition phenomena perform a promising influence on dynamic rock mechanics of tight shale. There are very different responses induced by air dry, oil, and water imbibition on dynamic rock strength, cutting size, and energy consuming. Oil imbibition is capable to enhance the strength, achieve uniform size of cutting, and consume less specific energy to break unit volume of rock. Comparatively, water imbibition sharply reduces the strength and consumes much less specific energy. Air dry condition makes much bigger and non-uniform cuttings and consumes the most specific energy. Those observations are no longer the same with obtained knowledge from sandstone and concrete, which attribute to the nanoscale pore distribution and fluid sensitivity of tight shale.

Effect of Microstructure Deficiency on Quasi-static and Dynamic Compressive Strength of Crystalline Rocks

Grain size and microdeficits control static and dynamic behavior of crystalline rocks. Therefore, characterization and evaluation of the impact of such intrinsic parameters on mechanical behavior of rocks are necessary. Two marble stones of Baghat and Maroon in Iran, which consist of a unique mineral composition but different microstructural features, were selected as test samples. Microstructures of rocks were characterized using different techniques such as scanning electronic microscopic, fluorescent replacement and polarization microscopic techniques. A number of both rock types with different sizes were prepared and subjected to quasi-static and dynamic loading by a split Hopkinson pressure bar under different strain rates conditions, and mechanical properties of both marbles were measured. Microscopic studies about microstructure deficits show that fractal dimension in Maroon marble is higher than in Baghat marble. The results also show that the dynamic strength in both marbles is higher than their static strength. Strain rate controls mechanical properties in both rock types; however, in Baghat marble with a relatively lower microstructure intensity, this phenomenon is more significant. With increasing sample volume, dynamic strength of rocks is decreased. Morphology study of fracture surface in microscale indicated that in dynamic loading the intensity of created microcracks particularly intercrystalline microcracks is increased compared to the quasi-static loading condition.

Dynamic Mechanical Property Deterioration Model of Sandstone Caused by Freeze–Thaw Weathering

In cold climate regions, rock engineering structures are subjected to repeated processes of freeze–thaw weathering and consequently the integrity of these structures will gradually deteriorate. The resulted reduction in rock strength makes the structures become increasingly more vulnerable to external loads, particularly to dynamic loads such as blasting or earthquakes, even when these loads are below the original designed capacity. In this work, the reductions in static and dynamic strengths of sandstones after they are treated with different number of freeze–thaw cycles were studied using conventional UCS experiments and impact tests with split Hopkinson pressure bar apparatus. Based on the experimental results, a decay model was used to describe the reduction of rock strength with the increasing number of freeze–thaw weathering cycles. For the prediction of the degradation of dynamic rock strength corresponding to freeze–thaw weathering, a model describing the dynamic increase factor for the dynamic rock strength corresponding to different strain rates and specimen sizes was proposed and its parameters are obtained by regression analysis of published experimental data. These two models were then combined into a unified model which can be used to describe the reduction in the dynamic strength of rocks when they are subjected to repeated freeze–thaw weathering processes. Though only tested on sandstones, the proposed unified model, with different parameters, is expected to be applicable to other types of rocks as long as the rocks undergo the same or similar damage mechanism when they are subjected to freeze–thaw weathering processes.

Dynamic strength of rock with single planar joint under various loading rates at various angles of loads applied

Intact rock-like specimens and specimens that include a single, smooth planar joint at various angles are prepared for split Hopkinson pressure bar (SHPB) testing. A buffer pad between the striker bar and the incident bar of an SHPB apparatus is used to absorb some of the shock energy. This can generate loading rates of 20.2–4627.3 GPa/s, enabling dynamic peak stresses/strengths and associated failure patterns of the specimens to be investigated. The effects of the loading rate and angle of load applied on the dynamic peak stresses/strengths of the specimens are examined. Relevant experimental results demonstrate that the failure pattern of each specimen can be classified as four types: Type A, integrated with or without tiny flake-off; Type B, slide failure; Type C, fracture failure; and Type D, crushing failure. The dynamic peak stresses/strengths of the specimens that have similar failure patterns increase linearly with the loading rate, yielding high correlations that are evident on semi-logarithmic plots. The slope of the failure envelope is the smallest for slide failure, followed by crushing failure, and that of fracture failure is the largest. The magnitude of the plot slope of the dynamic peak stress against the loading rate for the specimens that are still integrated after testing is between that of slide failure and crushing failure. The angle of application has a limited effect on the dynamic peak stresses/strengths of the specimens regardless of the failure pattern, but it affects the bounds of the loading rates that yield each failure pattern, and thus influences the dynamic responses of the single jointed specimen. Slide failure occurs at the lowest loading rate of any failure, but can only occur in single jointed specimen that allows sliding. Crushing failure is typically associated with the largest loading rate, and fracture failure may occur when the loading rate is between the boundaries for slide failure and crushing failure.

