Integrated reservoir characterization and uncertainty analysis, Heidrun Field, Norway
Authors: OLSEN T. , ROSVOLL K. J. , KJÆREFJORD J. M. , ARNESEN D. M. , SANDSDALEN C. , JØRGENVÅG S. H. , LANGLAIS V. , SVELA K. E. 
Published: April 2010

The Lower Jurassic Åre and Tilje Formations of the Heidrun Field contain some 350 × 10
6
Sm
3
(2160 × 10
6
BBL) of oil in place. The reservoirs are highly heterogeneous and represent deposition in a wide range of fluvial, tidal and marginal marine environments. This, together with a high level of faulting and a relatively viscous oil type, has led to low recovery estimates. To help improve recovery, an integrated reservoir description project including a thorough sedimentological analysis, stochastic modelling of genetic and diagenetic facies, and an associated uncertainty analysis also using stochastic modelling was initiated.

Facies architectures were modelled in 17 individual reservoir zones, each zone consisting of up to 10 × 10
6
grid blocks. The Åre Formation is characterized by alluvial plain and low energy deltaic systems, including incised valley fills, fluvial channels, crevasse splays, upward coarsening bayfills and large splay lobe deposits. The overlying Tilje Formation is characterized by mixed tidal and marginal marine depositional systems with tidal channels, tidal sand and mud flats, tidal shoals and shoreface and offshore sands. Several facies were modelled within each zone and the desired facies architecture was often achieved by merging several individual stochastic realizations. Input to the facies modelling was based on well data as well as information from outcrop analogues and modern depositional systems. Petrophysical attributes were distributed stochastically within each facies body type, using frequency distributions from the wells and interpreted variogram functions. The realizations were finally fitted to the structural maps and upscaled for flow simulation. This enabled the building of a ‘best guess’, or most likely, full-field geological and dynamic simulation model.

A subsequent uncertainty study integrated and evaluated the full spectrum of geological and petrophysical uncertainties related to the dynamic behaviour of the reservoir including facies geometries and facies volume fractions. Best, worst and intermediate case facies realizations with respect to fluid flow were first generated using stochastic modelling. These were combined with other major reservoir uncertainties (gross rock volume, petrophysical values) and 150 complete geological models were established. The hydrocarbon pore volume in each of these models was calculated, and the models were taken through a simplified flow simulation. The results from this process allowed a ranking of the models and a selection of representative models for further dynamic flow simulations. The uncertainty study shows that the uncertainty related to the gross rock volume is the most significant on a field-wide scale. Uncertainties in the facies input parameters (geometry, facies volume fraction) have relatively little impact at a field wide scale. However, their impact upon individual zones or segments (local scale) can be large.