Over the last 15 years, much research and many field application studies have led to considerable improvement in our understanding of the formation and mitigation of calcium naphthenate deposits.

In this field example, calcium naphthenates and stable emulsions are formed following mixing of fluids from different reservoir formations on a single FPSO. High TAN crudes containing low levels of ARN produce with low calcium formation waters whereas low TAN crudes are associated with high calcium formation waters. Mixing of these two systems has led to calcium naphthenate deposition and associated problems with its removal.

This paper outlines the challenges in this complex deepwater subsea production system and the interpretation of the cause of the deposit. A series of laboratory tests using a specialised flow rig were conducted to illustrate the effects of mixing different fluids and identify those mixtures with the largest naphthenate potential.

The work further illustrates the effect of bicarbonate ions on the system. Laboratory tests at low levels of bicarbonate (to prevent carbonate scaling at separator conditions) do not result in calcium naphthenate formation when mixing the high TAN crude with the current produced brine (moderate calcium). Naphthenates only formed when mixing with the high calcium brine. When bicarbonate is included at full field levels (in the presence of a scale inhibitor) significant calcium naphthenate formation is recorded with the lower calcium brines. The effect of CO2 within the produced fluids has also been evaluated.

The paper describes how several variables contribute to the likelihood of calcium naphthenate deposition and presents results from several naphthenate formation and inhibition tests covering a range of fluid compositions and mixtures. Chemical qualification in the lab using the worst case fluid mixtures has been conducted to select a calcium naphthenate inhibitor for field deployment. Field trials demonstrate both the effectiveness of the treatments and also the qualification exercise conducted for this field.

The results further indicate the complexity of accurately predicting a calcium naphthenate risk while illustrating that, even under challenging conditions, chemical inhibitors are effective in this system.

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