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Multiphase flow is ubiquitous in subsurface energy applications and natural processes, such as oil recovery, CO2 sequestration, and water flow in soils. Despite its importance, we still lack a thorough understanding of the coupling of multiphase flow and reaction of transported fluids with the confining media, including rock dissolution and mineral precipitation. Through the use of geomaterial microfluidic flow experiments and high‐performance computer simulations, we identify key pore‐scale mechanisms that control this coupling. We compare the reactivity of fractured limestone with CO2‐saturated brine (single phase) and a mixture of supercritical (sc) CO2 and CO2‐saturated brine (multiphase). We find that the presence of scCO2 bubbles significantly changes both the flow dynamics and the resulting reaction patterns from a single‐phase system, spatially homogenizing the rock dissolution. In addition, bubbles redirect oversaturated fluid into low‐velocity regions, thereby enhancing carbonate precipitation occurs. Plain Language Summary: The impact of pore‐scale multiphase flow on fluid‐solid reactions is poorly understood because direct observations of reactive multiphase fluids in real rock materials are not widely available and the necessary computing is intractable. Using high‐pressure/temperature geomaterial microfluidic experiments complemented by high‐performance computer direct numerical simulation of multiphase flow in those geometries, we elucidate the pore‐scale mechanisms that lead to homogenization of rock dissolution and enhancement of mineral precipitation. This study contributes to our ability to predict soil weathering and to optimize CO2 sequestration and hydrocarbon extraction. Key Points: Multiphase flow leads to homogenization of rock dissolution and enhancement of mineral precipitationThe identified pore‐scale mechanisms have fundamental implications for soil weathering and subsurface applicationsThe results provide a foundation for engineering design and predictive tools [ABSTRACT FROM AUTHOR]

The authors discuss how pore-scale approach affects macroscale geofluids in porous media. Topics discussed include the investigation of viscous-dominated two-phase flow using the pore-scale hydrodynamic direct numerical approach, the methods for porosity-permeability correlations, and the impact of pore-scale mineral spatial heterogeneity on the alterations of fractured rock.

Time-lapse electrical resistivity tomography (ERT) is a geophysical method widely used to remotely monitor the migration of electrically-conductive tracers and contaminant plumes in the subsurface. Interpretations of time-lapse ERT inversion results are generally based on the assumption of a homogeneous solute concentration below the resolution limits of the tomogram depicting inferred electrical conductivity variations. We suggest that ignoring small-scale solute concentration variability (i.e., at the sub-resolution scale) is a major reason for the often-observed apparent loss of solute mass in ERT tracer studies. To demonstrate this, we developed a geoelectrical milli-fluidic setup where the bulk electric conductivity of a 2D analogous porous medium, consisting of cylindrical grains positioned randomly inside a Hele–Shaw cell, is monitored continuously in time while saline tracer tests are performed through the medium under fully and partially saturated conditions. High resolution images of the porous medium are recorded with a camera at regular time intervals, and provide both the spatial distribution of the fluid phases (aqueous solution and air), and the saline solute concentration field (where the solute consists of a mixture of salt and fluorescein, the latter being used as a proxy for the salt concentration). Effective bulk electrical conductivities computed numerically from the measured solute concentration field and the spatial distributions of fluid phases agree well with the measured bulk conductivities. We find that the effective bulk electrical conductivity is highly influenced by the connectivity of high electrical conductivity regions. The spatial distribution of air, saline tracer fingering, and mixing phenomena drive temporal changes in the effective bulk electrical conductivity by creating preferential paths or barriers for electrical current at the pore-scale. The resulting heterogeneities in the solute concentrations lead to strong anisotropy of the effective bulk electrical conductivity, especially for partially saturated conditions. We highlight how these phenomena contribute to the typically large apparent mass loss observed when conducting field-scale time-lapse ERT. [ABSTRACT FROM AUTHOR]

