Naphthenate Formation in Oil Production: General Theories and Field Observations

Abstract

Naphthenate Formation in Oil Production: General Theories and Field Observations

Naphthenate formation in oil production has been an increasingly recognised problem over the last 5 to 10 years. It has broadly been ascribed to the formation of sodium and calcium salts of naphthenic acids that are found in the oil. This is thought to be triggered by the release of CO2 from the oil at sufficiently low pressures leading to a corresponding increase in pH which then leads to the formation of “soaps”. The form and behaviour of the observed soaps ranges from soft emulsion type materials to “harder” precipitate like solids depending on the cation composition of the brine and other factors.

At Heriot-Watt, initially an extensive series of experimental and theoretical studies has been carried out on model naphthenic acid/oil/brine systems in which the effects of a range of parameters on the naphthenate formation has been investigated, including acid type, cationic composition of the brine, initial pH, effect of CO2, etc. A number of systematic observations have been made which give some indication of an overall theory of naphthenate formation. This theory is by no means complete and its basis may be criticised since model naphthenic acids, brines and oils have been used. However, further studies using actual field crude oils and their corresponding soap deposits have also been performed to extend our findings to more realistic systems and our findings broadly support the conclusions from simpler model systems. The Oil Plus group has extensive experience of observing, assessing and monitoring field naphthenate problems in many parts of the world, on pressurised fluid streams. They too have arrived at some general views of how naphthenates form and what the key factors are in their formation (and also in how they may be avoided).

In this paper, we present a synthesis of the various theories of soap formation and how these correspond to the field observations from a number of reservoirs. Several “competing” theories are presented and in particular we discuss the role of the recently discovered molecular weight acids collectively known as ARN (M.Wt. » 1230). However, there are in our view three or four key mechanisms that are at work and the differences between observed field naphthenates reflects (a) the different oil compositions in terms of their naphthenic acid content, (b) the CO2 content of the system and the brine composition, particularly the sodium/calcium balance and the bicarbonate content. We believe that these factors affect the balance between the various mechanisms, which we describe in this paper. The ideas presented are supported with a number of field examples of naphthenate formation.