Dry Sandstone Research Articles

Mechanical and fracture characteristics of weak microwave-absorbing sandstone under microwave irradiation: influence of pore water

Microwave radiation is an effective method to weaken the strength of hard rock, but this weakening effect is not obvious enough for weak microwave-absorbing rocks. This work focuses on the role of water in amplifying the effect of microwaves on weak microwave-absorbing rocks, by examining the bursting characteristics of the rocks as well as their physical and mechanical properties before blasting. The experimental results show that when subjected to microwave irradiation, and the time taken for this bursting decreases with higher microwave power. In addition, the surface temperature of saturated sandstone gradually rises with increased irradiation power and duration. Under high microwave power, the uniaxial compressive strength (UCS) of heated saturated sandstone notably decreases within the same irradiation duration. When the microwave power is low, the UCS of saturated sandstone remains relatively unchanged over time. The UCS of dry sandstone exhibits no significant alteration under identical irradiation power and duration. In conclusion, sandstone shows no significant change in its physical and mechanical properties during brief exposure to low-power microwave irradiation in its dry state. As water within the sandstone undergoes a phase transition from liquid to gas due to microwave irradiation, the resulting steam pressure causes the sandstone to rupture.

Stress–strain hysteresis during hydrostatic loading of porous rocks

A micro-mechanical model is proposed to predict the stress–strain hysteresis during the cyclic hydrostatic loading of fluid-saturated rocks under drained or undrained conditions. A spherical pore is surrounded by a multi-cracked shell where local deviatoric stress develops despite the remote hydrostatic loading. The effective properties of the material composing the shell are constructed assuming an isotropic distribution of cracks with no interaction, and the overall properties thanks to the spherical assemblage approach. The fluid pressure in drained and undrained conditions is assumed to be uniform throughout the assemblage. A new analytical solution is proposed, assuming all cracks are closed and slipping either forwardly or reversely. It is shown with numerical simulations for drained conditions that this assumption is indeed respected for sufficiently small values of the crack friction angle. However, for reasonable values, the closed cracks during the unloading phase could slip in either direction: reversely close to the pore and still forwardly away from the pore. Moreover, at critical radii, the slip could occur in either direction depending on the crack orientation. A similar micro-structural response is observed for undrained conditions, although the remote confining stress required to close the cracks is much larger. The model’s predictions compare favourably with recent experimental data on dry sandstones and carbonates, which were presented in a study on the influence of strain amplitude on the transition between static and dynamic properties. The crack density and matrix elasticity modulus are sufficient fitting parameters to accurately predict the hysteresis loops, especially for porosity levels above 10%.

SELECTION OF A MRP SYSTEM FOR OIL AND GAS DEPOSITS

A number of serious problems arise in the development of sub-gas zones of oil and gas deposits due to the joint occurrence of oil and gas. The key problem is gas breakthrough from the gas cap to oil wells. This entails an increase in the gas factor (GOR), a rapid and intense decrease in pressure in the gas cap. If there is a reservoir pressure maintenance system or an active aquifer as a result of a decrease in reservoir pressure in the gas cap, oil may be pushed back into it, which leads to a rise in gas-oil contact, jamming of mobile oil in the «dry sandstones» of the gas cap, which in turn leads to a decrease in reserves of mobile oil and a decrease in the oil recovery factor (ORF). All this together allows us to consider oil in the sub-gas zones as hard-to-recover reserves.Preventing and combating the negative consequences of gas breakthrough from the gas cap to oil wells is a complex task and consists of several areas, such as selection of rational drilling of production and injection wells, selection of optimal operating modes for them, use of inflow control devices or performance of repair and isolation work to limit inflow from intervals with gas breakthrough, periodic shutdown of wells to disband gas cones. The ultimate goal of solving these problems is to increase the oil recovery factor and economic efficiency indicators for the development of sub-gas zones.Due to the variety of ways to optimize the development of oil and gas reservoirs, it is not possible to cover them all in one work, therefore, the tasks associated with optimizing the reservoir pressure maintenance system are considered. It is worth noting that in comparison with the development of oil reservoirs, the presence of a gas cap significantly complicates this task due to the lack of analytical solutions for three-phase filtration. For this reason, the selection of the optimal parameters of the reservoir pressure maintenance system is usually carried out either by multivariate calculations in the hydrodynamic model (HDM), or by the method of analogies or the implementation of pilot tests of various approaches.In this study, an approach based on multivariate numerical calculations using the sector model of the development element of the gas zone was used. As a result of calculations, it was shown that when choosing the optimal parameters of the reservoir pressure maintenance system, it is advisable to use a differentiated approach, which consists in the fact that these parameters must be set individually for zones with different ratios of oil-saturated and gas-saturated thicknesses.

