Rapid estimate of solid volume in large tuff cores using a gas pycnometer

Abstract

A thermally-insulated, rigid-volume gas pycnometer system has been developed. The pycnometer chambers have been machined from solid PVC cylinders. Two chambers confine dry high-purity helium at different pressure. A thick-walled design ensures minimal heat exchange with the surrounding environment and a constant volume system, while expansion takes place between the chambers. The internal energy of the gas is assumed constant over the expansion. The ideal gas law is used to estimate the volume of solid material sealed in one of the chambers. Temperature is monitored continuously, and incorporated into the calculation of solid volume. Temperature variation between measurements is less than 0.1 °C. The data are used to compute grain density for oven dried Apache Leap tuff core samples. The measured volume of solid and the sample bulk volume are used to estimate porosity and bulk density. Intrinsic permeability was estimated from the porosity and measured pore surface area, and is compared to in-situ measurements by the air-permeability method. The gas pycnometer accommodates large core samples (0.25 m length x 0.11 m diameter), and can measure solid volume greater than 220 cm³ with less than 1% error.