Temperature and salt resistant CO2 responsive gas well foam: Experimental and molecular simulation study

Abstract

Synergism has developed to be an important means to improve the foaming, stabilizing and responsive deforming characteristics of foam. Here, CO2/N2 responsive surfactants with tunable lengths of carbon chains are synthesized. Through forming and deforming testing, the obtained 11-dimethylamino-undecyl sulfate sodium (11-DUSNa) with 11 carbons on the linear tail chain has the best comprehensive performance. 11-DUSNa is then mixed with Cocamidopropyl Hydroxyl Sulfobetaine (CHSB) to produce a compound foam with enhanced temperature and salt tolerance. Experimental observations show that the compound foam has the largest stability by adding 3.2 mol/L divalent salts (Ca2+, Mg2+), and has improved temperature resistance from 20 to 100 °C compared to singular surfactant foam. The compound foam defoams in response to the injection of CO2 within 2200 s. The foaming and defoaming processes can be cycled many times by repeat addition of NaOH for deprotonation and CO2 for protonation of amino groups. Molecular dynamics simulations show that Ca2+ and Mg2+ cations distributing around the head groups of surfactants form a stable network liquid film wall, explaining why the compound foam is salt tolerant. The synergistic effect of CHSB and anions increases the liquid film wall thickness and therefore leads to better temperature resistance.