| dc.description.abstract | The composition of the water injected into a reservoir affects the oil recovery. Injecting water with low salinity (LowSal) has been proved by several studies to improve the water wetness of the rock and thereby increase the recovered oil in sandstone reservoirs. The mechanism behind the LowSal enhanced oil recovery (EOR) process has been discussed the last decade, but none of the suggested mechanisms has been accepted as the main one contributing for the observed LowSal EOR effect.
The decrease in salinity is not the main reason for the wettability alteration in a LowSal process. Previous work have concluded that adsorption and desorption of polar material onto clay are mainly pH dependent. In this experimental study, a sandstone reservoir core was waterflooded with sequence, FW-SW-d50SW-LowSal Al, at reservoir temperature 110°C. Both pH and the concentration of Ca2+ and SO42- of the effluent were recorded during the flooding. When switching from FW to SW and d50SW an increase in pH was observed, while switching to LowSal Al the pH decreased to the initial value of the injected brine. When the pH increased, the organic material which was initially adsorbed onto the clay, desorbed. The pH gradient, ΔpH, was linked to the concentration of Ca2+, salinity of the brine, temperature and the presence of anhydrite, CaSO4, in the rock material. A higher concentration of Ca2+ caused less adsorption, and a wetting alteration towards a more water-wet system. The high temperature caused both the ΔpH and desorption of Ca2+to decrease because the ions have increasing affinity towards the clay surface. Not having CaSO4 present in the core, a higher ΔpH and desorption of Ca2+ was observed, compared to rock material containing CaSO4. | no_NO |
