Abstract: Efficient removal of residual oil-based mud in wellbore is the key for excellent cementing and wellbore intergrity. To clean the oil-based mud at high temperature and high salt efficiently, the cleaning agent was used to form micro- and nanoemulsion in-situ with the oil-based drilling fluid based on the thermoresponsive phase inversion effect and cloud point effect. By simulating the law of temperature change in the well and testing the cloud point and conductivity, the cleaning agent formula was identified as the mixture of 3% fatty alcohol-polyoxyethylene ether and 2% isomeric carbonyl polyoxyethylene ether. The cleaning agent was thermoresponsive. It could seperate out from its aqueous solution at 75℃ and therefore facilitate the oil phase shedding; then the oil phase mixed with the cleaning fluid and was then solubilized in the cleaning fluid by forming the micro-and nano-emulsion. The effects of cleaning agent concentration, temperature, salinity and time on the cleaning results were investigated by testing the cleaning rate, contact angle and cementing strength. The results showed that the cleaning rate of over 99% of oil-based mud could be reached using 5% thermoresponsive cleaning agent at 50～90℃ and 20% CaCl₂ with 10 minutes, the wall surface could return to water wetting state from the oil wetting state, and the cementing strength was enhanced observably.

Key words: thermoresponsive cleaning agent, in-situ emulsifacation, oil-based mud, well washing, well cementation