Freshwater Aquifers Research Articles

CO2 effects on an unsaturated and saturated carbonate reservoir from geoelectric-geochemical coupling

The objective of this work is to understand and characterize how dissolved CO2 concentrations can influence physicochemical parameters, in particular the electrical conductivity and pH.In previous work, a CO2 injection experiment was carried out directly in a freshwater aquifer within an Oligocene carbonate underground quarry located in Saint-Emilion (Gironde, France). Different parameters emerged as excellent indicators for monitoring a gas plume during CO2 geological storage, especially the electrical conductivity and pH.We tried to reproduce field observations on a laboratory scale by batch experiments and measurements on Hassler cell, and to make the link between the geochemical results (electrical conductivity, pH, CO2 concentrations) and the electrical properties (electrical resistivity index) depending on the saturation state of the carbonate rock.A calibration line has been highlighted, making it possible to link the relation between electrical conductivity-CO2-pH. This line is only valid for Saint-Emilion limestones at the injection well because it is dependent on the petrophysical properties of the environment.As field observations are reproducible in the laboratory, this calibration line allows to pass from one scale to another by combining different results, and estimate the CO2 concentrations obtained for such values of electrical conductivity and pH.

Submarine groundwater discharge detection on the coast of Buenos Aires using 222Rn and geoelectrical methods

Submarine groundwater discharge (SGD) is a complex hydrological process which occurs in the continent–ocean interface and plays an important role in coastal dynamics. The detection of groundwater discharge from the sandy freshwater aquifer towards the Atlantic Ocean was proposed on the western coast of Buenos Aires Province (Argentina). There are different methods used to detect SGD. In this study, 222Rn as a tracer, electrical resistivity tomography (ERT) and flow maps were used as the methodology. 222Rn activity was measured in the wells, at the beach (tidal pools and surf zone) and along a transect 200 m from the coastline where geo-electrical method was also used. Groundwater depth was measured in the wells and groundwater contour maps were made .222Rn activity in the wells varies from 16 and 173 dpm/L, at the beach the values are between 28 and 48 dpm/L and along the coastline they oscillate between 1.3 and 20.5 dpm/L. The ERT shows a high resistivity layer close to a depth of 3-4 m from the sea floor, which would indicate the presence of freshwater. Groundwater contour maps show discharge toward the continental plain to the west and toward the sea to the east. There is no precedent related to the application of these methodologies in the study area, therefore this study is of interest to increase our knowledge of the coastal hydrodynamics.

Biogeochemical dynamics and microbial community development under sulfate- and iron-reducing conditions based on electron shuttle amendment.

Iron reduction and sulfate reduction are two of the major biogeochemical processes that occur in anoxic sediments. Microbes that catalyze these reactions are therefore some of the most abundant organisms in the subsurface, and some of the most important. Due to the variety of mechanisms that microbes employ to derive energy from these reactions, including the use of soluble electron shuttles, the dynamics between iron- and sulfate-reducing populations under changing biogeochemical conditions still elude complete characterization. Here, we amended experimental bioreactors comprised of freshwater aquifer sediment with ferric iron, sulfate, acetate, and the model electron shuttle AQDS (9,10-anthraquinone-2,6-disulfonate) and monitored both the changing redox conditions as well as changes in the microbial community over time. The addition of the electron shuttle AQDS did increase the initial rate of FeIII reduction; however, it had little effect on the composition of the microbial community. Our results show that in both AQDS- and AQDS+ systems there was an initial dominance of organisms classified as Geobacter (a genus of dissimilatory FeIII-reducing bacteria), after which sequences classified as Desulfosporosinus (a genus of dissimilatory sulfate-reducing bacteria) came to dominate both experimental systems. Furthermore, most of the ferric iron reduction occurred under this later, ostensibly “sulfate-reducing” phase of the experiment. This calls into question the usefulness of classifying subsurface sediments by the dominant microbial process alone because of their interrelated biogeochemical consequences. To better inform models of microbially-catalyzed subsurface processes, such interactions must be more thoroughly understood under a broad range of conditions.

Soluble Salts Quantitative Characterization and Thermodynamic Modeling on Roman Bricks to Assess the Origin of Their Formation.

