f The influence of the size and shape of grid blocks in representing geological heterogeneity

Traditionally, the size and shape of grid blocks owe more to the limitations of numerical simulation procedures than to any consideration of geological heterogeneity. Geological systems, and hence associated permeability and porosity fields, are inherently complex, being characterized by nested levels of heterogeneity. Moreover, for directionally dependent variables, such as permeability, a heterogeneous nature at smalt scales will lead directly to anisotropy in 'average' values at larger scales. The representation of complex geology as single horizontal and vertical permeability values has long been recognized as a (necessary) simplification. Recent advances in stochastic modelling have vastly improved the representation of complex geology in unknown interwell regions. The high level of detail provided such models often requires several million grid blocks, which in turn must be upscaled for input into flow simulations.