The use of non-conventional water (e.g., treated wastewater, desalinated water) for different purposes is increasing in many water scarce regions of the world. Its use for irrigation may have potential drawbacks, because of mineral dissolution/precipitation processes, such as changes in soil physical and hydraulic properties (e.g., porosity, permeability), modifying infiltration and aquifer recharge processes or blocking root growth. Prediction of soil and groundwater impacts is essential for achieving sustainable agricultural practices. A numerical model to solve unsaturated water flow and non-isothermal multicomponent reactive transport has been modified implementing the spatio-temporal evolution of soil physical and hydraulic properties. A long-term process simulation (30 years) of agricultural irrigation with desalinated water, based on a calibrated/validated 1D numerical model in a semi-arid region, is presented. Different scenarios conditioning reactive transport (i.e., rainwater irrigation, lack of gypsum in the soil profile, and lower partial pressure of CO 2 ( p CO 2 )) have also been considered. Results show that although boundary conditions and mineral soil composition highly influence the reactive processes, dissolution/precipitation of carbonate species is triggered mainly by p CO 2 , closely related to plant roots. Calcite dissolution occurs in the root zone, precipitation takes place under it and at the soil surface, which will lead a root growth blockage and a direct soil evaporation decrease, respectively. For the studied soil, a gypsum dissolution up to 40 cm depth is expected at long-term, with a general increase of porosity and hydraulic conductivity. [ABSTRACT FROM AUTHOR]

Microfluidic investigations of flow and transport in porous and fractured media have the potential to play a significant role in the development of future subsurface energy resource technologies. However, the majority of experimental systems to date are limited in applicability due to operating conditions and/or the use of engineered material micromodels. We have developed a high pressure and temperature microfluidic experimental system that allows for direct observations of flow and transport within geo-material micromodels (e.g. rock, cement) at reservoir conditions. In this manuscript, we describe the experimental system, including our novel micromodel fabrication method that works in both geo- and engineered materials and utilizes 3-D tomography images of real fractures as micromodel templates to better represent the pore space and fracture geometries expected in subsurface formations. We present experimental results that highlight the advantages of using real-rock micromodels and discuss potential areas of research that could benefit from geo-material microfluidic investigations. The experiments include fracture–matrix interaction in which water imbibes into the shale rock matrix from etched fractures, supercritical CO2 (scCO2) displacing brine in idealized and realistic fracture patterns, and three-phase flow involving scCO2–brine–oil. [ABSTRACT FROM AUTHOR]

For semi-arid regions, methods of assessing aquifer recharge usually consider the potential evapotranspiration. Actual evapotranspiration rates can be below potential rates for long periods of time, even in irrigated systems. Accurate estimations of aquifer recharge in semi-arid areas under irrigated agriculture are essential for sustainable water-resources management. A method to estimate aquifer recharge from irrigated farmland has been tested. The water-balance-modelling approach was based on VisualBALAN v. 2.0, a computer code that simulates water balance in the soil, vadose zone and aquifer. The study was carried out in the Campo de Cartagena (SE Spain) in the period 1999-2008 for three different groups of crops: annual row crops (lettuce and melon), perennial vegetables (artichoke) and fruit trees (citrus). Computed mean-annual-recharge values (from irrigation+precipitation) during the study period were 397 mm for annual row crops, 201 mm for perennial vegetables and 194 mm for fruit trees: 31.4, 20.7 and 20.5% of the total applied water, respectively. The effects of rainfall events on the final recharge were clearly observed, due to the continuously high water content in soil which facilitated the infiltration process. A sensitivity analysis to assess the reliability and uncertainty of recharge estimations was carried out. [ABSTRACT FROM AUTHOR]

• MPS is used to generate phases distributions from experimental images. • Similar scaling of mixing/reaction rate is observed regardless of phases distribution. • Phases distribution impacts the final mass of reaction product. The impact of phases distribution on mixing and reaction is hardly assessable experimentally. We use a multiple point statistical method, which belongs to the family of machine learning algorithms, to generate simulations of phases distributions from data out of laboratory experiments. The simulations honour the saturation of the laboratory experiments, resemble the statistical distributions of several geometric descriptors and respect the physics imposed by capillary forces. The simulated phases distributions are used to compute solute transport. The breakthrough curves reveal that different phases distributions lead to broad ranges of early arrival times and long-term tailings as saturation decreases. For a given saturation, a similar long-term scaling of mixing area, interface length, and corresponding reactivity is observed regardless of phases distribution. However, phases distribution has a clear impact on the final values (before breakthrough) of area of mixing, interface length and mass of reaction product. [ABSTRACT FROM AUTHOR]

