Effect Of Increase In Pressure Research Articles

Control of serpentinisation rate by reaction-induced cracking

Serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water. The solid volume increase during serpentinisation can lead to stress build-up and trigger cracking, which ease fluid penetration into the rock. The quantitative effect of this reaction-induced cracking mechanism on reactive surface generation is poorly constrained, thus hampering our ability to predict serpentinisation rate in geological environments. Here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid-ocean ridges. We develop a micromechanical model to quantify the propagation of serpentinisation-induced cracks in olivine. The maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses. The micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting, and thus bulk reaction rate. Our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation. We also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the Marum ophiolite and the Papuan ultramafic belt (Papua New Guinea). The natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40%. These results are in agreement with our model predictions, suggesting that reaction-induced cracking controls the serpentinisation rate. We use our model to estimate that, at mid-ocean ridges, serpentinisation occurs up to 12 km depth and reaction-induced cracking reduces the characteristic time of serpentinisation by one order of magnitude, down to values comprised between 10 and 1000 yr. The increase of effective pressure with depth also prevents cracking, which positions the peak in serpentinisation rate at shallower depths, 4 km above previous predictions.

Pore Compressibility of Shale Formations

Summary This study introduces a novel outlook on a shale-pore system and on the potential effect of pore compressibility on the production performance. We divide porosity of the system into accessible and inaccessible pores, and incorporate inaccessible pores with grains into the part of the rock that is not accessible. In general, accessible pores contribute to flow directly, whereas inaccessible pores do not. We present a mathematical model that uses mercury-injection capillary pressure (MICP) data to determine the accessible-pore and inaccessible part of the rock (IRP) compressibility as a function of pressure. During MICP testing in a typical shale sample, the rock sample experiences conformance, compression, and intrusion as effective pressure increases. We characterize the compressibility value dependent on MICP data as a function of pressure. The calculated compressibility values for accessible pores generally appear to be much greater (two to three orders of magnitude) than those of IRP. Next, we evaluate how calculated accessible-pore-compressibility values affect gas recovery in several shale-gas plays. Our results suggest that substitution of total pore compressibility with accessible-pore compressibility can significantly change the reservoir-behavior prediction. The fundamental rock property used in many reservoir-engineering calculations including reserves estimates, reservoir performance, and production forecasting is the total pore-volume (PV) compressibility, which has an approximate value typically within the range of 1 × 10−6 to 1 × 10−4 psi−1 (Mahomad 2014). By recognizing the part of the pore system that actually contributes to production and identifying its compressibility, we can substitute total pore compressibility with accessible-pore compressibility. The result changes the value by nearly two orders of magnitude. The outcome of the paper changes the industry's take on prediction of reservoir performance, especially the rock-compaction mechanism. This study finds that production caused by rock compaction is in fact much greater than what has often been regarded, which will change the performance evaluation on a great number of reservoirs in terms of economic feasibility.

Experimental investigation of the shear behaviour of hemp and rice husk-based concretes using triaxial compression

Lime and hemp concretes are used as infill materials manually tamped in timber stud walls or more recently in the form of precast blocks. In either case, the structural design practice of wood frame construction associated with hemp concrete does not assume any contribution of the plant-based concrete whereas it may contribute towards the racking strength of the wall.This work is intended to evaluate the shear behaviour of two different bio-based concretes by means of triaxial compression.Hemp shives and whole rice husks were mixed with a lime-based binder according to the same mix proportioning and mixes were vibro-compacted in cylindrical forms. Then, samples were cured at 23°C – 65%RH before being tested under unconfined and triaxial compression. The triaxial shear test was performed after 60days of curing on unsaturated specimens under drained conditions at air pressure and for increasing effective confining pressure (from 25 to 150kPa).It was possible to estimate the shear strength parameters (peak friction angle and cohesion) of the two plant-based concretes. Comparing results, it appears a consistent value of cohesion but a different friction angle related to the binder and the aggregate contributions respectively. This leads to a first analysis of the relationship between the composition of the concrete (plant aggregates cemented with a binder) and its shear strength. Furthermore, the ductility of plant-based concretes is markedly enhanced as the mean effective pressure increases. Finally, the shear strength of plant-based concretes should be considered as part of the design practice of building envelopes.

