Katherine J. Dobson

Dome-building eruptions are a complex expression of hybrid, effusive to explosive volcanism at the Earth’s surface. They are characterized by frequent and rapid shifts in eruptive style that make them highly unpredictable and hazardous. This multifaceted behavior is strongly influenced by the thermal/rheological evolution and mechanical response of lavas and rocks in the dome, and the efficiency of gas release through the edifice. Gas escape or entrapment in lava domes depends primarily on the permeability of the porous networks at different scale. Fracturing is crucial in developing connected and permeable pore pathways and thus in enabling and regulating gas escape efficiency in lava domes. In this study, we use X-ray micro-tomographic data obtained on a dacite from a block-and-ash flow deposit from the 1990-1995 eruption of Mt Unzen (Japan). We simulate pore-controlled fracturing in the 3D volume through a watershed-type image analysis, through which we explore the combined role of fractures and pore size distribution on pore connectivity and permeability. We use two different starting pore networks: one consisting of macro-porosity surrounded by micropores and a second set consisting of macropores only, yielding two distinct starting pore size distributions and pore connectivities. We quantify the influence of crack number density and width on the evolution of pore connectivity with porosity for the two pore networks. We then use Lattice Boltzmann simulations to quantify the porosity-permeability relationships of virtually cracked rocks. Our results show that as cracking progresses in a crystal-rich dome rock, connectivity and permeability strongly increase. The initial pore size distribution has a strong impact on crack propagation as well as connectivity and permeability. Our dataset also suggests that connectivity for a given pore network depends mostly on crack-number density, whereas permeability is more sensitive to crack width. Combining measurements of connectivity and permeability may be key when assessing the extent and mode of fracturing in volcanic edifices such as lava domes. We compare our simulations with mechanical tests of uniaxial compressive strength on similar Unzen dacites in order to link the impact of initial porosity and pore size distribution on crack propagation during failure. We finally discuss the implications of our results for the stability and eruptive style of silicic lava domes.

Percolation through magma mush is a key transport mechanism for melts in the crust and is influenced by the permeability of the crystal framework. Existing models for mush permeability do not account for the range of microstructures that can evolve as mushes crystallize or compact to low melt fractions. Here, we use numerically generated domains of cuboids at the random maximum packing as a starting geometry for a loose magma mush. We then expand the cuboid edges into the pore spaces sequentially, representing a geometrical simulation of crystal overgrowth and crystallization. At each iterative step, we measure the melt fraction, specific surface area, and melt permeability via 3D fluid flow simulations. We find that (a) the permeability drops proportional to the drop in surface area as the melt fraction reduces, (b) the permeability falls to zero at a percolation threshold that is independent of scale and insensitive to the starting cuboid geometry, and (c) once the percolation threshold is determined, our data match a universal percolation model without requiring any free fitting parameters. We show how this percolation model accounts for any 3D shape of the crystals that comprise the evolving mush. Importantly, this approach demonstrates that mush permeability can remain non‐zero in texturally unequilibrated mushes, down to very low melt fractions. Our model outperforms previous models, which overestimate mush permeability by up to three orders of magnitude, and our model can be used to accurately predict how mush permeability changes as mushes mature and crystallize, with implications for quantifying melt extraction, percolation rates, and melt reservoir assembly.

