Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Lessons from Field Studies at the Apache Leap Research Site in Arizona
Lessons from Field Studies at the Apache Leap Research Site in Arizona
Pages 295-334
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From page 295... ...
Thompson,4 Guoliang Chen,5 and Amado Guzman6 ABSTRACT This paper summarizes lessons learned from single-hole and cross-hole pneumatic injection tests recently completed by The University of Arizona in unsaturated fractured turfs at the Apache Leap Research Site (ALRS) near Superior, Arizona.
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From page 296... ...
Analyses of pressure data from individual monitoring intervals by the two methods, under the assumption that the rock acts as a uniform and isotropic fractured porous continuum, yield comparable results. These results include information about pneumatic connections between the injection and monitoring intervals, corresponding directional air permeabilities, and air-filled porosities.
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From page 297... ...
There is at present no well-established field methodology to characterize the fluid flow and contaminant transport properties of unsaturated fractured rocks. In order to characterize the ability of such rocks to conduct water, and to transport dissolved or suspended contaminants, one would ideally want to observe these phenomena directly by conducting controlled field hydraulic injection and tracer experiments within the rock.
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From page 298... ...
Pressure was said to have reached stable values within minutes in most test intervals, allowing the calculation of air permeability by means of steady-state formulae. Figure 5b of Rasmussen et al.
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From page 299... ...
In most tests, three or more such incremental steps were conducted in each borehole segment while recording the air injection rate, pressure, temperature, and relative humidity. For each relatively stable period of injection rate and pressure, air permeability was estimated by treating the rock around each test interval as a uniform, isotropic continuum within which air flows as a single phase under steady state, in a pressure field exhibiting prolate spheroidal symmetry.
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From page 300... ...
the time required for pressure in the injection interval to stabilize typically ranges from 30 to 60 min. increases with flow rate, and may at times exceed 24 h, suggesting that steady-state permeability values published in the literature for this and other sites, based on much shorter air injection tests, may not be entirely valid; 4.
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From page 301... ...
air permeabilities exhibit a hysteretic variation with applied pressure; 9. the pressure-dependence of air permeability suggests that it is advisable to conduct single-hole air injection tests at several applied flow rates and/or pressures; 10.
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A total of 44 cross-hole pneumatic interference tests of various types (constant injection rate, multiple step injection rates, instantaneous injection) have been conducted during 1995-1997 using various configurations of injection and monitoring intervals.
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interval. In this paper, we interpret transient pressure data from the single-hole air injection tests previously conducted at the ALRS by Guzman et al.
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From page 304... ...
Type-Curve Interpretation of Single-Hole Tests We have interpreted over 40 sets of 1-m scale single-hole pneumatic injection test data by means of the spherical, radial, vertical, and horizontal fracture flow models described in the previous section (Illman et al., 1998; Illman, 1999~. The majority of these data conform to the spherical flow model regardless of number or orientation of fractures in a test interval.
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From page 305... ...
The data exhibit a good match with type curves that correspond to zero skin (s = 0~; indeed, most test data from the ALRS show little evidence of a skin effect. The match yields an air permeability value of 1.56 x 1o-~5 m2.
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From page 306... ...
Figure 10-6 shows type-curve matches for single-hole test JGA0605 in a 1-m interval. In this case, the early and intermediate data appear to fit the radial flow model but the late pressure data stabilize, and the late pressure derivative data 1 .OE-13 1 .OE-1 4 N 1.OE-15 ct an 't 1.OE-16 an 1 .OE-17 1 .OE-18 ~ .
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. 1.0E-02 1.0E-01 1.0E+OO 1.0E+O1 Unit SIOPe behaV' 1.0E+O2 1.0E+O3 1.0E+O4 tJCD FIGURE 10-6 Type-curve match of data from single-hole pneumatic test JGA0605 to the gas, radial flow model.
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Figure 10-9 depicts an attempt on our part to match the corresponding incremental squared pressure data to type curves of pseudopressure based on the horizontal fracture flow model described in the previous section. Only the early time data appear to match one of these curves.
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LESSONS FROM FIELD STUDIES AT APACHE LEAP 1 .OE+01 1 .OE+OO 309 1 .OE-01 W/ hD=0.05 vertical fracture solution 1 .OE-02 , 1 .OE-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 tD FIGURE 10-9 Type-curve match of data from single-hole pneumatic test JHB0612 to the horizontal fracture model. 1 .OE+02 1 .OE+01 1.0E+OO 1 .OE-01 / / / / - o / O / O / O 1 .OE-02 1.0E-02 1.0E-01 1.0E+00 1.0E+01 l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l 1.0E+02 1.0E+03 1.0E+04 tD FIGURE 10-10 Type-curve match of data from single-hole pneumatic test JHB0612 to the vertical fracture model.
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From page 310... ...
intervals, air injection at a constant mass flow rate began. It generally continued for several days, until pressure in most monitoring intervals appeared to have stabilized.
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Test PP4 was conducted by injecting air at a rate of 50 slpm into a 2-m interval located 15-17 m below the lower lip of casing in borehole Y2, as indicated by a large solid circle in Figure 10-12. The figure also shows a system of Cartesian coordinates x, y, z with origin at the center of the injection interval, which we use to identify the placement of monitoring intervals relative to this center.
