The prediction and prevention of both sodium and calcium naphthenate "scales" is an important issue in oil production. A broad description of how these scales form has been available for some time although most experimental findings are still of a qualitative nature. In this paper, we present an equilibrium thermodynamic model for predicting naphthenate partitioning and precipitation in an oil/brine immiscible system from some chosen initial conditions (i.e. naphthenate initial concentration in oil, brine pH and [Ca2+] etc). This model has, with some assumptions, been applied to both model and real naphthenate system.

This model describes two types of naphthenate experiment, viz.

  • full naphthenate precipitation, and

  • simpler "pH change" experiments where no precipitation occurs.

In order to predict naphthenate precipitation, the theory suggest that we need to know

  • the partition coefficient of the naphthenic acid, HA, between the oil and the water phases, Kow;

  • the pKa of the naphthenic acid in water; and

  • the solubility product, KCaA2 (or other similar solubility parameter), of the naphthenate deposit.

In the simpler pH change experiments, we only require the first two of these parameters, i.e. Kow and pKa. Using the naphthenate model without precipitation, we studied the effect of varying parameters on the degree of pH change predicted at equilibrium in the oil/naphthenic acid/brine system. We also applied the model to examine the sensitivities of the various parameters on the final pH. The comparison between the model predictions and experiment at a higher brine pH value is overall satisfactory.

Background and Introduction

The development of acidic, sometimes biodegraded, crudes in different parts of the world may lead to naphthenate problems during oil production 1–11. These naphthenate deposits have become an increasing flow assurance problem due to deposition and process disruption in production facilities 1,6,8,9.

A general view of the deposition mechanisms across the spectrum of naphthenate "scales", from sodium rich emulsions to calcium naphthenate deposits, was recently put forward by Sorbie et al.11. This work took the view that, in order to prevent naphthenate soap problems, we needed to develop a view of both the basic mechanism(s) through which they formed and a thermodynamic model to quantitatively describe their formation. An outline mechanism for the formation of naphthenate deposits was presented previously by Rousseau et al.9. Subsequent studies have used this conceptual mechanism to study deposition in both model and field naphthenate systems 11.

Naphthenic acids in hydrocarbons are defined as compounds containing carboxylic groups attached to saturated cyclic structures.7,10–17 There is a tendency for the naphthenic acid to partition between the oil and water phases during production. Once in the water phase, the weak acids will dissociate in accordance with normal equilibrium. However, with the exception of low molecular weight acids, naphthenic acids are relatively insoluble in water.10 The partition coefficient from the oil phase into the water phase (Kow) therefore tends to increase as the molecular weight of the naphthenic acid decreases. For a single naphthenic acid, this mass transfer process is governed by the two main quantities the oil-water partitions coefficient, Kow, and the acid dissociation constant, Ka. The study is aimed at using the model to use measured final brine pH (pHf) in "pH change experiments" from which we can back calculate the quantities, Kow and Ka. These "pH change experiments" are where we allow a naphthenate containing oil to contact a brine of given initial pH (pHi) and [Ca2+] and then let the system equilibrate without the formation of naphthenate precipitates. Only this type of experiment is presented in this paper.

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