Numerical inverse interpretation of multistep transient single-hole pneumatic tests in unsaturated fractured tuffs at the Apache Leap Research Site

More than 270 single-hole multiple-step air-injection (pneumatic) tests have been conducted by A.G. Guzman and coworkers in six shallow vertical and slanted boreholes in unsaturated fractured tuffs at the Apache Leap Research Site near Superior, Arizona. Guzman and coworkers used steady-state formulae for single-phase air flow in a uniform, isotropic porous continuum to interpret late data from each step of an injection test. W.A. Illman and coworkers used transient type curves for single-phase air flow in a similar continuum to analyze all data from the first step of several injection tests; however, this analysis did not allow reliable identification of air-filled porosity and the dimensionless borehole storage coefficient. This chapter describes numerical inversion of multistep and recovery data from some of these same tests, based on similar assumptions, by means of a three-dimensional finite-volume code (FEHM) coupled with a parameter-estimation code (PEST). Our numerical inverse model accounts directly for the geometry, flow properties, and storage capabilities of open borehole intervals by treating them as high-permeability and high-porosity cylinders of finite length and radius. It also allows interpreting multiple injection-step and recovery data simultaneously, and yields information about air permeability, air-filled porosity, and the dimensionless borehole storage coefficient. Some of this is difficult to accomplish with the analytical type-curve method. Air permeability values obtained by our inverse method agree well with those obtained by steady-state and type-curve analyses.