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Intensification of agricultural practices is necessary to support a growing global population. Water movement and storage in soils are very crucial for successful intensification of agriculture. They are important for nutrient delivery to plant and overall crop productivity. Climate change and population growth have been predicted to limit water supply especially in arid regions. Therefore, there is an urgent need to proffer solutions that would help maintain or even increase soil moisture retention. The use of biochar for improving soil hydraulic properties is a current and growing area of research. Biochar is a stable form of charcoal gotten from heating natural organic materials in a high temperature and low oxygen process known as pyrolysis. Biochar physical and chemical properties vary due to the pyrolysis process conditions, the type of feedstock, and the ratios of lignin, cellulose, and hemicellulose in the biomass. These change the structure of the biochar and will invariably affect to what extent it can improve soil water retention. To understand how biochar affects soil water properties we must understand the specific characteristics of biochar that influence these changes. Understanding the mechanisms is important for easy prediction of when and by how much biochar will improve soil water properties. The aim of this thesis is to provide new and extensive insight into 4D structural changes of biochar produced from different feedstock and pyrolysis conditions and how these affects soil moisture characteristics of different soil textures. The first aspect of the thesis was to carry out a meta-analysis of the available literature to quantify the effect of biochar on soil hydraulic properties. To enable matching of biochar to soil constraints and application needs, a thorough understanding of the impact of biochar properties on relevant soil parameters is necessary. This meta- analysis of the available literature for the first time quantitatively assessed the effect of not just biochar application, but different biochar properties on the full sets of key soil hydraulic parameters. The review shows that the key factors influencing biochar performance were particle size, specific surface area and porosity indicating that both soil-biochar inter-particle and biochar intra-particle pores are important factors. Next, the role of biochar particle size and hydrophobicity in controlling soil water movement and retention was assessed. Softwood pellet biochar in five particle size ranges was used for the experiment. These particle sizes were tested on two soil types at four different application rates in the laboratory. The results clearly show that both biochar intraporosity (pores inside biochar particles) and interporosity (pore spaces between biochar and soil particles) are important factors affecting amended soil hydraulic properties. Biochar interpores affect mainly hydraulic conductivity; both interpores and intrapores control soil water retention properties. Our results suggest that for a more effective increase in soil water retention of coarse soils, the use of hydrophilic biochar with high intraporosity is recommended. The third aspect of the thesis was to assess the 4D structural changes and pore network model of standard biomass feedstock during pyrolysis. Biochar properties are highly variable and are dependent on the biomass feedstock and production conditions. Despite these variabilities, there has been no study that uses a single sample to study the development of microstructural properties of biomass and biochar through the full range of pyrolysis temperatures. In this study, synchrotron x-ray micro-tomography (SµCT) was used to visualize the internal structure and characterise pore structure of biochar from several feedstock during pyrolysis (50 - 800 °C). The results show a wide range of variation in the pore structural characteristics of the biochar depending on feedstock and pyrolysis temperature, with observed porosity in the range of 7.41 – 60.56 %. The results from this study are not only important for the use of biochar as soil amendments but also a range of other applications relying on biochar pore characteristics, such as biochar as habitat for microbes, water, and wastewater treatments, as an absorbent for the removal of acid gases, or an additive in construction or engineering materials. In addition, the in-depth insights into changes in biomass structure during heating are valuable for research and applications related to fire safety. Finally, the data set from the SµCT was used to produce statistical models enabling the prediction of the effect of biochar on soil water holding capacity from biochar microstructural properties. The results of this thesis, therefore, clearly establish the mechanisms with which biochar improves soil hydraulic properties. It also demonstrates that by appropriately matching microstructural properties of biochar to those of the target soil, it is possible to achieve considerable improvement in soil properties using relatively low application rates of biochar. The relationship between biochar structural properties and its effect on soil moisture characteristics of specific soil texture, is an attractive pathway towards development of precisely tailored biochars aimed to enhance water use efficiency and provide low-dose, high-efficiency benefits.

Understanding the dynamic evolution of hydraulic rock properties and the associated deformation mechanisms is a crucial factor in establishing the potential of groundwater resources, geological repositories as well as fossil fuel reservoirs. Chemical compaction by dissolution precipitation creep (DPC) is a major ductile deformation mechanism in the Earth’s upper crust that affects the porosity and permeability of its host rock and therefore influences transport properties and fluid flow herein. However, investigations of the process were in the past limited to observations on natural samples, laboratory experiments and numerical modelling, either lacking a temporal or spatial component or the validation of the model by direct observations. The link between space and time only became possible recently with the implementation of X-ray computed microtomography (microCT) in the earth sciences. By means of microCT imaging time-resolved, three-dimensional data (4D datasets) can be used to investigate the dynamic spatio-temporal evolution of hydraulic properties and the effect of mechanical-chemical feedback mechanisms. This thesis investigates slow chemical compaction mechanisms and their influence on porosity in NaCl-biotite rock-analogues. Oedometric compaction experiments on samples of different compositions and structures (layered vs. homogeneous) were conducted and time-resolved (4D) microtomographic data used to capture the dynamic evolution of the transport properties with progressing compaction. Furthermore, emphasis was placed on the effect of phyllosilicates upon deformation by dissolution precipitation creep, which is generally regarded to reinforce the process. Percolation analysis in combination with advanced digital volume correlation techniques showed that the presence of phyllosilicates influences the dynamic evolution of the porosity in the samples by promoting a reduction of porosity in their vicinity to the point of disconnecting the pore volume. However, the absence of strain localisation around those phyllosilicates and a homogeneous distribution of axial shortening across the samples invites a renewed discussion of the effect of phyllosilicates on dissolution precipitation creep, with a particular emphasis on the length scales of the processes involved. The results presented in this thesis, are among the first spatial-temporal data sets resolved on the microscale that demonstrate the crucial role of dissolution precipitation creep for the dynamic evolution of fluid transport properties. In addition to that, they encourage discussing classical theoretical models of diffusive transport and give an insight into the remaining challenges of experimental dissolution precipitation creep, especially concerning the nucleation and growth of stylolites.