The environmental weathering and the formation of efflorescences on the brick walls are studied at the “Casa di Diana” Mithraeum at Ostia Antica archaeological site. Previous studies on subsoil, bedrock, hydrological systems and environmental conditions, and new ion chromatography analysis combined with ECOS-RUNSALT and Medusa-Hydra thermodynamic modelling software, had allowed us to identify the subsoil contamination related to soluble salts. The atmospheric acidic gases, CO2 and SO2, are determined as the main salt weathering species. A dry deposition after a subsequent hydration action from the shallow freshwater aquifer that reaches up to 1 m on the walls is identified as the mechanism of salt formation. An evaluation of potential sources such as the nearby Fiumicino airport, CO2-rich gases inputs from fumaroles and CO2 inputs was also debated. The risk level of contamination the surfaces of the materials should be considered mildly/very polluted with a medium/high risk of hygroscopic moisture due to the high concentration of sulphates.

Groundwater extraction-induced seismicity around Delhi region, India

The non-tectonic deformation, either of natural or anthropogenic origin, may influence the earthquake occurrence process and seismicity rate along the plate-boundary or ‘stable’ plate-interiors domains. The low magnitude but moderate seismicity rate of Delhi region on the stable plate-interiors domains of India, exhibits significant variation both in short-term at annual seasonal scale and in long-term at decadal scale. It correlates with the anthropogenic groundwater pumping for the extensive irrigation, urban activities, and seasonally controlled hydrological loading cycle of Indo-Ganga Basin hosted freshwater aquifers. Our coupled hydro-mechanical simulation and poro-mechanical analysis of basement fault stability suggest that the combined aquifer contraction and basement rock expansion act together to modulate the effective stress regime and anthropogenic seismicity on the basement faults in Delhi region.

Salinization of groundwater under desert reclamation stress at Qena region, Egypt

The scarcity of water resources with increasing demand made the evaluation of groundwater quality an urgent matter. Thus statistical analyses and hydrochemical models were conducted on 111 wells sampled from Pleistocene aquifer at Qena Governorate. Multivariate statistics revealed mixed processes affecting groundwater chemistry. Such processes expressed in terms of four rotated factors; these are mineralization (salinization), anthropogenic surface recharge, dissolution of ferromagnesian minerals, and oxidation factor. Aquifer salinization is evidenced by a lateral change in the basic hydrochemical characteristics from low mineralized Ca–HCO3, Mg–HCO3, and Na–HCO3 water types to highly mineralized Ca–Cl, Mg–Cl, and Na–Cl facies. Chloride-water species are associated with high-impact areas of chloride-rich sediments (dissolution of overlain saline sediments and marine Pliocene clay underneath). Piper diagram indicates desert wells are dominantly affected by saltwater intrusion from deep aquifers along fault planes under over-pumping. On other hand, shallow wells are the most affected by surface activities. Likewise, wells located in the vicinity of the industrial zone and sites of wastewater disposal. This raises questions about desert reclamation strategy and its impact on groundwater quality; hence this approach can be used as a basic tool for reviewing desert reclamation management strategies. CapsuleQuaternary freshwater aquifer in Egypt was exposed to salinization under desert reclamation, via dissolution of marine deposits and intrusion from deeper aquifers.

Bivalve haemocyte adhesion, aggregation and phagocytosis: A tool to reckon arsenic induced threats to freshwater ecosystem

The freshwater aquifers of the Indo-Gangetic plains support rich biodiversity which is under the threat of arsenic contamination. The filter feeding bivalve mollusc Lamellidens marginalis is a sessile and sentinel resident of these freshwater habitats. In the present study, the classical cell behaviours of adhesion and aggregation were monitored in the circulating haemocytes of the freshwater bivalve under the exposure of sodium arsenite (NaAsO2) at sublethal concentrations in controlled laboratory conditions for a maximum time-span of sixteen days. The toxic metalloid significantly inhibited non-self adhesion, inter-haemocyte interactions and haemocyte aggregation in a dose and time dependent manner. The natural occurrence of the filopods on the haemocytes was significantly diminished in the bivalves exposed to the inorganic arsenite. Moreover, a significant fall in the kinetics of phagocytosis index and haemocyte adhesion was observed under the in vitro exposure to NaAsO2. Compromised non-self adhesion, cell-cell aggregation and phagocytosis of non-self particles by the bivalve haemocytes probably indicate susceptible immunological status of the bivalve. Such vulnerable immunity of the bivalve probably signifies the nature of imminent threat to the freshwater ecosystem as a whole under inorganic arsenite exposure. The findings would be helpful to design bivalve haemocyte based inexpensive biomonitoring tool to assess the health of freshwater ecosystem under potential arsenic threat.

