Static Reservoir Modelling in the Eastern Niger Delta: Assessing Reservoir Heterogeneity and Uncertainty for Improved Field Development Planning

Abstract

 A combined static reservoir modeling study was conducted for three hydrocarbon-bearing sands 1, 2 and 3 of the ‘X’ Field, eastern Niger Delta, Nigeria. A post-stack, time-migrated 3-D seismic volume of 234 km² with a complete open-hole log suite from three of four wells (KC1–KC4) was provided. Static reservoir modeling was executed in Schlumberger Petrel to integrate the structural, stratigraphic and petrophysical characteristics of the reservoirs and to quantify the hydrocarbon volume in place. The static model was assembled in four connected stages: structural modeling, stratigraphic modeling, lithofacies modeling using sequential indicator simulation (SIS), and petrophysical property modeling using sequential Gaussian simulation (SGS) conditioned to the SIS facies model. A variogram-based, two-point geostatistical approach was adopted, in preference to deterministic kriging or object-based modeling, because it honoured the hard well data exactly, reproduced the natural spatial variability of the reservoir properties away from the wells, and allowed multiple equiprobable realizations to be produced for uncertainty and risk assessment. Thirty-one faults were used as the structural skeleton of the model, with the mapped fault population decreasing from twenty on the depth structure map of Reservoir 1 to nineteen on Reservoir 2 and eight on Reservoir 3. The facies model showed that shale volume decreased with depth, so that Reservoirs 2 and 3 were of better reservoir quality than Reservoir 1. The petrophysical property models returned an effective porosity of 19% to 28% (average 23%), a permeability of 50 mD to 477 mD (averaging 222 mD in Reservoir 1), a hydrocarbon saturation of 56% to 66%, and a net-to-gross of 0.5–3.5 (average 2.0) in Reservoir 1 and 0.5–2.5 (average 1.5) in Reservoir 3. The static model was subsequently used in a Petrel model-dependent volumetric workflow, together with the interpreted oil–water contact, to calculate total oil in place of 6,090 × 10³ bbl, 4,381 × 10³ bbl and 5,658 × 10³ bbl for Reservoirs 1, 2 and 3 respectively. The respective recoverable oil volumes were 2,436 × 10³, 1,752 × 10³ and 2,263 × 10³ bbl. The results confirm that Reservoirs 1 and 3 are more prospective and economically viable than Reservoir 2, and demonstrate that a fully integrated static model, built from properly conditioned structural, facies and property models, is indispensable for reliable reserve estimation in structurally complex Niger Delta reservoirs.