Natural Rock Samples Research Articles

Measurement on Diffusion Coefficients and Isotope Fractionation Factors by a Through-Diffusion Experiment

For radioactive waste disposal, it is important that local groundwater flow is slow as groundwater flow is the main transport medium for radioactive nuclides in geological formations. When the groundwater flow is very slow, diffusion is the dominant transport mechanism (diffusion-dominant domain). Key pieces of evidence indicating a diffusion-dominant domain are the separation of components and the fractionation of isotopes by diffusion. To prove this, it is necessary to investigate the different diffusion coefficients for each component and the related stable isotope fractionation factors. Thus, in this study, through-diffusion and effective-porosity experiments were conducted on selected artificial materials and natural rocks. We also undertook measurements relating to the isotope fractionation factors of Cl and Br isotopes for natural samples. For natural rock samples, the diffusion coefficients of water isotopes (HDO and H218O) were three to four times higher than those of monovalent anions (Cl−, Br- and NO3−), and the isotope fractionation factor of 37Cl (1.0017–1.0021) was slightly higher than that of free water. It was experimentally confirmed that the isotope fractionation factor of 81Br was approximately 1.0007–1.0010, which is equivalent to that of free water. The enrichment factor of 81Br was almost half that of 37Cl. The effective porosity ratios of HDO and Cl were slightly different, but the difference was not significant compared to the ratio of their diffusion coefficients. As a result, component separation was dominated by diffusion. For artificial samples, the diffusion coefficients and effective porosities of HDO and Cl were almost the same; it was thus difficult to assess the component separation by diffusion. However, isotope fractionation of Cl and Br was confirmed using a through-diffusion experiment. The results show that HDO and Cl separation and isotope fractionation of Cl and Br can be expected in diffusion-dominant domains in geological formations.

The relationships among different abrasion tests on deteriorated and undeteriorated rocks

In ancient times, the most widely used rocks, especially in construction of roads and monuments, were sedimentary and magmatic rocks. Natural stones used in the buildings and historical monuments are subjected to atmospheric effects such as freeze-thaw, salt crystalisation and wetting and drying cycles which accelerates abrasion. Therefore, wear resistance of natural stone materials is important for selection of suitable materials in engineering projects. In order to investigate the effect of deterioration on abrasion properties of natural rock samples, three different abrasion tests (Bohme abrasion (BA), wide wheel abrasion (WWA) and aggregate impact value (AIV)) were applied to undeteriorated and artificially deteriorated (samples were subjected to accelerated weathering tests (AWT)) natural rock samples in this study. For the first time, the relationship between AIV-WWA and BA-AIV tests was experimentally investigated. Consequently, the relationships among BA-WWA, BA-AIV and AIV-WWA values were examined, the empirical formulas showing these relationships were developed and high correlations (R2 > 0.80) were obtained. Finally, an abrasion class was proposed using classification of WWA for determining BA and AIV of natural rock samples in this study.

Spectral X‐ray computed micro tomography: 3‐dimensional chemical imaging

AbstractWe present a new approach to 3‐dimensional chemical imaging based on X‐ray computed micro tomography (CT), which enables the analysis of the internal elemental chemistry. The method uses a conventional laboratory‐based CT scanner equipped with a semiconductor detector (CdTe). Based on the X‐ray absorption spectra, elements in a sample can be distinguished by their specific K‐edge energy. The capabilities and performance of this new approach are illustrated with different experiments, i.e. single pure element particle measurements, element differentiation in mixtures, and mineral differentiation in a natural rock sample. The results show that the method can distinguish elements with K‐edges in the range of 20 to 160 keV, this corresponds to an element range from Ag to U. Furthermore, the spectral information allows a distinction between materials, which show little variation in contrast in the reconstructed CT image.

