Effect of different salt ions with different concentrations on the stability of carbon dioxide-in-water foam fracturing fluids

Different salt ions with different concentrations can affect the fracturing effect by influencing the stability of the carbon dioxide-in-water (C/W) foam during C/W foam fracturing to increase the penetration of coalbed methane (CBM) reservoirs. Therefore, in this paper, a combination of experiments and molecular dynamics simulations was used to investigate the mechanism of the effect of the concentration of salt ions on the stability of C/W foam fracturing fluids. The initial foam volume and drainage half-life of C/W foam were measured using a C/W foam stability evaluation device to evaluate the foamability and foam stability of C/W foam. The surface tension and viscoelastic modulus of the C/W foam were measured by an interface rheometer (Tracker). Moreover, molecular dynamics simulations were used to obtain the radial distribution functions between different elements and the mean square displacement of water molecules. The results showed that salt ions could reduce the surface tension and change the viscoelastic modulus of the C/W foam. Magnesium and calcium ions reacted with sodium dodecylbenzene sulfonate (SDBS); thus, magnesium and calcium dodecylbenzene sulfonate, which are slightly soluble in water, were formed. Sodium ions improved the stability of the C/W foam fracturing fluid. The liquid-carrying capacity of SDBS was improved, the drainage rate of water on the C/W foam film was decreased, and the stability of the C/W foam fracturing fluid was improved when the concentration of magnesium ions was lower than 100 mg/L or the concentration of calcium ions was lower than 50 mg/L. However, with increasing ion concentration, the stability of the C/W foam continued to decrease, and the stability was the lowest when the concentration of magnesium or calcium ions reached 400 mg/L. The results provide guidance on the application of C/W foam fracturing to increase penetration in coal seams with salt ions.