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Abstract

Recent research findings suggest that wettability modification holds a great potential for increased oil production from mixed-wet and fractured carbonates. Our current knowledge of the field-scale performance of these processes is very limited despite very favorable results obtained in the laboratory-scale experiments. Reservoir simulation is required to properly scale up the process from laboratory to field conditions and to understand and interpret reservoir data. Without a mechanistic simulation tool and adequate scale up, it is unlikely that a cost-effective process can be developed and applied economically. The predictive simulation and methodology to scale up such a complex process will reduce the risk of failure of field projects. 

A chemical compositional reservoir simulator with the capability to model oil recovery from mixed-wet carbonate rocks in both static imbibition and dynamic fractured block experiments using chemicals to alter wettability is used for this scale up study. The simulator captured the key recovery processes of capillary and natural imbibition, wettability alteration, buoyancy, oil mobilization, and viscous pressure gradient in imbibition experiments.

Proper scaling from laboratory to field indicates that the synergy of wettability alteration, ultra low interfacial tension, and emulsification under small viscous pressure gradient provides an attractive and profitable opportunity in fractured carbonate reservoirs. Dimensionless scaling groups and numerical simulations are presented for each experimental condition to aid in understanding the time dependence and up-scaling of the laboratory results to field scale applications. 

The oil recovery results of the static imbibition experiment were successfully scaled using a reference time based on gravity emphasizing that the buoyancy was a dominant mechanism in this case.  The scale up simulations for the dynamic fractured block experiment indicated favorable conditions for field scale applications with more dominance of viscous forces.

Introduction

Laboratory alkali and surfactant floods have shown a great potential in increasing oil recovery for reservoirs that are naturally fractured with low permeability mixed-wet matrix rocks. Fractured, mixed-wet formations usually have poor waterflood performance because the injected water tends to flow in the fractures and spontaneous imbibition into the matrix is generally insignificant. Surfactants or alkalis have successfully been used to change the wettability and enhance oil recovery by increased imbibition of the water into the matrix rock. The oil recovery mechanisms using surfactant/alkali mixtures are enhanced imbibition and buoyancy due to combined effects of reduced interfacial tension, reduced mobility ratio, and wettability alteration.

Several authors evaluated the relative contribution of gravity and capillary forces on oil recovery in imbibition experiments (Hirasaki and Zhang, 2004; Babadagli and Boluk, 2005; Hognesen et al., 2004; Zhang et al., 2008).  The explanation common by these authors are that the surfactant or alkali chemicals modify the wettability of the rock to more water-wet allowing a counter current water imbibition as a capillary-driven imbibition while reducing interfacial tension by surfactant causes gravity forces to prevail. 

Hognesen et al. (2004) tested the dimensionless time correlation developed by Li and Horne (2006) for their imbibition experiments in carbonate rocks performed for a wide range of experimental conditions of interfacial tension, permeability, initial water saturation, core height and diameter, temperature, and sulfate concentration. All the parameters were scaled very well when the normalized oil recovery was plotted versus dimensionless time once the height of the core was used as the shape factor.  They concluded that gravitational forces were significant oil recovery mechanisms in their experiments.