Dynamic Rock Properties Research Articles

Strain Rate Effect on the Mechanical Properties and Fracture Surface Roughness of Sandstone Subjected to Dynamic Direct Tension

Dynamic properties of rocks are extremely important in a variety of rock mechanics and rock engineering problems. The split Hopkinson tensile bar (SHTB) system is used in this paper to measure the mechanical properties of sandstone specimens under dynamic direct tension, and the full stress-strain curves of the specimens at different strain rates is obtained. The experimental results indicate that the tensile strength, the tensile modulus and the peak strain of the specimen increase almost linearly with the strain rate. The increases in the tensile strength, the tensile modulus and the peak strain reach 125 %, 37 % and 98 % respectively as the strain rate increases by 252 %. The microscopic structure characteristics of the fracture surfaces after the tensile failure of the specimens are investigated by three-dimensional scanning. The results suggest that the fracture surface roughness of the sandstone under direct tension is significantly sensitive to the strain rate. Both the roughness coefficient and the fractal dimension of the specimen increase with the strain rate. The fracture surface of the specimen changes from a relatively flat two-dimensional state to a three-dimensional state, and its relative area gradually increases. Finally, it is manifested from the aspect of energy consumption that both the energy consumed in the fracture process and the dynamic direct tensile strength enhance with the fracture surface roughness. It is believed that the investigation results can provide an important reference for the research on dynamic properties of rocks involved in experimental research and engineering practice.

Physics and Dynamics Characteristics and Energy Analysis of Freeze‐Thaw Limestone

In order to study the physical and dynamic properties of rock after damage, an open‐type saturated water freeze‐thaw test at ±20°C was carried out on the limestone specimen, the size, quality, and longitudinal wave velocity with measured after freeze‐thaw cycles for 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 times, and the SHPB test device was used to carry out the impact compression test with eight kinds of loading rate. This text analyzes the damage evolution characteristics on the physical properties of limestone of cycle times of freeze‐thaw and discusses the dynamic compression mechanical characteristics and energy dissipation law of limestone specimens after freeze‐thaw cycles. The test results show that the mass and longitudinal wave velocity of the specimen decreased and the volume and density increased. The damage factors have the quadratic function positive correlation with the cycle time of freeze‐thaw. Moreover, the dynamic compression stress‐strain curves of the specimens under different loading rates are similar in shape, and the curve shows an upward trend with increasing loading speed. In addition, with the loading rate increasing, the dynamic compressive strength and dynamic elastic modulus of the specimen increased and the dynamic strain decreased. In the SHPB test, the reflected energy, transmitted energy, and absorbed energy all increased linearly with incident energy. The dynamic compressive strength and absorbed energy increase as a power function, and the strain rate and absorbed energy increase as a quadratic function.

Dynamic Mechanical Behavior of Granite under the Effects of Strain Rate and Temperature

AbstractThe dynamic properties of rock under different strain rates and temperatures play an important role in the stability of rock mass. To investigate the dynamic behavior of granite specimens s...

Mechanical earth modeling and fault reactivation analysis for CO2-enhanced oil recovery in Gachsaran oil field, south-west of Iran

There is a huge potential for CO2-EOR and CO2 storage in depleted carbonate reservoirs in the south-west of Iran. In the first step of a CO2-EOR operation, a geomechanical assessment is needed to find out geological conditions, mechanical and strength properties of formation rocks (e.g., reservoir rock and caprock), in situ stress magnitudes and orientation and in situ pore pressure profile. An integrated analysis is performed in this work for geomechanical assessment of a reservoir–caprock system in Gachsaran oil field, south-west of Iran. A one-dimensional mechanical earth model (MEM) is built for 47 wells in the studied field based on drilling and logging data, laboratory and in situ tests. Static elastic and strength parameters of various formation rocks (limestone, dolomite, anhydrite, gray marl and salt) are evaluated from laboratory experiments. Empirical correlations are obtained to convert dynamic rock properties and well-log data to static elastic properties and strength parameters. The initial in situ pore pressure is calculated using modified Eaton method. In situ stresses state is evaluated based on the poroelastic method and calibrated using LOT and XLOT tests. The orientation of in situ stresses is obtained based on image logs. Fractures and faults analysis is performed to determine their orientations. An analytical analysis is performed to estimate the maximum sustainable CO2 injection pressure to prevent fault reactivation. This study presents a comprehensive method to reservoir and caprock characterization using laboratory and well-log data and 1D mechanical earth model. It helps the analysis of the geomechanical problems during CO2-EOR and provides the necessary information to build 3D geomechanical model for numerical simulations.

