Impact of Aqueous Phase Ion Type and Concentration on Calcite Surface Wettability and Acidic Oil-Brine IFT

Research subject: 

In water injection process, part of the surface active agents that are naturally present in oil can be dissolved in injected water. The presence of these substances affects the surface phenomena of the three phase water-oil-rock system. The effect of these substances on modification of the ionic composition of injected water has been less investigated in previous studies.

Research approach:

 In this paper, first, the effects of acidic compounds in crude oil and connate water composition on initial wettability of carbonate rock were investigated. Then to investigate the wettability changes of oil-wet calcite rock, single-salt aqueous solutions of MgCl2, CaCl2, KCl, and Na2SO4 having concentrations of 0.1 to 1 molar were used.Oil–water interfacial tension (IFT) for distilled water and salt waters in various concentrations were measured and compared. Variations of calcite rock surface properties were investigated by contact angle measurement after aging of thin sections in oil and salt waters.

Results indicated that by increasing ion concentration and ionic strength of salt waters, IFT decreases due to dissolution of acidic compounds of oil at the interface of oil and water. Moreover, for high content of acidic compounds in the oil, Magnesium ion has the most impact on reducing IFT and altering the rock wettability. So that by increasing the concentration up to 1 molar, IFT and contact angle decrease to 1 mN/m and 42 degrees, respectively.Contact angle measurements revealed that the composition of connate water is effective in changing the initial wettability of the rock, and formation of ionic-organic layer adjacent to the rock surface is one of the most important factors in wettability alteration. The presence of Magnesium ion in connate water decreases the contact angle to 145 degrees and causes the rock surface to become more water wet.Also, increasing solubility of oil-soluble acidic compounds in the aqueous phase has a significant effect on the rock wettability. The experimental results showed that the interaction between ions in the aqueous phase, especially the divalent ions, and surface active agents (carboxylic acid) determine the surface behavior of water in contact with oil and rock.