Scale and Naphthenate Inhibition in Deep-Offshore Fields

Abstract

Abstract Many 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 CO2. 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. Introduction Many recent fields discovered in the deep offshore waters of Angola and Congo will produce acidic crude oils. These may be associated with gas at a high concentration of CO2. Such compositions are not limited to West Africa and are found in many other locations such as the North Sea or Venezuela. Reservoir waters are naturally saturated with carbon dioxide (CO2) in equilibrium with bicarbonate anion (HCO3-) according to the following reversible reaction:Equation During production, carbon dioxide is released whenever a significant pressure drop occurs, and this is accompanied by an increase in the pH value. The various naphthenic acids present in these crudes, together with the corrosion that may occur subsequently during refining, can also give rise to two other major problems resulting from an increase in the pH of the reservoir water [1]:formation of mixed carbonate and naphthenate deposits inside tubing or surface installationscreation of stable emulsions due to the surface-active naphthenate group RCOO-. This paper presents the results of a study into the behaviors of two crudes with respect to emulsion stability and deposit formation. It goes on to show that chemical prevention of scale and naphthenate formation by additives can be very efficient. Finally, synergetic effects (sometimes with side effects such as increased calcium concentration in the oil phase) of mixtures of additives are also highlighted. 1.Experimental part Two different crude oils were used in this study: a TotalFinaElf crude from Angola (Dalia) and a North Sea Crude (NSC). The acidity of each was evaluated by TAN measurements (Total Acid Number determined by ASTM standard D664–89):Equation