Case Study: Effect of Natural Fracture on Stimulation Strategy in Keshen Tight Gas

Aimed at problems of poor fracturing effects of fracture-network acid fracturing in three wells in the Keshen 5 block, a fractured and tight gas reservoir in Tarim Oilfield, the mechanical activity of natural fractures was studied. The fractures in this block were divided into three types: fully-filled, semi-filled, and unfilled. The analysis of the correlations showed that the correlation of the number of fully-filled fractures and absolute open flows (AOF) is poor, while the correlation between the number of unfilled and semi-filled fractures and AOF is the best. Core samples with different types of fractures were tested using conventional triaxial compression methods, and the experimental results showed that the friction coefficient of the fully-filled fracture is close to that of semi-filled fracture, while the cohesion of fully-filled natural fractures is close to that of intact rocks, which is much greater than that of semi-filled fractures. The effective stress test results shows that the effective stress coefficient of fully-filled natural fracture rock samples is lower than that of semi-filled fractures. Using Mohr–Coulomb theory to calculate the natural fracture mechanics of high-production wells and low-production wells, the results show that low-production wells are mainly fully-filled fractures, most of which are not activated, so the production is low; while high-production wells are semi-filled and unfilled fractures, most of which can be activated, so the production is high. Based on this, for a well dominated by fully-filled fractures, it is recommended to use conventional gel fracturing to replace the fracture-network acid fracturing process. The stimulation practice of two wells has proven the validity of this strategy. The filling degree of natural fractures affects the increasing production and choice of stimulation process. This may be a relatively new understanding and may provide insights to stimulation strategy decisions for other fractured tight reservoirs in the world.

Planetary Terrestrial Analogues Library project: 2. building a laboratory facility for MicrOmega characterization

Multiple spectroscopic techniques have been selected on previous, present and forthcoming missions to explore planetary surfaces in the Solar System. In particular, forthcoming ESA/Roscosmos and NASA missions to the surface of Mars will bring instruments capable of near-infrared (NIR), Raman and Laser Induced Breakdown Spectroscopies to analyze the mineralogy and chemistry of rocks. The PTAL (Planetary Terrestrial Analogues Library) project aims at building a multi-instrument spectral database of a large variety of natural Earth rock samples, including Mars analogues. The NIR hyperspectral microscope MicrOmega was selected to characterize the mineralogy of these analogues within the PTAL project. The instrument model used for the PTAL project is a spare flight model that requires specific care. For the safety of the instrument, and because of the large number of samples in the PTAL library and the requirement to optimize the observational conditions, a dedicated and semi-automated setup was built for the use of the MicrOmega instrument for this project. This paper presents the requirements specified for this setup, the technical solutions that have been selected, their implementation and the performances of the set-up. Sample preparation and operations during sample observations are explained, and a characterization example is presented to briefly illustrate the capabilities of MicrOmega in this set-up. The complete results from the MicrOmega characterizations of the PTAL rock analogues will be presented in a forthcoming paper (Loizeau et al. in prep).

Effect of heterogeneity on failure of natural rock samples

A carbonate sample extracted from the depth of about 10 kft was subjected to uniaxial loading while the confining stress remained constant. Post-experiment inspection of the sample showed an inclined crack at an angle less than 20° to the horizontal. This subhorizontal crack orientation was contrary to the expected 45° inclination, the plane of the maximum shear stress. Coincidentally, as shown by CT-scan prior to loading, there was a boundary between two layers of different density inside the sample located almost exactly where the crack appeared. This density difference has arguably translated into the contrast in the elastic properties at the boundary. The hypothesis is that because of this elastic heterogeneity, an incipient crack developed at the boundary due to the unavoidable tensile stressing of the sample as it was brought to the benchtop from its original state of high confining stress at depth. Controlled uniaxial compression made the sample slip along this crack, which then developed into a prominent feature. This assumption was corroborated by a numerical experiment showing a strong von Mises stress concentration at the elastic contrast boundary during hydrostatic tensile loading. Another sample, from the same formation, but without strong density heterogeneity, exhibited a classic 45° crack after uniaxial loading. These results provide a novel and important insight into the mechanics, breakage, and strength of natural rock.

