Closure Of Microcracks Research Articles

Experimental study and finite element simulation of hydrogen permeation resistance of Ti-6Al-4V alloy strengthened by laser peening

The hydrogen permeation resistance of Ti-6Al-4V alloy treated by laser peening (LP) was investigated. Electrochemical hydrogen charging, hydrogen permeation experiments were carried out respectively. The residual stress, micro-hardness of hydrogenated and non-hydrogenated specimens were analyzed. The apparent hydrogen diffusion coefficient, lattice hydrogen concentration and hydrogen trap density were compared. LP process and hydrogen diffusion process were numerically simulated by ABAQUS software. The results show that LP can effectively increase the compressive residual stress (CRS) and micro-hardness on the surface of Ti-6Al-4V alloy. LP also promoted the closure of the original micro-cracks by forming a dense strengthening layer on the surface of the material, which reduced the hydrogen concentration and the diffusion coefficient in the crystal lattice. The laser power was inversely related to the hydrogen diffusion coefficient. The numerical simulation results reveal that the hydrogen concentration in the depth direction of LPed specimen decreases significantly, and the maximum drop of 50% for hydrogen concentration was appeared at the depth of 0.6 mm from the top surface. Moreover, the distribution trend of hydrogen concentration in the LP area is consistent with that of CRS, which indicates that CRS plays a positive role in the inhibition of hydrogen diffusion.

Transport property evolution during hydrostatic and triaxial compression of a high porosity sandstone

Permeability and porosity are two crucial properties related to underground gas storage. This research investigates the permeability and porosity evolution of a Vosges sandstone under different stress paths. Under hydrostatic stress, the influence of irreversible deformation on permeability is more significant than on porosity. The permeability is more sensible to confining pressure than porosity. Under non-hydrostatic loading, permeability decreases continuously with deviatoric stress. At volumetric compression stage, the decrease is due to compression and closure of pores and micro-cracks. At volumetric dilatancy stage, sand production in the shear band causes the permeability decrease. Sand production mainly occurs at volumetric dilatancy stage when closing to failure.

A Spectral Microplane Model for the Anisotropic Damage Behavior of Shales

AbstractThe development of constitutive models for shales has been a challenge for decades due to the difficulty of characterizing the strongly anisotropic macroscopic behavior related to the inherent mesostructure and damage mechanisms. In this paper, a spectral microplane damage model is developed for the anisotropic damage behavior of shales. The modeling challenge of the anisotropic elasticity in the microplane model is theoretically overcome by the spectral decomposition theory without limitation on the degree of the anisotropy compared with other microplane models. The stiffness tensor of anisotropic shales is effectively decomposed into four different eigenmodes with the activation of certain groups of microplanes corresponding to the specific orientation of the applied stresses. The inherent and the induced anisotropic behavior is thus characterized by proposing suitable microplane relations on certain eigenmodes directly reflecting the initial mesostructure and the failure mechanisms. For the challenge of the postpeak softening behavior, two-scalar damage variables are introduced to characterize the tensile and the shear damage related to the opening and the closure of microcracks under different stress conditions. Comparison between numerical simulation and experimental data shows that the proposed model provides satisfactory predictions for both weakly and highly anisotropic shales including prepeak nonlinear behavior, failure strengths, and postpeak softening under different confining pressures and different bedding plane orientations.

High-efficiency techniques and micro-structural parameters to evaluate concrete self-healing using X-ray tomography and Mercury Intrusion Porosimetry: A review

Researchers have put significant effort into developing various healing agents for micro-crack closure to achieve a more durable structure and various destructive and non-destructive test methods are often used for the evaluation of concrete self-healing. In this paper, the use of the X-ray tomography and Mercury Intrusion Porosimetry is reviewed as the most advanced non-destructive test methodologies for the evaluation of concrete self-healing at the micro-structural level. The capabilities of the two test methods in correlation to self-healing in cementitious materials are presented via the void content, crack width/pore diameter and crack volume measurements. In addition, the paper suggests the geometrical aspect of voids as an alternative type of analysis for evaluating self-healing in cementitious materials.

