Characterization of hydraulic properties of Pocheon and Yangsan granites and Boryung and Berea sandstones by measuring porosity, permeability, and electric conductivity

Cracks and pores ubiquitous in rocks play a crucial role in storing and transmitting subsurface fluids. In order to understand the effect of geometric characters of such voids on the hydraulic behaviors using geophysical data, we measured porosity, permeability, and electric impedance of two granites (Pocheon and Yangsan granites) and two sandstones (Boryung and Berea sandstones). The dimension of cored rock samples is 38-50mm in diameter and 70-100mm in length. Because Berea sandstone shows a clear bedding planes, rock samples were prepared by coring in three directions; parallel, perpendicular, and oblique (45°) to the bedding. We obtained the connected porosity from dry and wet densities of each sample, hydraulic permeability from the steady-state flow of distilled water using Darcy's law, and the electric conductivity and capacitance by measuring electric impedance between the two ends of samples saturated with saline water with different salinity (0.1, 0.5, 1.0, 2.0, and 4.0% of NaCl). After all tests, SEM micrographs of a polished surface of each rock sample were taken to examine the geometry of pores and cracks. The connected porosity ranges from ~0.2% in Boryung sandstone, ~0.4% in Yangsan granite, ~1.8% in Pocheon granite, and up to 17-19% in Berea sandstone. According to the SEM micrographs, the original pores of Boryung sandstone were mostly filled with cementing materials, and Pocheon granite shows higher crack density and weathering than Yangsan granite does. The permeability shows a range of 4.2×10^-21 m² in Boryung sandstone, 1.3×10^-19 m² in Yangsan granite, 2.5×10^-17 m² in Pocheon granite, and 1.3×10^-14 m²-4.6×10^-16 m² in Berea sandstone depending on the direction of bedding with respect to flow. The porosity-permeability relation is approximated by a power function with the exponent of ~3. The electric conductivity of rock samples shows a linear relation with porosity with different slopes for different salinity of interstitial fluid. Electric conductivity-capacitance relation for all four rock types is delineated by a linear function with the same equation parameters when the salinity is larger than 0.5%. Some geometric parameters such as formation factor, cementation factor and tourtuosity of fluid passages were calculated using the geophysical data and hydraulic properties. The ratio of pore volume to surface area was obtained from the Kozeny-Carman equation. Formation factor and cementation factor are better correlated with porosity, while tortuosity and the ratio of pore volume to surface area are better correlated with permeability.