The reactive mixing between seawater and terrestrial water in coastal aquifers influences the water quality of submarine groundwater discharge. While these waters come into contact at the seawater groundwater interface by density driven flow, their chemical components dilute and react through dispersion. A larger interface and wider mixing zone may provide favorable conditions for the natural attenuation of contaminant plumes. It has been claimed that the extent of this mixing is controlled by both, porous media properties and flow conditions. In this study, the interplay between dispersion and reactive processes in coastal aquifers is investigated by means of numerical experiments. Particularly, the impact of dispersion coefficients, the velocity field induced by density driven flow and chemical component reactivities on reactive transport in such aquifers is studied. To do this, a hybrid finite-element finite-volume method and a reactive simulator are coupled, and model accuracy and applicability are assessed. A simple redox reaction is considered to describe the degradation of a contaminant which requires mixing of the contaminated groundwater and the seawater containing the terminal electron acceptor. The resulting degradation is observed for different scenarios considering different magnitudes of dispersion and chemical reactivity. Three reactive transport regimes are found: reaction controlled, reaction-dispersion controlled and dispersion controlled. Computational results suggest that the chemical components' reactivity as well as dispersion coefficients play a significant role on controlling reactive mixing zones and extent of contaminant removal in coastal aquifers. Further, our results confirm that the dilution index is a better alternative to the second central spatial moment of a plume to describe the mixing of reactive solutes in coastal aquifers.@en

Low salinity water flooding is a common technique for enhancing oil recovery; however, the mechanism behind the low-salinity effect, positive or negative, is still not fully understood. In the proposed mechanisms, osmosis and emulsification are considered as two potential reasons for explaining the oil remobilization, but the specific contributions on the remobilization are not well studied at pore-scale. In this article, we performed a series of microfluidic experiments to investigate the movement of constrained oil between invading low-salinity brine and residual high-salinity brine. We find that various salinity contrasts over oil films cause different water fluxes through the oil and swelling areas of the trapped brine, resulting in the relocation of oil phases within the pore spaces. A higher salinity contrast (1.7-170 g/L salt concentrations) provides a faster water penetration in oil phases. In the presence of an oil-soluble surfactant, spontaneous emulsification occurs at the interface of low-salinity brine/oil, which enhances almost 100 times the water flux in two oil phases (n-heptane and n-dodecane). We directly observe pore-scale spontaneous emulsification at the low-salinity brine/oil interface but not at the high-salinity brine/oil interface. Furthermore, two scenarios for explaining water transport through the oil phase are proposed: water diffusion due to chemical potential gradient and water transport via reverse micelle or microemulsions movement.@en

Recent insights suggest that the osteochondral interface plays a central role in maintaining healthy articulating joints. Uncovering the underlying transport mechanisms is key to the understanding of the cross-talk between articular cartilage and subchondral bone. Here, we describe the mechanisms that facilitate transport at the osteochondral interface. Using scanning electron microscopy (SEM), we found a continuous transition of mineralization architecture from the non-calcified cartilage towards the calcified cartilage. This refurbishes the classical picture of the so-called tidemark; a well-defined discontinuity at the osteochondral interface. Using focused-ion-beam SEM (FIB-SEM) on one osteochondral plug derived from a human cadaveric knee, we elucidated that the pore structure gradually varies from the calcified cartilage towards the subchondral bone plate. We identified nano-pores with radius of 10.71 ± 6.45 nm in calcified cartilage to 39.1 ± 26.17 nm in the subchondral bone plate. The extracted pore sizes were used to construct 3D pore-scale numerical models to explore the effect of pore sizes and connectivity among different pores. Results indicated that connectivity of nano-pores in calcified cartilage is highly compromised compared to the subchondral bone plate. Flow simulations showed a permeability decrease by about 2000-fold and solute transport simulations using a tracer (iodixanol, 1.5 kDa with a free diffusivity of 2.5 × 10−10 m2/s) showed diffusivity decrease by a factor of 1.5. Taken together, architecture of the nano-pores and the complex mineralization pattern in the osteochondral interface considerably impacts the cross-talk between cartilage and bone.@en

A modular pore-scale model is developed to assess the response of wellbore cement to geological storage of CO2. Numerical formulations for modeling of solute transport are presented and a methodology for coupling with geochemical processes is discussed, which includes: (1) advective and diffusive fluxes of solutes within the pore space, (2) aqueous phase speciation, (3) mineral dissolution-precipitation kinetics, and (4) the subsequent changes in pore space geometry. A Complex Pore Network Model (CPNM) is used to discretize the continuum porous structure as a network of pore bodies and pore throats, both with finite volumes. CPNM allows for a distribution of pore coordination numbers ranging between 1 and 26. This topological property, together with a geometrical distribution of pore sizes, enables the microstructure of porous media to be mimicked. For each pore element, transport of solute is calculated by solving the governing mass balance equations. Chemical reaction of the fluid phase with the main reactive solid components (portlandite and calcite) is incorporated through coupling with a geochemical reactive simulator. Average values and properties are obtained by integration over a large number of pores. Using this approach, we investigate how chemical reaction between water-bearing wellbore cement and supercritical CO2 can create a distribution of porosity in a direction parallel to the CO2 concentration gradient and transport path, at 50°C. The dynamics of this process involve interaction between diffusion dominated mass transport and the kinetics of dissolution and precipitation of portlandite and/or calcite. Simulation of unconfined chemical degradation, in a fluid of constant composition, shows development of different regions: (1) a zone adjacent to the inlet face, which is characterized by an increase in porosity due to extensive dissolution, (2) a carbonation zone with decreased porosity, (3) the carbonation front which made a thin layer with the lowest porosity due to calcium carbonate precipitation, and (4) dissolution zone. These results are in agreement with laboratory observations under similar conditions. This provides confidence that this pore-scale approach can ultimately be applied to model the progress of coupled CO2 transport and cement degradation at critical points along the length of cemented wellbore sections at CO2 storage sites.@en