Enhanced oil recovery in fractured low-permeability reservoirs by a novel gel system prepared by sustained-release crosslinker and water-soluble thixotropic polymer

Polymer gels have been widely used in improving oil recovery and decreasing excessive water production in heterogeneous reservoirs after the long-term water flooding process. However, the high initial viscosity of polymer and the short gelation time still restricts the performance of polymer-based in situ cross-linked gels for in-depth conformance control. Herein, a novel gel system formed by in-situ crosslinking of water-soluble thixotropic polymer (WTP) and sustained-release crosslinker (SRC) was proposed. The WTP was synthesized by acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and N-[3-(dimethylamino)propyl] methacrylamide. It exhibits low viscosity under high shear rates and becomes thick under low shear rates, which ensures good injectability during the injection process and high gelation strength after cross-linking. The SRC was prepared by coating poly(ethyleneimine) (PEI) into the W/O/W multiple emulsions. The sustained-release mechanisms were well elaborated by confocal laser scanning microscopy (CLSM) observations. Only after the demulsification of multiple emulsions, the PEI would be released from the internal aqueous phase to the external aqueous phase. Therefore, these emulsions can be employed to deliver crosslinkers to specified targets in deep regions and prolong the release of crosslinkers to prevent undesirable crosslinking reactions near the injecting wells. According to the environmental scanning electron microscope (ESEM) observations and viscosity measurements, the low viscosity and an incompact spatial network could be observed in SRC/WTP gel system in the first 17 d, after which the PEI molecules were released to the external aqueous phase and cross-linked with WTP, forming the solid gel with small pores and yielding a high viscosity of 8793 mPa⋅s. Rheology tests have demonstrated that the elastic modules and the liner viscoelastic region of the novel gel system was nearly two times and twenty times than the traditional polymer gel, indicating higher gelation strength and better shear resistance. The core flooding tests showed that the SRC/WTP gel system had good injectability and could effectively plug the millimeter and submillimeter-sized cracks in low-permeability cores. 0.6 pore volume (PV) of SRC/WTP gel system with 3 gel slugs and a low injection rate can yield the best plugging performance in fractured core samples. And the gelation time shortens with the decrease of fracture apertures. Most importantly, the field tests in Ordos Basin verified the desirable EOR performance of the SRC/WTP gel system in fractured low-permeability reservoirs.