Development of North Sea viscous-oil fields represents a new challenge to the industry. A number of these reservoirs comprise relatively clean homogeneous sands with high horizontal and vertical permeabilities. The oil viscosities are in the range of 5 to 200 cp and oil columns are typically < 200 ft thick and often partially underlain by water. Waterflooding is dominated by gravity in the interwell region, with water slumping to the bottom of the reservoir and coning up to producers. Development depends on the use of horizontal wells to reduce the coning.
The viscous oil and poor vertical sweep make these reservoirs possible targets for IOR with polymers. This study shows that polymer incremental recoveries can be high, well within the range of potential economic viability. The incremental recovery mechanism is not the usual improved Buckley-Leverett sweep efficiency resulting from correction of an adverse displacement ratio. A novel recovery mechanism is identified in which the polymer increases the ratio of viscous to gravity forces, reducing the water slumping. This leads to improved vertical sweep, recovering oil from regions of high oil saturation at the top of the reservoir. We simulated polymer flooding including temperature effects. Comparison of waterflood simulations including temperature effects with conventional isothermal simulations shows that, counter to normal expectations, isothermal simulations can underpredict recovery by> 10%.
Many polymer-augmented waterfloods have been performed in onshore fields, but offshore experience is far more limited. The only published applications in the North Sea have been profile modification treatments to alleviate water channeling through thief zones in heterogeneous formations. This paper reports a simulation study of polymer flooding in gravity-dominated, viscous-oil reservoirs that uses a conceptual reservoir model. Temperature effects are included because these can have a significant effect on injectivity and polymer stability. Waterflood and polymer-flood recoveries are compared, and the incremental oil recovery mechanism for polymer flooding is identified. Simulations are performed for a range of water-leg thicknesses to assess the effect of underlying water on the polymer-flooding efficiency. Waterflood simulations including temperature effects are compared with conventional isothermal simulations to assess the importance of temperature in conventional waterfloods.