Metals and radionuclides (MaR) in the Alum Shale of Denmark: Identification of MaR-bearing phases for the better management of hydraulic fracturing waters

doi:10.1016/j.jngse.2018.02.015

Journal of Natural Gas Science and Engineering

Volume 53, May 2018, Pages 139-152

https://doi.org/10.1016/j.jngse.2018.02.015 Get rights and content

Highlights

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Alum Shale quantitative mineralogy to identify metal bearing phases.
•
Pyrites and organic matter are the main metal bearing phase.
•
Precise localization of metals allows the management of the fracking fluids.

Abstract

Hydraulic fracking is used to enhance the production of tight gas reservoirs. Because shale reservoirs can contain toxic elements (metals and radionuclides), the release rates of these elements must be quantified in order to determine the possible environmental impact of fracking. This paper is devoted to the complete and precise determination of the mineralogy of the Alum Shale in Denmark, which is known for its high content of gaseous hydrocarbons. Its metal-bearing phases are identified and quantified using complementary analytical techniques (i.e., X-ray diffraction, electron microscopy and electron probe analysis, and X-ray tomography). A detailed quantitative mineralogical composition is calculated using three different approaches (i.e., matrix inversion, quantitative X-ray diffraction, and the MQ program), which is then used to determine the quantity of polluting elements in each phase. Pyrite (FeS₂) is the major metal-bearing phase (e.g., As, Cu, Co, Ni, Pb, Zn, V, U). Elements such as V, Ra, Cs, Be, Cr, Ba are trapped in clay minerals, whereas U, Cd, Mo, and Hg are present in organic matter. It is essential to better identify toxic element-bearing phases to formulate fracking fluids with the lowest possible chemical reactivity in order to avoid the release of pollution by flowback waters.

Introduction

The term “black shales” refers to dark detrital sediments with silt- and clay-size minerals in which the organic matter content is greater than 2% (Swanson, 1961). These deposits occur in anoxic or low-oxygen basins, thus explaining the conservation of organic matter and the formation of sulfides. Black shales constitute an important trap for Metals and Radionuclides (MaR) (Vine and Tourtelot, 1970; Levanthal, 1991). The major MaR-bearing phases are sulfides, organic matter and clay minerals (Levanthal, 1991; Tuttle et al., 2009).

These sediments may constitute source rocks for oil and gas; hence, they are referred to as oil shale and gas shale. Black shales are very low-permeability rocks; for this reason, it is particularly difficult to extract gases from them. Indeed, they do not naturally migrate to the wellbore, and their extraction is only possible by artificially increasing the permeability using horizontal drilling and hydraulic fracturing techniques (Clark, 1947). The fracking fluid comprises water mixed with sand and chemical additives that is injected under high pressure into the rock. Sand grains (proppant) are co-injected to keep fractures open. These artificial fractures drain liquids and gases from the rock. The fracturing sequence is usually described as follows: i) a small volume of acid is injected in order to clean the perforations of the well and increase the porosity and the permeability of the formation by dissolving carbonates, and ii) the fracturing fluid associated with the sand is injected in order to create multiple fractures. Following the fracturing treatment, the well is connected to the production network. Initially, some fracturing flowback water will be produced in association with hydrocarbons. The water that returns to the surface (i.e., the flowback water) is a brine solution that usually has a high salt content and may contain high concentrations of metals and organic compounds (Gregory et al., 2011; Yang et al., 2017). It should be highlighted that oxidizing and acidic fluids have a dissolving effect on minerals containing metals and radionuclides that are present in the shales (Gregory et al., 2011).

The contamination of fracturing flowback water can have one of two origins: (i) the chemicals added to the fracking fluids (Entrekin et al., 2011; Rozell and Reaven, 2012; Rahm, 2011) and (ii) the metals and radionuclides that may be released by the shales at the end of the fracking operation (Gregory et al., 2011; Gaucher et al., 2014). The concentration of total dissolved solids (TDS) in flowback waters can reach 5 times that of seawater (Gregory et al., 2011). MaR could pollute surface waters, aquifers and soils if flowback waters were accidentally dispersed within the environment. Risks for human health could also be evoked if workers were to experience dermal exposure (Durant et al., 2016). The high concentrations of TDS in flowback waters may also induce additional treatment costs prior to disposal or recycling.

The Alum Shale was targeted for gas shale exploration by the petroleum company TOTAL in Europe, using an exploration well in the Jutland area (August 2015). The Alum Shale, which covers an area of more than 10,000 km, has been exploited in both Sweden and Poland during the last 5 years (Gautier et al., 2013; Schovsbo et al., 2014). Interesting results but low production rates have been obtained in Sweden at a low depth (Pool et al., 2012). Alum Shale-equivalent shales have been explored for shale gas in the Baltic basin in the northernmost parts of Poland. Here, gas has been reported to flow from stimulated wells, albeit at low production rates. Outcrops also exist on Bornholm Island, where scientific boreholes were drilled in a joint-venture program led by the GEUS (Geological Survey of Denmark and Greenland).

In this paper, we present the results obtained from the Sommerodde-1 scientific well, which was drilled in Bornholm in November 2012 (Gaucher et al., 2014; Sterpenich et al., 2017). An exhaustive detailed mineralogical study (using XRD: X-ray Diffraction; SEM: Scanning Electronic Microscopy; EPMA: Electron Probe Micro-Analysis; TEM: Transmission Electronic Microscopy; bulk analysis; and X-ray tomography) of the Alum Shale is performed to quantify as precisely as possible the mineralogical and chemical composition of the rock, as well as the concentrations and partition coefficients of MaR. Having a better understanding of the associated downhole chemical reactions between fracturing fluid and geological formations will help improve the formulation of the fracturing fluid to minimize or eliminate the concentrations of metals in the fracturing flowback water.