Experimental and numerical investigation of crack propagation and dynamic properties of rock in SHPB indirect tension test

To explore the dynamic mechanical properties and crack propagation law of rock under high strain rate impact loading, an experimental investigation with 12 sets of Brazilian disk (BD) rock specimens under Split Hopkinson Pressure Bar (SHPB) loading was undertaken. Unlike the previous researches that study the dynamic mechanical properties and crack propagation law individually, the relationship of them has also been investigated and discussed. More specifically, a dynamic resistance strain gauge and a high-frame camera were employed to simultaneously record stress wave data and rock destruction process videos under different impact velocities. Based on image processing technique, a new calculation method of crack propagation velocity was proposed and then the stress-strain characteristics and crack propagation features were analyzed. The experimental results indicate that: (1) Crack propagation rate of the crack along X-Y direction both rises with the impact velocity increase; (2) the strain of rock along the loading axis is much larger than the vertical direction; (3) there is a certain quantitative relationship between the stress-strain state and the crack area of the rock specimen after it has been damaged. Finally, a numerical model based on Peridynamic theory was developed to simulate crack propagation and dynamic constitutive relationship of rock materials with BD configuration in indirect tension test under SHPB loading, and further reveal the fracture behavior and mechanism of rock materials under high strain rate loading.

A Comparative Study on Rock Properties in Splitting and Compressive Dynamic Tests

The dynamic properties of rock in splitting (tensile) and compressive tests are the focus of rock dynamic mechanics research. In this study, a split Hopkinson pressure bar (SHPB) apparatus was used to assess the comparability and strain rate effect of rock samples under two kinds of load conditions. With an increase in impact velocity under compression and splitting (tensile) tests, the strain rate of the samples increased continuously. The rock properties and strain rate in the two kinds of samples exhibited clear similarities: the dimensionless stress‐strain curves of the rock samples were similar under the same strain rates. The peak strength and elastic modulus of the two kinds of samples increased with an increase in the strain rate. The number of cracks increased from intergranular to transgranular failure. The rack initiation value of the two kinds of samples was close at the same strain rate and declined with an increase in the strain rate as evidenced by a power function.

Dimension Effect on the Ultrasonic Pulse Velocity

Ultrasonic pulse velocity test is non-destructive and relatively simple to carry out at both in-situ and laboratory conditions. The sonic velocity (SV) test has been used to determine the dynamic properties of rocks in different applications of rock and geotechnical engineering. In the laboratory conditions, the SV tests determine the velocity of propagation of elastic waves through the rock samples. The velocities of P (Vp) and S (Vs) waves are calculated from the measured travel times and distance between transmitter and receiver. The SV values of rocks are influenced and controlled by different factors such as rock type, density, grain size and shape, etc. This study focuses on the possible dimensional, sample diameter and sample length to sample diameter ratio, effect on the mean Vp, mean dynamic elastic modulus, Ed, mean shear modulus, Gd, mean Poisson’s ratio, υd and mean dry density, ρ. For sample preparation and the SV tests, the representative rock blocks of Upper Eocene-aged limestone, Dammam Formation, is collected from Jabal Hafeet in Al Ain City, United Arab Emirates (UAE) and taken to the laboratory. Forty-nine SV tests was carried out on the prepared core samples having different dimensions. The study reveals that the Vp, Ed, Gd, υd and ρ of limestone can be estimated using the simple linear mathematical equations. Even though the obtained equations are useful and practical, but it still needs better ratification with more samples having different dimensions in order to use confidently in engineering applications.

