A drilling technology of running through thief zone based on gas lift suitable for the fractured formations with permanent low pressure

In the process of drilling the fractured formations with permanent low pressure, circulation loss tends to happen easily and can be hardly plugged. In order to solve this problem, a new drilling technology of efficient depressurization and running through thief zone based on gas lift was developed. By virtue of this technology, a little gas is injected through the double-wall drill pipe to lift the drilling fluid in the upper part of the wellbore, so as to reduce the annulus ECD and run through the thief zone fast. Then, the effects of key technical parameters on the annulus ECD were analyzed by means of multi-phase flow simulation, including standpipe pressure, double-wall drill pipe depth, gas injection rate and drilling fluid displacement. Afterwards, the design optimization method with the optimal gas lift efficiency as the objective was developed. Finally, based on the optimized construction scheme, the pilot test was performed in one karst leaker with permanent low pressure. The experimental results show that this technology can build up stable circulation quickly without plugging, satisfy the requirements of cuttings carrying and trajectory control, and greatly shorten the non-productive time efficiency caused by circulation loss, presenting good popularization and application prospect.