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From page 312... ...
the rock as if it was pneumatically uniform. However, since the analyses of pressure data from different monitoring intervals yield different values of pneumatic parameters, our analysis ultimately yields information about the spatial and directional dependence of these parameters.
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. 1.0E+02 1.0E+03 1.0E+04 tD FIGURE 10-13 Type-curve match of pressure and its derivative from monitoring interval V1.
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From page 314... ...
1 .OE+01 1 .OE+OO 1.0E-01 p1 =6.29 p2=0.40 1 .OE-02 Q=1 Q=10 Q=100 Q=1000 sr-~ : : A A A 1 .OE-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 U/' ~ : <` '. \ 1.0E+02 1.0E+03 1.0E+04 tD FIGURE 10-16 Type-curve match of pressure and its derivative from monitoring interval Z3M.
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From page 315... ...
Our numerical model thus accounts more fully and accurately for nonlinear pressure propagation and storage through both the rock and the boreholes than do our type curves, which rely on linearized airflow equations and ignore the effect of boreholes (other than the injection interval) on pressure distribution through the system.
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The air permeability k and air-filled porosity ~ are taken to be uniform within the computational region. In our inverse analyses, these two parameters are adjusted simultaneously with the effective porosity by of the injection interval.
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From page 317... ...
The three-dimensional grid represents quite accurately the geometry, flow properties, and storage capabilities of vertical and inclined boreholes at the site; is capable of resolving medium heterogeneity on a support scale of 1 m; is able to represent, with a high degree of resolution, steep gradients around the injection test interval, as well as pressure interference between boreholes, no matter how closely spaced; and assures smooth transition between fine borehole grids having radial structures and surrounding coarser grids having regular structures. Inverse Analysis of Single-Hole Tests A steady-state interpretation of pressure buildup data recorded during the first injection step (labeled A)
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From page 318... ...
, which renders model sensitivity to so low as to preclude it from computing confidence intervals for either parameter. When the effect of all open borehole intervals is included and the effective porosity of the injection interval is allowed to vary simultaneously with k and ¢, the match improves, yielding k = 2.2 x 10-~4 + 4.4 x 1o-~6 m2, ¢= 6.7 x 10-3 + 4.7 X 10-3, and By = 7.0 x 10-i + 6.7 x 10-2.
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Pressure data recorded in the injection interval
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FIGURE 10-22 Pressure data from single-hole test JG0921 interpreted by inverse model incorporating all boreholes.
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, and transient type-curve analysis with a spherical flow model yielded k = 2.9 x 1o-~5 m2 (Illman et al., 1998~. In the absence of open borehole intervals, the inverse model yields a poor fit with k = 1.8 x 1o-~5 + 3.9 X 1o-~5 m2 and ¢= 5.0 x 10-i + 4.2 x 10-2.
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From page 323... ...
only data corresponding to step A exhibit a pronounced one-to-one slope at early time, while all other data appear to be free of borehole storage influence. We therefore expect a simultaneous analysis of pressure data from the entire test to yield a more reliable estimate of parameters, especially air-filled porosity, than is possible based only on data from the first step.
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Air-filled porosity estimated by our inverse model on the basis of pressure data from injection interval Y2M during cross-hole test PP4 is highly uncertain, due to a very rapid pressure buildup in this interval. The large air-filled porosity
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From page 326... ...
estimates (0.5) obtained on the basis of pressure data from monitoring intervals X3, Z2L, Z2B, and Z3B are equal to their specified upper bound; we consider them highly unlikely due to poor fits between calculated and observed pressure responses in these intervals (Figure 10-26)
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Issues associated with the site characterization of fractured rock terrains. the analysis of fluid flow and contaminant transport in such terrains, and the efficient handling of contaminated sites are typically very difficult to resolve.
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From page 328... ...
2. In order to characterize the ability of unsaturated fractured rocks to conduct water, and to transport dissolved or suspended contaminants, one would ideally want to observe these phenomena directly by conducting controlled field hydraulic injection and tracer experiments within the rock.
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From page 329... ...
In a few single-hole tests, where the injection intervals are intersected by widely open fractures, air permeabilities decrease with applied pressure due to inertial effects. This pressure-dependence of air permeability suggests that it is advisable to conduct single-hole air injection tests at several applied flow rates and/or pressures.
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From page 330... ...
Only in a small number of singlehole test intervals, known to be intersected by widely open fractures, have such features dominated flow as evidenced by the development of an early half-slope on logarithmic plots of pressure versus time; unfortunately, the corresponding data do not fully conform to available type-curve models of fracture flow. Some pressure records conform to the radial flow model during early and intermediate times, but none do so fully at late time.
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13. Cross-hole pneumatic injection test data from individual monitoring intervals at the ALRS have proven amenable to analysis by type-curve and numerical inverse models that treat the rock as a uniform and isotropic fractured porous continuum.
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Laboratory Analysis of Fluid Flow and Solute Transport Through a Variably Saturated Fracture Embedded in Porous Tuff.
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From page 333... ...
Summary of Air Permeability Data From Single-Hole Injection Tests in Unsaturated Fractured Tuffs at the Apache Leap Research Site: Results of Steady-State Test Intepretation.
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Simulation of Liquid and Vapor Movement in Unsaturated Fractured Rock at the Apache Leap Tuff Site: Models and Strategies.
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