Delineating saline and fresh water aquifers in Tuticorin of southern India by using geophysical techniques

Geo-electrical survey was conducted in a highly industrialized area to delineate the freshwater and saline water aquifers. A total of 30 vertical electrical soundings (VES) were made along the coastal aquifers of Tuticorin, which covers both sedimentary formation and crystalline rocks. By using VES data, GIS was used to create spatial maps for resistivity and thickness at various depths. The results show that Q and H were the dominant curve types in the study area. Q type curves indicate saline water by decreasing resistivity values with respect to depths. In H type curves, low resistivity saline water aquifer was sandwiched between high resistivity topsoil and hard rock at the deeper horizon. The geophysical indicators such as Dar-Zarrouk parameters were calculated using the VES results. The Longitudinal unit conductance (S) values ranged between 0.075 and 7500 S. The transverse unit resistance (T) of the study area varied from 16.06 to 5909.25 Ωm2. The anisotropy (λ) values ranged from 1.00 to 55. Interpretation of data reveals that the saline water intrusion and industrial impact on shallow aquifers were observed in the coastal region of the study area. Few pockets of the freshwater aquifer were also noted in deeper horizons. Since the quality of groundwater was deteriorating, proper management plans will be needed to save this fragile coastal aquifer.

Shale oil production and groundwater: What can we learn from produced water data?

As oil production in the Permian Basin surges, the impact of shale production on groundwater resources has become a growing concern. Most existing studies focus on the impact of shale production on shallow freshwater aquifers. There is little understanding of the shale development's impact on other groundwater resources (e.g., deep carbonate aquifers and deep basin meteoric aquifers). The possible natural hydraulic connections between shallow aquifers and formation water suggest such an impact can be consequential. This study explores the relationship between shale production and groundwater using produced water (PW) samples from active unconventional oil wells. Focusing on the most productive portion of the Permian Basin-the four-county region in Southeast New Mexico between 2007 and 2016, a large produced water dataset allows us to analyze the conditional correlations between shale oil production and PW constituents. The results suggest that (1) expanding from primarily conventional wells to unconventional wells during the recent shale boom has led to dramatic increases of the TDS, chloride, sodium, and calcium levels in groundwater (i.e., producing formation). (2) Nearby oil well density positively correlates with the TDS, chloride, and sodium levels in the PW samples.

Experimental and numerical assessment of Light Non-Aqueous Phase Liquid (LNAPL) subsurface migration behavior in the vicinity of groundwater table

Groundwater contamination by constituents of hydrocarbon fluids may occur from the earth’s surface or subsurface. Key sources of contamination include spills and leakages from oilwells, underground storage tanks (USTs), and oil transport-tankers. These light non-aqueous phase liquid (LNAPL) contaminants pollute the soil and its continuous migration, may even end up contaminating aquifer systems. This highlights the need to understand the fate and migration behavior of LNAPL contaminants in the subsurface, especially in the unsaturated zone – with a view to limiting the contaminants transport. This study presents a numerical approach for describing and predicting the fate of LNAPL contaminant-transport in the subsurface. Multiphase flow concept was adopted, where two LNAPL phases – oil, and gas – were mainly considered. Diesel and crude oil are the hydrocarbon contaminants used, while unconsolidated sand was used as the porous matrix. Two contaminant flow scenarios were simulated experimentally — surface and subsurface imbibition. Mass balance equations and constitutive functions from the several important phenomena that influence LNAPL contaminant flow, in the subsurface, are discussed. Extended Darcy’s law in combination with van Genuchten model were applied in the 2D numerical model description for mass balance equation and constitutive relationship, respectively. The numerical simulation was executed using COMSOL Multiphysics® v. 5.5. The numerical simulation results showed a good correlation with the experimental results, and suggest that exposure time, fluid viscosity, density, contaminant supply, amount released, and the hydraulic properties of the porous matrix, are the most important parameters in LNAPL contaminant subsurface migration. This study thereby concludes that if the fluid thermodynamic properties and the hydraulic properties of the porous matrix are known, the numerical model can accurately predict the migration behavior of hydrocarbon contaminants. In that way, it becomes possible to effectively track oil-spills and leakages propagating into the vicinity of groundwater table and freshwater aquifer. The study outcome is useful in designing and implementing efficient hydrocarbon contamination remediation approaches and saving time by taking the right action.

