Naphthenate Scale Formation – Examination of Molecular Controls in Idealised Systems

摘要

The formation of calcium naphthenate precipitates andemulsions during oil production is becoming an increasingproblem to the oil industry. Naphthenic acids, R-CO2H, arepresent in many crude oils and the hydrophilic nature of thecarboxylic acid group means that they congregate at the oilwaterinterface. As the pressure drops during production andcarbon dioxide is lost from solution, the pH of the brineincreases, which in turn leads to dissociation of the naphthenicacid (RCO2H -> RCO2-). The naphthenates can then act asnatural surfactants leading either to stabilised emulsions orsolid deposits following complexation with calcium cationspresent in the aqueous phase. The naphthenate depositscollect predominantly in oil / water separators and de-saltersbut can also deposit in the tubing and pipelines.This study has looked at a variety of conditions todetermine when certain carboxylic acids will formnaphthenate deposits under idealised laboratory conditions. Arange of naphthenic acids of different molecular structure weredissolved in an organic phase (toluene) and mixed withsynthetic brines containing a range of calcium concentrationstypical of oilfield production waters. These tests havedetermined that as the size of straight chain carboxylic acidsincreases 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 andmolecular structure on the formation of naphthenate depositswere more difficult to quantify. The work assists in increasingour understanding of the factors controlling the precipitationof naphthenate solids under controlled conditions and formsthe basis for future studies in real oilfield fluids.