Modeling Challenges and Geological Uncertainty Quantification in a Pre-Salt Discovery, Brazil

Zanao, Rodrigo¹; Real, Tacilaine S.; Algibez, Jose Luis A.; Bonillo, Pedro M.; and Perona, Ricardo J.
¹[email protected]

The pre-salt in Brazil is a new exploratory play in the South Atlantic Margin. Due to the fact that it is a new exploratory frontier, the quantity of data available is scarce and the associated uncertainty is proportionally inverse to the existing information. The present work is related to a geological 3D model of a recent pre-salt discovery located in Brazil. The uncertainties incorporated into the modeling process were quantified in three different steps: structural model, facies model and petrophysical model.

The objective of the Structural Model was to understand the uncertainty associated to Gross Rock Volume (GRV). Three scenarios were used considering different seismic interpretations and horizon picking for the three reservoir intervals. Structural model building involves challenges as faults with high angles can generate deformed cells which cause problems in the flow simulation and distinct approaches were tried to minimize the problem. Three different facies model scenarios, FS1, FS2 and FS3, were created varying the input data (reservoir probability cube and seismic facies maps) and assumptions. The porosity model used as input the statistical data delivered from the petrophysical interpretation and the Sequential Gaussian Simulation (SGS) to populate the model. For the saturation model two scenarios were built: (Sw1) Archie derived; (Sw2) Sw defined with NMR log. The permeability model was created using two different approaches: (K1) using Flow Units; (K2) using SGS to populate KSDR permeability log of the well constrained by porosity.

As result of the uncertainty related to the structural model, the GRV in the optimistic case is around 100% larger than the pessimistic case. For fault representation, the use of stair-stepped method minimizes the number of deformed cells compared to the pillar fault representation. The facies model results allow ranking the most pessimistic (FS2) and the most optimistic (FS3) scenarios. Related to the Sw model, the Sw1 scenario shows the most optimistic results than the Sw2. As result of the analyses of permeability models, the K1 scenario shows lower values of permeability compared to the K2 scenario.

It was possible to quantify that the variation of the hydrocarbon-in-place between the most pessimistic and the most optimistic scenario is around 320%. Structural model uncertainty has the greatest impact in the volumetric results at this early stage of a discovery

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013