Abstract
Hidden, blind faults have a strong seismic hazard potential. Consequently, there is a great demand for a robust geological indicator of neotectonic activity on such faults. Here, we conduct ...field measurements of disaggregation bands above known underlying blind faults at several locations in Central Europe. We observe that the disaggregation bands have the same orientation as that of the faults, indicating their close connection. Disaggregation bands develop in unconsolidated, near-surface, sandy sediments. They form by shear-related reorganization of the sediment fabric, as a consequence of grain rolling and sliding processes, which can reduce the porosity. Using an analogue shearing experiment, we show that disaggregation bands can form at a velocity of 2 cm h
−1
, which is several orders of magnitude slower than seismogenic fault-slip velocities. Based on the field data and the experiments, we infer that disaggregation bands can form in the process zone of active blind faults and serve as an indicator of neotectonic activity, even if the fault creeps at very low slip velocity. Disaggregation bands could open a new path to detect hidden active faults undergoing aseismic movements.
•Investigated Pannonian Basin's Dunántúl Group, noting high porosity and permeability.•Identified clogging risks from fine materials and weak cementation in geothermal wells.•Enhanced geothermal ...injection process understanding through advanced imaging.
This study focuses on the Pannonian Basin, specifically in Szentes, Hungary a region of significant geothermal potential, with particular emphasis on the Dunántúl Group; a collective name for the Zagyva and Újfalu formations, which consists of slightly consolidated delta-front sandstone sediments. This research is pivotal in understanding the challenges associated with clogging in geothermal wells, a problem that has led to the premature shutdown of injection wells in the region. Our approach integrates classical petrophysical and mineralogical methods with advanced techniques such as micro-Computed X-Ray Tomography imaging, 3D image analysis, and digital rock simulations. Our findings indicate that the target geothermal rock formations within the Dunántúl Group exhibit high porosity (27–31 %) and variable permeability (60–400 mD), dependent on the location and specific characteristics of the formation. Our micro-CT analyses further identified that the presence of fine-grained materials in smaller pores and generally weak cementation of grains substantially contributes to these challenges.
SUMMARY Clay minerals are extensively used in a wide range of applications. In particular, clay-bearing formations are considered as suitable radioactive waste repository. Electrical resistivity ...tomography is an appropriate tool to monitor the properties of clay-bearing locations. However, an inherent drawback of a conventional resistivity survey is its ambiguity in distinguishing between the effects of groundwater salinity, clay content and porosity. A discrimination can be achieved on the basis of the induced polarization method that provides a complex conductivity. The main purpose of this study is the investigation of the complex conductivity of clay samples with a special focus on the contribution of surface conductivity produced by an excess of ions in the electrical double-layer coating the solid particles. Six clay mixtures were selected that include an almost pure kaolinite sample, a sample consisting of a mixture of kaolinite, illite and smectite, a crushed saponite breccia, a Ca-bentonite sample and two illite clay samples. Besides the enriched kaolinite, the other samples are natural geomaterials that contain more than 40 weight per cent clay minerals. The mineralogical compositions of the samples were determined by quantitative X-ray diffraction analysis. The clay powder was mixed with a varying volume of sodium chloride solution to get plastic state clay samples with varying water content. The samples were investigated by the spectral induced polarization method in a frequency range between 1 mHz and 1 kHz. The resulting complex conductivity spectra indicate a decrease of the real part of the electrical conductivity with rising water content for the illite, bentonite and saponite breccia samples. The overall conductivity of these clay samples is dominated by their surface conductivity. In contrast, the electrical conductivity of kaolinite and kaolinite–illite mixture does not show any significant changes with the water content. For all samples, the imaginary part of electrical conductivity increases at low water content. The real part of the surface conductivity indicates a linear dependence on the volumetric clay content. The slope of this linear relationship can be used to distinguish the types of clay. The ratio between imaginary conductivity and surface conductivity, which decreases with increasing clay content, proves to be a suitable parameter that characterizes the connectivity of clay aggregates in the sample. The surface conductivity of the pure kaolinite sample has been determined in an additional multisalinity experiment. The resulting surface conductivity is in good agreement with the experiment of varying water content. The multisalinity experiment has shown that the resulting petrophysical parameters depend on the procedure of sample packing, which may lead to anisotropy. The effect of anisotropy is attributed to the alignment of the plate-like kaolinite particles in the course of the packing and consolidation procedure.
