Construct a comprehensive 3D geomechanical model
First, the multidisciplinary team built 1D mechanical earth models
(MEMs) along the trajectories of nine existing appraisal wells. Using cores,
wireline logs, leakoff tests, and hydraulic fracture pressures, 1D MEMs
described the mechanical stratigraphy, formation elastic properties, rock
strength parameters, pore pressure gradient, and in situ stress states at each
To fully characterize lateral variations in rock properties and
stresses between wells, advancements in seismic analysis and geostatistical
modeling were applied. Running 3D seismic data through a simultaneous inversion
workflow and calibrating the results against sonic and density logs, the team
mapped the distribution of porosity throughout the field and generated key rock
property volumes for use in 3D geomechanical modeling.
To understand how faults impact stress variations, the existing
geological model was enhanced by simulating two major bounding faults and more
than 24 faults within the reservoir based on seismic interpretation. To reduce
boundary effects, the existing model grid was extended below the reservoir and
in both lateral directions. Material properties were assigned to faults based
on intact rock properties, then rock elastic and strength properties were
distributed throughout the 3D model using seismic inversion for guidance.
Finally, equilibrium in stress magnitude and direction was achieved,
particularly around faults, by applying appropriate stress boundary conditions
to the model. Comparing simulated stress states with actual 1D stress gradients
validated the 3D geomechanical model.
Predict proper mud weights for planned development wells
Because the field was not yet under production, simulated stresses
accurately represented current in-situ stresses. Thus, they could be used to
predict wellbore stability at any subsequent well location.
Initial results from 1D modeling had indicated that inadequate mud
weight was the primary cause of wellbore instability. Using the 3D
geomechanical model, Schlumberger experts extracted rock mechanical properties,
pore pressures, and stress components from all cells along the trajectories of
several planned development wells. They predicted the minimum and maximum mud
weight windows needed to prevent both borehole breakouts and drilling-induced
fractures. Geomechanical specialists also predicted the hydraulic fracture
initiation pressures required to optimize stimulation of horizontal
3D geomechanical modeling with seismic inversion yielded more accurate
predictions than 1D modeling. Following recommendations from the Schlumberger
team, the operator encountered no significant drilling problems due to wellbore
instability—successfully mitigating the NPT incurred during drilling at
previous wells in the field.