Carbon ordering in an aseismic shear zone: Implications for Raman geothermometry and strain tracking

Determining the burial and strain history of sedimentary rocks is important for understanding crustal behaviour. When rocks contain organic carbon, increased temperatures at depth can alter the molecular structure of the carbon. This change, known as carbon ordering, can be detected using Raman spectroscopy. As a result, Raman spectroscopy is increasingly used to estimate burial depths and associated maximum temperatures in carbon-bearing rocks. It is known from experiments and natural samples that other factors can affect Raman-derived maximum temperatures, including frictional heating on fault planes and interaction with hot fluids. For faulted samples, a question remains as to whether it is purely frictional heating that causes carbon ordering or strain, or a combination of the two. In this study, we use a mid-crustal shear zone to show that strain-related carbon ordering occurs in natural rock samples during aseismic shear strain. A traverse across the shear zone, whose relative strain we quantify with respect to the surrounding less deformed rock, shows a marked decrease in Raman D/G peak intensity ratios indicating greater carbon ordering within the shear zone. We interpret this as evidence for carbon ordering as a result of aseismic shear strain, rather than inflated temperatures, due to frictional heating commonly associated with seismic strain rates on faults. Our results have implications for the further development of Raman spectroscopy as a geothermometer (which may yield erroneous results in strained rock samples) and for understanding strain localisation processes in the Earth's crust, and its associated rheological implications.

Dehydration at subduction zones and the geochemistry of slab fluids

Subducting oceanic slabs undergo metamorphic dehydration with the increase of temperature and pressure during subduction. Dehydration is an essential step for element recycling, and slab fluids are critical agents for mediating slab-mantle interaction. Dehydration is mainly controlled by the thermal structure of subduction zones and the stability of hydrous minerals. At fore-arc depths, slab dehydration produces aqueous fluid with dissolved salts such as NaCl. As subduction proceeds deeper, the content of silicate components increases. At sub-arc and post-arc depths, a hydrous silicate melt is likely to form, or a supercritical fluid could arise from complete miscibility between silicates and H2O. The partitioning of elements between slab fluid and the residual solid rock is controlled by the type of fluid, and generally it is the supercritical fluid that is the most capable of mobilizing trace elements, being an effective carrier even for high field strength elements. Understanding the chemistry of slab fluids relies on sophisticated integration of experiments, theoretical computation and investigation of natural rock samples. This contribution focuses on the content and speciation of key volatiles, including carbon, nitrogen and sulfur, in slab fluids as well as important fluid properties such as oxygen fugacity and acidity. The properties of slab fluids show complicated variation under the control of mineral assemblages and T-P conditions. Slab fluids at great depths of subductions have been inferred to be modestly alkaline and not necessarily very oxidizing as often assumed. Further progress in the research of slab dehydration and the chemistry and properties of slab fluids demands urgently the development of innovative experimental and computational technology including in situ analytical methods at high T-P.

Experimental study on dynamic acoustic characteristics of natural gas hydrate sediments at different depths

Acoustic properties are important geophysical parameters, compared to resistivity, pore water and other parameters that need to be obtained through drilling, the acoustic wave velocity of hydrate deposits is much easier to obtain. Therefore, the study of acoustic characteristics of hydrate reservoir is the basis for geophysical exploration and resource evaluation for hydrates. In this study, an experimental apparatus was developed to measure dynamic P- and S-wave velocity of gas hydrate bearing sediment. The effect of sensitivity factors including hydrate saturation, confining pressure, and reservoir solid phase particle size on the acoustic characteristics of hydrate reservoir were explored. The experimental results showed that the longitudinal and transverse wave velocities of natural gas hydrate rock samples correlated positively with hydrate saturation and confining pressure and the particle size of the solid phase exhibited little effect on the vertical and horizontal wave velocity of the gas hydrate reservoir. The results of this study indicated that the compression factor of different rock samples is the main factor affecting the vertical and horizontal wave velocity of the reservoir through mechanical experiments. At the same time, acoustic experiments and computed tomography results revealed the existence of different contact modes between natural gas hydrate and sediment particles under different saturations. Furthermore, the possibility of detecting the microscopic distribution of hydrates in sediments by acoustic waves was verified. The data can be used in well logging to determine hydrate saturation and other properties of hydrate bearing formations.

