Release of Particulate Iron Sulfide during Shale-Fluid Interaction

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Release of particulate iron sulfide during shale-fluid interaction Yevgeny Kreisserman, and Simon Emmanuel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05350 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Environmental Science & Technology

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Release of particulate iron sulfide during shale-fluid interaction

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Yevgeny Kreisserman, Simon Emmanuel*

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Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel

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*

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Abstract

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During hydraulic fracturing, a technique often used to extract hydrocarbons from

corresponding author, email: [email protected]

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shales, large volumes of water are injected into the subsurface. Although the injected

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fluid typically contains various reagents, it can become further contaminated by

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interaction with minerals present in the rocks. Pyrite, which is common in organic-

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rich shales, is a potential source of toxic elements, including arsenic and lead, and it is

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generally thought that for these elements to become mobilized, pyrite must first

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dissolve. Here, we use atomic force microscopy and environmental scanning electron

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microscopy to show that during fluid-rock interaction, the dissolution of carbonate

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minerals in Eagle Ford shale leads to the physical detachment, and mobilization, of

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embedded pyrite grains. In experiments carried out over a range of pH, salinity, and

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temperature we found that in all cases pyrite particles became detached from the shale

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surfaces. On average, the amount of pyrite detached was equivalent to 6.5×10-11 mol

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m-2 s-1, which is over an order of magnitude greater than the rate of pyrite oxidation

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expected under similar conditions. This result suggests that mechanical detachment of

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pyrite grains could be an important pathway for the mobilization of arsenic in

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hydraulic fracturing operations and in groundwater systems containing shales.

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1. Introduction

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Hydraulic fracturing is a commonly used technique to extract hydrocarbons from

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organic-rich shale formations deep in the subsurface. The method involves the

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injection of fluid into the shale, which induces rock fracture and the release of trapped

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oil and gas. The efficiency of this technique has increased significantly over the last

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decade due to the introduction of horizontal drilling techniques, and the number of 1 ACS Paragon Plus Environment


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such unconventional wells has jumped from 26,000 in 2000 to 300,000 by 20151.

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Although effective at extracting hydrocarbons, the use of hydraulic fracturing

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techniques has been controversial due to its potential environmental effects, which

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include the contamination of groundwater and surface water2–8

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One possible pathway for water reservoirs to become contaminated is via

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mixing with fracture fluids5,9. Such fluids contain contaminants both because of the

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reagents that are added prior to injection10,9, and the organic and inorganic

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contaminants that they acquire during water-rock interaction11,12. While the release of

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organic contaminants has been the focus of much recent research 13,2,14–18, the way in

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which inorganic contaminants enter the mobile fluid phase has received less attention.

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Ultimately, a better understanding of these pathways could help operators design

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injection fluids that will help minimize the levels of contamination.

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Fluids in contact with shales can react chemically with mineral assemblages

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within the rock, either by leaching sorbed species from mineral surfaces, or dissolving

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mineral phases. Some of these dissolving minerals, such as calcium carbonate, are

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relatively innocuous, while others, such as iron sulfides, can release toxic species into

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the fluid19,20. Dissolution of pyrite (FeS2) - which is a common component in organic

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rich shales, and typically has high concentrations of heavy metals and arsenic - has

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been implicated in the widespread contamination of groundwater in Bangladesh and

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India21,22,23. Furthermore, pyrite oxidation is often the main cause of pollution caused

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by acid mine drainage19,24.

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In addition to desorption and dissolution, fluid-rock interaction can also lead

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to the physical detachment of mineral grains and particles from the rock surface. It

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has been demonstrated that tiny clay and carbonate particles can be mobilized during

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the dissolution of limestone25,26, and a similar mechanism could also operate in shales.

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Importantly, colloidal particles released via this mechanism could be transported with

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the flowing fluid phase, contributing to the overall level of contamination. However,

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at present little is known about the amount of particulate matter that is released during

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shale-water interaction and what factors control its release.

