Produced Water Re-injection and its Impact on Reservoir Souring

Abstract

Produced Water Re-injection and its Impact on Reservoir Souring

Microorganisms are found throughout oil production systems, from the reservoir rock itself, through pipelines and topsides facilities. The sulphate-reducing bacteria (SRB) are responsible for the majority of the bacterial problems in oil production. Hydrogen sulphide is produced directly by SRB as a by-product of respiration. This hazardous gas, as a respiratory inhibitor, is volatile and toxic. It also ‘sours’ crude oil and gas, making it harder to refine into environmentally friendly, high quality fuels, hence reducing its value. In addition, sulphide concentrations even below 1 mg/l in the water phase may lead to high corrosion rates, and in the gas-phase, maximum allowable limits of H2S are often as low as 3ppm (v/v) for high pressure duties to avoid sulphide stress cracking (SSC) in susceptible steels.

Secondary oil production is often needed due to a lack of pressure in the reservoir; seawater is pumped down into the reservoir to increase pressure and drive more oil out. Introduced with seawater are high concentrations of sulphate and immigrant microorganisms, despite employment of biocides and filtration. The seawater also cools the reservoir where it enters it, resulting in a near well bore environment very different to that before human intervention. This environment can be ideal for the proliferation of SRB and results in sulphide biogenesis as bacteria reduce sulphate from the seawater in order to respire. Produced water is significantly different to seawater in terms of chemistry and physical characteristics. Therefore, produced water re-injection (PWRI) can change the bacterial environment and have a significant impact on the activity of SRB and so the generation of sulphide. Accurate predictions of reservoir souring due to SRB are essential for multi-million dollar decisions on oil field development and material selection. Furthermore, once the problem has been characterised properly it is possible to select options to mitigate the problem of sulphide generation by SRB.