Depogrid: Next – Generation Unstructured Grids for Accurate Reservoir Modeling and Simulation

Abstract

A depogrid is a new cut-cell unstructured grid that is demonstrably better suited than standard geocellular grids for reservoir modelling and dynamic simulation workflows. By construction, the depogrid honours the geological structure exactly by following the stratigraphic layering. It enables accurate property modelling in a flattened space (depospace), and can be simulated directly in a next generation simulator. The depogrid is constructed in the same flattened pseudo-depositional space, subject to geomechanical constraints, as is used in the construction of the structural geology model. A depogrid is called a cut-cell grid, because it is constructed first as a grid of regular cuboid cells, which are then cut by the features to be honoured (faults and unconformities) before being transformed back into geological space. Based on knowledge of this geological space–depospace transform, the depositional directional properties, e.g., permeability, are simulated and used in an accurate cell-cell transmissibility calculation within the reservoir simulator, thereby producing accurate dynamic results. The resulting grid has cell columns that are orthogonal to the horizon and layers that follow the stratigraphy; the cells are therefore generally closer to a cuboid shape. A depogrid can accurately represent geological structures, thus providing a good starting point to flow simulation workflows. The faults and fault-block volumes are accurately honoured, independent of the chosen grid resolution. At faults, where the cells have been cut, the cells are polyhedral. The multiple parts of these cut cells mean that the final geological grid has a globally unstructured topology. For each conformal sequence within a reservoir, the depospace, and therefore the depogrid, are constructed independently, so there can be cell column offsets at unconformable horizons. This complex set of non-neighbour connections at the faults and additional cross-sequence connections are handled robustly by the next- generation simulator. The depogrid has been demonstrated through examples to be well suited for modelling a variety of reservoirs and performing accurate flow simulations. We shall be focusing on examples representative of the reservoirs in the African continent. In structurally complex areas, the construction of accurate 3D models is often impeded by the limitations of standard geocellular models, which are successfully overcome with the introduction of the novel depogrid workflow.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018