Fluid-rock interaction represents a common geological process that is highly dynamic and may cause substantial microscale petrophysical and geochemical changes both in a static and syn-deformational environment. Understanding how these local microscale dynamics occur is crucial to comprehend macroscale behaviour of the lithosphere, and for advancing critical subsurface engineering challenges, such as carbon capture and storage; a process that, together with hydrogen storage and geothermal energy recovery, is vital for the energy transition. With the advance of non-destructive imaging techniques (X-ray Computed Tomography - XCT), we can image the evolution of these microscale dynamics and understand how they drive changes in crustal dynamics and subsurface engineering. We present two applications, in which we use XCT to characterise the evolution of reservoir storage properties, such as porosity and permeability, and provide further insights into carbon sequestration. In the first application, we used XCT to investigate the precipitation history of an amygdaloidal basalt now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations. In the second application, we considered the combined effect of upstream corrosion of the carbon capture and storage (CCS) infrastructure and pre-existing reservoir rock compositions on the evolution of reservoir storage properties. Reactions from the corroded pipelines can change the chemistry of injected brine, which can then react with the adjacent rock formations reservoir, affecting reservoir porosity, permeability and caprock integrity. These are important parameters that determine the injectivity and storage capacities of deep geological sites for long term CO2 storage. Reservoir rock samples are characterised before corrosion and after carbonation reactions using XCT and other micro-analytical techniques, to assess the changes in the rock storage capacity properties. Our preliminary results prompt further studies into the understanding of fluid-rock interactions for subsurface engineering challenges, with a particular focus to pre-existing microfractures and changes in the injected brine due to corrosion of the upstream pipelines and interaction between CO2 brine and reservoir rocks.

Engineered subsurface barriers with reduced porosity and permeability are critical for safe storage of CO2 and H2, for the prevention of pollutant transport, and for several other subsurface flow challenges. This study investigates enzyme-induced carbonate precipitation (EICP), a promising technique with the potential to achieve uniform precipitation in otherwise inaccessible regions, provided the mechanisms of pore-scale mixing are well understood. High-speed lab x-ray computed tomography and flow modelling were used to study the mechanisms of reagent mixing and precipitation. Our experiments show that initially, crystallization occurs homogeneously across grain surfaces, then localizes in­ pores with high enzyme concentrations. In these regions, we see crystal growth throughout the 65-minute experiment. Simulation of reagent injection produces a mixing front that matches the distribution of crystals seen in the experiments if we model mixing as a density driven flow. Overall, we see substantial reductions in simulated permeability (11-37%) depending on the efficiency of mixing. Our validated model allows us to predict and propose tailored injection strategies for optimizing mixing, bringing us closer to real-world deployment of EICP for subsurface barriers.

Biomineralization, through microbially, thermally, or enzyme induced carbonate precipitation (MICP/TICP/EICP), is a cost-effective cementation process for changing porosity and permeability in the subsurface. This study aims to optimize compositional and injection parameters for biomineralization fluids, and to develop understanding of the interactions between geochemical reactions and fluid transport properties at the pore (micron) scale. Utilizing real-time in situ X-ray computed tomography (XCT), we compare traditional Microbially Induced Calcium Carbonate Precipitation (MICP) with novel thermally delayed (TICP) and Enzyme Induced Calcium Carbonate Precipitation (EICP) in a range of lithologies. This allows us to investigate the impact of mineralogy, grain size distribution, and temperature as well as the injection composition and strategy. We present quantitative analysis of crystal locations, the volume of carbonate and of individual crystals, and the effect of crystals on permeability and flow localisation over time. Coupled to measured changes in microstructural and macroscopic properties over repeated precipitation and dissolution cycles we present refined models of reactive transport for different injection strategies, and identify the optimal treatment strategy for different subsurface applications. This includes validation of the durability of precipitated calcite seals during dissolution phase, simulating the behaviour of CO2-enriched brines.This work provides the underpinning understanding principles of crystal formation, growth and hydrodynamic feedback mechanisms necessary for accurate modelling of reservoir scale dynamic processes. However, we also show how TICP and EICP strategies can improve performance of real-world Carbon Capture and Storage systems, driving more homogeneous, widely distributed and larger volumes of precipitated CaCO3 by maintaining permeability during treatment at higher degrees of cementation when compared to MICP. We also show how variable injection strategies allow improvement of other physical properties (e.g. mechanical strength) and enables the addition of highly conductive additives or phase change materials without reducing precipitation and flow. Using CaCO3 precipitation we observed a 470% increase in the thermal conductivity of unsaturated quartz sand after 9 cycles of MICP, and an 800% increase following addition of 5 wt% expanded natural graphite (ENG). Our findings also demonstrate the compatibility of integrating paraffin as a phase-change material within the porous matrix of ENG prior to MICP/EICP treatment significantly increasing specific heat capacity. These new geomaterials have widespread implications for thermal energy storage, specialized geothermal grouts/backfill, shallower wells and reduced geothermal energy costs.The project's outcomes impact the commercialization of engineered biomineralization and its role in the subsurface energy transition.

