Depressurization of hydrate‐bearing sediments (HBS) can cause the movement of fine particles, and in turn, such fines migration affects fluid flow and mechanical behavior of sediments, ultimately affecting long‐term hydrocarbon production and wellbore stability. This study investigated how and to what extent depressurization of HBS causes fines migration using X‐ray computed tomography (CT) imaging. Methane hydrate was synthesized in sediments with 10% fines content (FC), composed of sands with silt and/or clay, and the hydrate‐bearing samples were stepwisely depressurized while acquiring CT images. The CT images were analyzed to quantify the spatial changes in FC in the host sediment and thus to capture the fines migration during depressurization. It was found that the FC changes began occurring from the hydrate dissociation regions. This confirms that the multiphase flow caused by depressurization accompanies fines migration. Depressurization of HBS with a hydrate saturation of ~20–40% caused FC reduction from ~10% to ~6–9%, and the extent of fines migration differed with the particle sizes of the host sands and the types of fines. It was found that fines migration was more pronounced with coarse sands and with silty fines. Such observed level of FC reduction is estimated to increase sediment permeability by several factors based on the Kozeny‐type permeability model. Our results support the notion that the extent of fines migration and its effect on fluid flow behavior need to be assessed in consideration of physical properties of host sediment and fine particles to identify optimum depressurization strategies.

This study explores the mechanical behavior of glass bead pairs cemented by microbial induced calcite precipitation (MICP) when subjected to tensile or shear loading. The mineral precipitation habit and contact area are also examined using X-ray computed tomography (X-ray CT). Examination of the failure surfaces reveals three distinctive failure modes: debonding failure at the precipitate-grain interface, internal failure within the precipitate, and mixed failure. The internal failure mode appears dominant when the calcite content (CC) of the bonded glass bead pair is greater than 17%, and it results in the smallest strengths: $\sim 8\mathrm{kPa}$ in tension and $\sim 7\mathrm{kPa}$ in shear. When CC is less than 17%, the debonding failure mode is mostly observed, and the debonding failure leads to the greatest strengths: $\sim 35\mathrm{kPa}$ in tension and $\sim 13\mathrm{kPa}$ in shear. The mixed failure mode occurs when $11\%<\mathrm{CC}<20\%$ , partly overlapping with the other two modes. The average tensile strength is greater than the average shear strength in all modes. The X-ray CT images demonstrate that the deposition of calcium carbonate first begins by coating the grain surface, and later shifts toward preferential precipitation at the grain contacts as the CC increases. Therefore, the relationship between contact radius and CC is bounded by the grain-coating and meniscus-filling models when CC < 20%; however, at a greater CC this relationship is bounded by the meniscus-filling and flat torus-filling models. This study presents unprecedented grain-scale mechanical responses associated with MICP-treated granular materials, which can be further extended to advance the understanding of the interplay between grain-scale cementation and the mechanical response of MICP-treated specimens, as well as the simulation of cemented soil behavior using discrete element modeling.

Fines migration driven by fluid flows often causes pore clogging in porous media and thus reduces the permeability of the porous media. The clogging can pose serious problems in various practices, such as formation damage in hydrocarbon reservoirs, degradation of fluid withdrawal or injection efficiency, particle‐induced fouling in filtration membranes, or even a risk of sediment failure with geostructures. However, the effect of seepage flow velocity on the spatial growth pattern of fines‐induced pore clogging remains poorly identified. This study explores the pore clogging growth behaviors caused by flow‐driven fines migration and accumulation in porous media and the associated permeability reductions through a series of column experiments and concurrent X‐ray computed tomography imaging. Particularly, the X‐ray image analysis captures the spatial distributions of fines, and thus the homogeneous or heterogeneous (localized) clogging growth pattern under various seepage velocities. Our results reveal that an increase in the seepage flow velocity increases the likelihood of localized clogging growth, attributable to the direct interception of fines by the sand grains. The localized clogging growth causes a faster reduction in permeability than the homogeneous clogging growth. The presented study provides well‐controlled experiment data on fines distribution and associated permeability change, which gives insights into fines‐induced clogging behavior in porous media.