Experimental Investigation of Crossflow in Layered Reservoir with High Permeability Contrast during Polymer Flooding

Abstract

When polymer flooding is applied in oil reservoir, polymer retention causes permeability reduction mainly due to polymer adsorption onto grain surface. The polymer adsorption especially in low permeable formation clogs the pore throat more seriously and decreases the permeability. Therefore, the EOR effect is rather decreased from particular value of polymer concentration. This phenomena can give rise to a severe crossflow problem in the layered reservoir containing low permeable layer. In this aspect, this study attempted to experimentally analyze polymer adsorption-induced permeability reduction, and also examine the crossflow problem in the layered reservoir. In order to improve these phenomena, crossflow-controlled polymer flooding was suggested as a remedial method to improve sweep efficiency. From the results of areal experiment for investigating the polymer adsorption-induced permeability reduction, the oil recovery was not proportionally enhanced with the increase in polymer concentration up to 1,500 ppm in this sandstone system of average 48 md which is relatively low for applying polymer flooding. That is, in this range of concentration, it is expected to produce more oil as the increase in polymer concentration. However, in this low permeable system, the effect of enhanced oil was rather decreased from the concentration of 1,000 ppm. This was because the effective permeability significantly reduced due to the polymer adsorption, and it was ascertained by estimating the adsorbed-layer thickness through the measured pressure data analysis. The afore-mentioned result may arise the severe crossflow problem in layered reservoir containing low permeable layer. Thus, this study investigated the crossflow-controlled polymer flooding method to prevent the crossflow for the two-layered vertical system with large permeability contrast including low permeable layer. Firstly, the experiments for analyzing the effect of polymer concentration design in each layer, individually, were conducted as waterflooding, conventional polymer flooding, and crossflow-controlled polymer flooding injecting polymer in high permeable layer and brine in low permeable layer. The results were interpreted by use of oil recovery, visual photos of flow paths and vectorial presentation with analysis of pressure data. It connoted that waterflooding and conventional polymer flooding were inefficient in sweep efficiency because of severe crossflow problem occurred almost from the beginning of injection. In crossflow-controlled polymer flooding, the oil recovery was significantly enhanced from the low permeable layer by preventing the crossflow, which was accomplished by controlling polymer concentration for each layer, individually. Also, from the results of actual flowing paths and vectorial presentation, the oil was successfully displaced within each layer, not showing crossflow. Secondly, the experimental results with regard to frontal velocity were analyzed in a similar manner to above polymer concentration case. When the same injection rate was implemented at each layer, the injected fluid moved upward almost directly as soon as injected, producing only a small quantity of oil from the low permeable layer. Meanwhile, the results for the case implementing the same frontal velocity at each layer indicated that the injected fluid moved virtually in horizontal direction, and oil was stably displaced without crossflow. As an overall result of crossflow-controlled polymer flooding as suggested in this study, it was shown that sweep efficiency and oil recovery were remarkably enhanced.