Effect of pressure on the stability and ability to drying of water-saturated gases

The inorganic salts-based solid phase adsorbents are often used as driers of gases or weakly polar liquids (for example – ethanol) as the cations of metal elements have the ability to readily form aquacomplex compounds, which easily solidify and form crystalline hydrates. However, since there is often a need to conduct drying gas from other pressure than atmospheric pressure, there is a need to carry out the dependence of properties of inorganic salts, which form crystalline hydrates, from the pressure. Results based on the use of four different inorganic adsorbents have shown that the pressure increase accelerates dehydration and generally deepens its extent. It is caused both by van der Waals bonds and a more effective mechanism for the formation of bonds of the donor-acceptor mechanism. Effect of pressure increase on various adsorbents is uneven and is linked to the strength of relationships, which are formed by a cationic mechanism, and is also associated with coordination number of the central atom, its radius, the distance to its electron shells and the position in the periodic table. The results make it possible to develop an adsorbent for effective drying gas to replace traditional gas drier, such as phosphorus (V) oxide, and sulfate (sulfuric) acid.

Research on the Effect of Water Draining Through Vertical Drainage Systems in the Process of Soil Consolidation

A saturated soil subjected to a growing total effort will reduce gradually the volume up to excess pore water pressure dissipation, interfering primary consolidation. This process can take a long time and when all excess pressure is dissipated, the soil is considered consolidated. In low permeability soils, from external loading, results in an increase of the normal effort at higher values than the earth preconsolidation pressure, thus having neutral increased pressure followed by a consolidation process where water is expelled from the pores of the soil. The decrease in porosity (soil volume), so produced, is accompanied by a gradual increase of effective pressure and a corresponding decrease in excess pore water pressure. The consolidation process continues until the excess pore water pressure dissipates only complete and upload translates into effective stress. Duration of the process depends on the consolidation characteristics of the soil and drainage ways (with how much drainage paths are longer, the longer is the process of consolidation). The purpose of installing vertical drains is to shorten the drainage ways and time needed to dissipate the neutral pressure. The time to dissipate the excess water pressure in the pores (time consolidation) is so short as the inter-distance between the drains is less. DOI: http://dx.doi.org/10.5755/j01.sace.12.3.12655

Thermo elastic-plastic transition of transversely isotropic thick-walled circular cylinder under internal and external pressure

Purpose – The purpose of this paper is to provide the guidance on a design and integrity evaluation of a cylinder under pressure, for which stress analysis has been done for transversely isotropic thick-walled circular cylinder under internal and external pressure with thermal effects. Design/methodology/approach – Transition theory has been used to evaluate plastic stresses based on the concept of generalized principal Lebesgue strain measure which simplifies the constitutive equations and helps to achieve better agreement between the theoretical and experimental results. Findings – It can be concluded that circular cylinder with thermal effects under internal and external pressure made of isotropic material (steel) is on the safer side of the design as compared to the cylinder made of transversely isotropic material (i.e. magnesium and beryl) because percentage increase in effective pressure required for initial yielding to become fully plastic is high for isotropic material (steel) as compared to transversely isotropic material (i.e. magnesium and beryl). It can also be concluded that out of two transversely isotropic materials, beryl is better choice for design of cylinder as compared to magnesium material because percentage increase in effective pressure required for initial yielding to become fully plastic is high for beryl as compared to magnesium. Originality/value – A detailed investigation of thermal transversely isotropic thick-walled circular cylinder under internal and external pressure has been done which leads to the idea of “stress saving” that minimizes the possibility of fracture of cylinder.

Collapse Pressure Analysis of Transversely Isotropic Thick-Walled Cylinder Using Lebesgue Strain Measure and Transition Theory

The objective of this paper is to provide guidance for the design of the thick-walled cylinder made up of transversely isotropic material so that collapse of cylinder due to influence of internal and external pressure can be avoided. The concept of transition theory based on Lebesgue strain measure has been used to simplify the constitutive equations. Results have been analyzed theoretically and discussed numerically. From this analysis, it has been concluded that, under the influence of internal and external pressure, circular cylinder made up of transversely isotropic material (beryl) is on the safer side of the design as compared to the cylinders made up of isotropic material (steel). This is because of the reason that percentage increase in effective pressure required for initial yielding to become fully plastic is high for beryl as compared to steel which leads to the idea of “stress saving” that reduces the possibility of collapse of thick-walled cylinder due to internal and external pressure.

