Use of FT-ICR MS measurement on offshore produced water to understand and mitigate oil-in-water issue

The main goal of this project is to identify water-soluble organics (WSOs) in production water, coming from oil and gas wells. The second is to investigate the source of WSOs and to provide potential mitigation strategies for WSOs removal. We used Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS), a powerful instrument to detect (polar) molecules with high mass resolution and accuracy. Specialized software Composer is used for formula assignment. The design of experiment (DOE) is the main design component in this project. Produced water is a brine containing salts, WSOs and other contaminants. It is necessary to desalt and concentrate the WSOs from the produced water for proper diagnosis and characterization. We prepared ‘brine simulant’ by equilibrate oil samples from different reservoirs with a synthetic brine, that has a composition based on the production water. In this work, solvent extraction and solid-phase extraction were used for the sample preparation/pre-treatment.
In the early stage of experimental work, solvent extraction was applied for the pre-treatment of produced water, water layers and brine simulants, except the oil samples. Solvent extraction results show that produced water is dominated by O2 and O2S1 acid compound classes (negative ion), which are likely naphthenic acids and heteroatomic naphthenic acids, respectively. No direct link between production water and oil samples could be found. However, various data such as spectra, distribution plots, bubble plots and bubble plots-cross section show that a brine simulant (representing one of the studied oils), is in good agreement with production water. This indicates that the WSOs in production water most likely originate from this particular oil. The O1S1 base compound class (negative ion) is also dominant and probably sulfoxides from crude oil, as found in brine simulants, but less abundant in produced water. Most likely, the O1S1 class was removed from produced water by hydro cyclone and flotation before sampling. Solid-phase extraction provides additional insight in fingerprinting produced water to oil samples. Although it gives some different results compared to solvent extraction, it mostly supports what has been found using the solvent extraction.
Presence of naphthenic acids in the production water is confirmed using molecular formula investigation with GC×GC TOF MS. Naphthenic acids are harmful to marine life and several potential mitigation solutions were considered to remove them, including wells shut-in, acidification, adsorption, fluid segregation, and activated carbon filtration. Nowadays, activated carbon filtration is considered as one of the best technologies for the removal of WSOs (NAs) due to its high efficiency of removal and relatively low cost of activated carbon.
Renting an activated carbon filtration equipment is a solution, with main disadvantages being the high initial startup cost and high monthly equipment rental cost. A more economical way could be to selectively segregate fluids from the main fluid train and only treat those with activated carbon filtration. In this way, activated carbon filtration can be considered as a long-term solution if no other novel regenerable filtration technologies or chemical precipitation methods are discovered.