Before making major decisions, it is important to fully understand the basis for the decisions. The oil and gas industry deals with large uncertainties. In Statoil it is a requirement to undertake a thorough uncertainty analysis of hydrocarbon volumes initially in place before deciding if and how a field should be developed.
This presentation will focus on a field case where the Statoil standard workflow for estimating the uncertainty in in-place volumes is used. It is exemplified by a complex field outside Norway. Uncertainties analyzed, modelled and combined are seismic time interpretation, depth conversion, isochore thickness, contacts, petrophysical uncertainties, uncertainties in geological mapping and uncertainties in fluid formation factors.
With only a few wells, the uncertainty in depth conversion is usually important. Hence a considerable effort is made to estimate depth conversion uncertainties. The geostatistical software Cohiba which utilizes Bayesian kriging and Gaussian simulation is used in combination with complex contact uncertainties to obtain gross rock volume uncertainties. The methodology used by Cohiba is a good way to analyse velocity data, combine seismic data and well data and to model uncertainties in trend and not only residual uncertainties (Gaussian random fields) giving a more realistic estimate of the total uncertainties in depth and gross rock volume.
The presentation will focus on the analysis and modelling of the depth conversion uncertainties, but modelling of uncertainty in petrophysics and facies distribution will be discussed and presented. Also here the difference between seed effects and trend uncertainties as volume fractions and different trend maps, is discussed. |
