Model-Parameter Sensitivity Analysis on Representative Elementary Volumes for Small-Scale Geological Heterogeneities.

Subsurface reservoir models typically use grid cells of tens to hundreds of meters in the horizontal directions, and several to tens of meters in the vertical direction. The effects of small-scale heterogeneities (below grid cell size) on fluid flow are often ignored or assumed to have an effective ’upscaled’ average in reservoir simulations. The used grid-cell size might not correspond to the scale at which these fluid-rock interactions can be accurately averaged. The smaller scale geological heterogeneities (from pore scale up to meter scale and tens of meters, or outcrop scale) and fluid behavior (e.g., capillary vs gravitationally dominant flow regimes) play an important role in CO₂ sequestration and hydrogen storage. Ideally, the grid cell size of a reservoir model is determined by the representative elementary volume (REV) which accurately captures the net effect of smaller scale structures on a certain fluid for a certain (representative) volume. We perform a rigorous analysis on the influence of sub-meter scale heterogeneities on REV scales for single-phase flow. Using generated 3D models of 2x2x2m size we vary the bed thickness, dip angle, azimuth angle, and permeability distribution (values and fining or coarsening upwards), and determining their impact on the REV. To do this a sensitivity analysis is performed on the parameters and the three calculated dimensions of the REV.