Refine
Document Type
- Article (4)
- Doctoral Thesis (1)
Language
- English (5)
Has Fulltext
- yes (5)
Is part of the Bibliography
- no (5)
Keywords
- SEM–EDS (2)
- bentonite (2)
- layer charge (2)
- metal substitution (2)
- organic supplements (2)
- smectite (2)
- - (1)
- Alkali-activated binder (1)
- Benchmark material (1)
- Clay mineral diagenesis (1)
Institute
Publisher
- MDPI (2)
- Springer Nature (2)
There is a current need for developing improved synthetic porous materials for better constraining the dynamic and coupled processes relevant to the geotechnical use of underground reservoirs. In this study, a low temperature preparation method for making synthetic rocks is presented that uses a geopolymer binder cured at 80 °C based on alkali-activated metakaolin. For the synthesised sandstone, the key rock properties permeability, porosity, compressive strength, and mineralogical composition, are determined and compared against two natural reservoir rocks. In addition, the homogeneity of the material is analysed structurally by micro-computed tomography and high-resolution scanning electron microscopy, and chemically by energy dispersive X-ray spectroscopy. It is shown that simple, homogenous sandstone analogues can be prepared that show permeability-porosity values in the range of porous reservoir rocks. The advance in using geopolymer binders to prepare synthetic sandstones containing thermally sensitive minerals provides materials that can be easily adapted to specific experimental needs. The use of such material in flow-through experiments is expected to help bridge the gap between experimental observations and numerical simulations, leading to a more systematic understanding of the physio-chemical behaviour of porous reservoir rocks.
Diagenetic illite growth in porous sandstones leads to significant modifications of the initial pore system which result in tight reservoirs. Understanding and quantifying these changes provides insight into the porosity-permeability history of the reservoir and improves predictions on petrophysical behavior. To characterize the various stages of diagenetic alteration, a focused ion beam – scanning electron microscopy (FIB-SEM) study was undertaken on aeolian sandstones from the Bebertal outcrop of the Parchim Formation (Early Permian Upper Rotliegend group). Based on 3D microscopic reconstructions, three different textural types of illite crystals occur, common to many tight Rotliegend sandstones, namely (1) feldspar grain alterations and associated illite meshworks, (2) tangential grain coats, and (3) pore-filling laths and fibers. Reaction textures, pore structure quantifications, and numerical simulations of fluid transport have revealed that different generations of nano-porosity are connected to the diagenetic alteration of feldspars and the authigenic growth of pore-filling illites. The latter leads to the formation of microstructures that range from authigenic compact tangential grain coatings to highly porous, pore-filling structures. K-feldspar replacement and initial grain coatings of illite are composed primarily of disordered 1Md illite whereas the epitaxially grown illite lath- and fiber-shaped crystals occurring as pore-filling structures are of the trans-vacant 1Mtv polytype. Although all analyzed 3D structures offer connected pathways, the largest reduction in sandstone permeability occurred during the initial formation of the tangential illite coatings that sealed altered feldspars and the subsequent growth of pore-filling laths and fibrous illites. Analyses of both illite pore-size and crystallite-size distributions indicate that crystal growth occurred by a continuous nucleation and growth mechanism probably controlled by the multiple influx of potassium-rich fluids during late Triassic and Jurassic times. The detailed insight into the textural varieties of illite crystal growth and its calculated permeabilities provides important constraints for understanding the complexities of fluid-flow in tight reservoir sandstones.
The deep geological underground represents an important georesource for the short-
term storage of renewable energy and the long-term reduction of greenhouse gas emissions. To ensure the economic viability and safety of any subsurface storage project, detailed characterisation of the quality and integrity of the reservoir and its cap rock is required. This characterisation includes the accurate determination of the petrophysical properties, such as porosity and permeability, as well as the potential mineral reactions, such as the dissolution of reactive phases, which may occur during the lifespan of such a project. Clay minerals are common components of many reservoir systems and, depending on their type and structure, can have a significant impact on storage and transport properties. These processes are, however, currently not well understood. In order to address these issues, the main focus of this thesis is on mineralogical analyses using X-ray diffraction (XRD) and microstructural studies using focused ion beam scanning electron microscopy (FIB-SEM) together with micro X-ray computed tomography (µXCT) to gain a better understanding of the influence of clay minerals on reservoir and cap rock properties.
