Experimental Investigation of the Underlying Mechanism Behind Improved Oil Recovery by Low Salinity Water Injection in Carbonate Reservoir Rocks

Abstract

Abstract It has recently been reported that the additional oil recovery that may be obtained by low salinity water injection (LSWI) is due to an interaction between the crude oil and injected brine. This mechanism, which is linked to natural surface active components of crude oil, is referred to as micro-dispersion. Previous studies have demonstrated the link between formation of micro-dispersion and the additional oil recovery in sandstone rocks but the existence of such links for carbonate rocks has not been reported before. In this work, we have investigated whether a similar oil/brine interaction can take place in carbonate reservoir rocks and whether that could lead to additional oil recovery by LSWI in carbonates. Using rock and oil samples from carbonate reservoirs, a number of crude oil samples were first tested and ranked based on their ability to form micro-dispersion when in contact with low salinity brine. Two samples were selected; one with high and the other with low propensity to form micro-dispersion. A third oil sample was then prepared by removing the micro-dispersions from the oil sample with high propensity to form micro-dispersion. Using these three oil samples, a number of coreflood experiments were performed on the same relatively large carbonate reservoir core 3.8 cm in diameter and 30cm long. Both secondary high salinity flood and tertiary low salinity flood were carried out in each case to determine whether the presence of micro-dispersion was associated with improved oil recovery for this carbonate system. In secondary high salinity water injection, in experiments using the three oil samples, all coreflood tests showed the same ultimate oil recovery. However, when the flood water was switched to low salinity, the highest amount of additional oil recovery was obtained for the oil that had shown a high tendency to form micro-dispersion in the fluid/fluid tests. The coreflood experiment that was performed with the treated oil sample (the oil sample which had its micro-dispersions removed prior to coreflood test), exhibited no additional oil recovery. The third oil sample, with a low tendency, also showed little improved oil recovery. Moreover, the pH and ionic composition analysis of the core effluents was found to be the same in all three experiments. This important observation indicates that the geochemical interaction played little role in the observed improved oil recovery compared to the micro-dispersion mechanism. The novel information obtained for carbonate reservoirs in this work helps us better understand the mechanisms involved in oil recovery by LSWI. This knowledge is important for developing screening tools for oil reservoirs that are being considered as potential candidates for EOR by LSWI.