Ability to Inhibit H+ Transmission of Gemini Surfactants with Different Chain Lengths under Different Ca2+ Circumstances

To study the ability to inhibit ion transmission of the Gemini surfactant under different Ca2+ circumstances, three kinds of Gemini surfactants with different alkyl chain lengths are synthesized (Cn-4-Cn, n = 12, 14, and 16), which are characterized using H-1 NMR, C-13 NMR, and Fourier transform infrared spectroscopy. To analyze the property of inhibition of the acid-rock reaction rate, surface tension and contact angle measurements and atomic force microscopy (AFM) results are obtained with different surfactants and under different Ca2+ concentrations. Inhibition rates with different alkyl chain lengths and an acid-etched surface morphology are also studied carefully. The result shows that all cationic Gemini surfactants significantly impact the control of the reaction rate, and the reaction rate decreased remarkably by 44.4% after adding 12-4-12. The Delta G and W-A indicate that 12-4-12 has the best adsorption ability on the rock with added Ca2+ compared with the other two Gemini surfactants. It is revealed through the AFM that Ca2+ can significantly change the adsorption morphology of the surfactant. The surfactant adsorption area decreased when Ca2+ is dispersed in the solution as well. These two phenomena can lead to the reduced ability to block H+ of 14-4-14 and 16-4-16. However, the presence of Ca2+ affects the adsorption area of 12-4-12 slightly. Thus, the reaction rate, including that of 12-4-12, is almost unchanged. Because 12-4-12 is adsorbed tightly on the rock surface, H+ can only react with the rock on the unabsorbed dot, resulting in rock surface nonuniformity after being etched, which is beneficial for maintaining the conductivity of the crack.