AN EXAMPLE OF WASHOVER TECHNOLOGY IN LARGE INTERVAL WITH HIGH DIFFERENTIAL PRESSURE AND NARROW SIZE DIFFERENCE IN YX-1 WELL

YX-1 well, an important exploratory well in Tarim Basin, encountered a serious gas cut at 6260 m and a differential sticking with φ213 mm bit at 5080 m when a mud treatment was contacted. The φ244 mm technical casing shoe was at 4820 m. The spotting a soak solution and reducing mud weight were failed as remedial measures and led to a more severe case-mud deeply contaminated and plug of bit nozzles. Two washover operations were applied with φ206 mm washover tool and two sets of fish of total 193.29 m long were twisted off and recovered. The third washover operation for the total remained fish of 192.48 m was successfully done after 32 hours manipulation.