ФІЛЬТРАЦІЙНО-ЄМНІСНІ ПАРАМЕТРИ НИЖНЬОПЕРМСЬКИХ КАРБОНАТНИХ ПОРІД ЗАХІДНОЇ ЧАСТИНИ ГЛИНСЬКО-СОЛОХІВСЬКОГО ГАЗОНАФТОНОСНОГО РАЙОНУ ДНІПРОВСЬКО-ДОНЕЦЬКОЇ ЗАПАДИНИ

The paper concerns the researches of the Lower Permian carbonate deposits reservoir properties of western part of Hlynsko-Solohivska area of gas-oil-bearing district of Dnieper-Donets depression. Such reservoir parameters as the open porosity factor, permeability coefficients and residual water saturation factor have been used to assess the promising rocks for the possible hydrocarbon reservoirs. Void structure of rocks with capillarimetric method and the correlation of rock density with their porosity were also studied. The porosity study was carried out in atmospheric and reservoir conditions. The bulk density of dry rock samples varies from 2212 kg/m3 to 2593 kg/m3 (mean 2413 kg/m3), water saturated rocks – from 2442 to 2642 kg/m3 (mean 2549 kg/m3), kerosene saturated rocks – from 2400 to 2622 kg/m3 (mean 2541 kg/m3); an apparent specific matrix density – from 2718 to 2828 kg/m3 (mean 2783 kg/m3). The open porosity coefficient of study rocks, saturated with the synthetic brine, varies from 0.045 to 0.181 (mean 0.127), if samples are saturated with kerosene then it varies from 0.049 to 0.184 (mean 0.128) and when N2 is used - from 0.063 to 0.217 (mean 0.149). The effective porosity has following values: 0.004-0.125 (0.036), and the residual water saturation factor - 0.4-0.97 (0.79). Analysis of reservoir conditions modeling revealed that porosity coefficient varies from 0.040 to 0.169 (mean 0.118). Due to the closure of microcracks under rock loading reduced to reservoir conditions the porosity decreases in comparison with atmospheric conditions, which causes a relative decrease in the porosity coefficient from 1.5 % to 11 % (mean 9.0 %). Capillar void of study rocks describes the prevalence of subcapillar pores (40-97 %, mean 79 %) while the overcapillars pores have range 1-22 % (mean 7 %) and the capillar pores - 2-38 % (mean 14 %). The permeability coefficient varies from 0.038 fm2 to 1.992 fm2 (mean 0.323 fm2). As result of petropysical researches the rocks have been classified with above mentioned reservoir parameters. The correlation analysis has allowed to establish a series of empirical relationships between the reservoir parameters (density, porosity coefficient, permeability coefficient and residual water saturation factor).

ЕЛЕКТРИЧНІ Й АКУСТИЧНІ ПАРАМЕТРИ НИЖНЬОПЕРМСЬКИХ КАРБОНАТНИХ ПОРІД ЗАХІДНОЇ ЧАСТИНИ ГЛИНСЬКО-СОЛОХІВСЬКОГО ГАЗОНАФТОНОСНОГО РАЙОНУ ДДЗ

The main objective of this article is to study electrical parameters of Lower Permian carbonate rocks of Western part of the Hlynsko-Solokhivskyi gas-oil-bearing district of the Dnieper-Donets Basin (DDB) in normal (atmospheric) and modeling (reservoir) conditions. In atmospheric conditions it has been revealed that the resistivity of dry extracted limestones (the specific electrical resistivity of framework of grains was measured) varies from 12.147 kΩ⋅m to 111.953 MΩ⋅m (mean 1.542 MΩ⋅m). The resistivity of saturated limestone samples with kerosene varies from 44.478 kΩ⋅m to 14.449 MΩ⋅m (mean 1.435 MΩ⋅m). The resistivity of dry and saturated with kerosene samples is almost the same. The resistivity of limestones saturated with model of reservoir water (salinity M = 190 g/l) is lower and varies from 1.11 Ω⋅m to 23.16 Ω⋅m (mean 3.12 Ω⋅m). It has been determined that formation resistivity factor of limestones in atmospheric conditions varies from 13.5 to 230 Ω⋅m (mean 32.5 Ω⋅m). In addition to resistivity, the parameter of relative permittivity of investigated limestones was studied. It was determined that relative permittivity of dry limestones varies from 3.0 to 7.5 (mean 4.2). Relative permittivity saturated samples of limestones with kerosene varies from 2.8 to 8.8 (mean 4.5) and practically does not differ from dry ones but significantly lower than values of samples saturated with model of reservoir water (from 655 to 9565, mean 4280). That means when pores of limestones are saturated with NaCl solution their relative permittivity increases rapidly – from hundreds to thousands times (on average 944 times). It can be explained by the high conductivity of model of reservoir water. Limestones saturated with NaCl solution have velocities of P-waves in the range from 3346 m/s to 4388 m/s (mean 4030 m/s), and velocities of Swaves – from 1753 m/s to 2121 m/s (average 1942 m/s). If rocks are saturated with kerosene then velocities have strictly higher values – P-waves from 3433 m/s to 4514 m/s (mean 4011 m/s) and S-waves – from 2137 m/s to 2464 m/s (average 2344 m/s). Physical modelling of reservoir conditions (temperature 50 °С, pressure 30 MPa) showed that the specific electrical resistivity varies from 0.81 Ω⋅m to 13.19 Ω⋅m (mean 2.67 Ω⋅m), and limestones – from 0.49 Ω⋅m to 7.81 Ω⋅m (mean 1.95 Ω⋅m). Also, "specific electrical resistivity – pressure" connection was investigated. Due to the closure of microcracks and the deformation of the pore space, the electrical resistance of rocks increases with increase of pressure. The regression dependence of the formation resistivity enlargement factor with pressure for the studied rocks has a linear character. It was determined that in reservoir conditions the range of the formation resistivity factor for limestones varied from 17.3 to 271.9 Ω⋅m (mean 50.7 Ω⋅m), and range of variation of porosity coefficient was from 0.040 to 0.169 Ω⋅m (mean 0.118 Ω⋅m). The comprehensive analysis of petrophysical data has been resulted in a set of correlation ratios between reservoir, electric and elastic parameters of studied limestones in normal and modeling conditions.

