Authors: Martin Elsner¹, Carsten Vogt², Anett Georgi², Frank-Dieter Kopinke², Wolfgang Calmano³, Kathrin Schreglmann¹, Axel Bergmann⁴, Bernhard Mayer⁵, Franziska D.H.Wilke⁶

The "Chemicals in Hydrofracking for Natural Gas Extraction" technical committee is a group of experts of the German Water Chemistry Society (specialist group of GDCh, Gesellschaft Deutscher Chemiker e.V.). It is our aim to form an independent committee of experts that deals with current research questions regarding the environmental chemistry and water chemistry of fracking chemicals. Our contributions are intended to assist in better understanding of processes, identification of risks and development of solutions.

Few technologies have established themselves as quickly in the USA as hydraulic fracturing ("Fracking") for the extraction of unconventional natural gas deposits. Within only a few years, shale gas has made the USA independent from imports, reduced natural gas prices and provided the chemical industry with low-cost raw materials. However, at the same time, the concern about possible environmental pollution has led to controversial public debates. The focus of these are on the chemicals that play a part in the fracking process. The aim of this brief article is to point out current knowledge gaps from the point of view of water chemistry.

Regarding the chemistry of fracking operations

In order to give stability to a drill hole, to cool the boring tool and remove drillings, bentonite (a clay material) or polymer-based drill mud are used. Newly drilled sections are immediately fitted with pipework for stability and to provide protection from shallow groundwater and the pipework is cemented in. At the depth of the gas deposit, the pipework is selectively perforated and the surrounded cement is dissolved with acid (e.g. HCl), where necessary, in order to come into contact with the shale. With actual hydraulic fracturing, a fracturing liquid is subsequently injected at high pressure, in order to create fine fissures and cracks, so that the enclosed gas can escape and be produced. In order to keep these open, proppants are used, such as sand, ceramic, etc. Friction reducers (e.g. water-based polyamide gels or guar rubber solutions) and surface-active substances (e.g. organosulphates) make the liquid 'slipperier' and ensure better wetting. Cross-linked polymer substances (crosslinkers, e.g. ethanolamine mixed with borates and transition metal complexes) increase the viscosity in order to transport proppants. "Breakers" (oxidants, acids/base, enzymes and similar) subsequently "break up" the viscosity again, so that the gas can escape from the formation. Corrosion inhibitors (quinoline salts, sulphites), clay stabilisers (quaternary ammonium salts), iron complexing agents (e.g. citric acid) and biocides accompany the gas extraction and prevent plugging of the production well. Between 7 and 18 million litres of water are required for performing a single fracking operation. A significant part of this returns to the earth's surface as flowback, mixed with a rising proportion of formation water from deep underground.

Knowledge gaps regarding fracking chemicals and the biogeochemistry of the subsurface

Numerous fracking chemicals that are used in the USA are disclosed in Congress reports [1] and online databases (e.g. FracFocus; NGS Facts). Alphabetical lists show strong acids and base, oxidants and reducing agents, petroleum components and alcohols, fatty acids and their esters. Organosulphates and phosphates appear, as well as amines and quaternary ammonium salts. The large number of possible substances currently makes a systematic overview difficult. This is intensified by the fact that many substance cocktails are trade secrets and individual substances that account for less than 0.1% of the total volume do not need to be declared. Therefore, the general public is not yet aware of a prominent part of the fracking chemicals. A cost-benefit dialogue, which has already been conducted for other substances (e.g. pesticides) ("Why is this substance used and not a more environmentally friendly alternative?") and would lead to more acceptance is only slowly beginning to play a part in public debates. [see SHIP News]

Furthermore, prior to the initial boring, it is also difficult to evaluate the biogeochemistry of the deep subsurface. Black shale is known for being able to contain a large amount of organic material, heavy metals and radioactive nuclides. Knowledge gaps exist regarding the mobility of organic compounds, of heavy metals and radioactive elements during hydraulic stimulation. In exactly the same way, the microbiology of the deep subsurface is still largely unknown. Microorganisms can either be introduced unintentionally with the fracking liquid or originate from the subsurface itself. Heat-resistant microorganisms can live in pore space and possibly be released by the hydraulic breakage of the rock. Not all of them are necessarily killed by biocides in the fracking liquid.

Knowledge gaps regarding the processes in the subsurface

At a high temperature, high pressure and high concentrations of salt, fracking chemicals can enter chemical reactions, which differ significantly from those that we are familiar with from shallow groundwater. In addition to this, there are changing redox conditions during the fracking process (due to the addition of oxidants and reducing agents), so that geogene substances can also possibly be transformed into new products. Both can lead to potentially new organic transformation products being formed in the subsurface. For the sorption, precipitation and release of inorganic substances such as heavy metals, their species is crucial. In order to model this behaviour under conditions that prevail during the fracking process, the framework conditions are little known and furthermore dependent on the location. Therefore, it is not yet possible to forecast the release of problematic substances with certainty or to even prevent it through optimisation of the fracking process. Similarly, it is difficult to forecast microbial activity. On the one hand, e.g. microbial sulphide production can lead to corrosion, while on the other hand, microorganisms can also degrade fracking chemicals and therefore take on a natural cleaning function.

Need for research

Knowledge about the chemicals used, a characterisation of the conditions in the subsurface and research on the processes that are taking place there are important (a) for a hazard assessment (which substances would be released in the worst case?), (b) for optimisation of the fracking process (how can we avoid their release?), (c) for effective monitoring concepts (which substances should be look out for?) and (d) for secure and cost-efficient wastewater treatment (which substances need to be eliminated?). Water chemistry can potentially make important contributions to this. However, for these questions to be investigated, the industry must be willing to share information about fracking chemicals and provide independent scientific access to current fracking operations [2]. Future research in this field is therefore not only a scientific challenge, but also depends crucially on the requirements under which these contributions can be made at all.

Literature

1. Waxman, H.A., E.J. Markey, and D. DeGette, Chemicals used in hydraulic fracturing. 2011, United States House of Representatives, Committee on Energy and Commerce
2. Jackson, R.E., et al., Groundwater Protection and Unconventional Gas Extraction: The Critical Need for Field-Based Hydrogeological Research. Groundwater, 2013. 51(4): p. 488-510.

Contact

¹Helmholtz Centre Munich, German Research Centre for Environmental Health, Institute of Groundwater Ecology; Germany

²Helmholtz Centre for Environmental Research – UFZ, Department Isotope Biogeochemistry; Germany

³Hamburg University of Technology, Institute of Environmental Technology and Energy Economics; Germany

⁴IWW Water Centre, Germany

⁵University of Calgary, Department of Geoscience; Canada

⁶Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Germany