Tilted oil–water contacts: modelling the effects of aquifer heterogeneity

Abstract

In the North Sea tilted oil–water contacts (OWCs) are recognized in fields of Paleocene, Cretaceous and Jurassic age. In areas where lateral pressure (potentiometric) gradients exist in the aquifer, OWCs dip in the direction of pressure drop and hydrodynamic flow occurs. The direction and magnitude of potentiometric gradients in the North Sea vary locally, but are generally observed to be associated with de-watering of the overpressured Central Graben. Local variations in the potentiometric gradient can be caused by heterogeneity in the aquifer, such as faults, channels, facies changes, pinchouts and/or thinning. These variations cause local irregularities in a field’s tilted OWC. Where an aquifer beneath a field becomes less permeable or thins, the dip of the OWC increases. Where the aquifer becomes more permeable or thickens, then the tilted OWC tends to flatten out. Channels and high-permeability zones tend to concentrate aquifer flow and direct the potentiometric gradient along their axes. Various combinations of aquifer heterogeneity have been modelled in 3D simulations. These models are described and compared with real examples from North Sea fields.