Multiphase immiscible fluid flow in porous media is an important physical phenomenon that impacting many natural and industrial processes such as groundwater migration and enhanced oil recovery. The scope of this study is pore-scale flow processes because it is the pore-scale behaviour of immiscible fluids in a porous material that controls the macro scale behaviour. The porous media focused on this study are carbonate rocks because they are of global importance as they contain about 50% of the world’s hydrocarbon reserves and are also a major host to groundwater resources globally. Carbonate rocks are much less understood than sandstones because they often have much more heterogeneous properties in porosity, pore size distribution and pore network structure. The experimental approach of this research consists of a series of core-flooding experiments using both in-house and synchrotron-based X-ray microtomography (X-ray µCT) facilities. The in-house X-ray micro CT imaging is used to image static rock samples. The synchrotron µCT with its high photon flux enables active processes of fluid flow to be imaged in-situ and in real-time, so this 4D time-resolved imaging technique was used to observe the impact of porous media properties on actual flow events in the experiments. Displacement processes that occur in a mixed-wet Indiana limestone plug under brine injection, oil injection and steady-state injection were investigated and imaged with synchrotron µCT. Many of the previous works have done unsteady-state injection. In this study, direct observation of two-phase flow in carbonate rocks and measurements of saturation and fluid topological connectivity are presented in both steady-state and unsteady-state flow. Fluid displacement and trapping processes, as well as the fluid connectivity, were compared during the three injection stages. The result confirmed connectivity hysteresis during drainage and imbibition. In the other core-flooding experiment, a carbonate ’shrub’ sample from the Brazilian Pre-salt reservoir was used, together with dead crude oil from the Pre-salt Lula field. The displacement processes were imaged with in-house conventional X-ray µCT. In the Pre-salt ’shrub’ sample, fluid displacement using crude oil and mineral oil was compared. The crude oil achieved higher saturation than the mineral oil during drainage and remained less residual saturation after the imbibition of brine. The present work demonstrates that the pore-scale fluid displacement and trapping in carbonates are much more complex than that occur in idealised or relatively homogeneous media such as sandstones. On the other hand, pore-scale multiphase flow phenomenon was numerically modelled, and validated by our experimental observation. The model simulation was studied by the collaborating research group at Heriot-Watt University by Dr Julien Maes. In this research, a Roof snap-off event was identified from the experiments, and the event was modelled and repeated using a direct numerical simulation method. A deep learning method was implemented to improve the image segmentation workflow. The deep learning approach increased the temporal efficiency of the conventional segmentation workflow by ten-fold with equally good accuracy and is proved to have a strong resistance to ring artefacts. It also minimised human intervention during segmentation and allow unsupervised processing. The deep learning approach provides a promising tool for the image processing of extra-large µCT datasets. To conclude, the study increased the understanding of immiscible multiphase flow in porous carbonates, and provided a promising approach of processing and segmenting large-scale X-ray µCT datasets.