Current Patterns in the Use of Domestic Wells in the Villages in Devinuwara Divisional Secretariat Division, Southern Sri Lanka

At present, freshwater is rapidly becoming scarce with increasing competition from humans for different purposes and climate change. In this context, sustainable use and conservation of freshwater aquifers, water use efficiency, and wastewater reuse at the domestic level play a significant role in solving water management challenges and food security. Domestic well use pattern, very unique to Sri Lankan villages for centuries, is mutually connected with the society, environment, and culture. However, nowadays the village lifestyle has been changing with increasing the standard of living of people. It leads to change their traditional behavior when they use natural resources like domestic wells. Accordingly, such changes affect the sustainability of wells and domestic food security. Therefore, this study examines the current patterns in the use of domestic wells aiming to identify the factors affecting such behavioral changes and their impacts. Five Grama Niladari Divisions like Pathegama East, Thalalla North, Naotunna, Uda Aparekka, and Kadawadduwa East in Devinuwara Divisional Secretariat Division were selected purposively. The sample consists of 100 dug well owners. While the sampling was stratified random, the data analysis was carried out using SPSS 17.0. Findings show that only 24 % of well owners directly accessed to their wells daily to fulfill their water requirement. 76% of well owners did not directly access to their wells daily as they have pumped water. The main reasons for that were due to easiness and save time (33%), easiness and water depth (24%), location and water depth (14%), and the location of the wells (8%). According to the chi-square test, it was statistically proved that there is no relationship with adoption of pumped water to the distance of the well from home (P value= .627), the number of family members (P-value = .089), well depth (P-value =.174) and occupation of the housewives (P value=.107). Further, it was revealed that 60% of well owners did not use land near wells for crop cultivation. In contrast, 39% of householders have planted crops near their water tanks while 77% of respondents planted crops near their outdoor taps. 60% of well owners’ wives maintained their kitchen gardens using well water. 47% of well owners’ wives used once used well water to maintain their kitchen gardens while 53% of them did not utilize kitchen greywater for any purposes. Accordingly, this study statistically proved that there is a relationship between the attitude of the housewives regarding water conservation at home and greywater reuse (P value= .000). Further, this study also found negative impacts of the absence of direct connection with wells such as lack of knowledge about the changes in water level, water quality, domestic well condition, and dryness of surrounding well environment. This study highlights the significance of enhancing knowledge about managing and protecting well resources, domestic water conservation, water use efficiency, kitchen garden maintenance for sustainable water, and food security at the domestic village level.

Modeling of groundwater yield by the electrical method case of the Triassic sandstone aquifer (Tataouine South-Eastern Tunisia)

Abstract The method of electrical resistivity has proven very effective in the evaluation of groundwater. This specialized technique uses Dar-Zarrouk (D-Z) parameters in the estimation of longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity to examine the groundwater level, to distinguish the fresh, brackish, and saline water interface, and to assess the storage capacity of groundwater in the Triassic sandstone aquifer system in the Tataouine region (South-Eastern Tunisia). In this context, 23 vertical electrical soundings (VESs) were carried out in the Tataouine region using the Schlumberger configuration with a current electrode with a maximum spacing of the current electrodes (AB) of 500–600 m. The results indicate that the study area consists of three types of aquifers: (i) silt/clay saline water (<20 Ωm), (ii) a mixture of sand and clay freshwater (20–40 Ωm), and (iii) sand freshwater (40–200 Ωm). These sand freshwater aquifers are characterized by low longitudinal unit conductance (0–2.8 S), high values of transverse unit resistance (more than 9,000 Ωm2), and longitudinal resistivity (more than 35 Ωm) and are mainly concentrated in the north, south, and south-west regions of the study area. It should also be noted that the coefficient of anisotropy (λ) overlaps and does not clearly differentiate the characteristics of the aquifers of fresh, brackish, and saline water. An interpretation of VESs can also determine the storage capacity of groundwater by determining yield index values. Groundwater supply for the entire study area was classified as low yield, with a percentage of 13% and a maximum of 31% of the study area and 56% of moderate yield. Lastly, the real data from the drilling confirm all these results presented previously. The findings suggest that D-Z parameters are useful for making a distinction of various aquifer zones.