Experimental study of the effect of ultralow temperature on the fracture toughness of sandstone

Fractures are widely present in subsurface formations, and fracture extension under ultralow temperature conditions is important for tasks such as liquid nitrogen fracturing and underground storage of liquid natural gas or liquid air. Therefore, the fracture toughness of sandstone under ultralow-temperature conditions was investigated using three-point bending tests of the NSCB specimens. The specimen damage processes were monitored using the DIC technique, and the changes in the pore microscopic characteristics after ultralow-temperature treatment were observed using CT and SEM. The results show that fracture toughness of dry sandstone was insensitive to the variations of the ultralow temperature (-30 to −120 ℃), whereas that of saturated sandstone increased gradually with the decrease of temperature due to the cementing effect of ice. DIC analysis showed that the peak strain around the fracture tip increased, and the fracture extension time was shortened with a decrease in temperature for both dry and saturated sandstone samples. The minerals shrank and the samples became more compacted with a decrease in temperature for the dry samples, resulting in their brittle characteristics under ultralow temperatures. The enhanced cementation effects of ice under ultralow temperatures induced high internal stress and rapid energy release upon failure of the saturated samples. Large pores shrank owing to pore collapse and filling after the one-cycle ultralow-temperature freeze–thaw treatment.

Experimental study on the flow characteristics of supercritical CO2 in reservoir sandstones from the Ordos Basin, China

AbstractUnderstanding the flow characteristics of supercritical CO2 in dry sandstones or those with low water content provides crucial information on the flow behavior in near‐wellbore zone. We conducted supercritical CO2 core flooding experiments using sandstone cores extracted from potential CO2 reservoirs in the Ordos Basin, China. During the experiments, we reduced the water content of saturated cores by flushing with dry CO2 and subsequently vacuumizing them at a temperature of 35°C to simulate sandstones with low water content. The experimental results demonstrate that the CO2 permeability was initially high during the low differential pressure stage and remained constant as the differential pressure increased. In the carbonic acid solution injection experiment, we observed an increase in the flow rate of the solution with the continuous interaction in the cores from the Shanxi and Shihezi groups, while the Yanchang group exhibited the opposite effect. This increase in permeability can be attributed to mineral dissolution and the loss of fine particles. Conversely, the blockage of fine particles or the precipitation of dissolved minerals may lead to a decrease in permeability. After the CO2–water–rock interaction, the CO2 permeability decreased compared to before the interaction, indicating that adsorbed water, the precipitation of dissolved mineral, or pore throat blockage by fine particles could induce this permeability decrease. The impact of adsorbed water on the decrease in CO2 permeability is significant. Additionally, the CO2–water–rock interaction caused corrosion on the anorthite surface. Furthermore, calcite dispersed in connected pores displayed a more pronounced dissolution compared to cemented calcite. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

Analysis of fracture characteristics of saturated sandstone based on infrared radiation variance

The use of infrared radiation technology for monitoring rock stability and failure has been a common practice. However, the presence of groundwater significantly compromises the precision of infrared monitoring. This study employs scanning electron microscopy technology and PFC simulation to examine the micro-fracture development and infrared radiation variance (IRV) characteristics in dry and saturated sandstone conditions, and establishes a direct relationship between the two. The findings of this research indicate the following: (1) Saturated sandstone exhibits a substantially weakened internal structure, with the number of cracks at peak strength is approximately twice that of dry sandstone，the strength, peak stress, and elastic modulus of the saturated samples decreased to 56% and 59%, respectively. (2) The surface IRV characteristics of saturated samples are notably more pronounced, with an IRV value 9.57 times that of dry rocks. (3) Two infrared radiation variance indicators have been introduced to delineate different fracture stages of sandstone: “slow fracture variance” (SFIRV) denotes the initial crack formation in sandstone, while “rapid fracture variance” (RFIRV) characterizes a rapid fracture stage marked by a significant increase in crack development, serving as a precursor to complete sandstone fracture. (4) The relationship between IRV and the quantity of internal fractures adheres to a power function. A mathematical model for Fracture-IRV has been established, demonstrating a high degree of consistency with actual experimental results. The outcomes hold valuable implications for infrared monitoring research in deep engineering that may encounter groundwater-related challenges, such as tunnel and coal mining.

