The relationship between the aperture and gas conductivity of a single natural fracture was investigated in the laboratory. Fracture conductivity was evaluated as a function of both the applied fluid pressure gradient and average fracture aperture, the latter ranging from 600 to 200 μm. Fracture apertures were determined independently on the basis of both fracture deformation and fracture volume measurements. Flow generally occurred in the linear and transitional flow regime between linear and fully nonlinear flow. The transition was found to be smooth and well described by an equation of the form: −(dp/dx) = av + bv2, where dp/dx is the pressure gradient and v is the fluid velocity. The linear and nonlinear fracture conductivities were found to be functions of the aperture and surface roughness of the fracture in agreement with existing empirical equations. A new physical model for fracture flow is also formulated based on an analogy to pipe flow.

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.

This report was prepared a s an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not. necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAI MEW Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

No abstract

Reconstruction of fracture apertures in a block of welded tuff has revealed two scales of autocorrelation, one on the order of millimeters, the other on the order of tens of centimeters. The spatial and frequency distributions of apertures changed drastically as a result of minor nondestructive shear.