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Information Content of Slug Tests for Estimating Hydraulic Properties In Journal of Hydrology 403 (2011) 66–82 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Information content of slug tests for estimating hydraulic properties in realistic, high-conductivity aquifer scenarios ⇑ Michael Cardiff a, , Warren Barrash a, Michael Thoma a, Bwalya Malama b a Boise State University, Center for Geophysical Investigation of the Shallow Subsurface (CGISS), Department of Geosciences, 1910 University Drive, MS 1536, Boise, ID 83725-1536, USA b Montana Tech of the Univ. of Montana, Dept. of Geological Engineering, 1300 West Park Street, Butte, MT 59701, USA article info summary Article history: A recently developed unified model for partially-penetrating slug tests in unconfined aquifers (Malama Received 12 August 2010 et al., in press) provides a semi-analytical solution for aquifer response at the wellbore in the presence Received in revised form 30 January 2011 of inertial effects and wellbore skin, and is able to model the full range of responses from over- Accepted 24 March 2011 damped/monotonic to underdamped/oscillatory. While the model provides a unifying framework for Available online 2 April 2011 realistically analyzing slug tests in aquifers (with the ultimate goal of determining aquifer properties This manuscript was handled by P. Baveye, such as hydraulic conductivity K and specific storage Ss), it is currently unclear whether parameters of Editor-in-Chief this model can be well-identified without significant prior information and, thus, what degree of infor- mation content can be expected from such slug tests. In this paper, we examine the information content Keywords: of slug tests in realistic field scenarios with respect to estimating aquifer properties, through analysis of Slug test both numerical experiments and field datasets. Aquifer characterization First, through numerical experiments using Markov Chain Monte Carlo methods for gauging parameter Wellbore skin uncertainty and identifiability, we find that: (1) as noted by previous researchers, estimation of aquifer Identifiability storage parameters using slug test data is highly unreliable and subject to significant uncertainty; (2) Kozeny–Carman joint estimation of aquifer and skin parameters contributes to significant uncertainty in both unless prior knowledge is available; and (3) similarly, without prior information joint estimation of both aquifer radial and vertical conductivity may be unreliable. These results have significant implications for the types of information that must be collected prior to slug test analysis in order to obtain reliable aquifer parameter estimates. For example, plausible estimates of aquifer anisotropy ratios and bounds on well- bore skin K should be obtained, if possible, a priori. Secondly, through analysis of field data – consisting of over 2500 records from partially-penetrating slug tests in a heterogeneous, highly conductive aquifer, we present some general findings that have applicability to slug testing. In particular, we find that aquifer hydraulic conductivity estimates obtained from larger slug heights tend to be lower on average (presumably due to non-linear wellbore losses) and tend to be less variable (presumably due to averaging over larger support volumes), supporting the notion that using the smallest slug heights possible to produce measurable water level changes is an important strategy when mapping aquifer heterogeneity. Finally, we present results specific to characterization of the aquifer at the Boise Hydrogeophysical Research Site. Specifically, we note that (1) K estimates obtained using a range of different slug heights give similar results, generally within ±20%; (2) correlations between estimated K profiles with depth at closely-spaced wells suggest that K values obtained from slug tests are representative of actual aquifer heterogeneity and not overly affected by near-well media disturbance (i.e., ‘‘skin’’); (3) geostatistical analysis of K values obtained indicates reasonable correlation lengths for sediments of this type; and (4) overall, K values obtained do not appear to correlate well with porosity data from previous studies. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Slug tests have become a primary method for analyzing aquifer transmissivity due to their relative speed and simplicity as com- ⇑ Corresponding author. Tel.: +1 208 426 4678; fax: +1 208 426 3888. pared with more labor-intensive tests such as pumping tests or E-mail addresses: [email protected] (M. Cardiff), wbarrash@ hydraulic tomography. Likewise, slug tests have proven beneficial boisestate.edu (W. Barrash), [email protected] (M. Thoma), bmalama@ at contaminated sites since they do not produce water during a test mtech.edu (B. Malama). 