Experimental investigation on the triaxial unloading mechanical characteristics of sandstone immersed in different brines

In this study, the stress–strain curves of sandstone soaked in 5% NaCl and 5% K2SO4 solutions under different loading and unloading paths were obtained by carrying out different paths of unloading confining pressure tests on sandstone specimens. Thereafter, the strength, deformation, and failure characteristics of the specimens were analysed. First, the unloading capacity of sandstone specimens soaked in 5% NaCl was found to be greater than those soaked in 5% K2SO4, and the unloading amount increased in the order of increasing axial pressure unloading confining pressure < constant axial pressure unloading confining pressure < constant deviatoric stress unloading confining pressure. When unloading the confining pressure, the strain change rate of the sandstone specimens soaked in 5% K2SO4 was greater than that of the sandstone specimen soaked in 5% NaCl, and the strain change rate had the following trend: increasing adding axial pressure and unloading confining pressure > constant axial pressure unloading confining pressure > constant deviatoric stress unloading confining pressure. The deformation modulus and Poisson's ratio of the sandstone specimens during unloading exhibited an exponential change with increasing unloading ratio. The deformation modulus and Poisson’s ratio of the sandstone soaked in 5% NaCl during the unloading period was respectively greater and less than those of the sandstone soaked in 5% K2SO4. Second, the total energy absorbed by 5% NaCl soaked sandstone specimens was always less than that soaked in 5% K2SO4 under different unloading paths. The effect of immersing the specimens in different solution on acoustic emission and failure characteristics was unclear. Compared with the dry sandstone specimens, the sandstone specimens soaked in brine solutions lower mineral composition and higher, clay mineral content. The contents of plagioclase and calcite contents in the sandstone specimens soaked in 5% NaCl was similar to those in dry sandstone, but no plagioclase or calcite was detected in the specimens soaked in 5% K2SO4.

Effect of strain amplitude and confining pressure on the velocity and attenuation of &lt;i&gt;P&lt;/i&gt; and &lt;i&gt;S&lt;/i&gt; waves in dry and water-saturated sandstone: an experimental study

In rock physics, much attention has been paid to the study of the processes of strain of natural materials at small strains. Experiments using high-precision measurements have allowed new knowledge at micro/nano level to be acquired. The microplasticity of solids is studied in materials science, but there is also data regarding some rocks. The property of microplasticity of natural materials is still little studied. The study was carried out on rock samples. The effect of strain amplitude and confining pressure on the velocity and attenuation of P and S waves in dry and water-saturated sandstone has been studied. The method of reflected waves was used in the frequency range of 0.5–1.4 MHz at four strain amplitudes (0.5–1.67)·10−6 Amplitude cycling causes an open and closed hysteresis effect for wave velocity and attenuation. This has been observed for both dry and water-saturated sandstone. The hysteresis loop overlaps in both states. The amplitude changes in the velocity of P-wave in dry sandstone is 1.12 %, and the attenuation of P-wave in dry sandstone is 5.43 %. As for S-wave, its maximum attenuation in dry sandstone reaches 8.81 %. The behavior of a wave velocity and attenuation can be explained by the combined effect of viscoelasticity and microplasticity. Elastoplastic transition strongly depends on the details of the microstructure, its defectiveness, and other parameters. The characteristics of the complications of wave parameters can be the signs of the internal structure of the subject.

Geoelectric and mineralogical studies for foundation soil characterization in new Luxor city, Upper Egypt

The new Luxor city was proposed by the Egyptian government as a new constructional and building site in order to face the increasing population density around the two banks of the river Nile. It is important prior construction of buildings to characterize the foundation soil. So, vertical electrical resistivity survey and mineralogical investigation were conducted for this purpose. The results obtained from the interpretation of the sounding data indicate that the lithological succession of the study area consists of four geoelectric layers. The top layer corresponds to the surface loose sediments of conglomerate, gravel, and sand. The second geoelectric layer is composed of sand and gravel. The third layer is composed of silty clay, sandy clay, and clayey sand. The fourth geoelectric layer is composed of dry sandstone. The X-ray diffraction (XRD) results of samples collected from the clay-rich layer revealed the presence of smectite mineral as predominate clay minerals which is characterized by its swell potentiality on wetting. Accordingly, the clay zone recorded within the third layer must be taken into consideration during designing the foundations at new Luxor city especially this layer was located at shallow depth range between 1.6 and 7.5 m and attains a relatively big thickness that reach up to 33.5 m in some parts. This study emphasizes the importance of the integration of geophysical investigation and mineralogical analysis of the foundation soil for optimal characterization of a site.

