The industry is moving away from overboard discharging of produced water and is increasingly selecting produced water reinjection (PWRI) as the preferred method for waterflooding projects. This does not come without risks as the produced water usually needs to be supplemented by seawater in order to meet injection volume requirements, which increases the risk of both scaling and souring.
PWRI supplemented by seawater was the selected produced water management strategy for Field M located in West Africa. Produced water from the adjacent Field K is also being considered for reinjection into Field M. Field M will be waterflooded from the beginning to maintain reservoir pressure close to the bubble point. Recent experiences of PWRI in other assets have created a challenging atmosphere as scaling posed a major risk to production. Field M had to go through very detailed and though souring and scaling evaluations to justify the feasibility of this project.
A consistent formation water chemistry analysis for these evaluations was obtained by incorporating thermodynamic equilibrium constraints on formation water composition from reservoir mineralogy and using knowledge of basin formation water contributions from proximal salt formations. The evaluation of scaling potential was done using ScaleSoftPitzer. The results show a low risk of scaling when the waters are commingled. Periodic formation scale squeezes are recommended to mitigate the scaling potential.
The souring potential study was conducted using a full field reservoir souring simulation model (SourSimRL). SourSimRL superimposes criteria for the generation, partitioning and transport of H2S. Based on the results, the souring potential is predicted to be generally moderate. The souring potential for the field is restricted by the availability of carbon nutrients in the injection waters and the high reservoir temperature of 250 °F.
These rigorous technical evaluations were instrumental to obtain the support for PWRI and could be used as guidance for other PWRI projects.