Effects of Petroleum Sulfonate-lignosulfonate Surfactant Mixtures On the Coalescence Behaviour of Oil Droplets
Authors: Son J.E. , Neale G.H. , Hornof V. 
Published: March 2010

Abstract
The coalescence characteristics of oil droplets dispersed in a petroleum sulfonate-lignosulfonate surfactant system have been investigated. The measurements, conducted in an inclined spinning drop tensiometer, involved the coalescence of two isolated oil droplets placed in a continuous aqueous solution of the surfactant. The apparatus permitted the determination of coalescence rate as a function of varying buoyancy forces and contact radii between the droplets. Both model oil (isooctane) and crude oil systems were studied.
The results show that, at a given contact radius, rapid coalescence rates are associated with low interfacial tension systems, indicating a strong influence of interfacial tension on coalescence rate behaviour. It is postulated that an ultra low tension system is in a state where the film interface becomes fully mobile due to the presence of the third phase generated at the oil-water interface, resulting in rapid drainage. For systems at low NaC1 concentration, the droplets are completely stabilized. The addition of lignosulfonate destabilizes the system and the droplets begin to coalesce. This provides new evidence that the salt (NaC1) and the lignosulfonate (Marasperse C-21) playa similar role in both phase and coalescence behaviour. Because the range of contact radii investigated was of the same order of magnitude as the pore size in many underground reservoirs, the coalescence data presented here could prove useful in reservoir simulation studies.

Introduction
The mobilization of oil droplets from the pores of the reservoir rock by low-tension surfactant flooding does not alone guarantee an efficient recovery. Model studies of ganglia dynamics (break-up and coalescence) by Payatakes et al.(2,3) have clearly demonstrated that without coalescence, isolated oil droplets or ganglia will eventually experience break-up and/or re-entrapment. The need for a better understanding of coalescence phenomena was recently underlined by Taber(4).
It has been known for some time that physico-chemical phenomena, which determine the phase behaviour of oil water- surfactant systems and interfacial effects, playa significant role in the coalescence process. These include the stability of oil-water emulsions and the properties of the oil-water interface. These properties include the composition, film thickness, interfacial tensions, charge, resistance to shear and expansion (interfacial viscosities), all of which influence the coalescence behaviour. These properties may ultimately influence oil bank formation and thereby the oil-recovery efficiency in chemical flooding processes.
A number of recent studies have suggested that poor oil recovery efficiency is due to emulsion stability problems. Schechter and Wade (5)observed that those systems which spontaneously emulsified and rapidly coalesced yielded better oil recovery than those systems which spontaneously formed stable emulsions. Other investigators (6,7), reporting on the results of the Salem low-tension water flood field tests (which used Witco TRS 10–80 petroleum sulfonate), found stable oil in- water emulsions at an observer well in addition to emulsion problems at the production well and observed that the actual oil recovery was only about one-quarter of the target value. Vinatieri (8) presented results of experiments dealing with the stability of crude oil/water emulsions similar to those that would be produced during a surfactant flood.