Introducing Seismic Inversion to Reduce Uncertainties in Static Modeling

Faical Ben Amor¹ and Ofonmbuk Ekpoudom^2
1Schlumberger, Lagos, Nigeria
²Schlumberger, Port-Harcourt, Nigeria

Uncertainty quantification is an integral part of a reservoir modeling study and the impact is even more pronounced when there is limited well coverage. The spatial distribution of petrophysical properties within a heterogeneous reservoir is also affected by uncertainties. Porosity modeling is generally considered as a critical step in volumetrics and in the population of other properties such shale volume, water saturation and reservoir permeability. Using seismic inversion to predict porosity has successfully reduced the uncertainty away from the well control.

Given the reservoir extent and lack of well coverage, an acoustic impedance inverted seismic cube was used as 3D soft constrain for porosity population. Impedance cube, after calibration with well logs, was critical to estimate horizontal variogram parameters rather than using analog data. For each mega structure compartment, likely to define an independent depositional environment, a variogram map was generated to provide valuable information on anisotropy direction, lateral heterogeneity, chronology of structural events and role of growth faults in controlling reservoir quality and thickness distribution. Vertical variogram was still derived directly from the well information. Impedance based co-kriging allowed guiding inter-well porosity distribution. Water saturation model was subsequently derived from plotting porosity vs. Sw above OWC.

Using Monte-Carlo methods, uncertainty parameters (structure, porosity and water saturation) were stochastically combined to assess volumetrics uncertainty. STOIIP from the impedance constrained model has been compared with the one constructed without impedance. The resulting histograms show that impedance based geomodel has less uncertainty on the STOIIP distribution. Using Experimental Design method, a ranking of the uncertainty parameters effect was possible.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery