Velocities Of Sandstones Research Articles

Abnormal variation of P-wave velocity of red sandstone after cyclical thermal shock in water

Hot dry rock (HDR) has great development potential because of its advantages of clean, environmental protection and renewable. The study of physical and mechanical properties of HDR is one of the important links in the process of geothermal energy development. Previous studies have mainly focused on the granite thermal reservoir but research on the sandstone thermal reservoir, especially the physical properties of sandstone after multiple thermal shocks, is scarce. Therefore, in this study, cyclic thermal shock experiments of sandstone at different temperatures are carried out, and the variation law of P-wave velocity of sandstone after heat treatment is revealed. It is found that the P-wave velocity of red sandstone decreases with the increase of temperature and cycle times, especially when the temperature is higher than 400 °C, the maximum change rate of wave velocity reaches 52.6%. It is particularly noteworthy that the P- wave velocity of sandstone increases abnormally at about 600 °C. And this article puts forward three hypotheses to explain the wave velocity anomaly. In addition, the nonlinear relationship model among P-wave velocity, temperature and number of cycles is established for the first time, and the correlation coefficient R2 is more than 0.9. This study serves as a reference for the development and utilization of the sandstone geothermal reservoir.

Experimental study on deterioration of bedrock strength and P-wave velocity by pressurized seawater

Bedrock has to be excavated before the construction of rock-socketed pile foundations for offshore wind power turbines, leading to the exposure of fresh and dry rocks to seawater. The pile-rock interface is partially separated under the long-term action of various cyclic loads during the operation stage of offshore wind turbines, and the infusion of seawater will further weaken the bedrock. To comprehensively investigate the deterioration of bedrocks by pressurized seawater, tests on point load strength and slake durability were carried out to study the long-term deterioration of mechanical properties of granite, sandstone, and tuff collected from an offshore wind power farm. The elastic P-wave velocity was measured to analyze the variation trend of P-wave velocity, the mechanism of which was revealed by means of Gassmann’s equation. The results of mechanical experiments indicate that granite is almost impervious to pressurized seawater, while the mechanical properties of sandstone and tuff are deteriorated by water, especially by the pressurized seawater. After 60 days of immersion in seawater at 0.5 MPa pressure, decreasing amplitude of point load strength and slake durability index for the sandstone specimens in seawater reaches 53.2% and 18.5%, respectively, and that for the tuff specimens is 14.6% and 1.52%, respectively. The elastic P-wave velocity of granite shows an upward trend with increasing immersion time in different environments, but there exists a different tendency between the P-wave velocity of sandstone and tuff specimens in water or seawater with normal pressure and that in seawater with 0.5 MPa pressure. The variation of P-wave velocity of different types of rocks is studied based on Gassmann’s equation that takes into account the change of saturation degree, porosity, and compressibility of the matrix material.

Degradation of physico-mechanical properties and damage mechanisms of sandstone under the combined action of freeze–thaw cycles, chemical solution, and compressive stress

Rocks in cold regions subjected to freeze–thaw cycles are often affected by the surrounding hydro-chemical solutions and external load. In this study, the evolution of physical and mechanical characteristics and the damage mechanism of sandstone under the combined action of freeze–thaw cycles, chemical solution, and compressive stress were investigated. A series of quick cyclic freeze–thaw tests in different chemical solutions and stress levels were conducted. The dry mass, wave velocity, and compressive strength decrease as the number of freeze–thaw cycles increases. The variations in dry mass, wave velocity, and rock appearance indicate that lower stress levels (<0.5σ0, σ0 is the saturated uniaxial compressive strength of the original specimen) can limit the effect of repeated freeze–thaw cycles and chemical corrosion, whereas higher stress levels (>0.5σ0) can exacerbate the combined effect on rock damage. In addition, a modified decay model is proposed that incorporates the initial properties and residual integrity of the rock to describe the evolution of rock strength and wave velocity. Both the residual strength and wave velocity of sandstone are constants that are independent of stress level and chemical solution. The degradation rate of compressive strength increases abruptly at higher stress, whereas that of wave velocity generally decreases with increasing stress level. The continuous action of the cyclic frost-heaving force, hydrolysis and chemical corrosion, and external compressive stress promote generation and propagation of pores and microcracks, causing deterioration of the mass, wave velocity, and compressive strength.

