Abstract
With changes in environmental regulations a number of operators have moved from seawater injection to produced water reinjection and others have used produced water reinjection as a way to reduce/mitigate the potential sulphate scale risk by reducing the amount of sulphate rich seawater being injected into their reservoirs.
The paper will focus on three case study fields looking into the control of the scale risk within the topside process, water injector wells and the evolving scale risk within the production wells.
Data from North Sea fields will be presented which shows the challenge experienced within the production wells as initial injected seawater (or low sulphate seawater) is displaced by injected produced water. The change in produced water composition with time showed a significant extension in the scale risk envelope due to produced water reinjection with respect to sulphate scale formation relative to what would have been expected with only seawater injection. The expected trend of reducing barium ion concentration with time was reversed with produced water breakthrough (enhancing the barium and strontium ion concentrations) leading to a much longer period over which scale squeeze treatments were required to be applied.
Along with the prolonged scale risk envelope the tracking of scale squeeze treatments became a challenge due to production of degraded phosphonate scale inhibitor which was present in the produced water (topside scale inhibitor and re injected squeeze scale inhibitor) leading to uncertainty in the actual scale squeeze inhibitor concentration where phosphonate scale inhibitor was being utilised.
The lessons learned from this study are that changes in scaling potential within a PWRI system require to be carefully assessed prior to the onset of PWRI. While control of scale issues within the topside process and water injector wells has proved easy to predict and manage, the extended scale risk and challenges of tracking squeeze treatment performance need to be considered as this may present a significantly higher operation cost for effective scale management.
