Technical Evaluations To Support the Decision To Reinject Produced Water

Abstract

Summary 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 because the produced water usually needs to be supplemented by seawater in order to meet injection-volume requirements, increasing 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 bubblepoint. Recent experiences of PWRI in other assets have created a challenging atmosphere because scaling posed a major risk to production. Field M had to go through very detailed and thorough 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 performed 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. On the basis of 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.