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In this paper, we quantify the release of particulate pyrite during experiments that

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simulate the interaction of shale and water in the subsurface. In addition, we

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determine the effect of different conditions such as salinity, temperature, and flow rate

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on the rate of pyrite release. Finally, we discuss how our findings could be used to

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assess groundwater contamination from organic rich shales and hydraulic fracturing

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operations.

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2. Materials and methods

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2.1 Sample characterization

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We carried out a series of experiments on shale from the Eagle Ford formation from

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the Del Rio quarry in Texas. For the last decade, the Eagle Ford shale has been one of

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the most actively drilled plays in North America27. X-ray diffraction analyses (Bruker

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D8 Advance) on our samples indicate that calcite is the dominant mineral (61%;

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Table 1), and that pyrite is present in minor amounts (0.8%). Using RockEval, the

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total organic content (TOC) was determined to be 5.7%, which is within the range

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reported for the Eagle Ford formation28.

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2.2 In situ atomic force microscopy (AFM) imaging experiments

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To provide high resolution in situ imaging during fluid-rock interaction, and to

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simulate the way a fracture surface might behave, we performed a limited number of

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experiments with atomic force microscopy (AFM; Figure S1a). This technique has

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been used successfully to examine grain detachment in carbonate rocks in fluid

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environments25,29, although the experiments shown here are the first to be reported for

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shale samples. Samples were prepared from a 5.5 mm diameter shale core that was

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embedded in a Plexiglas disk and then polished by hand using a diamond-impregnated

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textile (Policloth; Fischer). The surfaces were then rinsed with deionized water and

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dried in air.

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Once prepared, the sample was sealed in a fluid imaging cell on the AFM scanner

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(Veeco Multimode 8 AFM with a NanoScope V controller and NanoScope 8.15

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software). Deionized water equilibrated with the atmosphere (pH 5.5) was injected

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constantly into the system at a rate of 2 ml hr−1. To track the evolution of the surface

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of the shale, the sample was imaged continuously during the experiments with silicon

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nitride probes (Bruker ScanAsyst Fluid+). Scans of regions up to 20 μm × 20 μm

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were performed at a line rate of 1 Hz with a resolution of 256 × 256 pixels using the

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Peakforce ScanAsyst non-contact imaging method. At this rate, image acquisition

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takes 4 min 16 s. The imaging mode we used produced a topographical map which



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provides the precise height of the surface. The surfaces of the samples were imaged

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both before and after the experiments using environmental scanning electron

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microscopy (ESEM; FEI Quanta 200), and the chemical identification of individual

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grains was made using energy dispersive X-ray spectroscopy (EDS).

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2.3 Ex-situ environmental scanning electron microscopy (ESEM) experiments

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Due to the increasing surface roughness during the fluid experiments, the duration of

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the AFM measurements was limited to several hours. To determine changes to the

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surface over longer times, we developed a protocol in which the sample was sealed in

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a Plexiglass flow-through cell into which a fluid was injected at a constant rate (15 ml

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h-1) using a peristaltic pump (Figure S1b). The residence time of water in the fluid cell

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was estimated to be approximately 40 seconds. During the experiment, the fluid cells

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were immersed in a refrigerated circulating bath which maintained temperatures at 25

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±0.01°C. Samples were exposed to the fluid for a total of 6-9 days, after which the

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experiments were terminated.

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At the beginning and end of each experiment, the samples were imaged using E-

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SEM in the backscattered electron (BSE) mode. By comparing the before and after

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images, we were able to identify bright grains that had been detached during the

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experiment. Using EDS analysis to identify the composition of individual grains, we

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found that >95% of the bright grains in the BSE images comprised pyrite. For each

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sample, multiple images were obtained and then stitched together in Photoshop to

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create mosaics with areas ranging from ~500,000 m2 to ~5,000,000 m2.