Geoelectrical Investigation of Groundwater at Northern Gaza Strip, Palestine

A vertical electrical sounding survey was carried out in northern Gaza Strip (Gaza City, Jibalya, Bayt Lahya and Bayt Hanun), Palestine to study the groundwater conditions and evaluate its quality and distribution. The survey included twenty-six (26) traverses with variable lengths in the range 200-500 m. The geoelectrical survey revealed that the resistivity of the study area varies in a wide range mostly 9 to 850 Ωm, subdivided into three subsurface zones. The first zone represents the dry sandstone and alluvial that attains the highest resistivities (100-850 Ωm) with variable thicknesses in the range 26-80 m beneath the whole study area. This zone is characterized by the presence of thin layers of clay and silt in some localities that reduced the resistivity to 15-25 Ωm. The second zone represents the fresh water saturated calcareous sandstone (locally named Kurkur) with resistivities in the range 22 -39 Ωm). The thicknesses of this zone is about 55-70 m in the western part of the study area and increased to 110 m in the east. The surface of this zone shows a gentle westward gradient, which indicates a general flow of water towards the Mediterranean Sea. The third zone represents saline water saturation with low resistivities, mostly 9 Ωm -10 Ωm close to the Mediterranean shore, located at 60 m below sea level and 100 m beneath the eastern side of the study area. Beneath the eastern part of the study area, an anomalous low resistivity of 2-3 Ωm is observed within the fresh water zone, which indicates groundwater pollution near the buffer zone caused by agriculture activities and untreated pumped wastewater.

Experimental study on uniaxial compression failure modes and acoustic emission characteristics of fissured sandstone under water saturation

Widely distributed fissures in the open-pit slope (especially those which are in water-rich conditions) determine the slope stability. In order to study the effects of water and fissures on rock’s mechanical properties and corresponding acoustic emission characteristics, red sandstones containing dual-fissure with different angle configurations were prepared for uniaxial compression tests along with acoustic emission (AE) monitoring in this study. The impacts of water content and fissure angles on rock strength, failure mode and acoustic emission characteristics were analyzed. Moreover, the kernel density estimation (KDE) was accepted to quantify the AE events spatial distribution, then distinguishing difference between dry and saturated rocks’ failure modes. This study reveals that the presence of water in the rock lowers of the rock strength and decreases the amounts of secondary cracks and acoustic emission counts. As the fissure angle increases, both the rock strength and acoustic emission counts increase and the failure mode shifts from the tension failure to the shear failure. The b value was highly sensitive to crack propagation, reaching around 0.5 when the rock was finally destroyed under different water conditions. AE events mainly gathered in the location where the main cracks occurred. The maximum KDE density points of dry sandstone changed from concentrated state to dispersed state before failure, while the maximum density points of saturated sandstone always gathered in the middle of rock. The results of this paper lay theoretical foundations for monitoring and pre-warning of engineering rock failure in water environment.

A new species of Halecania (Leprocaulaceae, Lecanoromycetes) from eastern North America1

Halecania robertcurtisii is described as new to science from dry sandstone cliffs and overhangs in Ohio, USA, in eastern North America. It has a smooth and continuous to rimose-areolate thallus, pseudolecanorine apothecia with reddish to rust colored disks, and lacks lichen substances. The species is similar to Halecania subsquamosa but lacks the diagnostic unidentified terpenoid present in that taxon.

