Microstructural Controls on the Macroscopic Behavior of Analogue Rocks (Geo-architected Rocks)
Creators: name: Mitchell, Chven A
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Geophysics not elsewhere classified
Applied geophysics

description: Probing the subsurface for evidence related to the degradation of porous mediums and the evolution of damage mechanisms has been a long-standing challenge in geophysics. As such imaging and predicting fracture network development has remained a difficult area for subsurface science for decades despite the seminal and significant works put forward by many researchers. While this has provide great understanding about the behaviours and properties of natural porous media, there is still much that needs to be explored particularly in regard to the mineralogical composition and chemistry of clay-rich rocks. Despite the fact that argillaceous rocks which consist of different types of clays and varied mineral composition are ubiquitous in nature and are often the target of several technologies (e.g. geotechnical engineering, nuclear waste storage and disposal,hydrocarbon exploration and extraction, carbon capture and sequestration, etc.), many studies focus primarily on the bulk properties or the percentage of components in the matrix. For these reason and due to the problems that can be encountered with natural rocks that contain a swelling clay component whether randomly distirbuted or localized in consolidated globs in zones of the matrix, the influence of clay chemistry in relation to fracture development which is not well characterized, especially during desaturation is investigated with analogue rock samples which were systematically fabricated for this purpose. <br> The research performed in this dissertation investigated, the applicability of the fabrication protocol for developing synthetic rocks with desirable rock like features and behavior, the impact and relationship between the rock properties, the microstructural composition, water loss, and the macroscopic behavior of the analogue rocks, focusing on the structure and chemistry of the constituent clay materials. Synthetic rocks were fashioned with the necessary geometries, properties, and material compositions. On the macroscopic scale the fracture and drying behavior of the synthetic rocks were examined with 3D X-ray microscopy and further evaluated through the utility of acoustic emission monitoring, water loss monitoring, and unconfined compressive testing. On the finer scale (nano-microscale), the chemical and mechanical properties, and behavior of select clays was explored by exploiting several methods of material characterization which also included cation exchange experiments coupled with inductively coupled plasma – optical emission spectrometry (ICP-OES). <br> For the finer scale, experiments verified that calcined kaolinite clay had a different mineral structure and negligible to non-existence shrinkage abilities. In contrast, the montmorillonite clays possessed higher and similar moisture contents but, owing to the different principal cations these clays interacted a bit differently in the highly akaline environment, experienced varying degrees of shrinkage, and had observedly minor structural dissimilarities. For the relatively larger scale, the emergence of damage, extent of the damage network, and the patterns of the crack network mainly depended on the microstructural composition of the analogue rocks, particularly it's clay chemistry and/ or distribution. The location of damage depended on the emplacement and percentage of swelling clay in the matrix, and numerical investigations with peridynamics revealed that the observed damage was a consequence of the action of the swelling and non-swelling components of the matrix. Furthermore, if the microstructure consisted of no clay or calcined kaolinite the AE activity was solely attributed to interfacial processes that occurred during fluid front movement. If the microstructure consisted of a particular montmorillonite, the cracks propagated in the direction of the drying front. Conversely, for montmorillonite clay predominated by a different principal cation, the crack network developed and propagated differently during water loss. Additionally, on the laboratory core scale, properties and behavior similar to natural rocks were confirmed and the rock strength, porosity, AE activity, and velocities were primarily affected by the microstructural composition of the analogue rocks. <br> An added challenge for investigating and monitoring evolving systems and processes, whether on the laboratory or field scale, is the problem of extracting useful information from the physical data that can be used to identify signatures of developing processes, and changes in the properties or the behavior of a system. Here, data driven machine learning modeling and clustering techniques were undertaken to build a mechanistic understanding of the AE activity generated during drying. The intent is for this work to add to the fundamental research aimed at developing methods that will robustly detect and extract signatures related to evolutionary processes or features in the AE signals, and group them according to some degree of similarity. Such research will support reliable interpretations of the physical data for predictions of the behavior of systems, development of engineering controls, and improvement of the understanding of intrinsic dynamics related to complex processes particularly those that occur in clay-rich systems. <br> Combined chemical and mechanical investigations have great potential for unraveling practical challenges in subsurface science, especially regarding damage processes in clay-rich rock systems, and identifying and interpreting the presence of discontinuities from geophysical data. The present findings are useful for establishing a link between the constituent clay and observed damage, and improving our understanding of the development of damage in clay-bearing systems. These results provide insight on the influence of swelling clay and the chemistry of such clays on the generation of cracks and crack networks in rock like materials which can be useful for the characterization of damage in both laboratory and the field. The work presented here can also be a basis for further experiments that aim to uncover methods and protocols that will help with the indirect characterization of evolutionary processes, damage mechanisms, and damage in clay rich porous media. Additionally support for the use of analogue rocks in experimental rock physics, architected with specific material compositions, pore structures, crack systems, or clay fractions, is provided here.