Type-curve interpretation of transient single-hole pneumatic injection tests in unsaturated fractured tuffs at the Apache Leap Research Site

Abstract

More than 270 single-hole pneumatic tests have been conducted in 6 shallow vertical and inclined boreholes in unsaturated fractured tuffs at the Apache Leap Research Site near Superior, Arizona. Guzman et al. applied a steady-state analysis to late-time data from each test. In this paper, type-curve analyses of transient data from those tests are described, which yielded information about air permeability, skin factor, dimensionless wellbore-storage coefficient, and dimensionality of the flow regime on a nominal scale of 1 m in the immediate vicinity of each test interval. Significant nonlinear effects such as inertia, turbulence, air compressibility, and two-phase flow were expected from air injection into unsaturated fractured tuffs. Type-curve interpretation revealed that the single-hole test data analyzed show little sensitivity to those nonlinearities and the method of linearization. Type-curve analysis was accomplished with the aid of various analytical continuum (radial, spherical) and fracture (horizontal, vertical) flow models as the initial qualitative assessment showed considerable variability of the transient behavior of data at different injection intervals. The fact that transient pressure behavior is not entirely consistent across the site, but varies from one test interval to another, provided a qualitative indication that the site is pneumatically nonuniform and the local rock is heterogeneous. Results show that most data fit the spherical flow type-curve model, implying that flow on the 1-m scale is three dimensional. Air permeabilities obtained from type-curve analyses agree well with values obtained through steady-state analyses. Overall, considerable insight was gained in terms of the local properties of unsaturated fractured rocks, the importance of nonlinear effects, and the phenomenology of airflow through such rocks.