Drilling Geomechanics Successfully Applied in a High-Temperature High-Pressure Offshore Well, South China Sea

Abstract

Abstract Drilling in Yinggehai basin of South China Sea is challenging due to high pressure gradient (2.20-2.30 g/cm3) and high temperature (186-196 0C). Several wells did not reach target and were abandoned due to severe wellbore instability of gas kick and mud loss. In order to drill safely, drilling geomechanics techniques were applied in well LD-6, including: 1) real-time monitored pore pressure and fracture pressure with logging while drilling (LWD) technique; 2) predicted over-pressured reservoir depth and formation pressure ahead of bit with borehole vertical seismic profile (VSP); 3) wellbore stability analysis to reveal the mechanism of mud loss in Huangliu formation. With drilling geomechanics techniques, well LD-6 was successfully drilled and completed. The achievements were:1) formation pore pressure and fracture pressure were accurately monitored while drilling. The predicted reservoir pore pressure gradient was 2.27g/cm3 and actual measurement was 2.28 g/cm3; 2) The top depth of over-pressured reservoir, S1 Sand, was predicted within 3 meters versus actual depth. The 9-5/8" casing successfully set in shale formation above S1 Sand, which enabled a manageable pressure window for drilling 8-3/8" reservoir section; 3) the wellbore stability analysis with borehole resistivity image revealed that mud loss only occurred in sand and mainly caused by drilling-induced tensile fractures. Stresses computation results indicated that horizontal stress anisotropy in sand is larger than shale; and breakdown pressure in sand is smaller than that in shale. With this finding, drilling parameters was adjusted accordingly and managed downhole pressure within the narrow safe mud weight window. The drilling experiences in well LD-6 has proved the advantages of drilling geomechanics techniques, especially in HTHP area. It can predict and mitigate drilling risks, optimize mud weight and completion diagram, improve operational safety and reduce costs.