The Interfacial Shear Viscosity of Crude Oil/water Systems

Abstract

Introduction This paper reviews the developments in the measurement of interfacial shear viscosities. A laboratory approach has involved the use of a biconical bob tension pendulum viscometer. The problems encountered in its use are explained, the special characteristics related to interfacial behaviour are detailed, and the relevance of the method to emulsion stability and enhanced oil recovery processes are discussed. Introduction Measurements of liquid/liquid and liquid/air interfacial shear viscosities have found relevance in such diverse studies as molecular interactions in polymer systems(1), transport processes(2), foam stability(3), emulsion stability(4) and displacement mechanism in enhanced oil recovery processes(5). Over the past few years, there have been significant developments in the measurement of interfacial shear viscosities(6–14). In many of these instances, a theoretical hydrodynamic analysis of the flow patterns in the neighbourhood of the liquid interfaces is presented. However, with very exceptions, these new approaches still possess shortcomings when applied to crude oil/water systems. In particular, it is a difficult task to design practical surface viscometers capable of giving interfacial flows with constant shear rates. Thus, the investigation of non-Newtonian interfacial flows is plagued by interpretation problems(8). Most approaches assume a Newtonian film behaviour. Hedge and Slattery(15) have proposed a method whereby the non-Newtonian character, frequently encountered with crude oil systems, can be estimated from a study of the complete velocity profile across the channel of a viscous viscometer. However, this technique, as well as others that rely on centre-line velocity measurements using PTFE markers, become extremely difficult when dealing with one liquid which is opaque (eg. crude oil). Wasan et. al.(13) have proposed a modified approach for the study of crude oil/water interfaces based on the canal viscometer of Mannhehner and Schechter(16). Here they propose the measurement of the centre-line velocity in the canal at the crude oil/ air interface when the canal contains only crude oil. They then set up a crude oil/water interface in the canal and remeasure the centre-line viscosity at the crude oil/air interface. These two results are then mathematically treated to reveal the crude oil/water interfacial viscosity. In view of the special difficulties encountered with crude oil, one approach that has been used in this laboratory(4,5,17,18) has been to make use of a biconial bob torsion pendulum viscometer. Although it has been pointed out that such a (Equation in Full Paper) device fails in the region of low interfacial viscosity(8), it is very effective for measuring high surface viscosities. It is the belief of the authors that only high interfacial viscosities are of practical significance in crude oil/water systems. The instrument provides a suitable approach for their measurement. The use of this apparatus, however, poses some problems (see Figure 1 and appendix). One must especially ensure that a reliable torque transmission property of the interface is measured. In particular, three problems are frequently encountered, viz:Does slip occur at the extremities of the interfacial annulus (i.e. at the bicone bob or at the glass container wall)?