Mobilization of Remaining Oil - Emphasis on Capillary Number and Wettability
Authors: Johannesen Else Birbeland , Graue Arne 
Published: December 2007

The ratio between the viscous and capillary forces, commonly denoted the Capillary Number Nc, is crucial in determining the remaining oil saturation. The impact on residual oil saturation by a systematic increase in Nc is determined in homogeneous chalk at wettabilities varying from nearly neutral-wet to strongly-water-wet conditions. In fractured chalk reservoirs waterflood residual oil saturation is strongly dependent on the wettability. The current results provide assistance in determining the potential target for tertiary oil recovery by measuring the amount of mobile oil at various Nc. A series of displacements of oil by water injection at constant pressure were carried out to determine the relation between oil recovery and applied capillary number in waterfloods at different wettability conditions.

Maximum oil recovery at constant Nc occurred at wettability conditions reflecting an Amott Index to water at 0.3. The remaining oil decreased with increasing capillary number and significant trapped oil after completed spontaneous water imbibition was mobilized at moderately water-wet to nearly neutral-wet conditions. Similar results as reported in the literature for waterflooding residual oil saturations as function of wettability and PV water injected in sandstone were found for chalk at increasing capillary number. Distinct dome shaped curves of oil recovery as function of wettability, with consistent increase in oil recovery with increasing capillary number, reflected similarities to earlier results on waterflooding oil recovery.

Introduction

The opportunity window for implementing IOR schemes for a given reservoir in production is limited when reaching the tail production. It is vital that the amount of potential target oil for EOR is determined as early as possible, and in this respect that the ultimate immobile or residual oil saturation for the rock/crude/brine system at the wettability conditions present in the reservoir is determined. Oil recovery depends strongly on the wettability condition and the interaction between the capillary and viscous forces will accordingly change with wettability [1, 2]. This study emphasizes the impact from wettability on the residual oil saturation during increasing differential viscous pressure drops at various wettabilities.

Capillary forces are responsible for fluid entrapment during an immiscible displacement in porous media. Laboratory studies have shown that more of the remaining oil may be recovered in immiscible displacements if increased viscous displacement is applied. By exceeding the capillary forces trapped residual oil may be mobilized [3]. The capillary forces are determined by the wettability conditions and the oil/water interfacial tension (IFT). The potential to mobilize capillary trapped oil depends on the pore geometry. The required viscous force needed to mobilize trapped oil is determined by the fluid dynamics of the displacing phase. Thus an important parameter determining mobilization of capillary trapped oil during immiscible fluid displacements is the capillary number; exhibiting the ratio of viscous forces to the capillary forces.