Laminated clastic reservoirs: The interplay of capillary pressure and sedimentary architecture

Many scales of heterogeneity are present in clastic reservoirs. Important heterogeneities occur at the small-scale as a result of episodic or periodic fluctuations in the depositional process. These fluctuations are manifested as quasi-periodic grain size variations at scales of 10^-3 to 1 metre. In the hierarchical description of sediments (the basis of sequence stratigraphy), these periodic stratal elements are known as laminae or beds. The fluctuating grain size elements result in well determined variations in pore and pore-throat size distributions. The effects of pore throat distributions on two-phase flow are reflected in capillary pressure curves, which may differ strongly as a function of absolute permeability (especially for permeabilities less than 1 Darcy). When stratal elements of contrasting grain size, sorting and permeability occur in close proximity, significant differences in local capillary pressures may occur over short distances. In common flow situations, these capillary pressure gradients can be comparable to or greater than pressure gradients induced by either viscous or gravitational effects. Such conditions occur when (a) permeability varies over very short distances (e.g., centimetres for the lamina scale), (b) flow rates are low (i.e., frontal advance rates are less than 1 metre/day), and (c) permeability contrasts are significant (i.e., of one order of magnitude or greater). These conditions occur in many laminated clastic reservoirs. This paper shows how a knowledge of sedimentary architecture (stratal elements) can be exploited to scale-up these capillary effects. A multiphase flow upscaling methodology based firmly on the length scales represented within the hierarchy of stratal elements is presented. We refer to our approach as the geopseudo approach since it incorporates the flow/structure interactions at the lamina, bed and formation scales (i.e., it includes the geology in an appropriate manner). The Rannoch Formation of the Brent Group (North Sea) provides an interesting practical example where large scale production performance is seen to be affected by the interaction of capillary forces with small-scale structure.