An Integrated Workflow Combining Seismic Inversion and 3D Geomechanics Modeling - Bonga Field, Offshore Nigeria
Authors: Xiao Xiaohui , Jenakumo Timipere , Ash Charlie , Bui Huyen , Fakunle Oludayo , Weaver Sophia 
Published: April 2016

Abstract

Objectives/Scope
The deep water Bonga development is situated in block OML118 offshore Nigeria, . The Bonga Main Field was discovered in 1995 with first production in November 2005. The main reservoirs are channelized, unconsolidated, turbidite sandstones of Miocene age. While the field development has been successful, opportunities and challenges remain. Below the producing reservoir levels, there is potential for additional reservoirs - unlocking those deep hydrocarbons would require to drill beyond present well control. At the same time, drilling development wells cost effectively has remained challenging even for shallow intervals given subsurface heterogeneities, which often cause borehole stability issues.

Methods, Procedures, Process
This study introduces a novel workflow that allows the asset to leverage quantitative seismic interpretation, that is closely integrated with geomechanics modelling to address both the deep reservoir potential opportunity and the borehole stability related drilling cost challenge. Here we focus on the integration of the geomechanical and geophysical data and workflows rather than on the successful prediction of deep sand probabilities using seismic AvO inversion and Bayesian facies classification. As part of the seismic inversion, 3D dynamic Young's Modulus and Poisson's Ratio volumes were derived. In parallel, a finite-element mesh for geomechanical modelling was created from the structural interpretation and then populated with the seismic derived rock properties. The resulting field scale 3D geomechanics model helps to address production-related challenges such as top seal integrity, fault reactivation, compaction, subsidence, injection, depletion, borehole stability, and sand control.
For this study, seismic data needed to be inverted over an interval from near seabed to deep targets below well penetration - some 3 seconds TWT or 10,000ft, a much larger window than normal for single reservoir-focused studies. Seismic AvO inversion was run using overlapping, time windows from shallow to deep, to account for wavelet transmission effects. The resulting inversion outputs, acoustic and shear impedance, were used to derive shale and sand probability volumes. Well based analysis was used to determine the best relationship between acoustic and shear impedance and Young's Modulus for both sand and shale facies. Using the facies probability volumes from seismic inversion, 3D dynamic Young's Modulus and Possion's Ratio volumes were calculated from the acoustic and shear impedance volumes.

Results, Observations, Conclusions
A 1D geomechanics model, calibrated against drilling experience, was used to convert from dynamic to static Young's Modulus. Finite-element geomechanical modelling was used to produce the 3D stress model combining pore pressure, structural information, seismic-based static rock properties, and far-field horizontal stresses. The final stage of stress analysis involved calculating stresses that honor local field measurements and incorporate regional trends.

Novel/Additive Information
Utilizing 3D finite element models constrained by seismic yielded a high resolution predictive model that will significantly improve wellbore stability predictions along the paths of future development wells. The business impact for the Asset is reduced development well costs by having a more predictable geomechanics model, fully constrained by lateral variations from 3D seismic data, and greatly reduced cycle times for borehole stability predictions for future wells.