The effectiveness of any enhanced oil recovery process is influenced by the mixing of injected and reservoir fluids since the oil recovery efficiency of the injected fluids usually deteriorates with dilution. Reservoir heterogeneities affect this mixing, and some of the poorer-than-expected oil recoveries in pilot operations can be related to reservoir heterogeneities.
Composite, stratified cores can be used to model heterogeneous systems in the laboratory. Such cores are prepared by assembling two different core materials side by side in a way that assures capillary contact between the fluids in the two core components. This paper describes polymer floods in composite cores consisting of Berea sandstone and silicon carbide. There is an approximate tenfold difference between the permeabilities of these two materials.
Pfizer biopolymer Flacon 4800 was used in all of the polymer floods, and the waterflood oil recoveries were compared with the incremental oil recoveries resulting from polymer injection. In addition, polymer floods were conducted in composite cores in which the water-wet characteristics of either one or both materials were altered to predominantly oil-wet by treatment with a Quilon-S solution. Results show that the effectiveness of polymer flooding is enhanced in oil-wet situations.
A number of laboratory studies have shown that, in heterogeneous systems, injection of a polymer solution can significantly improve oil recovery over that of a waterflood.1,2 However, the improvement in oil recovery reported for field applications of polymers ranges from excellent to none at all. Such disparity in results has not stopped polymer flooding from being one of the most frequently used enhanced oil recovery methods. The main reason for this lies in the apparent simplicity of adding small amounts of polymer to the injection brine, a procedure requiring only minor modifications to existing injection facilities.
Although polymer flooding appears to be a simple process, the widely varying results of field applications indicate that a detailed reservoir description and evaluation of polymer-rock and polymer-reservoir fluids interactions is necessary in order to select the most suitable polymer and its mode of injection.
This paper is part of a general study evaluating and comparing sweep efficiencies in waterflooding and other enhanced oil recovery processes. In this part of the study, polymer flooding is evaluated in stratified cores comprised of two consolidated materials of widely differing permeabilities – Berea sandstone and silicon carbide. The mounting of the two materials in parallel, with the fluids in both parts of the core in capillary contact, approximates reservoir heterogeneities while permitting vertical crossflow. This is in contrast to the majority of laboratory studies on heterogeneous systems which used simultaneous injection into parallel, but separate cores. In such experiments, there is no capillary contact between the fluids in the different permeability zones, and therefore crossflow between reservoir layers is not represented.
The results reported here include comparisons of oil recoveries for the water and polymer floods, and the dependence of oil recovery on the size of the polymer slug. The role of wettability in polymer flooding was also evaluated.