Steady-State Measurements of Relative Permeability for Polymer/Oil Systems

Abstract

Abstract Means for increasing tertiary oil recoveries from previously waterflooded viscous oil reservoirs are receiving added attention today as a result of industry-wide efforts to improve U.S. oil producing rates and reserves. Injection of a bank of polymer solution that precedes injection of a miscible slug (e.g., a micellar fluid) can reduce reservoir permeability contrasts and result in improvement of the sweep efficiency of the process. To evaluate the potential magnitude of improved recovery and economics of prior polymer slug injection, there is a need for basic polymer/oil relative permeability data for use in performance evaluation calculations. Such relative permeability data were measured by steady-state procedures on a suite of 18 out-crop and formation core samples ranging, in permeability from about 50 to 1,200 md. Six different polyacrylamide polymers were tested, and resistance and residual resistance data were obtained on each. Data were obtained in both oil-wet and water-wet systems. The observation in these studies was that the presence of polymers in the water phase had a significant and consistent effect, lowering water relative permeability over the entire water saturation range. In many of the tests, the presence of flowing polymer or its residual effect during subsequent brine flow had no effect on oil relative permeability. In several tests, polymer contact actually improved oil mobility through increases in oil relative permeability at all levels of oil saturation. Permeability level and polymer type produced no clear-cut differences in flow behavior. The obvious differences in core wettability resulted in widely varying relative permeability characteristics, but again the effect of polymer contact was about the same, qualitatively, as obtained on the water-wet cores. Introduction The steady decline of U.S. oil reserves and rapidly, increasing, prices obtained for each barrel of crude produced are strong incentives to maximize recoveries for all reservoirs. Various enhanced oil recovery techniques are being tested and used for recovering some of the oil left behind after conventional waterflooding. The added recovery achievable with such processes, however, is influenced to a large degree by one of the same factors leading to inefficient waterflooding - i.e., reservoir heterogeneity. Numerous laboratory studies using, both physical and mathematical models, plus numerous field projects, have shown that when contrasts in reservoir permeability increase, recovered by any external injection recovery process decreases as a result of reduced sweep efficiency. Thus, if recoveries from the more heterogeneous reservoirs are to be maximized, procedures must be developed for reducing the permeability contrasts before application of an EOR process or by mobility adjustment within the process itself. Preinjection of polymers in advance of a micellar flood has been proposed as a means for improving reservoir sweep efficiency by reducing permeability contrasts. Laboratory tests of this process demonstrated that, in both linear and five-spot stratified systems, the residual resistance effect achieved by preinjection of poly-acrylamide polymers resulted in improved sweep and additional recovery by subsequent micellar flooding. In the one reported field test of this process, tertiary oil was mobilized and recovered, but insufficient data are available to indicate whether the preinjected polymer resulted in improved sweep efficiency. Mathematical model studies provide a reliable means for evaluating potential benefits of polymer preinjection. However, such studies require input data that permit the model to simulate the physical processes that may occur in the reservoir. This laboratory study was conducted to provide such data. SPEJ P. 79^