Controls on two-phase fluid flow in heterogeneous sandstones

The effective flow behaviour of two immiscible fluids in a permeable medium is a function of fluid/fluid as well as fluid/matrix interactions. These are described by relative permeability functions, which are strongly influenced by the spatial distribution of permeability in sedimentary media, given specified fluid and pore surface properties. For typical subsurface flow rates and patterns of rock heterogeneity in hydrocarbon reservoirs, capillary forces can result in significant amounts of trapping and bypassing of the non-wetting phase. Different types of sedimentary media (e.g. laminated, crossbedded and pervasively faulted strata) result in different, quantifiable, degrees of trapping. Examples of typical water-wet oil/water systems illustrate how the amount of trapped oil can vary between around 40% and 65% depending on the type of heterogeneity. Furthermore, the layered nature of most sedimentary strata means that the relative permeability of two immiscible phases is highly anisotropic. These findings have important implications for reservoir engineering and studies of oil migration.