Measuring well hydraulic connectivity in fractured bedrock using periodic slug tests

Abstract

6Periodic hydraulic experiments were conducted in a five-spot well cluster completed in a single 7 bedding plane fracture. Tests were performed by using a winch-operated slug (submerged solid 8 cylinder) to create a periodic head disturbance in one well and observing the phase shift and 9 attenuation of the head response in the remaining wells. Transmissivity (T) and storativity (S) 10 were inverted independently from head response. Inverted T decreased and S increased with 11 oscillation period. Estimated S was more variable among well pairs than T, suggesting S may be 12 a better estimator of hydraulic connectivity among closely spaced wells. These estimates 13 highlighted a zone of poor hydraulic connection that was not identified by a constant rate test 14 conducted in the same wells. Periodic slug tests appear to be a practical and effective technique 15 for establishing local scale spatial variability in hydraulic parameters. 16 1 Introduction 17 Flow channelization has long been recognized as a hallmark of groundwater flow in fractured 18 bedrock systems [Tsang and Neretnieks, 1998]. The physical nature of the problem is described 19 simply through the cubic law which dictates that water flow rate is related cubically to the local 20 fracture aperture. Prediction of flow in a natural bedrock system is not so simple, however, as 21 the distribution of aperture and its interaction with water flow is highly variable even within a 22 single fracture. Understanding flow in bedrock consequently requires site specific hydraulic 23 characterization to be conducted. 24Typical pumping and slug test configurations are not well suited to bedrock environments. 25Because of the small water storage in bedrock, the hydraulic radius of influence of a pumping 26 well extends rapidly outward implying that only the earliest drawdown contains local 27 information. Early drawdown is often dominated by well-bore storage and formation damage 28 effects in open boreholes. Slug test responses are weighted more toward local hydraulics but are 29 even more sensitive to borehole influences. 30 A periodic hydraulic test potentially overcomes some of the limitations of pumping and slug 31 tests. Periodic (also called harmonic, oscillatory, or sinusoidal) tests are conducted by creating 32 an oscillating head in one well and observing the corresponding oscillatory head response in one 33 or more observation wells. Because the head signal is in a constant state of transience, periodic 34 tests highlight the influence of formation storativity on drawdown response. The repeatability of 35 the transience allow initial effects of well bore storage and pump priming to be isolated. Most 36 interestingly, periodic tests are capable of interrogating different portions of the formation 37without the addition of observations wells. This is because the spatial weighting of hydraulic 38 response to transmissivity (T) and storativity (S) is sensitive to the frequency of the head 39 oscillation [Cardiff et al., 2013;Renner and Messar, 2006]. Periodi...

What does hydraulic tomography tell us about fractured geological media? A field study and synthetic experiments

“…For example, Day-Lewis et al (2003 combined geophysical data and a conventional hydraulic test (a cross-hole test, not hydraulic tomography) as well as tracer tests to characterize the fracture-rock aquifer heterogeneity. Using the head responses induced by periodic slug tests, Guiltinan and Becker (2015) independently inverted the transmissivity and storativity in a sandstone bedrock containing a single, isolated subhorizontal fracture at the Altona Flat Rocks experimental site. They found that different oscillation periods lead to different estimates and periodic slug tests may be useful to establishing local scale spatial variability in hydraulic parameters.…”

Strategies for avoiding errors and ambiguities in the analysis of oscillatory pumping tests

“…They can also be performed at multiple frequencies in order to assess impacts of heterogeneity or check for "frequency dependence" in aquifer parameters. While hardware for performing oscillatory pumping tests is more complex than the hardware needed for simple constant rate pumping tests, computer-controlled pumping systems using oscillating slugs (Guiltinan and Becker 2015) or pneumatic systems (the Pneusinehttp://hydroresolutions.com/hr_press/wp-content/uploads/2013/02/PneuSine-Test-HydroResolutions-LLC.pdf) are beginning to be "packaged" as tools for aquifer characterization.…”

“…Here we present concrete examples of this effect, using the 2-D solution. To demonstrate the issue, we utilize the same analysis approach applied by Rasmussen et al (2003), and later by Guiltinan and Becker (2015). As a review, the approach consists of the following steps which are used to estimate aquifer transmissivity T Kl  , storativity s S S l  , and diffusivity / D T S  :…”

“…Implementing steps 1-4 above in MATLAB, we then attempted to estimate aquifer parameters using the approach used by Rasmussen et al (2003) and Guiltinan and Becker (2015). The results are contained in Table 2.…”

“…Oscillatory pumping tests, in which water is alternately injected into and extracted from an aquifer sinusoidally, have seen increasing interest recently as a useful strategy for characterizing both sedimentary aquifers (Rasmussen et al 2003, Zhou et al 2016) and fractured bedrock environments (Guiltinan and Becker 2015). Relative to standard constant-rate pumping tests, oscillatory tests have several advantages that make them practical for field experiments including:…”