Heat capacity and thermal diffusivity of heavy oil saturated rock materials at high temperatures

As well-known the rate of heat extraction depends on the thermal conductivity, $$Q = - \, \lambda \left( T \right){\text{grad }}T$$ , and the fluids flow characteristics, while amounts of heat recovery depends on heat capacity of reservoir rocks, $$Q = \left( {1 - \phi } \right)\rho C_{\text{P}} T$$ , where $$\phi$$ is the porosity. In the present work the laser-flash (LFA 457) and differential scanning calorimeter (DSC 204 F1) techniques were employed on a heavy oil saturated natural rock sample for accurate measurements of the thermal diffusivity and heat capacity over a temperature range from (294 to 1024) K and from (306 to 771) K, respectively. The density of the sample at room temperature was 2300 kg m−3 and the porosity was 17.1%. The sample for the present study comes from Russian Oil Field (Eastern Siberia, Russia). The expanded uncertainty of the thermal diffusivity and heat capacity measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 3% and 1%, respectively. At low temperatures (below approximately 373 K), the sharp increases (up to η = 2.3) of the thermal diffusivity anisotropy was observed. Based on the measured thermal diffusivity and heat-capacity data, thermal conductivity of the same oil saturated rock sample was calculated using the thermodynamic relation $$\lambda = a\rho C_{\text{P}}$$ . The effect of temperature and various physical and chemical processes, such as thermal decomposition (chemical reactions) of pore heavy oil occurred in the sample during the heating in distinct temperature ranges were studied. The effect of pore heavy oil decomposition (under thermal stress) on the measured values of heat capacity and other thermophysical properties of rock sample at high temperatures (around 700 K) was experimentally observed. Also, we experimentally found weak temperature maximum of the heat-capacity of the sample under study in the low temperature range (around 380 K).

Experimental alteration of monazite in granitic melt: Variable U–Th–Pb and REE mobility during melt-mediated coupled dissolution-precipitation

Monazite U–Th–Pb geochronology of high-temperature and melt-bearing metamorphic rocks sometimes produces a continuum of concordant ages (‘smear’) without well-defined age populations. In combination with textural and geochemical criteria, these datasets are typically interpreted to infer either variable Pb-loss between two discrete events or protracted metamorphism lasting >50 Myr. We experimentally react monazite with granitic melt (0.7 GPa and 750 °C) to produce a range of complex monazite textures and age patterns similar to those found in natural high-temperature rock samples. Our observations show that within a short duration (≤24 h experiment time), variable REE, Y, Th, U and Pb mobility is achieved via coupled dissolution-precipitation in a melt-bearing system. An ultra-local interface melt is saturated in dissolved components, including the U–Th–Pb isotopic character of the original monazite, such that newly precipitated monazite may copy or partly inherit the age of a precursor grain. This implies that monazite grown via coupled dissolution-precipitation may not faithfully record the age or duration of metamorphism in melt-bearing rocks typical of a variety of geodynamic settings.

The use of operationally-defined sequential Fe extraction methods for mineralogical applications: A cautionary tale from Mössbauer spectroscopy

Reactive iron minerals are crucial components of global nutrient cycles, directly controlling carbon transport and storage in marine sediments. Sequential selective extraction is frequently used for quantitatively characterising, and chemically isolating, individual Fe mineral phases. Reagent-specific mineral solubility is fundamental to the success of any sequential extraction, but is strongly affected by the varying physical and chemical morphology intrinsic to natural mineral samples. Natural sediment, rock, and soil samples often contain a mineral mixture, which further modifies solvent efficacy. 57Fe Mössbauer spectroscopy only probes the hyperfine interactions between next-nearest neighbouring atomic nuclei in the crystal lattice and is less affected by variation in mineral grain size and crystallinity than conventional, X-ray-based methods. In this study, we used Mössbauer spectroscopy in a novel context to cross-calibrate and optimise a popular, but frequently misused, sequential Fe extraction protocol. Our results showed that incomplete and premature removal of the target Fe minerals could occur at nearly every stage of the extraction and, in many cases, the leachate Fe content did not represent the target phase at all. Crystalline, natural siderite and amorphous, synthetic goethite were detected in the Mössbauer spectrum of the ammonium oxalate extraction for magnetite, after which all reactive Fe minerals should have been removed. Consistent with previous studies, and unlike many other clay minerals, nontronite was extracted as part of the highly reactive Fe pool, and in fact our data indicate that this mineral was extracted by the initial Na-acetate extraction that targets ‘carbonate-bound Fe’. Matrix effects appeared to cause variable yield efficiencies: synthetic goethite was successfully removed when present as an individual mineral yet persisted beyond its target extraction when present in an Fe mineral mixture. Although suitable for the quantification of operationally-defined Fe pools, we caution the unverified use of sequential Fe extraction protocols for mineral specific applications. The application of sequential Fe extractions to define the reactive Fe pools as a paleoredox proxy of depositional conditions appears relatively robust. The premature removal of 2-line ferrihydrite observed in this study (due to the use of the more aggressive Na-acetate extraction for crystalline siderite), does not limit the quantitative use of the sequential Fe extraction in ancient sediments, where such ‘easily reducible’ oxides are unlikely to persist. In contrast, attributing the outcomes of operationally-defined Fe pools to specific Fe minerals is precarious and potentially entirely erroneous. Where Fe mineral specificity or separation is required, we recommend post-extraction validation by another secondary technique. Mössbauer spectroscopy offers such a method that can independently verify extraction stages and assess mineral specificity.

