Effect of low salinity water injection on capillary pressure and wettability in carbonates

In the past decade, much attention has been pointed toward Low Salinity Water Injection (LSWI) as an efficient oil recovery technique. Despite compelling laboratory evidence of LSWI and smart water potential, the underlying mechanisms causing the incremental oil recovery in carbonate rocks are still controversial. Several studies have indicated that wettability alteration is the primary mechanism for LSWI. However, high temperature was required to observe this wettability alteration in majority of cases. This paper presents the effect of LSWI and smart water on the wettability of limestone at ambient conditions at the laboratory scale.
An ultra-centrifuge was used to measure the capillary pressure curve prior and after LSWI on four limestone core samples at ambient conditions. The end point from the imbibition capillary pressure data were used to calculate incremental oil recovery. U.S. Bureau of Mines (USBM) method was deployed to calculate wettability of the limestone core samples from forced drainage and imbibition capillary pressure curve. The effect of brine salinity and ionic composition was investigated by diluting seawater and altering concentration of sodium chloride (NaCl) and sulphate (SO4 2-).
The result from imbibition capillary pressure data indicated a reduction in residual oil saturation as the salinity was reduced. Twenty times diluted seawater enriched with four times sulphate lead to an incremental oil recovery of 7% compared to twenty times diluted seawater. Moreover, depletion of NaCl in the low salinity brine enriched with sulphate lead to 1.6% additional oil recovery. Reducing salinity of the injected brine resulted in a more water-wet condition. The highest wettability index alteration towards water-wet condition was observed when sulphate was spiked in low salinity brine. The result obtained in this work provided direct evidence that LSWI could change the wetting condition of the rock toward a more water-wet state. It was concluded LSWI could be an effective oil recovery technique, which could be improved by altering ionic composition of the brine on limestone core samples.