Laboratory Investigation of Low Salinity Waterflooding using Carbonate Reservoir Rock Samples

Abstract

A laboratory study was performed to evaluate the possibility of performing a low salinity waterflood in an offshore carbonate reservoir using its rock and fluid samples. A series of spontaneous imbibition and core flood tests were conducted and both tests confirmed the incremental oil recoveries when the composition of the injection brine was diluted and modified. During the core preparation stage, three uncertainties were identified; the core cleaning procedure, the aging time and the reservoir heterogeneity. First, a core is conventionally cleaned with polar solvents. However, a new core cleaning procedure called the “mild cleaning” has been proposed by Austad, which recommends the use of non-polar solvents. In our laboratory studies, the similar core samples were cleaned by the two different techniques and subsequently the spontaneous imbibition test results were compared. It was found that the low salinity effect was confirmed regardless of the core cleaning procedure. Second, the wettability distribution is not clear in this field. To account for this, the core aging time was varied from eight weeks (more oil-wet) to no-aging (more water-wet). The similar cores were aged for different time and subsequently the spontaneous imbibition test results were compared. It was found that the low salinity effect was confirmed regardless of the aging time, although the core samples with longer aging time showed lower oil recoveries. And third, since the reservoir is heterogeneous, the above sensitivity investigations covered a wide range of rock types within the production intervals. It was found that all rock types showed the low salinity effect. In addition to the above investigations, a number of water recipes were tested. It was found that the sea water performed better than the formation water, while the diluted sea water performed better than the sea water. The effect of sulfate ions was also investigated. Some core plugs showed the low salinity effect when the concentration of the sulfate ion was spiked, while other core plugs did not respond. The effect of sulfate ions therefore, needs to be further investigated in this field. Based on the results from the zeta-potential and the contact angle measurements, the low salinity effect in this reservoir was considered to be due to a change in the surface-charge and the wettability, which is consistent with the mechanism proposed by Austad. The conclusion of this laboratory study highlighted the possibility of applying the low salinity waterflood in this field.