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Environmental conditions in the subsurface, as well as the chemistry of injected

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fluids, can vary significantly; to determine the influence of pH, salinity, and

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temperature on the release of pyrite particles, these three parameters were varied in

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the range 3-9, 0-35 g l-1, and 25-60oC respectively. To assess the variability of our

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results, repeat experiments were carried out, with a total of 16 experiments being

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performed (Table 2).

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To quantify the number and size of pyrite grains being detached from the surface,

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we developed a workflow based around the Matlab Image Processing Toolbox. The

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workflow involved four main steps: (i) registration of the before and after gray scale

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BSE images; (ii) subtraction of the before image from the after image; (iii) application

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of an intensity threshold to identify regions which had become significantly darker, 4 ACS Paragon Plus Environment

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which represented the possible formation of a pit in the surface; and (iv) identification

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of “blobs” in the thresholded image that were above a certain size, possessed a

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specified level of sphericity, and which had a similar level of brightness to pyrite in

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the original image. To filter out false positive identifications, each “blob” was

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inspected manually to determine whether or not a pyrite grain had indeed been

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removed. By normalizing to the total surface imaged, a minimum estimate was

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obtained for the number grains removed per unit area of shale surface with time. An

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equivalent grain diameter, D, was calculated from the area of each pyrite grain, Apy,

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according to D =(4Apy/)1/2. Assuming that the grains were spherical, the volume for

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each grain was calculated, and the molar amount was determined by dividing the

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volume by the molar volume of pyrite (2.39×10-5 m3 mol-1).

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To provide an indication of the structural and mineralogical changes occurring

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beneath the surface of the shale, we used a focused ion beam – scanning electron

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microscope (FIB-SEM; Raith e_LiNE - e-Beam Lithography System) to create, and

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then image, a 40 m wide by 20 m deep cross section. To determine the mineralogy

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of the grains exposed in the cross section, elemental maps of the cross section were

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obtained using energy dispersive X-ray spectroscopy.

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3. Results and discussion

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3.1 Pyrite dissolution and pyrite grain detachment

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During the in situ atomic force microscope experiments, we observed relatively rapid

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dissolution of carbonate phases, and even instances of carbonate grain detachment,

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which is consistent with the behavior reported for fine grain calcareous rocks30,31. By

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contrast, pyrite framboids and individual grains in the images did not appear to

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undergo significant chemical dissolution during the course of the experiments (Figure

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1). Over a 3 hour experiment, we estimated pyrite surfaces to dissolve less than 10 nm

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(the estimated uncertainty for the topographic measurements), indicating that

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dissolution rates must be below 10-3 nm s-1. From the rate the law proposed by

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Williamsen and Rimstidt (1994)32,

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,

Equation 1

is the dissolved oxygen concentration (estimated from Henry’s law to be

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where

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2.7×10-4 mol l-1) and

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oxidation for our experimental conditions is calculated to be 3.8×10-10 mol m-2 s-1.

is the proton concentration (10-5 mol l-1), the rate of pyrite



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Taking the molar volume of pyrite as 2.39×10-5 m3 mol-1, and assuming that all

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oxidized pyrite becomes effectively dissolved, pyrite surfaces should retreat at a

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maximal rate of ~9×10-6 nm s-1, which is entirely consistent with the upper bound we

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estimate from our measurements.

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Despite the very low rate of pyrite dissolution, we did observe instances of

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physical detachment of pyrite grains from the surface of the Eagle Ford shale. In

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Figure 1, a 4 m pyrite grain - surrounded by calcite – is visible in the first AFM

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image but is clearly absent from an image of the same area taken around 4 minutes

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later (Figure 1d). By contrast, no change is seen in the pyrite framboid present in the

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same image, indicating that rapid dissolution cannot account for the disappearance of

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the grain.

