Underlying mechanisms of shale wettability alteration by low salinity water injection (LSWI)

Abstract

Low salinity water flooding is an approach used during the EOR process to alter the surface wettability the IFT reduction. However, implementation of this method in shaly sandstone layers has not been totally successful due to the presence of clays and its active interaction with salt ions. In this study, attempts are made to evaluate the effect of ions on the surface wettability of shale rocks and the IFT of brine/oil. For this purpose, NaCl and MgCl2 were considered as two very common salts used in many studies. The results obtained indicated that in the low salinity water range, decreasing the concentration of monovalent ions changes the wettability of shale from neutral wet to weakly water wet where the contact angle reaches 63° in the 1000 ppm NaCl solution. On the other hand, decreasing the divalent concentration initially increases and then decreases the contact angle in the crude oil-brine-shale rock system, inducing an oil wet system in place. It was found that a favorable wettability is achieved when the concentration of NaCl and MgCl2 in the solution reaches 1000 ppm and 8000 ppm respectively. This could be due to several phenomena including the “salt in” effect. In addition, time appears to be an important factor in the shale wettability alteration and a significant contact angle reduction was observed over time. IFT was also sensitive to the salt ion concentration and reached its lowest value once the concentration of NaCl and MgCl2 in the solution decreased by 49.46% and 32.04% respectively.