Natural Sedimentary Rocks Research Articles

Anisotropic continuum framework of coupled gas flow – adsorption – deformation in sedimentary rocks

AbstractSolid deformation is always a crucial factor of gas transport in sedimentary rocks. While previous studies always adopt the assumption of isotropic poroelastic deformation, anisotropic poroelastoplastic deformation is rarely considered, despite anisotropy being a ubiquitous property of natural sedimentary rocks. In this work, an anisotropic poromechanical model is established to analyze the matrix porosity and apparent permeability evolutions during the process of gas migration. Using a thermodynamic formulation that treats the fluid–solid interface as an independent phase, we derive a rate form for matrix porosity and obtain the new dissipation function that contains three parts: dissipations from solid deformation, gas adsorption, and fluid flow. For gas adsorption, we justify the rationality of the adopted model; for fluid flow, the updated porosity can be substituted into sophisticated apparent permeability models for full‐scale analysis; and for solid deformation, a recently developed constitutive model appropriate for rocks exhibiting transverse isotropy in both the elastic and plastic responses is adopted in this work. Through the novel stress‐point simulation incorporating two effective stress measures and adsorption strain, new patterns of apparent permeability are obtained, which fit the experimental data quite well and cannot be reproduced from the assumption of isotropic poroelasticity. The advantages of our poromechanical model include thermodynamic consistency and the ability to employ finite‐element‐based formulation. Finally, an initial‐boundary value problem of gas production considering anisotropic plasticity is conducted, and the effects of the bedding plane and different adsorption models are highlighted.

The Influence of Microporous Cements on the Pore Network Geometry of Natural Sedimentary Rocks

We investigate the pore network geometry and permeability of six natural sandstones and carbonate rocks. Using 3D microtomographic images, we segment each rock sample into three phases: Solid matrix of grains, macropores containing void spaces, and a third microporous phase containing nanometer-sized pores beyond the resolution of the image. In the majority of our samples, the microporosity exists inside cements deposited as a secondary phase along the surface of grains. Within the macropores, the pore radius, coordination number, throat radius, and throat length display a power law relation with porosity. We also find that the permeability of the aggregate depends on the porosity following the relation k = k_0 \phi^{3.32}, where $\log{k_0} = 5.52$ (mD). The fraction of connected porosity shows a strongly nonlinear reduction with an increase in the volume fraction of microporous cement.

3D Printed Sandstone Strength: Curing of Furfuryl Alcohol Resin-Based Sandstones

Natural sedimentary rocks can be widely heterogeneous and often include discontinuities on many scales—no two samples are truly identical. This poses a major roadblock for geomechanical experiments since most of them are destructive in nature. Recent advances in additive manufacturing technology allow fabrication of identical sandstone analogs. The technology allows control over grain size, packing, mineralogy, cementing type and content, bedding orientation, and discontinuities. This article explored how curing temperature affects the strength of sand and furfuryl alcohol resin-based specimens. When cured at optimal oven temperature of 80°C, specimens reach unconfined compressive strength (UCS) of 19.0 MPa with only 1.1 MPa in standard deviation. Additionally, this article determines a minimal number of UCS test repetitions required to reach a desired degree of confidence in the strength results. Outcomes of this study can be used as a guide in preparing and strength testing of furfuryl alcohol resin and sand powder-based 3D printed rocks.

Pore-Size Distributions and Tortuosity in Heterogeneous Porous Media

The diffusion coefficient, measured at long observation times by pulsed-held-gradient NMR, can in principle be used to estimate the tortuosity of a porous medium. This method is useful for glass-sphere packs, but we find that it does not generally work for porous sedimentary rock. Natural sedimentary rocks are characterized by complex microgeometries and broad distributions of pore sizes, which cannot be adequately sampled by diffusing molecules in experimentally accessible observation times. The time-dependent diffusion coefficient D(t) can be distinctly irregular for rocks with very large pores. In heterogeneous porous media, determination of pore-size distribution by relaxation-time measurements and tortuosity by PFG diffusion measurements are mutually exclusive.

Depth Filtration of Clay in Rock Cores Observed by One-Dimensional 1H NMR Imaging

A previous study (M. A. Horsfield, E. J. Fordham, C. Hall, and L. D. Hall, J. Magn. Reson. 81, 593 (1989)) used one-dimensional NMR imaging to measure water/clay volume ratios in a colloidal filter cake; proton spin-lattice relaxation time ( T 1) was used as the indicator of moisture content within the cake. The present Note reports the application of that technique to depth filtration of clay within natural sedimentary rocks. A region of the filter medium exposed to pressurized clay suspension is found to exhibit significant depression in T 1 on a timescale of less than 15 min; little subsequent change is observed over 12 h. The depth of penetration was up to 3 cm in an oolitic limestone of exceptionally high permeability (> 10 Darcy), but less than 1 cm for a sandstone with a permeability more typical of natural sedimentary rocks.

