A NOVEL RECOVERY MECHANISM FOR POLYMER FLOODING IN GRAVITY-DOMINATED VISCOUS-OIL RESERVOIRS

Development of North Sea viscous-oil fields represents a new challenge to the industry. A number of these reservoirs comprise relatively cle an homogeneous sands with high horizontal and vertical permeabilities. The oil viscosities are in the range of 5 to 200 cp and oil columns a re typically < 200 ft thick and often partially underlain by water. Wa terflooding is dominated by gravity in the interwell region, with wate r slumping to the bottom of the reservoir and coning up to producers. Development depends on the use of horizontal wells to reduce the conin g. The viscous oil and poor vertical sweep make these reservoirs possi ble targets for IOR with polymers. This study shows that polymer incre mental recoveries can be high, well within the range of potential econ omic viability. The incremental recovery mechanism is not the usual im proved Buckley-Leverett sweep efficiency resulting from correction of an adverse displacement ratio. A novel recovery mechanism is identifie d in which the polymer increases the ratio of viscous to gravity force s, 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 w ith conventional isothermal simulations shows that, counter to normal expectations, isothermal simulations can underpredict recovery by > 10 %.