Type curve interpretation of a cross‐hole pneumatic injection test in unsaturated fractured tuff

Abstract

Abstract. Cross-hole pneumatic injection tests have been conducted in 16 vertical and inclined boreholes in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. Their purpose was to characterize the bulk pneumatic properties and connectivity of fractures at the site on scales ranging from meters to several tens of meters. We describe the design, conduct, and type curve interpretation of one of these tests. Our cross-hole type curves are modified after Hsieh and Neuman [1985] to consider single-phase airflow and extended to consider the effects of storage and skin in monitoring intervals. Cross-hole type curves of pressure derivatives and recovery are included for improved pneumatic characterization of the site. We analyze recorded pressures in each monitoring interval separately from those in other intervals while treating the fractured rock as a uniform, isotropic porous continuum. Each record yields an equivalent directional air permeability and air-filled porosity for fractures that connect the corresponding monitoring and injection intervals. Both parameters are found to vary considerably from one monitoring interval to another, reflecting the nonuniform nature of pneumatic rock properties at the ALRS. The geometric mean of these equivalent permeabilities is found to be larger by a factor of 50 than that obtained from single-hole pneumatic injection tests on a nominal scale of 1-3 rn (single-hole tests yield only limited information about porosities, which therefore cannot be meaningfully compared with cross-hole results). Our results find support in numerical inverse modeling of cross-hole ] that a similar scale effect is exhibited by fracture porosity at the ALRS and that both scale effects disappear when cross-hole tests at the site are interpreted by means of a numerical inverse model, which resolves heterogeneities down to a scale of 1 m. When considered jointly with these and other studies of the site, our analysis implies that the pneumatic pressure behavior of unsaturated fractured tuffs at the ALRS can be described quite accurately by means of linearized single-phase airflow equations; this behavior can be interpreted by treating the rock as a continuum on scales ranging from meters to tens of meters: the continuum is representative primarily of interconnected fractures; as these fractures are filled primarily with air, their pneumatic permeabilities and porosities are close to the bulk intrinsic properties of fractures at the site: these intrinsic properties vary randomly with location and direction across the ALRS, and they depend strongly on the scale at which they are determined. IntroductionIssues Apache Leap Research Site Early Work at the ALRSEarly work related to our area of study at the ALRS is described by Rasmussen and Evans [1987, 1989, 1992

Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 2. Equivalent parameters, high‐resolution stochastic imaging and scale effects

“…We view the collection of all parameter estimates obtained in this manner, from the totality of pressure records, as a sample from a random population of equivalent log air permeabilities and log airfilled porosities representing nominal scales of meters to tens of meters. We expect them to be comparable to pneumatic parameters determined by Illman and Neuman [2001] on the basis of individual pressure records from test PP4 by means of analytically derived type-curves. All log air permeability estimates appear to be reasonable for a fractured tuff.…”

“…The two sets of estimates are compared in Figure 5. Illman and Neuman [2001] concluded that a decrease in barometric pressure has no effect on estimates of k but causes the estimates of 4> to decrease by a factor of 1.1. We show in Appendix A that their conclusion, which is contrary to ours, stems from an artifact of their linearized analysis that is easy to eliminate.…”

Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 1. Methodology and borehole effects

“…In reality, we estimate these parameters from records of pressure variation with time in individual borehole intervals, one record at a time, by a numerical inverse procedure. This is akin in principle to interpreting each pressure record by the type curve method of Illman and Neuman [2001]. It yields a series of equivalent air permeabilities and air-filled porosities for rock volumes having length scales ranging from meters to tens of meters, represented nominally by radius vectors extending from the injection interval to the various monitoring intervals.…”

“…Their purpose was to characterize the bulk pneumatic properties and connectivity of fractures at the site on scales ranging from meters to several tens of meters. The design, conduct, and type curve interpretation of one of these tests have recently been described by Illman and Neuman [2001]. The type curves they used have been modified after to consider single-phase airflow and extended to consider the effects of storage and skin in monitoring intervals.…”

Semianalytical solutions for release of fluids from rock matrix blocks with different shapes, sizes, and depletion regimes

“…Illman and Neuman [2001] developed pressure and pressure derivative type curves for single-phase airflow. Illman [2005] analyzed the single-hole pneumatic injection and recovery tests for determining permeability, porosity, and skin.…”