The plastic flow model in the healing process of internal microcracks in pre-deformed TC4 sheet by pulse current

In this paper, the distribution of microcracks in titanium alloys (TC4) before and after electropulsing treatment (EPT) was observed by X-ray microscopy (XRM). The results show that EPT has obvious healing effect on internal microcracks in the TC4 tensile specimen. Based on Mises yield criterion and the results of finite element analysis in the internal microcrack healing process in titanium alloy, the elastic-plastic deformation characteristics during microcrack healing were analyzed. The results show that the plastic deformation around the microcrack is nonuniform due to the uneven distribution of temperature and thermal compressive stress during EPT. The region with large plastic deformation looks like a thick-walled ellipsoid-shaped ring surrounding the microcrack, where the elongation of the material along the normal direction of the microcrack surface is the main cause of the microcrack closure. The deformation ring plastic flow model was proposed to explain the microcrack closure mechanism by pulse current.

Impact of cooling on fracturing process of granite after high-speed heating

Only very few studies about the influence of cooling on the thermo-mechanical properties of heated rocks exist. This study focuses on high-speed heating followed by slow as well as rapid cooling of granite samples. Target temperatures of 400 °C and 800 °C were reached with heating rates of 200 °C/min, 300 °C/min and ISO 834 standard fire curve. Numerical simulations are used to investigate the temperature evolution inside the sample. The digital image correlation (DIC) technique was used to analyse the strain field evolution during air-cooling with furnace removed. DIC analysis reveals that the microcracks induced by high-speed heating and high temperatures (i.e. 800 °C) can close progressively due to the material contraction during cooling. The closing of microcracks caused by shrinkage plays a more important role than cracking behaviour caused by thermal gradients (i.e. thermal shock). Both rapid (furnace removed) and slow (furnace in place) cooling do not show obvious influence on the stress-strain behaviours of granite samples heated to 400 °C. The samples heated to 800 °C with rapid cooling have a higher uniaxial compression strength than in slow cooling case. This indicates that the contraction effect during rapid cooling might be more intense than in slow cooling cases.

Influence of microwave treatment on mechanical behaviour of compact basalts under different confining pressures

Abstract The realisation of microwave-induced fracturing of hard rocks has potential significance for microwave-assisted mechanical rock fracturing and stress release in deep rock masses. In this context, compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz, and then, we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression (CTC) tests. After microwave exposure, cracks appeared on the surface and inside of the rock sample, and the temperature of the sample's surface was unevenly distributed. The results show that the conventional triaxial compressive strength (CTCS) of basalt samples decreased linearly with microwave exposure time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. Under uniaxial compression, microwave exposure greatly affected the axial deformation, suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time. Under triaxial compression, some microcracks induced by microwave exposure closed due to the effect of confining pressure, resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve. Furthermore, under uniaxial compression, the elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time. Under triaxial compression, microwave exposure has slight influence on elastic modulus and Poisson's ratio. After microwave treatment, the changes in rock strength and deformation mainly result from changes in between the mineral structures. Confining pressure results in the closure of microcracks produced by microwave exposure, so that effects of microwave treatment on strength and deformation decrease, thus reducing the influence on elastic constants. The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time. In the basalt samples, new microcracks in various directions generated by microwave exposure can increase the discreteness of test results, while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.