Scrutiny of freeze-thaw cycles effect on physical and dynamic fracture characteristics of cracked sandstone

A series of freeze-thaw (FT) cycle experiments and drop-weight impact tests are performed to explore the dynamic fracture characteristics of fractured sandstone acted upon by FT cycles. To this end, an innovatively cracked sample, a tunnel with a single radial crack (TSRC), made from an intact sandstone rock is implemented. The experimental results reveal that for FT cycles 10, the dynamic elastic modulus and P/S wave velocities of sandstone exhibit the most drastically descending trend. In such a case, the decreasing percentage of the dynamic elastic modulus, P wave velocity, and S wave velocity reach 16.4%, 7.2%, and 8.3%, respectively. The percentage reduction of these three physical parameters is also tapered off as the number of FT cycles reaches 30 and 40. While the porosity of sandstone grows by 1%, the pore distribution substantially alters for the case of 10 cycles of FT. For the number of FT cycles greater than 30, the porosity of sandstone slightly increases. Further, the crack growth velocity generally grows with the number of FT cycles such that the highest crack growth velocity is detected for 40 cycles FT, and the corresponding velocity touches 427.6 m/s. The obtained results demonstrate that the dynamic fracture toughness lessens as the number of FT cycles increases, and the dynamic initiation toughness is less than the average dynamic propagation toughness in sandstone samples subjected to impact loads. The dynamic loading rate calculated from dynamic initiation toughness reduces with the number of FT cycles when the sandstone specimen experiences the same dynamic loading.

Calculation of P-Wave Velocity in Sandstones with Different Pore Size Distributions Using Digital Rock Model without Segmentation

Calculation of elastic wave velocity using digital rock models is a powerful method to understand physical properties of rocks. However, one difficulty of the approach is to construct a proper digital model due to the uncertainty by the segmentation process of X-ray CT image. Recently, the segmentation-less method has been proposed to avoid the uncertainty, and has shown to be useful in application to Berea sandstone. In this study, we assessed the method through the application to Shirahama sandstone in addition to Berea sandstone, and confirmed whether the method can be applied to other types of sandstones. We also applied and compared two different rock-physics models to obtain elastic properties of a digital rock model. Our results showed that the segmentation-less method can be successfully applied to Shirahama sandstone as well as Berea sandstone. However, when the pore diameter of rock sample is smaller than the spatial resolution of CT image, we found that a correction is required when mapping the CT number to a density. Our results would be useful when applying the segmentation-less method to other types of sandstones.

Calculation of P-Wave Velocity in Sandstones with Different Pore Size Distributions Using Digital Rock Model without Segmentation

Calculation of P-Wave Velocity in Sandstones with Different Pore Size Distributions Using Digital Rock Model without Segmentation

Experimental Investigation on the Correlation between Dynamic Ultrasonic and Mechanical Properties of Sandstone Subjected to Uniaxial Compression

Ultrasonic wave velocity is effective to evaluate anisotropy property and predict rock failure. This paper investigates the correlation between dynamic ultrasonic and mechanical properties of sandstones with different buried depths subjected to uniaxial compression tests. The circumferential anisotropy and axial wave velocity of sandstone are obtained by means of ultrasonic wave velocity measurements. The mechanical properties, including Young’s modulus and uniaxial compressive strength, are positively correlated with the axial P wave velocity. The average angles between the sandstone failure plane and the minimum and maximum wave directions are 35.8° and 63.3°, respectively. The axial P wave velocity almost keeps constant, and the axial S wave velocity has a decreasing trend before the failure of rock specimen. In most rock samples under uniaxial compression, shear failure occurs in the middle and splitting appears near both sides. Additionally, the dynamic Young’s modulus and dynamic Poisson’s ratio during loading are obtained, and the negative values of the Poisson’s ratio occur at the initial compression stage. Distortion and rotation of micro/mesorock structures may be responsible for the negative Poisson’s ratio.