Mechanical Properties of Sandstones after Freeze‐Thaw Cycles and Models for Their Strength Prediction

In cold regions, rock’s load‐bearing capacity will be greatly diminished in severe settings. This research investigates the influence of freezing and thawing on the physical and mechanical characteristics of sandstone, as well as the strength under complicated stress conditions. Uniaxial compression of sandstone samples was performed following freeze‐thaw cycles, and the changes in elastic modulus and peak stress of dry sandstone and saturated sandstone were investigated under various freeze‐thaw cycles. ANOVA was used to analyze whether there were significant differences between the number of freeze‐thaw cycles and the peak strength and elastic modulus. A three‐parameter strength prediction model based on σC, k0, and m is created based on the critical failure energy function. Experimental data is utilized to assess the model’s correctness, and the model is used to forecast the strength of dry and saturated sandstone during freeze‐thaw cycles. The result indicates that peak stress and elastic modulus of dry and saturated sandstone show a steady attenuation pattern as the number of freeze‐thaw cycles increases. Brittle failure is the failure mode of dry sandstone, whereas brittle failure to plastic failure is the failure mode of saturated sandstone. After 60 freeze‐thaw cycles, peak stress in dry and saturated sandstone was reduced by 55.3% and 56.8%, respectively. The three‐parameter model can predict the triaxial compressive strength of specimens under different confining pressures through the uniaxial compressive strength of specimens with an error range of 1% to 13%.

Nonlinear Young\u2019s Modulus of New Red Sandstone: Experimental Studies

Young’s modulus of New Red Sandstone was investigated experimentally to gain insight into its nonlinear nature. A large experimental programme was carried out by applying a controllable quasi-static and dynamic uniaxial loading to 286 dry sandstone samples of four different sizes. The static and dynamic tests, similar to those aiming at determining the uniaxial compressive strength, were conducted using the state-of-the-art experimental facilities at the University of Aberdeen including a custom-built small experimental rig for inducing a dynamic uniaxial compressive load via a piezoelectric transducer. The obtained results have confirmed a complex nature of Young’s modulus of sandstone. Specifically, under a harmonic dynamic loading, it shows strongly nonlinear behaviour, which is hardening and softening with respect to frequency and amplitude of the dynamic loading, respectively.

Infrared radiation test on the influence of water content on sandstone damage evolution

The rock mechanics uniaxial compression testing system and a thermal imaging camera were used to research the influence of water on sandstone damage evolution and the concurrent infrared radiation (IR) characteristics on sandstone samples with different water contents. The field of view of the thermal imaging camera divided the sandstone surface into several small areas of equal area. When one of the small areas of sandstone ruptured (that was when a damage point was generated in the sandstone) the IR information from that area underwent abrupt changes. Based on this monitoring, a quantitative index of infrared radiation, the IR count (IRC), was adopted and investigated as the indicator for damage evolution of rocks that also linked damage intensity with the corresponding IR temperature. The research results showed that when the sandstone stress experienced a stress drop, the IRC will simultaneously undergo a sudden increase indicating that the IRC was controlled by the stress. However, this stress control on IRC decreased with increasing water content due to the softening effect of water on the sandstone. Among them, the control effect of stress on IRC reaches 100% for dry sandstone, which is reduced to 75% for sandstone with a water content of 0.5% and 66.7% for sandstone with a water content of 1.5%. The control effect of stress of saturated sandstone on IRC is 57.1%. For sandstones with other water contents, but at the same damage evolution stage, the average IRC (IRC¯) for sandstones showed an upward trend as the water content increased. Compared with other damage evolution stages, the IRC¯ of the water-bearing sandstone had the most significant difference in the failure stage, indicating that water had the most significant influence on the damage evolution of the sandstone in the failure stage. This IRC was generated at the location of a new damage point on the sandstone surface. Therefore, based on the instant of time node when the sandstone IRC suddenly increased, the IRC spatial distribution map at that corresponding time was drawn to accurately locate the sandstone damage and fracture area. The characteristics of the damage evolution process of sandstones with different water contents were the same over the time before the sudden increase in IRC, the damage points formed were disorderly distributed, and the number of damage points was less. The difference was that as the water content increased, the number of sandstone damage points became larger and more concentrated.

