ABSTRACT: Calcium Naphthenate precipitation is often initiated by a rise in solution pH, as produced fluids are degassed. 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 employed to maintain a low pH and prevent solids precipitation. This has significant implications for the integrity of the process pipework and vessels. The impact of acetic acid on weld-corrosion under typical oilfield conditions is reviewed and data presented to demonstrate its effect on corrosion rates. Typically, this increased threat to system integrity is mitigated by corrosion inhibition. Laboratory test data demonstrated the importance of selecting the correct inhibitor chemistry in order to eliminate preferential corrosion of the weld or heat affected zones. The effects of acetic acid concentration and weld metallurgy on corrosion rates are discussed. The development of an environmentally acceptable alternative to the existing inhibitor chemistry is also described.
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 reservoir1. They are produced during in-reservoir biodegradation of petroleum hydrocarbons and are found predominantly in immature heavy crudes2,3. These naphthenic acids are complicated mixtures of cyclo-aliphatic monocarboxylic acids. An overview shows them to consist of C10 - C50 compounds with up to six fused saturated rings, with the carboxylic acid group attached to a ring by a short side chain2. 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 crudes4. 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 deposition5. 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. 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. A number of approaches have been shown to be effective at emulsion breaking and naphthenate deposit inhibition5.