Rock Fractures and Fluid Flow Contemporary Understanding and Applications (1996) / Chapter Skim
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5 Hydraulic and Tracer Testing of Fractured Rocks
5 Hydraulic and Tracer Testing of Fractured Rocks
Pages 243-306
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From page 243... ...
The discussion is limited to hydraulic and tracer tests under single-phase, isothermal flow conditions in which the solute concentration is sufficiently dilute that density effects can be neglected. Not covered are hydraulic and pneumatic tests for evaluation of the vadose zone.
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From page 244... ...
In the petroleum industry the properties obtained are permeability and total compressibility. The distinction between hydraulic conductivity and transmissivity, or specific storage and storativity, is clear-cut when testing a porous medium, but confusion may arise in fractured rocks, as illustrated by the following example.
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From page 245... ...
The point of this example is not to discourage the use of hydraulic conductivity in test analysis but to emphasize the importance of reporting the test results with sufficient detail on the method of analysis so that readers are not misled. Hydraulic Testing in a Single Borehole In many subsurface investigations, particularly during the initial exploratory phase, hydraulic measurements (e.g., hydraulic head and flow rate)
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From page 246... ...
Open-borehole tests are easy to set up and do not require expensive test equipment. They provide information on the hydraulic properties of the tested rock mass as a whole.
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From page 247... ...
Q h Q t II Ql n Rae . t h 1 t t a: constant-flow test b: constant-head test c: slug test d: pressure-pulse test e: drillstem test h = hydraulic head Q = flowrate into well t = time | b FIGURE 5.2 Schematic plots of hydraulic head and flow rate versus time for singlehole hydraulic tests.
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From page 248... ...
, in which fluid is withdrawn at a constant rate from the test interval, is the most common test method in the discharge tar _ _ _ _ _ L ~ ~ - # #I ~ ~ ~ ~ ~ ; g ~ "g ~ , ! '9 ~ WW' WE ; ~ g ~ ~ ~ ~ ~ ~ I packer FIGURE 5.3 Equipment setup for a constant-flow pumping test in a single borehole.
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From page 249... ...
A pressure pulse test (Bredehoeft and Papadopulos, 1980) is similar in concept to a slug test, with the exception that the downhole valve is closed after the hydraulic head in the test interval is abruptly increased or decreased.
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From page 250... ...
The duration of a pressure pulse test depends on the transmissivity and the "system compressibility" of the test interval. System compressibility is
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From page 251... ...
Because head recovery is controlled by compressibility effects rather than filling or draining fluid from the pipe string, the duration of a pressure pulse test is much shorter than that of a slug test, all other factors being equal. For this reason, the pressure pulse test is commonly applied to test intervals of very low transmissivity.
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From page 252... ...
For these reasons, models for single-borehole hydraulic tests are primarily concerned with fluid flow in the immediate vicinity of the test interval. Flow in the vicinity of the test interval can be envisioned in terms of three basic geometries: spherical flow, radial flow, and linear flow.
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From page 253... ...
When applied to fractured-rock testing, the layer of porous medium represents a horizontal fracture zone or a single fracture bounded by impermeable rock (Figure 5.7b)
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From page 254... ...
FIGURE 5.7 (a) Radial flow to a cylinder in a homogeneous porous medium.
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From page 255... ...
introduced the concept of a fractional flow dimension to hydraulic test analysis. This concept provides a novel approach to interpreting hydraulic tests.
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From page 256... ...
In introducing the concept of fractional flow dimension, Barker allowed d to take nonintegral values. For example, a dimension between 2 and 3 represents a hybrid flow geometry between radial and spherical.
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From page 257... ...
Knowing the characteristic forms of the analytical solutions can be highly beneficial for selecting a flow model to analyze the hydraulic tests. For a constantflow test a plot of the log of drawdown versus the log of time illustrates the contrasting behaviors for different flow geometries.
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From page 258... ...
Numbers are flow dimensions. TABLE 5.1 Characteristic Behavior of Constant-Flow and Constant-Head Tests with Different Flow Geometries Flow Geometry Constant-Flow Test Constant-Head Test Spherical Constant s at late time Radial s proportional to log t at late time Linear s proportional to tin at late time Constant Q at late time 1/Q proportional to log t at late time 1/Q proportional to to during entire test Note: s denotes drawdown, t denotes time, and Q denotes discharge rate.
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From page 259... ...
If the hydraulic test is of a short duration, the well response will be controlled by the hydraulic properties of the near-well region. The response will be similar to that of a flow domain with a fractional flow dimension larger than 2.
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From page 260... ...
The stabilized head change and flow rate constitute the data from the test. If radial flow is assumed, the Theim formula (e.g., Bear, 1979)
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From page 261... ...
In such situations one may have to accept a range of hydraulic properties values until additional information becomes available to better guide selection of the flow model. In principle, transient analysis yields the transmissivity and storativity if radial flow is assumed or hydraulic conductivity and specific storage if spherical flow is assumed.
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From page 262... ...
When wellbore storage dominates the test response, a plot of log drawdown versus log time is a straight line of unit slope. Neglecting this effect will result in erroneous interpretation of the hydraulic test.
