Direct Observation of Low-Salinity Water Effect: Relationship Between Micro-Dispersion Formation and Wettability Alteration

摘要

Improved oil recovery by low salinity water injection (LSWI) has been linked to wettability alteration. However, the main causes behind this are still debatable. Recently, formation of water-in-oil microdispersions has been put forward as the predominant mechanism behind the additional oil recovery by LSWI. This work aims to investigate the role of micro-dispersion formation in wettability alteration by LSWI by performing a series of contact angle measurements. A dynamic contact angle measurement technique was designed and performed on aged quartz substrates. The substrates were first covered with high salinity brine and then several oil drops were deposited on the substrates to assess the initial wettability of the substrates. Then, the high salinity brine was gradually replaced by a low salinity brine to obtain the response of the same droplets to reduction of water salinity. This approach enabled analysis of wettability alteration due to LSWI. Four crude oil samples with various tendencies to form micro-dispersion were used. Also, two blends of positive and negative crudes were used to test if adding a crude oil with surface active components could make apoor crude oil responsive to low salinity water. A positive oil isa crude oil that forms micro dispersion and a negative oil is one that does not. It was observed that there was a shift in wettability from mixed-wet and weakly-oil wet towards a more water-wet state once the high salinity brine was replaced by low salinity for the oils that were prone to formation of micro-dispersion. An average change of 10-20° in the value of contact angles was observed for these oil samples depending on initial wettability. However, the contact angles did not exhibit any detectable change for the oils with a weak tendency to form micro-dispersion. Furthermore, the mixing of positive and negative oils resulted in considerable changes in contact angles for the blended oil, which demonstrated the importance of polar components in the wettability alteration process caused by LSWI. In conclusion, the results indicate that the formation of micro-dispersion and the initial wettability are two dominant factors that determinethe outcome of LSWI.