Doubly cross‐linked polymer gels with ultra‐high‐thermal stability for hydrofracking

Abstract Cross‐linked polymer gels have been commonly utilized in the hydraulic fracturing process for stimulating oil and gas production; however, their weak thermal stability still impedes their more broadly use. Here we report doubly cross‐linked polymer gel consists of a high‐viscosity friction reducer (HVFR), poly‐(acrylamide‐co‐acrylic acid‐co‐2‐acrylamido‐2‐methyl‐1‐propane‐sulfonic acid), and crosslinkers zirconium ion (Zr 4+ ) and polyethyleneimine (PEI). The mixture solution of HVFR/Zr 4+ /PEI can be gelled spontaneously via heating to form double cross‐linked polymer gel due to the formation of physical and chemical crosslinks. The electrostatic interactions of Zr 4+ and carboxylate moieties from HVFR to form first physically cross‐linked network at low temperature; the transamidation reaction between amide groups of HVFR and primary amines of PEI to generate second chemically cross‐linked network at high temperature. More significantly, the physical and chemical crosslinks can work synergistically to improve thermal stability of double cross‐linked polymer gel as compared to single cross‐linked polymer gel counterparts, which is capable of reaching fluid service temperatures up to 190°C. The double cross‐linked polymer gel features delayed crosslinking performance, the physically cross‐linked reaction of HVFR and Zr 4+ occurs at approximately 52°C, while the onset of chemical crosslink of HVFR and PEI can be tuned from 154 to 180°C. Moreover, double cross‐linked polymer gel provides a high viscosity at ultra‐high temperatures, with remarkable heat‐ and shear‐resistance as well as proppant carrying capacity in hydrofracking process. The double cross‐linked polymer gel also can be completely broken with no apparent residues. This double cross‐linked polymer gel may provide a new approach for polymer gels used in ultra‐high‐temperature reservoirs.