The distribution characteristics of additional extracted oil displaced by surfactant–polymer flooding and its genetic mechanisms

Abstract The pilot test of surfactant–polymer flooding (S–P flooding) in the western part of Gudong 7th reservoir has achieved good development effect. In this work, the distribution characteristics of additional extracted oil (AEO) displaced by S–P flooding were firstly studied using the reservoir numerical simulation method, to improve the understanding of the complicated mechanism during the S–P flooding process. Based on the performance of the components in S–P flooding system, AEO regions were distinguished into three sub-regions, including one enhancing displacement efficiency (S region), one improving sweep efficiency (P region), and one integrating the two mechanisms mentioned above (PS region). S region which had a dam-like distribution and low AEO saturation was located in layers with high permeability around the injection well. PS region that obtained higher AEO saturation was located close to the S region. P region which achieved the highest AEO saturation was located in layers with low permeability furthest away from the injection well. It was vertically distributed in the upper parts of the fining upward sequence reservoir and took on the shape of ellipse between wells. Compared with water flooding, the flux increased in some parts of the reservoir during the S–P flooding process. In order to quantitatively characterize the flux difference between water flooding and S–P flooding in the same area of the reservoir, a new characterization parameter named increased flux percentage (IFP) was defined and then IFP region in the reservoir was established. There was a very good correlation between the AEO region and the IFP region, which revealed the formation mechanisms of the three AEO sub-regions. The formation of S region was mainly due to the low interfacial tension contribution of surfactant in improving displacement efficiency. The polymer in S–P flooding system can improve the injection profile and increase the viscosity of the S–P flooding system. These mechanisms were the main reasons for the formation of P region. The formation of PS region was due to the synergistic effect between surfactant and polymer in the S–P flooding system. This synergistic effect was considered as the key mechanism to achieving good performance in the S–P flooding pilot test. Some characterization parameters were defined, including area of the sub-region, degree of the circularity and position of the centroid. Some sensitivity studies were conducted to find out the influence of surfactant and polymer concentration on the distribution characteristics of AEO and IFP. The results showed that the increased extent of PS region area reduced and the cross flow phenomenon caused by low interfacial tension of surfactant became serious when surfactant concentration was higher than 0.45%. Within a certain range, the areas of three sub-regions all increased as the polymer concentration increased. However, when the polymer concentration increased from 2000 mg/L to 2500 mg/L, the effect of increasing IFP and improving sweep efficiency was significantly decreased. Moreover, the increased extent of PS region area also reduced.