Abstract
Most fractured carbonate reservoirs are characterized by a highly permeable fracture zone surrounded by a low-permeability oil-wet matrix. These features make the displacement of oil from the matrix into the fracture zone almost impossible during water flooding. This paper presents the results of flooding with the polymer polyacrylamide (PAM) and the biopolymer xanthan gum (XG) in combination with a biosurfactant to enhance water imbibition into oil-wet fractured carbonate rocks. Core flooding experiments were conducted on induced horizontally fractured (at 180°) carbonate cores in room conditions (20 ± 2 °C). The polymer or biopolymer was used to plug the fracture zones, while the biosurfactant was added to the system to alter the wettability state of the rock matrix from oil-wet to water-wet. Rock surface characterization before and after core flooding was conducted using scanning electron microscopy (SEM). The results indicate that PAM flooding led to a higher reduction of 35.6% in fracture-matrix permeability than that with XG at 18.3%. The monitoring of oil production also showed that ultimate oil recovery levels from oil-wet fractured carbonate cores for the aforementioned systems were 16 and 8.7%, respectively, which can be attributed to the drive mechanisms of temporary fracture plugging as well as mobility ratio improvement due to the polymer and wettability alteration by the biosurfactant. SEM images confirm the proposed mechanisms, where the presence of the polymer/biopolymer followed by the biosurfactant can be detected at the rock surface as a result of chemical flow through the system.