Innovative Logging-While-Drilling and Measurement-While-Drilling Technologies Enable Successful ERD Well Geosteering in High Gamma Ray, Low Resistivity Pay

Abstract

Typical oil reservoirs containing radioactive minerals, characterized by high gamma-ray readings and low resistivity logs, are developed within the ZJ1 Formation of the WC16 Oilfield, offshore South China Sea. Due to the lack of effective reservoir evaluation and geosteering technologies, this oilfield has remained undeveloped since its discovery. In 2026, to unlock reserves and enhance production, four horizontal wells were planned to develop this complex reservoir. All wells were classified as extended-reach drilling (ERD) due to the reservoir's shallow true vertical depth (TVD) and the resulting high step-out ratio. Geosteering and formation evaluation were critical for achieving the project's objectives, as the target sands exhibited only minimal resistivity and gamma ray (GR) contrast relative to adjacent layers. The target zone resistivity ranged from 1.5 to 2.0 Ω·m, while surrounding siltstones had slightly lower resistivity (1.1 Ω·m), and the water zone was measured at 0.9 Ω·m. Meanwhile, GR readings for the target zone were nearly indistinguishable from those of neighboring mudstones. These factors made delineating the complex target interval, and thus achieving optimal oil drainage especially challenging, directly impacting geosteering accuracy and overall well success in this low-permeability reservoir. During the planning phase, innovative Logging-While-Drilling (LWD) and Measurement-While-Drilling (MWD) technologies were selected to address these challenges and ensure project success. Initially, an ultra-high-definition deep azimuthal resistivity (DAR) mapping-while-drilling tool was deployed to deliver close-to-bit, high-resolution delineation of nearby boundaries. This technology enabled remote identification of reservoir and thin interlayer boundaries, fluid contacts, and resistivity distribution, which facilitated earlier proactive strategies for optimizing wellbore placement and enhanced understanding of reservoir distribution. Furthermore, a novel non-radioactive LWD Nuclear Magnetic Resonance (NMR) tool was introduced, enabling real-time measurement of T1/T2 distributions, NMR porosity, and permeability. This technology provided high-resolution, real-time formation evaluation and effectively differentiated fluid components adjacent to the well trajectory. Definitive dynamic surveys (DDS) MWD tool could take surveys while drilling instead of staying off bottom with the pump off by using advanced signal processing and modeling to remove the effect of the drilling environment. The synergy of these technologies provided substantial benefits to the drilling campaign. The DAR tool mapped not only the reservoir top, but also shale interbeds, calcareous dry layers, and water zones during both the landing and lateral sections, enabling proactive wellbore steering. The NMR tool delivered critical formation evaluation data, giving clear insight into fluid distribution without radioactive sources. The DDS tool allows more surveys to be taken along the trajectory and decreases depth uncertainty. Despite considerable technical challenges, the project was executed smoothly: all four laterals were drilled and completed as planned, achieving target production rates and reserve additions. The implementation of DAR, NMR, and DDS technologies saved 16 days of operations and reduced waste by 615 tons. In summary, the successful deployment of these innovative technologies has paved the way for efficient development of high-GR, low-resistivity pay zones in the South China Sea.