Monitoring geological storage of CO2: a new approach

Geological CO2 storage can be employed to reduce greenhouse gas emissions to the atmosphere. Depleted oil and gas reservoirs, deep saline aquifers, and coal beds are considered to be viable subsurface CO2 storage options. Remote monitoring is essential for observing CO2 plume migration and potential leak detection during and after injection. Leak detection is probably the main risk, though overall monitoring for the plume boundaries and verification of stored volumes are also necessary. There are many effective remote CO2 monitoring techniques with various benefits and limitations. We suggest a new approach using a combination of repeated seismic and electromagnetic surveys to delineate CO2 plume and estimate the gas saturation in a saline reservoir during the lifetime of a storage site. This study deals with the CO2 plume delineation and saturation estimation using a combination of seismic and electromagnetic or controlled-source electromagnetic (EM/CSEM) synthetic data. We assumed two scenarios over a period of 40 years; Case 1 was modeled assuming both seismic and EM repeated surveys were acquired, whereas, in Case 2, repeated EM surveys were taken with only before injection (baseline) 3D seismic data available. Our results show that monitoring the CO2 plume in terms of extent and saturation is possible both by (i) using a repeated seismic and electromagnetic, and (ii) using a baseline seismic in combination with repeated electromagnetic data. Due to the nature of the seismic and EM techniques, spatial coverage from the reservoir's base to the surface makes it possible to detect the CO2 plume’s lateral and vertical migration. However, the CSEM low resolution and depth uncertainties are some limitations that need consideration. These results also have implications for monitoring oil production—especially with water flooding, hydrocarbon exploration, and freshwater aquifer identification.

Pore resistivity variation by Resistivity imaging technique in sedimentary part of main Gadilam river basin, Cuddalore District, Tamil Nadu, India

Electrical resistivity is the only property of physics which give information of subsurface moisture content in the formation, Hence geophysical electrical resistivity survey was carried out to investigate the nature of shallow subsurface formations and geological contact in the main Gadilam river basin of Cuddalore District in Tamil Nadu. Twenty-seven vertical electrical soundings (VES) were conducted by Schlumberger configuration in the basin. Data is interpreted by curve matching techniques using IPI2 WIN software, layer parameters like apparent resistivity (?a) and thickness (h) interpretation were exported to Geographic Information System (GIS). Interpretation distinguishes three major geoelectric layers like topsoil, sandy clay layer, clayey sand layer along the contact zone in the basin. Interpreted VES sounding curves are mostly four-layer cases of QH, H, HA and KH type. Investigation demarcates lithology of subsurface and hydrogeological set up by employing maximum possible electrode sounding to infer saline water and freshwater occurrence based on resistivity signals. Zone of groundwater potential map was prepared with the combination of resistivity (?= ?1+ ?2+ ?3+ ?4) and corresponding thickness (T= T1+T2+T3+T4). High resistivity value of >200 ? m and low resistivity value of <10 ? m show the occurrence of alkaline and saline water within the formation aquifers as a result of possible rock water interaction and saline water dissolution. Four-layer resistivity cases from the matched curve (namely KH, AH, QA, and KA type) show the resistivity distribution/variation. It separates the freshwater depth wish from 1 to 140 ? m in fluvial sediments. Flood basin, sandstone and clay layer with low resistivity value of 3.16 - 7.5 ? m indicates contact with saline and freshwater aquifer. The Iso – resistivity map delineates saline water and freshwater zones with in the fourth layer cases in the same locations to indicate the irrational way of abstracting groundwater, resulting in saltwater ingress.

