Technology for Safe Drilling of an Offshore Well with Long Horizontal Section through Depleted Gas Reservoir

Extended reach horizontal drilling is used in developing the shallow gas reservoir buried in the 2nd segment of the Yinggehai Formation. The Y platform, where the horizontal wells were drilled, is located in an offshore gas field in the east of the Western South China Sea. The buried depth of the gas reservoir is about 1,300 m, the maximum horizontal length of the well is 3,783 m, with the maximum horizontal vertical length ratio of 2.73. After decades of production, the reservoir became depleted to some extent. The horizontal section of the wells drilled on the Y platform penetrated a fault zone, and the risk of mud losses during drilling existed. Mud losses in previous development well drilling have been encountered. A temporary plugging solids free drilling fluid was modified to address the mud loss problem based on the analyses of difficulties previously encountered and the characteristics of the formations around the Y platform. The temporary plugging solids free drilling fluid plus the technique of annular ECD real-time monitoring and precise controlling, successfully resolved the mud loss difficulty. Drilling efficiency of 5 wells drilled on the Y platform was obviously increased, the average rate of penetration (ROP) in drilling the φ215.9 mm horizontal section was 77.27 m/h, created a new record in extended reach horizontal shallow well drilling in the same area. The gas reservoir was well protected; the production rate tested was 25% higher than predetermined.