Geology, geometry and effective flow

The effects of small-scale structure are frequently ignored in reservoir simulation, although they may have a significant effect on hydrocarbon recovery. Many sandstones exhibit lamination, and in such rock structures, permeability may vary by an order of magnitude over distances of a centimetre or less. Frequently, laminations are inclined with respect to the pressure gradient, which gives rise to cross-flow within the unit. In this case a tensor may be used to represent the effective permeability of the bed. In this paper the effect of small-scale geological structure on single-phase flow is investigated using a model of a cross-bedded unit to determine which factors have the greatest effect on cross-flow. In addition, a range of other types of bedding are considered, including models with stochastic variation. We have found that, in general, the geometry of the sedimentary structure has a significant effect on flow. The cross-flow is greater (and therefore tensors are more likely to be necessary) when the angle of the laminae is large relative to the pressure gradient (up to 45 degrees ), when the permeability contrast between laminae is large, or when the structure is asymmetric.