Petrophysical parameters of rocks of the Visean stage (Lokhvytsky zone of the Dnieper-Donets Depression)

The results of complex investigations of petrophysical properties of the Visean rocks from wells in Lokhvytsky zone of the DDD (sandstones and siltstones from depths 5060―5600 m and limestones from depths 5960―6200 m) are presented. The open porosity coefficient, specific and relative electrical resistivity, velocity of P-waves and S-waves, three-dimensional magnetic susceptibility of rocks are determined. The studies of electric and acoustic properties were carried out in atmospheric and simulated stratal conditions on dry and water saturated stratal model rocks. Relative decrease of the porosity coefficient with changing from atmospheric conditions to reservoir conditions is from 5,5 to 25 % (mean 11,5 %). The average value of specific electrical resistivity of sandstones is 1.2 times more than of siltstones, and 12 times more than of limestones. It has been found that due to the closure of micro-cracks and the deformation of the pore space, the electrical resistance of rocks tends to increase with pressure. There is no significant distinction in velocity parameters between dry sandstones and siltstones, however, limestones are characterized by higher velocity (approximately 30 %) of transversal waves. Saturated rocks have approximately the same average values of S -wave velocity. The average value of velocity of S -waves in sandstones is 17 % higher than in siltstones. It was established that three-dimensional magnetic susceptibility varies in wide limits. Sandstones are characterized by the highest value, this parameter for limestones is 58 times lower than for sandstones. By the mean values of the physical parameters of sandstones and siltstones, there is clearly a direct relationship between the magnetic susceptibility, density and electrical resistivity, and the reverse one with porosity. The results obtained can be used to determine the perspectivity of rocks of different composition, to modeling their properties for different depths and to quantify the structure of their pore spaces. Received dependencies between the various parameters permit to clarify the interpretation of wells geophysical researching data of the studied area.

A novel constitutive modelling approach measured under simulated freeze–thaw cycles for the rock failure

In this study, we derived a computational modelling relation between the model parameters and characteristic parameters of rock deformation and failure via total differentials. We construct a damage model of rock under simulated freeze–thaw cycles and loading based on continuum damage mechanics theory that considers the influence of confining pressure and the random characteristics of rock material defects. This model reflects the variation in regulation between the internal mechanism of freeze–thaw damage and selected physical variables, making it more adaptable. We further analyze the evolution of microdamage and induced material mechanical properties of the rock using our proposed model, producing a total damage evolution curve under freeze–thaw cycles and loading that reflects the closure, initiation, propagation and coalescence of internal microcracks, as well as the subsequent appearance of macrocracks and rock failure. As the number of freeze–thaw cycles increases, rock damage intensifies, as demonstrated by the material’s deteriorating micromechanical properties. However, in later stages of deformation, both the strain and plasticity of the rock increase. With increasing confining pressure, rock damage and the damage accumulation rate, peak damage evolution ratio and descending segment after the peak decrease which manifest in the enhanced resistance of the rock to failure and increased macroscopic plastic deformation. Finally, we perform triaxial compression tests of rock under freeze–thaw cycles to validate our model. The macroscopic rock deformation and failure predicted by our model’s damage characteristics analysis are consistent with our experimental result.

Experimental Study on Mechanical Properties, Acoustic Emission Energies and Failure Modes of Pre-cracked Rock Materials under Uniaxial Compression

To better understand the basic properties and phenomena of compression failure in rock mass structures, a series of uniaxial compression tests on sandstone specimens was carried out using loading equipment, an acoustic emission system, a computed tomography scanning system and a high-speed video camera. The results showed that the fracture process of sandstone samples was divided into three stages: original micro-crack closure, micro-crack nucleation and macro-crack propagation. Avalanche energies of the intact and single-fissured specimens followed a power-law distribution, which revealed avalanche criticality. The optimal exponents ( $$r$$ ) of single-fissured specimens were about 1.64 and were slightly lower than that of the intact specimen. The fitted power-law exponents ( $$r^{\prime}$$ ) of sandstone specimens change in the different stages of experiments and gradually tend to a stable value, which acted as a “warning signal” for the impending major collapse. Based on histogram analysis and the maximum likelihood method, the presence of a flaw may affect the statistical properties of avalanche energies. The flaw inclination angle had little influence on the optimal exponent, but it had certain influence on the fracture modes of the specimens. This paper may provide a new theoretical basis for predicting the collapse failure of engineering rock mass structures.