Groundwater quality index as a hydrochemical tool for monitoring saltwater intrusion into coastal freshwater aquifer of Eastern Dahomey Basin, Southwestern Nigeria

Saltwater intrusion into coastal freshwater aquifers is a threat to groundwater quality globally. This study aims to determine the extent of saltwater intrusion into the coastal freshwater aquifer of the Eastern Dahomey Basin (EDB), Nigeria. Groundwater chemistry was sampled and analysed for ionic ratios and interpreted using a hydrochemical facie evolution diagram (HFE-D), the saltwater mixing index (SMI) and the Groundwater quality index for saltwater intrusion (GQIswi). High EC and TDS and the concentration of dissolved ions showed increased salinity as a result of seawater intrusion in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. Correlation of ions in the wet season also suggests higher salinities which originate partly from industrial and municipal effluents especially from wells which are close to river channels, while dry season groundwater shows the dominant influence of seawater intrusion. HFE-D revealed that mixed groundwater of Na + Ca–HCO3, Na–Cl and Ca–Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi shows areas within 3 km from the coastline that are more sensitive to saltwater intrusion based on abstraction rate and depth of the wells. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin.

Designing of Cement Slurry to Enhance Compressive Strength and Rheology: An Experimental Study

The most essential function of oil and gas well cement is to provide isolation of zones in a production well. Cement isolates the wellbore in completion of oil and gas wells, prevents failure of casing, and it also prevents wellbore fluids so that they cannot be contaminated from fresh water aquifers. In slurry state cement is distinguished by rheological properties like plastic viscosity and yield stress. Proper selection of cement slurries plays a vital role in well commenting and that could be possible with the utilization of efficient materials that may results the superior rheological properties of cement slurry i.e. plastic viscosity, gel and yield strength. The selection of cement has been done on account of higher values of compressive strengths for which quality of cement sheath will be enhanced. Cement Slurry design can be optimized by operators for which it is mandatory for operators that they have knowledge about actual temperature that cement will encounter inside wellbore. Down hole temperatures of cementing have impact on thickening time, rheology, and compressive strength development. In this research work Rice Husk Ash (RHA) and Egg Shell Powder (ESP) are used in cement slurry so that compressive strength of cement specimen can be increased. The cement slurry has been made from locally manufactured cement, water, RHA and ESP. Cement slurry of 600ml by volume has been prepared. Specimens with size of 2×2×2 inch cubic molds are made for compressive strength testing at different curing conditions. Three variants of the specimen with varying concentrations of RHA and ESP are made. In present work during slurry state the rheological properties are determined while in solid state compressive strength of cement specimen at three curing ages of 08 hours, 24 hours and 72 hours is determined.

Mechanical properties of grain contacts in unconsolidated sands

Unconsolidated sands provide zones of high porosity and permeability important for freshwater aquifers, hydrocarbon production, and CO2 sequestration. An understanding of the acoustics of unconsolidated sands enables the characterization of such formations using ultrasonics, borehole acoustics, and seismic methods. Inversion of ultrasonic compressional and shear velocities measured for unloading as a function of confining pressure for room-dry unconsolidated sands allows information on the mechanical properties of the grain contacts to be obtained using an approach based on the divergence theorem. This allows the effective compliance of sand to be written as the sum of the compliance of the pores and of the grain contacts, and it does not assume that the grains are identical spheres, in contrast to previous approaches. Grain contacts are found to be more compliant under shear than under normal compression, and the ratio of the normal-to-shear compliance decreases with decreasing confining pressure, implying that the shear compliance increases faster with decreasing confining pressure than the normal compliance. This is of importance in understanding the role of shear in the failure of unconsolidated sands, such as occurs in shallow water flow, sanding, and failure around injectors, where the change in stress is a function of the normal-to-shear compliance ratio of the grain contacts.

