Abstract

Organic field deposits from distinct geographical regions were analysed using a wide range of analytical techniques, viz. for cation composition (EDAX), diffraction patterns (XRD), thermal profiling (DSC/TGA), naphthenic acid distribution using electrospray mass spectrometry (ESMS), nuclear magnetic resonance 1H NMR and solid state 13C NMR. Clear distinctions for end member soap types were observed with regard to the type and amount of cations, the naphthenic acid content, as well as their thermal behaviour. Specific soap samples were analysed along with their parent soap forming crude oils collected from the same field over a period of one year. The nature of two of these soap samples were found to be related to the particular chemical treatment on site. There were clearly observable differences in the final location within the surface facility, as well as the final composition (calcium content, acid distribution, presence of other chemical families) of these samples. These were suggested to be related also to crude oil chemistry changes and mitigation (chemical) strategies used. The implications of these new findings on the basic mechanisms of soap formation are discussed.

Introduction

The occurrence of solid deposits downhole, in pipelines, or in surface facilities during crude oil production is one of many flow assurance problems facing operators. The origin of solid deposits can broadly be divided into inorganic (e.g. calcium carbonate, barium sulphate, collectively known as "scale") or organic (e.g. asphaltenes, waxes and soaps). The correct sampling of such solids, and their parent crude oils, in addition to their transportation and analysis in the laboratory, is central to the study of the various deposition mechanisms. Soaps represent a relatively new category of flow assurance problem which has been the focus of much recent study[1–8]. Soaps can essentially be described as either divalent (e.g. calcium) or monovalent (e.g. sodium) salts of naphthenic acids1. The former are usually referred to as calcium naphthenates (soap scales) and the latter as carboxylates (soap emulsions)[1,3]. From an operational standpoint, calcium naphthenates harden on exposure to air, for reasons not yet fully understood. Sodium carboxylates do not harden, but remain at the oil water interface enhancing the stability of emulsions, and they may incorporate other solids to form sludges throughout the production train[4,5]. Soaps are believed to be formed from the reaction between the produced water cations with naphthenic acids from the crude oil[1,6]. This takes place during fluid flow from reservoir to surface conditions, which results in significant brine pH changes. Variations in the system pH in turn trigger the partition and dissociation of the crude oil naphthenic acid content. In the literature, soap formation has been reported as only occurring in surface facilities, and to our knowledge there have been no confirmed cases of these solids occurring downhole or causing formation damage[3,5,7]. The basic soap deposition mechanism has been introduced by Rousseau et al6. More detailed attempts to study the underlying scientific fundamentals for soap deposition have been reported[1,2,8,9]. A preliminary thermodynamic model has also been used to describe the formation of soaps under controlled laboratory conditions[1].

There are many current challenges in the study of soaps including their initial identification by field personnel, the analysis of their precise naphthenic acid components and the development of prediction tools and mitigation strategies.

The ARN species is a particular naphthenic acid (or small family of compounds) which was first identified in soap scales and characterized by Baugh and coworkers10,11. More recently, certain geochemical implications of the presence of this acid family have been postulated[12]. It has been suggested that the ARN acid is always present in soap scale type deposits (calcium naphthenates) where it is believed to be play an integral role in the deposition mechanism. The acid is not thought to be an active contributor in soap emulsion formation (sodium carboxylate soaps). The ARN species is believed to be present in very low concentrations (< 20 ppm) in crude oils. It has also, however, been identified in crude oils in fields with no known deposition problem[13].

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