Element release and reaction-induced porosity alteration during shale-hydraulic fracturing fluid interactions

The use of hydraulic fracturing techniques to extract oil and gas from low permeability shale reservoirs has increased significantly in recent years. During hydraulic fracturing, large volumes of water, often acidic and oxic, are injected into shale formations. This drives fluid-rock interaction that can release metal contaminants (e.g., U, Pb) and alter the permeability of the rock, impacting the transport and recovery of water, hydrocarbons, and contaminants. To identify the key geochemical processes that occur upon exposure of shales to hydraulic fracturing fluid, we investigated the chemical interaction of hydraulic fracturing fluids with a variety of shales of different mineralogical texture and composition. Batch reactor experiments revealed that the dissolution of both pyrite and carbonate minerals occurred rapidly, releasing metal contaminants and generating porosity. Oxidation of pyrite and aqueous Fe drove precipitation of Fe(III)-(oxy)hydroxides that attenuated the release of these contaminants via co-precipitation and/or adsorption. The precipitation of these (oxy)hydroxides appeared to limit the extent of pyrite reaction. Enhanced removal of metals and contaminants in reactors with higher fluid pH was inferred to reflect increased Fe-(oxy)hydroxide precipitation associated with more rapid aqueous Fe(II) oxidation. The precipitation of both Al- and Fe-bearing phases revealed the potential for the occlusion of pores and fracture apertures, whereas the selective dissolution of calcite generated porosity. These pore-scale alterations of shale texture and the cycling of contaminants indicate that chemical interactions between shales and hydraulic fracturing fluids may exert an important control on the efficiency of hydraulic fracturing operations and the quality of water recovered at the surface

Ion Transport in Fractured Shale

International audienceRecent modeling and experimental studies elucidate the controls on ion transport in fractured shale. Modeling based on the code CrunchClay is presented for a fracture-clay matrix system that includes electrostatic effects on transport. The electrostatic effects include those associated with the development of a diffusion potential as captured by the Nernst-Planck equation, and the formation of a diffuse layer (EDL) bordering negatively charged clay particles within which partial anion exclusion occurs. A dual continuum formulation accounts for diffuse layer and bulk water pore space, with the diffuse layer model obtained by volume averaging ion concentrations in the Poisson-Boltzmann equation. The simulation results demonstrate the lack of retardation for anions (e.g., 36Cl‑) of the contaminant plume within the fracture flow system because they are largely excluded from the charged clay rock, while the migration of cations that accumulate in the EDL (e.g., 90Sr++) are strongly attenuated. This behavior has been validated experimentally in transport experiments conducted in the Wolfcamp Shale. Samples without fractures showing the typical behavior in shale, with strong anion retardation due to their exclusion in the EDL. In contrast, where fractures are present, bromide (the representative anion) breaks through earlier than uncharged solutes (D2O) and cations due to the fact that they can migrate down the electrically neutral fracture with no retardation from matrix diffusion

Ion Transport in Fractured Shale

International audienceRecent modeling and experimental studies elucidate the controls on ion transport in fractured shale. Modeling based on the code CrunchClay is presented for a fracture-clay matrix system that includes electrostatic effects on transport. The electrostatic effects include those associated with the development of a diffusion potential as captured by the Nernst-Planck equation, and the formation of a diffuse layer (EDL) bordering negatively charged clay particles within which partial anion exclusion occurs. A dual continuum formulation accounts for diffuse layer and bulk water pore space, with the diffuse layer model obtained by volume averaging ion concentrations in the Poisson-Boltzmann equation. The simulation results demonstrate the lack of retardation for anions (e.g., 36Cl‑) of the contaminant plume within the fracture flow system because they are largely excluded from the charged clay rock, while the migration of cations that accumulate in the EDL (e.g., 90Sr++) are strongly attenuated. This behavior has been validated experimentally in transport experiments conducted in the Wolfcamp Shale. Samples without fractures showing the typical behavior in shale, with strong anion retardation due to their exclusion in the EDL. In contrast, where fractures are present, bromide (the representative anion) breaks through earlier than uncharged solutes (D2O) and cations due to the fact that they can migrate down the electrically neutral fracture with no retardation from matrix diffusion

