Naphthenate Scale Formation - Examination of Molecular Controls in Idealised Systems

Abstract

Abstract The formation of calcium naphthenate precipitates and emulsions during oil production is becoming an increasing problem to the oil industry. Naphthenic acids, R-CO2H, are present in many crude oils and the hydrophilic nature of the carboxylic acid group means that they congregate at the oil-water interface. As the pressure drops during production and carbon dioxide is lost from solution, the pH of the brine increases, which in turn leads to dissociation of the naphthenic acid (RCO2H -> RCO2-). The naphthenates can then act as natural surfactants leading either to stabilised emulsions or solid deposits following complexation with calcium cations present in the aqueous phase. The naphthenate deposits collect predominantly in oil / water separators and de-salters but can also deposit in the tubing and pipelines. This study has looked at a variety of conditions to determine when certain carboxylic acids will form naphthenate deposits under idealised laboratory conditions. A range of naphthenic acids of different molecular structure were dissolved in an organic phase (toluene) and mixed with synthetic brines containing a range of calcium concentrations typical of oilfield production waters. These tests have determined that as the size of straight chain carboxylic acids increases so does the amount of naphthenate deposit. Increases in brine pH also increased the amount of deposit. However, the effects of changes in calcium concentration and molecular structure on the formation of naphthenate deposits were more difficult to quantify. The work assists in increasing our understanding of the factors controlling the precipitation of naphthenate solids under controlled conditions and forms the basis for future studies in real oilfield fluids. Introduction Naphthenic acids in crude oil are predominantly carboxylic acids with saturated cyclic structures.1–5 However, naphthenate deposits can also be formed from long-chain aliphatic carboxylic acids, as is seen in the soap industry.6 On report indicates crude oils can contain up to about 2 weight percent of naphthenic acids, consisting of a wide range of individual components.5 The low molecular weight naphthenic acids present in crude oils contain a wide range of complex structures including cyclopentane carboxylic acids and cyclohexane carboxylic acids. The potential exists for naphthenic acids to partition between the oil and water phases during production and once in the water phase the acids will dissociate in accordance with normal equilibrium reactions. However, with the exception of the low molecular weight acids, naphthenic acids are relatively insoluble in water. The partitioning coefficient from the oil phase into the water phase therefore tends to increase as the molecular weight of the naphthenic acid decreases. The naphthenic acids will also congregate at the oil-water interface, with the aliphatic tail in the oil phase and the anionic head in the water phase. Work has shown that the optimum molecular weight for interfacial activity is 300 to 400.5 Metal Naphthenates. The cations in formation brines and sea water can come into contact with the anionic head of the RCO2- ion at the oil-water interface and form a soap, such as RCO2Na or (RCO2)2Ca. Low molecular weight sodium naphthenates are soluble in the water phase, but calcium naphthenates are less soluble in water resulting in the formation of a precipitate under appropriate pH conditions.5,7,8 The insolubility of calcium soaps, even at high temperatures is well known in other industries.8–11 In the deinking process used in the paper recycling industry, the presence of calcium in solution from natural hard water can precipitate the soap.8 However, it has been suggested that metal soaps will not deposit when the solution pH is below pH 6.12 Research for the deinking industry has also shown that for straight chain hydrocarbons, increasing the number of C=C unsaturations increases the solubility of the calcium soap.13 This is because increasing the number of C=C unsaturations in the hydrocarbon chain decreases the chains hydrophobic character and therefore decreases the surfactant efficiency of the RCO2- ion.13 Metal Naphthenates. The cations in formation brines and sea water can come into contact with the anionic head of the RCO2- ion at the oil-water interface and form a soap, such as RCO2Na or (RCO2)2Ca. Low molecular weight sodium naphthenates are soluble in the water phase, but calcium naphthenates are less soluble in water resulting in the formation of a precipitate under appropriate pH conditions.5,7,8 The insolubility of calcium soaps, even at high temperatures is well known in other industries.8–11 In the deinking process used in the paper recycling industry, the presence of calcium in solution from natural hard water can precipitate the soap.8 However, it has been suggested that metal soaps will not deposit when the solution pH is below pH 6.12 Research for the deinking industry has also shown that for straight chain hydrocarbons, increasing the number of C=C unsaturations increases the solubility of the calcium soap.13 This is because increasing the number of C=C unsaturations in the hydrocarbon chain decreases the chains hydrophobic character and therefore decreases the surfactant efficiency of the RCO2- ion.13