Variation of failure properties, creep response and ultrasonic velocities of sandstone upon injecting CO2-enriched brine

Variation of failure properties, creep response and ultrasonic velocities of sandstone upon injecting CO2-enriched brine

Fracture Characteristics and Geometric Fractal of Damaged Sandstone under Impact Load

Before the rock mass in the engineering is broken under load, it is usually in a state of varying degrees of damage. Aiming at the fracture characteristics of damaged sandstone under impact load, this paper adopts a method of the cyclic static load to cause the sandstone specimens to have varying degrees of damage. Then, the wave velocity of sandstone before and after the damage is measured using the nonmetallic acoustic velocimeter, and the change rule of damage factor is analyzed. Finally, the split Hopkinson pressure bar (SHPB) is used to test the impact dynamics of sandstone with different damage degrees. The broken rock block is screened by a vibrating screen, and the crushing characteristics are analyzed. The results show that not only the damage factor of damaged sandstone but the growth rate increases, with the raising upper limit of stress. Under the impact load of the same incident energy, the fragmentation degree of the damaged sandstone increases with the increase of the upper stress limit, while the average diameter of the broken rock block decreases gradually, and the reduction rate increases first and then decreases.

Influence of Drying–Wetting Cycle of Acid Solution on the Mechanical Properties of Sandstone

Ensuring the stability of reservoir slopes requires a comprehensive understanding of the mechanical properties of rocks after the drying–wetting cycles under acidic conditions. In this work, sandstone samples were selected and subjected to cyclic drying–wetting treatments with aqueous solutions of pH 7.0, 6.0, 4.0, and 2.0. The uniaxial compression tests and P-wave velocity measurement were carried out to evaluate the variations in the physico-mechanical properties of sandstones with different treatments. The results revealed that the internal structure of sandstones is softened by the drying–wetting cycles, which deteriorate their physico-mechanical properties. As the acidity of the aqueous solution increases, porosity and Poisson’s ratio show an increasing trend, while the strength characteristics, Young’s modulus, and P-wave velocity of sandstones present a decreasing trend. The physico-mechanical properties of sandstones significantly deteriorate from neutral to weakly acidic solutions; however, the deterioration becomes slower as the acidity continues to increase. Moreover, the physico-mechanical properties of sandstones are markedly reduced under strong acidic conditions.

Inconsistency of changes in uniaxial compressive strength and P-wave velocity of sandstone after temperature treatments

It is generally accepted that the uniaxial compressive strength (UCS) and P-wave velocity of rocks tend to decrease simultaneously with increasing temperature. However, based on a great number of statistical data and systematic analysis of the microstructure variation of rocks with temperature rising and corresponding propagation mechanism of elastic wave, the results show that (1) There are three different trends for the changes of UCS and P-wave velocity of sandstone when heated from room temperature (20 °C or 25 °C) to 800 °C: (i) Both the UCS and P-wave velocity decrease simultaneously; (ii) The UCS increases initially and then decreases, while the P-wave velocity decreases continuously; and (iii) The UCS increases initially and then fluctuates, while the P-wave velocity continuously decreases. (2) The UCS changes at room temperature–400 °C, 400 °C–600 °C, and 600 °C–800 °C are mainly attributed to the discrepancy of microstructure characteristics and quartz content, the transformation plasticity of clay minerals, and the balance between the thermal cementation and thermal damage, respectively. (3) The inconsistency in the trends of UCS and P-wave velocity changes is caused by the change of quartz content, phase transition of water and certain minerals.

