A method is provided to deliver energy downhole with a heat and nitrogen generating reaction, wherein the heat and nitrogen generating reaction occurs in-situ with an acid-generating reaction. The heat and nitrogen generating reaction is initiated by injecting into a reservoir a) nitrogen gas-generating reactants which react to generate heat at a pH of less than 8.0; b) an acid precursor which is capable of hydrolyzing in-situ in the reservoir to generate a sufficient amount of acid to maintain the reactants at a pH of less than 8.0, for a reaction that generates at least 20 kcal of heat per mole of reactants.