摘要
The mechanism(s)of Low salinity water flooding(LSWF)has been extensively investigated for 15 e20 years,as a cost-effective and environmentally friendly technique for improved oil recovery.However,there is still no consensus on the dominant mechanism(s)behind low salinity effect due to the complexity of interactions in the Crude oil/Brine/Rock(COBR)system.While wettability is most agreed mechanism of low salinity EOR effect.Nevertheless,the mechanism(s)behind the wettability change is debated between multi-component ion exchange(MIE)and double layer expansion(DLE)in sandstone reservoirs.This paper aims to investigate the effectiveness of MIE with a coupled geochemical-reservoir model using published experimental data reported by Nasralla and Nasr-El-Din[1].We created core-scale numerical models with parameters identical to those used in the experiments.We simulated the low salinity effect using a commercial reservoir simulator,CMG-GEM,by coupling three chemical reactions:(1)aqueous reaction,(2)multi-component ion exchange,and(3)mineral dissolution and precipitation.We modelled the adsorption of divalent cations on the surface of the clay minerals during low salinity water injection.Simulation results were compared with the experimental results.Simulation results show that the fractional adsorption of divalent cations(Ca^2+)increased almost 25%by injecting a 2000 ppm NaCl solution,compared to initial 10,000 ppm NaCl.Injecting a 2000 ppm of CaCl2 solution,however,significantly increased the adsorbed Ca^2+from 0.1 to 1,which implies the complete saturation of mineral surface with divalent cations.Moreover,injecting 50,000 ppm of CaCl2 solution also demonstrated the same effect as the 2000 ppm CaCl2 solution but with a faster rate.Upon combining the simulation and experimental results,we concluded that the multicomponent ion exchange is not the sole mechanism behind low salinity effect for two reasons.First,almost 10%additional oil recovery was observed from the experiments by injecting the 2000 ppm CaCl2 compared with 50,000 ppm CaCl2 solutions.Even though in both cases the surface is expected to be fully saturated with Ca^2+according to the geochemical modelling.Second,6%incremental oil recovery was achieved from the experiments by injecting 2000 ppm NaCl solution compared with that of 50,000 ppm NaCl.Although 25%incremental adsorption of divalent cations(Ca^2+)were presented during the flooding of the 2000 ppm NaCl solution.Therefore,it is worth noting that the electrical double layer expansion due to the ion exchange needs to be taken into account to pinpoint the mechanism(s)of low-salinity water effect.
