Constraining uncertainty in volumetric estimation: A case study from Namorado Field, Brazil

This paper describes the reservoir-modeling case of Namorado, an oil field located in offshore Brazil, the workflow, tolls and benefits of a 3D integrated study with uncertainties. A geological uncertainty study was initiated to identify and quantify the input parameters of greatest impact in the reservoir model. In order to rank reservoir uncertainties, a series of static models was built and a method to quantify the uncertainty associated with geological parameters was tested. The proposed workflow was developed in the Irap-RMS software and comprised the following steps: construction of the structural model; construction of the geological model; population of the geological model with petrophysical parameters, and uncertainty analysis. To construct the static reservoir model, the low, base and high cases of each uncertainty parameter were defined and used, and all combinations of these parameters were tested. The uncertainties related to the choice of parameters such as the variogram characteristics (type, range, and sill) involved in each geostatistical iteration were included into the workflow. The highest ranked contributors to uncertainty in Stock Tank Oil Initially in Place (STOIIP) were oil–water contacts, range of variogram used to calculate porosity in possible-reservoir facies, and 3D water saturation. The uncertainties related to the main parameters that affect the volumetric calculation were incorporated into the proposed workflow. The hydrocarbon probabilistic volume established for the Namorado Field varies from 92.07 to 134.04 × 106 m3.