Effect of salinity contrast between formation water and injected low salinity water on low salinity waterflooding in shaly-sandstone formation
Authors:   , Phuenghansaporn Sarun 
Published: September 2023

Low salinity waterflooding is an improving oil recovery technique that is highly mentioned nowadays as it is cost-efficient and environmentally friendly. The technique involves injecting water with lower salinity compared to formation salinity to shift the surface equilibrium toward the liberation of oil from the rock surface. Nevertheless, successful of this technique depends on many parameters especially the presence of clays. Several types of clays can be found in oil reservoirs, and they may react to different cations in inject low salinity in different ways, resulting in different magnitudes of oil recovery. Moreover, the contrast of salinity between formation water and injected low salinity is also another important parameter as the contrast can result in different degrees of the dissolution of ions, leading to oil recovery mechanisms.

This study is therefore performed to investigate the dissolution of clays from different water formulations and the effect of the salinity contrast between formation water and injected low-salinity water. The first study was performed by filtration method combined with color titration to obtain important ions for different clays to identify water formulation and range of salinity of injected low salinity water in the following steps. After that, selected water formulations were tested in a core flooding test using shaly-sandstone core samples with different salinities of formation water to obtain various salinity contrasts.

From the study, the best concentration of injected low-salinity water is in the range of 1,000 to 5,000 ppm which is favorable for the dissolution of essential ions. Potassium ion is required to be mixed with calcium ion or magnesium ion to enhance oil recovery mechanism by Multi-component Ion Exchange. Based on the core flooding experiment, it can be concluded that K-Cl solution with calcium ions that replaces magnesium ion works well at higher salinity contrast. This can be explained that the dissolution of magnesium ions which is smaller in diameter must have the presence of calcium ions in specific contrast to allow three components to trigger the overall process including 1) high salinity contrast to allow the dissolution of magnesium ion; 2) adequate amount of calcium ions to induce reaction with an organic acid in oil and 3) presence of monovalent ion to substitute leaving magnesium ion. The K-Ca solution has the best salinity contrast ratio range of around 14, yielding additional oil recovery of around 0.1 percent, whereas too low or too high salinity contrasts would decrease its effectiveness. Magnesium ion replacing calcium ion occurs easily due to the larger size of calcium ion and hence, low salinity contrast allows magnesium ion to well. The salinity contrast ratio of 2.8 is found to be the best for K-Mg solution with an additional oil recovery of 4.6 percent.

Understanding the effects of both salinity contrast and chemical composition of injected water would lead to selecting the best water formulation, especially for fields having difficulties in the technique to dilute salinity water for injection or finding sources of fresh water to dilute salinity water.