Prof. Karl-Heinz Rosenwinkel, Universität Hannover
The expert report examines the environmental pollution that occurs in connection with the storing, transport and disposal of waste water and waste matter produced by deep drilling to extract natural gas from unconventional reservoirs, including hydraulic fracking. It focuses initially on the characterisation and overall balance of the frac fluids used and the waste water produced (especially flowback). The findings serve as a basis for recommendations on water management, treatment, utilisation and disposal in accordance with the state of the art. Previous experience, especially from the USA, is incorporated in the assessment. It should be mentioned that fracs (volume and composition of frac fluids, process conditions and number) are adapted to the local conditions in the reservoir. To reproduce the different geological conditions for red sandstone (medium drilling depth), shale (flat wellbore) and carbonate rock (deep-well drilling under high pressure and at a high temperature) the well sites Damme 3, Buchhorst T12 and Cappeln Z3a were selected as example from among the many available well sites (217 in Lower Saxony alone). The breakdown, selection and preparation of the results were likewise based on the different types of rock.
Numerous reports drawn up by external laboratories, which analysed the fluids from different reservoirs (conventional and unconventional) both before and after hydraulic fracking, were available for quantity balancing and characterisation purposes. The assessment was based on drilling logs for fracs in various reservoirs as well as on current descriptions, plan documents and process charts for methods that were supplied by ExxonMobil or its subcontractors.
No detailed and robust chemical analysis of a flowback during and shortly after fracking is available to date. This makes it difficult to distinguish reservoir water without frac fluid from flowback (reservoir water with frac fluid) and to assess the pollution of the environment. The laboratory reports make it clear that the substances in the extracted reservoir water and the flowback are reservoir-specific. Most of the fluids examined are extremely salty or saturated with salt, and the salt content is mostly introduced and represented by means of the chloride content of the reservoir water. Therefore, the development over time of the chloride content in the flowback was ascertained and plotted in order to obtain an estimated quantitative determination of the percentages of frac fluid and reservoir water in the flowback.
There is no marked presence of radioactive substances in the analytical reports, because they are generally extracted with the solids. Nonetheless, these substances form part of the ongoing investigation and assessment.
The relevant technical regulations of the German Association for Water, Wastewater and Waste (DWA) and scientific publications were consulted, analysed and evaluated for the treatment and disposal of the waste water produced. The water law framework, acts and ordinances (e.g. the Ordinance on the Requirements for the Discharge of Wastewater into Surface Waters and its appendices) were examined for their applicability and for parallel uses of waste water of other origins; concentrations from the Drinking Water Ordinance were used as comparative values.
At present, the following conclusions can be drawn from the data and the methods chosen.
The design of a well site for exploration and production purposes is standardised. Construction work goes hand in hand with water management measures (in particular surface water and drilling fluid) in compliance with transferable regulations and the state of the art. Substantial improvements are needed in water management during hydraulic fracking and production. This applies to the treatment potential as well as to material flow management, monitoring and documentation.
Although a large volume of data was available, no clear-cut and final balance or differentiation of the mass flows of frac fluid and flowback can be undertaken. The same applies to an unambiguous classification of the residues. The substances contained in the flowback and reservoir water can be grouped in clusters of ‘hydrocarbons’ (including BTEX, PAK, etc.), metals and salts; groups of soluble and insoluble substances; and volatile substances. This enables specific treatment methods to be elaborated.
The formulation of the frac fluid and the amount used are selected in consideration of the specific reservoir conditions. Flowback volumes in the region of 8% were determined for Damme3. In the Marcellus area in the USA they are between 10% and 12%. ExxonMobil assumes a figure of 23% and the German Federal Environment Agency 20% to 80%. The operators have reported a reduction of the volumes used and, in particular, of chemicals for the frac fluids.
Determination of the percentage of reservoir water in the flowback coupled with knowledge of the input volumes makes it possible to ascertain the amount of frac fluid remaining in the underground at the end of the observation period. In two flowback analyses increased values of acetate and formate were recorded compared to reservoir water. No concluding study was carried out on whether acetate and formate, as degradation products of the polymers introduced below ground with the frac fluid, might possibly serve as an indicator of frac fluid components in the flowback.
The results from observations of the contents in the flowback show a marked increase in the share of chloride over time. Hence the percentage of chloride could serve as an indicator for estimation of reservoir water in the flowback. It should be noted that the reservoir water percentage dominates in the flowbacks studied and corresponds more or less with the reservoir water that occurs in conventional reservoirs. At present no methods for the treatment of flowback and reservoir water can be described as state of the art. Relevant process technologies were highlighted that could, in principle, be considered. However, their specific suitability for these waste waters has yet to be established. There are no special German requirements for the methods used in the USA (for irrigation, treatment in municipal plants, treatment and direct discharge, processing and re-use). Here again, processing and re-use are only a partial solution to the problem. The same applies to deep well injection.
However, more precise experiments and analyses are needed for a reliable evaluation. The same is true of the treatment of radioactive polluted reservoir waste water.
ExxonMobil currently regards the treatment of waste water as possible, but not viable for economic reasons. Hence its preference is for the deep well injection, further research is needed for the development of a sustainable solution.