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In the ex-situ experiments, numerous instances of pyrite grain detachment

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were observed, and these grains were almost always embedded in calcite at the

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beginning of the experiment (Figure 2). By contrast, we did not observe any

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mobilization of pyrite grains surrounded by clay minerals and organic matter,

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suggesting that the dissolution of reactive carbonate phases is a critical step in the

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mobilization of pyrite grains. The effect of carbonate dissolution on the structure and

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porosity of the shale is evident from the FIB-SEM images (Figure 3). In the cross

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section cut at the surface of the sample that had not been exposed to fluid, pyrite

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framboids can be seen to be embedded in calcite. In addition, only a low level of

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porosity is visible. By contrast, in the cross section of the sample exposed to fluid for

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9 days, much less calcite is present and a well developed porous network has

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developed.

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3.2 Effect of salinity, temperature, and flow rate on pyrite grain detachment

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To determine the effect of environmental conditions on the amount of pyrite released,

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we carried out a series of experiments at varying pH, salinity, and temperature. Under

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all the different conditions, numerous instances of pyrite grain detachment were

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observed (Table 2; Figure 4). However, we observed significant variance between

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experiments carried out under identical conditions, making it impossible to identify

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any statistically significant trends (Figure 5). Such high variability is likely to be due

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to the difficulty of imaging representative regions in shale samples that exhibit a high

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degree of mineralogical and textural heterogeneity. Carrying out additional


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experiments, or imaging larger regions, could help reduce this uncertainty in future

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studies. Despite this high variability, what is clear from the experiments is that pyrite

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grain detachment occurs under a wide range of conditions that are likely to be

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encountered during hydraulic fracturing operations, as well as in other environmental

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scenarios.

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3.3 Implications of pyrite grain detachment for environmental systems

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When normalized to the area of shale exposed to fluid-rock interaction, the rate of

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release of particulate pyrite in our experiments is estimated to be in the range 2.8×10-

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12

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mol m-2 s-1). Assuming that the rate of pyrite oxidation represents the maximal rate of

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dissolution (i.e., all oxidized pyrite enters the solution), multiplying the rate of

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oxidation, calculated from Equation 1, by the proportion of the shale surface at which

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pyrite is exposed (0.8%) yields the effective rate of dissolution to be approximately

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3.0×10-12 mol m-2 s-1. Thus, in our experiments, the detachment of pyrite is on average

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22 times greater than the flux expected from pyrite oxidation. This simple calculation

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suggests that the physical detachment of pyrite grains could represent a significant

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mechanism for the mobilization of pyrite in subsurface systems. Crucially, pyrite

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often contains arsenic and toxic metals. As a result, during hydraulic fracturing

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operations, as well as in other groundwater systems exposed to pyrite (e.g., acid mine

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drainage), the mobility of these elements may be strongly influenced by the transport

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of colloidal iron sulfide particles.

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– 2.0×10-10 mol m-2 s-1, with an average of 6.5×10-11 mol m-2 s-1 (1 = 6.5×10-11

While our analysis provides insight into the mechanisms controlling pyrite

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mobilization, there are some clear limitations. Firstly, the rates of carbonate

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dissolution in our laboratory experiments may be more rapid than those found under

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field conditions33, especially after prolonged water-rock interaction. Thus, the actual

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rate of particulate release in the field, especially under high pH conditions, could be

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lower than that reported here. Moreover, carbonate minerals are not the dominant

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phase in many shales, and determining if pyrite particles detach from shales with

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different mineral assemblages will help us to assess the full extent of this mechanism

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in hydrogeological systems. Finally, even if the release of pyrite particles is

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widespread, it is unclear how far they can be transported within a real fracture

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network before settling or being oxidized. Addressing these questions, and looking for



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evidence for the mobilization of pyrite particles in the field, is the focus of ongoing

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research.

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Acknowledgments

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We thank the Israel Science Foundation for generous financial support and the Israel

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Ministry of National Infrastructures, Energy and Water. We also thank two

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anonymous reviewers and the associate editor, Richard Valentine, for their helpful

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comments.

