Scale and Naphthenate Inhibition in Deep-Offshore Fields

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMany of the fields that have been discovered recently in the West African deep-offshore will produce acidic crudes associated with gas containing a high concentration of CO 2 .During the oil production process, a pH increase due to decompression and carbon dioxide degassing may generate surface-active naphthenates that can drastically stabilize emulsified water in crude oil. These may also combine with metal cations present in the reservoir water and form deposits. In all cases, production operations may be seriously disturbed.The aim of the work conducted was to assess naphthenate and scale inhibition and the various factors that can affect its efficiency. In particular, we studied scale inhibitor interactions on naphthenate prevention. This paper presents the results of studies on emulsion stability and naphthenate deposit formation, evaluated for various acidic crudes. As the pH increased, various behaviors were observed: progressive emulsion stabilization or abrupt transitions from unstable to stable emulsions. Naphthenate deposits formed in some cases even at low pH. Such a diversity of behaviors was explained in terms of differences in acid natures.To prevent emulsion stability and naphthenate deposits, selected demulsifier and scale inhibitor additives were then tested. Several types of demulsifiers were found to be efficient at both emulsion-breaking and naphthenate deposit inhibition. In some cases, mixtures of demulsifiers and scale inhibitors produced very good results, highlighting a synergetic effect between the two additives. Unfortunately, the use of scale inhibitors generally increased the calcium content of the oil phase. Basically, the use of scale inhibitor with acidic crudes degrades the oil quality in terms of water cut and metal content.

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“…Naphthenate deposition is a problem only if the aqueous pH exceeds the pK a of the naphthenic acids. Anything which keeps the pH low, like injecting short-chain organic acids, may thus hinder formation of naphthenates (Goldszal et al 2002). In addition, acid demulsifiers consisting of acetic acid in an aromatic solvent mixture, and non-acid demulsifiers consisting of ethoxylates and alcohol, have shown to reduce naphthenate deposition significantly (Gallup, 2004).…”

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“…The development of acidic, sometimes biodegraded, crudes in different parts of the world may lead to naphthenate problems during oil production (Goldszal et al 2002;Laredo et al 2004;Havre et al 2003;Shepherd et al 2005;Ese and Kirkpatrick 2004;Vinstad et al 2003;Dyer et al 2003;Gallup et al 2002;Rousseau et al 2001;Havre 2002;Sorbie et al 2005). These naphthenate deposits have become an increasing flow assurance problem because of deposition and process disruption in production facilities (Goldszal et al 2002;Vinstad et al 2003;Gallup et al 2002;Rousseau et al 2001).…”

“…These naphthenate deposits have become an increasing flow assurance problem because of deposition and process disruption in production facilities (Goldszal et al 2002;Vinstad et al 2003;Gallup et al 2002;Rousseau et al 2001).…”

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“…[3] When pH increases the acids are ionized and concentrate at the oil-water interface but at high pH they might loose their surface activeness due to internal repulsion in the film destroying the mechanical properties that give rise to protection against coalescence, [4] It also has been shown that acidic crudes form very stable emulsions at high pH that destabilize when Na þ or Ca 2þ was added to the water phase. [5,6] The naphthenic acids may form salts with the cations and precipitate form solution. The salts will no longer contribute to the oil water interface.…”