AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Geomechanical modeling for exploration wells, 2D or 3D? A case study from the West African Coast


Geomechanical modeling for frontier hydrocarbon prospects can be used to obtain stress/strain information to complement the pore pressure and fracture gradient studies traditionally performed in these settings. Offset well, basin modeling and seismic velocity analysis are used to define the preliminary inputs to the first well design. Geomechanical models can also be built using these inputs to improve or increase confidence in the pore pressure and stress predictions. At this stage, the selection of a 3D versus 2D geomechanical model becomes important. Choosing one option can be influenced by time and budget constraints or required accuracy of the results. Geometric variability, complex fault networks, lateral changes in lithologies or salt versus sediment interaction can be factors that tip the balance from one approach to another. In this talk, a real case study from the West African Coast is presented where a 3D elastic geomechanical model was built to obtain a full stress/strain description of a salt-related structure to assess fault stability, as well as to compare the results to the offset well and basin model analysis. Comparisons between the 3D model and a 2D simplified version of this model are presented to study the differences in the results and conclude if simplifying one of the three dimensions negatively affects the results or if the 2D model still represents the geology encountered by the exploration well. To achieve such a comparison, a 2D representative section was chosen from the full 3D model and used as the geometric input for the construction of a geomechanical model. The rest of the inputs required are the same as the original 3D model, to ensure the minimum input variability and a proper comparison between models. The results of stress and displacements from the 3D model are compared to the 2D model allowing the driving mechanisms for both models to be identified and compared. The main objective of this study was to assess the impact of the 2D vs 3D approach for the portion of the model where the well was drilled, above the main salt body. In this area, the key difference between both models is that the 2D model shows higher stress ratio reduction over the crest of the main salt structure and has higher salt and sediment displacements around the well location when compared with the 3D model. The orientation of salt deformation is similar in both models, but the election of the orientation of the 2D section used for the model construction is an important factor when trying to simplify one of the 3 dimensions of the actual problem faced. Finally, the mechanism that drives the stress/strain variations across the model are identified to be the salt creep and the geometry of the salt bodies for both models. The results indicate that 2D geomechanical models, if selected correctly, can represent more complex 3D geometries. Moreover, the increased speed of construction and processing of 2D models allows more time for sensitivity analysis compared to 3D.