The Influence of Rheological Properties on Mobility Control in Polymer-Augmented Waterflooding

Abstract

Abstract One of the major restrictions to increased oil recovery from reservoirs is poor sweep efficiency, caused by an interfacial instability phenomenon termed "viscous fingering". This paper reports the results of an experimental study into viscous fingering in two-phase flow, using a one metre diameter Hele-Shaw cell. Newtonian and non-Newtonian fluids were tested, including shear-thinning and ideal elastic (Boger) fluids. The viscous fingering patterns exhibit fractal behaviour, which enable both characterisation of the displacement pattern and extrapolation to larger scales. Rheological properties can have a significant influence on the viscous fingering pattern and thus on sweep efficiency. Shear-thinning fluids exhibit significantly increased interfacial instability, especially at the rear interface with the chasing waterflood. Elastic properties have a negligible effect on the macroscale development of viscous fingers, but may have an important role at the microscale, by modifying the viscosity ratio for flow in porous media. Results to date suggest that the greatest improvement in sweep efficiency for EOR polymer flooding may result from the development of a pusher fluid with viscoelastic non-shear-thinning properties. Introduction Worldwide, the oil recovery from reservoirs averages less than 50% of original oil in place (OOIP). The remainder represents a target for a range of enhanced oil recovery (EOR) programs. One of the cheapest methods of EOR in terms of cost per barrel of oil produced (Figure 1) is polymer-augmented waterflooding, also caned polymer flooding. A simplified sketch depicts this process (Figure 2). Potential oil recovery from waterflooding is often severely reduced by an interfacial instability phenomenon termed "viscous fingering", where the driving fluid forms characteristic fingers extending into the displaced fluid, resulting in low sweep efficiencies and thus low oil recoveries. Methods of reducing the fingering (and thus increasing the sweep efficiency) include improved mobility control and reduced reservoir heterogeneity. Polymer solutions can be used to achieve either or both of these objectives. The main objective of polymer-augmented waterflooding is improved mobility control, achieved by boosting the viscosity of the driving fluid and thus reducing the viscosity ratio at the interface. The polymer solutions used for polymer-augmented waterflooding are non-Newtonian fluids, which can exhibit substantial shear-thinning behaviour and large extensional viscosities in extensional flow. Both of these properties can be important for flow of non-Newtonian fluid in packed beds. However, little work has been done on their influence on mobility control in EOR. Without this fundamental information, estimating the influence of different polymer additives and conditions will be inspired guesswork. An improved understanding of the link between non-Newtonian flow properties and interfacial instability (and thus mobility control) will improve the selection guidelines for polymer flooding operations and provide a basis for future industrial work. The results also have relevance to work on other forms of enhanced oil recovery, many of which involve two-phase flow and have potential interfacial instability problems. BACKGROUND THEORY Two-phase flow in porous media can exhibit unstable displacement which is generated either by the fluid properties, or by the porous medium properties. P. 449^