Permeability of Apache Leap Tuff: Borehole and core measurements using water and air

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Abstract
Field and laboratory methods for estimating and interpreting parameters obtained from field borehole and laboratory core experiments are examined using permeability data interpreted from air and water injection tests in variably saturated fractured tuff at the Apache Leap Tuff Site in central Arizona. The tuff at the field site has a matrix porosity of approximately 17.5% and contains numerous near‐vertical fractures at an average spacing of 1.3 m. More than 270 m of 6.4‐cm‐diameter oriented core were collected from boreholes drilled to a maximum depth below the surface of 30 m and at a vertical angle of 45°. Laboratory estimates of absolute permeabilities using air and water as the test fluids were acquired at a range of matric potentials for 105, 5‐cm‐long core segments extracted at approximately 3‐m intervals containing no obvious fractures. Field scale estimates of fractured rock permeabilities using air and water as test fluids were obtained at ambient matric suctions and water saturated conditions, respectively. The field tests were conducted along 3‐m intervals within boreholes with the intervals centered on core sampling positions. Borehole and core permeabilities demonstrate substantial spatial variability, with variations exceeding three orders of magnitude. Laboratory core data show a strong relationship between permeabilities using saturated water and oven‐dry air injection tests with the latter demonstrating the Klinkenberg effect. The influence of matric suction on permeabilities is used to demonstrate that relative permeabilities do not sum to a constant for a wide range of matric suction. Only weak relationships exist between permeabilities measured in boreholes versus cores for both water and air. Permeabilities measured in boreholes using air are shown to provide good estimates of permeabilities measured using water into initially unsaturated, fractured rock at the Apache Leap Tuff Site.