Abstract: Crystal scales were found at the diameter changing spots inside drill strings in Sichuan Basin during drilling with oil base drilling fluids. The scales blocked the circulation channel, resulting in pump overpressure or even pipe sticking, lost circulation and blowout. The scales were studied with water analysis and EDS to determine the main elements in them, and it was found that barium ions and titanium ions are the main metal ions forming the scales. By comparing with the drilled formation rocks and barite used to increase the density of the mud, it was determined that soluble salts had been drilled. These salts dissolve at high temperatures downhole and crystalize and separate out of the drilling fluid at low temperatures when circulate out to the surface. A salt re-crystallization inhibitor and a scale inhibitor for high valence metal salt scaling were selected through laboratory experiment. These additives were used on the Well ST-6, and were found compatible with the oil base drilling fluid used. Drill string block by the scales was successfully resolved. As a new technology for ultra-deep natural gas exploration and development in Sichuan Basin, it is of field application value.