Effect of inorganic salts in coal seams on the sand-carrying capacity of hydroxypropyl guar foam-fracturing fluid: An experimental study

Coal seam water typically contains inorganic salts, which could affect the sand-carrying capacity of foam-fracturing fluid. This study selected the standard cationic surfactant cetyltrimethylammonium bromide as the foaming agent, used the high molecular polymer hydroxypropyl guar gum as the foam stabilizer to prepare foam-fracturing fluid, and selected silica particles as the proppant. The effects of different CaCl2, NaHCO3, MgSO4, and KCl concentrations on sand-carrying sedimentation of foam-fracturing fluid were evaluated. The effects of inorganic salts on foam morphology and foam morphology on proppant sedimentation were obtained through microscope observation, and the mechanism of inorganic salts on proppant sedimentation in foam-fracturing fluids was explored. The results show that 1% CaCl2 and 0.1% NaHCO3 inhibited the sand-carrying capacity of the foam-fracturing fluid, whereas two concentrations of MgSO4 and KCl promote the sand-carrying capacity of the foam-fracturing fluid. The effect of inorganic salts on the sand-carrying properties of cationic surfactant foam-fracturing fluids is divided into two main stages. During the settling phase, inorganic salts affect settling by changing the viscosity of the foam-fracturing fluid and the repulsive force of the double electric layer on the proppant. During the suspension phase, the proppant improves the probability of collision between the proppant and the foam by promoting the foam particle size and improves the hindering effect of foam pressure and structural forces on the proppant. The results have a guiding significance for developing foam-fracturing technology.