Abstract
Produced water (PW) and formation water are complex mixtures of hydrocarbons and water produced at oil and gas upstream facilities. Submicron oil droplets represent a multitude of issues affecting the performance of downstream advanced water treatment processes, such as micro and ultra-filtration processes. Conventional de-oiling technologies do not efficiently remove submicron oil droplets in PW. An accurate characterization of submicron oil droplets and contaminants is required to improve PW treatment technology. In this study, a methodology for visualization and quantification of submicron oil droplets size distribution (DSD), using optical and electron microscopy techniques, was developed. Various microscopy techniques were evaluated, including epifluorescence microscopy (EpiFM), confocal laser scanning microscopy (CLSM), cryogenic scanning and transmission electron microscopy (cryo-SEM and cryo-TEM, respectively). Synthetic PW was used to improve and standardize the sample preparation and characterization methodology. The improved methodology was then tested with two PW samples from different oilfields in the Middle East region. Two methods were developed for the determination of DSD in oilfield PW samples. The first method is suitable for highly polydisperse PW samples with oil droplets larger than 250 nm. This method is based on using low-temperature agarose to immobilize the samples, avoiding coalescence, and allowing clear visualization of the oil droplets at high magnification in EpiFM. The second method is suitable for concentrated PW samples and oil droplets as small as 20 nm in size. This method is based on cryo-TEM with plunge freezing and without the use of agarose for sample immobilization. The agarose-immobilization technique was also applied for sample preparation in cryo-SEM. Cryo-SEM fixation by high-pressure freezing (HPF) preserved the morphology of oil droplets in synthetic oil-concentrated samples and allowed its visualization in a wide range of sizes from 50 nm up to 20 μm.
Cryo-TEM imaging of oilfield PW samples revealed the presence of oil droplets as small as 20 nm in size. However, no quantification of the DSD in the oilfield PW samples was possible due to the low oil concentration. Moreover, a relatively large amount of other submicron particulates/aggregates were observed in PW samples. Cryo-SEM coupled with energy-dispersive X-ray spectroscopy (EDS) provided valuable information on the morphology and elemental composition of the submicron-sized contaminants. These submicron contaminants would not be removed by conventional de-oiling technologies and might cause intrapore blocking in membrane filtration systems downstream. The effect of these submicron contaminants, other than oil droplets present in PW, needs to be addressed to improve the PW treatment technologies.
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•A new method based on EpiFM of immobilized gelled samples is proposed to determine the DSD.•Oil droplets as small as 20 nm were observed in oilfield PW samples by cryo-TEM.•Cryo-SEM-EDS complements information on the nature of the submicron contaminants in PW.•Submicron contaminants other than oil droplets should be further studied for improved PW treatment.