Evolution of Thermal Microcracking in Refractory ZrO₂-SiO₂ after Application of External Loads at High Temperatures.

Zirconia-based cast refractories are widely used for glass furnace applications. Since they have to withstand harsh chemical as well as thermo-mechanical environments, internal stresses and microcracking are often present in such materials under operating conditions (sometimes in excess of 1700 °C). We studied the evolution of thermal (CTE) and mechanical (Young’s modulus) properties as a function of temperature in a fused-cast refractory containing 94 wt.% of monoclinic ZrO2 and 6 wt.% of a silicate glassy phase. With the aid of X-ray refraction techniques (yielding the internal specific surface in materials), we also monitored the evolution of microcracking as a function of thermal cycles (crossing the martensitic phase transformation around 1000 °C) under externally applied stress. We found that external compressive stress leads to a strong decrease of the internal surface per unit volume, but a tensile load has a similar (though not so strong) effect. In agreement with existing literature on β-eucryptite microcracked ceramics, we could explain these phenomena by microcrack closure in the load direction in the compression case, and by microcrack propagation (rather than microcrack nucleation) under tensile conditions.

Thermal conductivity of thermally damaged Beishan granite under uniaxial compression

Investigations of the thermal conductivity of thermally damaged Beishan granite under uniaxial compression were conducted. Rectangular prism specimens preconditioned at 105 °C were heated at different temperatures (400, 550 and 650 °C) and then cooled to room temperature (27 °C) for testing. Using the transient plane source (TPS) method, in combination with a servo-controlled compression machine and an acoustic emission (AE) monitoring system, lateral thermal conductivities, stress–strain relations and AE responses of the thermally treated specimens during uniaxial compression were measured. The effects of thermal treatment temperature on the strength, deformation and AE parameters were analyzed. The lateral thermal conductivity evolution characteristics for the specimens in the deformation process were studied. The stress-and-deformation-dependent lateral thermal conductivities were also evaluated. The experimental results indicate that the lateral thermal conductivity varies nonlinearly with increasing axial stress due to closure, initiation and propagation of microcracks within the specimens. Moreover, significant differences exist in the lateral thermal conductivity behaviors between the specimens subjected to different temperature treatments. Furthermore, to reveal the influence of cracking behaviors on the thermal conductivity of the thermally damaged specimens in compression, numerical modeling was performed using PFC and the simulation results agree well with the experimental data.

Dynamic Fracture Test of Laurentian Granite Subjected to Hydrostatic Pressure

Dynamic fracture failure of rocks subjected to static hydrostatic pressure is commonly encountered in deep underground rock engineering. The static fracture behavior of rocks under hydrostatic stress has been well studied in the literature. However, it is desirable to investigate the dynamic fracture failure of rocks under various hydrostatic pressures. In this study, a triaxial split Hopkinson pressure bar (SHPB) system is used to measure the dynamic fracture toughness of rocks under five hydrostatic pressures. The results show that dynamic fracture toughness under a certain hydrostatic pressure enhances with the increase of the loading rate, and the dynamic fracture toughness at the similar loading rate increases with the hydrostatic pressure due to the closure of microcracks in rocks. An empirical formula is proposed to describe the influences of the loading rate and the hydrostatic pressure on the rock dynamic fracture toughness.

АНАЛІЗ АКУСТИЧНИХ ВЛАСТИВОСТЕЙ ПОРІД-КОЛЕКТОРІВ РУНОВЩИНСЬКОЇ ПЛОЩІ НА ОСНОВІ ПЕТРОФІЗИЧНИХ ДОСЛІДЖЕНЬ У РІЗНИХ БАРИЧНИХ УМОВАХ