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Figure 1: (a) Backscattered electron microscopy image of a shale surface before fluidrock interaction and (b) after 18 hours of contact with water. While the bright pyrite framboid in the center of the image remains unchanged, the pyrite grain, circled in red, disappears entirely. (c-d) Topographic maps of the same region imaged during the experiment with atomic force microscopy. The time elapsed between the two images is approximately 4 minutes, and the rapidity with which the pyrite grain (circled in red) disappears indicates that it has been physically detached from the surface.



Figure 2: Backscattered electron microscopy images of Eagle Ford shale (a,c,e) before interaction with fluid, and (b,d,f) after fluid contact. Note the complete removal of the pyrite grains circled in red.


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Figure 3: Back scattered electron microscopy images of cross sections of Eagle Ford shale (a) in a sample before interaction with fluid, and (b) in a sample exposed to 9 days of fluid flow. In the section imaged before exposure to fluid, pyrite framboids can be seen embedded in calcite and a relatively low level of porosity is present. By contrast, after exposure to fluid, the section is far more porous, presumably as a result of calcite dissolution. EDS analyses confirmed that the light gray phase in each image is pyrite.



Figure 4: (a)-(f) Histograms showing the sizes of pyrite grains released in the experiments. The pH, temperature, and salinity for each series of experiments is indicated. In (g), a summary of all the grains is given.


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Figure 5: Rate of pyrite particle release as a function of (a) pH; (b) salinity; and (c) temperature. Each point represents a single experiment. The high variance is probably a result of the heterogeneity of the shale samples and the relatively limited region imaged in each experiment. Unless otherwise indicated, experimental conditions were carried out at a pH of 5.5, at 25oC, and at a salinity of 0 g l-1.



Table 1: Mineral composition in the Eagle Ford shale sample. mineral calcite quartz kaolinite gypsum dolomite pyrite

% by mass 61.3 21.5 9.0 4.7 2.6 0.8

% 6.6 1.3 2.2 2.2 1.1 0.1


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Table 2: Summary of experiments. Experiment Salinity pH number [g/l]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5.5 5.5 5.5 5.5 3 3 9 9 5.7 5.7 5.8 5.8 6.1 6.1 6.1 6.1

0 0 0 0 0 0 0 0 3.5 3.5 35 35 0 0 0 0

Temperature [°C]

25 25 25 25 25 25 25 25 25 25 25 25 60 60 60 60

Duration Detachments [hr]

223 223 223 223 223 223 223 223 144 144 144 144 143 143 143 143

19 2 7 8 28 33 20 13 39 33 35 11 74 11 8 22

Area 2 [um ]

Detachments 2 per um

Volume of detached 3 pyrite [um ]

Rate of pyrite removal -2 -1 [mol m sec ]

4.7×105 7.1 ×105 8.4 ×105 1.1 ×106 2.6 ×106 1.6 ×106 1.0 ×106 1.7 ×106 3.7 ×106 3.5 ×106 2.9 ×106 3.3 ×106 4.4 ×106 4.4 ×106 4.5 ×106 4.5 ×106

4.1 ×10-5 2.8 ×10-6 8.3 ×10-6 7.6 ×10-6 1.1 ×10-5 2.0 ×10-5 1.9 ×10-5 7.7 ×10-6 1.0 ×10-5 9.3 ×10-6 1.2 ×10-5 3.3 ×10-6 1.7 ×10-5 2.5 ×10-6 1.8 ×10-6 4.8 ×10-6

471 73 126 390 3,711 816 1,243 178 6,738 6,377 6,138 415 10,775 513 157 5,806

5.3 ×10-11 5.4 ×10-12 7.8 ×10-12 1.9 ×10-11 7.3 ×10-11 2.6 ×10-11 6.2 ×10-11 5.5 ×10-12 1.5 ×10-10 1.5 ×10-10 1.7 ×10-10 1.0 ×10-11 2.0 ×10-10 9.4 ×10-12 2.8 ×10-12 1.0 ×10-10



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Page 18 of 18

Release of Particulate Iron Sulfide during Shale-Fluid Interaction

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