D.D. Evans

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

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.

LABORATORY ANALYSIS OF FLUID FLOW AND SOLUTE TRANSPORT THROUGH A FRACTURE EMBEDDED IN POROUS TUFF

Laboratory experiments were conducted to determine the flow and transport properties of a fractured porous tuff block measuring 20 × 20 × 50 cm. One porous ceramic plate was placed immediately above a fracture and two other plates were placed on either side of the fracture above the rock matrix. The plates control the pressure head applied to the upper surface of the fractured rock block. Laboratory results are simulated using the boundary integral method for a single saturated fracture with an assumed uniform transmissivity embedded within a porous tuff block. The simulation is used to identify the saturated hydraulic properties of the fracture. Matrix hydraulic conductivity is estimated as 50 × 10−9 m s−1 and the fracture transmissivity is estimated as 5.0 × 10 −9 m2 s−1. Much of the flow which exits the fracture at the lower surface first passes through the rock matrix even though a direct contact with a porous plate is present. Travel times and breakthrough curves are calculated by integrating the inverse velocity along a streamline, and then summing over all streamlines. Observed breakthrough curves were used to estimate fracture dispersivities which ranged from 0.0207 to 8.01 m. Breakthrough curves deviated from simulation results due to significant channeling of fracture flow.

Unsaturated fractured rock characterization methods and data sets at the Apache Leap Tuff Site

Performance assessment of high-level nuclear waste containment feasibility requires representative values of parameters as input, including parameter moments, distributional characteristics, and covariance structures between parameters. To meet this need, characterization methods and data sets for interstitial, hydraulic, pneumatic and thermal parameters for a slightly welded fractured tuff at the Apache Leap Tuff Site situated in central Arizona are reported in this document. The data sets include the influence of matric suction on measured parameters. Spatial variability is investigated by sampling along nine boreholes at regular distances. Laboratory parameter estimates for 105 core segments are provided, as well as field estimates centered on the intervals where the core segments were collected. Measurement uncertainty is estimated by repetitively testing control samples. 31 refs., 10 figs., 21 tabs.

Nonisothermal hydrologic transport experimental plan

A field heater experimental plan is presented for investigating hydrologic transport processes in unsaturated fractured rock related to the disposal of high-level radioactive waste (HLW) in an underground repository. The experimental plan provides a methodology for obtaining data required for evaluating conceptual and computer models related to HLW isolation in an environment where significant heat energy is produced. Coupled-process models are currently limited by the lack of validation data appropriate for field scales that incorporate relevant transport processes. Presented in this document is a discussion of previous nonisothermal experiments. Processes expected to dominate heat-driven liquid, vapor, gas, and solute flow during the experiment are explained, and the conceptual model for nonisothermal flow and transport in unsaturated, fractured rock is described. Of particular concern is the ability to confirm the hypothesized conceptual model specifically, the establishment of higher water saturation zones within the host rock around the heat source, and the establishment of countercurrent flow conditions within the host rock near the heat source. Field experimental plans are presented using the Apache Leap Tuff Site to illustrate the implementation of the proposed methodology. Both small-scale preliminary experiments and a long-term experiment are described.

Laboratory analysis of fluid flow and solute transport through a variably saturated fracture embedded in porous tuff

Laboratory techniques are developed that allow concurrent measurement of unsaturated matrix hydraulic conductivity and fracture transmissivity of fractured rock blocks. Two Apache Leap tuff blocks with natural fractures were removed from near Superior, Arizona, shaped into rectangular prisms, and instrumented in the laboratory. Porous ceramic plates provided solution to block tops at regulated pressures. Infiltration tests were performed on both test blocks. Steady flow testing of the saturated first block provided estimates of matrix hydraulic conductivity and fracture transmissivity. Fifteen centimeters of suction applied to the second block top showed that fracture flow was minimal and matrix hydraulic conductivity was an order of magnitude less than the first block saturated matrix conductivity. Coated-wire ion-selective electrodes monitored aqueous chlorided breakthrough concentrations. Minute samples of tracer solution were collected with filter paper. The techniques worked well for studying transport behavior at near-saturated flow conditions and also appear to be promising for unsaturated conditions. Breakthrough curves in the fracture and matrix, and a concentration map of chloride concentrations within the fracture, suggest preferential flows paths in the fracture and substantial diffusion into the matrix. Average travel velocity, dispersion coefficient and longitudinal dispersivity in the fracture are obtained. 67 refs., 54 figs., 23 tabs.