Radionuclide Fate of Naturally Occurring Radioactive Materials (NORM) in the Oil and Gas Industry

Abstract

The formation of naturally occurring radioactive materials (NORM) during oil extraction and operational marine discharges is a global ongoing problem encountered in the petroleum industry. The presence of 226Ra in effluent waters (e.g. produced water) and the mixing of incompatible waters (e.g. seawater and produced water) encountered at different stages of production drives the co-precipitation of radium into insoluble sulphate mineral phases (e.g. RaBaSO4 and RaBaSrSO4). These materials can accumulate in large volumes across an array of components in the production system (e.g. pipe lines, valves, storage tanks) but, may also form and deposit in the environment as a result of operational effluent marine discharges. The build-up and accumulation of naturally occurring radionuclides within solid and liquid waste materials during the extraction of oil (e.g. minerals, effluent waters and sediments) has been a recognised risk in the oil and gas industry during the past century. However, there is uncertainty about the speciation and fate of radium during offshore produced water discharges, and the nature of the material formed as a result of the mixing of these waters which is poorly constrained. An understanding of the fate and uptake of key radionuclides into NORM waste streams e.g. mineral scales formed in tubulars, and from operational marine discharge of produced water as a result of mixing of incompatible waters is key in order to develop the capability to predict their fate and environmental risk. In this study, the uptake and fate of radium in marine sediment samples obtained from a field site where produced waters are discharged were explored. A variety of techniques such as gamma spectroscopy was used to assess the radium activity concentrations in marine field samples. Radium was present in selected field samples at concentrations between 0.1 – 0.3 Bq g-1. Heavy liquid extractions were used to separate (radio)strontiobarite particles, a NORM component, from the marine sediments. The bulk chemistry, particle size (≤ 2 µm) and morphology (equant) were then characterised, and the radioactivity in these particles using autoradiography to provide validation. The uptake and fate of radium was explored in model systems where synthetic (full-component) and field produced waters were mixed with synthetic and field seawaters under laboratory conditions to mimic the formation of (radio)strontiobarite particles by produced water discharge into the marine environment. Experiments showed that a significant proportion of radium (up to 48 % in 1 hour) co-precipitates with barium during mixing. Solid samples were extracted and characterised using a variety of techniques including, FTIR, XRD, BET, XAS and SEM which confirmed the mineralogical (i.e. barite particles 1 - 6 μm in size) and chemical composition (Ba~75Sr~25SO4) of precipitates obtained from laboratory mixing regimes to be consistent with the particles found in field sediments. Additionally, the long term fate, speciation and mobilisation of radium once discharged to a marine setting as the inorganic solid and aqueous ion was investigated via a series of sediment microcosm experiments where progressive anoxia was stimulated using terminal electron acceptor additions. A range of techniques including radiochemical measurements, sequential extractions, SEM, heavy liquid extractions and DNA sequencing were performed to fully quantify, understand the partitioning of radium within sediment and the resultant effect on the microbial community. Experiments showed radium remained recalcitrant to dissolution and desorption as aqueous radium (Ra2+) and radiobarite (RaBaSrSO4) respectively during bioreduction and development of sulphate reducing conditions. Finally, a comprehensive investigation into the factors controlling the distribution of radionuclides in NORM samples (e.g. hard scales) obtained from tubulars from oil producing platforms (UK and Iraq) was performed to understand

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor), Samuel Shaw (Supervisor) & Katherine Morris (Supervisor)