The measurement of the seismic velocity of a medium is fundamental to many applications in geoscience and engineering. Examples include the monitoring of: ice sheet melting, the health of concrete structures, temperature in volcanic regions, and sub-surface fluid pressure due to hydrocarbon extraction or the injection of CO2 to mitigate climate change. Velocities are also used to infer elastic properties, such as bulk and shear moduli and density, which can then be used to develop a wide range of rock physics models. This thesis addresses two key areas of research related to the seismic velocity: first, the improvement in the methodology of measuring changes in velocity in the time-lapse or four dimensional mode; and second, the interpretation of changing velocity measurements in terms of underlying processes, using various rock physics models. First, I investigate the use of coda wave interferometry (CWI) for measuring temporal changes in bulk velocity, particularly in an experimental rock physics setting. CWI uses the diffuse, multiply-scattered waves that arrive in the tail of the seismogram, sampling the entire medium and sampling the same sub-volumes many times, thus coda waves are far more sensitive to changes in a medium compared to the first arriving ballistic waves. Compared to conventional methods of phase picking of first arriving waves, CWI provides significant improvements in the accuracy and precision of estimates of velocity changes and is far more robust in the presence of background noise. CWI is also capable of jointly estimating changing source locations, allowing the estimation of the relative locations of a cluster of acoustic emissions with simultaneous velocity perturbations, all with a single receiver. Previously, the estimate of velocity change made by CWI has been an average of changes in compressional (P) and shear (S) wave velocities, which has previously been a major limitation to the application of the CWI method. I present a new method to use CWI for estimating changes in both P and S wave velocities individually. I then validate this method using numerical simulations on a range of media and the results of triaxial rock deformation experiments. The second part of this thesis is based on understanding the relationship between seismic velocity and time-dependent variables, including the evolving differential stress during deformation and changes in porosity during cementation. I investigate the seismic velocity-differential stress relationship during the experimental deformation of two finely laminated carbonate samples, using CWI to measure the temporal changes in both P and S wave velocity, allowing the inversion of crack density to interpret the mechanical behaviour of these carbonate samples. I then investigate the velocity-porosity relationship with an entirely digital method, using digital rocks where deposition and cementation are computationally simulated. I then simulate wavefield propagation through the digital rocks using a 3D finite-difference method to estimate the velocity of the medium. I statistically test two competing inclusion models for modelling elastic moduli-porosity data and find one that allows variable inclusion aspect ratio to be the most appropriate for fitting the data. I find CWI to be an effective method characterising changes in a medium in a rock physics environment. By providing a method for estimating separate changes in P and S wave velocity, I greatly improve the relevance and applicability of CWI for experimental rock physics. The method can be extended for the characterisation of media for a variety of applications in geoscience and engineering.

Successful management of hydrocarbon reservoirs, geothermal energy extraction sites, radioactive waste and CO2 storage sites depends on a detailed knowledge of fluid transport properties, porosity and permeability. Amongst deformation processes, fluid-rock interaction plays an important role in controlling the petrophysical properties of a rock. The presence of fluids in the rocks induce chemical and physical changes in compositions and texture, affecting porosity and permeability, hence influencing dynamic transport properties and fluid flow. Fluid-rock interaction processes have been deeply investigated in nature and in numerous experimental and numerical modelling studies. However, these studies lack a spatio-temporal characterization of the dynamic evolution of porosity and reaction microfabrics. There is no clear understanding of the spatio-temporal evolution of these properties in three dimensions, and how this evolution affects fluid percolation in the rock. Computed X-ray micro-Tomography (μCT) was applied to investigate these processes in three dimensions and observe their evolution in time (4DμCT). The combination of μCT with 2D analytical techniques (e.g. scanning electron microscope, SEM, electron microcrobe, EMPA, electron backscatter diffraction, EBSD) furthermore enables the extrapolation of the information gained from 2D analyses to the 3rd an 4th dimension (4D μCT). The thesis investigates two different categories of fluid-rock interaction processes, by using 4DμCT to monitor the evolution of mineral reactions (in the first case) and porosity (second case) in relation to strain and time. In the first case study, natural rock samples were analysed. The samples show a compositional change along a strain gradient from olivinic metagabbros to omphacite-garnet bearing eclogites in a ductile shear zone. Synchroton-based x-ray microtomography (sμCT) was applied to document the 3D evolution of garnets along the strain gradient (which represent the 4th dimension). The 3D spatial arrangement of garnet microfabrics can help determine the deformation history and the extent of fluid-rock interaction active during deformation. Results from the sμCT show that in the low strain domain, garnets form a large and well interconnected cluster that develops throughout the entire sample and garnet coronas never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnets reflect a deformational origin of the microfabrics. EBSD analyses reveal that garnets do not show evidence for crystal plasticity, but rather they highlight evidence for minor fracturing, neo-nucleation and overgrowth, which points to a mechanical disintegration of the garnet coronas during strain localisation. In the second case study, pressure-solution processes were investigated using NaCl as rock-analogue, to monitor the evolution of porosity and pore connectivity in four dimensions, providing a time-resolved characterization of the processes. NaCl samples were uniaxially compacted and μCT scans were taken at regular interval times to characterize the evolution of grain morphologies, pore space and macro-connectivity of the samples. Different uniaxial loads, as well as different bulk sample compositions (phyllosilicates and/or glass beads) were used to investigate their effect on the process. Greater uniaxial loads, and the presence of phyllosilicates within the deforming NaCl columns were found to enhance pressure-solution processes. The pore space becomes highly disconnected in the presence of phyllosilicates, with important implications for fluid percolation and dynamic transport properties. Mean strain rates, calculated from volumetric Digital Image Correlation (3D-DIC) analyses, were found to be higher where phyllosilicates were located. The combination of μCT with volumetric DIC and SEM imaging proved to be an efficient analytical method for investigating the dynamic behaviour of porosity and permeability during ongoing pressure-solution processes. The results showed that fluid-rock interaction critically modifies the rocks at the pore/grain scale, with important consequences on dynamic fluid transport properties. The combination of μCT with classical 2D techniques provided a better understanding on the dynamic evolution of transport properties and fluid percolation during fluid-rock interaction processes, allowing the characterization in three dimensions of reaction microfabrics and porosity.