The effect of sandstone microfabric upon relative permeability end points

The nuances of relative permeability curves are commonly considered to be the product of variations in pore structure and wettability. Extrapolation of the results of a few flow tests into an entire reservoir for simulation purposes assumes that wettability does not change much over most of the reservoir and that the porous microstructure is relatively random and homogeneous. However, there is an increasing body of research indicating that the distribution of porosity is never random or homogeneous. Sandstone fabrics are a mixture of close-packed domains and packing flaws. This characteristic structure imparts a characteristic structure to the pore network that, in turn, defines fluid flow behavior (both single- and multiphase). Packing flaws are zones of oversized pores and pore throats with great spatial continuity. In lithified sandstones, virtually all of the single-phase flow occurs within packing flaws. The close-packed zones have much smaller pores and pore throats, and along with microporosity, tend to retain irreducible water. Results from a variety of quartz-rich sandstone reservoirs indicate that the domainal structure of porosity exerts a major influence upon Sor and Swi values observed in unsteady-state tests. The sample set is limited to quartz-rich reservoir sands with an induced water-wet condition. However, the results demonstrate the linkage between pore fabric and relative permeability end points, and may ultimately permit one to extrapolate those properties as a function of depositional fabric.