Single- and cross-hole pneumatic injection tests in unsaturated fractured tuffs at the Apache Leap Research Site near Superior, Arizona

Abstract

This dissertation documents research results from a series of field experiments and analyses used to test interpretive models for investigating the role of fractures in fluid flow through unsaturated, fractured tuffs. It summarizes the experimental design of single- and cross-hole pneumatic injection tests, including borehole configuration and testing schedules, data collection system, interpretive models developed and tested, data, and conclusions. Single-hole tests were interpreted by Guzman et al. (1996) by means of steady-state analysis to obtain permeability values based solely on late pressure data. This dissertation and Illman et al. (1998) employ pressure and pressure-derivative type-curves to analyze transient data. Air permeabilities determined from transient analyses agree well with those derived from steady-state analyses. Cross-hole pneumatic tests were analyzed by means of a graphical matching procedure using newly-developed pressure and pressure-derivative type-curves. Analyses of pressure data from individual monitoring intervals using these new type-curves, under the assumption that the rock acts as a uniform and isotropic fractured porous continuum, yield results that are comparable with parameters obtained from a numerical inverse procedure described in Illman et al. (1998). The results include information about pneumatic connections between the injection and monitoring intervals, corresponding directional air permeabilities, and air-filled porosities. Together with the results of earlier site investigations, single- and cross-hole test analyses reveal that at the Apache Leap Research Site in central Arizona: (1) the pneumatic pressure behavior of fractured tuff is amenable to analysis by methods that treat the rock as a continuum on scales ranging from meters to tens of meters; (2) this continuum is representative primarily, but not exclusively, of interconnected fractures; (3) its pneumatic properties vary strongly with location, direction and scale, in particular, the mean of pneumatic permeabilities increases, and their variance decreases with scale; (4) this scale effect is most probably due to the presence in the rock of various size fractures that are interconnected on a variety of scales; and (5) given a sufficiently large sample of spatially varying pneumatic rock properties on a given scale of measurement, these properties are amenable to analysis by geostatistical methods, which treat them as correlated random fields defined over a continuum.