Development and Field Application of a Novel Non-acid Calcium Naphthenate Inhibitor

Abstract

Abstract Current state of the art for treating calcium naphthenates requires addition of large doses of acid to the production fluids. Although effective in dissolving deposits, acid treatments result in increased corrosion in production equipment, decreased oil quality and increased transport and HSE costs. Consequently, an opportunity exists for the development of non-acid inhibitors to prevent calcium naphthenate formation without the drawbacks encountered with acid treatment. In this account, we report on the discovery and development of molecules that, when injected into a production stream, prevent the formation of large scale calcium naphthenate deposits. As a result, these molecules allow for treatment of calcium naphthenate deposits without treatment with large acid dosages. The purification scheme for naphthenic acids and a novel screening test method capable of selecting highly active naphthenate inhibitors will be detailed as well as field testing results in which no deposits formed and excellent water quality was obtained. Introduction Calcium naphthenates are sticky, sludgy solid deposits that form at the oil-water interface, aggregate in the production medium and then collect in oilfield production equipment. As the deposit amounts increase, they reduce the capacity of in-line separators and clog filters and screens. Clean up is exacerbated by the tendency of calcium naphthenate deposits to harden to a rock-like consistency, resulting in costly production shut downs. Calcium naphthenate solids are composed primarily of carboxylic salts and are similary but distinctly different from another naphthenate field deposit - sodium naphthenate. Calcium naphthenate solids are formed by the interaction of high molecular weight naphthenic acids present in crude oil and divalent ions (Ca2+, Mg2+) in the formation water, while sodium naphthenates are emulsions formed by the interaction of naphthenic acids and monovalent (Na+, K+) ions present in crude (Goldszal, 2002; Dzidic, 1988). The main difference between the two is the identity of the naphthenic acid forming the solids. Sodium naphthenates are generated by smaller, less substituted water soluble naphthenic acids. Calcium naphthenate formation, though, depends largely on the presence of tetra-acid naphthenic acids (ARN) in the crude (Fig. 1) (Baugh, 2005; Lutnaes, 2006). Tetra-acids, when they interact with divalent ions, form ionic crosslinks between the multiple acid groups on the tetra-acid. This results in a highly crosslinked network, that traps solid material at the interface resulting in the oilfield solid. For a solid to be considered a calcium naphthenate it must contain some quantity of the tetra-acid and a significant amount of calcium ion. Samples that are composed of carboxylic acids, but do not contain the tetra-acid are not considered calcium naphthenate solids. The tetra-acid is believed to be derived from a natural source - Archaea (Lutnaes, 2006). It is understood that molecules within the cell walls of Archaea undergo diagenetic and catagenic transformations to become tetra-acid naphthenic acid molecules. Arachae are prevalent throughout the biosphere and are concentrated in many extreme environments. It is natural then to expect that the occurrence of calcium naphthenates in production systems is worldwide and is a particular concern in the North Sea, offshore Africa, offshore China and the Gulf of Mexico.