High Confinement Pressure Research Articles

Experimental study on P-wave response characteristics of hot dry rock under high temperature and high confining pressure

In the exploration and development of hot dry rock reservoirs, accounting for lithology, temperature, and confining pressure is vital when predicting rock mechanical properties. Acoustic logging data is essential for accurate information in hot dry rock fracturing. However, under high-temperature and high-confining pressure conditions, logging signals in these reservoirs become more susceptible to interference, resulting in significant errors in rock mechanics data. This study employs the in-house CCZU-HTHP logging system to assess P-wave velocity in granite and sandstone over a temperature range of 25–400 °C and a pressure range of 0.1–110 MPa. Findings indicate that under normal temperature and pressure conditions, granite exhibits an average P-wave velocity 1.3 times higher than sandstone, with sandstone having a variation range 1.1 times that of granite. Furthermore, as temperature increases, both granite and sandstone experience logarithmic and linear reductions in P-wave velocity, with respective decreases of 24.3% and 29.67%. Notably, P-wave velocity and pressure curves demonstrate pronounced nonlinear characteristics as confining pressure rises. Under temperature-confining pressure coupling, P-wave velocity initially increases, then decreases. Sandstone's P-wave velocity increases by 12%, compared to 6.25% in granite. Conversely, P-wave velocity decreases by 12.04% in sandstone and 9.38% in granite. Importantly, the response threshold of temperature-confining pressure in granite exceeds that of sandstone.

Experimental study on brittle-to-ductile transition mechanism of lower Silurian organic-rich shale in south China

Brittle-ductile transition conditions and mechanisms are key scientific issues embedded in the reservoir deformation-related difficulties faced during shale gas exploration and exploitation. In this study, shale deformation experiments under different temperatures, confining pressure and differential stress were conducted using a NaCl solid confining pressure medium, taking the Lower Silurian marine shale from South China as an example. We aimed to clarify the characteristic textures and structures of brittle and ductile deformation in organic-rich shale and then, to reveal the conditions and mechanisms of the brittle-to-ductile transition under the deep in-situ environment of high-temperature, high-confining pressure and high-stiffness surrounding strata. Deformation characteristics of experimentally deformed shale showed significant differences between brittle and ductile deformed shale in terms of fracture development, failure combining form, deformation distribution homogeneity, shear angle and the structures and textures within intensely deformed zones. The micro-deformation structures of brittle deformed shale are characterized by the development of fractures and the simple mechanical crushing of shale blocks and mineral particles. The strong deformation zones of ductile deformed shale mainly show micro-deformation features, such as solid-state rheological deformation and accompanying fragmentation, rounding and directional arrangement of mineral particles. Comparison of control group experiments supported the conclusion that increasing confining pressure significantly improves the elastic modulus and compressive strength of shale and leads to a transition from large-area uniform deformation to narrow-band concentrated deformation. A confining pressure of 75–100 MPa is required for the brittle-to-ductile deformation transition of the target shale at 200 °C. An elevated temperature enhances shale deformation in the yield and creep stages. Finally, we revealed the progressive process and mechanism of brittle-to-ductile transition with the increase of confining pressure; that is, increasing confining pressure leads to a gradual transition of the shale micro-deformation mechanism, from brittle failures to ductile rheological deformation, by suppressing tension fracturing and enhancing friction.

Finite element method–discrete element method bridging coupling for the modeling of gouge

AbstractWe discuss the multiscale modeling of a granular material trapped between continuum elastic domains. The amorphous granular region, usually termed “gouge,” is under high confinement pressure, to represent the loading of faults at depth. We model the granularity of gouge using the discrete element method (DEM), while the elastic regions surrounding it are represented with two continuum domains modeled with the finite element method (FEM). We resort to a concurrent coupling of the discrete and continuum domains for a proper transmission of waves between the discrete and continuum domains. The confinement pressure results in the appearance of a new kind of ghost forces, which we address via two different overlapping coupling strategies. The first one is a generalization to granular materials of the bridging method, which was originally introduced to couple continuum domains to regular atomic lattices. This method imposes a strong formulation for the Lagrange constraints at the coupling interface. The second strategy considers a weak formulation. Different DEM samples sizes are tested in order to determine at which scale a convergence of the elastic properties is reached. This scale sets the minimal mesh element size in the DEM/FEM interface necessary to avoid undesirable effects due to an elastic properties mismatch. Then, the two DEM/FEM strategies are compared for a system initially at equilibrium. While the performance of both strategies is adequate, we show that the strong coupling is the most stable one as it generates the least spurious numerical noise. Finally, as a practical example for the strong coupling approach, we analyze the propagation of pressure and shear waves through the FEM/DEM interface and discuss dispersion as function of the incoming wave frequency.

