Type curve analyses of pneumatic single‐hole tests in unsaturated fractured tuff: Direct evidence for a porosity scale effect

Abstract

A large number of single‐ and cross‐hole pneumatic injection tests have been conducted in shallow vertical and inclined boreholes in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superior, Arizona. Previously, direct and indirect evidence for a permeability scale effect was provided through the analysis of these tests. Direct evidence was based on the comparison of small‐scale single‐hole and larger‐scale cross‐hole test results obtained through the type curve and steady state analysis of these data separately. Indirect evidence was provided by comparing cross‐hole test analyses conducted at fine and coarse scales of resolution by means of a three‐dimensional numerical inverse model. The latter study also provided indirect evidence for a porosity scale effect. However, there were no small‐scale porosity data from single‐hole tests to directly verify this indirect evidence. This paper presents such data generated through the type curve interpretation of the recovery phase of the single‐hole tests conducted at a nominal 1‐m scale providing direct evidence for a porosity scale effect at the site. Statistical analysis of results revealed a strong porosity scale effect confirming an earlier finding obtained indirectly. These results also showed that (1) the injection phase of the pneumatic single‐hole tests do not yield reliable estimates of porosity but the recovery phase are amenable to type curve interpretation, (2) flow dimensionality of single‐hole tests at 1‐m scale are three‐dimensional across the site except for a few tests which exhibited two‐dimensional and fracture flow behavior, (3) there is a very weak correlation between permeability and porosity, and (4) there is a lack of correlation between fracture density and both permeability and porosity.