Pulverized fly ash (PFA) is commonly used as a supplementary cementitious material (SCM) in cement and is permitted for use in cements and grouts used for the encapsulation of low-level nuclear waste. However, with a global decrease in availability of PFA predicted in the near future, alternative SCMs are required. In nuclear encapsulation, a key parameter for a new SCM is that it does not change the leaching behaviour of the grout. We present promising initial leaching data from two SCMs: metakaolin and waste paper fly ash. These were tested for 24 h in two leachants, deionized water and an artificial groundwater, tracking the pH and leachability of a range of elements as the samples equilibrated with the leachants. The study suggests that for many elements, both metakaolin and waste paper fly ash show a very similar leaching behaviour to PFA grout formulations despite differences in the pH equilibration. In general, the tests suggest that the composition of the elute is dominated by leaching from the cementitious phases, and not the SCM. This suggests that further longer-term leaching studies are required as part of testing for SCM suitability for nuclear grouts in order to understand the connections between leaching and grout microstructure.

Biomineralization, through microbially, thermally, or enzyme induced carbonate precipitation (MICP/TICP/EICP), is a naturally occurring and inexpensive cementation process that can seal microfractures and pore throats that are inaccessible to cement and chemical based grouts. The porosity, permeability and thermal conductivity of porous geomaterials can therefore be controlled.This project aims to determine the optimal compositional and injection parameters for biomineralization fluids in a range of subsurface applications relating to the low carbon energy transition. These include, improving the subsurface storage integrity of CO2 and H2 by reducing permeability around poorly sealed legacy wells, enhancing mineral trapping of geo-sequestered CO2, and improving the thermal performance of well casings and ground around low-high geothermal and thermal energy storage systems. We also assess the real time response of bio-cemented samples to harsh environmental conditions representative of those in the subsurface.Understanding the interactions between geochemical reactions and the transport properties of fluid at the reservoir scale first requires biomineralization experiments to be carried out at the pore (micron) scale. These studies are essential for understanding principles of crystal formation, growth and hydrodynamic feedback mechanisms. Using real-time in situ x-ray computed tomography, the complex and synergistic factors involved in the biomineralization process can be better understood. Correlation of microstructural and macroscopic properties during repeated precipitation and dissolution events will allow refinement of larger scale reactive transport models that assess the suitability of different injection strategies.Carbon Capture and Storage: The ability to create large, and spatially targeted low permeability regions could be a key tool in preventing leakage of geo-sequestered CO2 (and H2), as well as improving/restoring CO2 injectability and sweep efficiency. During 2-phase EICP a poor choice of injection angle and flow rate can inhibit the mixing of precipitation fluids, and therefore the efficiency of permeability reduction within a porous medium. The challenge of getting 2 fluids to mix uniformly in a tight pore space is only likely to get worse in high pressure, low permeability real world systems. We explore single-phase thermally-delayed, and pulsed EICP injection strategies that encourage better mixing within heterogeneous real-world systems. Injection cycles are repeated multiple times to target the larger (order of magnitude) reductions in permeability required to alter the flow behaviour of CO2 and other gases.Thermal: Cement and bentonite based grouts typically have low thermal conductivities (<1 W/m K), which is detrimental to subsurface heat exchange. They often form a poor seal at the host rock/soil interface which can increase interfacial resistance. Minerals formed by MICP at the contacts between soil grains can greatly increase the thermal conductivity of the ground, particularly in unsaturated conditions. We explore enhancing this effect further with inclusion of highly conductive additives. For thermal energy storage applications specific heat capacity can also be increased with integration of phase change materials. By developing these specialized geothermal grouts/backfill, shallower boreholes may be required, greatly reducing cost.The findings of this project have profound implications on the commercialization of engineered biomineralization, and its role in the subsurface energy transition.