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From page 263... ...
However, the highly heterogeneous nature of fractured rocks can give the impression of a skin effect. For example, if a borehole intersects a locally tight portion of a fracture, the region around the borehole may appear to
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From page 264... ...
As in the case of a slug test, a family of type curves exists for analyzing a constant-flow test with wellbore storage and skin effects ([Figure 5.16) , and the test data must be matched by using type curve analysis.
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From page 265... ...
If flow is controlled by several highly transmissive fractures and the test objective is to characterize the interconnection between these fractures, multiple-borehole testing is indispensable. Pumping at a constant rate is the common method to conduct multipleborehole tests, but varying the pumping rate in a systematic fashion can be advantageous when hydraulic tests are affected by nearby activities such as well drilling, testing, or underground operations.
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From page 266... ...
The systematic orientation of the fractures imparts anisotropy to the hydraulic conductivity. The test objective is to determine the hydraulic conductivity tensor of the rock mass, as opposed to the transmissivity of any individual
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From page 267... ...
Example 5. In this structurally complex setting, there are horizontal, inclined, and vertical fracture zones cutting through the rock mass, which is itself fractured (Figure 5.211.
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From page 268... ...
Hydraulic tests may be limited to a single borehole or a few nearby boreholes. As a conceptual picture of the underground begins to emerge, full-scale, multiple-borehole tests, such as the one illustrated in Figure 5.21, can be conducted to investigate how the fracture zones are interconnected.
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From page 269... ...
it\ \ ~ _ ~ - an_ ~ ~ ..... A: pumped intermit B observat~onmterval \ ~ \ ~ ~- \ \ \ \ \ ~ \ ~:e FIGURE 5.20 Example setup of multiple-borehole hydraulic test to determine the hydraulic conductivity tensor of a rock containing a dense and well-connected network of fractures.
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From page 270... ...
.. ~ :N at_ :~ -add ~ E FIGURE 5.21 Example setup of a multiple-borehole hydraulic test in a rock mass with multiple sets of fractures and fracture zones.
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From page 271... ...
To model a field setting such as that in Example 5, one approach is to use a numerical model (such as a finite-element model) that treats the fracture zones as highly transmissive elements and the fractured rock mass as porous blocks.
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From page 272... ...
Solute Transport Processes The discussion of tracer tests below is prefaced with a short review of key processes that control solute transport. Some of these processes are well known in porous media theory, for example, advection, dispersion, and adsorption.
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From page 273... ...
Because tracer tests are commonly conducted over a relatively short distance, the validity of assuming Fickian dispersion remains an open question that awaits further research. The magnitude of the dispersivity term depends on how much detail is known about the heterogeneity of the rock.
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From page 274... ...
As a general rule, the longer the duration of the tracer test, the more significant the effects of diffusion into stagnant water and rock matrix. Adsorption In the context of tracer testing, adsorption refers to the tendency of the solute to attach to solid phases in the host rocks.
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From page 275... ...
Line thickness is proportional to the square root of the flow rate. From Tsang et al.
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From page 276... ...
The natural gradient tracer test is generally expensive and difficult to apply in fractured rocks. In contrast to a granular aquifer at shallow depth, where multilevel samplers can be easily installed, fractured rock sites require numerous wells for sampling.
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From page 277... ...
In such a setting, sampling from a grid of wells may miss a significant portion, or even all, of the tracer mass. Another problem with conducting a natural gradient tracer test in fractured rocks is the difficulty of collecting water samples that are representative of in situ conditions.
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From page 278... ...
In addition, if multiple sampling wells are used, a larger volume of rock can be investigated compared to the convergent flow tracer test discussed below. The disadvantages of the divergent flow tracer test are that a large water supply may be required for injection, the recharge water may clog the fractures, and the tracer is left in the rock at the end of the test.
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From page 279... ...
Convergent Flow Tracer Test In the convergent flow tracer test, water is pumped from a well until a steady flow field is established. A tracer is then injected, ideally as a pulse, into the flow system through a "tracer injection" well (Figure 5.25~.
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From page 280... ...
Tracer arrival at the pumped well is monitored by sampling the pumped water. The two-well tracer test is attractive because the pumped water can be used as the recharge water, thus eliminating the logistical problem of obtaining an independent water supply for a divergent flow tracer test or disposing of the pumped water during a convergent flow tracer test.
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From page 281... ...
HYDRAULIC AND TRACER TESTING OF FRACTURED ROCKS 281 1 / / · O pig we · injection weB direction of groundwave Row tracer Plume FIGURE 5.26 Setup of a two-well tracer test.
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From page 282... ...
Despite the complexities of flow and transport in fractured rocks, simple models, which are derived from single- or dual-porosity media assumptions and are based on advection and Fickian dispersion, continue to be the popular choice for analyzing tracer tests. A good summary of these models is given by Welty
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From page 283... ...
(b) Distribution of normalized hydraulic head.
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From page 284... ...
The adequacy of using simple analytical models to analyze tracer tests in fractured rocks is open to question. Given the highly heterogeneous nature of fractured rocks, flow paths are likely to be highly complex, and therefore not well described by simple flow geometries such as those shown in Figures 5.25 and 5.26.