A central part of this thesis focuses on the analysis of clay minerals and pore structures of the Bebertal Sandstone of the Parchim Formation (Early Permian, Upper Rotliegend), which is considered a natural analogue for the tight reservoir sandstones of the North German Basin. Two illite polytypes with a variety of characteristic structures have been identified in the Bebertal sandstone. Disordered 1Md illite forms the majority of the observed structures, which include omnipresent grain coatings, altered permeable feldspar grains and pore-filling meshwork structures. Trans-vacant 1M illite represents the second and youngest generation of authigenic illite and occurs as fibrous to lath-shaped particles that grew into open pore spaces and led to a significant reduction in porosity and permeability during late diagenesis. Based on these results, a model for the formation of illite polytypes in the aeolian layers of the Bebertal sandstone was developed that describes the temporal and spatial evolution of porosity and permeability during diagenesis. Information from this model was then used to improve the prediction of permeability of the Bebertal sandstone based on µXCT pore space models and direct numerical simulations. To achieve this, a micro-scale pore space model was created that allowed the simulation of permeability reduction by clay minerals by including nanoporous illite domains based on a novel morphological algorithm. By performing Navier-Stokes-Brinkman simulations, more accurate predictions of permeabilities with respect to experimentally determined values were obtained compared to conventional Navier-Stokes simulations.
The detailed characterisation of the Bebertal sandstone has shown that natural reservoir rocks are usually complex heterogeneous systems with small-scale variations in texture,
composition, porosity and permeability. Flow-through experiments on the Bebertal sandstone revealed that the coupled geochemical and hydrodynamic processes that occur during the dissolution of calcite could not be predicted by reactive transport models. Therefore, as part of this thesis, a novel approach for developing synthetic sandstones at low temperatures based on geopolymer binder was developed. It is shown that simpler and more homogeneous porous materials can be produced with porosity and permeability values in the range of natural sandstones. These can be used to better understand the dynamic and coupled processes relevant to the storage of renewable energy in reservoir rocks through improved experimental constraints.
The final part of this thesis reports on a detailed clay mineral and pore space study of
three shale formations and one mudstone that were identified as potential seals for the Mt. Simon sandstone reservoir in the Illinois Basin. During the Illinois Basin - Decatur Project, this reservoir was used for the sequestration of one megaton of supercritical carbon dioxide. In order to better assess the quality of the sealing units and to better understand the role of the intergranular clay mineral matrix as potential pathway for fluid migration, a multi-scale evaluation was conducted that included thin section analysis, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), mercury intrusion capillary pressure (MICP) measurements, quantitative XRD and high-resolution FIB-SEM. The results allow for the classification of the studied formations into primary and secondary seals and emphasise the importance of three-dimensional clay-mineral-related pore structure characterisations in cap rock studies. XRD proved the most reliable method for the identification and quantification of clay minerals in the studied cap rocks and mudstones. In contrast, FIB-SEM and QEMSCAN provided the spacial constraints for reconstructing fluid flow pathways within the clay mineral matrix.
Overall, this thesis highlights the importance of the precise identification of clay minerals in geological reservoirs and their cap rocks. It also illustrates the need for three-dimensional characterisation and modelling of the associated small pore structures for an improved understanding of the rocks diagenetic history as well as the prediction of the transport and storage properties of these crustal reservoir systems.
Bentonite is currently proposed as a potential backfill material for sealing high-level radioactive waste in underground repositories due to its low hydraulic conductivity, self-sealing ability and high adsorption capability. However, saline pore waters, high temperatures and the influence of microbes may cause mineralogical changes and affect the long-term performance of the bentonite barrier system. In this study, long-term static batch experiments were carried out at 25 °C and 90 °C for one and two years using two different industrial bentonites (SD80 from Greece, B36 from Slovakia) and two types of aqueous solutions, which simulated (a) Opalinus clay pore water with a salinity of 19 g·L−1, and (b) diluted cap rock solution with a salinity of 155 g·L−1. The bentonites were prepared with and without organic substrates to study the microbial community and their potential influence on bentonite mineralogy. Smectite alteration was dominated by metal ion substitutions, changes in layer charge and delamination during water–clay interaction. The degree of smectite alteration and changes in the microbial diversity depended largely on the respective bentonite and the experimental conditions. Thus, the low charged SD80 with 17% tetrahedral charge showed nearly no structural change in either of the aqueous solutions, whereas B36 as a medium charged smectite with 56% tetrahedral charge became more beidellitic with increasing temperature when reacted in the diluted cap rock solution. Based on these experiments, the alteration of the smectite is mainly attributed to the nature of the bentonite, pore water chemistry and temperature. A significant microbial influence on the here analyzed parameters was not observed within the two years of experimentation. However, as the detected genera are known to potentially influence geochemical processes, microbial-driven alteration occurring over longer time periods cannot be ruled out if organic nutrients are available at appropriate concentrations.