Experimental investigation of the effect of acid and base on wettability alteration of calcite surface
Sheiam Mohammed Juma Al-Araimiyah , Mahvash Karimi, Rashid Al-Maamari
كلية الهندسة-جامعة السلطان قابوس · عمان
Altering the wettability of rock surfaces to a more favorable wetting condition is one of the primary mechanisms in enhanced oil recovery (EOR) methods applied to carbonate reservoirs. To achieve the desired wetting state, a clear understanding of the rock surface's initial wettability is essential. This study employed various analytical techniques to investigate the individual and combined effects of an organic acid and a base at different concentrations on the wettability alteration of calcite surfaces. The wettability of calcite was evaluated before and after treatment with different model oil solutions using contact angle measurements and zeta potential analysis. Additionally, Fourier Transform Infrared (FTIR) spectroscopy and Thermogravimetric Analysis (TGA) were used to identify the functional groups adsorbed on the calcite surface and quantify the amount of adsorbed organic material, respectively. Interfacial Tension (IFT) measurements were also conducted to assess the interfacial activity of the model oil systems. Stearic acid was used to represent the acidic component of crude oil, while N,N-dimethyldodecylamine (N,N-DMDA) represented the basic component. Various model oil systems with different acid-base combinations were prepared, keeping the acid concentration constant and adjusting the base concentration to achieve base-to-acid (BN/AN) ratios of 0.25, 0.5, 1, 2, and 4. The results showed that increasing the acid number shifted the calcite surface toward more oil-wet conditions and enhanced the adsorption of stearic acid. A relatively low acid concentration (AN = 0.25) was sufficient to alter the wettability from water-wet to oil-wet. TGA results indicated that stearic acid molecules were vertically adsorbed onto the calcite surface. Treatment with the basic component revealed that a low concentration of N,N-DMDA (BN = 0.25) could slightly shift the wettability from water-wet to oil-wet. As the base number increased, this shift toward oil-wetness improved slightly. TGA analysis showed a lower amount of base adsorption but a larger surface area coverage, suggesting horizontal orientation of the base molecules on the calcite surface. Possible mechanisms for wettability alteration were proposed based on interactions between base molecules or cations and the calcite surface. In acid-base mixtures, at a constant acid number, the wettability alteration toward oil-wetness increased with the presence of base until the BN/AN ratio reached 1, after which oil-wetness decreased. IFT measurements revealed that acid-base mixtures had lower interfacial tension compared to the individual acid or base systems. This reduction is likely due to a synergistic effect between acid and base, resulting in the formation of acid-base complexes. The presence of the base hindered acid adsorption on the surface. However, the base component contributed to wettability alteration by forming acid-base complexes that could adsorb onto the calcite surface and influence its wettability. Moreover, base molecule adsorption may increase at higher BN/AN ratios. Therefore, the BN/AN ratio plays a key role in determining the relative contributions of acid molecules, base molecules, and acid-base complexes in altering the wettability of calcite surfaces. The mechanisms of wettability alteration for acid-base mixtures at different BN/AN ratios are discussed in detail.