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From page 285... ...
to characterize transport in a different flow geometry. Theoretical development in this area would significantly advance the interpretation of tracer tests in fractured rocks.
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From page 286... ...
In this regard the development of efficient numerical models of solute transport, coupled with parameter estimation algorithms, will greatly facilitate the analysis of tracer tests. On a theoretical level, the development of alternative approaches that are not based on the classical advection/Fickian dispersion model may lead to new solutions.
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From page 287... ...
For the fractal network, however, the number of conductive elements intersected by the circle of radius r is proportional to r059. The flow dimension in this network is 1.59.
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From page 288... ...
. If the rock mass can be represented as a homogeneous porous medium, a three-dimensional plot of the square root of the directional hydraulic diffusivity versus direction should form an ellipsoid.
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From page 289... ...
-(c) Polar plots of square roots of directional hydraulic diffusivities (dots)
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From page 290... ...
To maintain a small number of model parameters, homogeneity was assumed in each fracture zone and in the rock mass, although hydraulic properties could differ from one zone to another. The initial model consisted of nine parameters, which were estimated by using a numerical inversion program.
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From page 291... ...
The internal variation of hydraulic conductivity in a fracture zone or in the rock mass has little effect on the observed flow pattern.
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From page 292... ...
~ E 0 012 _ 0 009 0.006 9 Coos 2 ~ ~ tlYt tN HOUR, O D~UY OIODrN£ ~C~ON-IS ~ UP FIGURE 5.D1 Breakthrough curves of four tracers during a radially converging flow tracer test performed by Gamier et al.
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From page 293... ...
To analyze this tracer test, Maloszewski and Zuber (1990) developed a onedimensional transport model that accounts for advection, Fickian dispersion, matrix diffusion, and matrix adsorption that involves both an instantaneous equilibrium reaction and a nonequilibrium kinetic reaction of the first order.
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From page 294... ...
(1991a, l991b) , illustrate the complexities encountered in the analysis of solute transport in fractured rocks.
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From page 295... ...
The complex's flow system suggests that knowledge of the groundwater hydraulics and flow paths is critical for the design and analysis of tracer tests in fractured rocks.
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From page 296... ...
Hydraulic tests were planned in two boreholes drilled from the two drifts using the 1987 tomography results (Figure 5.F1~. Each test consisted of pumping water in a given interval at a constant pressure and monitoring in all the other intervals.
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From page 297... ...
HYDRA URIC AND TRACER TESTING OF FRACTURED ROCKS 297 1987 ALL DATA DAMAGE ZONE Cal W -l o o o · _ of o B DAMAGE ° ZONE ACCESS TUNNEL o 2 4 ~8 10 i O 2 4 ~ 8 AU TUNNEL _ ~.
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From page 298... ...
296 # 3 o m P1.4 P1.3 P1.2 il.3 11.1 Pl.1 # _~ I ] Lower labora10~ funnel Hydraulic packer - ~echan~al packer Scale o am Upper access 1unnal 13~' P3.2 3.1~ P3.1~ 134.1 ~ ~1 o .` go m P2.3 12.2 2 P22 it 12.1 P2.1 AGUE 5.F2 Packer 1oC~ioDS DSOd in by~aulic rests at the Fag site.
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From page 299... ...
Although the no-skin curve is closer to the observed curve, it still does not explain the observed inflection in the flow rate curve. Geological observations indicate that the fracture zone may be highly anisotropic with the highest permeability in the vertical direction.
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From page 300... ...
Leakage into an infinite-sized rock was considered through a Laplace space solution for the normalized pressure in the fracture zone at a nondimensional distance, rD, under a constant-pressure test with leakage into an infinite-sized rock. Values of drawdown and flow for the Laplace space solution are plotted in Figures S.FS and S.F6 for various combinations of fracture and matrix proper
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From page 301... ...
This localized leakage is not taken into account by the analytical solution; it could explain the low pressure in 13.1 and the flattening of the flow rate curve. Hydraulic tests have confirmed the hydrological significance of this fracture zone, which was previously identified by seismic tomography.
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From page 302... ...
o 11 ROCK FRACTURES AND FLUID FLOW 1o2 1o1 10 1o-1 -- . 1o-2 , ,,1 , ,,,, , ,,1 l l 'to , , , , , ,lll ~'~'~` l l l l lull l 10-2 10-1 l Time Do-23 FIGURE S.F6 Comparison of drawdown and flow for leakage into an infinite-sized rock with data from 12.1.
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From page 303... ...
Flow measurements on this zone included piezometric measurements, single-hole hydraulic tests at different scales, interference tests, groundwater flow measurements, and tracer tests, radially converging and dipole. A number of hydrological modeling efforts were then undertaken.
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From page 304... ...
1988. A generalized radial flow model for hydraulic tests in fractured rock.
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From page 305... ...
Methodology and application to fractured rocks. Water Resources Research, 21(11)
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From page 306... ...
Interpretation of field tracer tests of a single fracture using a transient solute storage model. Water Resources Research, 24(12)
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