In this paper we will tell the story of a sub-sea well in the Oda field where the predicted downhole calcium sulphate (CaSO4) scale risk did not concur with observations in the field.

Seawater was injected into the oil leg and the first sample collected at 5% water cut had ~ 80% seawater breakthrough, with a harsh CaSO4 scaling tendency predicted (SR 3.5 at 120°C). A squeeze treatment was planned as part of the mitigation strategy and, in addition, continuous downhole injection of scale inhibitor (SI) was instigated to provide protection to the production tubing, flow line and topside facilities.

An extensive monitoring program consisting of productivity monitoring, weekly produced water ionic composition and ESEM/EDX bulk scale analysis (BSA) was instigated to confirm the scaling risk. The water cut increased as did the seawater content to ~90%, however, the results of the BSA did not indicate any significant amounts of CaSO4 scale on the filters indicating stable produced water.

Scale and 1D reservoir simulations also showed that to match the produced water composition with the correct FW:SW ratio, CaSO4 should have precipitated in the reservoir, but not to equilibrium resulting in a produced water with SR higher than one. In addition, a further increase in seawater content would lead to lower scale risk and the produced water was confirmed as stable even with a calculated SR of about 3-3.5. Several scale prediction models were evaluated with the same outcome.

A new scaling risk matrix created from the BSA field data, backed up by the trends in measured vs predicted ion data, indicated no significant scale risk to the Oda well which is in contrast to what was predicted.

Based upon this data it was agreed that the planned squeeze treatment was no longer required, but this still raised the question of why the well didn't scale. Several theories including metastability, natural inhibition and kinetic effects coupled with partial protection by downhole injection of SI will be discussed.

In addition, this paper will highlight the benefits of using a holistic, integrated scale management strategy, consisting of scale and reservoir simulations combined with field data (ions and BSA), to identify a more realistic field risk and save unnecessary treatment costs.

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