7,215 research outputs found
Recharge areas and geochemical evolution of groundwater in an alluvial aquifer system in the Sultanate of Oman
No full textA regional hydrogeochemical model was developed to evaluate the geochemical evolution of different groundwaters in an alluvial aquifer system in the Interior of Oman. In combination with environmental isotopes the model is able to extract qualitative and quantitative information about recharge, groundwater flow paths and hydraulic connections between different aquifers. The main source of water to the alluvial aquifer along the flow paths of Wadi Abyadh and Wadi M’uaydin in the piedmont is groundwater from the high-altitude areas of the Jabal Akhdar and local infiltration along the wadi channels. In contrast, the piedmont alluvial aquifer along Wadi Halfayn is primarily replenished by lateral recharge from the ophiolite foothills to the east besides smaller contributions from the Jabal Akhdar and local infiltration. Further down gradient in the Southern Alluvial Plain aquifer a significant source of recharge is direct infiltration of rain and surface runoff, originating from a moisture source that approaches Oman from the south. The model shows that the main geochemical evolution of the alluvial groundwaters occurs along the flow path from the piedmont to the Southern Alluvial Plain, where dedolomitization is responsible for the observed changes in the chemical and carbon isotope composition in these waters
Experimental evaluation of in situ CO2-water-rock reactions during CO2injection in basaltic rocks: Implications for geological CO2 sequestration
No full textDeep aquifers are potential long-term storage sites for anthropogenic CO2 emissions. The retention time and environmental safety of the injected CO2 depend on geologic and physical factors and on the chemical reactions between the CO2, the aquifer water, and the host rocks. The pH buffer capacity of the aquifer water and the acid neutralization potential of the host rocks are important factors for the permanent stabilization of the injected CO2. Mafic rocks, such as basalt, which primarily consists of Ca, Mg silicate minerals, have a high acid neutralization capacity by providing alkaline earth elements that form stable carbonate minerals. The carbonate minerals formed thus sequester CO2 in a chemically stable and environmentally benign form. In this study, we present results from a small-scale CO2 injection test in mafic and metasedimentary rocks. The injection test was conducted using a single-well push-pull test strategy. CO2 saturated water (pH = 3.5) was injected into a hydraulically isolated and permeable aquifer interval to study the acid neutralization capacity of Ca, Mg silicate rocks and to estimate in situ cation release rates. Release rates for Ca, Mg, and Na were calculated by use of solute compositions of water samples retrieved after the CO2 injection, the incubation time of the injected solution within the aquifer, and geometric estimates of the reactive surface area of the host rocks. Our results confirm rapid acid neutralization rates and water-rock reactions sufficient for safe and permanent storage of CO2. Carbonic acid was neutralized within hours of injection into a permeable mafic aquifer by two processes: mixing between the injected solution and the aquifer water, and water-rock reactions. Calculated cation release rates decrease with increasing pH that is confirmed by laboratory-based experiments. Large differences between release rates obtained from the field and laboratory experiments may be mainly due to uncertainties in the estimation of the reactive surface area in the field experiment and in hydrological and geological factors. Our results underscore the importance of defining bulk rock dissolution rates under in situ conditions in order to evaluate target formations for permanent mineral sequestration of carbon dioxide
Experimental study on mafic rock dissolution rates within CO2-seawater-rock systems
No full textFar-from-equilibrium batch experiments have been performed to study the low temperature dissolution potential of crystalline submarine basalts (from Juan de Fuca Plate and Mid-Atlantic Ridges) and of a highly altered gabbro from the Troodos ophiolite (Cyprus) in presence of seawater and carbon dioxide (CO2). The experiments have been carried out at 40 °C for up to 20 days with initial pH of ∼4.8 and under ∼1 bar pCO2 to identify the progressive water-rock interactions. Elemental steady-state release rates from the rock samples have been determined for silicon and calcium, the solution concentrations of which were found to be the most effective monitors of rock dissolution. Mass balance calculations based on dissolved Si and Ca concentrations suggest the operation of reaction mechanisms focussed on the grain surfaces that are characteristic of incongruent dissolution. Also, basic kinetic modelling highlights the role of mass-transport limitations during the experiments. Ca release rates at pH ∼ 5 indicate significant contributions of plagioclase dissolution in all the rocks, with an additional contribution of amphibole dissolution in the altered gabbro. Si release rates of all solids are found to be similar to previously studied reactions between seawater and basaltic glass and crystalline basalt from Iceland, but are higher than rates measured for groundwater-crystalline basalt interaction systems. This comparison with previous experimental results resumes the debate on the role of experimental variables, such initial rock mass and crystallinity, pCO2, and fluid chemistry on dissolution processes. Our new data suggest that CO2-rich saline solutions react with mafic rocks at higher rates than fresh water with low pCO2, at the same pH. Most significantly, both ophiolitic gabbro and Juan de Fuca basalts show Si and Ca release rates similar or higher than unaltered crystalline basalt from Iceland, highlighting the potential substantial role that ophiolitic rocks and offshore mafic reservoirs could play for the geological storage of CO2