Performance and Emission Characteristic on a Two Cylinder DI Diesel Engine Fuelled with Ethanol-Diesel Blends

An experiment on the application of diesel and ethanol blends as fuel in diesel engine was carried out at various engine loads and ethanol percentages. The experiments were performed using solar, E2.5%, E5%, E7.5%, and E10% ethanol-diesel blends and 0, 10, 20, 30, 40, 50 and 60 Nm engine loads. Several engine parameters i.e. power, brake specific fuel consumption, brake thermal efficiency, the exhaust gas temperature, and lubricating oil temperature were investigated. As a complement of the experiment, the exhaust emission characteristic of CO, HC and smoke were also investigated. The results indicate that the engine power and the indicated mean effective pressure increase with increasing of ethanol percentage. The brake specific fuel consumption and exhaust gas temperature decrease, meanwhile the lubricating oil temperature increase with increasing the ethanol content. From the experiments, as the increase of ethanol percentage content, the emission of CO, HC and smoke decrease.

Brittle creep in basalt and its application to time-dependent volcano deformation

Time-dependent brittle deformation is a fundamental and pervasive process operating in the Earth's upper crust. Its characterization is a pre-requisite to understanding and unraveling the complexities of crustal evolution and dynamics. The preferential chemical interaction between pore fluids and strained atomic bonds at crack tips, a mechanism known as stress corrosion, allows rock to fail under a constant stress that is well below its short-term strength over an extended period of time; a process known as brittle creep. Here we present the first experimental measurements of brittle creep in a basic igneous rock (a basalt from Mt. Etna volcano) under triaxial stress conditions. Results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress (and can be equally well fit by a power law or an exponential law); with a 20% increase in stress producing close to three orders of magnitude increase in creep strain rate. Results from stress-stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure. We show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure, with the remainder interpreted as due to a reduction in stress corrosion reactions; the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips. Overall, our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure, regardless of the level of applied stress and the time taken to reach this point. The experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law, and compared with the observed deformation history at Mt. Etna volcano.

Deformational characteristics of rock in low permeable reservoir and their effect on permeability

In the development of oil and gas the pressure in the rocks of reservoir changes constantly and rocks are compressed and deformed. So their permeability reduces. And the production capacity of oil and gas well is affected by the permeability. This paper deals with the deformational characteristics of rocks in low permeable reservoir and their effect on the permeability. The permeability of the rocks in low permeable reservoir is sensitive to effective pressure. The greater permeability, the more sensitive the effect of effective pressure on permeability is. The permeability of rock decreases gradually with the increase of effective pressure and tends to be stable when effective pressure reaches up to a certain amount. And the permeability cannot be recovered after the reduction of overburden pressure and the relationship between the porosity and the permeability is not linear. It can cause rocks to be deformed under different overburden pressure and the change of fissures in low permeable media leads to different permeable characteristics.

Effect of stress sensitivity on displacement efficiency in CO2 flooding for fractured low permeability reservoirs

Carbon dioxide flooding is an effective means of enhanced oil recovery for low permeability reservoirs. If fractures are present in the reservoir, CO2 may flow along the fractures, resulting in low gas displacement efficiency. Reservoir pore pressure will fluctuate to some extent during a CO2 flood, causing a change in effective confining pressure. The result is rock deformation and a reduction in permeability with the reduction in fracture permeability, causing increased flow resistance in the fracture space. Simultaneously, gas cross flowing along the fractures is partially restrained. In this work, the effect of stress changes on permeability was studied through a series of flow experiments. The change in the flowrate distribution in a matrix block and contained fracture with an increase in effective pressure were analyzed. The results lead to an implicit comparison which shows that permeability of fractured core decreases sharply with an increase in effective confining pressure. The fracture flowrate ratio declines and the matrix flowrate ratio increases. Fracture flow will partially divert to the matrix block with the increase in effective confining pressure, improving gas displacement efficiency.