Discrepancy between measured dynamic poroelastic parameters and predicted values from Wyllie\u2019s equation for water-saturated Istebna sandstone

The drilling-related geomechanics requires a better understanding of the encountered formation properties such as poroelastic parameters. This paper shows set of laboratory results of the dynamic Young’s modulus, Poisson’s ratio, and Biot’s coefficient for dry and water-saturated Istebna sandstone samples under a series of confining pressure conditions at two different temperatures. The predicted results from Wyllie’s equation were compared to the measured ones in order to show the effect of saturation on the rock weakening. A negative correlation has been identified between Poisson’s ratio, Biot’s coefficient and confining pressure, while a positive correlation between confining pressure and Young’s modulus. The predicted dynamic poroelastic rock properties using the P-wave value from Wyllie’s equation are different from measured ones. It shows the important influence of water saturation on rock strength, which is confirmed by unconfined compressive strength measurement. Linear equations have been fitted for the laboratory data and are useful for the analysis of coupled stress and pore pressure effects in geomechanical problems. Such results are useful for many drilling applications especially in evaluation of such cases as wellbore instability and many other drilling problems.

Effect of Water Content on Dynamic Fracture Characteristic of Rock under Impacts

Water contents in rocks vary with local hydrogeological conditions and may significantly affect the stability of rock mass engineering. For example, geological disasters are usually occurring after rainfall, such as landslides, karst collapse. In this paper, drop plate impact (DPI) tests were performed on single cleavage triangle (SCT) red sandstone specimens with dry, natural, absorbed and saturated conditions, which has a great guiding significance for deep understanding of dynamic failure of rock materials. The crack initiation time was obtained by crack propagation gauges (CPGs), and crack propagation speeds were computed by fractal method. The dynamic stress intensity factors (DSIFs) were calculated by ABAQUS code. Meanwhile, the microstructure of the fracture surface was scanned and was analyzed. The results show that the average critical DSIFs and crack propagation speeds vary with water content significantly. For the dry sandstone, the average critical DSIF is the highest, whereas the average crack speed is the lowest. For the natural sandstone, the average critical DSIF is the lowest, whereas the average crack speed is the highest. For the water-bearing sandstone, i.e., the natural, absorbed and saturated sandstone, the average critical DSIF increases with water content, whereas the crack speed decreases with increasing water content.

Effect of Temperature on Stiffness of Sandstones from the Deep North Sea Basin

Development of high-pressure, high-temperature (HPHT) petroleum reservoirs situated at depths exceeding 5 km and in situ temperature of 170 °C increases the demand for theories and supporting experimental data capable of describing temperature effects on rock stiffness. With the intention of experimentally investigating temperature effects on stiffness properties, we investigated three sandstones from the deep North Sea Basin. As the North Sea Basin is presently undergoing substantial subsidence, we assumed that studied reservoir sandstones have never experienced higher temperature than in situ. We measured ultrasonic velocities in a low- and high-stress regime, and used mass density and stress–strain curves to derive, respectively, dynamic and static elastic moduli. We found that in both regimes, the dry sandstones stiffens with increasing testing temperature and assign expansion of minerals as a controlling mechanism. In the low-stress regime with only partial microcrack closure, we propose closure of microcracks as the stiffening mechanism. In the high-stress regime, we propose that thermal expansion of constituting minerals increases stress in grain contacts when the applied stress is high enough for conversion of thermal strain to thermal stress, thus leading to higher stiffness at in situ temperature. We then applied an extension of Biot’s effective stress equation including a non-isothermal term from thermoelastic theory and explain test results by adding boundary conditions to the equations.

Experimental Investigation on Static and Dynamic Bulk Moduli of Dry and Fluid-Saturated Porous Sandstones

Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (Ks) were estimated from stress–volumetric strain curves while dynamic bulk moduli (Kd) were derived from the changes in ultrasonic P- and S- wave velocities (~ 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (Kd/Ks) reduces rapidly from about 1.5 − 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 − 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting that Kd/Ks is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space, Kd/Ks is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori–Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions.

Aquifer characterization using vertical electrical sounding in Auchi polytechnic, Auchi, Edo State, Nigeria

The Schlumberger array method was used to carry out a total of eight (8) Vertical Electrical Sounding (VES) with a spread of 350 m in the study area. The Pasi 16-GL Terrameter was used to acquire the data. The data obtained were analyzed with the Winresist 1.0 software to obtain the curves of best fit to the theoretical models. The modeling program converted the apparent resistivity into true resistivity values in 1-D model curves. Five (5) to seven (7) layers were delineated which correspond to the topsoil, sandy clay, clayey sand, sand, dry sandstone and saturated sandstone. The depth to aquifer ranges from 95.0 to 169.6 m with resistivity values ranging from 324.3 to 1524.7 Ωm.&#x0D; Keywords: Apparent resistivity, Aquifer, lithology and vertical electrical sounding.