Gravitational trapping of carbon dioxide in deep ocean sediments: hydraulic fracturing and mechanical stability
No full textGravitational trapping of carbon dioxide in deep ocean sediments is attractive both for the long term stability provided by gravity as well as the large volume and hence storage capacity of deep ocean sediments at necessary depths. Unfortunately, most pelagic sediments suffer from extremely low permeability and are not expected to have an overlying mechanical seal, making emplacement of CO2 contingent upon large scale hydraulic fracturing and some mechanism of arresting fracture growth before reaching the seafloor. An experimental design is presented with the capability of testing a variety of proposed fracture arrest mechanisms
Chemical and morphological changes during olivine carbonation for CO2 storage in the presence of NaCl and NaHCO3
No full textThe increasing concentrations of CO2 in the atmosphere are attributed to the rising consumption of fossil fuels for energy generation around the world. One of the most stable and environmentally benign methods of reducing atmospheric CO2 is by storing it as thermodynamically stable carbonate minerals. Olivine ((Mg,Fe)2SiO4) is an abundant mineral that reacts with CO2 to form Mg-carbonate. The carbonation of olivine can be enhanced by injecting solutions containing CO2 at high partial pressure into olivine-rich formations at high temperatures, or by performing ex situmineral carbonation in a reactor system with temperature and pressure control. In this study, the effects of NaHCO3and NaCl, whose roles in enhanced mineral carbonation have been debated, were investigated in detail along with the effects of temperature, CO2 partial pressure and reaction time for determining the extent of olivine carbonation and its associated chemical and morphological changes. At high temperature and high CO2 pressure conditions, more than 70% olivine carbonation was achieved in 3 hours in the presence of 0.64 M NaHCO3. In contrast, NaCl did not significantly affect olivine carbonation. As olivine was dissolved and carbonated, its pore volume, surface area and particle size were significantly changed and these changes influenced subsequent reactivity of olivine. Thus, for both long-term simulation of olivine carbonation in geologic formations and the ex situ reactor design, the morphological changes of olivine during its reaction with CO2 should be carefully considered in order to accurately estimate the CO2 storage capacity and understand the mechanisms for CO2 trapping by olivine
Reactive transport modelling insights into CO2 migration through sub-vertical fluid flow structures
No full textSub-vertical geological structures that cut through the overburden, usually called chimneys or pipes, are common in sedimentary basins. Chimneys behave as conduits that hydraulically connect deep strata with the overburden and seabed. Hence, if stored CO2 migrates to a sufficiently high permeability chimney the risk of CO2 leakage at the seabed increases. Despite the possible negative effects these structures may have on the integrity of CO2 storage sites, little is known about (i) their effective permeability distribution, controlled by the combined role of fractures and matrix, and (ii) feedback mechanisms between porosity-permeability, CO2 reactivity and mineralogy within them. Reactive transport modelling is used to perform 2D axisymmetric radial simulations of geological systems containing chimneys. CO2 saturations of 10%, 30% and 50% are imposed on a cell located next to the symmetry axis at the base of the model. Under hydrostatic conditions, CO2 reaches the seabed, at 500 m above the injection point, in less than 100 yr using injected CO2 saturations at or above 30% and with overburden isotropic permeabilities and chimney vertical permeabilities above 10−14 m2. Vertical fractures with apertures larger than 0.05 mm for volume fractions below 1% are sufficient to sustain such high vertical permeabilities in the chimney with a relatively high cap rock matrix permeability of 10−16 m2. Over 100 yr of CO2 injection, changes in porosity and permeability due to mineral precipitation/dissolution are negligible. For this time scale, in systems containing chimneys sufficiently far away from the injection well, the risk of CO2 leakage at the seabed is primarily controlled by the pre-existing hydrogeological state of the system
CO2 ionic trapping at meta-sedimentary aquifer, following a CO2 injection push-pull test
No full textIn order to study CO2-water-rock reactions relative to effectiveness of CO2 geological storage, small-scale CO2 injection experiments were performed, as single well push-pull tests, at the Lamont Doherty Earth Observatory test well site (New-York, USA). The injection interval was located at the contact zone between the chilled dolerite and the underlying metamorphosed sedimentary rocks. The variations of post-injection chemical and isotopic characteristics of retrieved water samples (major ions, DIC, 13CDIC) underline the CO2 reactivity in the aquifer and allow to identify reactions of the dissolved CO2 with the surrounding rocks, mainly the dissolution of carbonate minerals and complementary cation exchange
Elucidating the differences in the carbon mineralization behaviors of calcium and magnesium bearing alumino-silicates and magnesium silicates for CO<sub>2</sub> storage