Effects of time lag and stress loading rate on permeability in low permeability reservoirs

In order to study the effect of time lag and stress loading rates on rock deformation, the conventional stepped stress loading mode was changed into a continuous mode to investigate the effect of effective pressure on permeability and porosity. The time lag effect of rock deformation illustrating the relationship between changes in permeability and steady time was studied. Permeability reduction ratios were measured under different stress loading rates which were achieved by different pump rate settings. The results show that permeability and porosity gradually decrease with increases in effective pressure. Permeability at high effective pressure attains stability quickly. Steady times at low effective pressure are very long. Reduction in permeability at lower stress loading rates is small, while, in contrast, it is large at high stress loading rates.

Parametric study of an ethanol–water distillation column with direct vapour recompression heat pump

Heat pumps have been used to reduce energy consumption in distillation columns. However, the effects of some parameters have not been taken into consideration in the determination of energy consumption in such systems. Perhaps this is one of the reasons for little or no application of the system in ethanol which is fast becoming a competitive fuel to petrol. The study reported here considers the effect of pressure increase across the compressor, column heat loss and the overall heat transfer coefficient of reboiler–condenser as an explicit function of Prandtl, Reynolds and Nusselt numbers. This was to study the overall influence of these parameters on energy consumption and energy effectiveness (coefficient of performance). An ethanol–water conventional distillation column with direct vapour recompression was used as a case study. Results showed that when these parameters were included in the model equation, there was an increase in compressor power input and total energy consumption while the energy effectiveness reduced. Also, the comparison of the two systems showed that while the values of the column heat loss, reboiler heat transfer rate, total energy consumption and overall heat transfer coefficient were greater for the actual (VRC Δ P ) system, the coefficient of performance and the heat pump distribution factor were more for the control (VRC) system. The significance of this work is the provision of information for the determination of energy effectiveness as well as appropriate capacity of the auxiliary equipment.

Contact analyses and permeability measurements of polished fracture surfaces in Sambagawa metapelitic schists and Ryoke mylonites from the Median Tectonic Line, Japan

Fractures play a vital role in fluid flow through rocks, and highly fractured zones can be the most permeable part of a fault zone. However, the evaluation of the hydraulic properties of natural fractured rocks at great depths is not easy because of the complexity of fracture geometry and the scaling problem from laboratory to nature. As a first step toward the estimation of flow rate through fractured zones at depth, this paper examines the issue of permeability estimation in the case of flow between polished surfaces of known topography (or roughness) via an integrated study of permeability measurements at effective pressures up to 180 MPa, surface roughness measurements, and contact analyses using the Hertzian theory of surface asperities of fractures as a function of stress. Apertures between two polished surfaces in specimens of Sambagawa pelitic schist and Ryoke mylonite were estimated from permeability measurements assuming the cubic law. The aperture of contacting surfaces has also been calculated from contact analyses of representative asperities. Despite some imperfections in the agreement between these two approaches, the overall features of progressive reduction in aperture upon an increase in effective pressure were successfully predicted. Deviations between the two approaches are probably due to complex deformation of multiple contacting asperities or limitations of our contact analysis method.

Knock Investigation of a Direct Injection−Homogeneous Charge Compression Ignition Engine Fueled with Dimethyl Ether and Liquefied Petroleum Gas

An experimental investigation of knock was conducted on a DI−HCCI (direct injection−homogeneous charge compression ignition) engine fueled with DME (dimethyl ether) and LPG (liquefied petroleum gas). The engine’s HCCI fuel system provided a homogeneous charge of DME−LPG blend and the DI fuel system injected DME into the cylinder. The in-cylinder pressure was measured and processed using Fourier transform and discrete wavelet transform. The analysis shows that the main frequency band of knock is included between 3.5 and 10 kHz in DI−HCCI combustion with DME as DI and HCCI fuel. As the brake mean effective pressure increases, the main frequency band changes little. With DME as DI fuel and DME−LPG blend as HCCI fuel, the main frequency bands are between 4 and 10 kHz. The cetane number of HCCI fuel has little effect on the main frequency band. With no knock, light knock, and heavy knock, the greatest wavelet relative energy of in-cylinder pressure are at detail D2, D4, and D3, respectively. The distribution o...