Seepage characteristics of fractured rock mass with non-equal width filling under cyclic loading

Fractured rock mass with non-equal width filling often appears in the tunel engineering, the non-equal width ratio is the ratio of the filling width at the seepage inlet to the outlet. To study the seepage characteristics of FFRM (filling fracture rock mass) in the surrounding rock of the Gaoligongshan railway tunnel, the samples of FFRM with different non-equal width ratios were prepared indoors. The permeability coefficient formula of the non-equal width ratio FFRM was deduced for the permeability measurement. Then the triaxial seepage apparatus was used to conduct high and low confining pressure test. Results show that the permeability coefficient of FFRM with non-equal width ratios is a function of multi-parameter interaction, such as seepage outflow, inlet width and filling porosity. The permeability and confining pressure at LCP (low confining pressure) and HCP (high confining pressure) is the power and translation exponential function, respectively. While the relationship between the permeability and non-equal width ratio under confining pressure-seepage condition is the quadratic function. After the first cycle, the permeability loss of the samples under HCP is larger than that of the LCP, and the recovery degree is less than that of the LCP. The SSC (stress sensitivity coefficient) decreases with the increase of the confining pressure, the recovery degree is poor after the first cycle unloading. The trend of the subsequent cycle is almost consistent and the recovery degree is good, which is more obvious under the action of the HCP. The conclusions obtained in this study provide a reference for the stability evaluation of the surrounding rock in the underground engineering.

Perforation studies of concrete panel under high velocity projectile impact based on an improved dynamic constitutive model

The finite-depth concrete panels have been widely applied in the protective structures, and its impact resistance and dynamic fracture failures, especially the scabbing/perforation limits, under high velocity projectile impact, are mainly concerned by protective engineers, which are numerically studied based on an improved dynamic concrete model in this study. Firstly, based on the framework of the KCC (Karagozian & Case concrete) model, a dynamic concrete model is proposed which considers an independent tensile damage model and a continued transition between dynamic tensile and compressive properties. Secondly, the strength surface, equation of state and damage parameters of the proposed model are comprehensively calibrated by a triaxial compressive test with high confinement pressure, the rationality of which is further verified based on the single element tests, e.g., uniaxial and triaxial compression as well as uniaxial, biaxial and triaxial tension. Thirdly, a series of projectile high velocity impact tests on thin and thick concrete panels are simulated, which indicates that the projectile residual velocity and dynamic fracture failures are reproduced satisfactorily, while the KCC model underestimates both the spalling and scabbing dimensions severely. Finally, based on the validated concrete model and finite element analyses approach, the validations of the existing five empirical formulae are evaluated, in terms of the depth of penetration (DOP) and scabbing/perforation limits of concrete panel. Both the Army corps of engineers (ACE) and modified National Defense Research Committee (NDRC) formulae are recommended in the design of the protective structure to avoid scabbing failure.

Quantification of Plasticity and Damage in Berea Sandstone through Monotonic and Cyclic Triaxial Loading under High-Confinement Pressures

Quantification of Plasticity and Damage in Berea Sandstone through Monotonic and Cyclic Triaxial Loading under High-Confinement Pressures

Compression and Deformation Characteristics of Hydrate-Bearing Sediments under High Effective Confining Pressure

The depressurization method that is used to produce methane gas can increase the effective stress in hydrate-bearing sediments and induce deformation and settlement of reservoirs, which has adverse effects on safe and stable exploitation. A series of isotropic consolidation and depressurization dissociation tests under high effective confining pressures on methane hydrate-bearing specimens were conducted to investigate compressive characteristics and deformation behaviors. Isoconsolidation test results demonstrated that the rise in effective confining pressure-induced increasing compressibility and particle crushing of sediment, while the presence of hydrates can suppress compressibility and crushing. The compression index tends to increase with increasing effective stress but decrease with increasing hydrate saturation. Dissociation test results suggested that higher effective stress during dissociation and higher deviatoric stress led to more significant deformation and particle breakage. A specimen can exhibit serious deformation (ɛv ≈ 20%) after dissociation when the effective stress is extremely high (17 MPa), even if the stress state is in the stable zone. During water recovery after dissociation, axial and volumetric strains may further increase under the combined effect of deviatoric stress and high effective confining pressure.

