Managing Uncertainties in Wellbore Placement while Drilling Complex Reservoir: Case Study- Onshore, Niger Delta

The uncertainties associated with wellbore placement especially in terms of subsurface geology must be understood and minimized to maximize reserves for high returns. In the study area, the target reservoir is characterized as deep-seated hot sand body with intercalations of shale and siltstone. It is directly overlain by a thin bed of calcite. A closer look at the gamma ray log response of the sand puts a question to the reservoir presence, thus making landing point uncertain. Drilling to the target reservoir requires traversing through several depleted reservoirs sandwiched by shale formations that retain their original pressures. This narrows the safe mud weight window that could lead to wellbore instability issues. Other uncertainties include depth and fluid contacts. The objective of the well was to land and geosteer the wellbore in an optimal position within the oil leg. This paper reveals how the above-mentioned uncertainties were managed while drilling GX-04h infill well. The methodologies adopted include the use of reliable rotary steerable system, improved formation evaluation tools, real-time shale volume (Vshale) modeling, real-time pore pressure/ fracture gradient/ shear collapse pressure monitoring, real-time geological modeling, mud logging and effective communication protocols. Real-time Vshale computed and mudlog data helped in differentiating the top of deep-seat hot sand from the overlying shale. The at-bit resistivity measurement and neutron/density data aided in fluid typing/contacts. The approaches employed helped in proper landing of the well and successful drilling/ geosteering of 1000 feet lateral hole section within the oil leg of the target reservoir.