De-Risking Horizontal Well Challenges in a Compartmentalized Multifaulted Reservoir of Lower Gabus Formation, West Natuna Basin Indonesia
Authors: Permanasari Dian , Wilasari Ni Made , Azwar Olivia , Bonafid Kalim , Vataneta Rendiza , Murti Komang Wisnu , Sihombing Irene , Wijayanti Retno , Akbar Dino , Wibowo Ari , Prajitno Mudji , Putranto Heru 
Published: October 2023

A horizontal well was recently drilled in the F-Field, West Natuna Basin Indonesia, to drain oil from a compartmentalized multifaulted reservoir in the Lower Gabus formation. The risks associated with high-uncertainty faults were successfully mitigated by applying a comprehensive well placement method to optimize the well position within the good quality reservoir while ensuring that the well was drilled and completed safely.

The target Zone 5 reservoir thickness is on the order of 60 ft with some presence of shale intercalations of the Fan Delta deposit. At least three normal faults were encountered by the well trajectory. The faults included a major fault with approximately 100 ft displacement during landing and two normal faults in the lateral interval, all with associated depth uncertainties of ±50 ft. Considering these challenges, an ultradeep high-definition reservoir mapping-while-drilling (HD-RMWD) technology was used for the first time in the field to reduce depth uncertainty during landing, map the reservoir boundaries along the lateral interval, and identify the formation displacements accurately after crossing the faults. An iterative well planning process was performed to include the expected depth uncertainties in the well plan and manage the dogleg requirement during landing.

The HD-RMWD technology was used to successfully map the top of reservoir during landing, which was found to be 30 ft shallower after crossing the footwall block with approximately 120-ft fault throw. With this early indication, the trajectory was adjusted using a rotary steerable system (RSS) to land smoothly in the upper part of Zone 5. In the lateral, the HD-RMWD continuously mapped the reservoir boundaries, the resistivity property variations within the sand bodies, the change of formation dipping in the proximity of faults, and clearly defined the fault displacements. Based on all this information, the trajectory was optimized to reenter and increase the exposure in the good sand. The multifunction logging-while-drilling (LWD) tool that was run together in the bottomhole assembly (BHA) provided petrophysical data to evaluate reservoir quality and calculate total net pay length.

The precise placement of the well trajectory was considered with limitation on the applied dogleg severity (DLS) to fit 3°/100 ft as the packer requirement from casing shoe to landing point and 1.5°/100 ft DLS as completion requirement in the lateral. As a result, the completion string was successfully run and installed as planned. Monitoring the equivalent circulating density (ECD) and torque-drag became critical to prevent induced losses (cutting production or packoff) as part of the precaution approach when crossing faults.

This paper presents the results of the comprehensive well placement methods using the ultradeep HD-RMWD, and multifunction LWD and RSS in a single BHA run that proved to enable optimal well placement in a compartmentalized multifaulted Lower Gabus reservoir. Based on this success, similar methodology was applied to the subsequent horizontal wells in this field. It can also be applied to any other horizontal wells with similar geological challenges.