Type‐Curve Interpretation of Multirate Single‐Hole Pneumatic Injection Tests in Unsaturated Fractured Rock

Abstract

AbstractMore than 270 multirate single‐hole pneumatic injection tests were conducted by Guzman et al. (1996) within six shallow vertical and inclined boreholes in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superor, Arizona. The authors used steady‐state formulae to obtain air permeability values for the rock based solely on late pressure data from each test. We developed pressure and pressure‐derivative type‐curves for the interpretation of transient data from these tests, which account for storage in the test interval due to air compressibility and skin effect due to damage to the surrounding rock. We applied our type‐curves to pressure data from more than 40 of the tests to obtain information about air permeability, skin factor, phenomenology, and dimensionality of the flow regime on a nominal scale of 1 m in the immediate vicinity of each test interval. Our air permeabilities agreed well with those determined previously by means of steady‐ state formulae but correlated poorly with fracture density data. Nonlinear effects due to air compressibility were pronounced, but skin effect was generally small or nonexistent. Nonlinear two‐phase flow and inertia were evident but did not have a significant impact on type‐curve interpretation of the test data. Most of these data fit a continuum model of spherical airflow around the test interval, but a few fit a continuum model of radial flow or suggest the dominance of a single fracture. We take this to imply that fractures at the site are pneumatically well connected and can, for the most part, be treated as a porous continuum when analyzing airflow on scales of 1 m or more. This is supported by the finding that 1 m scale air permeability data from single‐hole tests at the ALRS are amenable to geostatistical analysis (Chen et al. 2000), which views them as a sample from a random field defined over a continuum. That the same holds true on larger scale is indicated by our ability to interpret cross‐hole pneumatic injection tests at the site by means of type‐curves and inverse methods (Illman et al. 1998), which treat the entire rock mass as a porous continuum.