Experimental and Numerical Evaluation of Rock Dynamic Test with Split-Hopkinson Pressure Bar

Feasibility of rock dynamic properties by split-Hopkinson pressure bar (SHPB) was experimentally and numerically evaluated with ANSYS/LS-DYNA. The effects of different diameters, different loading rates, and different propagation distances on wave dispersion of input bars in SHPB with rectangle and half-sine wave loadings were analyzed. The results show that the dispersion effect on the diameter of input bar, loading rate, and propagation distance under half-sine waveform loading is ignorable compared with the rectangle wave loading. Moreover, the degrees of stress uniformity under rectangle and half-sine input wave loadings are compared in SHPB tests, and the time required for stress uniformity is calculated under different above-mentioned loadings. It is confirmed that the stress uniformity can be realized more easily using the half-sine pulse loading compared to the rectangle pulse loading, and this has significant advantages in the dynamic test of rock-like materials. Finally, the Holmquist-Johnson-Concrete constitutive model is introduced to simulate the failure mechanism and failure and fragmentation characteristics of rock under different strain rates. And the numerical results agree with that obtained from the experiment, which confirms the effectiveness of the model and the method.

Analysis of the Behavior of Sedimentary Rocks Under Impact Loading

In multiple engineering fields such as rock drilling or building constructions or extreme events like earthquakes or impacts, the dynamic properties of rock play an important role. A way to model these events and define measures to minimize the damage derived from these events is created by means of numerical analysis. Hence, the knowledge of the dynamic material behavior is essential for studying the effects of such a loading scenario. Solid geological materials, from the family of the sedimentary rocks, have been analyzed under quasi-static loads. However, there is a lack of knowledge when high strain rate loadings are involved. Within this context, the paper focuses on the experimental characterization of two sedimentary rocks, sandstone and limestone, under impact loading using the Hopkinson-Bar spallation and compression tests. The analysis encompasses the determination of the tensile and compressive properties as well as the comparison between the quasi-static and dynamic behavior (dynamic increase factors). The paper fills the gap of information existing about dynamic behavior of sedimentary rocks under strain rates between 100 and 5.2 × 102 s−1. Furthermore, the fragmentation under different strain rates is investigated and conclusions with respect to energy absorption capacity are drawn.

Advanced methods for the evaluation of a hybrid-type unconventional play: The Bakken petroleum system

Hybrid-type plays such as the Bakken petroleum system (BPS) can be particularly challenging from an interpretation, completion, or production perspective due to the mix of conventional and unconventional elements coexisting within a relatively short depth interval. In the BPS, conventional aspects include the presence of separate reservoir intervals, which, depending on your location within the basin, may include the Scallion, Middle Bakken, Sanish, and Three Forks. Unconventional aspects include the Lower Bakken and Upper Bakken shales, which are organic-rich shales comprising source rock and reservoir. Developing an accurate petrophysical evaluation of these formations requires a priori knowledge of the mineralogy, fluids, and geomechanical properties such that appropriate logging measurements, core analysis methods, and interpretation techniques can be obtained and used. During the development phase of an oil field, the log and core measurements being acquired and the petrophysical evaluation being performed may vary significantly from well to well across the field. Some wells may have triple-combo wireline or logging-while-drilling measurements consisting of bulk density, neutron porosity, and induction or laterolog resistivity, supplemented with a total gamma ray measurement. Borehole sonic logs may also have been acquired in some wells primarily for seismic calibration, geomechanical modeling, and hydraulic stimulation design. If the “standard” suite of measurements and petrophysical evaluation being provided fail to accurately represent the true complexity of the formations being evaluated, the asset valuation will, in most cases, be negatively impacted. Our formation evaluation of the BPS led to the identification of unique petrophysical challenges for each of the formations comprising the BPS. Traditional formation evaluation methods were applied to the BPS based on triple-combo measurements, a traditional petrophysical analysis, and the evaluation of net feet of pay. Advanced evaluation methods and techniques were then applied to address the petrophysical complexities identified with core evaluation, advanced log measurements, and discrepancies between the two. New petrophysical models were developed and fine-tuned to address the shortcomings of the simple models, and the net feet of pay were reevaluated using these new models. The detailed formation evaluation program used to characterize the BPS consisted of standard triple-combo logs supplemented with advanced downhole measurements including: (1) triaxial resistivity for thin-bed analysis, (2) nuclear magnetic resonance for porosity, free-fluid, and kerogen identification, (3) dielectric dispersion for water saturation, (4) geochemical spectroscopy for mineralogy and total organic carbon, and (5) dipole sonic for dynamic rock properties. Petrophysical models were developed using deterministic and probabilistic methods to integrate the measurements acquired for the most accurate analysis of porosity, saturation, and mineralogy and to best describe the hydrocarbon production potential of the BPS.

