A method is provided for stimulating a subterranean formation penetrated by a wellbore by injecting a pill through the wellbore at an injection pressure below hydraulic fracking pressure. This is accomplished by introduction of a first volume of non-aqueous liquid to the wellbore, followed by introduction of anhydrous liquid ammonia, and this followed by introduction of a second volume of a non-aqueous liquid into the subterranean formation, wherein the anhydrous liquid ammonia is sandwiched between the first volume of non-aqueous liquid and the second volume of non-aqueous liquid, thereby forming the pill. A volume of push-fluid is then introduced to the wellbore to push the pill into the formation and allow the anhydrous ammonia to react with water in the formation to increase the pressure and form microfractures. However, the pill is introduced into the subterranean formation at injection pressure, and substantially below the hydraulic fracking pressure of the formation.