The porosities were determined by measurements of the total volume and pore space volume of the samples. We prepared right cylindrical cores using a core drill press, a rock saw, and a surface grinder.
Measuring sample volume: Calculated by measuring the length and diameter of the cylinders using a calipers. Most samples were a nominal 2-inch diameter and 1 to 3 inches long.
Drying the samples: Samples were oven-dried at 70°C (158°F) for at least 24 hours before testing.
Measuring pore space volume: Pore space volume was determined using a helium pycnometer. The helium pycnometer makes use of Boyle’s Law (P1V1=P2V2) and helium gas, which quickly penetrates small pores and is nonreactive, to determine the solid portion of a sample. The core is placed in a sample chamber of known volume. A reference chamber, also of known volume, is pressurized. The two chambers are then connected, allowing the helium gas to flow from the reference chamber to the sample chamber. The ratio of the initial and final pressures is used to determine the volume of the sample solid. The pore volume is the difference between the total volume and the solid volume as determined by the helium pycnometer. This technique can only be used to measure pores that are interconnected. Helium and water do not penetrate into isolated pores, so these pores are not included in the porosity measurement.
Measuring density: Dry densities were determined by weighing the samples after drying and dividing the mass by the total sample volume. Wet densities were then calculated by assuming the porosity of the sample was filled with water, adding that mass to the dry measured mass and dividing the sum by the total sample volume. Grain density was calculated by subtracting the pore space volume from the total sample volume and then dividing the difference by the dry mass.
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