Immiscible flow behaviour in laminated and cross-bedded sandstones

In this paper, we describe models of water/oil displacement in typical, geologically-structured media. We focus specifically on laminated and cross-bedded structures, since these are almost ubiquitous in clastic sedimentary reservoirs. The importance, for field-scale models, of properly representing the interaction of viscous, capillary and gravitational forces with small-scale heterogeneity is clearly demonstrated. Because the length-scale, delta x, of sedimentary lamination is of order 10(-3) to 10(-2) m, capillary forces, which are inversely proportional to delta x, may play a very significant role in determining the effective flow behaviour at larger scales. We show that there are important differences in oil recovery between cross-layer flow and along-layer flow. Differences of up to a factor of two in ultimate recovery may be produced by different representations of realistic clastic sedimentary structure (such as parallel lamination, cross-lamination and small-scale faulting). The significance of these findings is in determining the correct scale-up procedure for the multiphase effective flow parameters. We use the term geopseudo to describe correctly-scaled, multi-phase, pseudofunctions which capture the effects of small-scale sedimentary structure. Field-scale reservoir models must take account of these small-scale effects in order to lay claim to reasonable accuracy in production forecasts.