Irrigation with reclaimed water is becoming a common practice in arid- and semi-arid regions as a consequence of structural water resource scarcity. This practice can lead to contamination of the vadose zone if sewage-derived contaminants are not removed properly. In the current work, we have characterized soils from the Guadalete River basin (SW Spain), which are often irrigated with reclaimed water from a nearby wastewater treatment plant and amended using sludge. Physico-chemical, mineralogical and hydraulic properties were measured in soil samples from this area (from surface up to 2 m depth). Emerging contaminants (synthetic surfactants and pharmaceutically active compounds, or PhACs) were also determined. Synthetic surfactants, widely used in personal care products (PCPs), were found in a wide range of concentrations: 73–1300 μg kg − 1 for linear alkylbenzene sulfonates (LAS), 120–496 μg kg − 1 for alkyl ethoxysulfates (AES), 19–1090 μg kg − 1 for alcohol polyethoxylates (AEOs), and 155–280 μg kg − 1 for nonylphenol polyethoxylates (NPEOs). The presence of surfactant homologues with longer alkyl chains was predominant due to their sorption capacity. A positive correlation was found between LAS and AEOs and soil organic carbon and clay content, respectively. Out of 64 PhACs analyzed, only 7 were detected occasionally (diclofenac, metoprolol, fenofibrate, carbamazepine, clarithromycin, famotidine and hydrochlorothiazide), always at very low concentrations (from 0.1 to 1.3 μg kg − 1 ). [ABSTRACT FROM AUTHOR]

The simultaneous presence of liquid and gas in porous media increases flow heterogeneity compared to saturated flows. However, the impact of saturation on flow and transport has so far remained unclear. The presence of gas in the pore space leads to flow reorganization. We develop a theoretical framework that captures the impact of that reorganization on pore‐scale fluid velocities. Preferential flow is distributed spatially through a backbone, and flow recirculation occurs in flow dead‐ends. We observe, and predict theoretically, that this previously identified flow structure induces a marked change in the scaling of the velocity probability density function compared to the saturated configuration and a sharp transition to strongly anomalous transport. We develop a transport model using the continuous time random walk theory that predicts advective transport dynamics for all saturation degrees. Our results provide a new modeling framework linking phase heterogeneity to flow heterogeneity in unsaturated porous media. Plain Language Summary: The unsaturated zone, where water and air coexist in the pore space, extends between the soil surface and the groundwater level. Its pronounced structural heterogeneity induces complex flow patterns, which lead to rich solute transport behaviors. Inputs (precipitation) and outputs (evaporation and deep drainage) induce spatiotemporal variability in water saturation (i.e., fraction of the pore space occupied by water), which impacts flow, transport, and biochemical reactions. It has been observed that water‐unsaturated conditions lead to a strong separation of flow in regions of high velocity, where most of the fluid is transported, and regions of low velocity. We identify the spatial distribution and size of the low‐velocity regions as key control features on water flow and transport of dissolved chemical species, leading to transport behaviors that differ from those described by classical transport formulations. We use these findings to develop a theoretical framework that allows us to predict flow and advective transport under unsaturated conditions, based on parameters that describe the heterogeneity in phase distribution within the pore space and that are directly linked to the geometry of the system. These results represent a decisive step toward the prediction of fate and transport phenomena from structural properties in unsaturated porous media. Key Points: The presence of an immiscible phase in porous media leads to an abrupt shift in the scaling of the liquid‐phase velocity distributionDispersion is quasi‐Fickian in saturated systems but becomes quasi‐ballistic under even slightly unsaturated conditionsWe predict flow and advective transport based on phase distribution and porous medium geometry [ABSTRACT FROM AUTHOR]

