Corrosion Management of Lines With Comingled Produced Water And Sea Water

ABSTRACT To enhance asset capacity utilization and operational flexibility, produced water (PW) and sea water (SW) were comingled for several years as part of water flooding large Alaskan oil fields. Comingling led to severe corrosion and the functional loss of several pipelines. Detailed investigations revealed that inorganic scale, solids, low velocities, lack of supplemental corrosion inhibition, maintenance pigging ineffectiveness, and bacteria were principal factors that resulted in the lack of adequate corrosion control. A rigorous management of change process and aggressive corrosion mitigation efforts were implemented to separate PW and SW. Lessons learned from the above experiences were also applied to temporary PW-SW comingling efforts necessitated by business/operational constraints as well as emergency situations leading to inadvertent mixing of PW and SW. A mitigation matrix was developed and successfully implemented as an asset integrity management tool in addressing risks of PW and SW mixing temporarily or under emergency conditions. A set of key performance indicators specific to monitoring the effectiveness of mitigation measures were also implemented. INTRODUCTION Waterflooding is the most prevalent means of Secondary Recovery in the oilfield, sweeping hydrocarbons remnant in the rock after the well ceases to flow sufficiently under its own reservoir pressure – the production phase known as Primary Recovery. Production leaves voidage leading to subsurface mechanical instability, especially in unconsolidated rock. Thus, waterflooding also serves to maintain pressure support and mitigate reservoir subsidence.^1,2 To preserve the existing reservoir permeability, the quality of the injected water must be carefully controlled, so that it should be low in solids.^3,4,5,6 Nutrients such as sulfates in the water enhance bacterial activity and corresponding hydrogen sulfide (H₂S) generation caused by reservoir souring can increase the risk of corrosion in downhole and surface equipment.^7.8.9 Bacterial activity may produce biomass leading to operational problems.