Small-scale studies of roasted ore waste reveal extreme ranges of stable mercury isotope signatures

Active and closed Hg mines are significant sources of Hg contamination to the environment, mainly due to large volumes of mine waste material disposed of on-site. The application of Hg isotopes as source tracer from such contaminated sites requires knowledge of the Hg isotope signatures of different materials potentially released to the environment. Previous work has shown that calcine, the waste residue of the on-site ore roasting process, can exhibit distinct Hg isotope signatures compared with the primary ore. Here, we report results from a detailed small-scale study of Hg isotope variations in calcine collected from the closed New Idria Hg mine, San Benito County, CA, USA. The calcine samples exhibited different internal layering features which were investigated using optical microscopy, micro X-ray fluorescence, micro X-ray absorption spectroscopy (mu-XAS), and stable Hg isotope analysis. Significant Fe, S, and Hg concentration gradients were found across the different internal layers. Isotopic analyses revealed an extreme variation with pronounced isotopic gradients across the internal layered features. Overall, delta Hg-202(+/- 0.10 parts per thousand, 2 SD) describing mass-dependent fractionation (MDF) ranged from -5.96 to 14.49 parts per thousand, which is by far the largest range of delta Hg-202 values reported for any environmental sample. In addition, Delta Hg-199 (+/- 0.06 parts per thousand, 2 SD) describing mass-independent fractionation (MIF) ranged from -0.17 to 0.21 parts per thousand. The mu-XAS analyses suggested that cinnabar and metacinnabar are the dominant Hg-bearing phases in the calcine. Our results demonstrate that the incomplete roasting of HgS ores in Hg mines can cause extreme mass-dependent Hg isotope fractionations at the scale of individual calcine pieces with enrichments in both light and heavy Hg isotopes relative to the primary ore signatures. This finding has important implications for the application of Hg isotopes as potential source tracers for Hg released to the environment from closed Hg mines and highlights the need for detailed source signature identification. (C) 2014 Elsevier Ltd. All rights reserved

Arsenic sequestration by sorption processes in high-iron sediments

High-iron sediments in North Haiwee Reservoir (Olancha, CA), resulting from water treatment for removal of elevated dissolved arsenic in the Los Angeles Aqueduct system, were studied to examine arsenic partitioning between solid phases and porewaters undergoing shallow burial. To reduce arsenic in drinking water supplies, ferric chloride and a cationic polymer coagulant are added to the aqueduct upstream of Haiwee Reservoir, forming an iron-rich floc that scavenges arsenic from the water. Analysis by synchrotron X-ray absorption spectroscopy (XAS) showed that the aqueduct precipitate is an amorphous hydrous ferric oxide (HFO) similar to ferrihydrite, and that arsenic is associated with the floc as adsorbed and/or coprecipitated As(V). Arsenic-rich floc and sediments are deposited along the inlet channel as aqueduct waters enter the reservoir. Sediment core samples were collected in two consecutive years from the edge of the reservoir along the inlet channel using 30- or 90-cm push cores. Cores were analyzed for total and extractable arsenic and iron concentrations. Arsenic and iron speciation and mineralogy in sediments were examined at selected depths by synchrotron XAS and X-ray diffraction (XRD). Sediment–porewater measurements were made adjacent to the core sample sites using polyacrylamide gel probe samplers. Results showed that sediment As(V) is reduced to As(III) in all cores at or near the sediment–water interface (0–4 cm), and only As(III) was observed in deeper sediments. Analyses of EXAFS spectra indicated that arsenic is present in the sediments mostly as a bidentate–binuclear, inner-sphere sorption complex with local atomic geometries similar to those found in laboratory studies. Below about 10 cm depth, XAS indicated that the HFO floc had been reduced to a mixed Fe(II, III) solid with a local structure similar to that of synthetic green rust (GR) but with a slightly contracted average interatomic Fe–Fe distance in the hydroxide layer. There was no evidence from XRD for the formation of a crystalline GR phase. The release of dissolved iron (presumably Fe^(2+)) and arsenic to solution, as monitored by in situ gel probes, was variable but, in general, occurred at greater depths than arsenic reduction in the sediments by spectroscopic observations and appears to be near or below the depth at which sediment GR was identified. These data point to reductive dissolution of the sorbent iron phase as the primary mechanism of release of sorbed arsenic to solution