Sampling and preparation of c.200 mm diameter cylindrical rock samples for geomechanical experiments

Experimental investigation of rock mechanical properties of real and artificial samples often requires much care and attention to detail during sample preparation. This especially applies to high fidelity state of the art complex experimental apparatuses where sample tolerance is low due to the complexity of the measuring and stress control devices as well as the nature of the experiments to be conducted. Although sometimes mundane, the sample preparation methodology is as equally important as the experimental apparatus itself, and can require several new technological developments. The methodology and technical developments required to prepare realistic heterogeneous, fractured and natural reservoir analogue rock samples for coupled thermo-hydro-mechanical-chemical process experimental investigation is described here. We present the sample recovery and preparation procedures for large (c.200 mm diameter), cylindrical samples of 200 mm ± 5 mm length, with variable composition and mechanical properties e.g. rock strength, existing fractures/fracture networks, macro-porosity, or lithic fragments. Although the technology demonstrated is for a specific application, the procedures developed, equipment and methodology are applicable to multiple sizes of sample requirements.

Temperature effect on thermal-diffusivity and heat-capacity and derived values of thermal-conductivity of reservoir rock materials

A laser flash method (micro-flash apparatus LFA 457) and differential scanning calorimeter (DSC 204 F1) were employed to study of the temperature effect on the thermophysical properties (thermal diffusivity $$a$$, heat capacity $$C_{\text{P}}$$ and thermal conductivity $$\lambda$$) of the natural reservoir rock sample. A relationship between the thermophysical properties behavior and the physical–chemical processes (thermal decomposition of pore heavy oil and volatilization of pore fluids) occurring in the rock’s pore fluids during heating in distinct temperature ranges was established. The measurements of the thermal-diffusivity have been made over the temperature range from 295 to 774 K. The isobaric heat capacities (CP) of the same sample were measured in the temperature range from 308 to 768 K. Uncertainties of the measurements are 3% and 1% for $$a$$ and $$C_{\text{P}}$$, respectively. The significant effect of thermal decomposition on the measured values of heat-capacity of reservoir rock sample at high temperatures (above 680 K) was experimentally found. We experimentally observed temperature anomaly of the heat capacity of rock sample in distinct temperature ranges, around 380 K (low-temperature range) and 680 K (high-temperature range).We attribute these anomalies to the dehydration (evolution of the volatile matter, VM, devolatilization) and aromatization of the carbon (thermal decomposition), which are known to occur under heat treatment. This leads to unusual increasing the heat capacity at high temperatures. Measured values of thermal diffusivity ($$a$$) and heat capacity ($$C_{\text{P}}$$) together with density data ($$\rho = { 2210}\;{\text{kg}}\;{\text{m}}^{ - 3}$$) were used to calculate the derived key properties, thermal conductivities ($$\lambda$$) of the rock sample, using very well-known relation, $$\lambda = a\rho C_{\text{P}}$$.

