An Experimental Evaluation Of Viscosity Grading For Controlling Fingering In Miscible Displacements
Authors: Sarma Hemanta K. , Maini Brij B. 
Published: March 2010

Abstract
Displacement of oil by miscible flooding processes is adversely affected by the hydrodynamic instability at the flood from whenever the viscosity ratio is unfavourable. Instabilities cause viscous fingers to grow and make the displacement process inefficientand uneconomical. Therefore, it is necessary to develop ways of reducing or eliminating the detrimental effects of viscous instabilities to make miscible flooding processes economically viable under adverse viscosity ratio conditions. Grading the viscosity of the displacing fluids appears to be an effective technique for controlling viscous instabilities.
This paper presents an experimental evaluation of the efficacy of graded-viscosity solvent banks for improving the performance of solvent flooding under very adverse viscosity ratios. A scaled physical model with transparent top surface was developed for visual examination of viscous fingers during solvent floods. Base case displacement experiments were carried out to quantify the effects of viscosity ratio and flood velocity on the development of viscous fingers and the resulting displacement efficiency. The displacement experiments were then repeated with graded-viscosity solvent slugs to determine the effectiveness of such viscosity grading. Two different methods of designing graded-viscosity banks (the Claridge and the Coskuner- Bentsen methods) were examined.
Results show that the extent of viscous fingering present and the resulting displacement efficiency, in the base case displacements, varied significantly with viscosity ratio and flood velocity. The use of graded-viscosity banks always reduced, and often, totally eliminated the viscous fingering.
In this study, the viscous target oil was used as an additive for increasing the solvent viscosity. For field application of the graded-viscosity bank process, less expensive additives for increasing the viscosity of the solvent will be needed. Until such additives become available or the procedure can be improved upon, the use of graded-viscosity banks in miscible solvent flood appears to be uneconomical.

Introduction
Miscible flooding has received considerable attention in recent years because of its inherent potential to achieve a higher recovery fficiency. A 1989 survey by Thomas et al.(1) suggests that miscible flooding was the most popular method with operators in [he United States during 1980–1981, However, because of the high material and operational costs the process has become less attractive at the current uncertain oil prices.
Although the miscible displacement process is not considered an obvious choice for heavy oils, it may have some potential in thin heavy-oil reservoirs where thermal methods are not suitable because of excessive heat losses to the underlying and overlying strata. However, in the case of miscible floods, high viscosity ratios between the oil and solvent often result in viscous instabilities at the front, and lead to a lower recovery efficiency. Therefore, it is important to minimize the extent and effects of viscous instabilities.
Fingers in a miscible flood tend to generate a transition zone at the interface due to mass transfer. The transition zone, whose omposition exhibits continuous gradation in viscosity due to blending, arrests fingering of displacing fluid to an extent(J).