Impacts of Water-In-Oil Emulsion and Stage Equilibrium for Produced Water Analysis with Benchtop XRF
Authors: Gallo V. C. C. , Alvim E. D. , Dias Neto J. M , Simoes Neto S. , Moura V. B , Ursini C. V , Yamamura V. D , Andrela D. H 
Published: October 2023

This paper presents the main findings of continuous monitoring with benchtop XRF (X-ray Fluorescence) to assess produced water composition from different process streams in a FPSO operating in Brazil (offshore).

Produced water analysis is a vital aspect of scaling control protocols and is widely adopted in the offshore industry to conduct routine laboratory analysis based on ICP (Inductively Coupled Plasma) and IC (Ion Chromatography) for evaluating water composition. However, due to extended lead times and logistical costs, an offshore analysis method has been used for assessing produced water composition using XRF. The main purpose of an on-site approach is to provide fast and accurate results for produced water composition, which allows to identify subtle changes in composition due to changes in well composition or process arrangement.

The continuous monitoring with XRF revealed the existence of a non-equilibrium condition at the separation stages (process vessels) related to the existence of a "persistent" (treatment-resistant) water-in-oil emulsion. This condition caused the composition of the produced water to largely deviate from the expected values based on well composition. As the emulsion presented high ionic concentration (due to a specific well), it was verified higher concentration throughout the plant. At the latter separation stages, ionic concentration (e.g., calcium) was 30% higher than the blend would suggest.

To understand the condition observed at the separation vessels, it was performed an ionic characterization of the emulsified water at the oil phase (water below 1% volume) and the separated free aqueous phase that were in contact at each stage, revealing that they didn't have the same compositions (i.e., were not in equilibrium). XRF analysis revealed a higher ionic concentration in emulsified water than in the free aqueous phase, indicating an ionic distribution profile at each stage caused by the presence of emulsion.

The possibility to perform on-site water compositional analysis was also important during the early production days of a new water producing well, as it was possible to closely monitor its produced water composition evolution over time and its impact on the production blend overall composition, which would take months to be assessed with the current procedure due to logistics and laboratory lead time.

It is a common approach to assume that water composition through the separation stages is the same, being the direct result of the contribution of each well. This study shows that depending on the water cut, well characteristics and process conditions, there is a significant variation on the cations and anions distribution. The results show that emulsion and stage equilibrium are key factors on how the ions behave among water/oil phases. It is also presented the importance of early monitoring during the start of the water production by new wells and its contribution to the overall composition, as well as sudden changes in well composition may also happen due to reservoir changes and can be prompt identified with an on-site monitoring equipment.