Basaltic rocks are considered excellent candidates for CO2 storage by in situ mineral trapping, due to their large presence on Earth’ surface and their higher reactivity with CO2 to form calcium-rich minerals. Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems. When the vesicle network has been filled by earlier mineralization the basalts can act as impermeable seals and traps. Characterizing the spatial and temporal evolution of the porosity and permeability is critical to understand the CO2 storage potential of basalts. We exploited X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro-fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro-fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.

Research into the therapeutic roles and clinical potential of CD34+ cells is growing [Weissman and Shizuru, Blood. 2008;112(9):3543-3553]. Accordingly, advances in cell isolation methods have risen to meet the research demand. Cord blood banks have a unique opportunity to source CD34+ cells because of their relatively high prevalence in cord blood [Dauber et al. Cytotherapy. 2011;13(4):449-458]. However, current isolation systems are not optimized for processing cord blood, and even blood-compatible systems typically require the blood to be processed before use. FerroBio (www.42bio.com) has developed a magnetic separation system that facilitates scalable, high-volume cell isolation from complex biological fluids, such as blood, without pre-processing. This technology is now being adapted for CD34+ cell isolation from cord blood. In this study, the conditions in which magnetic beads efficiently bind target cells were explored. The objective of this work was to evaluate optimal blood mixing conditions to bring beads into contact with cells in suspension in the blood. This will allow unmodified cord blood units (CBUs) to flow directly into our apparatus. The FerroBio separation process is comprised of three simple steps (Figure 1A). Here, conditions of step 1 were explored to maximize target cell recovery and viability. Variables included type of agitation equipment (Figure 1B), temperature, time, and relative air volume in the blood bag. Recovery and viability were measured via flow cytometry and calculated relative to unprocessed blood from the same CBU. Two mixing conditions were identified for maximum recovery of CD34+ cells (Figure 1C, blue bars). While not statistically different from each other, the Drum Roller method only requires one piece of standard equipment, is gentler, and can accommodate multiple CBUs simultaneously. No statistical difference in viability was observed in any conditions (90.7% ± 16.3%, n = 9). Increasing incubation time and the relative amount of air in the bag did not increase recovery (not shown). (A): Overview of the FerroBio Magnetic Separation Process, highlighting step (1) in which magnetic beads are mixed within the CBU to bind the target CD34+ cells. (B): Illustrations of the agitation equipment used to mix the CBU during step (1). (C): The resulting CD34+ recovery (1-way ANOVA, P = .0245). Image credit for panel (A): BioRender. The FerroBio separator can streamline CD34+ cell isolation by processing high blood volumes in a fast, closed system with high recovery rates. Additional studies are under way to evaluate the functionality of the isolated CD34+ cells and efficiency of bead removal.

Large-volume rhyolitic eruptions are characteristically crystal-poor yet are thought to originate from crystal rich magma mush bodies. This contradiction is explained by the interstitial melt being extracted prior to the eruption, generating large volumes of crystal-poor magmas. The timescale for melt extraction is inversely correlated to the permeability of the mush, defined by the shape of the crystals. Yet, existing approaches for estimating the crystal framework permeability do not account for crystal shape. Here, we represent magma mush by using numerically generated packs of hard cuboids with a range of aspect ratios and at their maximally dense random packing. We use lattice-Boltzmann simulations to constrain the permeability of the cuboid packs, showing that crystal shape exerts a first-order control on both the melt fraction at maximum packing, and on the constitutive relationship between permeability and melt fraction. Using percolation theory and a validation dataset, we develop a predictive scaling framework to compute permeability for mush comprised of crystals that can be approximated by cuboids, valid at melt fractions down to, and including the random maximum packing of crystals. We show that for packs of prolate cuboids, the melt extraction timescale can be reduced by almost two orders of magnitude relative to a pack of oblate cuboids, implying that rejuvenation timescales leading to eruption could be much shorter than previously predicted, using our novel permeability model that is sensitive to crystal shape.