Numerical Tests on Failure Process of Rock Particle under Impact Loading

By using numerical code RFPA2D(dynamic version), numerical model is built to investigate the failure process of rock particle under impact loading, and the influence of different impact loading on crushing effect and consumed energy of rock particle sample is analyzed. Numerical results indicate that crushing effect is good when the stress wave amplitude is close to the dynamic strength of rock; it is difficult for rock particle to be broken under too low stress wave amplitude; on the other hand, when stress wave amplitude is too high, excessive fine particle is produced, and crushing effect is not very good on the whole, and more crushing energy is consumed. Secondly, in order to obtain good crushing effect, it should be avoided that wavelength of impact load be too short. Therefore, it is inappropriate to choose impact rusher with too high power and too fast impact frequency for ore particle.

Calibration of Rebound Hardness Numbers to Unconfined Compressive Strength in Shale Formations

Young Technology Showcase Today, unconventional-reservoir characterization is actively studied; however, rock-mechanics tests are still restricted to intact rocks. Intact intervals are necessary to acquire plug specimens (Fig. 1a). Anisotropy caused by the presence of various discontinuities affects rock-mechanical properties significantly. Shale formations consist largely of laminations, pre-existing microcracks, and fractures along bedding planes (Fig. 1b), and they inhibit sample preparation with the standard geometry (1:2 ratio of diameter/length). The primary purpose of this study is to develop a method that overcomes the restrictions of rock-mechanics tests with respect to unconventional shale formations. This study applies an Equotip Hardness Tester (EHT) and uses its rebound hardness numbers (RHNs) to estimate unconfined compressive strength (UCS) with the recovered cores from shale formations. The application enhances laboratory-testing results to calibrate petrophysical models that show higher dynamic rock strength generated by acoustic logging or ultrasonic-wave-velocity measurement. It can also support fracture designs by providing a continuous strength- contrast profile to identify where fracturable zones and fracture barriers exist. RHNs The EHT was originally developed by Dietmar Leeb (1978) to measure the hardness of metallic materials. Recently, its use has been extended to various intact rocks to estimate their UCS values (Meulenkamp 1997; Meulenkamp and Grima 1999; Papsworth 2014; Verwall and Mulder 2000). The EHT is a small, portable, electronic, battery- operated, spring-loaded device. A spherical tungsten carbide tip impacts the metal or rock surface driven by a constant spring force, then rebounds. The impact and rebound velocities of the tip are measured and converted into RHNs (Proceq 2014). Fig. 2 presents a schematic of the impact device.

A systematic approach to unconventional play analysis: the oil and gas potential of the Kockatea Shale and Carynginia Formation, North Perth Basin, Western Australia

Key formations throughout the North Perth Basin have been mapped from 3D and 2D seismic data to define depth grid inputs to a 3D basin model calibrated with temperature and maturity data from 45 wells, plus an additional 27 pseudo well models. The Permian Carynginia Formation and Early Triassic Hovea Member of the Kockatea Shale have been defined in this model as unconventional shale reservoir targets. Basin-wide pyrolysis data have been used to construct kinetics curves for both the Carynginia Formation and Kockatea Shale, which define Type D/E and mixed B, and D/E kerogen types, respectively. When combined with thermal history inputs, these source rocks expel and retain significant volumes of hydrocarbons, of which the free hydrocarbons in the retained components reach 22 BCF/km2 for the Carynginia Formation gas and 8 MMBBLS/km2 and 21 BCF/km2 for the Hovea Member liquids and gas, respectively. The defined kinetics relationships allow the estimation of kerogen-specific oil and gas windows, which have been applied across the study area to map unconventional play fairways for both formations, and to calculate the initial total organic carbon (TOC) and hydrogen index (HI) for each unit prior to significant maturation. This study employs a mass balance approach through basin modelling as a means of estimating likely retained hydrocarbon volumes in key unconventional reservoirs in the basin. Sonic and density data from 28 wells in the basin have been used to calculate theoretical porosity to determine likely areas of overpressure. When combined with observed connection gas peaks and modelled maturity, there is a reasonable correlation suggesting that the basin exhibits modest overpressure of 2–6 MPa associated with the main gas window at 1.2 Ro% and this observation is applied to the play fairway mapping process. Play fairways are further constrained through geomechanical and stress considerations from mechanical earth models (MEMs) built from log and image data for wells in the basin. These data define an overall strike-slip stress regime with SHmax consistently oriented east to west with the exception of local perturbations. Dynamic rock strength calculated from the same MEM process shows target zones in the Kockatea Shale and Carynginia Formation ranging from ~60–130 MPa unconfined compressive strength (UCS), calibrated against available static data. The net thickness of rock with a UCS &gt;75 MPa is mapped and overlain on retained in place hydrocarbon maps to restrict the area of likely economically extractable resource. While unconventional play cut-offs in the Perth Basin are notably lower than those commonly used in shale gas plays in the US, successful stimulation of Perth Basin rocks has been demonstrated by substantial flows from wells such as Arrowsmith–2. This study outlines a new workflow for mapping unconventional resources and suggests that Australian rocks are unique in both depositional environment and mechanical properties such that unconventional assessment using US play cut-offs may be misleading.