Compressive failure of silica-filled epoxy resins: influence of matrix strength, interfacial bond strength and porosity

Composites containing a high proportion of hard inert filler particles in a cold-curing polymer resin are relatively inexpensive materials, easily cast to shape and with mechanical strength and durability suitable for use as flooring and grouting compounds. They are also used.in the mineral-handling and mining industries as abrasion-resistant materials, to line plant handling or processing rocks, coal or ores, either dry or in slurry form [1]. Materials of this type are also useful to simulate some of the properties of natural sedimentary rocks, such as sandstones and limestones, for experimental purposes. For example, in studying the abrasive wear of steels against natural sandstone and limestone [2] it is not possible to obtain reproducible samples of rock with independently varied values of interparticle bond strength, porosity and binder strength. Silica-filled epoxy resins, however, can be made which behave in many respects like sandstones, with similar contents of silica particles, values of compressive strength and with the potential for controlled variation of their microstructure and properties. Previous work has shown that the addition of hard filler particles to an epoxide resin generally leads to increased elastic modulus and compressive strength, although the tensile strength may be reduced [3]. Compressive strength is a particularly important property for some applications, including the simulation of abrasive natural rocks, but only limited information is available about the effect on the compressive strength of composites with high proportions of particulate filler of the properties of the matrix material itself, the porosity of the composite or the strength of the bonding between the filler particles and the matrix. A systematic investigation was therefore made of the effects of these variables on the uniaxial compressive strengths of composites containing a constant high volume fraction (72%) of silica sand particles in an epoxy resin matrix.

Applicability of the Biot theory. I. Low-porosity materials

The Biot theory of wave propagation in porous, saturated materials contains 13 parameters. We discuss empirical and theoretical ways to predict values of these parameters for natural materials. The self-consistent theory of composites is used to predict the elastic moduli of the skeletal frame assuming that the inclusions (pore fluid, sediment grains) are needle-shaped at low concentrations. The Biot theory is then used to predict compressional and shear wave speeds in consolidated materials. In a man-made material (porous sintered glass), the predictions agree with experimental data to within 3% (experimental uncertainty is also 3%); these samples were distilled-water-saturated. The predictions agree with data to within 1% for Berea sandstone, and to within 5% for Bedford limestone; all of these samples were brine-saturated. The predictions agree with data to within 8% for distilled-water-saturated Bedford limestone. In the natural sedimentary rocks, the experimental uncertainty in measured wave speeds is estimated at 5%, and the comparisons apply at high effective pressures (>0.3 kbar), where the experimentally determined wave speeds approach ‘‘limiting velocities.’’ In distilled-water-saturated Massilon sandstone, predicted shear wave speeds are about 25% to 35% higher than measured speeds; this may indicate a reduction of the frame shear modulus due to interaction of the pore fluid with the minerals of the rock.

Lithification analysis — Experimental observations

Thin sections prepared from laboratory compacted dry sediment mixtures are compared with selected thin sections from natural sedimentary rocks. Compression of a quantity of calcium carbonate bioclasts at a pressure of 1,000 kg/cm2 gives rise to plastic deformation of grain contacts, penetration contacts, and the reduction of porosity and permeability. Compressing a mixture of carbonate fragments and quartz grains at a pressure of 1,000 kg/cm2 causes the quartz grains to penetrate the softer carbonate bioclasts. Compression of a dry, well-rounded quartz sand at a confining pressure of 3,000 kg/cm2 leads to the crushing of the majority of the grains. An artificial sediment consisting of a mixture of quartz spheres and gem fragments demonstrates upon compaction by a pressure of 3,000 kg/cm2 the capacity of the harder gem minerals to penetrate quartz grains by way of plastic deformation of the grain contacts. Differences in hardness between dolomite and calcite/aragonite fragments are responsible for the creation of lithification textures of the dolomite crystals penetrating the softer carbonate grains.

Magnetic Grain Fabric of Sedimentary Rocks: ABSTRACT

It is well known that measurement of magnetic anisotropy indicates the dimensional grain fabric of natural rocks. Although the directional results are easily interpreted, the quantitative descriptive parameters of the magnetic fabric require more care--but they can give valuable information as to the nature of the grain fabric. In direct grain-shape-related magnetic anisotropy, e.g., in magnetite, it is possible to compare magnetic and optical fabric results by using a density function to describe the distribution of the dimensional axes of an aggregate of similar particles. The data are tested with sand deposited at the angle of repose. By using this method to produce a standard depositional End_Page 561------------------------------ mechanism, it is seen that the magnetic fabric resulting from magneto-crystalline anisotropy within particles such as hematite has similar directional properties, but the descriptive parameters differ from the magnetite example. In natural sedimentary rocks the source mineral grains causing the magnetic fabric are more elusive. Nevertheless, simple deposition tests with crushed samples confirm that the directional data are correct. Also the descriptive parameters can be used to distinguish between primary and various deformational fabrics. End_of_Article - Last_Page 562------------

LIESEGANG’S STRATIFICATION DEVELOPED IN THE DIATOMACEOUS GYTTIA FROM LAKE HARUNA, AND PROBLEMS RELATED TO IT

Abstract (1) An example of Liesegang’s stratification of ferric hydroxide is given, which was formed half experimentally in the medium of diatomaceous gyttia from Lake Haruna. (2) An evidence is given which strengthens the conviction that true Liesegang’s stratifications can be found in natural sedimentary rocks. (3) It is pointed out that there are two kinds of ways possible, besides those hitherto established, in which stratifications are developed in lake deposits. (4) The cycle of iron in a lake is dealt, and a reasoning is given, which supports the view that a considerable portion of the deposited iron is redissolved and is carried back to the main water.