Interference of Potential Determining Ions in Low Salinity Waterflooding in Dolomite Reservoirs

Abstract

Low salinity waterflooding is one of the most concerned improved oil recovery methods nowadays. Many studies indicated that its effect on increment of oil recovery is depended on various parameters such as total salinity of injected brine, concentration of potential determining ions, injection rate and also temperature. This study emphasizes on performance comparison of low salinity waterflooding in dolomite reservoirs with different brine formulations as well as injecting conditions by the use of coreflooding experiment. First, optimum total salinity of injected brine is determined and used in further modification. After that, the ratio between potential determining ions together testing temperature is adjusted in order to observe interference. Finally, the effect of injection rate is also studied in order to obtain the highest benefits from low salinity waterflooding. The results show that the highest oil recovery is obtained when injecting with optimum total salinity of 80,000 ppm because decreasing of Calcium and Magnesium ions results in dissolution mechanism of rock matrix that helps to liberate adsorbed oil at the rock surface. However, products from dissolution mechanism at higher temperature may act as inhibitor of the process and could terminate the dissolution mechanism once the equilibrium is attained. For the study of effects of divalent ions, oil recovery factor is at the lowest when the ratio of Calcium ion to Magnesium ion is 1:1 because when both ions in injected brine are comparatively equal, this might inhibit dissolution of both Calcium and Magnesium from rock matrix. The highest value of oil recovery factor is obtained from the case that Sulfate ion concentration is twice compared to the original concentration because Calcium ions can approach closer to surface, resulting in an ease to form more Calcium Carboxylate complex with the least amount of inactive Calcium ion. Last, the highest value of oil recovery factor is obtained from the smallest brine injection rate because the diffusion rate of ions from the brine solution to rock surface is greater. Nevertheless, injection rate should not be too low in real implementation as water can underrun to bottom zone of reservoir, decreasing volumetric sweep efficiency. According to the results, the ratio of potential determining ions in injected brine is sensitive to effectiveness of the process and it should be principally considered in low salinity waterflooding process performed in dolomite reservoir.