Screening of Inhibition of Calcium Naphthenate Deposition in Crude Oil

Abstract

One of the initial steps in calcium naphthenate deposition is the reaction between ARN and Ca2+ leading to the formation of an interfacial gel. After growing, this gel will form calcium naphthenate deposits that are a threat for irregularities in crude oil production and processing. It has been previously shown that crude oil components such as asphaltenes and naphthenic acids can weaken and inhibit the formation of the interfacial gel. Based on this observation, the inhibition efficiency of four crude oils is compared. The variations of interfacial tension (IFT) and interfacial dilational modulus E′ of ARN solution (concentration = 10 μM) in contact with Ca2+-containing aqueous solution (pH = 8) with varying crude oil concentration allow us to define a parameter characterizing the interfacial gel inhibition efficiency of crude oils to compare the systems with each other. The determined inhibition efficiency order of crude oils is not directly related to the total asphaltene and naphthenic acid contents in the crude oils and, therefore, cannot be directly predicted by performing simple chemical analysis of the oils. By selectively removing asphaltenes and/or naphthenic acids, it was found that naphthenic acids are the most prevalent inhibitors followed by asphaltenes in the tested crude oils. Other compounds present in crude oil also have a weak inhibition activity on the ARN/Ca2+ interfacial gel. Finally, bottle tests were performed to upscale the results obtained at the interfacial scale. Bottle tests were performed at an ARN concentration 10 times higher than that used for IFT and interfacial rheology measurements. The tests indicate the presence of a soft solid material formed by the reaction between ARN and Ca2+ even in the presence of the crude oil that was found to have the highest interfacial gel inhibition efficiency. This apparent contradiction is not understood at the moment, and hence, a better understanding of the ARN/Ca2+ system is required to extrapolate the results obtained at the interface to bigger scales.