Effect of Rock Heterogeneity on Relative Permeability: Implications for Scale-up

Abstract

Abstract Relative permeability and capillary pressure are important parameters in reservoir engineering calculations and numerical simulation of reservoir performance. Heterogeneities are often avoided during core plug screening and selection for relative permeability and capillary pressure measurements. However, sandstone rocks in many depositional environments show significant small-scale laminations that affect the measured relative permeability. This report demonstrates the length scale dependence of relative permeability data that results from cm to mm scale rock laminations and patterns of initial and final oil saturation distribution in a 108-cm long laminated core. It shows in a quantitative way the capillary, trapping of water in low-permeability lamina during primary drainage and of oil in high permeability lamina during water imbibition. Steady-state water-oil imbibition relative permeability data and unsteady-state drainage and imbibition data were collected using linear x-ray and x-ray CT scanning for in situ fluid saturation measurement. Numerical simulations of the core floods show that relative permeabilities and capillary pressures that are correlated with small-scale differences in porosity and permeability are necessary to reproduce the observed saturation distributions. Thus, the relative permeability length-scale dependence, combined with anisotropy data presented elsewhere (SPE 27968), imply that scaled-up effective relative permeability must account properly for heterogeneity. Assignment of core-plug relative permeability to simulator grid blocks may not capture the correct effective fluid flow performance in rocks that are heterogeneous with correlation length greater than the plug dimensions, and thus lead to erroneous fluid flow performance predictions. Using numerical simulation, a number of papers have reported the importance of core-scale geologic heterogeneity to relative permeability and its ultimate impact to the prediction of reservoir-scale flow behavior. Jones et al. showed that within a channel environment the choice of relative permeability scale-up from small-scale to larger scales is very important to accurate prediction of water breakthrough and produced oil volume and is more important than are spatial arrangement of lithofacies within channel sandbodies.