Reservoir Souring

Biological hydrogen sulphide production in oil reservoirs and pipelines is an unfavourable phenomenon that leads to higher operational costs of oil production. The produced hydrogen sulphide is a toxic gas leading to health and safety issues; it also enhances infrastructure corrosion among other negative effects. Souring in such systems is mainly due to respiration of anaerobic microorganisms (sulphate-reducing prokaryotes, SRP) that reduces sulphate to sulphide. These microorganisms may exist indigenously in oil reservoirs or/and may be introduced into the system during drilling and waterflooding.

Utilising seawater for waterflooding provides additional sulphate and SRP in the system. It also lowers reservoir temperature providing a favourable condition for SRP activity. The main strategies to reduce the probability of souring are: injecting nitrate and/or nitrite to inhibit SRP activity and/or oxidize sulphide; removing sulphate from injection water; and biocide treatments.

In the Halfdan oilfield, controlling the microbial souring by injecting nitrate to shift the indigenous microbial community away from sulfide production has become an established method. However, more work is needed to predict and design the cost effective long-term strategies for the souring control process.

DHRTC Research

The reliable quantification of the mechanism of nitrate/nitrite inhibition under altered reservoir conditions is still a challenge and requires more examinations. The effects of induced fractures, reservoir heterogeneity, short circuits between wells, bioavailability, self-inhibition, and altered reservoir temperature on the souring control process have not been studied in detail in the Halfdan oilfield. Among others, the research is focused on developing detailed reservoir simulations incorporating all the relevant available information (tracer tests, the updated static model, measured nitrate, nitrite, H2S and CO2 concentrations in the wells), which is required to predict the success of various treatment options while reducing the uncertainties. Microbial and biochemical research is conducted to elucidate the complex mechanisms of souring and inhibition leading to enhanced prediction and optimisation of inhibition processes.