Abstract: Stuck drill pipe during the drilling of horizontal wells has become a common problem in the Changxing Formation of the Yuanba Gas Field. Conventional foamed acids are prone to lost circulation and cannot eliminate the risk of stuck drillpipe. In order to solve the problem of stuck drillpipes during horizontal drilling, a new technology using mud loss-proof emulsified acid was tried in an ultra-deep horizontal well after being developed by means of an in-depth analysis of the cause. For the basic stuck drillpipe freeing principle, it is important to recognize that acid fluids can return to stuck point by retarding of emulsified acids and inert circulation lost materials can be used to reduce mud loss rate so as to increase time of reaction between acid fluids and base rock to release stuck drillpipe. A mud loss-proof emulsified acid formula has been developed through laboratory tests. Test results show that the emulsified acid can achieve an acid solubility of 50% in cuttings and 40% in mud cakes of the Changxing Formation;circulation lost materials of acid fluid system can effectively improve plugging effects with a plugging ratio up to 99%. Further, the emulsified acid system achieved good retarded acid performance and its demulsification temperature can be as high as 140degrees. The emulsified acid has been successfully used in drillpipe stuck due to differential pressure in the Changxing Formation in Well YB205-1. According to research results, mud loss-proof emulsified acid can solve differential pressure which causes drillpipe to get stuck in ultra-deep horizontal wells in highly permeable carbonate rocks.