Numerical simulation on rock failure under combined static and dynamic loading during SHPB tests

The failure process of rock subjected to combined static and dynamic loading constitutes the mechanism of many engineering applications such as rockburst prediction, rock fragmentation, as well as rock drilling and blasting. In this study, the basic principle for simulating the damage and failure process of rock under combined static and dynamic loading is introduced, and it is implemented into RFPA-Dynamics (Rock Failure Process Analysis for Dynamics), a finite element package to analyze the failure process of rock-like media subjected to dynamic loading. The extended RFPA-Dynamics code is firstly validated by simulating the stress distribution and damage evolution in rock specimens during conventional SHPB (Split Hopkinson Pressure Bar) tests. Then, it is utilized to simulate the failure process of rock under combined static and dynamic SHPB tests, and the effects of axial static stress and dynamic stress on the damage and failure process of rock are examined. The strength increase factor (SIF) under combined static and dynamic loading, depending on the rock heterogeneity, static stress and strain rate, is predicted using numerical simulations, and the mechanisms associated with the increase of dynamic strength of rock subjected to the combined static and dynamic loading are clarified. The rock strength predicted with numerical simulations, which is closely related to the rock heterogeneity, is in qualitative agreement with that observed in laboratory SHPB tests, although the predicted rock strength under combined static and dynamic loading is still lower than those from experiments.

Dynamic strength of rocks and physical nature of rock strength

Time-dependence of rock deformation and fracturing is often ignored. However, the consideration of the time-dependence is essential to the study of the deformation and fracturing processes of materials, especially for those subject to strong dynamic loadings. In this paper, we investigate the deformation and fracturing of rocks, its physical origin at the microscopic scale, as well as the mechanisms of the time-dependence of rock strength. Using the thermo-activated and macro-viscous mechanisms, we explained the sensitivity of rock strength to strain rate. These mechanisms dominate the rock strength in different ranges of strain rates. It is also shown that a strain-rate dependent Mohr-Coulomb-type constitutive relationship can be used to describe the influence of strain rate on dynamic rock fragmentation. A relationship between the particle sizes of fractured rocks and the strain rate is also proposed. Several time-dependent fracture criteria are discussed, and their intrinsic relations are discussed. Finally, the application of dynamic strength theories is discussed.

Energy consumption in rock fragmentation at intermediate strain rate

In order to determine the relationship among energy consumption of rock and its fragmentation, dynamic strength and strain rate, granite, sandstone and limestone specimens were chosen and tested on large-diameter split Hopkinson pressure bar (SHPB) equipment with half-sine waveform loading at the strain rates ranging from 40 to 150s^(-1). With recorded signals, the energy consumption, strain rate and dynamic strength were analyzed. And the fragmentation behaviors of specimens were investigated. The experimental results show that the energy consumption density of rock increases linearly with the total incident energy. The energy consumption density is of an exponent relationship with the average size of rock fragments. The higher the energy consumption density, the more serious the fragmentation, and the better the gradation of fragments. The energy consumption density takes a good logarithm relationship with the dynamic strength of rock. The dynamic strength of rock increases with the increase of strain rate, indicating higher strain rate sensitivity.