Contribution à la modélisation des vitesses de propagation des ondes P et S dans les formations sédimentaires argileuses

Acoustic P and S wave propagation in the argillaceous sediments is related to the media porosity as well as on clay-mineral composition. The propagation-velocity prediction of these waves thus requires knowledge of these two parameters and their impact on the velocities. Porosity decreasing with effective pressure increasing, due to the burial, follows an exponential law. We will limit our analysis to the three clays: kaolinite, illite and montmorillonite. The variations of porosity between various clays are strongly reduced with the increase in effective pressure, which tends to minimize the impact of mineralogy with the burial. By integrating these equations in a model of Reuss and Voigt, in which we consider an association mineral argillaceous/fluid, we extract a law of evolution for bulk and shear moduli of clay according to the pressure, for each type of clay. By associating clay moduli calculation results to qualitative and quantitative predictions from spectral nuclear-radiations tools and X-ray-diffraction measurements on cores, it is possible to predict sonic velocities in clays with a given effective pressure. An example of P and S velocity modelling is presented on a well in the deep offshore-turbiditic environment.

Effects of pore fluid pressure on the seismic response of a fractured carbonate reservoir

An effective medium model for the stress-dependent seismic properties of fractured reservoirs is developed here on the basis of a combination of a general theory of viscoelastic waves in rock-like composites with recently published formulae for deformation of communicating and interacting cavities (interconnected pores/cracks and fractures at finite concentration) under drained loading. The inclusion-based model operates with spheroidal cavities at two different length scales; namely, the microscopic scale of the pores and (grain-boundary) cracks, and the mesoscopic scale of the fractures (controlling the flow of fluid). The different cavity types can in principle have any orientation and aspect ratio, but the microscopic pores/cracks and mesoscopic fractures were here assumed to be randomly and vertically oriented, respectively. By using three different aspect ratios for the relatively round pores (representing the stiff part of the pore space) and a distribution of aspect ratios for the relatively flat cracks (representing the compliant part of the pore space), we obtained a good fit between theoretical predictions and ultrasonic laboratory measurements on an unfractured rock sample under dry conditions. By using a single aspect ratio for the mesoscopic fractures, we arrived at a higher-order microstructural model of fractured porous media which represents a generalization of the first-order model developed by Chapman et al. (2002,2003). The effect of cavity size was here modelled under the assumption that the characteristic time for wave-induced (squirt) flow at the scale of a particular cavity (pore/crack vs. fracture) is proportional with the relevant scale-size. In the modelling, we investigate the effect of a decreasing pore pressure with constant confining pressure (fixed depth), and hence, increasing effective pressure. The analysis shows that the attenuation-peak due to the mesoscopic fractures in the reservoir will move downward in frequency as the effective pressure increases. In the range of seismic frequencies, our modelling indicates that the P-wave velocities may change by more than 20% perpendicular to the fractures and close to 10% parallel to the fractures. In comparison, the vertical S-wave velocities change by only about 5% for both polarization directions (perpendicular and parallel to the fractures) when the effective pressure increases from 0 to 15 MPa. This change is mainly due to the overall change in porosity with pressure. The weak pressure dependence is a consequence of the fact that the S waves will only sense if the fractures are open or not, and since all the fractures have the same aspect ratio, they will close at the same effective pressure (which is outside the analysed interval).

Semi-brittle flow during dehydration of lizardite–chrysotile serpentinite deformed in torsion: Implications for the rheology of oceanic lithosphere

