An operator drilling offshore South China Sea encountered many challenges during the exploration and evaluation stages. The two main issues involved the narrow mud weight window and the presence of a high-pressure formation. The need for high-quality logging-while-drilling (LWD) sonic measurements was critical to managing the overall risks of the drilling operations.
SLB used SonicScope™ 825 multipole sonic-while drilling service to deliver robust real-time and memory sonic data, including downhole slowness-time-coherence data, for real-time data quality control. We acquired excellent match between real-time and memory compressional and shear slowness in high stick and slip environment; accurately evaluated high-porosity gas formation despite mud invasion; successfully reached total well depth with geomechanics analysis.
Geomechanic research is performed before drilling the well to reduce the associated drilling risks and save rig time. Reliable, real-time sonic data is an integral part of the evaluation. Information from offset wells is compared with surface seismic data, and a good match between the two reduces uncertainty and boosts the confidence of operators.
Using SonicScope 825 service, the operator obtained reliable LWD sonic data for both real time and memory. The real-time slowness acquired matched the memory data, and the time-lapsed logging measurement helped quantify the impact of mud invasion in the high-porosity formations, as a reduced slowness was acquired in the gas zone. This data helped the operator successfully reach total depth with reduced risk and uncertainty. Having proven success after first-time use of the service, the operator has decided to run the latest LWD sonic service in both 14 3/4-in and 12 1/2-in hole sections in future HPHT deepwater wells.