The release of CO 2 from the soil into the atmosphere, due to soil respiration, is a major, yet poorly understood flux that regulates the terrestrial carbon balance. In particular, the apparent insensitivity of soil respiration to emerging combined extreme events of heatwave and drought makes terrestrial ecosystems likely to shift from being carbon sinks to sources. Limited understanding of the interactions underlying this response represents one of the key sources of uncertainty in forecasts of atmospheric CO 2. Here, we explore plant–microbe–soil interactions under heat and drought using a millifluidic setup that enables direct observations of hydration and oxygen content (the primary factors controlling soil respiration) in the root zone. Our observations reveal movement of water between soil regions via roots (termed root hydraulic redistribution), creating soil respiration hotspots and becoming a carbon source under combined heat and drought. [Display omitted] • Root hydraulic redistribution explains soil respiration insensitivity to extremes. • Plant–microbe–soil interactions with direct observations of oxygen content • Root-mediated control on soil biogeochemical fluxes [ABSTRACT FROM AUTHOR]

Abstract The Mar Menor is the largest lagoon along the Spanish Mediterranean coast. It suffers from eutrophication and algal blooms associated with intensive agricultural activities and urban pressure in the surrounding Campo de Cartagena plain. A balanced discharge of groundwater, carrier of algal nutrients such as nitrate, is essential to ensure the integrity of the coastal lagoon and the availability of groundwater resources inland. We here present a 3D hydrogeological model of the unconfined Quaternary aquifer that discharges into the lagoon. The model couples both surface water balance and groundwater dynamics and has been calibrated to available data in the period 2000–2016. The calibrated model allows understanding of the current state of the aquifer and its link to the lagoon. The potential discharge has been quantified in both space and time and falls between 69.5 and 84.9 hm3/yr during dry and wet periods, respectively (with values of nitrate discharge of 11.4–11.8 Mkg/yr in the absence of aquifer sink terms, e.g., leakage to deeper aquifers and pumping from groundwater wells). The predictive capabilities of the calibrated model can be used to test the impact of different integrated management scenarios on the surface-groundwater dynamics of the catchment. Three plausible management scenarios are proposed that include localized and distributed groundwater pumping (drains and groundwater wells, respectively). Results show the effectiveness of the scenarios in reducing the groundwater and nitrate discharge into the lagoon. The disadvantages of the proposed scenarios, including potential seawater intrusion, need to be balanced with their relative merits for the sustainable development of the region and the survival of the Mar Menor ecosystem. The modelling approach proposed provides a valuable tool for the integrated and holistic management of the Campo de Cartagena-Mar Menor catchment and should be of great interest to similar hydrological systems with high ecological value. Graphical abstract Unlabelled Image Highlights • Mar Menor coastal lagoon is a key example of eutrophication by agricultural activity. • The sustainable development of Mar Menor must be considered as a land-sea continuum. • A numerical model tests the effectiveness of groundwater management strategies. • The model provides a decision support tool for policy makers. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing of shale formations in the United States has led to a domestic energy boom. Currently, water is the only fracturing fluid regularly used in commercial shale oil and gas production. Industry and researchers are interested in non-aqueous working fluids due to their potential to increase production, reduce water requirements, and to minimize environmental impacts. Using a combination of new experimental and modeling data at multiple scales, we analyze the benefits and drawbacks of using CO 2 as a working fluid for shale gas production. We theorize and outline potential advantages of CO 2 including enhanced fracturing and fracture propagation, reduction of flow-blocking mechanisms, increased desorption of methane adsorbed in organic-rich parts of the shale, and a reduction or elimination of the deep re-injection of flow-back water that has been linked to induced seismicity and other environmental concerns. We also examine likely disadvantages including costs and safety issues associated with handling large volumes of supercritical CO 2 . The advantages could have a significant impact over time leading to substantially increased gas production. In addition, if CO 2 proves to be an effective fracturing fluid, then shale gas formations could become a major utilization option for carbon sequestration. [ABSTRACT FROM AUTHOR]