Experimental Investigation of the Mechanical Properties of Methane Hydrate–Bearing Sediments under High Effective Confining Pressure

A significant increase in effective stress can be induced in hydrate-bearing reservoirs when the depressurization method is applied. A series of drained triaxial shear tests were performed on hydrate-bearing sediments with various hydrate saturations to investigate their mechanical characteristics under effective confining pressures of up to 20 MPa. The results show that significant particle crushing of the host sand occurs during shearing under high pressures, and there is no remarkable effect of hydrate saturation on the degree of particle breakage. As the effective confining pressure increases, the stress–strain curves of the hydrate-bearing specimen transformed from strain-softening to strain-hardening. The peak stress ratio and internal friction angle of the sediments gradually decrease and tend to be constant with the increased confining stress, whereas the cohesion in hydrate-bearing sediments exhibits an increasing tendency. The critical state line (CSL) of hydrate-bearing sediments in the e-lnp′ space under low-to-high effective confining pressures intersects with the normal consolidation line under the same hydrate saturation. Furthermore, the CSL moves upward and rotates clockwise as the hydrate saturation increases.

Cross-Scale Characteristics of Damage Evolution in Granite under High-Confining Pressure Cyclic Loading

The rock properties under periodic load are important to the stability of engineering works. It needs to analyze the mechanical properties and damage of deeply buried rock under periodic load. Taking granite in a mine as the research object, designed the experiment involving different cycles under different confining pressures and constant periodic upper and lower limit loads, carried out corresponding high-confining pressure cyclic loading experiment. The results show that a closed stress hysteresis loop is formed during loading and unloading cycles under different confining pressures. The hysteresis loop area of axial deformation decreases gradually with an increase of cycles. With the confining pressure increasing, the deformation modulus decreases gradually. However, under the same confining pressure, the deformation modulus remains almost unchanged as the number of loading–unloading cycles increase. As the number of cycles increase, the residual displacement shows an increasing trend whereas the axial and circumferential strains decrease. This can be explained by primary cracks gradually becoming compacted with the increase of cycles so that the proportion of axial elastic strain increases, the proportion of circumferential elastic strain increases. Fractal analysis of the T2 spectrum shows that the sample has fractal characteristics before and after cyclic loading.

A brittleness index evaluation method for weak-brittle rock by acoustic emission technique

Brittleness is a significant indicator of a series of engineering problems in the oil and gas community. However, the majority of brittleness evaluation methods focus on brittle rock, but few pay much attention to weak-brittle rock. In this study, an attempt has been made to study the relationship between released acoustic emission (AE) energy and brittleness during the fracture process of rock samples. An accumulative acoustic energy-based brittleness evaluation method was proposed according to the energy accumulation and dissipation in the pre-peak and post-peak stages. Moreover, the suitability and reliability of this method was verified by tri-axial compression tests on shale, tight sandstone, and glutenite. The experimental results indicate that both the complete stress vs. strain brittleness method and the accumulative acoustic energy-based brittleness method can reflect the effect of confining pressure, but deformation and failure characteristics that transition from brittle to weak-brittle behaviors with increasing confining pressure make the post-peak feature difficult to distinguish. Thus, the conventional complete stress vs. strain brittleness method fails to determine brittleness. However, the accumulative acoustic energy-based brittleness evaluation method is more sensitive to rock failure, even if the rock is weak-brittle or ductile, while acoustic energy can be directly monitored rather than calculated by a mathematical model. Thus, the accuracy of brittleness evaluation can be improved. The good match between the fracture morphology and accumulative acoustic energy-based brittleness results demonstrate that this novel method is more suitable for brittleness evaluation in weak-brittle or even ductile rock; therefore, this brittleness method can be efficiently used in deep formations under high-confining pressure, benefitting various engineering applications.