Seismic velocity change in sandstone during CO2 injection

This paper presents analytical and experimental studies of the effects of supercritical CO2 injection on the seismic velocity of sandstone initially saturated with saline water. The analytical model is based on poroelasticity theory, particularly the application of the Biot-Gassmann substitution theory in the modeling of the acoustic velocity of porous rocks containing two-phase immiscible fluids. The experimental study used a high pressure and high temperature triaxial cell to clarify the seismic response of samples of Berea sandstone to supercritical CO2 injection under deep saline aquifer conditions. Measured ultrasonic wave velocity changes during CO2 injection in the sandstone sample showed the effects of pore fluid distribution in the seismic velocity of porous rocks. CO2 injection was shown to decrease the P-wave velocity with increasing CO2 saturation whereas the S-wave velocity was almost constant. The results confirm that the Biot-Gassmann theory can be used to model the changes in the acoustic P-wave velocity of sandstone containing different mixtures of supercritical CO2 and saline water provided the distribution of the two fluids in the sandstone pore space is accounted for in the calculation of the pore fluid bulk modulus.

An effective method for shear-wave velocity prediction in sandstones

Shear-wave velocity is essential in various seismic exploration applications, including seismic modeling, amplitude variation with offset analysis, multicomponent seismic data interpretation and other exploration applications. This study presents a simple but effective method for S-wave velocity prediction from P-wave velocity based on granular media model. In the proposed method, the soft-sand, intermediate stiff-sand and stiff-sand models are unified with the expression of the soft-sand model with an effective coordination number. The shear modulus of dry rocks can be related to bulk modulus through the coordination number in the unified model. Elastic-wave velocities of water-saturated rocks at low frequencies can be predicted from the moduli of dry rock using Gassmann’s equation. Thus, the coordination number can be inverted from P-wave velocity of saturated rocks by combining the unified granular media model and Gassmann’s equation. Eventually, S-wave velocity of saturated rock is computed with the inverted effective coordination number. The numerical results indicate that the predicted S-wave velocities agree well with the measured velocities for the laboratory data and well logging data. The proposed method is applicable to sandstones with lithification of large ranges because the unified granular media model accounts for the media between Hashin–Shtrikman lower bound and upper bound. Moreover, the new method is quite suitable for the prediction of S-wave velocity in sandstones deposited in deep-water environment, particularly for turbidite sediment.

Thermal effect on wave velocity of sandstone after high-temperature treatment: a review

Variations in wave velocity and porosity of sandstone after high-temperature treatment are investigated. The results for normalized wave velocity and porosity show that 400 °C is the critical threshold temperature of sandstone. Below 200 °C, porosity increases slowly as the treatment temperature increases, while wave velocity gradually decreases. When the temperature is higher than 400 °C, porosity quickly increases, and wave velocity sharply decreases. The “room temperature–200 °C” and “200–400 °C” temperature ranges correspond to the undamaged state and the microdamage state, respectively.

Study of the effect of micro-pore characteristics and saturation degree on the longitudinal wave velocity of sandstone

In an attempt to investigate the effect of different saturation degrees and micro-pore characteristics on longitudinal wave velocity, the present study selects two groups of sandstones with different micro-pore structure characteristics as research objects. In view of the different degrees of saturation of the samples, improved acoustic emission is employed to measure longitudinal wave velocity and a comprehensive analysis is conducted on the variation of longitudinal wave velocity. The test results show that the longitudinal wave velocity of the sample with diverse porous structure, at different saturation levels, manifests itself in three stages. At the saturation ratio of 0–30%, the wave velocity of the sandstone samples presents an increasing tendency, while there is a slight change given for the saturation ratio of 30–70%, first declining and then slowly increasing. In the third stage, the velocity significantly increases with the saturation ratio of 70–100%. Regarding the samples with a single porous structure, the measured longitudinal wave velocity presents two stages. When the saturation ratio is below 75%, the longitudinal wave velocity decreases slowly with the increase of saturation degree, while there is a sharp increase of the velocity as the saturation ratio increases when the ratio exceeds 75%. Based on the capillary model and the test results of the samples with a single pore structure, a relationship is established between the micro-pore structure characteristics and the macroscopic longitudinal wave velocity of the samples at different water saturation degrees, and the longitudinal wave velocity displays a satisfied consistency with that of the samples with diverse pore structure.