Class I and Class II Rocks: Implication of Self-sustaining Fracturing in Brittle Compression

Tests to determine the complete stress–strain curve of rocks indicate whether the rocks can be classified a Class I or Class II. Class II rocks exhibits the potential for self-sustained failure in the post-peak region. The purpose of the research described in this paper was to investigate whether or not this self-sustained failure characteristic is related to the fragmentation of the rock. The aim of the research was, therefore, to determine possible relationships between fragmentation and various properties of several rocks types, including the influence of the Class II characteristic. Fragmentation of rock depends on its self-sustaining failure behaviour and the energy available in the post-peak region to shatter the rock. The correlation of static and dynamic rock properties with size of fragments resulting from compression tests demonstrate clear relationships of Class II rocks, but the same cannot be said for Class I rocks. Analyses of test results show that fragmentation increases with an increase in rock strength, and is explosive for Class II rocks. Probability density distributions were constructed to show the overall comparison of fragment sizes produced during failure of Class II and Class rocks. The calculated probability of passing at X50 and X10 sieve sizes show that Class II rocks as a group are more finely fragmented. It can therefore be concluded that, when breaking rocks under the same steady loading conditions, Class II rocks will show greater fragmentation than Class I rocks.

Determination of Atterbeg Limits Based on Wave Propagation Techniques

Wave propagation techniques have been extensively used for many years in geotechnical engineering. Since these techniques are nondestructive and easy to apply, they have been used increasingly for determination of dynamic properties of rocks and soils. In this study, a seismic refraction survey line was conducted across two boreholes, and the Atterberg limits were calculated using laboratory test, and equation related between Atterberg limits and Vp which identify from seismic refraction method. Experimental results showed that the Liquid limit have a good correlation while Plastic limit and Plasticity index show moderate and poor correlation, respectively. Finally, the seismic refraction method can be used for Liquid limit and Plastic limit calculation.

Digital Rocks: Emerging Technology to Business-Proven Capability

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170752, “Digital Rocks: Developing an Emerging Technology Through to a Proven Capability Deployed in the Business,” by J.T. Fredrich, D.L. Lakshtanov, N.M. Lane, E.B. Liu, C.S. Natarajan, D.M. Ni, and J.J. Toms, BP, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed. This paper describes the development of “digital-rocks” technology, in which high-resolution 3D image data are used in conjunction with advanced modeling and simulation methods to measure petrophysical rock properties. This technology has developed into a proven capability sanctioned for use in the operating assets of a large multinational oil company. Digital-rocks technology can improve evaluation of reservoir quality and quantification of hydrocarbons in place, and it can help inform the development of improved-recovery methods. Introduction The premise of the technology is to apply state-of-the-art high-resolution 3D imaging technologies to derive digital descriptions of reservoir rocks, typically at a resolution of approximately 1–5 Μm, or even lower for complex rocks with significant microporosity less than 1 Μm in dimension. These digital descriptions can be thought of as a numerical grid or mesh and, in conjunction with the appropriate models and numerical algorithms, can be used to perform massively parallel simulations of pore-scale processes of interest. These simulations can then be interpreted to derive macroscopic static and dynamic rock properties. Because the geometrically and topologically complex pore-scale architecture of the rock is captured in the 3D image and faithfully represented in the computational mesh, the numerical solution of the underlying physics is not compromised as long as the image is acquired with the requisite fidelity, and at a suitable resolution and over a representative elementary volume. In the field of computational physics, this general approach is known as direct numerical simulation (DNS). While DNS is often used to describe a solution method for problems in computational fluid dynamics, there exists an underlying generality and the same concept can be applied to other partial-differential equations (PDEs) of interest, thereby expanding application to various rock properties in addition to intrinsic permeability.

