Abstract

Naphthenate precipitation is encountered when the naphthenic acids, present in acidic crude oils, interact with calcium and, to a lesser extent, other metal ions that are present in connate waters.Precipitation is often associated with a rise in solution pH.It can cause severe problems during the separation of oil and water, forming very stable emulsions and/or insoluble deposits in separators or crude dehydration/desalting vessels.

Acetic acid injection is widely used to inhibit naphthenate precipitation and remove calcium from crude oils.This has significant implications for the integrity of the process pipework and vessels.The impact of acetic acid on the corrosion of carbon steels under typical oilfield conditions is discussed. Laboratory and field data are presented to demonstrate the impact of acetic acid on corrosion rates.

A variety of mitigation options are discussed, including the impact of acetic acid on corrosion inhibition.These include the use of novel surfactants to inhibit calcium naphthenate precipitation.This type of naphthenate inhibitor has been shown to provide effective naphthenate control in the absence of any pH reduction, eliminating the detrimental effect of acetic acid injection on process pipework and vessels.

Introduction

Many recently discovered fields produce oil with a high acid content.The term naphthenic acid is used to account for all the carboxylic acids present in crude oils.Naphthenic acids in a crude oil are considered to be a class of biological marker closely linked to the maturity of the reservoir[1].They are produced during in-reservoir biodegradation of petroleum hydrocarbons and are found predominantly in immature heavy crudes[2,3].These naphthenic acids are complicated mixtures of cyclo-aliphatic carboxylic acids.An overview shows them to consist of C[10] - C[50] compounds with up to six fused saturated rings, with the carboxylic acid group attached to a ring by a short side chain[2].Traditionally the term naphthenic acid refers to all organic acids found in crude oil, with the acidity of crude oils often expressed as Total Acid Number (TAN).

Naphthenic acids are responsible for corrosion problems during the refining of acidic crudes[4].They are also responsible for a number of other problems in crude oil processing.The surface activity of naphthenic acids will influence the stability of crude oil emulsions and solids deposition[5].This is exacerbated by the dissociation of the acids to form the naphthenate anion as pH increases.As a result, the degassing of produced fluids during the separation process can form strong emulsions and naphthenate scales.In the absence of bicarbonate buffering, dissociation of the naphthenic acids will counteract the pH increase and impede naphthenate precipitation.If bicarbonate is available to buffer the solution, naphthenates will be produced.Monovalent salt bicarbonate structures (Figure 1) strongly stabilise emulsions, which can often constitute a significant fraction of production[6].The formation of the dissociated acid also favours the precipitation of metal ion soaps, principally calcium naphthenates.These calcium salts are neither oil or water-soluble and their relative density means they tend to accumulate at the oil/water interface in separators or dehydrators.These ‘chewing-gum’ like precipitates can fill vessels, block heat exchangers and clog control equipment.Exposure to air allows these scales to solidify, rendering them particularly difficult to remove.Naphthenate deposits are also found to co-precipitate with carbonate scales[7].

A number of approaches have been shown to be effective at emulsion breaking and naphthenate deposit inhibition[5].Several types of demulsifier were found to effective and mixtures with scale inhibitors produced very good results highlighting a synergistic effect between the two additives.Unfortunately the use of scale inhibitors with acidic crudes degraded the oil quality, increasing the water cut and calcium content of the crude oil.Although demulsifiers and dispersants have given encouraging results, they need to be adapted to the specificity of the crude[7].Any procedure that can prevent the pH from rising should be effective at preventing naphthenate formation.Acidification of the produced fluids is the most obvious solution[8], but a significant portion of the acid injected is consumed overcoming the bicarbonate buffering.Hydrochloric acid is the most cost effective option, but corrosion issues often result in the use of acetic acid. This also carries an increased corrosion threat.

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