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Gas bubble in aquatic sediments has a significant effect on its geophysical and geomechanical properties. Recent studies have shown that methane gas and hydrate can coexist in gas hydrate–bearing sediments. Accurate calibration and understanding of the fundamental processes regarding such coexisting gas bubble dynamics is essential for geophysical characterization and hazard mitigation. We conducted high-resolution synchrotron imaging of methane hydrate formation from methane gas in water-saturated sand. While previous hydrate synchrotron imaging has focused on hydrate evolution, here we focus on the gas bubble dynamics. We used a novel semantic segmentation technique based on convolutional neural networks to observe bubble dynamics before and during hydrate formation. Our results show that bubbles change shape and size even before hydrate formation. Hydrate forms on the outer surface of the bubbles, leading to reduction in bubble size, connectivity of bubbles, and the development of nano-to micro-sized bubbles. Interestingly, methane gas bubble size does not monotonously decrease with hydrate formation; rather, we observe some bubbles being completely used up during hydrate formation, while bubbles originate from hydrates in other parts. This indicates the dynamic nature of gas and hydrate formation. We also used an effective medium model including gas bubble resonance effects to study how these bubble sizes affect the geophysical properties. Gas bubble resonance modeling for field or experimental data generally considers an average or equivalent bubble size. We use synchrotron imaging data to extract individual gas bubble volumes and equivalent spherical radii from the segmented images and implement this into the rock physics model. Our modeling results show that using actual bubble size distribution has a different effect on the geophysical properties compared to the using mean and median bubble size distributions. Our imaging and modeling studies show that the existence of these small gas bubbles of a specific size range, compared to a bigger bubble of equivalent volume, may give rise to significant uncertainties in the geophysical inversion of gas quantification.

Zero carbon energy generation from renewable sources can reduce climate change by mitigating carbon emissions. A major challenge of renewable energy generation is the imbalance between supply and demand. To overcome the energy imbalances, subsurface storage of hydrogen in porous mediais suggested as a large-scale and economic solution, yet its mechanisms are not fully understood. Important unknowns are the effect of the high migration potential of the small and mobile hydrogen molecule and the volume of recoverable hydrogen.We conducted non-steady state, cyclic hydrogen and brine injection experiments at 2-7 MPa and flow rates of 2-80 µl min-1 using water-wet Clashach sandstone cylinders of 4.7 mm diameter and 53-57 mm length (Clashach composition: ~96 wt.% quartz, 2% K-feldspar, 1% calcite, 1% ankerite). Two sets of experiments were performed using our new transparent flow-cell designed for x-ray computed microtomography: 1) Experiments using a laboratory x-ray source (University of Edinburgh) imaged the flow, displacement and capillary trapping of hydrogen by brine as a function of saturation after primary drainage and secondary imbibition. 2) Experiments using synchrotron radiation (Diamond Light Source, I12-JEEP tomography beamline) captured time-resolved hydrogen and brine flow and displacement processes. Pressure and mass flow measurements across the experimental apparatus complemented the microtomography volumes in both sets of experiments.Results from a water-wet rock show that hydrogen behaves as a non-wetting phase and sits in the centre of the pore bodies, while residual brine sits in corners and pore throats. Hydrogen saturation in the pore volume is independent of the injection pressure and increases with increasing hydrogen/brine injection ratio up to ~50% saturation at 100 % hydrogen. Capillary trapping of hydrogen during brine imbibition occurs via snap off and is greatest at higher brine injection pressures, with 10 %, 12% and 21% hydrogen trapped at 2, 5 and 7 MPa, respectively. Higher brine flow rates reduce capillary trapping and increase hydrogen recovery at any given injection pressure. Based on these results, future hydrogen storage operations should inject 100% hydrogen and manage the reservoir pressure to avoid high pressures and minimize capillary trapping of hydrogen during brine reinjection.Ongoing analysis of time-resolved experimental data will provide further insight into the critical pore-scale processes that ultimately influence the potential for geological hydrogen storage and recovery.

Multi-pile foundation systems are used to support offshore infrastructure in areas of Northern Europe underlain by Chalk. During installation, the tip of the tubular open-ended ‘small displacement’ pile crushes and destructures the Chalk forming a ‘putty’ interface or ‘annulus’ around it. Pile resistance to uplift (i.e. shaft capacity) results from sliding friction between the pile shaft and this putty chalk annulus. The limited knowledge of the characteristics of this interface is associated with pile design guidelines that may be overconservative, which could result in unnecessary increases in economic costs for the offshore renewable energy industry.The present Thesis investigates the prospect of using soil mechanics to describe shaft friction in small displacement piles in Chalk as function of the critical state strength of the chalk putty that composes the annulus. To this end, oedometer and triaxial tests were conducted on low and medium density destructured White Chalk samples to determine the suitability of a critical state framework to characterise the mechanical behaviour of the material. Monotonic and cyclic simple shear tests were then used to simulate load transfer through the chalk-pile interface and examine the compatibility of interface performance with the prospective critical state framework. Thereafter, model piles of different geometries were installed in intact chalk cores and micro-focus X-ray computed tomography (XCT) was applied to measure the density of the chalk putty annulus. The piles were then tested in tension.Oedometer and triaxial results demonstrate that a unique one-dimensional normal compression line and a unique critical state line (CSL) exist for the tested materials, regardless of their origin and the varied preparation methods used. This ‘uniqueness’ is associated with comparable grain shapes and size distributions, and to grain breakage processes that resemble those reported for sands. Simple shear experiments show that interface failure states comply with the CSL, although strain localisation precludes the attainment of critical state strengths when shearing initiates in significantly dense of critical conditions. A CSL-based framework defining interface strength as a function of void ratio was produced and found to compare well with the small number of early-life shaft capacity measurements of piles available in the literature. Yet, this framework could not predict the shaft capacity of the model pile experiments, due to heavily dense of critical states at the end of installation. XCT further revealed that pile penetration processes in chalk and sand are generally comparable, and that design approaches based on the cone penetration test might successfully estimate early-life shaft capacity. Though pile set-up was found to possibly result from annulus consolidation, its prediction may need to rely on empiricism for the foreseeable future.