Abstract

Naphthenic acids in a crude from field A in West Africa in combination with high pH connate waters cause severe problems during the separation of oil and water. The neutralization products of those acids with basic components in the water tend to form very stable water-in-oil emulsions and insoluble sticky naphthenate sludge. These phenomena considerably hamper oil production and result in high treatment costs using acetic acid.

This paper discusses the ways problems were prevented during a large field trial with a combined demulsifier/naphthenate inhibitor based on a physical and chemical approach. Innovative technology in surface-active chemistry has demonstrated an effective way to reduce or eliminate the problems described.

In addition, lab tests were performed on an instrumented bottle test that enables the different phases of decantation (Coalescence, sedimentation, dense emulsion) to be measured. Two conventional demulsifiers used in the field were tested and compared to this new combined demulsifier/naphthenate inhibitor. The new chemical prevented the formation of dense emulsion and resulted in better water quality.

Further experiments demonstrating the unique challenges associated with naphthenates in different fields around the world are also presented.

Introduction

Crude oils usually contain naphthenic acids. While in most crudes these small amounts of acids are of no importance, in some crudes they can become a nuisance if they come in contact with the produced water. Depending on the type and amount of naphthenic acid and the type of water, the problem can be four-fold:

  • Very tight and strong emulsions formed by the salts of the more aliphatic organic acids in the crude. In the worst case this type of emulsion behaves like sludge (1).

  • The large quantities of organic acids in oilfield produced waters, formed by the water soluble salts of very low molecular weight naphthenic acids cause significant oil in water problems(2).

  • The oil soluble calcium salts of the low molecular weight naphthenic mono-acids generate high calcium content in crudes.

  • Sticky deposits formed by the polymeric salts of calcium and/or iron with ARN, a high molecular weight naphthenic acid with four acid groups(3,4).

For the field A only the first two points are of interest (5): strong emulsions over the entire field and large quantities of organic acids in the produced water. Dehydration using conventional demulsifiers was difficult, resulting in very inefficient separation of the 1st separator and addition of considerable amounts of acetic acid downstream of the 2nd stage separator to achieve an export crude oil that meets the specification.

Drawing on the know-how of treating crudes with similar problems in the North Sea and in the South China Sea, and the R&D currently underway in different parts of the world to understand the behaviour of naphthenic acids in crudes, it should be possible to develop a product which would reduce or eliminate the use of the corrosive acetic acid in the field. The product to be developed should essentially break the existing tough emulsion into clean water and dry oil without inducing the formation of any remaining slop oils or sludges.

To obtain the required effect, the product to be designed has to be a combination of demulsifying agents and naphthenate inhibitors. Experience in treating emulsions for 27 years in the neighboring country of Nigeria offered a vast pool of demulsifier chemistry for building the required product. After an intensive off-shore testing period, a new, completely neutral non-corrosive product was formulated.

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