Investigating the effects of micro-defects on the dynamic properties of rock using Numerical Manifold method

In this study, the Numerical Manifold method (NMM) is extended to investigate the effects of micro-defects on the dynamic mechanical properties of rock under different strain rates. The displacement decomposition technique is incorporated in the NMM to estimate the mixed mode stress intensity factors at the crack tip. A dynamic crack growth criterion is also incorporated in the NMM for crack growth analysis. The developed NMM is first validated by a simple sliding crack model. The developed model is then applied to investigate the effects of the micro-cracking properties such as initial micro-crack length, initial micro-crack inclination angle and initial micro-crack separation distance on the dynamic mechanical properties of a granite under different strain rates ranging from 10−4/s to 100/s. The effect of confining stress on the granite dynamic strength is also investigated. Simulation results illustrated that the initial micro-crack length and the confining stress have a significant effect on the dynamic strength. The effect of micro-crack separation distance, on the other hand, is heavily dependent on the ratio of separation distance to the initial micro-crack length.

Dynamic properties of anhydrites, marls and salts of the Gachsaran evaporitic formation, Iran

A large carbonate oil field in Iran is suffering from severe casing collapses and related operational problems in anhydrite, marl and salt sequences of the Gachsaran cap rock formation. To investigate the causes and cures of operational problems, specifically casing collapse, knowing geomechanical properties of anhydrite, marl and salt of this formation is a prerequisite. However, taking cores in this formation is virtually impossible due to high solubility and weakness of the rocks. Moreover, there are insufficient well log data in this formation and the only available running well log is Vp. In this paper, in order to obtain the dynamic parameters of the Gachsaran formation, Vp, Vs and ρb in anhydrite, marl and salt cores, which had been taken from depths up to 300 m, were measured. Moreover, Vp and Vs in salt cores under different triaxial and hydrostatic stress conditions were obtained. The Vp‐Vs, (Vp/Vs)‐Vp and Vp‐ρb relations in anhydrite, marl and salt were investigated. The established relations in these anhydrite samples were verified by the data derived from limited cores which were taken from 3600 m depth. The relations between dynamic properties of salt with lateral and hydrostatic stresses were investigated. In conclusion, Vs, ρb and the ratio of Vp/Vs in anhydrite and marl can be estimated through the established relations and having Vp logs in the Gachsaran formation. As a result, the dynamic properties of these rocks can be calculated in different depths of this evaporitic formation. Furthermore, the dynamic properties of salt rock seem to be constant in various depths and under differing triaxial and hydrostatic stress conditions.

Establishment of a Dynamic Mohr–Coulomb Failure Criterion for Rocks

Abstract Static Mohr–Coulomb Failure Criterion for rocks has been used extensively in various rock engineering applications. In this model, the compressive strength, tensile strength, and shear strength are related. To investigate the applicability of the Mohr–Coulomb model to dynamic failures, we studied the correlation of the three dynamic rock properties: compressive strength, tensile strength and punch shear strength. The strengths are quantified using a split Hopkinson pressure bar (SHPB) system. The methods for acquiring these strengths were briefly discussed. A fine-grained sandstone, Longyou sandstone (LS) was studied using these methods. The results showed that the compressive strength calculated from the punch shear strength of LS matched well with the experimental UCS results, thus the punch shear strength could be effectively used to predict the UCS of rocks. The tensile strength calculated from the punch shear strength of LS also has exhibited a good trend with the experimental results of Brazilian tensile strength. We concluded that the famous Mohr–Coulomb criterion can be extended to the dynamic range. However, caution has to be taken in determining the loading rates for different strength results.

