A flexible cement stable at high temperatures contains an extender that is a finely ground polyetheretherketone. The cement retains its flexibility for long times at high temperatures, for example those used for steam assisted gravity drainage of heavy oil formations, for example from about 480 to about 644.degree. F. (249 to 340.degree. C.). Compressive strength, tensile strength, permeability and porosity are all stable and suitable for high temperature down-hole oilfield use.