The Kuga thrust belt in Tarim Basin is supposed to be one of the areas which concentrated difficult and problems for drilling on land of China. Reservoirs here are deeply buried and with extremely thick salt bed, which is comprised of salt rock, salt lime rock and soft mudstone, micro-cracked sandstone between salts and high pressured saltwater layer. Drilling in ultra-deep salt layer has become a extremely difficult work, especially when the depth is over 6500 meters and the temperature is over 180 Celsius.
According to the difficult of drilling in extremely thick salt layer in Tarim, a safe density design plate has been created, casing program has been optimized and a new type drilling fluid with high quality has been imported after the creep behavior of salt formation has been analyzed. In this paper, many matched drilling technologies used in KeShen Well 7 have been introduced. The designed depth of KeShen 7 is 8100 meters, the salt layer buried at 7211-7944m, the maximum pressure coefficient of saltwater layer between salts is 2.42 g/cm3, maximum mud density is 2.55 g/cm3 and the maximum bottom hole temperature is 186 Celsius. To improve the ability of dealing with complex geological condition, an unusual casing program has been developed, and a high-density oil-based drilling fluid has been applied. Precipitation Isolation Plugging Technology had been innovatively applied when the loss circulation and blowout happened simultaneously in the high pressure saltwater layer. As a result, the thick salt layer had been drilled through successfully at 7944 meters and the 81/8™ liner had been tripped in 7945 meters smoothly.
All these new technologies will bring a complete set of demonstration drilling engineering technology for foreland thrust belt engineering technology research.