Section snippets

Geological setting

The study area is located in the southeastern part of Bornholm Island (Denmark), which is located to the south of Sweden (Fig. 1A). The rock samples were collected from the Sommerodde-1 well. This well is located in the southeastern region of Bornholm (Fig. 1B). The stratigraphic subdivision of the Sommerodde-1 well was described by Schovsbo et al. (2015).

The Alum Shale Formation comprises a dark, organic-rich mudstone (up to 25 wt.% total organic carbon, TOC) with abundant disseminated pyrite

Samples

A total of 6 core samples were investigated. Two samples were collected from black shales (i.e., the Rastrites Shale and the Dicellograptus Shale) and are compared here with four ones that were collected from the Alum Shale (Fig. 1C).

In the Alum Shale, the core samples were chosen based on their GR values (API units), which represent their concentrations of radioelements (²³²Th, ²³⁸U and ⁴ K, Ellis and Singer, 2007): these samples include SOM1-218 and SOM1-238, which have intermediate GR values

Petrographic analysis

X-ray tomography was performed on the Alum Shale samples to observe their internal features. Fig. 2A shows a rock sample with a small grain size, considering the millimeter scale of the picture. At the observation scale, no visible porosity or fractures are observed. The shale is rich in heavy phases (the denser or heavier a phase, the whiter it appears in tomography), which are mainly sulfide minerals, such as pyrite (and, to a lesser extent, barite). Pyrites are concentrated in layers that

Quantitative mineralogy

A precise quantification of shale mineralogy is required to prevent the fracking of zones that are rich in swelling clays; it is also essential to calculate mass balances during the numerical modeling of the interactions between fracking fluids and rock. The proportions of mineral phases were quantified using three different methods that will be compared here, as it is difficult to determine the quantitative mineralogy of clay-rich rocks using XRD alone. The chosen methods are:

(i)
Matrix

Conclusion and perspectives

The Alum Shale was studied for its potential as a gas resource in Europe. This mineralogical study shows that the Alum Shale in the Bornholm area is composed of quartz and micas, which are the main minerals, and carbonates (i.e., calcite, dolomite, ankerite, siderite), Na/K-feldspars, pyrite and rutile, which are the secondary minerals. The mineralogical assemblage of the Alum Shale is similar to the assemblages observed in other shale gases around the world (e.g., the Marcellus Shale, Barnett

Aknowledgements

Our thanks go to Sandrine Vidal-Gilbert, Adeline Garnier, Emmanuel Derbez, Patrick Baldoni-Andrey, and Monica Burgos from Total for their technical assistance. Christophe Morlot is acknowledged for assisting with X-ray tomography images, Olivier Rouer is acknowledged for assisting with EPMA analyses, and Lise Salsi is acknowledged for assisting with SEM images. We also thank four anonymous reviewers who helped to improve the quality of this paper.

The study was financially supported by the TOTAL

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Metals and radionuclides (MaR) in the Alum Shale of Denmark: Identification of MaR-bearing phases for the better management of hydraulic fracturing waters

Highlights

Abstract

Introduction

Section snippets

Geological setting

Samples

Petrographic analysis

Quantitative mineralogy

Conclusion and perspectives

Aknowledgements

Geochem. Cosmochim. Acta

Chem. Geol.

Org. Geochem.

Chem. Geol.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

J. Nat. Gas Sci. Eng.

J. Geochem. Explor.

Int. J. Coal Geol.

Ore Geol. Rev.

Geochem. Cosmochim. Acta

Int. J. Coal Geol.

Chem. Geol.

Geochem. Cosmochim. Acta

Geochem. Cosmochim. Acta

Int. J. Coal Petrol.

Energy Pol.

Geochim. Cosmochim. Acta

Procedia Earth and Planetary Science

Appl. Geochem.

Geochim. Cosmochim. Acta

J. Nat. Gas Sci. Eng.

The scandinavian alum shales

Sveriges Geologiska Undersökning Ca

IUCr Commission on Powder Diffraction Newsletter

Pyrite geochemistry in the Toarcian Posidonia Shale of southwest Germany: evidence for contrasting trace-element patterns of diagenetic and syngenetic pyrites

Sedimentology

The inorganic geochemistry of Cretaceous black shales (DSDP Leg 41) in comparison to modern upwelling sediments from the Gulf of California

Geol. Soc. Lond. Spec. Publ.

Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian-Ordovician Alum Shale, southern Scandinavia

AAPG (Am. Assoc. Pet. Geol.) Bull.

Carbon and oxygen isotope composition of Cambro-Silurian limestone and anthraconite from Bornholm: evidence for deep burial diagenesis

Bull. Geol. Soc. Den.

Alun Skiferen i Skandinavien

Geologisk Tidskift

On-thefly Control of Delayed Borate-crosslinking of Fracturing Fluids. US Patent 5 877 127, Assigned to Schlumberger Technology Corporation, Sugar Land, TX, 2 March 1999

Routine analyses of trace element in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS: a study of geochemical reference materials BR, DR-N, UB-N, AN-G and GH

Geostand. Newsl.

A hydraulic process for increasing the productivity of wells

J. Petrol. Technol.