Super absorbent microsphere used for slow-release thickening.

Traditional linear polymer is commonly used for polymer flooding in tertiary oil recovery. However, it faces several problems, such as early injection allocation before use and viscosity reduction caused by high-speed shear. In this paper, a novel method of polymer flooding was proposed by using a super absorbent microsphere emulsion. Inverse emulsion polymerization method was adopted to obtain core polymer with acrylamide (AM), acrylic acid (AA) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as monomers through one-pot two-step approach. In order to prepare polymer microsphere, AM and N-isopropylacrylamide (NIPAM) were used as shell monomers, which can aggregate on core polymer surface by secondary polymerization. The functional groups, morphology and heat resistance of water-absorbing core-shell microspheres was characterized by FT-IR, SEM, TEM and TGA respectively. The amount of emulsifier used and shell-core ratio were optimized by particle size analyzing and solid content calculation. It is found that the particle size distribution of super absorbent microsphere was the most concentrated with emulsifier 10 wt%. Meanwhile, the average particle size of super absorbent microsphere was about 220 nm when the shell-core mass ratio was 1:10. The resulted microsphere samples display spherical shape and possess relatively high pyrolysis temperature. After aging at 80 ℃ for 48 h, the microsphere size can enlarge 10 times than that of initial one. Moreover, the apparent viscosity of microsphere emulsion dispersion was only 1.78 mPa·s at 80 ℃, which was just seventh of that without being microencapsulated core polymer. After aging at 70 ℃ for 48 h, its viscosity increased up to 9.06 mPa·s, indicating good slow-release and thickening properties. Under a low shear rate of 0-72 s^-1, the microsphere emulsion dispersion exhibited shear thinning characteristics. While under a high shear rate of 72-600 s^-1, with the increase of shear rate, the microsphere emulsion dispersion revealed a shear thickening property. Compared with traditional linear polymer, super absorbent microsphere takes on excellent water absorption performance in relatively high temperature environment, namely 80 ℃. In addition, its thickening by absorbing water to reach equilibrium is relatively slow, showing a slow-release feature. Therefore, super water absorbent thickened system prepared in this paper is expected to be used in promoting oil recovery based on improved polymer flooding.