The results of rock physics study of 68 core samples from well No. 110 of the Runovshchynska field of the Dnipro-Donets depression in Ukraine are presented. Investigation of the P-waves on samples under different pressure conditions with the use of 'Kern-4' and high pressure VSC-1000 was performed. Analysis of the obtained data and calculated reservoir values of P-waves was performed. The character of the change in velocity of P- and S-waves for atmospheric conditions is considered. It is shown that the predominant amount of water saturated samples has a velocity of P-waves 3200–3500 m/s (dry samples 2100–2550 m/s), and the S-wave velocity for saturated samples is 2100–2550 m/s (dry specimens 1400–1500 m/s). For a collection of samples, which were measured in atmospheric conditions, the correlation dependence between velocities of P-waves and their density with a close correlation was established. Correlation dependences between elastic wave velocities and the connected porosity of saturated samples were investigated. The dependences of type Vð = f (Kð) with high correlation coefficient for three separate picks of the homotypic sandstones were established. During the analysis of the acoustic studies results under conditions of variable pressure for the majority of samples from the studied intervals, the authors obtained the following common factors. The values of the P-wave velocity, measured in atmospheric conditions, are always smaller than the values obtained after the removal of the pressure; however, there are sometimes quite noticeable fluctuations in their difference, which can be explained by a sharp (possibly hopping) closure of microcracks in the rock with increasing pressure and their delayed opening or non-disclosure when it is reduced. The most contrasting changes in the behavior of the P-wave velocities are haracteristic for several samples (Nos. 27, 48, 50, 53/1), which is most likely due to the void space structure in the rocks, possibly with an increased number of microcracks compared with other samples. On the basis of a priori data and the results of researches of samples at variable pressures, the authors calculated the P-wave velocities in reservoir conditions, conducted their comparative analysis with velocities that are characteristic for samples in atmospheric conditions, built a tight (R² = 0,85) correlation dependence of the investigated parameters.

ЕЛЕКТРИЧНІ ПАРАМЕТРИ ПОРІД ВЕРХНЬОГО КАРБОНУ РУНОВЩИНСЬКОЇ ПЛОЩІ ДНІПРОВСЬКО-ДОНЕЦЬКОЇ ЗАПАДИНИ

The main objective of this article is to study electrical parameters of sandstones and argillites of the Upper Carbon rocks in the Runovshchynska area of the Dnieper-Donets basin. It has been determined that specific electrical resistivity of dry rock samples (specific electrical resistivity of rock matrix) varies from 44,802 kΩ·m to 6,115 МΩ·m (average 751,328 kΩ·m). Specific electrical resistivity of sandstones is 3,45 times more than argillitesdue to different shaliness of studied rocks. Specific electrical resistivity of saturated rocks samples varies from 0,54 Ω·m to 10,46 Ω·m (average 1,23 Ω·m). Specific electrical resistivity of argillites is 2,46 times more than sandstones because the latter had high content of reservoir water in their pores (sandstones had better conductivity). It has been determined that formation resistivity factor of sandstones in atmospheric conditions varies from 6,05 to 33,71 (argillites 11,8), and argillites – from 4,76 to 51,47 (average 17,4). Physical modelling of reservoir conditions (temperature t = 78,5°С, pressure p = 31–31,9 MPa, mineralization M= 170 g/l) showed that specific electrical resistivity varies from 0,3 Ω·m to 3,0 Ω·m (average 0,75 Ω·m). Sandstones in reservoir conditions had the range from 0,3 Ω·m to 2,3 Ω·m (average 0,7 Ω·m), and argillites – from 0,5 Ω·m to 3,0 Ω·m (average 1,2 Ω·m). In this case, specific electrical resistivity of argillites is 1,6 times more than sandstones. Due to the closure of microcracks and the deformation of the pore space, the electrical resistance of rocks increases with increasing pressure. The dependence of formation resistivity enlargement factor on pressure for the studied rocks is expressed by 2-order polynomials. The formation resistivity factor of the studied rocks in reservoir conditions has been determined. It was defined that sandstones in reservoir conditions had the range of the formation resistivity factor from 5,4 to 63,3 (average 20,3), and porosity coefficient – from 0,038 to 0,175 (average 0,113). The range of the formation resistivity factor for argillites was from 13,4 to 88,7 (average 34,3), and porosity coefficient – from 0,043 to 0,115 (average 0,086). Analysis of data of laboratory electrometric investigations has allowed establishing correlations between the porosity coefficient and formation resistivity factor. In addition, the correlation of electrical parameters of rocks in atmospheric and reservoir conditions and the formation resistivity enlargement factor from the water saturation coefficient, taking into account the lithological varieties of the studied rocks, was established.

