Clastic rocks with variable grain sizes exhibit different flow properties depending on how the various grain sizes are geometrically arranged within the grain pack. Grain arrangement must be assessed to quantify dynamic petrophysical properties when rocks exhibit non-unimodal grain-size distributions. A non-unimodal throat-size distribution will be observed, for instance, in cases where different grain sizes are arranged in laminated form (e.g., aeolian sandstones). Clastic rocks that have been subject to extreme diagenesis and recrystallization, such as tight-gas sandstones, often exhibit bimodal grain and pore-throat size distributions.
This paper investigates the impact of grain arrangement on permeability and capillary pressure in clastic rocks that exhibit multiple grain sizes. Two extreme cases are studied for grain packs that include variable grain sizes: when grains are
(i) randomly dispersed, and
(ii) laminated in the grain pack.
Equations are derived to calculate permeability in each case. Additionally, shale concentration is accounted for in the calculation of permeability for both laminated and poly-dispersed grain arrangements. A three-dimensional chart is constructed to illustrate the behavior of permeability with respect to rock type fraction and shale concentration. The assessment includes the calculation of permeability anisotropy resulting from grain-size laminations. Synthetic samples of grain packs are also constructed and subject to pore-scale fluid flow simulations to calculate permeability and throat-size distribution and to examine how these properties change with different grain-size arrangements.
Finally, a new rock classification method that considers grain arrangement, capillary pressure, shale concentration, and permeability is introduced and verified with measurements acquired in a Carboniferous tight-gas sandstone from northern Germany. Our method of rock classification yields improved permeability calculations compared to widely used classification methods such as Winland R35, which implicitly assume a unimodal throat-size distribution. The new rock classification method can be readily adapted to calculate more specialized fluid-transport properties such as relative permeability. It can also be modified to account for capillary pressure during both imbibition and drainage and their consequence of saturation-height behavior.