Multistep Parametric Inversion of Scanning Magnetic Microscopy Data for Modeling Magnetization of Multidomain Magnetite

AbstractDevelopment in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source material, from the mineral to the crustal anomalies scale. High‐resolution magnetic mapping techniques allow for accurate investigation of the magnetization in natural rock samples and particularly of their remanence carriers, which can record geologically meaningful information. Multidomain magnetic grains are expected to retain a remanence that is susceptible to change by exposure to magnetic fields or by changes in temperature. Although this makes multidomain grains less reliable remanence carriers for paleomagnetic studies, their magnetization contributes to rocks bulk magnetization and to induced anomalies in the Earth crust. Here, we investigate the fine‐scale magnetization of a sample that exhibits a multidomain behavior. We used a scanning magnetic microscope, equipped with a room temperature magnetic tunnel junction sensor, to map the magnetic field over a petrographic thin section of the sample containing large magnetite grains (>100 μm) surrounded by serpentine and carbonate. We modeled the fine‐scale remanent magnetization of the magnetite by inverting the magnetic scans data acquired in near‐field‐free conditions. We applied a multistep inversion, a priori tested on a synthetic model, with a controlled range on the intensity of the magnetization. Modeling results on the study sample suggest homogenously magnetized regions within the magnetite grains with variable remanent magnetization intensities and directions coherent with the multidomain behavior inferred from bulk measurements.

Experimental Evaluation of Remanence Carriers Using the Microcoercivity‐Unblocking Temperature Diagram

AbstractKnowledge of remanence carriers in recording media is crucial for obtaining reliable paleomagnetic data. In this paper, we report a newly developed technique to determine remanence carriers in natural rock samples that uses anhysteretic remanent magnetization (ARM) spectra in the microcoercivity (hc) and unblocking temperature (TUB) planes. The distribution of ARM carriers was determined via systematic stepwise demagnetization experiments of ARM combining thermal demagnetization and alternating field demagnetization treatments and calculating the change in ARM intensity on the hc‐TUB plane (hc‐TUB diagram). Samples from three basaltic lavas and two welded tuffs were analyzed. Combining the hc‐TUB diagram with the change in ARM coercivity spectra, both decreases and increases in magnetic minerals due to the thermal alteration during laboratory heating could be identified. From experimental samples, ARM carrier components were identified as being original rather than produced as a result of thermal alteration. In the case of the Izu‐Oshima 1986 A lava flow (1986La) sample, it was confirmed from the hc‐TUB diagram that high‐hc components can be separated using stepwise alternating field demagnetization treatment but not stepwise thermal demagnetization treatment. Paleointensity experiments were also conducted on Izu‐Oshima 1986 A lava flow samples. Two of the three paleointensity estimates were greater than the expected field intensity, suggesting a certain rock‐magnetic property that affects the paleointensity estimation. The hc‐TUB diagrams of the Izu‐Oshima 1986 A lava flow samples suggest that the differences in the paleointensity estimates were likely caused by different degrees of the high‐temperature oxidation state.

Implementation of overlap degree in sphere packed sedimentary rock model and its impact to pore structure characteristics

Computerized rock modelling is studied and developed because natural rock samples can sometimes be difficult and expensive to obtain. Sedimentary rock modelling is a process of generating a digital sedimentary rock model using numerical methods which resembles the ideas and processes of natural sedimentary rock formation. One of the most important physical characteristics in generating the model is how the pore structure is affected by overlap degree among each grain. The overlap degree simulates the output of compaction process in natural sedimentary rock formation. The approach to implement such idea is by using spherical grains so that the degree of overlap could exactly be determined quantitatively. Stochastic method is used in the algorithm to map each grain positions based on a desired range of overlap degree. From previous trials it shows that higher overlap degree produces lower porosity. The algorithm is implemented in generating two dimensional cross sections of the model as well as in generating three-dimensional structure. Characteristics of pore structure are analyzed for various modelling parameters to obtain better understanding of the modelling nature.