Establishment of a Dynamic Mohr–Coulomb Failure Criterion for Rocks

Abstract Static Mohr–Coulomb Failure Criterion for rocks has been used extensively in various rock engineering applications. In this model, the compressive strength, tensile strength, and shear strength are related. To investigate the applicability of the Mohr–Coulomb model to dynamic failures, we studied the correlation of the three dynamic rock properties: compressive strength, tensile strength and punch shear strength. The strengths are quantified using a split Hopkinson pressure bar (SHPB) system. The methods for acquiring these strengths were briefly discussed. A fine-grained sandstone, Longyou sandstone (LS) was studied using these methods. The results showed that the compressive strength calculated from the punch shear strength of LS matched well with the experimental UCS results, thus the punch shear strength could be effectively used to predict the UCS of rocks. The tensile strength calculated from the punch shear strength of LS also has exhibited a good trend with the experimental results of Brazilian tensile strength. We concluded that the famous Mohr–Coulomb criterion can be extended to the dynamic range. However, caution has to be taken in determining the loading rates for different strength results.

Improving reservoir models of Cretaceous carbonates with digital outcrop modelling (Jabal Madmar, Oman): static modelling and simulating clinoforms

ABSTRACT In Jabal Madmar in the Sultanate of Oman, Cretaceous epeiric carbonate platform architectures were characterized by employing a digital outcrop modelling workflow. A framework model for Natih Sequence I (Natih E member) was established, which embeds a meticulously studied platform-top incision and shoal complex. Outcrop-scale clinoforms are recognized in these shoals by hectometre-scale (100 m long) medium to high-angle (1–5°) inclined stratal surfaces comprising texture-based facies transitions. These clinoforms are usually beneath the resolution of seismic data and as such are not easily recognized and correlated between wells. Geologically realistic clinoform models were built using a well-defined stratigraphic model that incorporated inclined surfaces in the model grid and if available, data on lateral facies transitions. Waterflood simulations demonstrated improved sweep efficiency in these models. In contrast, simple models without clinoform heterogeneities resulted in less efficient piston-like patterns of sweep. The study presented in this paper demonstrates an outcome contrary to previous studies, as in this study, barriers to flow are absent. Complex clinoform models must be considered in reservoir modelling workflows to correctly derive static and dynamic rock properties. This is because outcrop-scale clinoforms have a potential impact on reservoir behaviour under secondary and tertiary recovery mechanisms.

Dynamic Properties of Intact Rock Samples Subjected to Cyclic Loading under Confining Pressure Conditions

Recently, attention has focused on the dynamic properties of rocks with the goal of understanding their dynamic mechanical features under different loading histories and conditions (e.g., Stavrogin and Tarasov 2001; Bagde and Petros 2009). Researchers have carried out studies on different types of rocks to examine the loading effects and loading strain rates on their strength and deformation characteristics (e.g., Zhao 2000; Cho et al. 2003; Mahmutoglu 2006; Wang et al. 2009; Liang et al. 2010). More extensive work on cyclic loading has explored whether rocks are subject to weakening from fatigue (e.g., Burdine 1963; Prost 1988; Singh 1989; Tien et al. 1990; Li et al. 2001; Fuenkajorn and Phueakphum 2010).

Pore-facies as a tool for incorporation of small-scale dynamic information in integrated reservoir studies

In this study, the quantification and incorporation of pore geometry, a qualitative parameter, and a source of dynamic information, will be demonstrated in the integrated reservoir studies. To quantify pore geometry, mercury injection capillary pressure (MICP) curves have been exploited. For each MICP curve, 20 parameters were derived and multi-resolution graph-based clustering was applied to classify the curves into nine representative distinct clusters. The number of clusters was determined based on petrography and cluster analysis. The quantified pore geometry in terms of discrete variable has been called pore-facies, and like electro-facies and litho-facies could be used in facies modelling and rock typing phases of an integrated study. The dependence of dynamic reservoir rock properties on pore geometry makes the pore-facies an interesting and powerful approach for incorporation of small-scale dynamic data into a reservoir model. A comparison among various facies definitions proved that neither litho-facies nor electro-facies is capable of characterizing dynamic rock properties, and the best results were achieved by the pore-facies method. Based on this study, it is recommended that for facies analysis in reservoir modelling, methods based on pore characteristics such as pore-facies, introduced in this paper, be used rather than traditional facies that rely on matrix properties. The next generation of the reservoir models which incorporate pore-facies-based rock types will provide a basis for more accurate static and dynamic models, a narrower range of uncertainty in the models, and a better prediction of reservoir performance.