Paralic and Tidal Reservoirs of the Heidrun Field, Offshore Mid-Norway - Integrated Reservoir Characterization and Uncertainty Analysis Using Stochastic Modelling

Abstract

The Lower Jurassic Are and Tilje formations of the Heidrun Field contain some 2160 million barrels of oil in place. However, 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 simulated recoveries. 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 modeled in 17 individual reservoir zones, each zone consisting of up to 10 million grid blocks. The Are 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 modeled within each zone and the desired facies architectures were 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 Recent 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. The aim of a subsequent uncertainty study was to integrate and evaluate the entire spectrum of uncertainties related to the dynamic behavior 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 reflecting the total uncertainty as a function of the geological input parameters. The hydrocarbons pore volume in each of these models was calculated, and the models were also taken through a simplified flow simulation. The results from this process allowed a ranking of the models. The selection of representative models for further full-field dynamic flow simulations was based on this ranking. 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.