Experimental investigation of the fractal dimension of the pore surface of sedimentary rocks under pressure

A new experimental technique has been developed and applied to study the changes in the fractal dimension of the pore surface of sedimentary rocks under increasing confining pressures. Twenty sandstone, and twenty limestone reservoir rock samples were studied; the pressure was step-wise increased from 4 to 83 MPa. At any given pressure step, and for a given lithology, the porosity Φ and hydraulic permeability k were found related as a power law k ∝Φ. For sandstone, the exponent v smoothly increased from ≈2 to ≈3 as the pressure increased from 4 to 83 MPa. For limestone, for the same pressure range v smoothly decreased from ≈5.7 to ≈5. Assuming two plausible fractal models of fluid flow in rocks, the exponent v was converted to pore-surface dimension D, and it was found that D increases with pressure from D≈2 to D≈2.5 for sandstone, and decreases from D≈2.7 to D≈2.65 for limestone. The difference between the pressure-dependencies of the fractal dimension of the pore surfaces in case of the two lithologies is due to the different mechanisms governing compression in the various rock types.