Ethylene glycol monoethyl ether adsorption by interlayer montmorilonite-organic matter complexes: Dependence on the organic matter content and its alkyl chain length

The ethylene glycol monoethyl ether (EGME) adsorption method is an essential and available technique for measuring the total specific surface area (TSSA) of natural rock samples that usually contain an abundant amount of clay-organic matter (clay-OM) complexes. Occurrence sites and types of OM are believed to influence the EGME adsorption behaviors of clay-OM complexes. However, the influence of OM properties on the EGME adsorption mechanism remains unclear until now. In this study, interlayer montmorillonite (Mt)-OM complexes were prepared using the intercalation of OM with various contents and different length of alkyl chains, such as tetramethylammonium bromide (TMAB) and dodecyl trimethyl ammonium bromide (DTAB), based on the cation exchange. EGME adsorption experiments were performed on Mt, OM, and Mt-OM complexes. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the structural properties of the samples before and after EGME adsorption. The EGME adsorption capacities and the corresponding TSSA decreased after OM intercalation into the interlayer space of Mt. The OM content and the amounts of alkyl chains significantly affected the EGME adsorption behaviors of Mt-OM complexes. For short alkyl chain OM (TMAB), TSSA values of the Mt-OM complexes decreased with the increase of the interlayer OM content. However, for long alkyl chain OM (DTAB), the TSSA values showed a different variation tendency where the smallest TSSA did not appear in the Mt-OM complex with the highest DTAB content. The initial interlayer distance of Mt-OM complexes and the arrangement of interlayer OM are the primarily factors for EGME adsorption behaviors of the Mt-OM complexes. The large initial interlayer distance favored for incremental expansion of Mt. after EGME adsorption, which promoted EGME molecules entering into the interlayer space. The influence of the arrangement of interlayer OM are primarily involved in some newly formed micropores preventing EGME molecules from entering and more adsorption sites were occupied by OM with a higher ordered mode for the conformation of the alkyl chain and a higher packing density for the arrangement.

Comparison of stress‐dependent geophysical, hydraulic and mechanical properties of synthetic and natural sandstones for reservoir characterization and monitoring studies

ABSTRACTSynthetic rock samples can offer advantages over natural rock samples when used for laboratory rock physical properties studies, provided their success as natural analogues is well understood. The ability of synthetic rocks to mimic the natural stress dependency of elastic wave, electrical and fluid transport properties is of primary interest. Hence, we compare a consistent set of laboratory multi‐physics measurements obtained on four quartz sandstone samples (porosity range 20–25%) comprising two synthetic and two natural (Berea and Corvio) samples, the latter used extensively as standards in rock physics research. We measured simultaneously ultrasonic (P‐ and S‐wave) velocity and attenuation, electrical resistivity, permeability and axial and radial strains over a wide range of differential pressure (confining stress 15–50 MPa; pore pressure 5–10 MPa) on the four brine saturated samples. Despite some obvious physical discrepancies caused by the synthetic manufacturing process, such as silica cementation and anisotropy, the results show only small differences in stress dependency between the synthetic and natural sandstones for all measured parameters. Stress dependency analysis of the dry samples using an isotropic effective medium model of spheroidal pores and penny‐shaped cracks, together with a granular cohesion model, provide evidence of crack closure mechanisms in the natural sandstones, seen to a much lesser extent in the synthetic sandstones. The smaller grain size, greater cement content, and cementation under oedometric conditions particularly affect the fluid transport properties of the synthetic sandstones, resulting in lower permeability and higher electrical resistivity for a similar porosity. The effective stress coefficients, determined for each parameter, are in agreement with data reported in the literature. Our results for the particular synthetic materials that were tested suggest that synthetic sandstones can serve as good proxies for natural sandstones for studies of elastic and mechanical properties, but should be used with care for transport properties studies.

Contributions of 3D Printed Fracture Networks to Development of Flow and Transport Models

Conventional experiments using natural rock samples have trouble in observing rock structures and controlling fracture properties. Taking advantage of 3D printing technologies, a complex fracture network was made by using a 3D printer. This approach allowed us to control the properties of the fracture networks and to prepare identical geometries for both simulation and experiment. A tracer response curve from the flow experiment was obtained and compared with numerical simulations. The result of the computational fluid dynamics (CFD) simulation based on the Navier–Stokes equations was in good agreement with experimental result, which suggested that the results of experiment and the CFD simulation are reliable. On the other hand, comparison with an equivalent permeability model based on the cubic law showed a discrepancy from the experimental result. This validation approach enabled discussion of the limitation of the flow model. Because 3D printed fracture networks could reduce uncertainty between numerical simulation and laboratory experiment, they will be useful for understanding more detailed and more complicated phenomena in fracture networks.