A New Approach to Thin Oil Zone Modeling in High-Dipping Multilayered Reservoirs

Abstract

This paper focuses on maximizing oil recovery from thin oil rim in a high-dipping, multilayered reservoir in the North Sea. Oil output from thin oil zones between a gas cap and an aquifer is limited to uneconomical rates caused by water coning and gas cresting. Horizontal wells are widely used in thin oil rim development due to the limited pressure drawdown applied to the formation. The horizontal wells should be optimally positioned within the oil zone to avoid premature gas and water breakthrough. Critical issues for thin oil zone reservoir modeling are correct grid representation of internal heterogeneities, fluid saturation distribution, fluid contacts, horizontal well trajectories and flow displacement properties. In a stratigraphic grid, the lack of resolution affects the fluid contacts movement within the high-dipping reservoir layers. Similarly for a horizontal grid, thin reservoir intervals are truncated due to a 'stair-casing' effect, which leads to loss of communication within the layers. A hybrid grid was constructed to combine the benefits of the horizontal grid in the oil zone together with the stratigraphic grid in the water and gas zones. Such a grid framework provides the best modeling approach for representing the transition zone and fluid contact movements. In the study, the prediction of oil, gas and water production in the transition zone was difficult because the crude oil altered formation wettability after hydrocarbon migration and reservoir structure tilting. Multiphase flow was modeled by integrating the key dynamic reservoir controls, translated into end-point correlations above the free water level based on multivariate SCAL data analyses. Comparative analyses of alternative modeling techniques exhibited clear differences in the response for coning development and optimal well placements.