Equilibrium Partitioning of Naphthenic Acid Mixture Part 2: Crude Oil-Extracted Naphthenic Acids

Abstract

Crude oil naphthenic acids can partition into the water phase during oil production. Variations in production parameters such as temperature, pressure, pH, and water cut affect partitioning. In a previous article, a method was developed to determine the partition ratio for mixtures of naphthenic acids and tested on a commercial acid mixture. In this study, the method is implemented on a naphthenic acid mixture extracted from an acidic crude oil. Compositional analysis of the extracted acid mixture reveals a broad structural distribution consisting mainly of saturated ring structures with 2 to 3 rings. Examination of the GC/MS method revealed a distribution bias toward low molecular weight compounds, and a correction method was explained and applied. The equilibrium partitioning as a function of pH for the acid mixture and molecular mass ranges within the acid mixture was determined by GC/MS. An extracted crude oil acid mixture dissolved in toluene was used as oil phase, and 3.5 wt % NaCl aqueous buffers were used as water phase. Dividing the acid mixture into molecular weight ranges, characterized by a single partition ratio Pwo allowed the oil–water partitioning to be successfully modeled. The variation of the cologarithm of the partition ratio pPwo with naphthenic acid molecular weight was compared with previously published experimental and simulated data. The presence of calcium reduced the partitioning of the acid mixture at high pH for larger acid molecules. Analysis of the resulting oil phase revealed a calcium content consistent with oil-soluble calcium naphthenate in agreement with results found for a commercial acid mixture.