Chemical and mineralogical characterization of waste generated in the petroleum industry and its correlation with ²²⁶Ra and ²²⁸Ra contents

Abstract

Scales and sludge are commonly formed during oil and gas extracting and processing operations. They usually appear when injection and formation water with different chemical characteristics come into contact. When the produced water is brought to the surface alongside with the oil, a precipitate can be deposited on the walls of pipes and equipment, forming the so-called scales. Otherwise they can accumulate in the form of sludge on the bottom of storage tanks, separators, and other equipment. Radium is the main radionuclide brought to the surface with oil and produced water and it co-precipitates with barium forming complex compounds of sulfates, carbonates and silicates. These compounds are the main constituents of scale and sludge. The objective of this work was to relate the radium content of scales and sludge to their chemical and mineralogical composition. Samples were taken from a PETROBRAS Unit in the State of Sergipe, in Northeast Brazil. They were collected either from the inner surface of water pipes or from containers in the waste storage area of the Unit. Oil was separated from the solid material in a Soxhlet extractor equipment by using aguarras as solvent. The concentrations of ²²⁶Ra and ²²⁸Ra were determined by gamma spectrometry. Both the mineralogical and chemical composition of samples were determined by X-ray diffraction and X-ray fluorescence, respectively, and used to characterize samples as scales or sludge. The results showed that scales are mainly formed by BaSO₄ and CaCO₃ while sludge composition is quite variable, having higher contents of SiO₂ and FeO₃ than those observed on scale samples. The measured activity concentrations of ²²⁶Ra and ²²⁸Ra are strongly correlated for both kinds of samples.