Produced water chemistry - Analysis of produced water from the Danish North Sea: Inorganic ions, heavy metals and organic compounds

Sofie Nitsche Bergfors*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Water constitutes approximately half of the fluid volume produced at part of the oil production in the Danish North Sea. The presented PhD study examines the chemical composition of produced water, both experimentally and computationally, to shred light on the temporal and regional variations in the water composition. The findings are discussed in a context of production strategies, scale and corrosion mitigation strategies and adds to a pure geochemical understanding. Included in the study are the fours fields: Dan, Halfdan, Kraka and Valdemar. At Dan and Halfdan, seawater is injected for improved oil recovery and pressure support, but the accompanying consequences for reservoir quality and the potential for further recovery are not fully evaluated. Also, a need for a more sensitive analytical workflow for the analysis of Ba2+ has been identified for an accurate prediction of hard scale formation. Principal component analysis is used to study an extensive data set for the four included fields covering the produced water chemistry (Na+, K+, Ca2+, Mg2+, Sr2+, Ba2+, Cl− and SO2− 4 ) from start of production up to and including 2015. Five water types are classified; these differ mainly by salinity and the concentration of the divalent ions. These are used as input for scale predictive calculations, which show a high risk of potential hard scale formation in many wells. Additionally, the findings suggest a connectivity between the close-lying fields, Dan and Halfdan. The quality of the data set discussed above has been examined experimentally from a comparison between data and new analytical results of similar samples; the Halfdan field was used as a case study. For this purpose, and to improve the analytical sensitivity for all ions, but especially Ba2+, a new analytical workflow is introduced utilising the benefits of inductively coupled plasma - optical emission spectroscopy (ICP-OES). This is combined with routine analysis of the included anions using ion chromatography. In short, good agreement between data and new results were observed This was confirmed by an inter-laboratory and inter-instrumental test. The analytica workflow using ICP-OES showed an increased sensitivity for all cations and provided accurate measurements of the Ba2+ concentrations despite the high salinity matrix which is an inevitable condition of in the produced water samples. This is extremely valuable for future analysis of produced water; for example for the study of (exchange) mechanism in the reservoirs and in the prediction of scale formation. A study of changes in produced water chemistry as a consequence of sample storage suggested no major changes, thus proving the value of stored samples for many purpose, eg. barium scale prediction. By the use of inductively coupled plasma - mass spectrometry (ICP-MS), the content of five heavy metal ions (Mn2+, Pb2+, Zn2+, Hg2+ and As2+) were studied in a subset of samples including samples from the Halfdan, Kraka and Valdemar fields, as these three fields showed the largest produced water composition variations. The acquired results showed a clear distinction between the fields based on the concentration of the heavy metals ions. Last, comprehensive two-dimensional gas chromatography (GCxGC-HRMS) is used for a non-target screening study of dissolved organic species in produced water samples from the Halfdan field. 18 distinct organic compounds are observed, including alcohols and saturated and aromatic acids. Further analysis of the achieved results from this study may yield some interesting geographical differences. We successfully classified five types of produced water from the Dan, Halfdan, Kraka and Valdemar field, which are used as input for decision making on production strategies, and scale and corrosion prediction and mitigation strategies. The water types also add input to geochemical studies. The findings are confirmed by an experimental study, which presents a new analytical workflow that was successfully developed and validated for the analysis of produced water samples - combining the benefits of ICP-OES and IC. A study of heavy metals ions in the produced water suggests that the concentrations of these also display regional variations; thus linking the heavy metal ions to the classified water types. A presented experimental technique for the analysis of dissolved organic species in produced water shows great potential and may be taken further.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages231
Publication statusPublished - 2020

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