Modelling damage mechanisms of concrete under high confinement pressure

Dynamic failures (e.g., crater, crack and spall) can be observed in concrete structures when suffered from possible blast and impact loadings. Understanding the physical mechanisms behind these failures needs a sound material model that thoroughly considers the damage mechanisms of concrete material, especially under the high confinement situation existed near the projectile and explosion. Modelling the damage mechanisms is a challenging problem that is not fully resolved in the commonly-used concrete material models. In this study, three kinds of damage mechanisms of concrete material, i.e., the hydrostatic damage due to pore collapse, shear damage due to shear-induced microcracking and tensile damage due to tensile microcracking are identified and physically modelled respectively using corresponding damage indexes. The interaction of the three damage is also considered by introducing two stress-dependent factors. The damage model is implemented into the Kong-Fang material model recently proposed, in which improvements on the strength surfaces, strain-rate enhancement and erosion criterion are accompanied made based on a few years use of this model. Numerical triaxial compression test and hydrostatic compression to unconfined compression test are conducted using a single element to validate the proposed hydrostatic damage model and the shear damage model. And the competition mechanism between hydrostatic damage and shear damage during the triaxial compression test is discussed. Then the modified Kong-Fang model is used to numerically simulate the reinforced concrete slabs subjected to a contact explosion load and projectile impact load, in which the physical mechanisms of the experimentally observed crater, crack, tunnel and spall failures are revealed.

A novel triaxial apparatus to evaluate the effect of high temperature nitrogen injection concept and design

High temperature nitrogen, injection into coal seams is supposed to improve the per?meability and thus maintain the safety of underground mining. A novel triaxial appa?ratus is recently developed, aiming at providing the effective method to evaluate the effect of high temperature nitrogen injection. The main feature of this novel appara?tus is its high confining pressure, gas injection with high pressure as well as the high temperature. This new device can be either used for natural coal samples (e.g. intact or fractured) or the synthetic coal samples. A series of leakage tests were conducted to verify the feasibility of this instrument, the results of which have confirmed that the maximum pressure (i.e. 10 MPa) can be reached. In addition, the high temperature and pressure of nitrogen gas can also be sustain at the requested level. Based on the preliminary tests on the instrument, a large amount of tests were carried out to eval?uate the effect of nitrogen injection in enhancing the permeability of coking coal from the Pingdingshan coalfield, China, and the influence of high temperature nitrogen injection on mechanical parameters of coal was obtained.

THE EFFECT OF INCREMENTAL CONFINING PRESSURE ON THE HYDRAULIC PROPERTIES OF PVD

Prefabricated Vertical Drains (PVD) has become a standard method in soft soil improvementsproject to accelerate the soil consolidation time. Due to PVD’s primary function as drainage, it should haveadequate hydraulic properties, i.e., discharge capacity and transmissivity. This paper evaluates the effect ofincremental confining pressure on three types of PVD using an apparatus adopting ASTM D4716, a mechanismto determine the in-plane flow rate and transmissivity of geosynthetics. The apparatus is originally designed toobserve the discharge capacity of the PVD. Three types of PVD with different thickness and box channel coreshapes were tested under incremental confining pressure in the range of 50 to 200 kPa and varied hydraulicgradients. The results indicated that the reduction of discharge capacity under incremental confining pressureis higher for lower hydraulic gradients, and the reduction of discharge capacity is significant at high confiningpressure. The thickness of PVD held an essential role in transmissivity since PVD with greater thicknessprovides higher discharge capacity and transmissivity.

Tracking Metamorphic Dehydration Reactions in Real Time with Transmission Small- and Wide-Angle Synchrotron X-ray Scattering: the Case of Gypsum Dehydration

AbstractWe present a review of a unique non-destructive method for the real-time monitoring of phase transformations and nano-pore evolution in dehydrating rocks: transmission small- and wide-angle synchrotron X-ray scattering (SAXS/WAXS). It is shown how SAXS/WAXS can be applied to investigating rock samples dehydrated in a purpose-built loading cell that allows the coeval application of high temperature, axial confinement, and fluid pressure or flow to the specimen. Because synchrotron sources deliver extremely bright monochromatic X-rays across a wide energy spectrum, they enable the in situ examination of confined rock samples with thicknesses of ≤ 1 mm at a time resolution of order seconds. Hence, fast kinetics with reaction completion times of about hundreds of seconds can be tracked. With beam sizes of order tens to hundreds of micrometres, it is possible to monitor multiple interrogation points in a sample with a lateral extent of a few centimetres, thus resolving potential lateral spatial effects during dehydration and enlarging sample statistics significantly. Therefore, the SAXS/WAXS method offers the opportunity to acquire data on a striking range of length scales: for rock samples with thicknesses of ≤ 10-3 m and widths of 10-2 m, a lateral interrogation-point spacing of ≥ 10-5 m can be achieved. Within each irradiated interrogation-point volume, information concerning pores with sizes between 10-9 and 10-7 m and the crystal lattice on the scale of 10-10 m is acquired in real time. This article presents a summary of the physical principles underpinning transmission X-ray scattering with the aim of providing a guide for the design and interpretation of time-resolved SAXS/WAXS experiments. It is elucidated (1) when and how SAXS data can be used to analyse total porosity, internal surface area, and pore-size distributions in rocks on length scales from ∼1 to 300 nm; (2) how WAXS can be employed to track lattice transformations in situ; and (3) which limitations and complicating factors should be considered during experimental design, data analysis, and interpretation. To illustrate the key capabilities of the SAXS/WAXS method, we present a series of dehydration experiments on a well-studied natural gypsum rock: Volterra alabaster. Our results demonstrate that SAXS/WAXS is excellently suited for the in situ tracking of dehydration kinetics and the associated evolution of nano-pores. The phase transformation from gypsum to bassanite is correlated directly with nano-void growth on length scales between 1 and 11 nm for the first time. A comparison of the SAXS/WAXS kinetic results with literature data emphasises the need for future dehydration experiments on rock specimens because of the impact of rock fabric and the generally heterogeneous and transient nature of dehydration reactions in nature. It is anticipated that the SAXS/WAXS method combined with in situ loading cells will constitute an invaluable tool in the ongoing quest for understanding dehydration and other mineral replacement reactions in rocks quantitatively.

