Integrated Stress Calibration by Utilizing Advanced Acoustic Information - A Case Study from Deepwater Malaysia

Abstract

Abstract "Block X," located in offshore Sabah is operated by JX Nippon Oil & Gas Exploration (Deepwater Sabah) Limited., which had drilled several exploration wells and acquired extensive wireline and LWD information to evaluate the block for further exploration and future development. Drilling exploration wells in a deepwater environment can lead to drilling difficulties. For example, a very narrow safe mud weight window was encountered in the discovery Well X and subsequent drilling difficulties delayed the well delivery. In orderto minimize the drilling risk for upcoming prospects, detailed petrophysical, geological and geomechanical studies have been conducted based on extensive evaluation of wireline and LWD data. Determination of horizontal stress directions and magnitudes are an important part of building the 1D mechanical earth model, which allows optimizing perforation and completion design, and production planning. Knowledge of the stress directions also helps to optimize well trajectory design so that wellbore instability can be minimized. Several methods for identifying stress directions are available and investigated, including borehole breakout orientation and shear sonic anisotropy information. For complementing the existing drilling data (leak-off and formation integrity tests), the horizontal stress magnitudes are determined using the dipole radial profiles and the sonic anisotropy using acoustoelasticity theory. The variation in shear sonic slowness within the elastic region of the rock is compared to the stress distribution caused by the presence of the wellbore. The relationship between shear slowness (modulus) and stress is then established to provide the acoustoelastic coefficient and horizontal stress magnitudes (both minimum and maximum) are then determined. These results were used to reduce uncertainty in the state of stress, and used in optimizing the wellbore trajectories for future development wells. In the current study, besides conventional stress calibration data such as LOT/FIT and calliper information, advanced sonic information helped to calibrate further the stress profile, which resulted in a comprehensive understanding of the stress regime of the study area. These results are utilized to better design the mud weight program for the drilling of future Deepwater prospects in Block X.