Abstract: A novel reversible emulsion drilling fluid which is useful in resolving the contradiction between drilling efficiency,cementing and environmental effect has been invented based on the protonation reversibility of amine.The new drilling fluid system has reversible emulsion-phase behavior.The formulation of the reversible emulsion drilling fluid was:5# white oil+25.0% CaCl2 brine+1.0% organoclay+0.5% lime+4.0% reversible emulsifier ET+1.0% wetting agent RS,oleaginous fluid to non-oleaginous fluid-ratio was 50/50,drilling fluid density was 1.2 kg/L.Comprehensive performance evaluation with laboratory test showed that the new drilling fluid could be readily and reversibly converted from a water-in-oil emulsion to an oil-in-water emulsion and back to a water-in-oil emulsion using an acid-base chemical switch.And before and after converting the emulsion mud maintains good performance,with emulsion-breaking voltage of 500-900 V,resistance to high temperature of 180 ℃,HTHP filter loss of less than 15 mL,rheology comparable with traditional invert emulsion drilling fluid,and resistance to 10% calcium sulfate and 20% cuttings pollution.Additionally,both permeability recovery value and rolling recovery ratio were above 90%,and the filter cake and oily cuttings could be treated easily.This suggests that the reversible invert emulsion drilling fluid has both merits of oil-based fluid and water-based fluid,and all properties are excellent.The reversible invert emulsion drilling fluid is capable of resolving the problems with the application of conventional oil-based drilling fluid.