Enhancing Oil Recovery from Chalk Reservoirs by a Low-Salinity Water Flooding Mechanism and Fluid/Rock Interactions
Different Low Salinity Waters (LSWs) are investigated in this work to understand the role of some ions, which were recognized from our previous work and the literature for their effect on wettability alteration. Different flooding stages were followed. The primary stage was by injecting synthetic seawater (SSW) and the secondary stage was with SSW diluted by 10 (LSW 1:10) and 50 (LSW 1:50) times, single and two salt brines, such as Na 2 SO 4 , MgCl 2 , and NaCl+MgCl 2 at 70 • C. The flooding
... C. The flooding sequence was due to that most of the fields in the North Sea were flooded with seawater. Two flooding rates were followed, 4 PV/day (PV = Pore Volume) and 16 PV/day in all the experiments. One of the observations was the increase of the pH during the flooding with LSW and single salt brines. The increase of the pH was attributed to mineral precipitation/dissolution as the results of ionic interactions. The effluent ion concentrations measured to understand the most likely oil recovery mechanisms. The results showed that the higher the SSW dilution the slower the oil recovery response. In presence of SO 4 2− , Ca/Mg, higher oil recovery. The exchange between Ca/Mg, was in line with field observations. A geochemical simulation was done for a comparison with the experimental data. Energies 2017, 10, 576 2 of 16 help increase oil recovery. A number of low salinity water flooding mechanisms have been proposed, i.e., fine migration (dispersion of rock minerals), pH increase, double layer expansion effect and wettability alteration, including adsorption of SO 4 2− with co-adsorption of Ca 2+ and replacement of Ca 2+ by Mg 2+ on chalk surface because of increase in ion reactivity at higher temperature , but a concise mechanism which conforms to the LSW effect is still debatable. Based on whatever has been published so far in the literature, the mechanisms are mainly related to the presence of clay minerals, oil composition, and presence of formation water. Concentration of divalent ions (Ca 2+ , Mg 2+ , SO 4 2− ) and salinity level of high saline water in range of 1000 ppm-3000 ppm also play an important role in LSW effects  . This work is focused mainly on observing the effects of the type and concentration of ions on the increase in secondary recovery. From the experiments it has shown that the injection of water/brine with modified composition alters the wettability and enhances the oil recovery. These effects are linked to exchange of effective ions (Ca 2+ , Mg 2+ , SO 4 2− ) from/to the reservoir surface and brine. This work is concerned towards secondary injection of brine containing only a single ion (Mg 2+ /SO 4 2− ) and brines diluted in proportion of 1:10 and 1:50. Carbonate samples were recovered from the Bu-Hasa field in Abu Dhabi from the experiments observed by . They observed an optimum concentration of SO 4 2− at which highest recovery is obtained, which was 47 ppm. This work also contributes towards confirming the best possible dilution ratio of SSW to observe optimum oil recovery. In addition, all the experiments were performed at 25 bar and 70 • C/90 • C. SSW is used as primary injection brine and brines with different monovalent and divalent cations were used as secondary injection brines. Effluent's pH, pressure drop, and oil recovery were measured. Ion tracking results were obtained by ion chromatography and analyzed to assess the ion exchange between rock and brine.