Influence of particle morphology simplification on the simulation of granular material behavior

Morphology is an important grain scale property that substantially influences the macroscopic behavior of granular materials. Particle-based methods such as discrete element method are suitable to study the morphology effect because the particle geometry can be systematically controlled. However, coupling realistic particle morphology with simulation involves significant computational costs. Therefore, considerably simplified particle morphology has been typically adopted in the particle-based simulations to keep the computational costs manageable. This paper aims to systematically address how modeling error results from the morphology simplification and investigates how the modeled fidelity of particle morphology influences the simulated granular material behavior. To this end, a spectrum of particle morphology is developed by gradually simplifying a polyhedral particle model that represents a realistic particle morphology. The modeled fidelity is then quantified in term of Shape Factor and Angularity Factor to characterize the global form and the local angularity of the particle morphology. A set of 3D printed particles is developed from the numerical particle models and used in laboratory testing for systematic parametric study. A series of triaxial compression tests is performed on the synthetic specimens with the 3D printed particles. The result indicates the relative importance of high-fidelity modeling of (1) local angularity under low-confining pressure and (2) global form under high-confining pressure. This study also demonstrates the potential of leveraging 3D printed synthetic particles to study the morphology-dependent mechanical behavior of granular materials.

First experimental measurements of a new fast ion driven micro-burst instability in a field-reversed configuration plasma

In modern field-reversed configuration (FRC) experiments (Binderbauer et al 2015 Phys. Plasmas 22 056110) at TAE Technologies, classical FRC instabilities are suppressed by advanced neutral beam injection and edge biasing methods, leading to high plasma confinement and fast ion pressure built-up which is comparable to the bulk plasma pressure. In some of these high performance FRC plasmas, a new macroscopically non-destructive fast ion driven micro-burst instability is observed as periodic small amplitude bursts with frequency down chirping in the diamagnetic drift frequency range, repeating about every 0.1 to 0.5 ms. The occurrence of these micro-bursts and burst-free operation can be controlled by changing the injected neutral beam energy. Major observed characteristics of this new instability are presented. Possible explanation of the phenomenon is suggested.

Hydro-mechanical behavior of a lateritic fiber-soil composite as a waste containment liner

In waste disposal landfill projects, the hydraulic conductivity of the barriers is a major consideration. The use of fibers mixed with backfill may improve the overall performance of the barriers. Fiber-soil composites show a more resistant and ductile behavior than the soil alone. The presence of fibers may reduce cracking problems related to shrinkage or traction in liners or covers. In this study, laboratory tests were performed to evaluate the use of fiber-soil composites as a containing barrier. Hydraulic conductivity and diametral-compression tests were carried out on PET fiber reinforced and unreinforced compacted soil specimens. The tests were conducted under confinement conditions similar to those found in the field. Diametral-compression tests were used to induce cracks in the specimens. Hydraulic conductivity was measured at different stages during the diametral loading. In the tests performed under low confinement pressure (10 kPa), the crack openings led to a significant increase in hydraulic conductivity. The results showed that the addition of fibers increases the tensile strength of the soil-fiber mass and delays the opening of cracks. Moreover, in the tests under high confinement pressure (100 kPa), a decrease in hydraulic conductivity occurred at all stages of the diametral load application.

