The competing hypotheses for gas hydrate formation at the particle scale in sediments describe processes of pore‐filling, frame‐building, or cementation. New measurements of compressional (V_P) and shear wave (V_S) velocities in fine‐grained sands subjected to low confinement and monitored during formation of tetrahydrofuran hydrate indicate that hydrate nucleates in the pore space (presumably at grain boundaries) and grows with limited impact on the sediment shear stiffness, V_P, and V_S until crystals begin to interact with the granular skeleton at ∼40% hydrate concentration. V_S increases significantly more than V_P at higher hydrate concentrations, reflecting larger changes in the specimen's shear stiffness than its bulk stiffness. The results indicate that seismic velocities and/or their ratio (V_P/V_S) have limited capability for locating hydrate or constraining hydrate concentrations.

Gas hydrates consist of guest gas molecules inside hydrogen-bonded water lattices. Natural gas hydrates are found in offshore and permafrost regions. The large amounts of gas hydrate reserves suggest the potential of gas hydrates as an energy resource if economically viable production methods were developed. The proper understandings of hydrate formation/dissociation are important for the drilling and oil production applications. The investigations of physical and geotechnical properties provide the in-depth understandings of the in-situ hydrate formation mechanism and the associated production technologies. The purpose of this review paper is to provide a starting kit for civil engineers who have recently started the research related to the hydrate development and production and want to have insights on the general trends of the hydrate research and the relevant knowledge needed for their research. Gas hydrate explorations include the geophysical explorations, such as the seismic survey, the borehole logging and the geological and geochemical explorations. Gas hydrate productions require the dissociation of gas hydrates, and the production technologies are categorized based on the dissociation techniques involved: depressurization method, thermal stimulation method, and inhibitor injection method. Establishing safe and efficient gas production technology requires the extensive information on the geotechnical characteristics of hydrate reservoirs. Flow assurances, the integrity of sediment formation and the well bore stability are crucial for the safe and efficient productions of gases from gas hydrates in sediments. The strength and deformation characteristics, the fluid migration characteristics, and the thermal conduction characteristics are key factors for controlling the above.

Carbon dioxide sequestration into geological formations has been identified as an alternative to mitigate the global climate change. The CO₂ invasion pattern is dependent on various factors such as fluid viscosity, interfacial tension, injection rate, and the characteristics of porous media. Among these variables, we provide a systematic experimental study on the influence of the injection rate and the phase of CO₂ invading into a brine‐saturated microfluidic chip in order to quantitatively assess the displacement ratio. Interfacial tension and contact angle are accurately measured under the temperature and pressure conditions relevant to in situ conditions. The injection rate varies 3 orders of magnitude for gaseous, liquid, supercritical CO₂, and CO₂‐water foam invasion. The capillary number and the viscosity ratio are calculated for each experimental condition, and the displacement ratio (CO₂ saturation) is obtained after CO₂ invasion. The results show that the saturation of injected CO₂ is controlled by manipulating the injection rate and the phase of invading fluid, which can be used to optimize the in situ storage capacity. Especially, the CO₂‐water foam displaces almost all brine out of the microfluidic chip, but the increase in CO₂ saturation is 23% ~ 53% compared to pure gaseous CO₂ injection due to the water initially mixed in the CO₂‐water foam. The potential advantages of using CO₂‐water foam in the geological CO₂ sequestration were also discussed.