In the context of global efforts to transition toward renewable energy and reduce greenhouse gas emissions, geothermal energy is increasingly recognized as a viable and sustainable option. This paper ...presents a comprehensive assessment derived from a subset of a larger sample collection within the Dunántúli Group of the Pannonian Basin, Hungary, focusing on optimizing micro-computed tomography (µ-CT) resolution for analyzing pore structures in sandstone formations. By categorizing samples based on geological properties and selecting representatives from each group, the study integrates helium porosity and gas permeability measurements with µ-CT imaging at various resolutions (5 µm, 2 µm, and 1 µm). The findings reveal that µ-CT resolution significantly affects the discernibility and characterization of pore structures. Finer resolutions (2 µm and 1 µm) effectively uncovered interconnected pore networks in medium- to coarse-grained sandstones, suggesting favorable properties for geothermal applications. In contrast, fine-grained samples showed limitations in geothermal applicability at higher resolutions due to their compact nature and minimal pore connectivity, which could not be confidently imaged at 1 µm. Additionally, this study acknowledges the challenges in delineating the boundaries within the Dunántúli Group formations, which adds a layer of complexity to the characterization process. The research highlights the importance of aligning µ-CT findings with geological backgrounds and laboratory measurements for accurate pore structure interpretation in heterogeneous formations. By contributing vital petrophysical data for the Dunántúli Group and the Pannonian Basin, this study provides key insights for selecting appropriate µ-CT imaging resolutions to advance sustainable geothermal energy strategies in the region. The outcomes of this research form the basis for future studies aimed at developing experimental setups to investigate physical clogging and enhance geothermal exploitation methods, crucial for the sustainable development of geothermal resources in the Pannonian Basin.
Recent years have seen a growing interest in the characterization of the pore morphologies of reservoir rocks and how the spatial organization of pore traits affects the macro behavior of rock-fluid ...systems. With the availability of 3-D high-resolution imaging, such as x-ray micro-computed tomography (mu-CT), the detailed quantification of particle shapes has been facilitated by progress in computer science. Here, we show how the shapes of irregular rock particles (pores) can be classified and quantified based on binary 3-D images. The methodology requires the measurement of basic 3-D particle descriptors (length, width, and thickness) and a shape classification that involves the similarity of artificial objects, which is based on main pore network detachments and 3-D sample sizes. Two main pore components were identified from the analyzed volumes: pore networks and residual pore ganglia. A watershed algorithm was applied to preserve the pore morphology after separating the main pore networks, which is essential for the pore shape characterization. The results were validated for three sandstones (S sub(1), S sub(2), and S sub(3)) from distinct reservoirs, and most of the pore shapes were found to be plate- and cube-like, ranging from 39.49 to 50.94% and from 58.80 to 45.18% when the Feret caliper descriptor was investigated in a 1000 super(3) voxel volume. Furthermore, this study generalizes a practical way to correlate specific particle shapes, such as rods, blades, cuboids, plates, and cubes to characterize asymmetric particles of any material type with 3-D image analysis.
We investigate the pore space of rock samples with respect to different petrophysical parameters using various methods, which provide data on pore size distributions, including micro computed ...tomography (μ-CT), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and spectral-induced polarization (SIP). The resulting cumulative distributions of pore volume as a function of pore size are compared. Considering that the methods differ with regard to their limits of resolution, a multiple-length-scale characterization of the pore space is proposed, that is based on a combination of the results from all of these methods. The approach is demonstrated using samples of Bentheimer and Röttbacher sandstone. Additionally, we compare the potential of SIP to provide a pore size distribution with other commonly used methods (MIP, NMR). The limits of resolution of SIP depend on the usable frequency range (between 0.002 and 100 Hz). The methods with similar resolution show a similar behavior of the cumulative pore volume distribution in the overlapping pore size range. We assume that μ-CT and NMR provide the pore body size while MIP and SIP characterize the pore throat size. Our study shows that a good agreement between the pore radius distributions can only be achieved if the curves are adjusted considering the resolution and pore volume in the relevant range of pore radii. The MIP curve with the widest range in resolution should be used as reference.