0022-1694/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2011.03.044 M. Cardiff et al. / Journal of Hydrology 403 (2011) 66–82 67 and thus may reduce characterization costs. In addition, partially- and wellbore skin may contribute significantly to uncertainty in penetrating slug tests such as those that can be performed within aquifer hydraulic parameters. packed-off intervals in a borehole are a beneficial source of infor- In this work, we undertake a study of slug test response in mation about depth variations in hydraulic conductivity, which is highly conductive aquifers under realistic field conditions (i.e., tak- not generally obtainable with traditional fully-penetrating pump- ing into account wellbore skin as well as timing issues that may be ing tests. In order to obtain aquifer parameter estimates from slug present in field data). In the first section of this work, we perform test records, curve fitting is generally carried out using one of a numerical experiments with the goal of understanding how esti- variety of analytic or semi-analytic models which assume homoge- mates of aquifer properties (and their associated uncertainty) are neous aquifer properties within the volume interrogated by slug affected by such conditions. We utilize the above-mentioned uni- test measurements. fied model in order to determine the sensitivity of estimated aqui- Depending on the type of aquifer being investigated (confined/ fer parameters and their uncertainty (as determined via inversion unconfined), the type of slug response observed (overdamped/ of synthetic data) under several different types of ‘‘prior’’ informa- underdamped), the location of the test (shallow/deep), the type tion. Then, in the second section of this work, we analyze a large of slug test performed (fully-penetrating/partially-penetrating), set of slug test field data from the Boise Hydrogeophysical and the existence of near-well disturbance (skin/no skin) a variety Research Site (BHRS) using the unified model with prior informa- of models may be used to analyze slug test data. Relatively simpler tion from studies at the site (Barrash et al., 2006). slug-test models may be used for analysis when the response ob- served is non-oscillatory or ‘‘overdamped’’ – as is generally the case in very shallow or low-conductivity aquifers – and where ver- 2. Mathematical model tical flow in the aquifer is deemed insignificant (Hvorslev, 1951; Cooper et al., 1967; Bouwer and Rice, 1976). In the overdamped re- Building on the work of Hyder et al. (1994) and Butler and Zhan sponse case where partial penetration produces significant vertical (2004) among others, Malama et al. (in press) developed a semi- flow, more complex models may be used (e.g., Hyder et al., 1994, analytical model for slug test response in unconfined aquifers that which also incorporates wellbore skin). takes into account partial penetration as well as skin effects at the While partially-penetrating slug tests present a potentially source well and inertial effects within the borehole. The solution quick and information-rich source of data for estimating depth- presented by Malama et al. (in press) satisfies the governing partial dependent aquifer heterogeneity, several difficulties are associ- differential equations (PDEs), boundary conditions, and initial con- ated with their implementation in highly conductive aquifers. ditions described below. Firstly, in highly conductive aquifers, slug response may be so fast that inertial effects within the wellbore become important, result- ing in oscillatory well water level responses. In order to duplicate such responses in numerical or analytical models, both the head response in the aquifer and inertial balances in the wellbore must be modeled, resulting in more computationally complex solutions (Bredehoeft et al., 1966; Van Der Kamp, 1976; Kipp, 1985; Spring- er and Gelhar, 1991; Hyder et al., 1994; Zlotnik and McGuire, 1998; Butler and Zhan, 2004). In the case of extremely fast-mov- ing wellbore water, even turbulent energy loss may contribute to slug response, resulting in non-linear responses (see, e.g., McEl- wee and Zenner, 1998). Secondly, the fast response of highly con- ductive systems means that, simply due to finite measurement frequency, it may be difficult to exactly define both the initial slug height and the time at which the test began (see, e.g., Butler, 1996, 1998), both of which are required as parameters for slug test modeling. Thirdly, since most slug test setups perform both slug injection/extraction and measurement at the same well, wellbore skin effects may contribute prominently to slug re- sponse (Faust and Mercer, 1984; Hyder et al., 1994; Malama et al., in press), meaning analysis
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