Low salinity effect after sea water flooding in sandstone reservoirs

Abstract

Clays are the main wetting minerals and have permanently negative surface charges. The negative charges must be balanced by; active cat ions, polar components or H+. pH changes is observed in the effluent by flooding in a sequence of FW-SW-LS-FW. The concentration of ions in the Low Salinity brine are lower than in the formation water and sea water brines, especially Ca2+, Mg2+. In the proposed chemical mechanism on Low Sal EOR effects in sandstone reservoir, it is the effects of pH, both for the adsorption of acidic and basic organic components onto clay minerals, to create initial low water wetness, and also for the desorption of the polar components when the smart water is introduced. When injecting Low Salinity fluid with low Ca2+ concentration, it will promote desorption of Ca2+ from the clay surface which consequently creates a local increase in pH close to the brine-clay interface due to H+ from the water compensates the negative charges at the clay surface. A fast reaction between OH- and the absorbed acidic and protonated basic material, it will cause desorption of organic material from the clay surface, and as the results, the water wetness of the rock is improved and increased in oil recovery is observed due to increased positive capillary pressure. Mostly all sandstone reservoirs in North Sea have already been flooded with Sea Water. Is it likely to observe Low Salinity EOR effect after the reservoir have been Sea Water flooded? Through the combination of theoretical knowledge, and detailed low salinity experiments carried out in the lab, both pH screening tests and oil recovery tests on reservoir cores confirmed the possibility to observe Tertiary LS EOR effects in a High Temperature Sandstone Reservoir.