New Understanding of Bar Top Hollows in Dryland Sandy Braided Rivers from Outcrops with Unmanned Aerial Vehicle and Ground Penetrating Radar Surveys

Bar top hollows (BTHs) are morphological elements recognized in modern braided rivers; however, information regarding their depositional features and types of filling units in ancient strata is unclear. This is an important reason behind why it is difficult to identify BTH units in subsurface reservoirs. A Middle Jurassic dryland sandy braided river outcrop in northwestern China is characterized in this study through the application of an unmanned aerial vehicle (UAV) surveying and mapping, and ground penetrating radar (GPR). A workflow of UAV data processing has been established, and a 3D digital outcrop model has been built. By observation and measurement of the outcrop model and GPR profiles, two types of BTH filled units were found: (a) sandstone-filled, and (b) mudstone-filled hollows. Both of these units were located between two adjacent bar units in an area that is limited to a compound bar, and were developed in the upper part of a braided bar depositional sequence. The ellipse-shaped, sandstone-filled unit measures 10 m × 27 m in map view and reaches a maximum thickness of 5 m. The transversal cross-section across the BTHs displays a concave upward basal surface, while the angle of the inclined structures infilling the BTHs decreases up-section. The GPR data show that, in the longitudinal profile, the basal surface is relatively flat, and low-angle, inclined layers can be observed in the lower- and middle part of the sandstone-filled BTHs. In contrast, no obvious depositional structures were observed in the mudstone-filled BTH in outcrop. The new understanding of BTH has a wide application, including the optimization of CO2 storage sites, fresh water aquifers exploration, and oil and gas reservoir characterization.

Numerical Investigation of Wellbore Methane Leakage From a Dual‐Porosity Reservoir and Subsequent Transport in Groundwater

AbstractThree‐dimensional, multiphase simulations are used to analyze migration of methane leakage from a hydrocarbon wellbore. The objective is to evaluate the relevance and importance of coupling fast, advective transport of methane through fractures with slower, diffusive transport in the shale matrix below a freshwater aquifer on water quality assuming dual‐domain mass transfer (DDMT) in the reservoir by using the multiple interacting continua (MINC) as implemented in TOUGH2. The conceptual model includes a methane gas‐phase leak from a wellbore 20–30 m below an aquifer; multiphase, buoyant transport through shale partially saturated with brine; and, after methane leakage reaches groundwater, multiphase transport under varying lateral groundwater flow gradients. Results suggest that DDMT affects the rate of methane reaching groundwater by (i) providing long‐time secondary storage in less‐mobile pore space and (ii) creating larger methane‐plume diameters than those predicted by a single‐domain advection‐diffusion equation. Compared to models without DDMT, these factors combine to increase methane flow rates by an order of magnitude across the base of the aquifer 100 years after leakage begins. In the simulated aquifer, dissolution of gas‐phase plumes leads to bimodal aqueous‐phase methane breakthrough curves in a simulated water well 100 m downstream from leakage, with peak concentrations appearing decades after a 1‐year pulse of leakage. The major implication is that DDMT in the reservoir can explain newly discovered methane concentrations in water wells attributable to older leakage events. Therefore, remediation of abandoned or legacy wells with wellbore integrity loss may be necessary to prevent future incidents of groundwater contamination.

Aquifer-CO2 leak project: Physicochemical characterization of the CO2 leakage impact on a carbonate shallow freshwater aquifer

This work is part of the Aquifer CO2-Leak project and aims to understand, quantify and model the environmental impact of a CO2 leak on water quality in the carbonate freshwater aquifer as well as CO2-water-carbonate interactions. The experiment has been performed within an Oligocene carbonate underground quarry located in Saint-Emilion (France).A water charged with dissolved CO2 was injected in the aquifer through a borehole. Downstream, seven wells were fitted with in-situ probes which automatically measured physicochemical parameters. Periodic water samplings in all wells have been undertaken to determine the elemental concentrations by ion chromatography.The spread of CO2 in the groundwater was monitored as a function of time and was observed to influence the various physicochemical parameters. Five parameters seem to be excellent indicators for monitoring a gas plume during CO2 geological storage in regard to our results: electrical conductivity and pH, and Ca2+, HCO3−, and CO2(aq) concentrations.The interaction between CO2 and limestone is highlighted by a saturation index (SI) calculated with PhreeqC software. It shows (i) a slight trend to dissolution of calcite in the injection well (SI < 0) linked to the reaction process between CO2-H2O-CaCO3 and (ii) a transport process via diffusion for the observation wells with a SI≃0.The evolution of physicogeochemical signatures in the aquifer allows us to understand the reactive and transport processes that occur during the migration of a gasified water plume in the context of leakage from a geological storage reservoir. Our results will make possible to model a leakage in a complex natural reservoir.