Serpentinite is a significant component of the oceanic lithosphere and plays an important role in subduction dynamics. However, the rheology of serpentine-bearing rocks is poorly understood, especially at large strains and during the dehydration of serpentine. We have investigated the mechanical behavior and microstructural evolution of serpentinite during dehydration reaction to olivine, talc and water at temperatures of 550 and 600 °C, pressures of 300 and 400 MPa and shear strain rates of 1 × 10 − 5 to 1 × 10 − 4 s − 1 under drained conditions. The non-coaxial deformation experiments were performed in a gas-medium apparatus equipped with a torsion system to shear strains of up to 3.3. Strong crystallographic preferred orientations (CPO) of partially dehydrated serpentine, shear localization through the development of S–C structures and widespread microfracturing in deformed specimens indicate that deformation took place in the semi-brittle field. Microstructural observations reveal that the CPO results from slip and rotation of (001) planes in partially dehydrated lizardite, possibly assisted by fluid released by the reaction. Despite the development of a strong CPO and shear localization, strain hardening accompanied by repeated transitory stress drops of a few MPa was observed and may be explained by the following combined effects: (1) progressive decrease of excess local pore pressures due to specimen dilatancy and the formation of pore networks during the reaction, leading to an increase in effective pressure, (2) subsequent collapse of this pore space by shear-enhanced compaction, resulting in work hardening, and (3) formation of a mechanically stronger assemblage of reaction products. The experiments imply that the bulk strength of a serpentinized subducting slab increases during dehydration as fluid escapes from the slab, while local embrittlement and shear localization take place.

Viscoelasticity of precompacted unconsolidated sand with viscous cement

The elastic properties of sand strongly depend on the grains’ contact stiffness, which can be increased significantly by solid matter and, depending on frequency, viscous fluid acting as contact cement. To calculate seismic velocities in precompacted fluid-cemented sand, we examine how a small amount of viscous fluid at the grain contacts influences their normal and tangential stiffnesses as a function of effective pressure. Using the Hertz-Mindlin approach and considering oscillatory loading in addition to precompaction of a combination of two elastic spheres, we extend the dry-contact elastic theory by a viscoelastic formulation. Here, we describe the radial flow of the fluid cement induced by the oscillations of the grains’ surfaces around the direct contact, a process that leads to a complex normal stiffness and stiffness/frequency dispersion. In the resulting combined model, the low-frequency real part of the complex normal stiffness identical is to the original Hertz-Mindlin expression. The magnitude of the dispersion is governed by the amount of viscous cement; magnitude decreases as effective pressure increases. The frequency of the maximum imaginary part of the normal stiffness is determined mainly by cement viscosity and contact geometry. The tangential contact stiffness virtually is not influenced by the viscous fluid. Comparison of predicted results with data from pulse transmission experiments (500 kHz) on glass beads with two different fluids shows an excellent fit in P-wave velocities [Formula: see text], whereas S-wave velocities [Formula: see text] are systematically overestimated by the model. The experimental results confirm, however, the predicted change with effective pressure in the [Formula: see text] ratio for both examined cases as well as reflect the predicted increase in [Formula: see text] and [Formula: see text], respectively, between the two cases. This implies that our viscoelastic formulation represents a reasonable way to describe the role of viscous cement in sand.

Pore fabric shape anisotropy in porous sandstones and its relation to elastic wave velocity and permeability anisotropy under hydrostatic pressure

To understand the relationship between pore space anisotropy and petrophysical properties, we developed a novel apparatus capable of simultaneously measuring permeability, porosity and ultrasonic velocities at hydrostatic pressures up to 100 MPa. First, we use magnetic susceptibilities and acoustic wave velocities to identify the principal anisotropy axes under ambient laboratory conditions. This directional anisotropy data is then used to guide experiments on two sandstones (Bentheim and Crab Orchard) under hydrostatic pressure from 5 to 90 MPa. We find the structural anisotropy formed by the void space is well described by velocity anisotropy in both cases. Under hydrostatic pressure, the acoustic anisotropy of Crab Orchard sandstone (COS) decreases from 3% and 7% at 5 MPa (P-wave and S-wave) to 1.5% and 1%, respectively, at effective pressures over 40 MPa; for Bentheim sandstone the decrease is considerably less. Permeability of COS is 125×10 −18 m 2, decreasing rapidly as effective pressure increases, with permeability parallel to bedding approximately twice that normal to bedding. In contrast, permeability of Bentheim sandstone is 0.86×10 −12 m 2, and varies little with effective pressure or coring direction. We relate many of our measurements made under hydrostatic pressure to the contrasting pore fabric between the two rock types, and infer that a critical pressure is required for the initiation of crack closure.