Thermal resilience of microcracked andesitic dome rocks

The strength of the rocks forming a lava dome informs on its structural stability, important for volcanic hazard assessments. Dome-forming rocks are persistently challenged by thermal stresses from recurring eruptive events that may reduce their strength and jeopardise the structural stability of the dome. Here, we present a series of experiments to better understand the impact of thermal stresses on the strength of an andesitic dome rock from Volcán de Colima (Mexico), a volcano that has witnessed some substantial dome collapses in recent years. Uniaxial compressive strength (UCS) was first tested at room temperature on as-collected samples and samples that had undergone either slow (heated and cooled at 1 °C/min) or shock (heated at 1 °C/min and shock-cooled in cold water) thermal stressing to target temperatures of 400–700 °C. Slow- and shock-cooling thermal stressing did not measurably alter sample strength, connected porosity, or permeability. UCS tests performed at high in-situ temperatures (400–700 °C), however, showed an increase in sample strength and stiffness. We interpret that the resistance of this rock to thermal stresses results from both the presence of abundant pre-existing microcracks and the thermal stability of its mineral assemblage. Unchanged physical properties for the thermally stressed samples deformed at room temperature suggests that the pre-existing microcracks close and reopen, respectively, as the rock expands and contracts during heating and cooling to accommodate the volumetric changes without further microcracking. The increase in strength and stiffness at high in-situ temperatures can be explained by the closure of microcracks due to thermal expansion. These observations suggest that the strength of microcracked dome rocks (1) may be slightly higher when hot (below the glass transition of the groundmass glass), although “upscaled” strength estimates highlight that dome strength will be largely unchanged by an increase in temperature, (2) may only be reduced following the first thermal stressing event, and (3) may not be further reduced by repeated thermal stressing events. Therefore, thermal perturbations, often observed at active domes, may not, as perhaps expected, repeatedly degrade the strength of individual blocks forming the lava dome and therefore may not jeopardise dome stability.

Damage Evolution and Failure Behavior of Sandstone under True Triaxial Compression

Abstract In the present study, a series of conventional triaxial tests (CTTs) and true triaxial tests (TTTs) were performed on sandstone through a self-developed true triaxial testing system, which was combined with acoustic emission (AE) monitoring and X-ray computed tomography (CT) scanning technique to systematically study the damage and failure behavior of sandstone in different triaxial stress states. The results show that the damage evolution process of the specimen under the CTT and TTT conditions could be divided into four stages based on the AE monitoring observations: microcrack closure, stable growth of microcracks, unstable growth of microcracks, and post-peak phase. During the first three stages, the spatial-temporal distribution of the AE activity under the two loading conditions was nearly equal. However, in the post-peak stage, abrupt changes in the AE locations occurred more often under the CTT conditions than under the TTT conditions. The overall AE activity under the CTT conditions decreased with increasing confining pressure, whereas under the TTT conditions, it tended to strengthen first and then weaken with increasing intermediate principal stress. The post-test CT scanning observations show that the fracture morphology of the specimen under the TTT conditions was more regular and simpler than that under the CTT conditions. The fracture surface area of the specimen under the CTT conditions tended to decrease with increasing confining pressure, whereas that of the specimen under the TTT conditions tended to decrease first and then increase with increasing intermediate principal stress.

Poromechanical Properties of a Sandstone Under Different Stress States

Poromechanical properties of a sandstone from an underground gas storage site are investigated with the use of a neutral gas to control the pore pressure. The poroelastic theory of anisotropic medium is used to evaluate elastic properties and coupling coefficients under different stress states. In-situ CT observations have been made under uniaxial compression test and they are used to underline some effects due to the occurrence of damage and microcraks. Under hydrostatic loading, pores and micro-cracks are gradually compressed. The initial state of the sandstone is “slightly” transversely isotropic but in a first approach the Biot’s tensor reduces to a scalar. This coefficient decreases with the increase in confining pressure that can be attributed to the closure of micro-cracks. The effect of damage and cracking is investigated with two series of conventional triaxial tests conducted at different confining pressures, either with an increase in pore pressure (series 1) or with a decrease in pore pressure (series 2), to evaluate the coupling coefficients. The results, derived from these two test series, are consistent as regards the effects of cracking on the material behavior. There is an obvious damage due to the triaxial loading which induces a parallel decrease in the ratio E3/(1 − ν3) and in the modulus H3. Axial compaction results in the continual increase in the H1 modulus observed for every sample whatever the measurement technique was.