Effect of Chloride on the Dissolution Rate of Silver Nanoparticles and Toxicity to E. coli

PMID: 23641814International audiencePristine silver nanoparticles (AgNPs) are not chemically stable in the environment and react strongly with inorganic ligands such as sulfide and chloride once the silver is oxidized. Understanding the environmental transformations of AgNPs in the presence of specific inorganic ligands is crucial to determining their fate and toxicity in the environment. Chloride (Cl–) is a ubiquitous ligand with a strong affinity for oxidized silver and is often present in natural waters and in bacterial growth media. Though chloride can strongly affect toxicity results for AgNPs, their interaction is rarely considered and is challenging to study because of the numerous soluble and solid Ag–Cl species that can form depending on the Cl/Ag ratio. Consequently, little is known about the stability and dissolution kinetics of AgNPs in the presence of chloride ions. Our study focuses on the dissolution behavior of AgNPs in chloride-containing systems and also investigates the effect of chloride on the growth inhibition of E.coli (ATCC strain 33876) caused by Ag toxicity. Our results suggest that the kinetics of dissolution are strongly dependent on the Cl/Ag ratio and can be interpreted using the thermodynamically expected speciation of Ag in the presence of chloride. We also show that the toxicity of AgNPs to E.coli at various Cl– concentrations is governed by the amount of dissolved AgClx(x–1)– species suggesting an ion effect rather than a nanoparticle effect

Theoretical and experimental investigations of mercury adsorption on hematite surfaces

<p>One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior and speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with laboratory-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe₂O₃). According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The projected density of states (PDOS) analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary, α-Fe₂O₃ has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl. For the laboratory-scale experiments, three types of α-Fe₂O₃ nanoparticles were prepared using the precursors Fe(NO₃)₃, Fe(ClO₄)₃, and FeCl₃, respectively. The particle shapes varied from diamond to irregular stepped and subrounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m²/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 μg Hg/g. The Hg L_III-edge extended X-ray absorption fine structure spectroscopy also indicated that HgCl₂ physisorbed onto the α-Fe₂O₃ nanoparticles.</p> <p><i>Implications</i>: Atomic-scale theoretical simulations proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. Following laboratory-scale experiment of Hg sorption also shows that HgCl₂ physisorbs onto α-Fe₂O₃ nanoparticles which have highly stepped structure.</p

Shale Kerogen: Hydraulic Fracturing Fluid Interactions and Contaminant Release

The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for 2 weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen isolates were in fact altered, although by apparently different mechanisms. In particular, hydrophobic functional groups decreased in the Marcellus kerogen, which suggests the wettability of shale organic matter may be susceptible to alteration during hydraulic fracturing operations. About 1% of organic carbon in the more immature and Type I Green River kerogen isolate was solubilized when it was exposed to fracturing fluid, and the released organic compounds significantly impacted Fe oxidation. On the basis of the alteration observed in both kerogen isolates, it should not be assumed that kerogenic pores are chemically inert over the time frame of hydraulic fracturing operations. Shifts in functional group composition and loss of hydrophobicity have the potential to degrade transport and storage parameters such as wettability, which could alter hydrocarbon and fracturing fluid transport through shale. Additionally, reaction of Green River and Marcellus kerogen isolates with low pH solutions (full fracturing fluid, which contains hydrochloric acid, or pH 2 water) mobilized potential trace metal­(loid) contaminants, primarily S, Fe, Co, Ni, Zn, and Pb. The source of trace metal­(loid)­s varied between the two kerogen isolates, with metals in the Marcellus shale largely sourced from pyrite impurities, whereas metals in the Green River shale were sourced from a combination of accessory minerals and kerogen