Effect of salinity contrast between formation water and injected low salinity water on low salinty water flooding in dolostone formation
Authors:   , Junput Nuttapol 
Published: September 2023

Low Salinity Waterflooding (LSWF) has gained substantial prominence in current scientific method for oil recovery improvement, owing to its simplicity, cost-effectiveness, and environmental friendliness as an Enhanced Oil Recovery (EOR) technique. Various studies have highlighted the crucial role of injected brine salinity and the concentration of Potential Determining Ions (PDI) in stimulate oil recovery mechanisms. However, the majority of these studies have predominantly concentrated on sandstone reservoirs. In light of the fact that over 50% of hydrocarbon reserves are situated in carbonate reservoirs, this research focuses specifically on dolomite reservoirs to examine the applicability and feasibility of implementing LSWF in such contexts.

First, Dolomite powder was treated with crude oil to simulate oil-wet condition and treated samples were tested with different water formulations to identify their capacities in Multi-component Ion Exchange (MIE). The second part was performed onto Silurian dolostone core samples. Results from the first part were then used to select the appropriate water formulation for the core flood experiment. Core flooding experiment was performed to assess the impact of salinity contrast between formation water and injected water on the oil recovery mechanism during low salinity waterflooding in dolomite formations.

From this study, the combination effect between calcium ion and sulfate ion yielded benefits in low salinity waterflooding in dolostone formation. This led to the dissolution of magnesium ion while calcium ion was not largely consumed due to the neutralization of positive charges of dolostone surface. Nevertheless, the addition of magnesium ion into the solution enhanced the effectiveness of the solution as magnesium ion could replace calcium ion in calcium carboxylic complex in oil droplets, resulting in more active calcium ion in the system. Based on the core flooding experiment, it was concluded that the presence of calcium, magnesium, and sulfate ions had a combined effect that enhanced the oil recovery mechanism through Multi-component Ion Exchange (MIE). The core flooding experiment demonstrated that the overall process required an adequate amount of calcium ion and sulfate ion to decrease the strength between the adsorbed oil layer and external dolostone surface. The optimal salinity contrast ratio for the injection of low-salinity water into the formation water in this study ranged from 5.64 to 14.10 when the salinity of formation water was 28,196 ppm. This salinity contrast ratio led to an incremental recovery factor (RF) of more than 25% after conventional waterflooding. For a formation water salinity of 56,392 ppm, the ideal salinity contrast ratio was approximately 11.28, resulting in an incremental RF of about 8.6%. Multi-component Ion Exchange (MIE) cannot occur effectively at extremely low or high salinity contrasts, resulting in lower oil production compared to the optimal salinity contrast range. The performance of LSWF was less effective at very high formation water salinity (56,392 ppm) compared to low formation water salinity (28,196 ppm). At very high formation water salinity, the magnitude of improvement or incremental oil recovery was much smaller in all cases. Besides the salinity contrast between formation water and injected water, the magnitude of formation water can be another controlling factor of the process. The ratio of dissolved magnesium ion to consumed calcium ion showed similar results in both high and low salinity of formation water. A moderate ratio of ion replacement (in this study 1.49 and 2.37 for the formation water salinity of 28,196 and 56,392 ppm respectively) occurred at the optimum salinity, resulting in an obvious increment of oil recovery.