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 Ca 2+ 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 Ca 2+ -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/Ca 2+ 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 Ca 2+ 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/ Ca 2+ system is required to extrapolate the results obtained at the interface to bigger scales.

Prevention and Removal of Naphthenate Deposits in Oil and Gas Production—Historical Background and Novel Attitude towards Inhibition and Solution

“…Simon [13] described the formation of high-molecular-weight ARN acid and calcium ions; here, three parameters were taken as important to estimate the effectiveness of naphthenate inhibitors: interfacial tension, interfacial rheology measurements, and bottle tests. In the case of the bottle test, a mixture of an aqueous phase and a xylene phase (xylene + ARN acid + crude oil, in a ratio of 1:1) was shaken overnight.…”

“…The blank sample contained no extra ingredients (oil and water without any acidic additive). The use of hydrochloric, acetic, and ABS acids is discussed in the literature [3,[9][10][11]13]. No references to citric or formic acid were found.…”

“…Based on the photos shown in Figure 4 and Figure 5 and visual investigations of the separation surface, we can infer that the inhibitors NI-2 and NI-6 were the most effective. Simon et al [13] stated that in the absence of ARN naphthenic acid, the interfacial tension between brine and an xylene/oil solution is lower when the crude oil content in xylene is increased. Furthermore, some naphthenic acids and asphaltenes inhibit the formation of calcium naphthenate salts.…”