Acid solution decreases the compressional wave velocity of sandstone from the Yungang Grottoes, Datong, China

To understand the effects of an acidic environment on the internal structure of sandstone from the Yungang Grottoes, Datong, China, the physicochemical properties of fresh and weathered sandstone samples and their compressional wave (P-wave) velocities in response to different concentrations of H2SO4 and HNO3 solution were investigated. X-ray diffraction (XRD) and a scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM–EDX) were used to determine grain morphology and chemical composition. The results show that the sandstone, which mainly consists of silicon dioxide and calcium carbonate, became more complicated in composition after weathering. A nonmetallic ultrasonic detector was used to measure the P-wave velocities of sandstones in a natural state and soaked with ultrapure water and with H2SO4 and HNO3 at concentrations of 0.2 mol L−1, 0.4 mol L−1, and 0.8 mol L−1, respectively. For the acid-treated groups, with increasing acidity, P-wave velocity decreased significantly, compressive strength decreased and effective porosity increased; these behaviours are different from those of the water-treat group, implying that the acid solutions damaged the microstructure of the sandstone. The results suggest that the deterioration risk of H2SO4 and HNO3, or sulfates and nitrates converted from ambient SO2, NOx and PM2.5, on the stone relics in the Yungang Grottoes should be a cause for concern.

An Experimental Study Among P-wave Velocity of Sandstones, Pressures and Water Saturation in Hami, Xinjiang, China

Seismic velocity of rocks have a wide range of applications in regional tectonic research and shallow seismic exploration. Hami area is one of the important production bases for oil and gas in China, but there is short of seismic velocity. This paper contributes to the measurement of Pwave velocity of sandstones from Hami under different conditions by applying AutoLab 2000 computer-controlled servo hydraulic triaxial test system. Specifically, it shows the variation of seismic wave velocity of sandstones with respect to the water saturation under various pressures. The results show that P-wave velocity of sandstones is remarkably correlated linearly with the increasing water saturation and the correlation coefficient is over 0.9. Besides, the change rate between P-wave velocity and water saturation will decrease with the increasing pressure. The fitting formulas among pressures, water saturation and P-wave velocity are suitable for Hami region. Finally, we get a new method to predict water saturation by combining the experimental result with logging curve in Hami area.

Correlation of physical and mechanical properties with ultrasonic pulse velocities of sandstones in Çenedağ, Kocaeli-Turkey

Correlation of physical and mechanical properties with ultrasonic pulse velocities (UPV) of sandstones in Çenedağ, Kocaeli-Turkey, NW was performed in this study. Physical and mechanical properties were defined and the relationships among the uniaxial compressive strength (UCS), porosity, void ratio, point load strength index Is(50),Schmidt hardness (RN) and bulk mass density by weight with ultrasonic pulse velocity of pink and cream colored sandstone unit of Lower Ordovician Çenedağ formation were investigated. In the scope of this research, 16 sandstone specimens were collected from various locations of Çenedağ formation in Çenedağ-Kocaeli, Turkey and laboratory experiments were implemented. Later, the statistical correlations were performed by regression analysis to evaluate the relationships between these properties and ultrasonic pulse velocity. Reasonably good correlations were determined between the UPV and physical and mechanical properties.

Experimental study of the mechanical behavior of sandstone affected by blasting

• Investigate the static uniaxial compressive behavior and longitudinal wave velocity of sandstone affected by blasting.

Variation of Wave Velocity and Porosity of Sandstone after High Temperature Heating

This paper reports the variations of mass, porosity, and wave velocity of sandstone after high temperature heating. The range of temperature to which the sandstone specimens have been exposed is 25–850°C, in a heating furnace. It has been shown that below 300°C, porosity and wave velocity change very little. Above 300°C, there is a rapid increase in porosity, but the wave velocity decreases significantly. The results of thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC) and mercury intrusion porosimetry (MIP) suggest that a series of changes occurred between 400 and 600°C in sandstone could be responsible for the different patterns of variation in porosity and wave velocity.