# Sulfide Production and Microbial Dynamics in the Water Reinjection System from an Offshore Oil-Producing Platform

## TL;DR

This study examines hydrogen sulfide production and microbial changes in water systems used for oil recovery, highlighting risks of reservoir souring.

## Contribution

The study identifies Desulfobacterota as a dominant sulfate-reducing bacteria in reinjected water systems, despite flotation treatment.

## Key findings

- PW samples showed the highest average H2S concentration.
- Flotation treatment was linked to enrichment of SRB and other bacteria in PWRI.
- H2S levels exceeded safety thresholds in 18% of PW and 16% of PWRI samples.

## Abstract

In addition to seawater in the injection header (IH) to enhance oil recovery, oil companies reuse produced water (PW), a byproduct of oil extraction, and implement produced water reinjection systems (PWRI). Although the microorganisms in IH are controlled by biocides, PW is generally treated by flotation to remove oil residues before PWRI. However, IH, PW, and PWRI can be sources of sulfate-reducing bacteria (SRB) related to oil reservoir souring. Here, we evaluated hydrogen sulfide (H2S) production in IH, PW, and PWRI, as well as the microbial dynamics (most probable number–MPN, quantitative PCR, and amplicon sequencing), of a Brazilian oil reservoir. Results revealed that the highest average H2S concentration occurred in PW samples. However, the dissolved H2S threshold concentration of 2 mg L−1 was exceeded in 18% of PW and ~16% of PWRI samples, respectively. Although MPN showed no correlation between H2S and the number of SRB or total anaerobic heterotrophic bacteria (TAHB), qPCR and microbiome data revealed that the SRB Desulfobacterota was the most abundant in PW and PWRI. Overall, flotation was associated with (i) low microbial control in PW; and (ii) the enrichment of SRB (mainly Desulfobacterota), Thermotogota, and Proteobacteria groups in PWRI.