In the past years X-ray computed tomography (CT) has became more and more common for geoscientific applications and is used from the µm-scale (e.g. for investigations of microfossils or pore-scale ...structures) up to the dm-scale (full drill cores or soil columns). In this paper we present results from CT imaging and mineralogical investigations of an Opalinus Clay core on different scales and different regions of interest, emphasizing especially the 3-D evaluation and distribution of cracks and their impact on mechanical testing of such material. Enhanced knowledge of the testing behaviour of the Opalinus Clay is of great interest, especially since this material is considered for a long-term radioactive waste disposal and storage facility in Switzerland. Hence, results are compared regarding the mineral (i.e. phase) contrast resolution, the spatial resolution, and the overall scanning speed.With this extensive interdisciplinary scale-down approach it has been possible to characterize the general fracture propagation in comparison to mineralogical and textural features of the Opalinus Clay. Additionally, and as far as we know, a so-called mylonitic zone, located at an intersect of two main fractures, has been observed for the first time for an experimentally deformed Opalinus sample. The multi-scale results are in good accordance to data from naturally deformed Opalinus Clay samples, which enables us to perform systematical research under controlled laboratory conditions. Accompanying 3-D imaging greatly enhances the capability of data interpretation and assessment of such a material.
This paper presents a detailed description and evaluation of a
multi-methodological petrophysical approach for the comprehensive multi-scale
characterization of reservoir sandstones. The suggested ...methodology enables
the identification of links between Darcy-scale permeability and an extensive
set of geometrical, textural and topological rock descriptors quantified at
the pore scale. This approach is applied to the study of samples from three
consecutive sandstone layers of Lower Cretaceous age in northern Israel. These
layers differ in features observed at the outcrop, hand specimen, petrographic
microscope and micro-CT scales. Specifically, laboratory porosity and
permeability measurements of several centimetre-sized samples show low
variability in the quartz arenite (top and bottom) layers but high variability
in the quartz wacke (middle) layer. The magnitudes of this variability are
also confirmed by representative volume sizes and by anisotropy evaluations
conducted on micro-CT-imaged 3-D pore geometries. Two scales of directional
porosity variability are revealed in quartz arenite sandstone of the top
layer: the pore size scale of ∼0.1 mm in all directions and
∼3.5 mm scale related to the occurrence of high- and
low-porosity horizontal bands occluded by Fe oxide cementation. This
millimetre-scale variability controls the laboratory-measured macroscopic rock
permeability. More heterogeneous pore structures were revealed in the quartz
wacke sandstone of the intermediate layer, which shows high inverse
correlation between porosity and clay matrix in the vertical direction
attributed to depositional processes and comprises an internal spatial
irregularity. Quartz arenite sandstone of the bottom layer is homogenous and
isotropic in the investigated domain, revealing porosity variability at a ∼0.1 mm scale, which is associated with the average pore size. Good
agreement between the permeability upscaled from the pore-scale modelling and
the estimates based on laboratory measurements is shown for the quartz arenite
layers. The proposed multi-methodological approach leads to an accurate
petrophysical characterization of reservoir sandstones with broad ranges of
textural, topological and mineralogical characteristics and is particularly
applicable for describing anisotropy and heterogeneity of sandstones on
various rock scales. The results of this study also contribute to the
geological interpretation of the studied stratigraphic units.
The presence of a wellbore skin layer, formed during the drilling process, is a major impediment for the energy-efficient use of water wells. Many models exist that predict its potential impacts on ...well hydraulics, but so far its relevant hydraulic parameters were only estimates or, at best, model results. Here, we present data on the typology, thickness, composition, and hydraulic properties obtained from the sampling of excavated dewatering wells in lignite surface mines and from inclined core drilling into the annulus of an abandoned water well. Despite the limited number of samples, several types of skin were identified. Both surface cake filtration and particle straining in the aquifer occur. The presence of microcracks may be a determining feature for the hydraulic conductivity of skin layers. In the case of the well-developed water supply well, no skin layer was detected. The observed types and properties of wellbore skin samples can be used to test the many mathematical skin models.
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