Alternative Geomechanics Analysis for Optimal Wellbore Stability in Sabah's Challenging Multi-Stack Reservoir Gas Fields

ABSTRACT: This study presents geomechanical analyses for two gas projects off the coast of Sabah, offshore Malaysia: Field X and Field Y. Field X, a new development with high formation pressure, features a deep-water turbidite environment with varying stress regimes. Field Y, a brownfield with normal and HPHT regimes, has been producing for over ten years, with some reservoirs depleted by up to 6000 psi, resulting in a narrow drilling window. Conventional pore pressure prediction and wellbore stability analyses revealed narrow drilling windows in both fields, leading to high safety and cost concerns. To address these, an alternative study using Gassman fluid substitution on density and sonic logs, along with PVT data and 1D geomechanical models, was conducted. The models, validated through wellbore stability analysis and calibrated with core data, incorporated stress magnitudes, orientations, and rock properties. By applying fluid substitution, improvement on drilling windows were achieved, reducing the need for Managed Pressure Drilling (MPD) and significantly improve the project cost. This case study demonstrates that fluid substitution enhances 1D geomechanics analysis and wellbore stability, improving drilling efficiency and reducing project expenses for gas fields. 1. INTRODUCTION A new development campaign for Field X and an infill drilling program for Field Y with three wells has been planned for each field. Field Y has been in production for over a decade where significant reservoir depletion, with pressure drops of up to 6000 psi, has created challenging drilling conditions due to a narrow margin for drilling mud. A 1D Geomechanical Earth Model (1D GEM) has been developed for both Field X and Field Y, representing the vertical variation of subsurface mechanical properties along the wellbore. The 1D GEM integrates all the data from well logs, well tests, core samples, and seismic surveys, focusing on critical geomechanical parameters such as rock mechanical properties, pore pressure, in-situ stresses, and elastic moduli.