Shear Rheology Using De Noüy Ring To Evaluate Formation and Inhibition of Calcium Naphthenate at the Water/Oil Interface

Abstract

The extraction of crude oils rich in tetraprotic naphthenic acids along with formation water under pH conditions above 6.5 favors the formation of naphthenic acid salts that can generate deposits with high consistency and very low solubility in the oil and water phases. There is still much uncertainty regarding the behavior of naphthenates, and only a small number of tests exist to evaluate it. The aim of this study was to obtain the parameters necessary to assess the formation of calcium naphthenate film through shear rheology (Du Noüy ring method) and to assess the rheological behavior of this film at the water/oil interface as a function of the ARN acid concentration, calcium concentration, presence of magnesium ions, and addition of commercial chemical inhibitors. We observed an increase in the elastic modulus (G′) and a reduction of the viscous modulus (G″) with the formation of the calcium naphthenate film and that the viscous deformation is restricted, which is coincident with the formation of a cross-linked network between the calcium ions and ARN acids. We also found that G′ increases with an increasing calcium ion concentration and salinity. The addition of the commercial inhibitors tested caused a reduction of G′ in relation to the control formulation (no inhibitor), with it being possible to distinguish their different performances in film formation prevention. However, we did not observe a correlation between the results of the biphasic mixture test and rheological assay throughout the concentration range tested.

Solubility behavior of amphiphilic sulfonated copolymers based on styrene–stearyl methacrylate and styrene–stearyl cinnamate

“…Amphiphilic polymers have widespread applications. In the petroleum industry, they are used in drilling fluids and for demulsification of water‐in‐oil emulsions, inhibition of wax deposition, stabilization of asphaltenes in crude oil, flocculation of asphaltenes, inhibition of naphthenate deposition, enhanced oil recovery, and water treatment . In drilling, hydrophobically modified starches can be used to control filtrate from emulsified fluids .…”

Oilfield metal naphthenate formation and mitigation measures: a review

“…This is due to increase in dissociation rate of naphthenic acid. However, the increase in dissociation rate is not monotonic, but increases sharply at the point where pH of brine equals the pKa of the dominant naphthenic acid (Bertelli et al 2014). Even at low calcium concentration in water, CaN formation can still occur substantially if pH is high (> 8) (Igwebueze et al 2013).…”

“…Also the kinetics of partitioning and dissociation of naphthenic acid is much slower in the presence of sodium ions than calcium ions because divalent calcium ions complex twice as much naphthenate ion as the monovalent sodium ion, so the rate of calcium naphthenate formation is higher than that of sodium naphthenate (Bertelli et al 2014). The strong attractive forces between naphthenic acid and Ca 2+ at the interface induce a chemical reaction which proceeds in two stages by sequential binding of the acid to the metal cation.…”

“…Mg 2+ with higher hydration number of 6 compared to 2 for Ca 2+ experiences greater electrostatic shielding. Therefore, Mg 2+ has a lower tendency to complex with ARN and will barely participate in the formation of an interfacial film (Bertelli et al 2014). In contrast to the weakly soluble coordination compounds formed by Ca 2+ , Mg 2+ tends to form uncoordinated, water-soluble ionic species (Taylor and Hiu 2018).…”

“…The unique property of CaN to harden as it cools makes removal difficult and introduces the additional problem of solid disposal. It had been speculated that this behaviour is as a result of oxidation of the surface of the deposit on exposure to air, but the process is more likely related to the glass transition temperature of the material (Bertelli et al 2014;Junior et al 2013). Metal naphthenate removal and disposal comes at a considerable cost in addition to lost production during shutdown period.…”