An Integrated Workflow Combining Seismic Inversion and 3D Geomechanics Modeling - Bonga Field, Offshore Nigeria

摘要

Objectives/Scope: The deep water Bonga development is situated in block OML118 offshore Nigeria,Figure 1. The Bonga Main Field was discovered in 1995 with first production in November 2005. Themain reservoirs are channelized, unconsolidated, turbidite sandstones of Miocene age. While the fielddevelopment has been successful, opportunities and challenges remain. Below the producing reservoirlevels, there is potential for additional reservoirs - unlocking those deep hydrocarbons would require todrill beyond present well control. At the same time, drilling development wells cost effectively hasremained challenging even for shallow intervals given subsurface heterogeneities, which often causeborehole stability issues.Methods, Procedures, Process: This study introduces a novel workflow that allows the asset toleverage quantitative seismic interpretation, that is closely integrated with geomechanics modelling toaddress both the deep reservoir potential opportunity and the borehole stability related drilling costchallenge. Here we focus on the integration of the geomechanical and geophysical data and workflowsrather than on the successful prediction of deep sand probabilities using seismic AvO inversion andBayesian facies classification. As part of the seismic inversion, 3D dynamic Young’s Modulus andPoisson’s Ratio volumes were derived. In parallel, a finite-element mesh for geomechanical modellingwas 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 astop seal integrity, fault reactivation, compaction, subsidence, injection, depletion, borehole stability, andsand control.For this study, seismic data needed to be inverted over an interval from near seabed to deeptargets below well penetration - some 3 seconds TWT or 10,000ft, a much larger window than normal forsingle reservoir-focused studies. Seismic AvO inversion was run using overlapping, time windows fromshallow to deep, to account for wavelet transmission effects. The resulting inversion outputs, acoustic andshear impedance, were used to derive shale and sand probability volumes. Well based analysis was usedto determine the best relationship between acoustic and shear impedance and Young’s Modulus for bothsand and shale facies. Using the facies probability volumes from seismic inversion, 3D dynamic Young’sModulus 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 geomechanicalmodelling 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 analysisinvolved calculating stresses that honor local field measurements and incorporate regional trends.Novel/Additive Information: Utilizing 3D finite element models constrained by seismic yielded a highresolution predictive model that will significantly improve wellbore stability predictions along the pathsof future development wells. The business impact for the Asset is reduced development well costs byhaving a more predictable geomechanics model, fully constrained by lateral variations from 3D seismicdata, and greatly reduced cycle times for borehole stability predictions for future wells.