A Study of Coupled Creep Damaged Constitutive Model of Artificial Frozen Soil

Artificial frozen soil is a kind of typical creep material, and the frozen clay under the unloading stress paths of high‐confining pressure conforms to the improved the Zienkiewicz–Pande parabola‐type yield criterion, and the Mohr–Coulomb yield function can describe the shear yield surface of artificial frozen clay under low‐confining pressure. Based on the results of triaxial creep and shear tests for artificial frozen soil, the viscoplastic damage variable and evolution rule of artificial frozen clay were obtained by using the theory of viscoelastic‐plastic mechanics and damage mechanics. An improved Zienkiewicz–Pande parabola‐type yield criterion was used instead of a linear Newton body to obtain a coupled constitutive model of viscoelastic‐plastic damage in the frozen soil under the unloading stress paths and to derive the coupling flexibility matrix for viscoelastic and viscoplastic damage. A finite element program of artificial frozen soil considering creep damage was written in the Visual Fortran 6.6A environment and embedded into the nonlinear finite element software ADINA as a user subroutine. The results of numerical simulation and laboratory testing were identical, with a maximum error of no more than 4.8%. This work shows that it is reasonable to describe the creep constitutive model of frozen soil with the viscoelastic‐plastic‐coupled constitutive model.

Characterization of transport and water retention properties of damaged Callovo-Oxfordian claystone

Abstract In the context of the underground storage of radioactive waste, the aim of this experimental study is to characterize the effect of damage on transport and water retention properties of Callovo-Oxfordian (COx) argillite. The originality of the study is to simultaneously investigate the pore-size distribution, water retention, the dry, effective and relative gas permeability, and the gas breakthrough pressure (GBP) of damaged COx argillite. These different properties are all relevant to characterizing the fluid transport ability of COx argillite. Results show that the damage has a significant impact on the properties of the COx argillite. It induces a decrease in its water retention capacity and GBP, and it increases its gas permeability and apparent porosity available to water owing to the creation of micro-cracks. Another objective is to show which of these properties is the most suitable to detect early damage states in COx argillite, with a potential use being to identify them in situ . GBP appears to be the best ‘detector’ of damage because of its sensitivity to damage even under high confinement pressures. Gas permeability could be a good indicator of damage, as it increases significantly (one or several orders of magnitude) after the damage. Finally, the water permeability curve is a poor indicator of COx argillite damage.

Effect of confining pressure on diffusion coefficients in clay-rich, low-permeability sedimentary rocks

The effect of confining pressure (CP) on the diffusion of tritiated-water (HTO) and iodide (I−) tracers through Ordovician rocks from the Michigan Basin, southwestern Ontario, Canada, and Opalinus Clay from Schlattingen, Switzerland was investigated in laboratory experiments. Four samples representing different formations and lithologies in the Michigan Basin were studied: Queenston Formation shale, Georgian Bay Formation shale, Cobourg Formation limestone and Cobourg Formation argillaceous limestone. Estimated in situ vertical stresses at the depths from which the samples were retrieved range from 12.0 to 17.4MPa (Michigan Basin) and from 21 to 23MPa (Opalinus Clay). Effective diffusion coefficients (De) were determined in through-diffusion experiments. With HTO tracer, applying CP resulted in decreases in De of 12.5% for the Queenston Formation shale (CPmax=12MPa), 30% for the Georgian Bay Formation shale (15MPa), 34% for the Cobourg Formation limestone (17.4MPa), 31% for the Cobourg Formation argillaceous limestone (17.4MPa) and 43–46% for the Opalinus Clay (15MPa). Decreases in De were larger for the I− tracer: 13.8% for the Queenston shale, 42% for the Georgian Bay shale, 50% for the Cobourg Formation limestone, 55% for the Cobourg Formation argillaceous limestone and 63–68% for the Opalinus Clay. The tracer-specific nature of the response is attributed to an increasing influence of anion exclusion as the pore size decreases at higher CP.Results from the shales (including Opalinus Clay) indicate that the pressure effect on De can be represented by a linear relationship between De and ln(CP), which provides valuable predictive capability. The nonlinearity results in a relatively small change in De at high CP, suggesting that it is not necessary to apply the exact in situ pressure conditions in order to obtain a good estimate of the in situ diffusion coefficient. Most importantly, the CP effect on shale is reversible (±12%) suggesting that, for argillaceous rocks, it is possible to obtain De values that are representative of the in-situ condition by conducting measurements on re-pressurized samples that were obtained with standard drilling practices. This may not be the case for brittle rock samples as the results from limestone suggest that irreversible damage occurred during the pressure cycling.