No full textEngineering the permanent storage of CO2 in earth-abundant Ca- and Mg-bearing silicate and alumino-silicate rocks and minerals as carbonates requires a fundamental understanding of the extents of carbonate conversion that can be achieved at conditions relevant to geologic formations. While many studies have reported the reaction rates and the carbonation extents of specific minerals, the data is limited in terms of reaction conditions and the mineral samples were relatively pure to start with. Thus, understanding of the effect of the chemical and mineralogical heterogeneity on the carbon mineralization behaviors of various minerals and rocks in geologic conditions is lacking. Therefore, this study investigated the reactivities of a selection of minerals and rocks such as (a) Mg-rich olivine (Mg1.74Fe0.26SiO4) as previously reported by Gadikota and co-workers (2014), [1] labradorite (plagioclase feldspar with Ca0.53Na0.47Al1.53Si O8), (b) anorthosite (a mixture of plagioclase (Ca0.98Na0.02Al1.98Si2.02O8), olivine (Mg1.32Fe0.68SiO4) and magnetite (Fe3O4)), and (c) basalt (a fine-grained volcanic rock containing a mixture of plagioclase (Ca0.6Na0.4Al1.6Si2.4O8), calcic pyroxene (~Mg0.48, Fe0.52CaSi2O6) and low Ca pyroxene (~Mg0.48Fe0.52SiO3)), that are relevant to CO2 storage. The reaction conditions were also selected to mimic the conditions relevant to geologic CO2 storage sites (Tmax = 185 °C, Pmax = 164 bar, 0–1 M NaHCO3, 0–1 M NaCl, 1.0 M NaCl + 0.64 M NaHCO3). Our studies show that the extents of carbonation of olivine, labradorite, anorthosite, and basalt are 85, 35, 19 and 9%, respectively, when reacted for three hours at 185 °C, PCO2 of 139 atm in 1.0 M NaCl + 0.64 M NaHCO3 with 15 wt% solid reactant and a stirring rate of 800 rpm. Further, our results indicate that increasing the reaction temperature over the range of 90 to 185 °C, and increasing the concentration of NaHCO3 over the range of 0 to 1 M, both enhance the extent of carbon mineralization. On the other hand, increasing the partial pressure of CO2 from 64 atm to 169 atm and raising the concentration of NaCl to 1.0 M have no significant effects within the time-scale of these experimental studies. Comparison of our results with previous studies suggests that the reactivity of Ca- and Mg-bearing alumino-silicates is lower compared to Ca- and Mg-bearing silicates
Multimodal imaging and stochastic percolation simulation for improved quantification of effective porosity and surface area in vesicular basalt
No full textImproved methods for predicting fluid transport and vesicle connectivity in heterogeneous basalts are critical for determining the long-term reaction and trapping behavior of sequestered carbon dioxide and maximizing the efficiency of geothermal energy production. In this study we measured vesicle geometry, pore connectivity, and vesicle surface area of three basalt cores from the CarbFix carbon storage project in Iceland using a combination of micro-computed tomography, clinical computed tomography, and micro-positron emission tomography. A vesicle percolation simulator was then constructed to quantify vesicle connectivity across a complete range of porosities, pore size distributions, and microporosity conditions. Percolation simulations that incorporate important geologic features such as microporosity are able to describe the trend of experimental measurements made in this study and in previous work, without relying on statistical or empirical techniques. Simulation results highlight and quantify the trade-off between storage capacity and reactive surface area in high porosity basalts. Experiment and simulation results also indicate that there is very limited connected pore space below total porosity values of 15%, guiding improved site selection for large scale CO2 storage projects. Use of this stochastic percolation simulation method for basalt storage reservoir evaluation will enable more accurate storage capacity and mineral trapping estimates.</p
Parapatric speciation of Meiothermus in serpentinite-hosted aquifers in Oman
No full textThe factors that control the distribution and evolution of microbial life in subsurface environments remain enigmatic due to challenges associated with sampling fluids from discrete depth intervals via boreholes while avoiding mixing of fluids. Here, using an inflatable packer system, fracture waters were isolated and collected from three discrete depth intervals spanning >130 m in a borehole intersecting an ultramafic rock formation undergoing serpentinization in the Samail Ophiolite, Sultanate of Oman. Near surface aquifer waters were moderately reducing and had alkaline pH while deeper aquifer waters were reduced and had hyperalkaline pH, indicating extensive influence by serpentinization. Metagenomic sequencing and analysis of DNA from filtered biomass collected from discrete depth intervals revealed an abundance of aerobes in near surface waters and a greater proportion of anaerobes at depth. Yet the abundance of the putatively obligate aerobe, Meiothermus, increased with depth, providing an opportunity to evaluate the influence of chemical and spatial variation on its distribution and speciation. Two clades of Meiothermus metagenome assembled genomes (MAGs) were identified that correspond to surface and deep populations termed Types I (S) and II (D), respectively; both clades comprised an apparently Oman-specific lineage indicating a common ancestor. Type II (D) clade MAGs encoded fewer genes and were undergoing slower genome replication as inferred from read mapping. Further, single nucleotide variants (SNVs) and mobile genetic elements identified among MAGs revealed detectable, albeit limited, evidence for gene flow/recombination between spatially segregated Type I (S) and Type II (D) populations. Together, these observations indicate that chemical variation generated by serpentinization, combined with physical barriers that reduce/limit dispersal and gene flow, allowed for the parapatric speciation of Meiothermus in the Samail Ophiolite or a geologic precursor. Further, Meiothermus genomic data suggest that deep and shallow aquifer fluids in the Samail Ophiolite may mix over shorter time scales than has been previously estimated from geochemical data
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