Modelling Fine-scale Sedimentological Heterogeneity for Optimal Production from the Neptune Field, Aeolian Gas Reservoir UKCS

The Neptune Field lies on the NW margin of the Southern North Sea gas basin in block 47/4b and extends into blocks 47/5a and 42/29. Following development drilling of the field, a rebuild of the deterministic reservoir simulation model failed to provide a reasonable history match except for Flowing Bottom Hole Pressures (FBHP). The poor dynamic history match was due to the modelling not incorporating the permeability heterogeneity seen in the aeolian sandstones of the Neptune Field. A stochastic reservoir model was produce that captured the broad range of scales of critical facies and petrophysical heterogeneities present within this aeolian reservoir. The challenge for the stochastic reservoir model was to capture the key heterogeneities in this high net to gross system. Facies modelling was undertaken using the General Mark Point Processing in Roxar's RMS. Dunes were modelled as prolate ellipsoids with long axes alignment constrained by dip-meter interpretation. Dune dimensions were estimated from outcrop analogues and were conditioned to well data. Aeolian sandsheet and damp interdune facies were modelled as sheet-like oblate ellipsoids inclined at similar dips to the aeolian dunes. Permeability variations due to lamina grain size segregation in aeolian dune foresets were modelled using sector models to capture cross lamina permeability (kx, kz) and lamina parallel flow (ky). The results of sector modelling were incorporated into the stochastic petrophysical realisations where spatial variation in permeability within dunes was modelled using object specific linear piecewise trend porosity and permeability transformations. Dunes were simulated with low permeability dune bases, high permeability lower dune slipfaces and moderate to low permeability upper dune slipfaces. Other minor aeolian and fluvial facies were assumed to be petrophysically isotropic. The stochastic permeability map gave a better BHP response and history match at the development wells than any deterministic permeability distribution modelled.