Characterization, modelling, prediction and inhibition of Naphthenate deposits in oilfield production

Abstract

The production of acidic or sometimes heavier crudes often leads to the formation of either calcium naphthenate or sodium carboxylate soap emulsions during oil production operations. This problem has been recognised as a major flow assurance problem in the oil industry. In this work, both harder calcium naphthenates and more emulsion–like sodium carboxylates have been studied and these are viewed as “end members” in the spectrum of field naphthenate deposits which may occur. In this thesis, four related aspects of naphthenate deposits are studied, as follows: (i) Characterization of various field naphthenates samples from different parts of the world, involving naphthenic acid extraction method development and subsequent extract analysis using electrospray mass spectrometry (ESMS) and atmospheric pressure chemical ionisation mass spectrometry (APCI MS) techniques. It was observed that different naphthenic acid extraction and analysis methods affected the naphthenic acid identification and apparent composition; (ii) Geochemical studies have been carried out on various crude oils which form calcium naphthenate deposits, sodium carboxylate soaps and emulsions. Biomarker correlation analysis was employed on the aliphatic and aromatic fractions of the crude oil samples using gas chromatography (GC) and gas chromatography mass spectroscopy (GCMS); (iii) Thermodynamic modelling of naphthenate formation was carried out, and both a simple thermodynamic pH change naphthenate model and a full naphthenate precipitation model have been developed. This thermodynamic model describes the chemistry of a simple partitioning “pseudo” naphthenic acid which may form calcium naphthenate “precipitate”, according to a solubility product type description. This model has been used in a wide sensitivity study which makes some predictions about the way a precipitating Ca-Naphthenate (CaN) system behaves and these predictions have been confirmed experimentally; (iv) Naphthenate chemical inhibition studies have been performed in order to develop a methodology for the mitigation of these deposits. Static “inhibition efficiency” tests have been conducted using 8 different naphthenate inhibitors in order to determine the most appropriate methodology for testing and also the effectiveness of these chemicals in preventing the formation of calcium naphthenates and sodium carboxylate soap emulsions.